Field of the Invention
The invention relates in general to a signal receiver of a communication system, and more particularly to a signal receiver with adaptive soft information adjustment and an associated signal processing method.
Description of the Related Art
FIG. 1 shows a functional block diagram of a signal receiving end of a conventional communication system. A transmission signal S transmitted from a transmitting end (not shown) is transmitted through a channel 110 (having a channel effect H) and added by noise N (represented by an adder 120) during the transmission process, and becomes an input signal Y=HS+N received at a signal receiving end 130. A channel estimating unit 131 estimates a channel effect H′ that the input signal Y underwent according to the input signal Y, and an equalizer 132 restores the input signal Y according to the channel effect H′ to obtain a modulated signal S′=Y/H′=(HS+N)/H′. Assuming that H′ is close to H, the modulated signal S′ may be approximately S′=S+N′, where N′ is equalized noise. A soft-decision demodulating circuit 133 demodulates the modulated signal S′ to generate a plurality of sets of soft information SI. FIG. 2 shows a diagram of a relationship between soft information and hard information. In hard decision, if the value of the modulated signal S′ is greater than a threshold η, the modulated signal S′ is determined as S1, and if the value of the modulated signal S′ is smaller than the threshold η, the modulated signal S′ is determined as S0. The threshold η is usually an average of S1 and S0, i.e., η=(S0+S1)/2. Thus, if the modulated signal S′ happens to be near the threshold η, a misjudgment is likely incurred. On the other hand, in soft decision, the modulated signal S′ is determined according to a maximum a posterior (MAP) principle, and a similarity level between the modulated signal S′ and S1 or S0 can be obtained. For example, if the modulated signal S′ is located at a position indicated by y, the modulated signal S′ is determined as S0 as far as hard decision is concerned. As far as soft decision is concerned, the probability (P1) of determining the modulated signal S′ as S1 is greater than the probability (P0) of determining the modulated signal S′ as S0, and so the modulated signal S′ is eventually determined as S1. According to the MAP principle, soft decision has a higher reliability. However, the accuracy of soft decision is frequently affected by the channel or noise and thus becomes lowered. Therefore, soft information S1 is conventionally compensated.
Again referring to FIG. 1, an output of the soft decision demodulating circuit 133 is coupled to a multiplier 134. The multiplier 134 multiplies the soft information SI by a constant coefficient A to generate adjusted soft information SI′. A quantizer 135 quantizes the adjusted soft information SI′ by its adjustment range to generate quantized data. The decoder 136 then decodes the quantized data to generate decoded data. However, adjusting the soft information SI by the constant coefficient A suffers from certain drawbacks. When the soft information SI is good enough (meaning that the decoded data having undergone quantization and decoding has a very low bit error rate (BER)), multiplying such soft information SI by the constant coefficient A may contrarily cause an increase in the BER. Therefore, there is a need to improve the conventional signal receiving end 130, such that the quantizer 135 may obtain more appropriate soft information to further reduce the BER of the signal receiving end 130.