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1、精品论文An Idea on Single Carrier Frequency EqualizerKedi Wang, Xin Zhang, Dacheng YangBeijing University of Posts and Telecommunications, Beijing (100876)E-mail: jhony1AbstractThe application of frequency domain equalization techniques makes single carrier modulation a potentially valuable alternative
2、to OFDM, especially in regard to its lower peak-to-average transmitpower ratio and less sensitivity to carrier frequency offset. In this paper, we bring up a novel single carrier frequency domain equalizer (SC-FDE), this new SC-FDE needs not a cyclic prefix (CP. The cyclic prefix not only wastes tra
3、nsmits power, but also degrades transmission efficiency. As cyclic prefix is not required, we can reduce the system complexity greatly. We will simulate the conventionalscheme and the new scheme over Rayleigh fading channel, performance and complexity comparisonsare presented between the traditional
4、 scheme and the proposed scheme.Keywords: single carrier frequency domain equalization (SC-FDE); frequency domain equalization1. IntroductionFuture wireless systems must provide high data rate services to satisfy the increasing needs of the next-generation wireless networks. As the bit rate increase
5、s, the problem of inter-symbol interference (ISI) becomes more severe. Conventional equalization in the time domain may become impractical as it requires one or more transversal filters with the tap number covering the maximum channel impulse response length 1. OFDM is an attractive technology to de
6、al with the detrimental effects of multi-path fading 2. But it has several inherent disadvantages such as large peak-to-average ratio and sensitivity to carrier frequency offsets. Recent research has shown that single carrier frequency domain equalizer (SC-FDE) is an attractive technology for broadb
7、and wireless communications because it does not have the problems of OFDM systems such as large peak-to-average ratio and the sensitivity to carrier frequency offsets 3.By inserting a cyclic prefix in front of each data block in an FDE system, single carrier modulation with frequency domain equaliza
8、tion (SC-FDE) is an alternative solution 1 2. A cyclic prefix, which is a copy of the last part of the transmitted block, is appended to each block. The length of the cyclic prefix is the maximum expected length of the channel impulse response. In general, the length of cyclic prefix is 4 times the
9、length of the channel impulse response. In the receiver, the received cyclic prefix id discarded, and FFT processing is done on each block. The cyclic prefix at the beginning of each block has two main functions: (1) it prevents contamination of a block by inter-symbol interference from the previous
10、 block; and (2) it makes the transmitted block appear to be periodic 4. This cans the conversion from linear convolution between the transmitted signal and the fading channel impulse response to circular convolution between them. By signal processing technology the FFT can be easily accomplish. Code
11、d linear SC-FDE systems have been shown to offer similar, (and in high code rate/high SNR case) better, bit error rate performance to coded OFDM 1 5 6 7. The remainder of this paper is organized as follows: Section II describes the simulation system model and the new SC-FDE scheme in detail. Section
12、 III presents the simulation results. Section IV concludes the paper.- 4 -2. system modelFigure 1: Transmit and receive part of the system model.Figure 1 shows the system model. In our simulation system, we investigate the downlink single carrier system supporting K active users, in the transmit par
13、t, each active users data shall be encoded by turbo with code rate of R=1/3. The output of the encoder shall be fed into a BPSK/QPSK modulator. Each modulated symbol sequence shall be covered with a distinct pseudorandom sequence of length 31.then pseudorandom-coded symbols of all users shall be sum
14、med together to form a single stream. For the conventional single carrier frequency domain equalization, we divide the stream into 31 blocks; each block needs a cyclic prefix whose length is4 times of the length of the multi-path channel. Finally, the stream passes through a six paths Rayleigh fadin
15、g channel and additive white Gaussian noise (AWGN) channel. At the receiver, in the first place, the received signal shall pass through SC-FDE to combat the Rayleigh fading. The MMSE linear equalization criterion is used in our SC-FDE system. Then, by means of parallel interference cancellation (PIC
16、) technique; we can subtract multi-access interference (MAI) from the signal output from SC-FDE. In traditional SC-FDE scheme, we need a cyclic prefix insertion at the transmit part, and extract the cyclic prefix from the receive signal at the receive side. Asfigure 2 shows.Figure 2: conventional SC
17、-FDE scheme modelIn our system we need not a cyclic prefix, which is a repetition of the lastNCPchipsin the block, as shown in Fig.1; this will reduce the complexity of the system. It is known toall that relation between the linear convolution and circular convolution 8, if there are twofinite seque
18、ncesx(n)and h(n) , the length of them are N and M respectively., toimplement a linear convolution using the circular convolution operation, we need to extend both sequences by zero-padding with sufficient number of zero-valued samples. Denotes thelength of the sequence sequences:yLnwhere L = M + N 1
19、 . Define two LengthL = xn,0 n N 1,xe nhen= 0, N n L 1,hn,0 n M 1,0, M n L 1,Appendingxnandhnwith zero-valued samples. Then, at the receiver, we can useynandhnto restore the transmit signal sequenceL 1xn. We can get:Yn = DFT ( yn) = DFT (l = 0hlxn l )= HenXen0 n N + M 1(1)As appendxn with 0 when N n
20、 L 1, we can discard them after IFFTof X n . Taking the noise into consideration, we shall employ the SC-FDE based on MMSEcriterion instead of zero-forcing criterion to determine the coefficients of the frequency-domain filter.Then,itsfrequencyresponsewillbeW =H*/(H 2+2/2),k =0,1,.,N+M1(2)kkknxWhere
21、 the superscript * denotes the complex conjugate,n 2 denotes the noise power,x 2represents the signal power. In our system, we make the signal power normalized, so (2)can be written as follows:W =H*/H 2+1/ SNR,k =0,1,.,N+M1kkk3. Experiment Results and AnalysisAs figure 1 shows, we use TURBO encode,
22、QPSK modulation and perfect channel estimation. We simulate 500 frames. In every frame, we simulate 16 users with each generates 2000 bits. For the cyclic prefix scheme, we divide the data into 31 blocks. Then bit error ratio and frame error ratio comparison are displayed as follows:4. ConclusionFig
23、ure 3: BER performance comparison.A new single carrier frequency domain equalizer scheme which doesnt need a cyclic prefix is proposed in this paper, the cyclic prefix not only wastes signal power, but also degrades the transmission efficiency. We employ the relationship between the line convolution
24、 and the circular convolution. At the receive part, by means of signal processing technique, we can restore the transmit signal. Our simulation result shows the BER performance of the proposed equalizer scheme is close to the conventional single carrier domain frequency equalizer, but not needs a cy
25、clic prefix.References1D.Falconer, S.L. Ariyavisitakul, A. Benyamin-Seeyar, and B. Eidson, “Frequency domain equalization for single-carrier broadband wireless systems,” IEEE Commun. Mag, Vol. 40, No. 4, pp. 58-66, April 2002.2D.Huang and K. B. Letaief, “Symbol Based Space Diversity for Coded OFDM S
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28、strialBroadcast Channels,” Proc. Globecom94, Nov-Dec.1994, pp. 1-5.6H. Sari, G. Karam and I. Jeanclaude, “Transmission Techniques for Digital Terrestrial TV Broadcasting,” IEEE Comm. Mag., Vol.33, No.2, Feb. 1995, pp.100-109.7V. Aue, G.P. Fettweis and R.Valenzuela, “Comparison of the Performance of Linearly Equalized Single Carrier and Coded OFDM over Frequency Selective Fading Channels Using the Random Coding Technique,” Pro. ICC98, pp. 753-757.8Sanjit K. Mitra “Digital Signal Processing a Computer-Based Approach,” published by The McGraw-HillCompanies, 2005.
