101 Report and Small Change Professor Ramos Blog

101 Report and Small Change Chp 5, â€Å"And Yet† Quick Write What are questions or issues with diversity that would benefit from us researching them? Chp 5, â€Å"And Yet† Distinguishing What  You  Say from What  They  Say Chapter 5 (p. 68) introduces you to the term  voice markers  in order to help you distinguish the â€Å"I say† from the â€Å"They say.† This is a very important move since we are now including the â€Å"They say† in your writing. If you do not do this clearly, the reader will be confused as to your position and you may seem to contradict yourself. The templates help you with specific ways of signaling who is saying what, and to embed the voice markers. Being able to distinguish your own view from the common view is a â€Å"sophisticated rhetorical move.† Using â€Å"I† or â€Å"We† The chapter also covers using the first person in academic writing, â€Å"I† or â€Å"we.† You have likely been told to not or never use the I in college writing. The book argues that well-supported arguments are grounded in persuasive reasons and evidence, not in the use of nonuse of pronouns. Grossman â€Å"From Scroll to Screen† https://www.nytimes.com/svc/oembed/html/?url=https%3A%2F%2Fwww.nytimes.com%2F2011%2F09%2F04%2Fbooks%2Freview%2Fthe-mechanic-muse-from-scroll-to-screen.html From Scroll to Screen by Lev Grossman  was first published in the New York Times. What is Grossman’s report about? What is his purpose? Does this sound like an argument or a thesis? How is he organizing the information? Gladwell â€Å"Small Change† Malcolm Gladwell â€Å"Small Change: Why the Revolution Will Not Be Tweeted† (399) Gladwell discusses the arguments promoting social media as a key component in social activism starting in paragraph 7. He says, â€Å"The new tools of social media have reinvented social activism. With Facebook and Twitter and the like, the traditional relationship between political authority and popular will has been upended, making it easier for the powerless to collaborate, coordinate, and give a voice to their concerns.† He discusses revolutions in Moldova and Iran and quotes a former senior State Department official who believes social media can be used to fight terrorism. He  brings up these opposing views (his â€Å"they say†) after an extended description of the Woolworth’s lunch counter sit-ins. He begins to refute these views in paragraph 8. 2/2/60 Greensboro, N.C.: A group of Negro students from North Carolina AT College, who were refused service at a luncheon counter reserved for white customers, staged a sit-down strike at the F.W. Woolworth store in Greensboro 2/2. Ronald Martin, Robert Patterson and Mark Martin are shown as they stayed seated throughout the day. The white woman at left came to the counter for lunch but decided not to sit down. Sit-in participants are bullied and have food and drinks dumped on them. Chp 6 Skeptics May Object Chapter 6 introduces a different sort of â€Å"they say†: the naysayer. The naysayer, or counterargument, appears after the conversation and after you have made some points. Including what the objections might be helps you make a more thorough point and adds credibility to the writing. Be careful to treat the objection carefully and fairly. Do not present a weak argument or a simplification of it because that can lead to a number of fallacies including the Strawman fallacy. The book offers suggestions for including the Naysayer or Skeptic. Anticipate Objections Entertain Objections in your own writing. Name the Naysayers. Introduce objections formally or informally Represent Objections Fairly Answer Objections Make concessions and stand your ground.

Political Career Timeline of US President Barack Obama

Political Career Timeline of US President Barack Obama    Barack Hussein Obama II graduated high school with honors in 1979 and was president of the Harvard Law Review long before he ever decided to enter politics. When he decided he wanted to run for the Illinois Senate in 1996, he ensured his candidacy by successfully challenging the nomination petitions of his four competitors. This marked his entry into federal politics.   Timeline of Barack Obama's Political Career 1988: Obama is a summer associate at the Chicago law firm Sidley Austin.1992: Obama graduates from Harvard and returns to Chicago.1995: In July, Obama - at 34 years of age - publishes his first memoir, Dreams From My Father: A Story of Race and Inheritance. In August, Obama files paperwork to run for incumbent Alice Palmers Illinois Senate seat.1996: In January, Obama has his four competitor petitions invalidated; he emerges as the only candidate. In November, he is elected to the Illinois Senate, which is controlled by Republicans.1999: Obama begins running for Congress.2000: Obama loses his challenge for the congressional seat held by Rep. Bobby Rush.2002: In November, Democrats usurp Republican control of the Illinois Senate.2003-2004: Obama amasses his legislative record and serves as chair of the Health and Human Services Committee.2003: Obama begins running for US Senate; the leading Democratic candidate withdraws in 2004 due to a sex scandal.  David Axelrod [begins having ] camera crews video virtually everything Obama does in public. He uses this footage to create a five-minute online video for the Jan. 16, 2007, announcement that Obama is running for president. 2004: In March, Obama wins the primary with 52% of the vote. In June, his Republican opponent Jack Ryan withdraws due to a sex scandal. He delivers the Democratic National Convention address in July 2004, and in November he is elected to the US Senate with 70% of the vote.2005: Obama files paperwork for his leadership PAC, The Hope Fund, in January. Shortly after his election to the US Senate, he delivered a well-received address arguing faith should have a greater role in public discourse.2006: Obama writes and publishes his book, The Audacity of Hope. In October, he announces he is considering a run for the presidency of the United States.2007: In February, Obama announces his candidacy for US president.  2008: In June, he becomes the Democratic Partys presumptive nominee. In November, he defeats Republican presidential nominee John McCain to become the first African-American president of the United States of America and the 44th president of the country.2009: Obama is inaugurate d in January. In his first 100 days in office, he expands health care insurance for children and provides legal protection for women seeking equal pay. He gets Congress to pass a  $787 billion stimulus bill  to promote short-term economic growth, and he also cuts taxes for working families, small businesses and first-time home buyers. He loosens the ban on embryonic stem cell research and improves relations with Europe, China, Cuba and Venezuela. The president is awarded the  2009 Nobel Peace Prize  for his efforts. 2010: Obama delivers his first State of the Union speech in January. In March, he signs his health care reform plan, known as the Affordable Care Act, into law. Opponents of the act claim that it violates the US Constitution.2011: Obama signs the Budget Control Act to rein in government spending.  He also signs a repeal of the military policy known as Dont Ask, Dont Tell, which prevents openly gay troops from serving in US Armed Forces. In  May, he green lights a covert operation in Pakistan that leads to the killing of al-Qaeda leader  Osama bin Laden  by a team of US Navy SEALs.2012: Obama began running for his second term, and in November, he wins with nearly 5 million more votes than his Republican counterpart.2013: Obama gets a legislative victory with a bipartisan agreement on tax increases and spending cuts, which is a step toward keeping his re-election promise of reducing the federal deficit by raising taxes on the wealthy. In June, his approval ratings tank because of an alleged cover-up of events in Benghazi, Libya; because of allegations that the IRS is targeting conservative political organizations seeking tax-exempt status; and due to revelations about the US National Security Agencys surveillance program. The Obama administration struggles with many domestic and international problems. 2014: Obama orders sanctions on Russia because of its annexation of Crimea. John Boehner sues the president, claiming he has overstepped his executive powers regarding some parts of the Affordable Care Act. Republicans gain  control of the Senate, and now Obama has to contend with the fact that Republicans control both houses of Congress during the final two years of his second term.2015: At his second State of the Union address, he claims that the United States is out of the recession. With Democrats outnumbered, he threatens to use his executive powers to stave off any potential Republican interference in his agenda. Obama has two major Supreme Court victories in this year: The Affordable Care Acts tax subsidies are upheld, and marriage equality becomes reality. Also, Obama and the five world powers reach a historic nuclear deal with Iran. And Obama launches his Clean Power Plan to reduce greenhouse gases and emissions.2016: In his final year in office, Obama tackles gun control but is met with strong opposition from both parties. He delivers his final State of the Union address on January 12, 2016. In March, he becomes the first sitting US president since 1928 to visit Cuba. 2017: Obama delivers his farewell address in January in Chicago. During his last day in office on January 19 - he announces that he will commute the sentences of 330 nonviolent drug offenders. Also in his final days, Obama  presented Vice President Joe Biden with the Presidential Medal of Freedom with Distinction.

Trade relations between USA and China Essay Example | Topics and Well Written Essays - 1000 words

Trade relations between USA and China - Essay Example Ranging from the ongoing currency war to accusations of immoral trade policies, factors which have led towards this destabilization of trade relations are not only economical but, perhaps, have a political milieu too. Firstly, one of the major challenges faced by the USA-China trade is the prominent USA accusation of ‘manipulating’ the Chinese currency. Over the past few years, the official currency of China, Yuan, has been held in a floating exchange rate system; though it has only been allowed a narrow margin to fluctuate. Many analysts and economists in the USA however urge for a more flexible exchange rate which pivots around the market equilibrium determined by the market forces of currency demand and supply. Despite of the fact that Yuan has been allowed to fluctuate more liberally and has even risen by 20% in value against the USA Dollar since 2005, Japan, European Union and USA yet accuse China that it manipulates its currency by deliberately keeping it at a low exchange rate in relation to the USA Dollar with a view to make the USA products less competitive and, simultaneously, the Chinese goods and services more competitive and relatively cheaper than the USA goods. This lowered currency rate leads to a rise in Chinese exports whereas USA exports decline. A large amount of trade deficit suffered by the USA in trade with China adds to the gravity of the accusation. Concerns for China regarding this issue are that the USA, World Trade Organization or APEC might impose trade sanctions on China for its ‘problematic’ currency exchange rate. These sanctions may prove very costly for the growing Chinese economy as the world’s second largest economy is principally led by mass amount of exports to its trading partners. Restrictions over free trade for China may cause the deceleration of its growth or may even put it to a halt. The trade balance between USA and China in 2011 exceeded $295 billion and any probable sanctions may d rastically affect this huge amount of traded commodities between the two giants. Moreover, China’s Current Account surplus has also been declining during the past few years and the consequences of USA pleas regarding the unfair trade advantage China has may turn the ‘already withering’ surplus into a trade deficit, whereas China may also lose its most important trade partner (Davis). The list of allegations upon China is elongated and does not restrict itself to the currency devaluation. China is also accused by the USA for adopting an unfair exchange rate regime. Yuan’s rate of exchange has been, contrarily to the US allegations, in conformity with the international monetary system which is operating at the time. Observing the variations from a fixed to a floating and then from the fixed peg to a basket peg, China’s exchange rate valuation has always been repudiated by USA. This might lead to a further withering of trade relations between the two n ations in the near future. (Moosa viii) Also, on the accusation list is the allegation upon China of piling up its foreign reserves. Again, the charge seems to be lacking ample rationality as any nation is allowed to purchase foreign currency reserves and to accumulate those reserves in order to minimize its risk of a speculative economic slump in the future. Similarly, China has done the same and has acquired pretty huge amounts of US Dollars in the

Loaded Spring Oscillator, Hooke's Law Assignment

Loaded Spring Oscillator, Hooke's Law - Assignment Example The motion involves attachment of simple harmonic oscillator to the spring with the other end on the wall or any other rigid support system. The oscillator’s motion is repetitive at constant frequency hence periodic (Serway & Jewett, 2006 p 54). When the oscillator passes through the equilibrium its velocity is maximum and zero when passing through the extreme positions in its oscillation. The acceleration experienced by the oscillator is proportional to the negative of its displacement from the midpoint of its motion. A system in equilibrium and at rest has no net force acting on the mass. Displacement of the mass from equilibrium causes a restoring elastic force which obeys Hooke’s to be exerted by the spring. The restoring force F, is found by multiplying the spring constant K, to the displacement from equilibrium x; F=-Kx. The extension of a spring is directly proportional to the load applied to it. This is referred to the Hooke’s Law of elasticity. The mater ial’s elastic limit is the maximum load that when exceeded the material will not be able to gain its original form. Therefore, Hooke’s Law do not apply on the material. The elastic limit varies among the materials. The materials following Hooke’s Law are known as Hookean materials or linear elastic materials. The materials regain their original form after deformation by the load on it. In the formulae used to determine Hooke’s Law a negative sign is added because the restoring force acts in an opposite direction of displacement. The formula was stated by Robert Hooke, a British physicist in the 17th century hence its name; Hooke’s Law. A spring of length L and cross-sectional area A, is considered a linear elastic material since its extension is linearly proportional to tits tensile stress by a constant. Materials such as rubber are regarded as non-linear or non-Hookean since the load is not proportional to the extension that occurs. The material c hanging least in extension when load is applied is regarded to have the greatest elastic force. Elasticity would be described in four ways; compression, flexure or bending, stretching or extension, torsion or twisting. Elasticity has two main kinds namely elasticity by volume and elasticity of form or shape. For example, elasticity of volume is mainly experienced by the gases and liquids. Elasticity of the two is considered perfect since when the load is applied or removed there is no lost of volume. Increase in temperature of the material would cause increased extension. Therefore, factors such as temperature are to be kept constant during the experiment to ensure the results are not misleading. The graph is expected to be as shown below: Figure 1 The springs are found to obey the Hooke’s law in combinations. Therefore the springs can be combined to cater for specific spring constant. For springs in series, the equivalent constant is equal to the following: 1/Keq = 1/K1 + 1/ K2 Therefore the equivalent spring constant is the reciprocal of the answer from above. If the springs are in parallel the equivalent spring constant is equal to the sum of the spring constants of the springs used. Keq = K1 + K2 The Apparatus The requirements for the experiment are the steel springs, tensile. Mass hangers with slotted masses, 100g. Retort stand base, rod, boss and clamp. Short length of stiff wire to combine springs in parallel. G-clamp if the retort stand base is

I will attach 2 pages Essay Example | Topics and Well Written Essays - 1000 words

I will attach 2 pages - Essay Example However, her son Bailey has already decided firmly to travel to Florida. On the other hand, the grandmother tries to persuade her son, in whatever sort of persuasion available, to change course. She made two arguments why they should go to Tennessee: first, the evil Misfit and his gang are headed toward Florida; and second, the children need to broaden their world by going to Tennessee since they haven’t gone there before. Evidently, the two arguments are contradictory; thus, the old woman deliberately lies to her family for the selfish reason of wanting to go to Tennessee. She could have been a good woman -- i.e., honest and truthful -- if there is just someone who will â€Å"shoot her.† On the road towards Florida, the grandmother keeps on talking about the good-old-days. For instance, she speaks of an old term which refers to an African American boy: pickaninny -- such term means â€Å"little† or â€Å"small.† The way the grandmother utters the word â⠂¬Å"pickaninny,† however, sounds like she is mocking the â€Å"little Negro.† Indeed, that was the olden time when black slavery or the pickaninny was popular among the white American race. Furthermore, the grandmother only tells stories of the past that greatly benefits her. When her grandchildren laugh at her, the grandmother reacts against their behavior by saying that â€Å"children were more respectful [in the past]† (O’Connor 231). A manipulative old woman, she could have been good to other people, especially the young ones, if somebody â€Å"shoot[s] her.† During the pit-stop, the old lady talks to the owner of The Tower, Red Sam; unsurprisingly, they considerably discuss about the â€Å"better times† (O-Conner 234). The grandmother appears to agree to Red Sam’s observation, that good men are hard to find these days. The Tower owner tells the old woman about the better times when people could go out without even locking their ho use’s door; nowadays, he adds up, no one can be trusted anymore. Here, the grandmother’s view of the world in general and men in particular is subtly revealed. She considers the present-day people as inherently evil. As we shall see, however, the old lady’s perception towards men as evil dramatically changes; she could have been good -- i.e., possessing positive outlook in life and the world -- if someone â€Å"shoot[s] her.† Before the accident, the old woman tells her grandchildren about a house in Georgia that has a secret panel. The main motive of the old lady why she mentions this secret panel is to convince her grandchildren to go to Georgia. In the succeeding incident, however, it becomes clear that the house â€Å"she had remembered so vividly† was in Tennessee after all. In the ditch after the accident, the grandmother keeps to herself about her deliberate deception to Bailey and others. And in order to avoid Bailey’s anger due to t he grandmother’s cat that primarily cause the accident, the old lady has hoped that â€Å"she was injured† (O’Connor 236). Again, one observes that the old woman keeps on looking for ways, whatever they are, in order to save her butt, so to speak. The grandmother’s manipulative character could have been altered if there is just somebody who will â€Å"shoot her.† When the Misfit and his gang arrived in the scene, the old lady speaks of lie again.

Mathematical description of OFDM

Mathematical description of OFDM When we talk about the Mathematical description of OFDM then we cannot neglect the following mathematical treatments: The Fourier transform The use of the Fast Fourier Transform in OFDM The guard interval and its implementation As we have discussed above that a large number of narrowband carriers which are spaced close to each other in frequency domain are transmitted by OFDM. The modern digital technique that is used in the OFDM is FFT i-e Fast Fourier transform (FFT) and due to the use of FFT it reduces the number of modulators and demodulators both at the receiver and transmitter side. Fig. 4 Examples of OFDM spectrum (a) a single subchannel, (b) 5 carriers At the central frequency of each subchannel, there is no crosstalk from other subchannels. Mathematically, each carrier can be described as a complex wave: (1) sc(t) = the real part of original signal. Ac(t) = the Amplitude f c(t) = Phase of carrier (t)= symbol duration period Ac(t) and f c(t) use to fluctuate on symbol by symbol basis. Parameter values are constant over (t). As we know that OFDM posses many carriers. So the complex signals ss(t) is represented as: (2) where This is of course a continuous signal. If we consider the waveforms of each component of the signal over one symbol period, then the variables Ac(t) and f c(t) take on fixed values, which depend on the frequency of that particular carrier, and so can be rewritten: If the signal is sampled using a sampling frequency of 1/T, then the resulting signal is represented by: (3) At this point, we have restricted the time over which we analyse the signal to N samples. It is convenient to sample over the period of one data symbol. Thus we have a relationship: t =NT If we now simplify eqn. 3, without a loss of generality by letting w 0=0, then the signal becomes: (4) Now Eq. 4 can be compared with the general form of the inverse Fourier transform: (5) In eq. 4, the function is no more than a definition of the signal in the sampled frequency domain, and s(kT) is the time domain representation. Eqns. 4 and 5 are equivalent if: (6) This is the same condition that was required for orthogonality (see Importance of orthogonality). Thus, one consequence of maintaining orthogonality is that the OFDM signal can be defined by using Fourier transform procedures. The Fourier transform Fourier transform actually relate events in time domain to events in frequency domain. There are different version of FFT which are used according to requirement of different sort of work The conventional transform provide the relation of continuous signals. Note that Continuous signals are not limited in both time and frequency domain. Though, it is better to sample the signal so that the signal processing becomes simpler. But it lead to an aliasing when we sample the signals with infinite spectrum and the processing of signals which are not time limited can lead to another problem that is referred to as space storage. DFT (discrete Fourier transforms) is use to overcome the above problem of signal processing. The original definition of DFT reveals that the time waves have to repeat frequently and similarly frequency spectrum repeat frequently in frequency domain. Basically in DFT the signals can be sampled in time domain as well as in frequency domain. The Fourier transform is the process in which the signal represented in the time domain transformed in frequency domain, while the reverse process uses IFT which is the inverse Fourier transform. The use of the Fast Fourier Transform in OFDM The main reason that the OFDM technique has taken a long time to become a prominence has been practical. It has been difficult to generate such a signal, and even harder to receive and demodulate the signal. The hardware solution, which makes use of multiple modulators and demodulators, was somewhat impractical for use in the civil systems. The ability to define the signal in the frequency domain, in software on VLSI processors, and to generate the signal using the inverse Fourier transform is the key to its current popularity. The use of the reverse process in the receiver is essential if cheap and reliable receivers are to be readily available. Although the original proposals were made a long time ago [Weinstein and Ebert], it has taken some time for technology to catch up. At the transmitter, the signal is defined in the frequency domain. It is a sampled digital signal, and it is defined such that the discrete Fourier spectrum exists only at discrete frequencies. Each OFDM carrier corresponds to one element of this discrete Fourier spectrum. The amplitudes and phases of the carriers depend on the data to be transmitted. The data transitions are synchronised at the carriers, and can be processed together, symbol by symbol (Fig. 5). Fig. 5 Block diagram of an OFDM system using FFT, pilot PN sequence and a guard bit insertion [Zou and Wu] The definition of the (N-point) discrete Fourier transform (DFT) is: (DFT) (7) and the (N-point) inverse discrete Fourier transform (IDFT): (IDFT) (8) A natural consequence of this method is that it allows us to generate carriers that are orthogonal. The members of an orthogonal set are linearly independent. Consider a data sequence (d0, d1, d2, †¦, dN-1), where each dn is a complex number dn=an+jbn. (an, bn= ± 1 for QPSK, an, bn= ± 1,  ± 3 for 16QAM, †¦ ) k=0,1,2, †¦, N-1 (9) where fn=n/(ND T), tk=kD t and D t is an arbitrarily chosen symbol duration of the serial data sequence dn. The real part of the vector D has components k=0,1,..,N-1 (10) If these components are applied to a low-pass filter at time intervals D t, a signal is obtained that closely approximates the frequency division multiplexed signal (11) Fig. 5 illustrates the process of a typical FFT-based OFDM system. The incoming serial data is first converted form serial to parallel and grouped into x bits each to form a complex number. The number x determines the signal constellation of the corresponding subcarrier, such as 16 QAM or 32QAM. The complex numbers are modulated in a baseband fashion by the inverse FFT (IFFT) and converted back to serial data for transmission. A guard interval is inserted between symbols to avoid intersymbol interference (ISI) caused by multipath distortion. The discrete symbols are converted to analog and low-pass filtered for RF upconversion. The receiver performs the inverse process of the transmitter. One-tap equalizer is used to correct channel distortion. The tap-coefficients of the filter are calculated based on the channel information. Fig. 6 Example of the power spectral density of the OFDM signal with a guard interval D = TS/4 (number of carriers N=32) [Alard and Lassalle] Fig 4a shows the spectrum of an OFDM subchannel and Fig. 4b and Fig. 6 present composite OFDM spectrum. By carefully selecting the carrier spacing, the OFDM signal spectrum can be made flat and the orthogonality among the subchannels can be guaranteed. The guard interval and its implementation The orthogonality of subchannels in OFDM can be maintained and individual subchannels can be completely separated by the FFT at the receiver when there are no intersymbol interference (ISI) and intercarrier interference (ICI) introduced by transmission channel distortion. In practice these conditions can not be obtained. Since the spectra of an OFDM signal is not strictly band limited (sinc(f) function), linear distortion such as multipath cause each subchannel to spread energy into the adjacent channels and consequently cause ISI. A simple solution is to increase symbol duration or the number of carriers so that distortion becomes insignificant. However, this method may be difficult to implement in terms of carrier stability, Doppler shift, FFT size and latency. Fig. 7 The effect on the timing tolerance of adding a guard interval. With a guard interval included in the signal, the tolerance on timing the samples is considerably more relaxed. Fig. 8 Example of the guard interval. Each symbol is made up of two parts. The whole signal is contained in the active symbol (shown highlighted for the symbol M) The last part of which (shown in bold) is also repeated at the start of the symbol and is called the guard interval One way to prevent ISI is to create a cyclically extended guard interval (Fig. 7, 8), where each OFDM symbol is preceded by a periodic extension of the signal itself. The total symbol duration is Ttotal=Tg+T, where Tg is the guard interval and T is the useful symbol duration. When the guard interval is longer than the channel impulse response (Fig. 3), or the multipath delay, the ISI can be eliminated. However, the ICI, or in-band fading, still exists. The ratio of the guard interval to useful symbol duration is application-dependent. Since the insertion of guard interval will reduce data throughput, Tg is usually less than T/4. The reasons to use a cyclic prefix for the guard interval are: to maintain the receiver carrier synchronization ; some signals instead of a long silence must always be transmitted; cyclic convolution can still be applied between the OFDM signal and the channel response to model the transmission system. http://www.wirelesscommunication.nl/reference/chaptr05/ofdm/ofdmqual.htm Multipath Challenges In an OFDM-based WLAN architecture, as well as many other wireless systems, multipath distortion is a key challenge. This distortion occurs at a receiver when objects in the environment reflect a part of the transmitted signal energy. Figure 2 illustrates one such multipath scenario from a WLAN environment. Figure 2: Multipath reflections, such as those shown here, create ISI problems in OFDM receiver designs. Click here for larger version of Figure 1b Multipath reflected signals arrive at the receiver with different amplitudes, different phases, and different time delays. Depending on the relative phase change between reflected paths, individual frequency components will add constructively and destructively. Consequently, a filter representing the multipath channel shapes the frequency domain of the received signal. In other words, the receiver may see some frequencies in the transmitted signal that are attenuated and others that have a relative gain. In the time domain, the receiver sees multiple copies of the signal with different time delays. The time difference between two paths often means that different symbols will overlap or smear into each other and create inter-symbol interference (ISI). Thus, designers building WLAN architectures must deal with distortion in the demodulator. Recall that OFDM relies on multiple narrowband subcarriers. In multipath environments, the subcarriers located at frequencies attenuated by multipath will be received with lower signal strength. The lower signal strength leads to an increased error rate for the bits transmitted on these weakened subcarriers. Fortunately for most multipath environments, this only affects a small number of subcarriers and therefore only increases the error rate on a portion of the transmitted data stream. Furthermore, the robustness of OFDM in multipath can be dramatically improved with interleaving and error correction coding. Lets look at error correction and interleaving in more detail. Error Correction and Interleaving Error correcting coding builds redundancy into the transmitted data stream. This redundancy allows bits that are in error or even missing to be corrected. The simplest example would be to simply repeat the information bits. This is known as a repetition code and, while the repetition code is simple in structure, more sophisticated forms of redundancy are typically used since they can achieve a higher level of error correction. For OFDM, error correction coding means that a portion of each information bit is carried on a number of subcarriers; thus, if any of these subcarriers has been weakened, the information bit can still arrive intact. Interleaving is the other mechanism used in OFDM system to combat the increased error rate on the weakened subcarriers. Interleaving is a deterministic process that changes the order of transmitted bits. For OFDM systems, this means that bits that were adjacent in time are transmitted on subcarriers that are spaced out in frequency. Thus errors generated on weakened subcarriers are spread out in time, i.e. a few long bursts of errors are converted into many short bursts. Error correcting codes then correct the resulting short bursts of errors. OR for guard interval Handling ISI The time-domain counter part of the multipath is the ISI or smearing of one symbol into the next. OFDM gracefully handles this type of multipath distortion by adding a guard interval to each symbol. This guard interval is typically a cyclic or periodic extension of the basic OFDM symbol. In other words, it looks like the rest of the symbol, but conveys no new information. Since no new information is conveyed, the receiver can ignore the guard interval and still be able to separate and decode the subcarriers. When the guard interval is designed to be longer than any smearing due to the multipath channel, the receiver is able to eliminate ISI distortion by discarding the unneeded guard interval. Hence, ISI is removed with virtually no added receiver complexity. It is important to note that discarding the guard interval does have an impact on the noise performance since it reduces the amount of energy available at the receiver for channel symbol decoding. In addition, it reduces the data rate since no new information is contained in the added guard interval. Thus a good system design will make the guard interval as short as possible while maintaining sufficient multipath protection. Why dont single carrier systems also use a guard interval? Single carrier systems could remove ISI by adding a guard interval between each symbol. However, this has a much more severe impact on the data rate for single carrier systems than it does for OFDM. Since OFDM uses a bundle of narrowband subcarriers, it obtains high data rates with a relatively long symbol period because the frequency width of the subcarrier is inversely proportional to the symbol duration. Consequently, adding a short guard interval has little impact on the data rate. Single carrier systems with bandwidths equivalent to OFDM must use much shorter duration symbols. Hence adding a guard interval equal to the channel smearing has a much greater impact on data rate. http://www.commsdesign.com/design_corner/showArticle.jhtml?articleID=16504605 As we know that cyclic prefix is used to restore the orthogonality and preserve ISI, but the question that arises is that how the orthogonality destroyed between the subcarriers and how cyclic prefix restore the orthogonality. [1] [2] The orthogonality between subcarriers is destroyed due to the channel dispersion whenever the signal is transmitted over a channel and this cause ICI and due to the longer delay ISI occur among the OFDM symbols which are in sequence. [1] Further more there is no any interference in uncorrupted OFDM signal when they are demodulated but when we talk about the time dispersive channel the OFDM subcarriers lost there orthogonality. The main cause behind this is that the demodulator correlation interval for one path will overlap with the symbol boundary of a different path as show in the figure [ ] [ 2] Fig. 4.11 16QAM constellation We will see that this makes equalization in the receiver very simple. If multipath exceeds the CP, then constellation points in the modulation is distorted. As can be seen from Fig. 4.11, when multipath delay exceeds the CP, the subcarriers are not guaranteed to be orthogonal anymore, since modulation points may fall into anywhere in the respective contour. As delay spread gets more severe, the radius of the contour enlarges and crosses the other contours. Hence, this causes error. The CP is utilized in the guard period between successive blocks and constructed by the cyclic extension of the OFDM symbol over a period Ï„ : (4.3) The required criteria is that Ï„ is chosen bigger than channel length Ï„h so as not to experience an ISI. The CP requires more transmit energy and reduces the bit rate to (Nb/NT +Ï„ ), where b is the bits that a subcarrier can transmit. The CP converts a discrete time linear convolution into a discrete time circular convolution. Thus, transmitted data can be modeled as a circular convolution between the channel impulse response and the transmitted data block, which in the frequency domain is a pointwise multiplication of DFT samples. Then received signal becomes Where (4.5) Hence, kth subcarrier now has a channel component Hk, which is the fourier transform of h(t) at the frequency fk. The OFDM symbol is sampled (t = nT and fk = k/NT) in the receiver and demodulated with an FFT. Consequently, the received data has the following form yk = Hk xk, k = 0, . . . ,N −1. (4.6) The received actual data can be retrieved with N parallel one-tap equalizers. One-tap equalizer simply uses the estimated channel ( ˆHk) components and use it to retrieve estimated ˆ xk as follows (4.7) Also note that the spectrum of OFDM decays slowly. This causes spectrum leakage to neighboring bands. Pulse shaping is used to change the spectral shape by either commonly used raised cosine time window or passing through a filter. An OFDM system design considers setting the guard interval (Ï„ ) as well as the symbol time (T) and FFT size with respect to desired bit rate B and given tolerable delay spread. The guard interval is selected according to delay spread, and typically it is 2–4 times the root-mean-squared delay spread with respect to chosen coding and modulation. Symbol time is set with respect to guard time and it is desirable to select much larger than the guard time since the loss in SNR in the guard time is compensated. Symbol time as we know determines the subcarrier spacing ( fb = 1/T). Number of subcarriers N is found with respect to desired bit rate, since total number of bits (bT ) to carry in one symbol is found with B/(T +Ï„ ) and selected coding and modulation determines the number of bits (b) in one subcarrier. Hence, the number of subcarriers is N = bT /b. For instance, b is two for 16QAM with rate 1/2. The required bandwidth (W) is then N âˆâ€" fb. Alternatively, this method is reversed to find out the symbol time starting from the given bandwidth. OR Cyclic-prefix insertion As I m talking about the time dispersive channel I want to include that in time dispersive channel the subcarrier not only have inter symbol interference within them but they also posses interference between them. As we know that in case of time dispersive channel the frequency-selective channel frequency response is equivalent to time dispersion on the radio channel. There are two reasons of orthogonality between OFDM subcarriers. Due to frequency-domain separation. The specific frequency-domain structure of each subcarrier. Even if the frequency-domain channel is constant over a bandwidth corresponding to the main lobe of an OFDM subcarrier and only the subcarrier side lobes are corrupted due to the radio-channel frequency selectivity, the orthogonality between subcarriers will be lost with inter-subcarrier interference as a consequence. Due to the relatively large side lobes of each OFDM subcarrier, already a relatively limited amount of time dispersion or, equivalently, a relatively modest radio-channel frequency selectivity may cause non-negligible interference between subcarriers. Time dispersion and corresponding received-signal timing Figure 9 Time dispersion and corresponding received-signal timing. To deal with this problem and to make an OFDM signal truly insensitive to time dispersion on the radio channel, so-called cyclic-prefix insertion is typically used in case of OFDM transmission. As illustrated in Figure 10, cyclic-prefix insertion implies that the last part of the OFDM symbol is copied and inserted at the beginning of the OFDM symbol. Cyclic-prefix insertion thus increases the length of the OFDM symbol from Tu to Tu +TCP, where TCP is the length of the cyclic prefix, with a corresponding reduction in the OFDM symbol rate as a consequence. As illustrated in the lower part of Figure 10, if the correlation at the receiver side is still only carried out over a time interval Tu =1/∆f , subcarrier orthogonality will then be preserved also in case of a time-dispersive channel, as long as the span of the time dispersion is shorter than the cyclic-prefix length. Cyclic-prefix insertion Figure 10. Cyclic-prefix insertion Cyclic-prefix insertion is beneficial in the sense that it makes an OFDM signal insensitive to time dispersion as long as the span of the time dispersion does not exceed the length of the cyclic prefix. The drawback of cyclic-prefix insertion is that only a fraction Tu /( Tu +TCP) of the received signal power is actually utilized by the OFDM demodulator, implying a corresponding power loss in the demodulation. In addition to this power loss, cyclic-prefix insertion also implies a corresponding loss in terms of bandwidth as the OFDM symbol rate is reduced without a corresponding reduction in the overall signal bandwidth. One way to reduce the relative overhead due to cyclic-prefix insertion is to reduce the subcarrier spacing ∆f , with a corresponding increase in the symbol time Tu as a consequence. http://wirelesscafe.wordpress.com/2009/04/20/ofdm-as-downlink-transmission-scheme-for-lte/

Relational Model :: essays research papers

The Relational Database Model A database can be understood as a collection of related files. How those files are related depends on the model used. Early models included the hierarchical model (where files are related in a parent/child manner, with each child file having at most one parent file), and the network model (where files are related as owners and members, similar to the network model except that each member file can have more than one owner). The relational database model was a huge step forward, as it allowed files to be related by means of a common field. In order to relate any two files, they simply need to have a common field, which makes the model extremely flexible. Poet Code First Name Surname Age 1 Mongane Afrika 62 2 Stephen Serote 58 3 Tatumkhulu Watson 29 Poem Title Poet Wakening Night 1 Thrones of Darkness 2 Once 3 These two tables relate through the code field in the poet table, and the poet field in the poem table. We can see who wrote the poem 'Once' by following the relationship, and see that it was poet 3, or Tatumkhulu Watson. In 1970, when E.F. Codd developed the model, it was thought to be hopelessly impractical, as the machines of the time could not cope with the overhead necessary to maintain the model. Of course, hardware since then has come on in huge strides, so that today even the most basic of PC's can run sophisticated relational database management systems. Together with this went the development of SQL. SQL is relatively easy to learn and allows people to quickly learn how to perform queries on a relational database. This simplicity is part of the reason that relational databases now form the majority of databases to be found. Basic Terms An understanding of relational databases requires an understanding of some of the basic terms. Data are the values stored in the database. On its own, data means very little. "43156" is an example. Information is data that is processed to have a meaning. For example, "43156" is the population of the town of Littlewood. A database is a collection of tables. Each table contains records, which are the horizontal rows in the table. These are also called tuples. Each record contains fields, which are the vertical columns of the table. These are also called attributes. An example would be a product record.