Based on the received sequence,the encoding rules and the noise characteristics of the channel,the receiver makes a decision which message was actually transmitted.This ecision making operation is called decoding Hard-decision When binary coding is used,the modulator has only binary inputs (M-2).If binary demodulator output quantization is used(=2),the decoder has only binary inputs.In this case.the demodulator is said to make hard decision.decoding based on hard decisions made 2yhedcgotlaoricalkeihaadkaiwonbcodis Soft-decision If the output of demodulator consists of more than two quantization levels (Q>2)or is left unquantized,the demodulator is said to make soft decisions.Decoding based on this is called soft-decision decoding O电平 硬判决(Q=2) 解调器 量化器 多 软判决 器 Hard-decision decoding is much easier to implement than soft-decision decoding However,soft-decision decoding offers significant performance improvement over hard-decision decoding.See figure 2. -BPSK soft-decision boun 1.2 -BSC Non sol-decision- 0.8 hard-deision 0.6 Ep/N.(dB) Figure2软判决与硬判决译码的信道容量 ■Optimal decoding 9 „ Based on the received sequence, the encoding rules and the noise characteristics of the channel, the receiver makes a decision which message was actually transmitted. This decision making operation is called decoding. „ Hard-decision When binary coding is used, the modulator has only binary inputs (M=2). If binary demodulator output quantization is used (Q=2), the decoder has only binary inputs. In this case, the demodulator is said to make hard decision. Decoding based on hard decisions made by the demodulator is called hard-decision decoding. „ Soft-decision If the output of demodulator consists of more than two quantization levels (Q>2) or is left unquantized, the demodulator is said to make soft decisions. Decoding based on this is called soft-decision decoding. 解调器 Q 电平 量化器 译 码 器 硬判决(Q=2) 软判决 „ Hard-decision decoding is much easier to implement than soft-decision decoding. However, soft-decision decoding offers significant performance improvement over hard-decision decoding. See figure 2. -2 0 2 4 6 8 10 0 0.2 0.4 0.6 0.8 1 1.2 Eb /N0 (dB) Capacity (bits/symbol) soft-decision hard-decision BPSK soft-decision bound Shannon bound BSC bound Figure 2 软判决与硬判决译码的信道容量 „ Optimal decoding