Reverse tracing of forward state metric in Log-Map and MAX-Log-MAP decoders

Serious BER performance degradation due to finite numeric precision in VLSI implementations of Log-MAP and MAX-Log-MAP decoders, where the forward state metric is calculated using a reverse tracing method, is analyzed, and two methods are proposed to overcome this problem; the loser storing method for MAX-Log-MAP and the periodic storing method for Log-MAP and MAX-Log-MAP. Both methods can reduce memory storage size effectively by half, but with additional circuit overhead. Our VLSI implementation examples show that, compared with the original method, both methods give about 15% improvement in area and power consumption with identical BER performance.