1. A symbol estimation method implemented at a receiver, comprising: calculating, based on an estimate of a communication channel and an initial estimate of a set of received symbols, a refined estimate of the set of received symbols, by performing, for each symbol in the set of received symbols, refinement operations comprising: parameterizing, for the symbol, a contribution to intersymbol interference (ISI) by the remaining symbols in the set of received symbols; selecting a refined estimated value for the symbol by evaluating an optimization function using the estimate of the communication channel and one or more symbol values from the initial estimate of the set of received symbols; and computing a log-likelihood value for the selected refined estimated value for the symbol based on, at least in part, the contribution of ISI by the remaining symbols in the set of received symbols and the estimate of the communication channel; wherein the set of receive symbols comprise two possible values and wherein, in the refinement operation of selecting the refined estimate value, the evaluating the optimization function comprises evaluating a hyperbolic tan function.