OFDM的原理.doc

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1、如有侵权，请联系网站删除，仅供学习与交流OFDM的原理【精品文档】第 19 页1.In modern society more high dependence and the requirements of communication, communication system and the design and development of high efficiency has become the constant pursuit of communication engineering. The efficiency of the communication system, in t

2、he final analysis is the spectrum utilization and power efficiency. Especially in wireless communication scenarios, the requirements of these two indicators more often, especially in spectrumefficiency. Because of the space available spectrum resources are limited, and wireless applications are more

3、 and more, making use of wireless spectrum of strict management by governments and unified planning. Thus, a wide variety of communication technology has high spectrum efficiency continues to be developed, OFDM(Orthogonal Frequency Division Multiplexing) is knownspectral efficiency of a communicatio

4、n system is the highest, the digital modulation, digital signal processing, multi carrier transmission technology combine together, make it in the system spectrum utilization rate, power utilization, systemcomplexities has very strong competitive power, is support for future mobile communication esp

5、ecially is one of the main technology of mobile multimedia communications.OFDM is a multi carrier transmission technology, the N sub carrier channel is divided into N sub channel, N sub channel parallel transmission.OFDM system has many remarkable advantages. First, OFDM has very high spectrum utili

6、zation. The common FDM system to signal from each sub channel, need to set the protection interval in the adjacent channel (band), so that the receiver can use the band-pass filter to isolate signals corresponding to sub channels, which not only protect the sub band waste.OFDM system spectrum resour

7、ces between channels, and between adjacent channels The mainlobe signal spectrum is overlapping (see Figure 1.5), but the spectrum of each sub channel signal is orthogonal to each other in the frequency domain, each subcarrier is orthogonal in time domain, separating each sub channel signals of the

8、OFDM system (demodulation) by the orthogonality to complete. In addition, sub channel OFDM can also be used M-ary modulation (such as high spectrum efficiency, QAM) to further improve the spectrum efficiency of OFDM system. Second, relatively simple to achieve. When the channel using QAM or MPSK mod

9、ulation, modulation demodulation process can be completed by IFFT. The process can be completed by FFT, without oscillation source group without band-pass filter group signal separation. Third, anti multipath interference and anti fading ability. Because of the general OFDM system with cyclic prefix

10、 (Cyclic Prefix, CP), so that it can completely eliminate the multipath propagation caused by code interference under certain conditions, completely eliminate the damage of multipath on the inter carrier orthogonality of subcarriers, so OFDM system has good ability of anti multipath interference.OFD

11、M the whole channel into many narrow channel, although the channel is available It can be a very flat fading channel, but the fading on each subchannel is approximately flat (see Figure 1.6), which makes the equalization of subchannels in OFDM system very simple, and usually requires only one tap eq

12、ualizer Of course, with the single carrier system, OFDM also has some difficult problems to be solved. These problems are mainly: first, synchronization problem. Theoretical analysis and practice show that the OFDM system of synchronous system requires higher accuracy, synchronization error not only

13、 caused the output SNR drop, will destroy the orthogonality of the sub the inter carrier, causing inter carrier interference, which greatly affect the performance of the system, and even make the system does not work. Second, the peak to average power ratio of OFDM signal (Peak-to-Average Power, Rat

14、io, PAPR) are often large, making it The linear range of amplifier requirements, but also reduces the efficiency of the amplifier in the future application of.OFDM in communication system, especially mobile applications in multimedia communication in the future will depend on the degree to solve the

15、 above problems. OFDM technology has been or is gaining some applications, for example, ETSI (European) in broadcast applications in EuropeTelecommunication Standard Institute, the European Telecommunications Standards Institute, has developed the number of OFDM technologies Digital Audio Broadcasti

16、ng (DVB) standard, digital video broadcasting (Digital VideoBroadcasting，DVB）The standards are being developed; in broadband wireless access applications, IEEE 802.11a and IEEE 802.16 are based on the advice of ETSI OFDM technology, HiperLAN II is a OFDM technology based on the standard; used in dig

17、ital cellular mobile communication, OFDM technology is one of the hot research at present; in the wired broadband access technology for example, xDSL (high speed digital subscriber line) technology, a special form of OFDM (Discrete Multito - DMT NE) to get widely used in these applications and so on

18、;.OFDM has shown strong vitality, with solving some key problems which restrict the application of OFDM, I believe OFDM will play an increasingly important role in future communication applications.2.OFDM 发展简史OFDM3.Multicarrier modulation and FFTOFDM is a multi carrier transmission technology. Let F

19、K (k = 1,2),OFDM is a multi carrier transmission technology. Let FK (k = 1,2),The carrier modulated signal can be expressed in the interval of I symbolsN 1si (t) = X i (k,t) exp( j2fk t)（1.2.1）k =0Among them, X I (k, t) is carried by the signal in the first I symbol interval information, it determin

20、es the Si (T) amplitude and phase, generally they are only with the symbol label I related complex constant, they carry the information to be transmitted; for example, if the K is carrier using QPSK modulation, set by pi / 4 way sign, when the I code is 00, according to the mapping between the symbo

21、ls and signs can know, X I (k, t) = 22 (1 + J). For simplicity, in just a When the symbol of multi carrier signal, often omit the symbol label I; and when the subcarrier with ordinary (without using waveform formation) QAM or MPSK modulation, X I (k, t) has nothing to do with the T, which will X I (

22、k, t) or X (k), according to the context of such ambiguity not. According to the above agreement, (1.2.1) can be written asN 1s(t) = X (k) exp( j2fk t)（1.2.2）k =0We hope that the spectrum utilization of this multi carrier transmission method is high, that is to say, the subcarrier interval should be

23、 as small as possible, and the system is easy to implementIn order to realize the multi carrier transmission system, the general need N oscillator and the corresponding bandpass filters, the complex structure of the system, does not embody the advantages of multi carrier transmission. However, after

24、 careful analysis can be found, the modulation and demodulation of multi carrier transmission system can use the discrete Fourier transform (Discrete Fourier Transform. DFT), due to DFTA fast algorithm of FFT (Fast Fourier Transform) famous, the multi carrier transmission system implementation is gr

25、eatly simplified, especially the OFDM system using FFT to achieve, with its simple structure, high spectrum efficiency and attention2.This paper analyzes the conditions that multicarrier transmission systems can be implemented with DFT In order to determine the frequency interval between subcarriers

26、, we consider how the receiver for signal demodulation of the received signal. We (not to consider the influence of noise and distortion) f s sampling to sampling rate, sampling to demodulate the signal by DFT. Using the N point DFT can calculate the signal frequency spectrum component for kN 1S(kf

27、) = s(n / f s ) exp( j2nk / N )（1.2.3）n=0Here, S (the K F) is the first k frequency; s (n / f s) (n = 0,1,2, N - 1) is the sampling signal; F = FS / NIt is the resolution of DFT. In order to make DFT correctly calculate the spectrum, the signal must be periodically repeated outside the N point sampl

28、ing, when the signal is repeatedWhen the number contains only the harmonic component of the DFT, the condition can be satisfied. The T = n / Fs substitution formula (1.2.2) is obtainedN 1s( n / f s ) = X ( l ) exp( j 2 f l n / fs )（1.2.4）L0将式（1.2.4）代入式（1.2.3）得N 1 N 1S ( k f ) = X ( l ) exp( j 2 f l

29、n / f s ) exp( j 2 nk /N )n = 0 l =0N 1N 1= X ( l ) exp( j 2 f l n / f s ) exp( j 2 nk / N )L0n=0N 1flk= X ( l) ()（1.2.5）fsl =0N其中0,m n(m, n) =m = n1,观察上式可以发现，当多载波已调信号的频率fk =kf s（1.2.6）N时，There is S (the K F) = CX (k), where C is a constant, that is when each subcarrier frequency demodulation with D

30、FTK can be demodulated by DFT in the case of integer multiples of the discrimination rate. From the above analysis, it is shown that in order to ensure correct demodulation, X (a) is in the process of demodulationOne symbol interval is constant is necessary, if the QAM or MPSK subcarrier modulation

31、uses a waveform shaping technology, such as the use of cosine waveform, but also specifically by DFT demodulation.From the above analysis, when the frequency of each subcarrier is integer multiples of the DFT resolution for demodulation, the DFT can be used to carry multiple loadsWave modulated sign

32、al sampling demodulation. In particular, the frequency interval of the subcarrier is f s / N, by type (1.2.4) isN 1s(n / f s ) = X (k) exp j2(kfs / N )n / fs k =0N 1= X (k) exp j2n / N （1.2.7）k =0Type is X (k) (k = 0,1,2, N - 1) sequence (the sequence we denoted as X (N) IDFT(Inverse Discrete Fourie

33、r Transform), namely the subcarrier frequency spacing of F S / N, multi carrier modulated signal time domain sampling sequence can be calculated by IDFT.The sequence of X carrying information (N) is a multi carrier modulated signal sampling sequence DFT, so we say that the modulation multi carrier m

34、odulation system based on FFT is carried out in the frequency domain.From the above analysis, the modulation of multi carrier modulation system can be completed by IDFT, demodulation can be completed by DFT, by the knowledge of digital signal processing, we can know that IDFT and DFT can be implemen

35、ted by efficient FFT4.Composition of OFDM systemThe block diagram of the OFDM system is shown in Figure 1.1After the input bit sequence is completed and transformed, the corresponding modulation mapping is completed according to the modulation mode adopted, and the modulation is formedThe informatio

36、n sequence X (N) is used to carry out IDFT on X (N), and the time domain sampling sequence of OFDM modulated signal is calculatedCP (cyclic prefix cyclic prefix can enable the OFDM system to completely eliminate multipath propagation caused by intersymbol interference (ISI) and inter carrier interfe

37、rence (ICI) analysis on 1.5 section 1.4 and section), and then D/A transform, OFDM modulated signal waveform. The receiving end of received signal in A/D remove the CP transform, cyclic prefix, OFDM modulated signalThe sampling sequence is DFT, and the original modulation information sequence X (N)

38、is obtained 输入信号 串并变换.符号映射.IFFT.加入数/模射频CP转换调制并串信道输出信号并串变换 .符号判决.均衡.FFT.去CP模/数射频串并转换解调图 1.1 OFDM 系统的结构The introduction of CP PR 1 cyclic prefix, the OFDM transmission can be completely eliminated due to intersymbol interference caused by multipath propagation under certain conditions (ISI) and inter

39、channel interference (ICI) effect, greatly promote the practical use of OFDM technology in the process. Figure 1.2 is a schematic diagram of the cyclic prefix.4图 1.2 CP 示意图OFDM symbol (symbol) is an ambiguous concept. In most of the OFDM literature, OFDM symbol refers to the modulation information s

40、equence X (N), and X (N) of each component (i.e. on each sub carrier modulation information) also used the symbol (symbol) said. To avoid this confusion, we will X (N) together with a cyclic prefix called OFDM frame symbols, referred to as the symbol, called X (N) component of the intra symbol.OFDM

41、in the literature of the inter symbol interference (ISI)Refers to the inter symbol interference frame, in particular to remove the interference between the cyclic prefix symbol frame, but also refers to the same symbol synchronization symbol synchronization. This frame is consistent with the name of

42、 OFDM in literature, and will not cause misunderstanding.5. Time continuous system model of OFDMThere are some different forms of OFDM system. We first establish the corresponding mathematical model ESBL 1. on the most popular OFDM system using cyclic prefixThe original OFDM system using digital mod

43、ulation and demodulation technology, so the OFDM model can be regarded as the ideal model of OFDM system, of course, is generally achieved by digital synthesis technology. Figure 1.3 is a continuous time model of OFDM baseband system.。图 1.3 OFDM 连续系统基带模型z transmitterThe OFDM system has a N subcarrie

44、r, the system bandwidth is W Hz, the symbol length is Ts, and the length of the cyclic prefix CP is Tcp, that is, the transmission time of a OFDM frame symbol is T = Ts + Tcp, considering the influence of the cyclic prefix, the K carrier wave waveform emitted by the transmitter is as follows1exp j2

45、Wk(t T) t 0,T TsNcp（1.4.1）k (t) =0,t 0,T Note that when the T, 0, Tcp, a diameter of K (T) = K (T + N = /W) with K (T + Ts), which is the cyclic prefix, which makes the signal within a certain period of time. This article looks with periodic I OFDM frames have symbols adjustable waveformN 1si (t) =

46、X i (k)k (t iT )（1.4.2）k =0When the transmission is an infinite OFDM symbol sequence, the OFDM modulated signal waveform can be expressed ass(t) = si (t)i= z channel N 1= X i (k)k (t iT )（1.4.3）i= k =0We assume that the channel impulse response g (tau; t) the support of less than the cyclic prefix C

47、P, namely r e 0, Tcp, the signal is received by the receiver forTcp（1.4.4）r(t) = (g s)(t) = g(;t)s(t )d + n (t)0Here, n(t) is the additive white noise (complex form) of the channel Gauss.z receiverThe OFDM receiver consists of a filter bank, where the first k filter is matched with the back part Tcp, T of the carrier wave waveform