On Low-Complexity Soft-Input Soft-Output Linear Equalizers

The design of low-complexity soft-input soft-output (SISO) linear equalizer (LE) for turbo equalization can be classified into two categories: the Cat-I assuming random a priori soft input (from the decoder) and the Cat-II treating the a priori input deterministic. There is a lack of systematic comparison between the two categories. In this letter, a comprehensive comparison between the two categories of SISO LE is provided for multiple-input multiple-output (MIMO) systems. It is shown the two categories are equivalent for constant-modulus modulations. For non-constant-modulus modulations, however, they are generally not the same, even though the two achieve similar performance. In terms of complexity, the Cat-I is more efficient, especially for high-order modulations.