Direct‐conversion‐based wireless communication systems suffer from I / Q imbalances in the front‐end analog processing. In this study, the authors investigate the impact of frequency‐independent (FI) I / Q imbalance on the channel reciprocity of time‐division‐duplexing (TDD) multiple‐input multiple‐output (MIMO) systems over frequency flat‐fading channels. Moreover, in order to compensate the channel non‐reciprocity caused by FI I / Q imbalance, the authors propose two pre‐compensation algorithms with and without I / Q imbalance parameters. The performance of the TDD MIMO system under study is evaluated in terms of downlink capacity and bit‐error rate. Simulation results show that the proposed algorithms can effectively mitigate the negative effect of FI I / Q imbalance on the channel reciprocity.

In multiuser multiple-input multiple-output (MIMO) downlink communications, system performance suffers from multiuser interference (MUI). Contrary to common practice where MUI is suppressed, correlation rotation (CR) linear precoding makes MUI constructive via rotating the angle of correlation between transmitted symbols. With the derived closed-form instantaneous SINR expression, we analyze the unified SER performance for CR precoding, and identify the channel condition under which the MUI exploitation via CR precoding will fail and consequently lead to the unified SER performance degradation. To solve the problem, two power allocation schemes that are subject to power constraint are proposed: one is iterative scheme that directly minimizes the unified SER, and the other has a closed-form solution that maximizes the minimum user SINR. The asymptotic behavior and computational complexity of the proposed schemes are analyzed. Numerical and simulation results show that both schemes can avoid the degradation of unified SER performance and furthermore offer performance gains. Moreover, the closed-form solution with much lower complexity can provide a comparable performance to that of the iterative scheme.

This letter proposes a novel algorithm to obtain a suboptimal solution for the balance of bit distribution after Huffman coding. The algorithm is simple, and can be embedded in the conventional Huffman coding process. In addition, the letter also discusses the bit-balance problem for reversible variable-length codes (RVLCs) based on Huffman coding. Analytical and experimental results suggest that the new algorithm is very useful in improving the 0/1 balance property for Huffman codes and RVLCs.