The authors present an efficient, sub-optimal, soft-in-soft-out decodi
ng rule for single parity check (SPC) codes, which requires only three
addition-equivalent-operations per information bit. Its application i
s demonstrated by the simulation results of a rate 5/6 four-dimensiona
l concatenated SPC code, for which performance of BER = 10(-5) at E-b/
N-0 = 3.5dB is observed, which is only similar to 1.2dB from the theor
etical limit.