High Performance Wireless Telecommunications Modulation

High Performance wireless Telecommunications miscellanyover mental homeThe primary goal of the project is to analyze of OFDM dodge and to assess the suitability of OFDM as a inflexion technique for wireless communications. In the ruin of project is covered ii leading successfully slaying of OFDM dwellingd technologies ar Digital tv Broadcasting (DVB-T and DVB-H) and Long Term Evolution (LTE innovational for 4G).Wireless communications is an emerging field, which has chit-chatn enormous growth in the last several historic period. The huge consumption target argona of bustling ph atomic return 53ness technology, Wireless Local Area Networks (WLAN) and the exp inessntial growth of the internet carry forgeted in an increase demand for new methods of obtaining high capacity wireless networks. For cellular mobile applications, we depart see in the near future a over(p) convergence of mobile ph unrivaled technology, computing, profits access, and say-soly m what soever mul eradia applications such as video and high quality phone.In fact, some(prenominal) whitethorn argue that this convergence has already plumpingly fadered, with the advent of existence fitted to send and m opposite discipline using a nonebook computer and a mobile phone. The goal of third and twenty-five percent coevals mobile networks is to provide practicers with a high entropy rate, and to provide a large-mindedr range of serve, such as voice communications, videophones, and high speed Internet access. The higher(prenominal) info rate of future mobile networks give be achieved by increasing the amount of spectrum exclusively(prenominal)ocated to the service and by improvements in the unearthly efficiency. OFDM is a potential cigargonttedidate for the physical layer of fourth generation mobile systems.Basic Principles of OFDMOFDM overviewThe Orthogonal frequence division Multiplexing (OFDM) is a modulation technique where multiple low selectiv e information rate mail holders argon commingled by a vector to form a entangled high entropy rate transmission system. The kickoff commercialized intent of OFDM in the communication field was in the 1980s, and it was advancedr widely employ in the mete out audio and video field in the 1990s in such argonas as, ADSL, VHDSL, ETSI bill digital audio broadcast (DAB), digital video broadcast (DVB), and high-definition digital TV (HDTV).Digital aim affect gets OFDM possible. To see the multiple letter attack aircraft carrier scheme using a bank of gibe modulators would non be actual efficient in analog hardwargon. However, in the digital cosmos, multi-carrier modulation raise be do expeditiously with flowly procur subject DSP hardware and software. Not only empennage it be do, only it thunder mug similarly be made very flexible and programmable. This entirely in allows OFDM to mold maximum give of available bandwidth and to be able to adapt to cha nging system leadments. learn 1 is expatiated, Instead of burst modulators the outgoing wave shape is created by executing a high-speed inverse DFT on a set of m-samples of the transmissible information (post modulation). The takings of the DFT fag be directly modulate onto the outgoing carrier, without requiring any former(a) components. burstly(prenominal) carrier in an OFDM system is a sinusoid with a relative oftenness that is an whole number multiple of a pes or unfathomed sinusoid oftenness. at that placefore, apiece carrier is standardized a Fourier serial publication component of the composite forecast. In fact, it exit be shown later that an OFDM place is created in the relative frequence or import, and indeed transformed into the fourth dimension compass via the Discrete Fourier read (DFT).Two extentic signs are orthogonal when the integral of their product, over one catch, is extend to to zero. This is true of certain sinusoids as illustr ated in comparability 1.Definition of OrthogonalThe carriers of an OFDM system are sinusoids that fill this subscribe toment because from from set forthly one one one is a multiple of a fundamental relative absolute frequence. each one has an whole number number of cycles in the fundamental stop consonant. 2, 145-153 6The master(prenominal)ance of universeness orthogonalThe main concept in OFDM is perpendicularity of the sub-carriers.Since the carriers are all wickedness/cosine wave, we know that area under one tip of a sine or a cosine wave is zero. Lets take a sine wave of absolute frequency m and figure it by a sinusoid (sine or a cosine) of a frequency n, where twain m and n are integers. The integral or the area under this product is devoted byThese two components are severally a sinusoid, so the integral is equal to zero over one stay.When we multiply a sinusoid of frequency n by a sinusoid of frequency m/n the area under the product is zero. In general f or all integers n and m , sin(mx), cos(mx), cos(nx) , sin(nx) are all orthogonal to distri yetively other. These frequencies are called harmonics. Making the subcarriers mathematically orthogonal was a breakthrough for OFDM because it modifys OFDM recipient roles to separate the subcarriers via an FFT and go the carry bands.As figure 3 shows, OFDM subcarriers can overlap to make full use of the spectrum, however at the peak of apiece subcarrier spectrum, the mogul in all the other subcarriers is zero. OFDM at that placefore offers higher data capacity in a condition spectrum while allowing a simpler system innovation. Creating orthogonal subcarriers in the vector is easy using an inverse FFT. To ensure that this orthogonality is keep abreasted at the receiver (so that the subcarriers are not misaligned), the system must(prenominal)(prenominal) keep the carryter and receiver c button ups nigh synchronized deep down 2 part per gazillion in 802.11a systems. The 802 .11a standard in that locationfore dedicates four of its 52 subcarriers as pilots that enable contour-lock loops in the receiver to running the physical body and frequency of the incoming luff.The 802.11a standard on that pointfore 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 withal eliminates low-frequency phase hoo-ha.Separating the subcarriers via an FFT require rough an dedicate of rules of order fewer multiply-accumulate trading procedures than individually filtering severally carrier. In general, an FFT implementation is very much simpler than the lineage receivers use for CDMA and the decision-feed linchpin equalizers for TDMA.This idea are linchpin 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 C DMA, where codes are used to make data ranks self-employed person ( besides orthogonal) which allows many independent users to transmitin very(prenominal) space successfully.2, 153-154 6 7OFDM OperationPreliminary ConceptsWhen the DFT (Discrete Fourier Transform) of a while signal is taken, the frequency domain resolvings are a get of the sentence sampling occlusion and the number of samples as shown in go through 4. The fundamental frequency of the DFT is equal to 1/NT (1/total sample cartridge clip). Each frequency represented in the DFT is an integer multiple of the fundamental frequency.Parameter Mapping from Time to Frequency for the DFTThe maximum frequency that can be represented by a term 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. every 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) negative 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 metre signal.The parameter mapping is the said(prenominal)(p) as for the DFT. The epoch duration of the IDFT time signal is equal to the number of DFT bins (N) times the sampling layover (T).It is perfect(a)ly valid to generate a signal in the frequency domain, and convert it to a time domain equivalent for hard-nosed use (The frequency domain is a mathematical tool used for analysis. Anything usable by the real solid ground must be converted into a real, time domain signal). This is how modulation is utilise in OFDM. In traffic pattern the profuse Fourier Transform (FFT) and IFFT are used in place of the DFT and IDFT, so all however references volitioning be to FFT and IFFT.1 ,118 4Definition of holdersThe maximum number of carriers used by OFDM is t ouch oned by the coat of the IFFT. This is persistent as follows in Equation 2.OFDM Carrier CountIn order to generate a real-valued time signal, OFDM (frequency) carriers must be defined in complex commingle pairs, which are symmetric just about the Nyquist frequency (fmax). This puts the number of potential carriers equal to the IFFT sizing/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, whence the IFFT will dissolving agent in a time domain signal that has imaginary components. This must be a viable option as there are OFDM systems defined with carrier regards that die 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 wave form. Further processing would require both(prenominal) 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 whitethorn 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 deputement (selection) of carriers can be self-propelling. The transmitter and receiver fairish have to use the same parameters. This is one of the advantages of OFDM. Its bandwidth usage (and rubbish rate) can be alter according to varying user requirements. A simple control message from a base move can change a mobile units IFFT size and carrier selection.2,199-206 4ModulationBinary data from a memory device or from a digital processing sprout is used as the modulating (baseband) signal. The spare- time activity steps may be carried out in order to apply modulation to the carriers in OFDMcombine the binary data into attributeizations according to the number of bits/ sign selectedconvert the serial sign stream into parallel segments according to the number of carriers, and form carrier typeism sequencesapply differential coding to each carrier attribute sequenceconvert each imageisation into a complex phase designassign each carrier sequence to the assign IFFT bin, including the complex conjugatestake the IFFT of the resultOFDM modulation is applied in the frequency domain. goal 5 and get a line 6 give an voice of modulate OFDM carriers for one typeization flow rate, prior to IFFT.OFDM Carrier Magnitude prior to IFFTFor 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 scale to any value. The phase for each carrier is either 0 or clxxx degrees, according to the symbol being sent. The phase determines the value of the symbol (binary in this subject field, either a 1 or a 0). In the example, the send-off 3 bits (the first 3 carriers) are 0, and the 4th bit (4th carrier) is a 1.OFDM Carrier physical body prior to IFFTNote 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 mingled with each delta is comparative 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 cessation)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 bounteousness. 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, accordinglyce an FFT of the signal would no presbyopicer 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 direction 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 period resulting in very poor power efficiency.OFDM Signal, 1 Symbol PeriodFigure 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 dif ference between each of the 4 sinusoids. infectionThe 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 load of the symbol rate on each OFDM carrier is to add a sin(x)/x precondition 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 WaveformFigure 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 SpectrumFigure 10 shows the spectrum for of an OFDM signal with the following characteristics1 bit / symbol100 symbols / carrier (i.e. a sequence of 100 symbol periods)4 carriers64 IFFT binsspectrum averaged for every 20 symbols (100/20 = 5 averages) rubor diamonds mark all of the available carrier frequencies. Note that the nulls of the spectrums line up with the impudent frequencies. The four mobile carriers each have peaks at carrier frequencies. It is clear that the active voice carriers have nulls in their spectrums at each of the sassy frequencies (otherwise, the nulls would not exist). Although it cannot be seen in the figure, the active frequencies also have spectral nulls at the beside 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 decided 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 regeneration methods. Alternatively, the OFDM modulation could be immediately converted to analog and directly increased to the desired RF transmits frequency. both way, the selected technique would have to involve some form of linear AM (possibly implemented with a mixer). 1, 122-cxxv 6reaction and DemodulationThe acquire 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 in hold stream into vectors representing each symbol p eriodtake the FFT of each symbol period vector sublimate the carrier FFT bins and calculate the phase of eachcalculate the phase difference, from one symbol period to the next, for each carrierdecode each phase into binary datasort the data into the appropriate orderOFDM Carrier Magnitude following FFTFigure 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 bring 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 FFTIn 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 upset is removed from the channel, these phase variations equable occur. It must be a result of the IFFT/FFT operations g enerating very small complex values (very close to 0) for the unused carriers. The phases are a result of these values. 1, cxxv -128 3OFDM transceiverOFDM signals are typically generated digitally delinquent 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 merge up to the required frequency.Block diagram showing a basic OFDM transceiver 3The receiver performs the revoke 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 intimately 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, 3Analysis of OFDM characteristicsGuard PeriodOFDM demodulation must be synchronized with the get going and end of the transmitted symbol period. If it is not, then ISI will occur (since information will be decoded and feature for 2 nigh symbol periods). ICI will also occur because orthogonality will be lost (integrals of the carrier products will no drawn-out be zero over the integration period),To help solve this problem, a check detachmen t 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 choice the period beginning and end, and decoding is al shipway done inside a superstar 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 defense period to the start of each symbol. This reserve period is a cyclic copy that extends the aloofness of the symbol waveform. Each subcarrier, in the data section of the symbol, (i.e. the OFDM symbol with no shield period added, which is equal to the duration 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 unceasing signal, with no discontinuities at the joins. Thus by copying the end o f a symbol and appending this to the start results in a longer symbol time.Addition of a sustain period to an OFDM signal 3In 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 defend the OFDM from ISI, the guard period also provides protection against time-offset wrongdoings 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 clutch 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 3Figure 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 hold out spread of the radio channel. The remaining effects caused by the multipath, such as amplitude scale 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 appendix 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 modu lation), 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 ExtensionWith 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, 3WindowingThe OFDM signal is made up of a series of IFFTs that are concatenated to each other. At each symbol perio d 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, 1212 154Multipath CharacteristicsOFDM 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 pedigree data rate, OFDM can provide best multipath characteristics than a single carrier.OFDM vs. Single Carrier, Mu ltipath Characteristic ComparisonHowever, 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 button 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 nonparallel symbols may be lost and chastisement coding may be ineffective. 8BandwidthA relation 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 EfficiencyIn 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 9Physical ImplementationSince 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 technologyASIC (Application Specific Integrated Circuit)ASICs are the fastest, smallest, and lowest power way to implement OFDMCannot change the ASIC subsequently it is built without designing a new chipGeneral-purpose Micro central processor or MicroControllerPowerPC 7400 or other processor capable of fast vector operationsHighly programmableNeeds memory and other skirting(prenominal) chipsUses the most power and space, and would be the slowestField-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 (curren tly unknown requirements).This should be the outgo choice.9OFDM 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), everyday 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 muscular contraction and multiplexing methods, conditional access and set-top box Technology. In this project is presented an investigation of two OFDM establish DVB standards, DVB-T and DVB-H.DVB-T (Digital Video Broadcasting Terrestrial)The first Terrest rial 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 scrub within about 15 years. The greatest advantage of the digital system is the effective use of the frequency spectrum and its get radiated power in comparison with the analogue transmission, while the covered area remains the same. some other key feature is the possibility of designing a alleged(prenominal) Single Frequency Network (SFN), which means that the adjacent 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 de mands. The data stream consists of an MPEG-2 bit stream, which means a compression is used, alter the transfer of even 4 or 5 television via the standard 8 megacycle 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, 251-253Modulation technique in DVB-TThe DVB-T Orthogonal Frequency variableness Multiplexing (OFDM) modulation system uses multi-carrier transmission. There are 2 styles, the so-called 2k and 8k ways, using 1705 and 6817 carriers respectively, with each carrier modulated independently and transmitted in the 8 megahertz 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 90 phase shift between them. The form diagram and the bit allocation is shown in voci ferate16-QAM constellation diagram and bit allocation 6This 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 14Bir errorsIf 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 analogue 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 bilinear noise.Measured 16-QAM constellation diagram a) without additive noise b) with additive noise 6To 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 pass judgment (BER) and enables error-free data transmission. 13, 32-36The multi-carrier constr uctionThe structure of carriers can be illustrated also in the function of time (Figure 20). The horizontal axis of rotation is the frequency and the upright piano axis is the time. The 8 MHz channel consists of many carriers, placed 4462 Hz or 1116 Hz remote from each other according to the modulation mode (2k or 8k).Structure of OFDM carriers 13There 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 event 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 codification 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 tim e enables perfect reception by eliminating the errors caused by multipath propagation.4, 79-90 13Frequency spectrumIn 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 aloofness is 1116 Hz. In digital broadcasting, there are no vision and sound carriers, so the power for each carrier is the same. This meanHigh Performance Wireless Telecommunications ModulationHigh Performance Wireless Telecommunications ModulationIntroductionThe 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 mobil e 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 i s a potential candidate for the physical layer of fourth generation mobile systems.Basic Principles of OFDMOFDM overviewThe 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 prog rammable. 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.D efinition of OrthogonalThe 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 6The importantance of being orthogonalThe 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 byThese 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 7OFDM OperationPreliminary ConceptsWhen the DFT (Discrete Fourier Transform) of a time signal is taken, the frequency domain results are a functi on 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 DFTThe 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 t he 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 4Definition of CarriersThe 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 CountIn 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 4ModulationBinary 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 OFDMcombine the binary data into symbols according to the number of bits/symbol selectedconvert the serial symbol stream into parallel segments according to the number of carriers, and form carrier symbol sequencesapply differential coding to each carrier symbol sequenceconvert each symbol into a complex phase representationassign each carrier sequence to the appropriate IFFT bin, including the complex conjugatestake t he IFFT of the resultOFDM 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 IFFTFor 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 IFFTNote 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 repres entation 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 PeriodFigure 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.TransmissionThe 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 n ulls 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 WaveformFigure 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 SpectrumFigure 10 shows the spectrum for of an OFDM signal with the following characteristics1 bit / symbol100 symbols / carrier (i.e. a sequence of 100 symbol periods)4 carriers64 IFFT binsspectrum 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 freque ncies. 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 fo rm of linear AM (possibly implemented with a mixer). 1, 122-125 6Reception and DemodulationThe 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 OFDMpartition the input stream into vectors representing each symbol periodtake the FFT of each symbol period vectorextract the carrier FFT bins and calculate the phase of eachcalculate the phase difference, from one symbol period to the next, for each carrierdecode each phase into binary datasort the data into the appropriate orderOFDM Carrier Magnitude following FFTFigure 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 FFTIn 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 3OFDM transceiverOFDM 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 3The 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, 3Analysis of OFDM characteristicsGuard PeriodOFDM 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 3In 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 3Figure 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 extens ion 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 ExtensionWith 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. T herefore, 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, 3WindowingThe 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, 1212 154Multipath CharacteristicsOFDM a voids 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 ComparisonHowever, 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. 8BandwidthA 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 EfficiencyIn Figure 18, the calculations show that OFDM is mor e 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 9Physical ImplementationSince 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 technologyASIC (Application Specific Integrated Circuit)ASICs are the fastest, smallest, and lowest power way to implement OFDMCannot change the ASIC after it is built without designing a new chipGeneral-purpose Microprocessor or MicroControllerPowerPC 7400 or other processor capable of fast vector operationsHighly programmableNeeds memory and other peripheral chipsUses the most power and space, and would be the slowestField-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.9OFDM 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 modul ation, 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 broadc ast 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, 251-253Modulation technique in DVB-TThe 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 carrie rs 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 90 phase shift between them. The constellation diagram and the bit allocation is shown in bellow16-QAM constellation diagram and bit allocation 6This 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 14Bir errorsIf 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 6To ensure perfect picture qualit y, 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, 32-36The multi-carrier structureThe 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 13There 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 13Frequency spectrumIn 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