tailieunhanh - Code Division Multiple Access (CDMA) phần 4

Tái sử dụng các kênh có nghĩa là nhiễu đồng kênh là nhận được mỗi người nhận trong hệ thống. Các mô hình tái sử dụng được sử dụng phụ thuộc vào tỷ lệ tín hiệu để can thiệp tối thiểu (SIR) có thể được dung thứ. | 48 CODE DIVISION MULTIPLE ACCESS CDMA The performance of a spread spectrum signal in flat Rayleigh fading can be determined similar to the AWGN case with the exception that due to fading we now must integrate the probability of error over the SNR distribution. The decision statistic for BPSK modulation can be written as the output of the matched filter which is also a correlator output Z 1 i r t a z dt T Jo 1 f T _ . ypb t a t Y t n t j a t dt Y ỉ b t a t a t dt i n t a t dt T 0 T 0 a P y b n where we have assumed that the channel Y t remains constant over a symbol interval. Since the baseband channel is complex we must remove the channel-induced phase modulation to perform phase detection. Multiplying the matched filter output by the complex conjugate of the channel gain Y and assuming BPSK modulation the probability of error conditioned on Y is then P 1 Y q arỀ Iy I2 No Since Y is a complex Gaussian random variable Iy 2 is a Chi-square random variable with two degrees of freedom. The average probability of error is the conditional probability of error averaged over the distribution of IY 2. The probability density function pdf of p NN IY 2 is p p 4 e p where p NIY I2 is the average SNR at the output of the matched filter. The average probability of error is then Pe l p p Q ụĩp dp 1 p Q T2p dp e 0 p SPREAD SPECTRUM TECHNIQUES FOR CODE DIVISION MULTIPLE ACCESS 49 If V - 1 _ 1 2 1 1 1 21 1 4 The probability of error and the high SNR approximation are shown in Figure . As can be seen the performance is dramatically degraded as compared to the AWGN case. As we can see from Figure multipath is a serious impairment to wireless communications. However the previous analysis assumes flat Rayleigh fading . the entire frequency band of the signal fades coherently . The use of spread spectrum allows for substantial improvement in performance one of the greatest benefits of spread spectrum communication. Specifically the preceding .