On the combining schemes for MIMO systems with hybrid ARQ

Multiple-input multiple-output (MIMO) systems with hybrid automatic-repeat-request (HARQ) promises high throughput with high reliability. However, combining-scheme design for such systems faces challenges, the presence of interference and the existence of multiple signal-to-interference-and-noise power ratios (SINRs). To overcome these challenges, this paper suggests to design combining schemes with the objective of directly optimizing the log-likelihood ratio (LLR) values. Using this approach, this paper proposes several combining schemes and then analyzes them based on three key design factors: decoding performance, scalability, and memory requirement. Computer simulations under the IEEE 802.16e standard settings show the performances of the proposed combining schemes, which align well with the analysis. Based on the analyses and the simulation results, this paper suggests preferable combining schemes respectively for MIMO systems with HARQ-chase combining (HARQ-CC) and HARQ-incremental redundancy (HARQ-IR).