c2) 8-PSK c=[ccc] (a)Encoder structure 0 E 101 110 8-PSK (b)Signal mapping rule and the trellis diagram of the coded scheme Fig.5.1.2 A rate-2/3 convolutional coded 8-PSK scheme To compare the coded and uncoded schemes it is common to use the coding gain parameter,which is defined as the difference in SNR for an objective target bit error rate between a coded system and an uncoded system. coding gainSNR-SNR At high SNR,this gain is termed the asymptotic coding gain (ACG)and is expressed as d1E,) y=10log (diE. dB For the coded scheme above.-23 dB.This result shows the 2 performance degradation of the coded scheme (optimized based on the free Hamming distance)compared to the uncoded one Massey pointed out that it was necessary to integrate the design of encoder and modulator. and to treat the code and modulation scheme as an entirety,as shown in Fig.5.1.Thus, 整体方案就应该基于maximizing the minimum Euclidean distance between coded signal 5-3 5-3 T T 8-PSK Signal (1) Set i c (2) i c (3) i c (1) i a (2) i a xi (3) (2) (1) [ ] iii c = ccc (a) Encoder structure 8-PSK Δ0 Es Δ1 Δ2 Δ3 (000) (001) (010) (011) (100) (101) (110) (111) 4 0 0 6 2 6 2 5 1 0 4 3 7 1 5 7 3 1 0 2 S0 S1 S2 S3 (b) Signal mapping rule and the trellis diagram of the coded scheme Fig. 5.1.2 A rate-2/3 convolutional coded 8-PSK scheme To compare the coded and uncoded schemes it is common to use the coding gain parameter, which is defined as the difference in SNR for an objective target bit error rate between a coded system and an uncoded system. uncoded coded coding gain | |  SNR SNR − At high SNR, this gain is termed the asymptotic coding gain (ACG) and is expressed as ( ) ( ) 2 10 2 ,min / 10log / free s coded E s uncoded d E d E γ = dB For the coded scheme above, 10 1.172 10log 2.3 dB 2 γ = =− . This result shows the performance degradation of the coded scheme (optimized based on the free Hamming distance) compared to the uncoded one. Massey pointed out that it was necessary to integrate the design of encoder and modulator, and to treat the code and modulation scheme as an entirety, as shown in Fig. 5.1. Thus, 系统 整体方案就应该基于maximizing the minimum Euclidean distance between coded signal