QPSK 调制器 FEC 8-PSK 编码器 调制器 Figure5.1.1 从第二章中的调制信号星座的容量分析可知，信号星座点个数增加一倍所提供的冗 余己足够实现在不增加系统带宽的条件下，逼近容量限的性能。 再进 一步扩展星座，所 得到的性能增益将很少。因此，在通常的编码调制系统中采用的是码率为kk+1)的信道 码。 ■将编码与调制作为一个整体进行联合优化设计。 Although the expansion of a signal set (e.g.,from QPSK to 8-PSK)provides the redundancy required for coding.it shrinks the distance betweer the ignal points if the average signal energy is kept constant.This reduc ction in distance should be compensated by coding advantage if the coded scheme is to provide a benefit. 如果是按照传统方法，简单地在一个纠错编码器后级联一个M元调制器，而纠错编码器 是基于汉明距离准则进行设计，则所得到的结果往往会令人失望。 ,The use of hard-decision demodulation prior to the decoding in a coded scheme causesalosofSNRToavoidsuchahard-decsionlos,itism aryt女 soft-outpu detector decoder directly on the soft-output samples of the channel.The decision rule of the optimum decoder depend on the Euclidean distance. Example5.2:对于上例中的) For the coded 8-PSK scheme above.if we choose the rate-2/3 convolutional code of Fig.5.1.2(a)which is designed based on maximizing free Hamming distance,and the mapping of 3 output bits of the convolutional encoder to the 8-PSK signal points is done as shown in Fig.5.1.2(b).then we can find that the minimum Euclidean distance between a pair of paths forming an error event(which is sometimes called free Euclidean distance)is d=0∑d,0 =d(xo)+d(xo.x)+d(x2x) =△+0+=1.172E,5-2 Figure 5.1.1 从第二章中的调制信号星座的容量分析可知，信号星座点个数增加一倍所提供的冗 余已足够实现在不增加系统带宽的条件下，逼近容量限的性能。再进一步扩展星座，所 得到的性能增益将很少。因此，在通常的编码调制系统中采用的是码率为k/(k+1)的信道 码。 „ 将编码与调制作为一个整体进行联合优化设计。 因为 Although the expansion of a signal set (e.g., from QPSK to 8-PSK) provides the redundancy required for coding, it shrinks the distance between the signal points if the average signal energy is kept constant. This reduction in distance should be compensated by coding advantage if the coded scheme is to provide a benefit. 如果是按照传统方法，简单地在一个纠错编码器后级联一个M元调制器，而纠错编码器 是基于汉明距离准则进行设计，则所得到的结果往往会令人失望。 另外，The use of hard-decision demodulation prior to the decoding in a coded scheme causes a loss of SNR. To avoid such a hard-decision loss, it is necessary to employ soft-output detector. TCM integrates demodulation and decoding in a single step and decoder operates directly on the soft-output samples of the channel. The decision rule of the optimum decoder depend on the Euclidean distance. Example 5.2：（对于上例中的） For the coded 8-PSK scheme above, if we choose the rate-2/3 convolutional code of Fig.5.1.2(a) which is designed based on maximizing free Hamming distance, and the mapping of 3 output bits of the convolutional encoder to the 8-PSK signal points is done as shown in Fig.5.1.2(b), then we can find that the minimum Euclidean distance between a pair of paths forming an error event (which is sometimes called free Euclidean distance) is 2 2 { }{ } min ( , ) i i free E i i x x i d d xx ≠ ′ = ∑ ′ 222 07 00 21 2 2 0 0 (,) (,) (,) 0 1.172 EEE s d xx d xx d xx E =++ =Δ + +Δ =