tailieunhanh - Code Division Multiple Access (CDMA) phần 5

phân phối và được cho là ngẫu nhiên và memoryless, và (e) kiểm soát quyền lực hoàn hảo là đạt được nhận. Xác suất của lỗi cho FH-CDMA có thể được viết như [35] Pe = Po (1 - Ph) + P1 Ph (2,89) | 68 CODE DIVISION MULTIPLE ACCESS CDMA diversity. distributed and are assumed random and memoryless and e perfect power control is achieved at the receiver. The probability of error for FH-CDMA can be written as 35 Pe Po 1 - Pi P1 Ph where Po is the probability of error when there are no collisions or hits between users . when two users avoid hopping to the same frequency at the same time Ph is the probability of at least one hit and P1 is the probability of error when at least one hit has occurred. With BFSK and non-coherent reception the probability of error in the absence of collisions is 22 Po 2 exp 1 El 2 N Now with independent random hopping codes the probability of at least one hit can be written as Ph 1 - 1 - P SPREAD SPECTRUM TECHNIQUES FOR CODE DIVISION MULTIPLE ACCESS 69 where P2 is the probability of any two users hopping to the same frequency in the same symbol duration. For independent random hop codes this probability is P2 1 N for synchronous hopping and P2 2 Nfor asynchronous hopping 1 . We will examine the problem by determining upper and lower bounds for the bit error probability. Clearly an upper bound can be formed by assuming that the probability of error whenever a hit occurs is 50 . P1 1 2 35 Pb Po 1 - Ph 2 Ph A lower bound can be determined by examining the case when a collision between the desired user and exactly one other user occurs. Clearly this probability is less than the probability of one or more collisions occurring . Ph K - 1 P2 1 - P2 K-2 Further the probability of error when a collision occurs between two users depends on the relative value of the bits. If the two values are the same which occurs with probability 1 2 the probability of error is Po. If the two values are different the probability of error is 1 2. This leads to a lower bound P Po 1 - Ph 1 1 pJ K - 1 P2 1 - P2 K-2 This lower bound tends to be accurate whenever K is small relative to N since it assumes that the probability of .