A Comparison Between Unitary and Non-Unitary Precoder Design for MIMO Channels with MMSE Detection and Limited Feedback

In English This work studies the design of linear precoder codebooks for N-r x N-t MIMO channels with MMSE detection at the receiver. A natural split of precoder-design is unitary precoding and non-unitary precoding. Unitary precoding is only performing rotation of the data in a way beneficial for the channel. Non-unitary precoding additionally also uses power-loading to further improve the performance. Somewhat surprisingly, unitary precoding facilitates a performance boosting by a re-enumeration of the antenna elements at the receiver side that can not be accomodated in the non-unitary precoding setting. This operation leads to substantial performance gains. The question investigated in this paper is whether this re-enumeration can compensate for the lack of power-loading. The outcome is that for small precoder codebooks, unitary precoding performs as good as non-unitary, while for larger codebooks non-unitary precoding outperforms unitary precoding.