optimum coded modulation scheme,its capacity strongly depends on the particular labeling of signal points.Caire et al.showed that the gap to an optimum scheme is surprisingly small with Gray labeling of the signal points.Howe ,it has been illustrated that th gap significantly if finite length of the code is taken into account.码长越短，性能差别越大。 Figure 5.11.8 shows the BICM mutual information for some signal constellations different binary labeling rules and uniform inputs for the AWGN channel,as a function of SNR for illustration simplicity.we have only plotted the information rate for the gray and set partitioning binary labeling g rules from n Figure x Ohserve that bina reflected Gray labeling pays a negligible penalty in information rate,being close to the coded modulation capacity. -CM 16QAM. BICM(Gray BICM(SP) 8PSK QPSK SNR(dB) Fig.5.11.8 Coded modulation and BICM capacities (in bits per channel use)for muliple signal constellations with uniform inputs in the AWGN channel.Gray and set partitioning labeling rules correspond to dashed and dash-dotted lines,respectively.The thick solid lines indicate CM channel capacity. -CM -BICM(Gray 320AM 16QAM 3.5 8PSK 2.5 QPSK 05 SNR(dB) Fig.5.11.9 27 27 optimum coded modulation scheme, its capacity strongly depends on the particular labeling of signal points. Caire et al. showed that the gap to an optimum scheme is surprisingly small with Gray labeling of the signal points. However, it has been illustrated that the gap increases significantly if the finite length of the code is taken into account. 码长越短，性能差别越大。 Figure 5.11.8 shows the BICM mutual information for some signal constellations, different binary labeling rules and uniform inputs for the AWGN channel, as a function of SNR. For illustration simplicity, we have only plotted the information rate for the Gray and set partitioning binary labeling rules from Figure x. Observe that binary reflected Gray labeling pays a negligible penalty in information rate, being close to the coded modulation capacity. -10 -5 0 5 10 15 20 0 0.5 1 1.5 2 2.5 3 3.5 4 SNR (dB) C (bits / channel use) CM BICM(Gray) BICM(SP) 16QAM 8PSK QPSK Fig. 5.11.8 Coded modulation and BICM capacities (in bits per channel use) for multiple signal constellations with uniform inputs in the AWGN channel. Gray and set partitioning labeling rules correspond to dashed and dash-dotted lines, respectively. The thick solid lines indicate CM channel capacity. -10 -5 0 5 10 15 20 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 SNR (dB) C (bits / channel use) CM BICM (Gray) 32QAM 16QAM 8PSK QPSK Fig. 5.11.9