Vector Precoding with MMSE for the Fast Fading and Quasi-Static Multi-User Broadcast Channel

Vector precoding is arguably the best form of precoding for the multi-user multiple input multiple output (MIMO) broadcast channel. However, conventional vector precoding schemes are designed to minimize the transmit energy, which is suboptimal in terms of the received signals mean square error (MSE). This paper proposes modifications to vector precoding to overcome this shortcoming. Improvements of about 2 dB are realizable in a fast fading environment. Also, conventional vector precoding schemes usually result in unbalanced levels of interference, resulting in a poor performance for some users. Noting the above, another improvement is proposed by directly minimizing the bit error rate rather than the MSE. This improvement adds another 1 dB of gain, resulting in an overall gain of 3 dB in a quasi static fading environment. These improvements are also applied to Tomlinson-Harashima precoding with similar results.