each label bit of the signal point by an individual binary code,so mutiple encoders (at different levels)are employed.At the receiver,the received sequence of signal points are sually decoded by a multis age decoder In multilevel coding(MLC)schemes,any code,e.g,block codes,convolutional codes,or concantenated codes.can be used as component codes.Since in early MLC schemes the FEC codes used were usually block codes,the MLC scheme is also referred to as block coded modulation (BCM) 5)Bit-interleaved coded modulation(BICM)(with iterative decoding) BICM was first proposed by Zehavi in199 for coding for fading channels,in which the output stream of a binary encoder is bit-interleaved and then mapped to an M-ary constellation.Its basic idea is to increase the code diversity.(For Rayleigh fading channels, the code performance depends strongly its minimum Hamming distance rather than the minimum Euclidean distance.)The information-theoretic aspects of BICM have been analyzed by Caire Recently,it has been recognized that the BICM based on turbo-like codes and iterative decoding provides an effective realization method for Gallager's coding theorem(proposed in 1968 for discrete memoryless channels).With this scheme,very good performance can be achieved on both awgn and fading channels Fig.5.1.7 depicts the performances of some of typical coded modulation schemes 7 M-QAM bound V33 64-QAM <4M4.37 32.0AM ·e0(4,3 + 16QAM 404,36) BPSK 43为 TCW32.7刀Mc OPSK P=10 40 12 14 16 20 E/N (dB) spectral efficiencies achieved by 5.2 Coding Gain and Shaping Gain In coded modulations,we can use the following approaches for improving signal 5.7 5-7 each label bit of the signal point by an individual binary code, so mutiple encoders (at different levels) are employed. At the receiver, the received sequence of signal points are usually decoded by a multistage decoder. In multilevel coding (MLC) schemes, any code, e.g., block codes, convolutional codes, or concantenated codes, can be used as component codes. Since in early MLC schemes the FEC codes used were usually block codes, the MLC scheme is also referred to as block coded modulation (BCM). 5) Bit-interleaved coded modulation (BICM) (with iterative decoding) BICM was first proposed by Zehavi in 1992 for coding for fading channels, in which the output stream of a binary encoder is bit-interleaved and then mapped to an M-ary constellation. Its basic idea is to increase the code diversity. (For Rayleigh fading channels, the code performance depends strongly its minimum Hamming distance rather than the minimum Euclidean distance.) The information-theoretic aspects of BICM have been analyzed by Caire. Recently, it has been recognized that the BICM based on turbo-like codes and iterative decoding provides an effective realization method for Gallager’s coding theorem (proposed in 1968 for discrete memoryless channels). With this scheme, very good performance can be achieved on both AWGN and fading channels. Fig. 5.1.7 depicts the performances of some of typical coded modulation schemes. Figure 5.1.7: Theoretical limits on spectral and power efficiency for different signal constellations and spectral efficiencies achieved by various coded and uncoded transmission methods. 5.2 Coding Gain and Shaping Gain In coded modulations, we can use the following approaches for improving signal