A new strategy to design and implement the optimal Multi-Antenna Broadcast Channel precoder

The sum-capacity of decentralized communication schemes, either at transmission or reception, has already been computed in the literature. However, there is a lack of practical implementation schemes that achieve this sum-capacity. This paper presents a novel precoder design for the multi-antenna Broadcast Channel (BC) in order to attain the BC sum-capacity. It is a non-iterative BC architecture, which first implements the so-called embedded power loading. This consists in including the equivalent point-to-point (PtP) multiple-input-multiple-output (MIMO) optimal power waterfilling and eigenbeamforming. However, in contrast to the PtP MIMO case, the resulting equivalent channel is not diagonal and the decentralized receivers cannot cooperate to cancel the interference. Therefore, the rest of the precoder is devoted to reduce the interference to the same residual values as in the optimal dual MAC, which has been previously computed. In order not to introduce additional power, only orthogonal transformations together with a THP (Tomlinson Harashima Precoding) are used for that. A numerical example illustrates the proposed new strategy and implementation. In contrast to the existing implementations of the optimal BC multi-antenna precoder, this paper presents a very simple way to apply its duality with the equivalent MAC.