Position Paper Essay

It is truly a T.G.I.F for elementary students in public schools because last September 16, 2010 Department of Education released a memorandum, called Memorandum 392 that orders elementary teachers to avoid giving their students home works during weekends. The Memorandum states that ‘ 1. Homework or assignments have been part of the pupils’ lives in their schooling. Common homework/assignment may include a period of reading to be done and writing to be completed, problems to be solved and projects to be worked on among others. The purpose of which is for the pupils to increase their knowledge and improve their abilities and skills. see more:should hw be banned 2. However, it has been observed that parents complain about too many pupils’ homework or assignments which rob themselves and their children quality time to be together in more enjoyable activities. 3. Hence it is advised that the teachers limit the giving of homework to reasonable quantity to give their ample time to rest and relax at home for the rest of the day. 4. Therefore no homework or assignment shall be given during weekends for pupils to enjoy their childhood and spend quality time with their parents without being burdened by the thought of doing lots of homework. Since teachers, parents and students always debate about this in the internet, I would like to share my stand in this issue. As a student and a future educator, I am in favor for the implementation of the No home works on weekend’s policy in grade school. First, according to number two guideline of the Deped Memorandum 392 S.2010; homework or assignment on weekends robs quality time of the parents and the children to be together in more enjoyable activities. I personally believe that weekends are for relaxing and of course bonding time with the family. According to a parent from www.debate.org, kids study 5 days a week with only a few hours with their family. It’s a difficult to manage, when teachers give home work over the weekend it takes away time from them doing sports, going outside, and even family time. According to a PNU Student, particularly a P.E Major she said that ‘we spend a lot more time awake at school. When we go home, we just sleep. We barely had time with our family’.  Also, according to a tumblr blogger which is a teacher; The State should see the value of quality Family bonding especially today when there seems to be a little time and opportunity for Parents to talk to their children. This quality time is a means to discuss sensitive issues that only parents can deliver and to strengthen the family ties. I believe that family time would be a productive and this time could help the children develop not only mentally but of course emotionally. Also according to an article in empoweringparents.com assignments always cause nightly conflicts among the parents and the child so I think that weekends can be a timeout and an exemption for this situation. According to the Assistant Secretary of DEPED, Mr. Toni Umali children should not only learn from school itself, they must also learn the importance of the family bonding. Aside from family matters, when kids are bombarded with a lot of assignments during weekend it does not only affects their bonding time with their family but also their health. I conducted a survey among PNU students and some of the answers concerns about the health of the students. According to a History major he is in favor of the policy because, in today’s educational system, a student’s week is crowded with too much school activities, leaving the student no room for recreation. Disallowing home works on weekends will be in favor of the student’s proper development not only mentally but also physically, emotionally and socially. Another response that came from our fellow English Major was ‘Yes, ‘coz students should relax their mind every weekend. Doing homework in weekends would stress out students and if students will be stressed out, their health would be at risk. According to teachthought.com Kids needs time to be kids, they need unstructured playtime; they need to play outside with their friends and of course physical exercise. Also a student answered that kids are studying 5 straight days a week and she don’t think that having 2 days break from it would be a reason for the kids to fail. A point was well raised by our fellow English major and a STA Major. The English major said that having no home works on weekends would inculcate laziness in the minds of our students So that students, in an early stage will learn how to prioritize and manage time. According to the STA Major ‘No, because as students it is our responsibility to do our assignments that  would enhance our deeper analysis or understanding in our lessons. It doesn’t mean that if it is weekend, we should enjoy or rest all through. As students, we need those home works for our weekend to be productive’ For me having no homework on weekend does not promote the habit of laziness, instead it promotes the importance of family bonding and that family should be a priority as mentioned above that is what memorandum 392 was meant for; to increase the time for the children to bond with their parents/family. Another well raised point was from a male student from University of Santo Tomas he said that he is not favor in No Homework Policy, because how come that the students understand the Lesson and also force them to reread again the lesson, so that they can deeply understand it. Well according to opencolleges.edu.au home works does not really equate to higher achievement. Study show that in elementary there is a weak link between achievement and home works. Most countries with less home works lead to have higher scores in achievement tests. Countries with more home work assigned like Greece, Thailand and Iran have the worst average scores. The last point that I would like to mention was a point raised by a fellow English Major that is not in favor of No Home works on weekend’s policy, she answered: No. Because home works act as supplements/follow-ups to the lessons taught inside the classroom. I believe so but I think it would be a great help if the assignments are not given during Fridays that would be pass on Monday the coming week. Let’s be considerate and give our kids free time to explore. Kids would also learn with life experiences that could be related with their topic in school. After all, kids are kids. They may not be able to absorb all the stress and the pressure in the environment maybe we’ll be surprised if one day our kids would be afraid to go to school just because he/she was not able to answer the assignments. According to teachthought.com homework may be a form of intrusion on family life, and may increase the drop-out rate in schools. The bottom line of this is we want what’s good for kids. We don’t train robots, we train humans. We don’t only fill the brains of our students; we should be able to fill their hearts. Let’s make learning easy and enjoyable  let us not take times in which children could do something great with their family and treasure the memory for the rest of their lives. Let us not make learning a horrible experience for our students, the government, the community, the teachers and most especially the parents should do their part to be able to develop a child into a intelligent, responsible and loving human being that could do something to promote excellence to the following generations. References: http://www.teachthought.com/teaching/20-reasons-you-shouldnt-assign-homework-over-the-holidays/ stophomework.com/fact.pdf http://juanrepublic.tumblr.com/post/1137647987/deped-memo-no-homework-shall-be-given-during-weekends http://www.debate.org/opinions/should-kids-have-homework-on-the-weekends http://www.debate.org/debates/should-Students-be-given-homework-during-the-weekends/1/ http://www.smartparenting.com.ph/kids/big-kids/poll-no-homework-on-weekends-policy-yes-or-no http://depedteacher.blogspot.com/2010/09/no-homework-shall-be-given-during.html http://www.empoweringparents.com/Homework-Hell-Part-l-How-to-Turn-It-Around.php http://happychild.mobi/articles/what-is-the-value-of-homework-research-and-reality http://www.youtube.com/watch?v=8xMo-QFB11E http://newsinfo.inquirer.net/inquirerheadlines/nation/view/20100919-293072/TGIF-No-homework-on-weekendsDepEd

Political Influences of Constructivist Theory Essay

There are several political influences that underlie the constructivist curriculum theory. Before the 1900s, the principles of constructivism were not very well regarded because it was generally thought that classroom instruction should be rigid and that teachers should be the sole generator of knowledge and while student’s only task in the classroom is to absorb that knowledge (Kitchener, 1986). During the 1920’s, educators such as Jean Piaget and John Dewey, began calling for educational reforms to adapt more â€Å"democratic† curricula that make use of constructivist ideals (Kitchener, 1986). These people inspired political movements that called for a different approach to learning. These political influences prompted the establishment of organizations that sought to veer away from the traditional learning methods. One such organization which was formed is the National Council of Teachers in Mathematics (NCTM). The NCTM saw how traditional methods called â€Å"drill and kill† numbed student’s minds and made learning inapplicable to practical circumstances and began to launch political and educational movements intended to help teachers facilitate learning more effectively in the classroom (NCTM, 2008). However even at the present, constructivist educational reforms have yet to take root in American legislation. Still, there are instances when political will is raised by the public to find new ways to deliver quality education. One such instance was in the latest release of the Trends in International Mathematics and Science Study (TIMMS) worldwide testing which showed that American students were performing poorly based on the world standards (AGI, 2008). This prompted movements that yet again called for a different approach to education outside of the traditional lecture methods. In 2002, the U. S. House Subcommittee on Education Reform introduced legislation that sought to overhaul the current research methods for deriving effective teaching practices and better explore constructivist approaches (AGI, 2008). This legislation is known as the Education Sciences Reform Act (AGI, 2008). In conclusion, it can be said that the constructivist curriculum is politically tied with the public’s desire to have better working educational models for their children. This want is what has driven movements to exist and call for government to address the problem.

Health Information Management System Proposal Essay

Health information technology can be best described as the point where information science, medicine, and healthcare all meet. The foundation of healthcare delivery consists of three major elements: cost, access, and quality. The U.S. government reportedly â€Å"hopes most Americans have electronic health records by 2014† (Ramachandran, 2013) .It is my proposal to upgrade to a current, cutting-edge health information management system in our office. Potential Benefits to the Practice Private practices are not a thing of the past. According to the AMA’s recent survey,†53.2% of physicians were self-employed in 2012, 41.8% were employed and 5% were independent contractors† (Robeznieks, 2013). Keeping that in mind, I have listed a few of the potential benefits of HIT to our practice: †¢Increased access to care – Having patient records stored electronically is in many cases, distance becomes irrelevant when consulting with a physician or the practice. For the patients that are out of the country or simply in another town, we can access their medical records and help them over the phone or allow other physicians access to their records at the patient’s request. †¢Allows for faster diagnosis when knowing the patient’s medical history and prescription history. Could possibly mean the difference between life and death. †¢Allows for a patient to record health information they measure at home, i.e. blood pressure, pulse, weight, blood glucose levels. This updated information can be used by the practice to keep tabs on patients for future visits or the need to schedule a visit. Potential Benefits to Staff In an online article written by Ken Terry, he stated that â€Å"41% of those who don’t have online access to records would consider switching physicians to obtain it† (Terry, 2013). Therefore, we need to look at other aspects that will benefit our staff in the long run. †¢Time equals money. Staff will spend less time retrieving, finding and filing patient charts. Now they can enter a patient’s name into the system and all their information will populate the screen. †¢Less money will be spent through the elimination of a transcription service as well as the transferring and transporting of patient charts. †¢Automated billing and accounting Necessary Training As one consulting company I spoke with stated, â€Å"understanding that one of the most important factors in a successful implementation is the quality of user training that takes place prior to the system activation† (Coastal Healthcare Consulting Inc., 2013). Due to the fact that we are a small practice, it will benefit everyone who has any contact with patients will be included in the training. Challenges with Implementing a New System †¢Over reliance on the accuracy of EMRs may lead to significant errors if a patient record contains false information. †¢Privacy and security risks due to hackers can include: identity theft, unauthorized access and corruption of patient data. †¢The cost of the equipment as well as the training that needs to be done. According to the Congressional Budget Office report written back in 2008 stated, â€Å"No aspect of health IT entails as much uncertainty as the magnitude of its potential benefits† (Devon M. Herrick, 2010). Our world has been radically changed in the last 5-10 years due to digital technology. Smart phones, tablets and web-enabled devices have changed the face of health care as we know it. This is our time to get involved and bring our practice into the present and solidify our future. References Coastal Healthcare Consulting Inc. (2013). Training. Retrieved September 22, 2013, from Coastal Healthcare Consulting Inc.: http://www.coastalhealthcare.com/services/training/ Devon M. Herrick, L. G. (2010). Health Information Technology: Benefits and Problems. Dallas: NCPA. Ramachandran, V. (2013, August 15). Technology Changing Healthcare. Retrieved September 20, 2013, from Mashable: http://mashable.com/2013/08/15/healthcare-and-it-health-informatics-infographic/ Robeznieks, A. (2013, September 17). Not All Doctors Giving Up Private Practice. Retrieved September 20, 2013, from Modern Health Care.com: http://www.modernhealthcare.com/article/20130917/BLOG/309179996 Terry, K. (2013, September 17). Patients Seek More Online Access To Medical Records. Retrieved September 19, 2013, from Information Week: http://www.informationweek.com/healthcare/policy/patients-seek-more-online-ac

Student with pervasive developmental disorders Essay

Student with pervasive developmental disorders - Essay Example st of them are not able to combine words into meaning by this age, while others are able either to repeat same words or phrases over and over or speak single words only. Various studies have indicated that PDD can considerably affect the progress of children in their academic, social, and personal development particularly within a standard environment where other normal students are growing and developing in (Syriopoulou- Deli, 2011). In order to have a better understanding of PDD and its impact on children, this paper will discuss the condition in respect to students suffering from the disorder. It is important to begin by noting that PDD is not a single disorder but rather it’s a category that comprises a range of delays in children of varying magnitude in various domains. The most severe of PDD is autism which indicates a primary or chief disturbance in the ability of a child to relate to others. Cognitive and language delays are the other primary characteristics of autism (Delfos, 2005). Students suffering from PDD may not be able to achieve academic and co-curricular success that students with normal abilities can achieve. For any student to be successful he or she needs normal abilities and cognitive skills. Nonetheless, if corrective measures are taken in time, students suffering from PDD may be able to achieve success in classroom and outside classroom. In particular, cognitive skills training can help a student with PDD to develop the necessary skills and abilities to succeed in classroom. Numerous studies have indicated that cognitive skills training in a gr eat way deliver essential skills and abilities to students with PDD (Holzhauser-Peters and True, 2008). There are symptoms associated with pervasive developmental disorder and parents and teachers should take immediate action once they notice them in a child. The first one is a child having difficulty in understanding and using language. Secondly, a child suffering from this disorder may display

Eusebius - History of the First Church Term Paper

Eusebius - History of the First Church - Term Paper Example Nothing is known of the parents of Eusebius, though, it is clear from his works that he must have received an extensive education in philosophy, biblical and theological sciences. He was greatly admired and cherished as a man and regarded as one who was very influential over his colleagues. At one time he was Presbyter of the Church, though it is difficult to narrow down an exact date for this. Eusebius was born C. 260 and lived through May of 339. He was baptized and ordained at Caesarea where Pamphilus gave him the name ‘Eusebius Pamphili’, meaning â€Å"son or servant of Pamphilus† (Eusebius Of Caesarea). It is possible that Eusebius was imprisoned for some time by Roman authorities as he was taunted years later for having escaped through some acts of submission though this is verified. He began to write the Ecclesiastical History during the Roman prosecutions and it was revised several times between 312 and 324. He described this work as a full and complete documented history of the Christian Church in which he used many paraphrasing and quotes from outside sources. This allowed earlier portions of work to be preserved which adds significant historical value. Eusebius’ role in the doctrinal debates and disputes within the Catholic Church during the fourth century included issues regarding the nature of Jesus Christ, enquiring as to the possibility that he had always existed in some form prior to human. Eusebius knew nothing of the Western Church and his strength was not in being a historian. Eusebius became the Bishop of Caesarea in 313 and in 318 became involved in controversy stemming from the views of a Priest from Alexander who taught subordination of the son to the father, and Eusebius, accordingly, soon became a leading supporter of Arius. Eusebius’ style was one that â€Å"weaved short entries into a broader scheme† (Ferguson), though some considered

Sociology Essay Example | Topics and Well Written Essays - 1000 words - 3

Sociology - Essay Example However, this can also be seen in the use of economic sanctions before the Iraq war, as Joy Gordon described in â€Å"Cool war: Economic sanctions as a weapon of mass destruction† (Gordon, 2002) While it is easy to see the destructive power of war as it impacts a country like Iraq economically through the destruction of resources, it should be noted that even the victorious power in a war may suffer economic consequences, such as the U.S. following WWII or Iraq. This is because of the great social cost of war production and mobilization; funds that could have been put to productive use in society instead were wasted and destroyed through bombs and war. The New York Times in 2009 described the total cost of the Iraq War at $860 billion USD, and though this may have helped military contractors it may have also reduced the ability of the country to grow economically through the effects of the redistribution and large-scale waste of resources. (Glanz, 2009) War itself may cause mi llions of deaths in a society. People who were once producers culturally may see economic destruction that takes years to rebuild. Examples in Asia show how landmines such as were used in Laos and Cambodia can disfigure populations for years, creating disabilities. Similarly, populations suffering wars in Africa have suffered mass-disfigurement through machete attacks and other weapons that leave people scared and disfigured, both physically and psychologically. These psychological forces can further reduce economic development following a war out of feelings of grief, depression, or loss that are felt in the victims. Question 2: what is the economic role of transnational corporations in integrating global economy? Multinational corporations currently play a role in developing economies and labour forces in the global economy, and their trading patterns furthers social integration internationally through the exchange of products and ideas. Dicken (1992) writes in â€Å"Global Shift ,† â€Å"The dominance of the industrialised countries’ markets is not as overwhelming as commonly perceived and the market potential of the developing and transition countries is becoming more impressive. The IMF defines as transition economy as an economy transitioning from a centrally planned economic system towards a free market system.† (Dicken, 2003) Dicken defines the modern economy as being related to over $54 trillion USD in economic activity. He notes that since the industrial revolution, the MNC or Multi-National Company has risen to represent the driving force of economic expansion internationally. These companies have the capital resources to build new factories, offices, and large production facilities that oftentimes developing nations or the small business owners in a society cannot manage. One reason for this is the ease through which MNCs can receive large financing through banks and corporate bonds. The other reason is more fundamental, in that rational management standards lead to an efficiency of operations, a specialization of labour, and the pursuit of profit in ever more scientific ways. MNCs drive technological expansion and innovation both through research and development in new products and also the introduction of these products into new markets. They can also promote jobs internationally by exploiting lower cost labour in

Discuss the implications of globalisation for both rich and poor Essay

Discuss the implications of globalisation for both rich and poor countries - Essay Example United Nation Development Program (UNDP) is concerned that inequalities and poverty level have increased in poor countries. UNDP indicates the major cause is global trade and finance system (World Bank, 2000). Another implication of globalization for the rich and the poor countries is that there is increased global protest and chaos. For instance, global trade means that investors from different countries have access to rich or poor countries thus increasing the chances of insecurity and drug trafficking. Security is important to poor and rich countries. While most countries join together to improve trade, in some cases insecurity is experienced. Likewise, illegal transactions have been reported for example importing of guns and elephant trunks which is mostly done by the residents of the countries (David, 2001). Drug trafficking like cocaine among other hard drugs is a threat to the countries residents especially to the youths since in most cases the drugs are sold to the youths thu s making a country lowers the economic status. This is based on the fact that young people support a country with the required output in the factories and in plantations among other areas. The rules and ideologies of globalization are that free trade is allowed, deregulation, privatization and structural adjustment which has left the poor countries residents to be homeless, landless and hungry. Since investors from other countries look for cheap labor, no training when recruiting new resources making the matter worse in poor countries. This means that access to health facilities, education, water, sanitation among other things is not easy thus resulting to negative effects on the productivity in such countries (Pritchett, 1998). On the other hand, the rich countries are not affected but rather they keep on increasing their wealth. For example, from 1986 to 2000, the total US wealth that was controlled by people increased from $7.2 trillion to $27 trillion. This

Cover letter Essay Example | Topics and Well Written Essays - 250 words - 7

Cover letter - Essay Example My lecturer recommended my work and asked me to assist him in some of his private assignment to gain more experience in the business world. I have taken internship exercises in two companies as indicated in the resume. The internship programs sharpened my financial skills by introducing me to the real world. In one firm, the firm assigned us an outdoor job description that required us to work in groups. As chairperson for the group, I ensured that every member committed to his assignment as described. In return, the firm awarded me with a certificate and a good recommendation for future endeavors. I have attached the certificates with the resume. I thank you in advance for considering my interview request. I would highly appreciate if you reach back on me after considering my request. Please feel free to reach me through my cell phone on 555-5678-786 anytime. I will follow up at the website to see the way forward decided by the firm in a week’s

Opportunities are at the heart of entrepreneurship. Critically discuss Essay

Opportunities are at the heart of entrepreneurship. Critically discuss this statement and explain how entrepreneurial opportunities are identified - Essay Example Entrepreneurial ventures provide more jobs than large firms. In the 70s and 60s, small enterprises had a net gain, despite severe recessions, in job creation. In the US, small firms account for two thirds of jobs created in the private sector (Sahlman, 2009 p26). Interestingly, over the same period, an approximated 500 companies lost five million jobs. Small and medium enterprises provide the local population with employment. This helps promote the development of such areas as inner cities. Sponsorship of local events also helps discover new talent which when developed proves beneficial to the local economy. Entrepreneurial opportunities can be defined as situations whereby new organizational methods, markets, raw materials, services, and goods can be introduced to existent ways of business operations. The introduction is via formation of new ends, means or means-ends relationships. The situations do not require a change in economic exchange terms in order to become entrepreneurial opportunities. Rather, all they need is potential to change the economic exchange terms. Entrepreneurial decisions, unlike satisfying or optimizing decisions, are creative decisions. This means that the entrepreneur creates the ends, means, or both in the chain. The creation of these frameworks of new means-ends in entrepreneurial making of decisions marks a crucial difference in optimization of previously established frameworks. They cannot be optimized for exploitation since they are unknown. Entrepreneurial decisions involve identification or creation of new ends or means previously undetected by participa nts in the market (Audretsch, 2010 p67). This paper aims to explain the role of opportunities in the process of entrepreneurship. It also seeks to examine entrepreneurship through a framework focused on the existence and characteristics of entrepreneurial opportunities. Finally, the paper seeks to discuss

The Knights Templar Research Paper Example | Topics and Well Written Essays - 1000 words

The Knights Templar - Research Paper Example Being a monastic military, the Knight Templar’s mandate was to protect Christian’s pilgrims on the route to the holy land. King Richard 1 commonly known as Richard the lion Hearted fought along side the Templars in the battle to the holy lands. The Templar was interesting because it had both soldiers and monks, making some of the earliest soldiers in the Western World. Orders played a vital role in many battles of the crusade to the extent that it improved modern banking, which could be considered as a notable improvement. The knights of the Templars started in small group, and as time progressed it grew in membership. The members of the group possessed certain duties and lifestyles, which made them unique in all their endeavors. As the military fighting for Christ, the Templars kept aside every temptations of the ordinary secular life, for a dedicated life of holiness and service to God. Since then, the concept of service has been the legacy of the Templars whereby mos t people emulate the military group. To many, the military group fought for Christ extraordinarily putting aside every secular life for an arduous life of service. In reality, the Templars’ services were greatly appreciated given the fact that, they gave their lives fully for the sake of Christ. At this point, the following paper aims to discuss the Knights Templar through giving out a brief history, its mandatory purpose, and its religious benefits. To Hale (1), the Knight Templars were the soldiers of the Christ throughout the medieval world of Solomon’s Temple. This implies that the Knight Templars were devoted soldiers who had set their lives for the service. Historically, the Templars originated from the Kingdom of Jerusalem around 118 AD whereby nine members vowed to protect the pilgrim of the dangerous road leading to Jerusalem. The Templars members combined the rules of knight in a remarkable way that the western world had never seen before. Their passion to se rve was so notable that, King Baldwin 11 of Jerusalem granted part of his palace to them for their headquarters, which thereafter a became a stable place of the Temple Mount which was referred to as Solomon’s Temple. After a while, the temple was captured by the Muslims whereby they built their holiest site commonly known as the Temple Mount. Despite controversial that emerged from who possessed the temple, the site remained sacred to the Jews, Muslims, and Christians. Many assumed the temple was significant to Christians since it was the storage of the Ark of the Covenant with other biblical treasures. After twenty years of crusaders conquest of Jerusalem, the noble French knight collected eight knights where several of them were relatives. Their stated assignment was to protect pilgrims of the holy land in service as military members gave their lives fully to the commitment of Christ. The knights were seen as complementary to the society in the sense that, they cared for si ck and weary pilgrims while the Templars guarded those approaching and those who left the city. The Knights Templar was emulated by many, particularly the official blessings of the church of the Council of Troyes, which grew dramatically hence the group was showered with blessings and donations. The group grew exceptionally, and as time progressed, they received land and money to assist the fight in the holy land. The group was so respected that, no princes, kings, or bishop could command

Ethical and Social Responsibility at Hyundai Motor Company Essay

Ethical and Social Responsibility at Hyundai Motor Company - Essay Example By being ethically and socially responsible, the company ensures that it maximizes positive impact on the society, while reducing negative societal impact. Accordingly, a company is supposed to ensure legal, economic and charitable humanitarian responsibilities (Ihlen, Bartlett & May, 2011). The Hyundai motor company is an automobiles manufacturing and distribution company, of South Korean origin. This company has dealings in many other world regions including; Europe, America as well as the Middle East. According to Lansbury et al. (2007), among the company’s main concerns is, corporate social responsibility via corporate citizenship, through which, the company makes its contribution to the global society. For Hyundai motor company, ethical responsibility encompasses social contribution, environmental management and trust-based administration. As far as social contribution is concerned, the company has enlarged its capacity and obtained expertise for effective global projects (Kaslow, 2006). Hyundai also participates in charitable initiatives that contribute to the improvement of the lives of the local population. Trust-based administration focuses on the improvement of labor dealings, mutual benefit between them and their suppliers, as well as transparent management. Lastly, environmental management deals with responding to global tendencies, as well as environmentally related laws. It recognizes that the environment is an essential element in any business, therefore, promotes the manufacturing and supply of environmentally safe goods. The Hyundai Company also ensures the reduction of pollutants, while preserving sources of energy, from product manufacture to its disposal. Needle (2010) states that, this is achieved through training programs offered to all employees on international laws and regulations concerning the environment. Generally, the company aims at gaining trust from both the public and the government and pursuing qualitative and quantitat ive growth. In addition, Hyundai seeks to lead the industry in sustainable management, improve corporate competitiveness and grow as a revered company. Hyundai concerns itself with communities suffering the aftermaths of natural disasters, by providing quick and effective assistance. In the summer of 2005, Austria was hit by unusual floods on the Alps. In response, the Hyundai Motor Company sent financial help and provided vehicular support to help in the evacuation efforts. According to Hyundai worldwide (2010), this gesture was well received by Austria and served to solidify the Hyundai presence in that region. Through its subsidiary company in America, Hyundai has positively impacted on the society, especially through its â€Å"hope on wheels† foundation that has funded the field of pediatric cancer study and treatment. Hyundai is also an ardent supporter of the American society against cancer, to whose end; it has given financial aid since 2001. Hyundai is also a corporat e sponsor of the association that calls for equal rights for all Americans, thus ensuring political, social, educational and economic rights of the minority in the American system. These activities have helped Hyundai gain many accolades globally. The Hyundai Motor

African Literature Essay Example for Free

African Literature Essay Despite the ignorance of most so called literati to the domain of African literature, African literature in fact is one of the main currents of world literature, stretching continuously and directly back to ancient history. Achebe did not invent African Literature, because he himself was inundated with it as an African. He simply made more people aware of it. The Beginnings of African Literature The first African literature is circa 2300-2100, when ancient Egyptians begin using burial texts to accompany their dead. These include the first written accounts of creation the Memphite Declaration of Deities. Not only that, but papyrus, from which we originate our word for paper, was invented by the Egyptians, and writing flourished. In contrast, Sub-Saharan Africa feature a vibrant and varied oral culture. To take into account written literary culture without considering literary culture is definitely a mistake, because they two interplay heavily with each other. African oral arts are arts for lifes sake (Mukere) not European arts for arts sake, and so may be considered foreign and strange by European readers. However, they provide useful knowledge, historical knowledge, ethical wisdom, and creative stimuli in a direct fashion. Oral culture takes many forms: proverbs and riddles, epic narratives, oration and personal testimony, praise poetry and songs, chants and rituals, stories, legends and folk tales. This is present in the many proverbs told in Things Fall Apart, and the rich cultural emphasis of that book also is typically African. The earliest written Sub-Saharan Literature (1520) is heavily influenced by Islamic literature. The earliest example of this is the anonymous history of the city-state of Kilwa Kisiwani. The first African history, History of the Sudan, is written by Abd al-Rahman al-Sadi in Arabic style. Traveling performers, called griots, kept the oral tradition alive, especially the legends of the Empire of Mali. In 1728 the earliest written Swahili work,Utendi wa Tambuka borrows heavily from Muslim tradition. However, there are little to no Islamic presence in Things Fall Apart. The Period of Colonization With the period of Colonization, African oral traditions and written works came under a serious outside threat. Europeans, justifying themselves with the Christian ethics, tried to destroy the pagan and primitive culture of the Africans, to make them more pliable slaves. However, African Literature survived this concerted attack. In 1789, The Interesting Narrative of the Life of Olaudah Equiano, or Gustava Vassa was the first slave narrative to be published. Kidnapped from Nigeria, this Ibo man wrote his autobiography in Great Britain in English, and like Achebe used his narrative as a platform to attack the injustices of slavery and cultural destruction. Back in Africa, Swahili poetry threw off the dominating influence of Islam and reverted back to native Bantu forms. One exemplar of this was Utendi wa Inkishafi (Souls Awakening), a poem detailing the vanity of earthly life. The Europeans, by bringing journalism and government schools to Africa, helped further the development of literature. Local newspapers abounded, and often they featured sections of local African poetry and short stories. While originally these fell close to the European form, slowly they broke away and became more and more African in nature. One of these writers was Oliver Schreiner, whose novel Story of an African Farm (1883) is considered the first African classic analysis of racial and sexual issues. Other notable writers, such as Samuel Mqhayi and Thomas Mofolo begin portraying Africans as complex and human characters. Achebe was highly influenced by these writers in their human portrayal of both sides of colonization. Emerging from Paris in the 1920s and 1930s, the negritude movement established itself as one of the premiere literary movements of its time. It was a French-speaking African search for identity, which ofcourse took them back to their roots in Africa. Africa was made into a metaphorical antipode to Europe, a golden age utopia, and was often represented allegorically as a woman. In a 1967 interview, Cesaire explained: We lived in an atmosphere of rejection, and we developed an inferiority complex. The desire to establish an identity begins with a concrete consciousness of what we are†¦that we are black . . . and have a history. . . [that] there have been beautiful and important black civilizations†¦that its values were values that could still make an important contribution to the world. Leopold Sedar Senghor, one of the prime thinkers of this movement, eventually became president of the country of Senegal, creating a tradition of African writers becoming active political figures. Achebe was doubtless familiar with the negritude movement, although he preferred to less surrealistic and more realistic writing. In 1948, African literature came to the forefront of the world stage with Alan Patons publishing of Cry the Beloved Country. However, this book was a somewhat paternalistic and sentimental portrayal of Africa. Another African writer, Fraz Fanon, also a psychiatrist, becomes famous in 1967 through a powerful analysis of racism from the African viewpoint Black Skin, White Masks. Camara Laye explored the deep psychological ramification of being African in his masterpiece, The Dark Child (1953), and African satire is popularized by Mongo Beti and Ferdinand Oyono. Respected African literary critic Kofi Awoonor systematically collects and translates into English much of African oral culture and art forms, preserving native African culture. Chinua Achebe then presents this native African culture in his stunning work, Things Fall Apart. This is probably the most read work of African Literature ever written, and provides a level of deep cultural detail rarely found in European literature. Achebes psychological insight combined with his stark realism make his novel a classic. Post-Achebe African Literature Achebe simply opened the door for many other African literati to attain international recognition. East Africans produce important autobiographical works, such as Kenyans Josiah Kariuki’s Mau Mau Detainee (1963), and R. Mugo Gatheru’s Child of Two Worlds (1964). African women begin to let their voice be heard. Writers such as Flora Nwapa give the feminine African perspective on colonization and other African issues. Wole Soyinka writes her satire of the conflict between modern Nigeria and its traditional culture in her book The Interpreters (1965). A prolific writer, she later produces famous plays such as Death and The Kings Horseman. Later, in 1986, she is awarded the Nobel Prize in Literature. African Literature gains more and more momentum, and Professor James Ngugi even calls for the abolition of the English Department in the University of Nairobi, to be replaced by a Department of African Literature and Languages. African writers J. M. Coetzee, in his Life and Times of Michael K. written in both Afrikaans and English for his South African audience, confronts in literature the oppressive regime of apartheid. Chinua Achebe helps reunite African Literature as a whole by publishing in 1985 African Short Stories, a collection of African short stories from all over the continent. Another African writer, Naguib Mahfouz, wins the Nobel Prize in literature in 1988. In 1990 African poetry experiences a vital comeback through the work I is a Long-Memoried Woman by Frances Anne Soloman. African Literature is only gaining momentum as time marches onwards.

High Performance Wireless Telecommunications Modulation

High Performance Wireless Telecommunications Modulation Introduction The primary goal of the project is to analyze of OFDM system and to assess the suitability of OFDM as a modulation technique for wireless communications. In the part of project is covered two leading successfully implementation of OFDM based technologies are Digital Video Broadcasting (DVB-T and DVB-H) and Long Term Evolution (LTE advanced for 4G). Wireless communications is an emerging field, which has seen enormous growth in the last several years. The huge uptake rate of mobile phone technology, Wireless Local Area Networks (WLAN) and the exponential growth of the Internet have resulted in an increased demand for new methods of obtaining high capacity wireless networks. For cellular mobile applications, we will see in the near future a complete convergence of mobile phone technology, computing, Internet access, and potentially many multimedia applications such as video and high quality audio. In fact, some may argue that this convergence has already largely occurred, with the advent of being able to send and receive data using a notebook computer and a mobile phone. The goal of third and fourth generation mobile networks is to provide users with a high data rate, and to provide a wider range of services, such as voice communications, videophones, and high speed Internet access. The higher data rate of future mobile networks will be achieved by increasing the amount of spectrum allocated to the service and by improvements in the spectral efficiency. OFDM is a potential candidate for the physical layer of fourth generation mobile systems. Basic Principles of OFDM OFDM overview The Orthogonal Frequency Division Multiplexing (OFDM) is a modulation technique where multiple low data rate carriers are combined by a transmitter to form a composite high data rate transmission. The first commercial use of OFDM in the communication field was in the 1980s, and it was later widely used in the broadcast audio and video field in the 1990s in such areas as, ADSL, VHDSL, ETSI standard digital audio broadcast (DAB), digital video broadcast (DVB), and high-definition digital TV (HDTV). Digital signal processing makes OFDM possible. To implement the multiple carrier scheme using a bank of parallel modulators would not be very efficient in analog hardware. However, in the digital domain, multi-carrier modulation can be done efficiently with currently available DSP hardware and software. Not only can it be done, but it can also be made very flexible and programmable. This allows OFDM to make maximum use of available bandwidth and to be able to adapt to changing system requirements. Figure 1 is illustrated, Instead of separate modulators; the outgoing waveform is created by executing a high-speed inverse DFT on a set of time-samples of the transmitted data (post modulation). The output of the DFT can be directly modulated onto the outgoing carrier, without requiring any other components. Each carrier in an OFDM system is a sinusoid with a frequency that is an integer multiple of a base or fundamental sinusoid frequency. Therefore, each carrier is like a Fourier series component of the composite signal. In fact, it will be shown later that an OFDM signal is created in the frequency domain, and then transformed into the time domain via the Discrete Fourier Transform (DFT). Two periodic signals are orthogonal when the integral of their product, over one period, is equal to zero. This is true of certain sinusoids as illustrated in Equation 1. Definition of Orthogonal The carriers of an OFDM system are sinusoids that meet this requirement because each one is a multiple of a fundamental frequency. Each one has an integer number of cycles in the fundamental period. [2, 145-153; 6] The importantance of being orthogonal The main concept in OFDM is orthogonality of the sub-carriers.Since the carriers are all sine/cosine wave, we know that area under one period of a sine or a cosine wave is zero. Lets take a sine wave of frequency m and multiply it by a sinusoid (sine or a cosine) of a frequency n, where both m and n are integers. The integral or the area under this product is given by These two components are each a sinusoid, so the integral is equal to zero over one period. When we multiply a sinusoid of frequency n by a sinusoid of frequency m/n the area under the product is zero. In general for all integers n and m , sin(mx), cos(mx), cos(nx) , sin(nx) are all orthogonal to each other. These frequencies are called harmonics. Making the subcarriers mathematically orthogonal was a breakthrough for OFDM because it enables OFDM receivers to separate the subcarriers via an FFT and eliminate the guard bands. As figure 3 shows, OFDM subcarriers can overlap to make full use of the spectrum, but at the peak of each subcarrier spectrum, the power in all the other subcarriers is zero. OFDM therefore offers higher data capacity in a given spectrum while allowing a simpler system design. Creating orthogonal subcarriers in the transmitter is easy using an inverse FFT. To ensure that this orthogonality is maintained at the receiver (so that the subcarriers are not misaligned), the system must keep the transmitter and receiver clocks closely synchronizedwithin 2 parts per million in 802.11a systems. The 802.11a standard therefore dedicates four of its 52 subcarriers as pilots that enable phase-lock loops in the receiver to track the phase and frequency of the incoming signal. The 802.11a standard therefore dedicates four of its 52 subcarriers as pilots that enable phase-lock loops in the receiver to track the phase and frequency of the incoming signal. This method also eliminates low-frequency phase noise.Separating the subcarriers via an FFT require about an order of magnitude fewer multiply-accumulate operations than individually filtering each carrier. In general, an FFT implementation is much simpler than the RAKE receivers used for CDMA and the decision-feedback equalizers for TDMA.This idea are key to understanding OFDM. The orthogonality allows simultaneously transmission on a lot of sub- carriers in a tight frequency space without interference form each other. In essence this is similar to CDMA, where codes are used to make data sequences independent (also orthogonal) which allows many independent users to transmitin same space successfully.[2, 153-154; 6 ; 7] OFDM Operation Preliminary Concepts When the DFT (Discrete Fourier Transform) of a time signal is taken, the frequency domain results are a function of the time sampling period and the number of samples as shown in Figure 4. The fundamental frequency of the DFT is equal to 1/NT (1/total sample time). Each frequency represented in the DFT is an integer multiple of the fundamental frequency. Parameter Mapping from Time to Frequency for the DFT The maximum frequency that can be represented by a time signal sampled at rate 1/T is fmax = 1/2T as given by the Nyquist sampling theorem. This frequency is located in the center of the DFT points. All frequencies beyond that point are images of the representative frequencies. The maximum frequency bin of the DFT is equal to the sampling frequency (1/T) minus one fundamental (1/NT).The IDFT (Inverse Discrete Fourier Transform) performs the opposite operation to the DFT. It takes a signal defined by frequency components and converts them to a time signal. The parameter mapping is the same as for the DFT. The time duration of the IDFT time signal is equal to the number of DFT bins (N) times the sampling period (T).It is perfectly valid to generate a signal in the frequency domain, and convert it to a time domain equivalent for practical use (The frequency domain is a mathematical tool used for analysis. Anything usable by the real world must be converted into a real, time domain signal). This is how modulation is applied in OFDM. In practice the Fast Fourier Transform (FFT) and IFFT are used in place of the DFT and IDFT, so all further references will be to FFT and IFFT.[1 ,118 ; 4] Definition of Carriers The maximum number of carriers used by OFDM is limited by the size of the IFFT. This is determined as follows in Equation 2. OFDM Carrier Count In order to generate a real-valued time signal, OFDM (frequency) carriers must be defined in complex conjugate pairs, which are symmetric about the Nyquist frequency (fmax). This puts the number of potential carriers equal to the IFFT size/2. The Nyquist frequency is the symmetry point, so it cannot be part of a complex conjugate pair. The DC component also has no complex conjugate. These two points cannot be used as carriers so they are subtracted from the total available. If the carriers are not defined in conjugate pairs, then the IFFT will result in a time domain signal that has imaginary components. This must be a viable option as there are OFDM systems defined with carrier counts that exceed the limit for real-valued time signals given in Equation 2.In general, a system with IFFT size 256 and carrier count 216. This design must result in a complex time waveform. Further processing would require some sort of quadrature technique (use of parallel sine and cosine processing paths). In this report, only real-value time signals will be treated, but in order to obtain maximum bandwidth efficiency from OFDM, the complex time signal may be preferred (possibly an analogous situation to QPSK vs. BPSK). Equation 2, for the complex time waveform, has all IFFT bins available as carriers except the DC bin. Both IFFT size and assignment (selection) of carriers can be dynamic. The transmitter and receiver just have to use the same parameters. This is one of the advantages of OFDM. Its bandwidth usage (and bit rate) can be varied according to varying user requirements. A simple control message from a base station can change a mobile units IFFT size and carrier selection.[2,199-206; 4] Modulation Binary data from a memory device or from a digital processing stream is used as the modulating (baseband) signal. The following steps may be carried out in order to apply modulation to the carriers in OFDM: combine the binary data into symbols according to the number of bits/symbol selected convert the serial symbol stream into parallel segments according to the number of carriers, and form carrier symbol sequences apply differential coding to each carrier symbol sequence convert each symbol into a complex phase representation assign each carrier sequence to the appropriate IFFT bin, including the complex conjugates take the IFFT of the result OFDM modulation is applied in the frequency domain. Figure 5 and Figure 6 give an example of modulated OFDM carriers for one symbol period, prior to IFFT. OFDM Carrier Magnitude prior to IFFT For this example, there are 4 carriers, the IFFT bin size is 64, and there is only 1 bit per symbol. The magnitude of each carrier is 1, but it could be scaled to any value. The phase for each carrier is either 0 or 180 degrees, according to the symbol being sent. The phase determines the value of the symbol (binary in this case, either a 1 or a 0). In the example, the first 3 bits (the first 3 carriers) are 0, and the 4th bit (4th carrier) is a 1. OFDM Carrier Phase prior to IFFT Note that the modulated OFDM signal is nothing more than a group of delta (impulse) functions, each with a phase determined by the modulating symbol. In addition, note that the frequency separation between each delta is proportional to 1/N where N is the number of IFFT bins. The frequency domain representation of the OFDM is described in Equation 3. OFDM Frequency Domain Representation (one symbol period) After the modulation is applied, an IFFT is performed to generate one symbol period in the time domain. The IFFT result is shown in 7. It is clear that the OFDM signal has varying amplitude. It is very important that the amplitude variations be kept intact as they define the content of the signal. If the amplitude is clipped or modified, then an FFT of the signal would no longer result in the original frequency characteristics, and the modulation may be lost. This is one of the drawbacks of OFDM, the fact that it requires linear amplification. In addition, very large amplitude peaks may occur depending on how the sinusoids line up, so the peak-to-average power ratio is high. This means that the linear amplifier has to have a large dynamic range to avoid distorting the peaks. The result is a linear amplifier with a constant, high bias current resulting in very poor power efficiency. OFDM Signal, 1 Symbol Period Figure 8 is provided to illustrate the time components of the OFDM signal. The IFFT transforms each complex conjugate pair of delta functions (each carrier) into a real-valued, pure sinusoid. Figure 8 shows the separate sinusoids that make up the composite OFDM waveform given in Figure 7. The one sinusoid with 180 phase shift is clearly visible as is the frequency difference between each of the 4 sinusoids. Transmission The key to the uniqueness and desirability of OFDM is the relationship between the carrier frequencies and the symbol rate. Each carrier frequency is separated by a multiple of 1/NT (Hz). The symbol rate (R) for each carrier is 1/NT (symbols/sec). The effect of the symbol rate on each OFDM carrier is to add a sin(x)/x shape to each carriers spectrum. The nulls of the sin(x)/x (for each carrier) are at integer multiples of 1/NT. The peak (for each carrier) is at the carrier frequency k/NT. Therefore, each carrier frequency is located at the nulls for all the other carriers. This means that none of the carriers will interfere with each other during transmission, although their spectrums overlap. The ability to space carriers so closely together is very bandwidth efficient. OFDM Time Waveform Figure 9 shows the OFDM time waveform for the same signal. There are 100 symbol periods in the signal. Each symbol period is 64 samples long (100 x 64 = 6400 total samples). Each symbol period contains 4 carriers each of which carries 1 symbol. Each symbol carries 1 bit. Note that Figure 9 again illustrates the large dynamic range of the OFDM waveform envelope. OFDM Spectrum Figure 10 shows the spectrum for of an OFDM signal with the following characteristics: 1 bit / symbol 100 symbols / carrier (i.e. a sequence of 100 symbol periods) 4 carriers 64 IFFT bins spectrum averaged for every 20 symbols (100/20 = 5 averages) Red diamonds mark all of the available carrier frequencies. Note that the nulls of the spectrums line up with the unused frequencies. The four active carriers each have peaks at carrier frequencies. It is clear that the active carriers have nulls in their spectrums at each of the unused frequencies (otherwise, the nulls would not exist). Although it cannot be seen in the figure, the active frequencies also have spectral nulls at the adjacent active frequencies. It is not currently practical to generate the OFDM signal directly at RF rates, so it must be up converted for transmission. To remain in the discrete domain, the OFDM could be upsampled and added to a discrete carrier frequency. This carrier could be an intermediate frequency whose sample rate is handled by current technology. It could then be converted to analog and increased to the final transmit frequency using analog frequency conversion methods. Alternatively, the OFDM modulation could be immediately converted to analog and directly increased to the desired RF transmits frequency. Either way, the selected technique would have to involve some form of linear AM (possibly implemented with a mixer). [1, 122-125; 6] Reception and Demodulation The received OFDM signal is down converted (in frequency) and taken from analog to digital. Demodulation is done in the frequency domain (just as modulation was). The following steps may be taken to demodulate the OFDM: partition the input stream into vectors representing each symbol period take the FFT of each symbol period vector extract the carrier FFT bins and calculate the phase of each calculate the phase difference, from one symbol period to the next, for each carrier decode each phase into binary data sort the data into the appropriate order OFDM Carrier Magnitude following FFT Figure 11 and Figure 12 show the magnitude and spectrum of the FFT for one received OFDM symbol period. For this example, there are 4 carriers, the IFFT bin size is 64, there is 1 bit per symbol, and the signal was sent through a channel with AWGN having an SNR of 8 dB. The figures show that, under these conditions, the modulated symbols are very easy to recover. OFDM Carrier Phase following FFT In Figure 12 that the unused frequency bins contain widely varying phase values. These bins are not decoded, so it does not matter, but the result is of interest. Even if the noise is removed from the channel, these phase variations still occur. It must be a result of the IFFT/FFT operations generating very small complex values (very close to 0) for the unused carriers. The phases are a result of these values. [1, 125 -128; 3] OFDM transceiver OFDM signals are typically generated digitally due to the difficulty in creating large banks of phase lock oscillators and receivers in the analog domain. Figure 13 shows the block diagram of a typical OFDM transceiver. The transmitter section converts digital data to be transmitted, into a mapping of subcarrier amplitude and phase. It then transforms this spectral representation of the data into the time domain using an Inverse Discrete Fourier Transform (IDFT). The Inverse Fast Fourier Transform (IFFT) performs the same operations as an IDFT, except that it is much more computationally efficiency, and so is used in all practical systems. In order to transmit the OFDM signal the calculated time domain signal is then mixed up to the required frequency. Block diagram showing a basic OFDM transceiver [3] The receiver performs the reverse operation of the transmitter, mixing the RF signal to base band for processing, then using a Fast Fourier Transform (FFT) to analyze the signal in the frequency domain. The amplitude and phase of the subcarriers is then picked out and converted back to digital data. The IFFT and the FFT are complementary function and the most appropriate term depends on whether the signal is being received or generated. In cases where the Signal is independent of this distinction then the term FFT and IFFT is used interchangeably. [1, 125 -128, 3] Analysis of OFDM characteristics Guard Period OFDM demodulation must be synchronized with the start and end of the transmitted symbol period. If it is not, then ISI will occur (since information will be decoded and combined for 2 adjacent symbol periods). ICI will also occur because orthogonality will be lost (integrals of the carrier products will no longer be zero over the integration period), To help solve this problem, a guard interval is added to each OFDM symbol period. The first thought of how to do this might be to simply make the symbol period longer, so that the demodulator does not have to be so precise in picking the period beginning and end, and decoding is always done inside a single period. This would fix the ISI problem, but not the ICI problem. If a complete period is not integrated (via FFT), orthogonality will be lost. The effect of ISI on an OFDM signal can be further improved by the addition of a guard period to the start of each symbol. This guard period is a cyclic copy that extends the length of the symbol waveform. Each subcarrier, in the data section of the symbol, (i.e. the OFDM symbol with no guard period added, which is equal to the length of the IFFT size used to generate the signal) has an integer number of cycles. Because of this, placing copies of the symbol end-to-end results in a continuous signal, with no discontinuities at the joins. Thus by copying the end of a symbol and appending this to the start results in a longer symbol time. Addition of a guard period to an OFDM signal [3] In Figure 14, The total length of the symbol is Ts=TG + TFFT, where Ts is the total length of the symbol in samples, TG is the length of the guard period in samples, and TFFT is the size of the IFFT used to generate the OFDM signal. In addition to protecting the OFDM from ISI, the guard period also provides protection against time-offset errors in the receiver. For an OFDM system that has the same sample rate for both the transmitter and receiver, it must use the same FFT size at both the receiver and transmitted signal in order to maintain subcarrier orthogonality. Each received symbol has TG + TFFT samples due to the added guard period. The receiver only needs TFFT samples of the received symbol to decode the signal. The remaining TG samples are redundant and are not needed. For an ideal channel with no delay spread the receiver can pick any time offset, up to the length of the guard period, and still get the correct number of samples, without crossing a symbol boundary. Function of the guard period for protecting against ISI [3] Figure 15 shows this effect. Adding a guard period allows time for the transient part of the signal to decay, so that the FFT is taken from a steady state portion of the symbol. This eliminates the effect of ISI provided that the guard period is longer than the delay spread of the radio channel. The remaining effects caused by the multipath, such as amplitude scaling and phase rotation are corrected for by channel equalization. In order to avoid ISI and ICI, the guard period must be formed by a cyclic extension of the symbol period. This is done by taking symbol period samples from the end of the period and appending them to the front of the period. The concept of being able to do this, and what it means, comes from the nature of the IFFT/FFT process. When the IFFT is taken for a symbol period (during OFDM modulation), the resulting time sample sequence is technically periodic. This is because the IFFT/FFT is an extension of the Fourier Transform which is an extension of the Fourier Series for periodic waveforms. All of these transforms operate on signals with either real or manufactured periodicity. For the IFFT/FFT, the period is the number of samples used. Guard Period via Cyclic Extension With the cyclic extension, the symbol period is longer, but it represents the exact same frequency spectrum. As long as the correct number of samples are taken for the decode, they may be taken anywhere within the extended symbol. Since a complete period is integrated, orthogonality is maintained. Therefore, both ISI and ICI are eliminated. Note that some bandwidth efficiency is lost with the addition of the guard period (symbol period is increased and symbol rate is decreased) [2,154-160, 3] Windowing The OFDM signal is made up of a series of IFFTs that are concatenated to each other. At each symbol period boundary, there is a signal discontinuity due to the differences between the end of one period and the start of the next. These discontinuities can cause high frequency spectral noise to be generated (because they look like very fast transitions of the time waveform). To avoid this, a window function (Hamming, Hanning, Blackman, ) may be applied to each symbol period. The window function would attenuate the time waveform at the start and the end of each period, so that the discontinuities are smaller, and the high frequency noise is reduced. However, this attenuation distorts the signal and some of the desired frequency content is lost.[1, 121;2 154] Multipath Characteristics OFDM avoids frequency selective fading and ISI by providing relatively long symbol periods for a given data rate. This is illustrated in Figure 17. For a given transmission channel and a given source data rate, OFDM can provide better multipath characteristics than a single carrier. OFDM vs. Single Carrier, Multipath Characteristic Comparison However, since the OFDM carriers are spread over a frequency range, there still may be some frequency selective attenuation on a time-varying basis. A deep fade on a particular frequency may cause the loss of data on that frequency for a given time, but the use of Forward Error Coding can fix it. If a single carrier experienced a deep fade, too many consecutive symbols may be lost and correction coding may be ineffective. [8] Bandwidth A comparison of RF transmits bandwidth between OFDM and a single carrier is shown in Figure 18 (using the same example parameters as in Figure 17). OFDM Bandwidth Efficiency In Figure 18, the calculations show that OFDM is more bandwidth efficient than a single carrier. Note that another efficient aspect of OFDM is that a single transmitters bandwidth can be increased incrementally by addition of more adjacent carriers. In addition, no bandwidth buffers are needed between transmit bandwidths of separate transmitters as long as orthogonality can be maintained between all the carriers.[2, 161-163; 8; 9] Physical Implementation Since OFDM is carried out in the digital domain, there are many ways it can be implemented. Some options are provided in the following list. Each of these options should be viable given current technology: ASIC (Application Specific Integrated Circuit) ASICs are the fastest, smallest, and lowest power way to implement OFDM Cannot change the ASIC after it is built without designing a new chip General-purpose Microprocessor or MicroController PowerPC 7400 or other processor capable of fast vector operations Highly programmable Needs memory and other peripheral chips Uses the most power and space, and would be the slowest Field-Programmable Gate Array (FPGA) An FPGA combines the speed, power, and density attributes of an ASIC with the programmability of a general purpose processor. An FPGA could be reprogrammed for new functions by a base station to meet future (currently unknown requirements).This should be the best choice.[9] OFDM uses in DVB (Digital Video Broadcasting) DVB (Digital Video Broadcast) is a set of standards for the digital transmission of video and audio streams, and also data transmission. The DVB standards are maintained by the DVB Project, which is an industry-led consortium of over 260 broadcasters, manufacturers, network operators, software developers, regulatory bodies and others in over 35 countries. DVB has been implemented over satellite (DVB-S, DVB-S2), cable (DVB-C), terrestrial broadcasting (DVB-T), and handheld terminals (DVB-H). the DVB standard following the logical progression of signal processing steps, as well as source and channel coding, COFDM modulation, MPEG compression and multiplexing methods, conditional access and set-top box Technology. In this project is presented an investigation of two OFDM based DVB standards, DVB-T and DVB-H. DVB-T (Digital Video Broadcasting Terrestrial) The first Terrestrial Digital Video Broadcasting pilot transmissions were started in the late 90s, and the first commercial system was established in Great Britain. In the next few years the digital broadcasting system has been set up in many countries, and the boom of the digital terrestrial transmission is estimated in the next few years, while the analogue transmission will be cancelled within about 15 years. The greatest advantage of the digital system is the effective use of the frequency spectrum and its lower radiated power in comparison with the analogue transmission, while the covered area remains the same. Another key feature is the possibility of designing a so-called Single Frequency Network (SFN), which means that the neighboring broadcast stations use the same frequency and the adjacent signals dont get interfered. The digital system transmits a data stream, which means that not only television signals but data communication (e.g. Internet service) may be used according to the demands. The data stream consists of an MPEG-2 bit stream, which means a compression is used, enabling the transfer of even 4 or 5 television via the standard 8 MHz wide TV channel. For the viewer, the main advantages are the perfect, noise-free picture, CD quality sound, and easier handling, as well as services like Super Teletext, Electronic Programme Guide, interactivity and mobility.[11, 251-253] Modulation technique in DVB-T The DVB-T Orthogonal Frequency Division Multiplexing (OFDM) modulation system uses multi-carrier transmission. There are 2 modes, the so-called 2k and 8k modes, using 1705 and 6817 carriers respectively, with each carrier modulated separately and transmitted in the 8 MHz TV channel. The common modulation for the carriers is typically QPSK, 16-QAM or 64-QAM. Each signal can be divided into two, so-called „In Phase (I) and „Quadrature Phase components, being a 90Â ° phase shift between them. The constellation diagram and the bit allocation is shown in bellow 16-QAM constellation diagram and bit allocation [6] This modulation can be demonstrated in the constellation diagram, where the 2 axes represent the 2 components (I and Q). In case of using 16-QAM modulation, the number of states is 16, so 1 symbol represents 4 bits. [11, 255; 6; 14] Bir errors If we simulate all the carriers in the constellation diagram we get not just 1 discrete point, but many points, forming a „cloud and representing each state. In case of additive noise the „cloud gets bigger and the receiver may decide incorrectly, resulting in bit errors. Figure 2 shows the measured constellation diagram without and with additive noise. Measured 16-QAM constellation diagram a) without additive noise b) with additive noise [6] To ensure perfect picture quality, the DVB-T system uses a 2 level error correction (Reed-Solomon and Viterbi). This corrects the bad bits at an even 10-4 Bit Error Rate (BER) and enables error-free data transmission. [13, 32-36] The multi-carrier structure The structure of carriers can be illustrated also in the function of time (Figure 20). The horizontal axis is the frequency and the vertical axis is the time. The 8 MHz channel consists of many carriers, placed 4462 Hz or 1116 Hz far from each other according to the modulation mode (2k or 8k). Structure of OFDM carriers [13] There are some reserved, so-called Transmission Parameter Signalling (TPS) carriers that do not transfer payload, just provide transmission mode information for the receiver, so the total number of useful carriers is 1512 and 6048 respectively in the two transmission modes, and the resultant bit rate is between 4,97 and 31,66 Mbit/s, depending on the modulation (QPSK, 16-QAM or 64-QAM), the transmission mode (2k or 8k), the Code Rate (CR) used for error correction and the selected Guard Interval (GI). This guard interval means that there is a small time gap between each symbol, so the transmission is not continuous. This guarding time enables perfect reception by eliminating the errors caused by multipath propagation.[4, 79-90; 13] Frequency spectrum In 2k mode, 1705 carriers are modulated in the 8 MHz TV channel, so each carrier is 4462 Hz far from its neighbor, while in 8k mode this distance is 1116 Hz. In digital broadcasting, there are no vision and sound carriers, so the power for each carrier is the same. This mean High Performance Wireless Telecommunications Modulation High Performance Wireless Telecommunications Modulation Introduction The primary goal of the project is to analyze of OFDM system and to assess the suitability of OFDM as a modulation technique for wireless communications. In the part of project is covered two leading successfully implementation of OFDM based technologies are Digital Video Broadcasting (DVB-T and DVB-H) and Long Term Evolution (LTE advanced for 4G). Wireless communications is an emerging field, which has seen enormous growth in the last several years. The huge uptake rate of mobile phone technology, Wireless Local Area Networks (WLAN) and the exponential growth of the Internet have resulted in an increased demand for new methods of obtaining high capacity wireless networks. For cellular mobile applications, we will see in the near future a complete convergence of mobile phone technology, computing, Internet access, and potentially many multimedia applications such as video and high quality audio. In fact, some may argue that this convergence has already largely occurred, with the advent of being able to send and receive data using a notebook computer and a mobile phone. The goal of third and fourth generation mobile networks is to provide users with a high data rate, and to provide a wider range of services, such as voice communications, videophones, and high speed Internet access. The higher data rate of future mobile networks will be achieved by increasing the amount of spectrum allocated to the service and by improvements in the spectral efficiency. OFDM is a potential candidate for the physical layer of fourth generation mobile systems. Basic Principles of OFDM OFDM overview The Orthogonal Frequency Division Multiplexing (OFDM) is a modulation technique where multiple low data rate carriers are combined by a transmitter to form a composite high data rate transmission. The first commercial use of OFDM in the communication field was in the 1980s, and it was later widely used in the broadcast audio and video field in the 1990s in such areas as, ADSL, VHDSL, ETSI standard digital audio broadcast (DAB), digital video broadcast (DVB), and high-definition digital TV (HDTV). Digital signal processing makes OFDM possible. To implement the multiple carrier scheme using a bank of parallel modulators would not be very efficient in analog hardware. However, in the digital domain, multi-carrier modulation can be done efficiently with currently available DSP hardware and software. Not only can it be done, but it can also be made very flexible and programmable. This allows OFDM to make maximum use of available bandwidth and to be able to adapt to changing system requirements. Figure 1 is illustrated, Instead of separate modulators; the outgoing waveform is created by executing a high-speed inverse DFT on a set of time-samples of the transmitted data (post modulation). The output of the DFT can be directly modulated onto the outgoing carrier, without requiring any other components. Each carrier in an OFDM system is a sinusoid with a frequency that is an integer multiple of a base or fundamental sinusoid frequency. Therefore, each carrier is like a Fourier series component of the composite signal. In fact, it will be shown later that an OFDM signal is created in the frequency domain, and then transformed into the time domain via the Discrete Fourier Transform (DFT). Two periodic signals are orthogonal when the integral of their product, over one period, is equal to zero. This is true of certain sinusoids as illustrated in Equation 1. Definition of Orthogonal The carriers of an OFDM system are sinusoids that meet this requirement because each one is a multiple of a fundamental frequency. Each one has an integer number of cycles in the fundamental period. [2, 145-153; 6] The importantance of being orthogonal The main concept in OFDM is orthogonality of the sub-carriers.Since the carriers are all sine/cosine wave, we know that area under one period of a sine or a cosine wave is zero. Lets take a sine wave of frequency m and multiply it by a sinusoid (sine or a cosine) of a frequency n, where both m and n are integers. The integral or the area under this product is given by These two components are each a sinusoid, so the integral is equal to zero over one period. When we multiply a sinusoid of frequency n by a sinusoid of frequency m/n the area under the product is zero. In general for all integers n and m , sin(mx), cos(mx), cos(nx) , sin(nx) are all orthogonal to each other. These frequencies are called harmonics. Making the subcarriers mathematically orthogonal was a breakthrough for OFDM because it enables OFDM receivers to separate the subcarriers via an FFT and eliminate the guard bands. As figure 3 shows, OFDM subcarriers can overlap to make full use of the spectrum, but at the peak of each subcarrier spectrum, the power in all the other subcarriers is zero. OFDM therefore offers higher data capacity in a given spectrum while allowing a simpler system design. Creating orthogonal subcarriers in the transmitter is easy using an inverse FFT. To ensure that this orthogonality is maintained at the receiver (so that the subcarriers are not misaligned), the system must keep the transmitter and receiver clocks closely synchronizedwithin 2 parts per million in 802.11a systems. The 802.11a standard therefore dedicates four of its 52 subcarriers as pilots that enable phase-lock loops in the receiver to track the phase and frequency of the incoming signal. The 802.11a standard therefore dedicates four of its 52 subcarriers as pilots that enable phase-lock loops in the receiver to track the phase and frequency of the incoming signal. This method also eliminates low-frequency phase noise.Separating the subcarriers via an FFT require about an order of magnitude fewer multiply-accumulate operations than individually filtering each carrier. In general, an FFT implementation is much simpler than the RAKE receivers used for CDMA and the decision-feedback equalizers for TDMA.This idea are key to understanding OFDM. The orthogonality allows simultaneously transmission on a lot of sub- carriers in a tight frequency space without interference form each other. In essence this is similar to CDMA, where codes are used to make data sequences independent (also orthogonal) which allows many independent users to transmitin same space successfully.[2, 153-154; 6 ; 7] OFDM Operation Preliminary Concepts When the DFT (Discrete Fourier Transform) of a time signal is taken, the frequency domain results are a function of the time sampling period and the number of samples as shown in Figure 4. The fundamental frequency of the DFT is equal to 1/NT (1/total sample time). Each frequency represented in the DFT is an integer multiple of the fundamental frequency. Parameter Mapping from Time to Frequency for the DFT The maximum frequency that can be represented by a time signal sampled at rate 1/T is fmax = 1/2T as given by the Nyquist sampling theorem. This frequency is located in the center of the DFT points. All frequencies beyond that point are images of the representative frequencies. The maximum frequency bin of the DFT is equal to the sampling frequency (1/T) minus one fundamental (1/NT).The IDFT (Inverse Discrete Fourier Transform) performs the opposite operation to the DFT. It takes a signal defined by frequency components and converts them to a time signal. The parameter mapping is the same as for the DFT. The time duration of the IDFT time signal is equal to the number of DFT bins (N) times the sampling period (T).It is perfectly valid to generate a signal in the frequency domain, and convert it to a time domain equivalent for practical use (The frequency domain is a mathematical tool used for analysis. Anything usable by the real world must be converted into a real, time domain signal). This is how modulation is applied in OFDM. In practice the Fast Fourier Transform (FFT) and IFFT are used in place of the DFT and IDFT, so all further references will be to FFT and IFFT.[1 ,118 ; 4] Definition of Carriers The maximum number of carriers used by OFDM is limited by the size of the IFFT. This is determined as follows in Equation 2. OFDM Carrier Count In order to generate a real-valued time signal, OFDM (frequency) carriers must be defined in complex conjugate pairs, which are symmetric about the Nyquist frequency (fmax). This puts the number of potential carriers equal to the IFFT size/2. The Nyquist frequency is the symmetry point, so it cannot be part of a complex conjugate pair. The DC component also has no complex conjugate. These two points cannot be used as carriers so they are subtracted from the total available. If the carriers are not defined in conjugate pairs, then the IFFT will result in a time domain signal that has imaginary components. This must be a viable option as there are OFDM systems defined with carrier counts that exceed the limit for real-valued time signals given in Equation 2.In general, a system with IFFT size 256 and carrier count 216. This design must result in a complex time waveform. Further processing would require some sort of quadrature technique (use of parallel sine and cosine processing paths). In this report, only real-value time signals will be treated, but in order to obtain maximum bandwidth efficiency from OFDM, the complex time signal may be preferred (possibly an analogous situation to QPSK vs. BPSK). Equation 2, for the complex time waveform, has all IFFT bins available as carriers except the DC bin. Both IFFT size and assignment (selection) of carriers can be dynamic. The transmitter and receiver just have to use the same parameters. This is one of the advantages of OFDM. Its bandwidth usage (and bit rate) can be varied according to varying user requirements. A simple control message from a base station can change a mobile units IFFT size and carrier selection.[2,199-206; 4] Modulation Binary data from a memory device or from a digital processing stream is used as the modulating (baseband) signal. The following steps may be carried out in order to apply modulation to the carriers in OFDM: combine the binary data into symbols according to the number of bits/symbol selected convert the serial symbol stream into parallel segments according to the number of carriers, and form carrier symbol sequences apply differential coding to each carrier symbol sequence convert each symbol into a complex phase representation assign each carrier sequence to the appropriate IFFT bin, including the complex conjugates take the IFFT of the result OFDM modulation is applied in the frequency domain. Figure 5 and Figure 6 give an example of modulated OFDM carriers for one symbol period, prior to IFFT. OFDM Carrier Magnitude prior to IFFT For this example, there are 4 carriers, the IFFT bin size is 64, and there is only 1 bit per symbol. The magnitude of each carrier is 1, but it could be scaled to any value. The phase for each carrier is either 0 or 180 degrees, according to the symbol being sent. The phase determines the value of the symbol (binary in this case, either a 1 or a 0). In the example, the first 3 bits (the first 3 carriers) are 0, and the 4th bit (4th carrier) is a 1. OFDM Carrier Phase prior to IFFT Note that the modulated OFDM signal is nothing more than a group of delta (impulse) functions, each with a phase determined by the modulating symbol. In addition, note that the frequency separation between each delta is proportional to 1/N where N is the number of IFFT bins. The frequency domain representation of the OFDM is described in Equation 3. OFDM Frequency Domain Representation (one symbol period) After the modulation is applied, an IFFT is performed to generate one symbol period in the time domain. The IFFT result is shown in 7. It is clear that the OFDM signal has varying amplitude. It is very important that the amplitude variations be kept intact as they define the content of the signal. If the amplitude is clipped or modified, then an FFT of the signal would no longer result in the original frequency characteristics, and the modulation may be lost. This is one of the drawbacks of OFDM, the fact that it requires linear amplification. In addition, very large amplitude peaks may occur depending on how the sinusoids line up, so the peak-to-average power ratio is high. This means that the linear amplifier has to have a large dynamic range to avoid distorting the peaks. The result is a linear amplifier with a constant, high bias current resulting in very poor power efficiency. OFDM Signal, 1 Symbol Period Figure 8 is provided to illustrate the time components of the OFDM signal. The IFFT transforms each complex conjugate pair of delta functions (each carrier) into a real-valued, pure sinusoid. Figure 8 shows the separate sinusoids that make up the composite OFDM waveform given in Figure 7. The one sinusoid with 180 phase shift is clearly visible as is the frequency difference between each of the 4 sinusoids. Transmission The key to the uniqueness and desirability of OFDM is the relationship between the carrier frequencies and the symbol rate. Each carrier frequency is separated by a multiple of 1/NT (Hz). The symbol rate (R) for each carrier is 1/NT (symbols/sec). The effect of the symbol rate on each OFDM carrier is to add a sin(x)/x shape to each carriers spectrum. The nulls of the sin(x)/x (for each carrier) are at integer multiples of 1/NT. The peak (for each carrier) is at the carrier frequency k/NT. Therefore, each carrier frequency is located at the nulls for all the other carriers. This means that none of the carriers will interfere with each other during transmission, although their spectrums overlap. The ability to space carriers so closely together is very bandwidth efficient. OFDM Time Waveform Figure 9 shows the OFDM time waveform for the same signal. There are 100 symbol periods in the signal. Each symbol period is 64 samples long (100 x 64 = 6400 total samples). Each symbol period contains 4 carriers each of which carries 1 symbol. Each symbol carries 1 bit. Note that Figure 9 again illustrates the large dynamic range of the OFDM waveform envelope. OFDM Spectrum Figure 10 shows the spectrum for of an OFDM signal with the following characteristics: 1 bit / symbol 100 symbols / carrier (i.e. a sequence of 100 symbol periods) 4 carriers 64 IFFT bins spectrum averaged for every 20 symbols (100/20 = 5 averages) Red diamonds mark all of the available carrier frequencies. Note that the nulls of the spectrums line up with the unused frequencies. The four active carriers each have peaks at carrier frequencies. It is clear that the active carriers have nulls in their spectrums at each of the unused frequencies (otherwise, the nulls would not exist). Although it cannot be seen in the figure, the active frequencies also have spectral nulls at the adjacent active frequencies. It is not currently practical to generate the OFDM signal directly at RF rates, so it must be up converted for transmission. To remain in the discrete domain, the OFDM could be upsampled and added to a discrete carrier frequency. This carrier could be an intermediate frequency whose sample rate is handled by current technology. It could then be converted to analog and increased to the final transmit frequency using analog frequency conversion methods. Alternatively, the OFDM modulation could be immediately converted to analog and directly increased to the desired RF transmits frequency. Either way, the selected technique would have to involve some form of linear AM (possibly implemented with a mixer). [1, 122-125; 6] Reception and Demodulation The received OFDM signal is down converted (in frequency) and taken from analog to digital. Demodulation is done in the frequency domain (just as modulation was). The following steps may be taken to demodulate the OFDM: partition the input stream into vectors representing each symbol period take the FFT of each symbol period vector extract the carrier FFT bins and calculate the phase of each calculate the phase difference, from one symbol period to the next, for each carrier decode each phase into binary data sort the data into the appropriate order OFDM Carrier Magnitude following FFT Figure 11 and Figure 12 show the magnitude and spectrum of the FFT for one received OFDM symbol period. For this example, there are 4 carriers, the IFFT bin size is 64, there is 1 bit per symbol, and the signal was sent through a channel with AWGN having an SNR of 8 dB. The figures show that, under these conditions, the modulated symbols are very easy to recover. OFDM Carrier Phase following FFT In Figure 12 that the unused frequency bins contain widely varying phase values. These bins are not decoded, so it does not matter, but the result is of interest. Even if the noise is removed from the channel, these phase variations still occur. It must be a result of the IFFT/FFT operations generating very small complex values (very close to 0) for the unused carriers. The phases are a result of these values. [1, 125 -128; 3] OFDM transceiver OFDM signals are typically generated digitally due to the difficulty in creating large banks of phase lock oscillators and receivers in the analog domain. Figure 13 shows the block diagram of a typical OFDM transceiver. The transmitter section converts digital data to be transmitted, into a mapping of subcarrier amplitude and phase. It then transforms this spectral representation of the data into the time domain using an Inverse Discrete Fourier Transform (IDFT). The Inverse Fast Fourier Transform (IFFT) performs the same operations as an IDFT, except that it is much more computationally efficiency, and so is used in all practical systems. In order to transmit the OFDM signal the calculated time domain signal is then mixed up to the required frequency. Block diagram showing a basic OFDM transceiver [3] The receiver performs the reverse operation of the transmitter, mixing the RF signal to base band for processing, then using a Fast Fourier Transform (FFT) to analyze the signal in the frequency domain. The amplitude and phase of the subcarriers is then picked out and converted back to digital data. The IFFT and the FFT are complementary function and the most appropriate term depends on whether the signal is being received or generated. In cases where the Signal is independent of this distinction then the term FFT and IFFT is used interchangeably. [1, 125 -128, 3] Analysis of OFDM characteristics Guard Period OFDM demodulation must be synchronized with the start and end of the transmitted symbol period. If it is not, then ISI will occur (since information will be decoded and combined for 2 adjacent symbol periods). ICI will also occur because orthogonality will be lost (integrals of the carrier products will no longer be zero over the integration period), To help solve this problem, a guard interval is added to each OFDM symbol period. The first thought of how to do this might be to simply make the symbol period longer, so that the demodulator does not have to be so precise in picking the period beginning and end, and decoding is always done inside a single period. This would fix the ISI problem, but not the ICI problem. If a complete period is not integrated (via FFT), orthogonality will be lost. The effect of ISI on an OFDM signal can be further improved by the addition of a guard period to the start of each symbol. This guard period is a cyclic copy that extends the length of the symbol waveform. Each subcarrier, in the data section of the symbol, (i.e. the OFDM symbol with no guard period added, which is equal to the length of the IFFT size used to generate the signal) has an integer number of cycles. Because of this, placing copies of the symbol end-to-end results in a continuous signal, with no discontinuities at the joins. Thus by copying the end of a symbol and appending this to the start results in a longer symbol time. Addition of a guard period to an OFDM signal [3] In Figure 14, The total length of the symbol is Ts=TG + TFFT, where Ts is the total length of the symbol in samples, TG is the length of the guard period in samples, and TFFT is the size of the IFFT used to generate the OFDM signal. In addition to protecting the OFDM from ISI, the guard period also provides protection against time-offset errors in the receiver. For an OFDM system that has the same sample rate for both the transmitter and receiver, it must use the same FFT size at both the receiver and transmitted signal in order to maintain subcarrier orthogonality. Each received symbol has TG + TFFT samples due to the added guard period. The receiver only needs TFFT samples of the received symbol to decode the signal. The remaining TG samples are redundant and are not needed. For an ideal channel with no delay spread the receiver can pick any time offset, up to the length of the guard period, and still get the correct number of samples, without crossing a symbol boundary. Function of the guard period for protecting against ISI [3] Figure 15 shows this effect. Adding a guard period allows time for the transient part of the signal to decay, so that the FFT is taken from a steady state portion of the symbol. This eliminates the effect of ISI provided that the guard period is longer than the delay spread of the radio channel. The remaining effects caused by the multipath, such as amplitude scaling and phase rotation are corrected for by channel equalization. In order to avoid ISI and ICI, the guard period must be formed by a cyclic extension of the symbol period. This is done by taking symbol period samples from the end of the period and appending them to the front of the period. The concept of being able to do this, and what it means, comes from the nature of the IFFT/FFT process. When the IFFT is taken for a symbol period (during OFDM modulation), the resulting time sample sequence is technically periodic. This is because the IFFT/FFT is an extension of the Fourier Transform which is an extension of the Fourier Series for periodic waveforms. All of these transforms operate on signals with either real or manufactured periodicity. For the IFFT/FFT, the period is the number of samples used. Guard Period via Cyclic Extension With the cyclic extension, the symbol period is longer, but it represents the exact same frequency spectrum. As long as the correct number of samples are taken for the decode, they may be taken anywhere within the extended symbol. Since a complete period is integrated, orthogonality is maintained. Therefore, both ISI and ICI are eliminated. Note that some bandwidth efficiency is lost with the addition of the guard period (symbol period is increased and symbol rate is decreased) [2,154-160, 3] Windowing The OFDM signal is made up of a series of IFFTs that are concatenated to each other. At each symbol period boundary, there is a signal discontinuity due to the differences between the end of one period and the start of the next. These discontinuities can cause high frequency spectral noise to be generated (because they look like very fast transitions of the time waveform). To avoid this, a window function (Hamming, Hanning, Blackman, ) may be applied to each symbol period. The window function would attenuate the time waveform at the start and the end of each period, so that the discontinuities are smaller, and the high frequency noise is reduced. However, this attenuation distorts the signal and some of the desired frequency content is lost.[1, 121;2 154] Multipath Characteristics OFDM avoids frequency selective fading and ISI by providing relatively long symbol periods for a given data rate. This is illustrated in Figure 17. For a given transmission channel and a given source data rate, OFDM can provide better multipath characteristics than a single carrier. OFDM vs. Single Carrier, Multipath Characteristic Comparison However, since the OFDM carriers are spread over a frequency range, there still may be some frequency selective attenuation on a time-varying basis. A deep fade on a particular frequency may cause the loss of data on that frequency for a given time, but the use of Forward Error Coding can fix it. If a single carrier experienced a deep fade, too many consecutive symbols may be lost and correction coding may be ineffective. [8] Bandwidth A comparison of RF transmits bandwidth between OFDM and a single carrier is shown in Figure 18 (using the same example parameters as in Figure 17). OFDM Bandwidth Efficiency In Figure 18, the calculations show that OFDM is more bandwidth efficient than a single carrier. Note that another efficient aspect of OFDM is that a single transmitters bandwidth can be increased incrementally by addition of more adjacent carriers. In addition, no bandwidth buffers are needed between transmit bandwidths of separate transmitters as long as orthogonality can be maintained between all the carriers.[2, 161-163; 8; 9] Physical Implementation Since OFDM is carried out in the digital domain, there are many ways it can be implemented. Some options are provided in the following list. Each of these options should be viable given current technology: ASIC (Application Specific Integrated Circuit) ASICs are the fastest, smallest, and lowest power way to implement OFDM Cannot change the ASIC after it is built without designing a new chip General-purpose Microprocessor or MicroController PowerPC 7400 or other processor capable of fast vector operations Highly programmable Needs memory and other peripheral chips Uses the most power and space, and would be the slowest Field-Programmable Gate Array (FPGA) An FPGA combines the speed, power, and density attributes of an ASIC with the programmability of a general purpose processor. An FPGA could be reprogrammed for new functions by a base station to meet future (currently unknown requirements).This should be the best choice.[9] OFDM uses in DVB (Digital Video Broadcasting) DVB (Digital Video Broadcast) is a set of standards for the digital transmission of video and audio streams, and also data transmission. The DVB standards are maintained by the DVB Project, which is an industry-led consortium of over 260 broadcasters, manufacturers, network operators, software developers, regulatory bodies and others in over 35 countries. DVB has been implemented over satellite (DVB-S, DVB-S2), cable (DVB-C), terrestrial broadcasting (DVB-T), and handheld terminals (DVB-H). the DVB standard following the logical progression of signal processing steps, as well as source and channel coding, COFDM modulation, MPEG compression and multiplexing methods, conditional access and set-top box Technology. In this project is presented an investigation of two OFDM based DVB standards, DVB-T and DVB-H. DVB-T (Digital Video Broadcasting Terrestrial) The first Terrestrial Digital Video Broadcasting pilot transmissions were started in the late 90s, and the first commercial system was established in Great Britain. In the next few years the digital broadcasting system has been set up in many countries, and the boom of the digital terrestrial transmission is estimated in the next few years, while the analogue transmission will be cancelled within about 15 years. The greatest advantage of the digital system is the effective use of the frequency spectrum and its lower radiated power in comparison with the analogue transmission, while the covered area remains the same. Another key feature is the possibility of designing a so-called Single Frequency Network (SFN), which means that the neighboring broadcast stations use the same frequency and the adjacent signals dont get interfered. The digital system transmits a data stream, which means that not only television signals but data communication (e.g. Internet service) may be used according to the demands. The data stream consists of an MPEG-2 bit stream, which means a compression is used, enabling the transfer of even 4 or 5 television via the standard 8 MHz wide TV channel. For the viewer, the main advantages are the perfect, noise-free picture, CD quality sound, and easier handling, as well as services like Super Teletext, Electronic Programme Guide, interactivity and mobility.[11, 251-253] Modulation technique in DVB-T The DVB-T Orthogonal Frequency Division Multiplexing (OFDM) modulation system uses multi-carrier transmission. There are 2 modes, the so-called 2k and 8k modes, using 1705 and 6817 carriers respectively, with each carrier modulated separately and transmitted in the 8 MHz TV channel. The common modulation for the carriers is typically QPSK, 16-QAM or 64-QAM. Each signal can be divided into two, so-called „In Phase (I) and „Quadrature Phase components, being a 90Â ° phase shift between them. The constellation diagram and the bit allocation is shown in bellow 16-QAM constellation diagram and bit allocation [6] This modulation can be demonstrated in the constellation diagram, where the 2 axes represent the 2 components (I and Q). In case of using 16-QAM modulation, the number of states is 16, so 1 symbol represents 4 bits. [11, 255; 6; 14] Bir errors If we simulate all the carriers in the constellation diagram we get not just 1 discrete point, but many points, forming a „cloud and representing each state. In case of additive noise the „cloud gets bigger and the receiver may decide incorrectly, resulting in bit errors. Figure 2 shows the measured constellation diagram without and with additive noise. Measured 16-QAM constellation diagram a) without additive noise b) with additive noise [6] To ensure perfect picture quality, the DVB-T system uses a 2 level error correction (Reed-Solomon and Viterbi). This corrects the bad bits at an even 10-4 Bit Error Rate (BER) and enables error-free data transmission. [13, 32-36] The multi-carrier structure The structure of carriers can be illustrated also in the function of time (Figure 20). The horizontal axis is the frequency and the vertical axis is the time. The 8 MHz channel consists of many carriers, placed 4462 Hz or 1116 Hz far from each other according to the modulation mode (2k or 8k). Structure of OFDM carriers [13] There are some reserved, so-called Transmission Parameter Signalling (TPS) carriers that do not transfer payload, just provide transmission mode information for the receiver, so the total number of useful carriers is 1512 and 6048 respectively in the two transmission modes, and the resultant bit rate is between 4,97 and 31,66 Mbit/s, depending on the modulation (QPSK, 16-QAM or 64-QAM), the transmission mode (2k or 8k), the Code Rate (CR) used for error correction and the selected Guard Interval (GI). This guard interval means that there is a small time gap between each symbol, so the transmission is not continuous. This guarding time enables perfect reception by eliminating the errors caused by multipath propagation.[4, 79-90; 13] Frequency spectrum In 2k mode, 1705 carriers are modulated in the 8 MHz TV channel, so each carrier is 4462 Hz far from its neighbor, while in 8k mode this distance is 1116 Hz. In digital broadcasting, there are no vision and sound carriers, so the power for each carrier is the same. This mean