hat is best viewed in -space context.所以编码调制也称为信号空间第码。 In TCM schemes,the code and an expanded signal set are jointly designed as a physical unit.The design criterion is to maximize the free Euclidean distance between coded signal sequences rather than Hamming distance.The resulting code can provide a significant coding gain and the loss from the expansion of the signal set can be overcome. Example 5.3: 我们从编码调制的角度，考虑图5.1.1中的编码器与调制器的联合设计。 As an alternative coded scheme we may use the 8-PSk TCM scheme shown in Fig 5 1 3 which was introduced by Ungerboeck.We will see in Section 5.5 that this TCM scheme can provide an asymptotic coding gain of=3(dB). a c) 8-PSK T e=Icoccm] Fig.5.1.3 The 4-state TCM encoder for 8-PSK The performances of various TCM schemes are shown in Fig.5.1.4.It is seen from Fig. 5.1.4 that the improvement of coding is evident.Note that the coding schemes shown in Figure5 achieves the codingins withour requring more bandwidth than the unc uncoded 545-4 sequences rather than Hamming distance来设计. More recently, it has been recognized that the design of coded modulation schemes for the AWGN channel is a problem that is best viewed in the geometric signal-space context. 所以编码调制也称为信号空间编码。 In TCM schemes, the code and an expanded signal set are jointly designed as a physical unit. The design criterion is to maximize the free Euclidean distance between coded signal sequences rather than Hamming distance. The resulting code can provide a significant coding gain and the loss from the expansion of the signal set can be overcome. Example 5.3: 我们从编码调制的角度，考虑图 5.1.1 中的编码器与调制器的联合设计。 As an alternative coded scheme, we may use the 8-PSK TCM scheme shown in Fig. 5.1.3, which was introduced by Ungerboeck. We will see in Section 5.5 that this TCM scheme can provide an asymptotic coding gain of γ = 3 (dB). 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 Fig. 5.1.3 The 4-state TCM encoder for 8-PSK The performances of various TCM schemes are shown in Fig. 5.1.4. It is seen from Fig. 5.1.4 that the improvement of coding is evident. Note that the coding schemes shown in Figure 5.1.4 achieves the coding gains without requiring more bandwidth than the uncoded QPSK system