Efficient Soft Demodulation in MIMO-OFDM Systems with BICM and Constant Modulus Alphabets

MIMO-OFDM using bit-interleaved coded modulation (BICM) is an attractive scheme for wireless communications in frequency-selective fading channels. The BICM decoder requires log-likelihood ratios (LLRs) whose exact computation is extremely costly. In this paper, we present a novel method termed soft sphere-projection algorithm (SSPA) that provides approximate LLRs for the case of constant modulus symbol alphabets. The SSPA has very low computational complexity and performs nearly as well as the list sphere decoder. These properties are demonstrated by numerical simulations using synthetic and measured MIMO channels