Method of normalization of forward metric (alpha) and reverse metic (beta) in a map decoder

摘要

A method of normalization of forward metric (alpha) and reverse metric (beta) in a MAP decoder. In a map decoder log values of probabilities may be continually added. This continual addition can overflow limited size registers set aside to hold the alpha or beta values. This overflow may be overcome by subtracting a constant value from all of the alpha or beta values when they have reached a certain value, a process called normalization. Subtracting a constant value however may slow down the computation. Instead of adversely affecting the computation speed however the detection of a constant value may occur on one decoding cycle and the normalization may occur on the succeeding decoding cycle. Additionally instead of using a traditional subtraction circuit a multiplexor type circuit can be used to direct either zeros, in the normalization case, or a most significant bit(s), in the case where the computation were proceeding without normalization, into the register holding the alpha or beta values. Additionally to minimize the impact on the computation by the normalization process, the multiplexor circuit can be set by the previous decoder cycle so that the computation does not have to wait for the multiplexor to be set to normalization or normal computation.