1. A method in a communications receiver for decoding received communications data, the method comprising, for each of at least one iteration in at least one soft-input soft-output decoder: calculating intermediate probability values associated with each possible transition between a first plurality of candidate decoder states at a first time and a second plurality of candidate decoder states at a second time; computing two or more partial sums from the intermediate probability values, wherein the partial sums correspond to possible combinations of two or more systematic bits, two or more parity bits, or at least one systematic bit and at least one parity bit; estimating a soft value for each of at least one systematic bit of the received communications data corresponding to the interval between the first and second times, as a first function of the partial sums; and estimating a soft value for each of at least one parity bit of the received communications data corresponding to the interval between the first and second times, as a second function of the partial sums.