tailieunhanh - Space-Time Coding phần 7

nơi ct là biểu tượng tin nhắn tại thời điểm t và N là độ dài chuỗi, được mã hóa bằng một mã tuyến tính. Nói chung các biểu tượng tin nhắn ct có thể được nonbinary nhưng để đơn giản chúng tôi giả định rằng họ là độc lập tạo ra các ký hiệu nhị phân và có tương đương với một xác suất tiên nghiệm. | Appendix MAP Algorithm 175 Appendix MAP Algorithm We present a MAP decoding algorithm for a system depicted in Fig. . In order to simplify the analysis the following description of the MAP algorithm is specific to n 1 m binary convolutional codes though it could easily be generalized to include k n rate convolutional codes as well as decoding of block codes. A binary message sequence denoted by c and given by c C1 C2 . Ct . cN where ct is the message symbol at time t and N is the sequence length is encoded by a linear code. In general the message symbols Ct can be nonbinary but for simplicity we assume that they are independently generated binary symbols and have equal a priori probabilities. The encoding operation is modeled as a discrete time finite-state Markov process. This process can be graphically represented by state and trellis diagrams. In respect to the input ct the finite-state Markov process generates an output vt and changes its state from St to St 1 where t 1 is the next time instant. The process can be completely specified by the following two relationships Vt f St Ct t St 1 g St ct t The functions f and g are generally time varying. The state sequence from time 0 to t is denoted by St and is written as S0 S0 S1 . St The state sequence is a Markov process so that the probability P St 1 S0 S1 . St of being in state St 1 at time t 1 given all states up to time t depends only on the state St at time t P St 1 S0 S1 . St P St 1 St The encoder output sequence from time 0 to t is represented as V0 V0 V1 . Vt Memoryless noise Figure System model 176 Space-Time Turbo Trellis Codes where Vt Vt 0 Vt 1 . Vt n-1 is the code block of length n. The code sequence V is modulated by a BPSK modulator. The modulated sequence is denoted by X and is given by X0 X0 X1 . Xt where Xt xt 0 xt 1 . xt n-1 and xt i 2vt i - 1 i 0 1 . n - 1 As there is a one-to-one correspondence between the code and modulated sequence