Energy efficient MIMO OFDM multi-user downlinks: THP based versus soft precoding

In 1993 Nakagawa and Esmailzadeh first proposed the generation of radio signals by precoding. As the asset of such schemes, they excel by a low receiver complexity, which makes them attractive for applications like mobile radio downlinks with their demand of inexpensive terminals. Unfortunately, linear precoding does not offer any transmit power reductions as compared to transmission schemes not resorting to precoding. However, such reductions become feasible - under virtually maintaining the low receiver complexity of linear precoding -, if we use non-linear instead of linear precoding. In the paper two downlink transmission concepts following this rationale will be comparatively assessed, namely a scheme termed Transmit Nonlinear Zero Forcing, and a concept termed soft precoding. Primarily, the assessment focuses on the required transmit powers. However, the aspects of the amplitude dynamic of the received signals and the impact of imperfect channel state information are addressed as well. The target systems of the investigations are MIMO OFDM multi-user downlinks. It turns out that the proposed non-linear precoding schemes not only excel by reduced transmit powers, but also offer increased robustness with regard to imperfect channel state information.