Per-antenna rate and power control for MIMO layered architectures in the low- and high-power regimes

In a MIMO layered architecture, several codewords are transmitted from a multiplicity of antennas. Although the spectral efficiency is maximized if the rates of these codewords are separately controlled, the feedback rate within the link adaptation loop is reduced if they are constrained to be identical. This poses a direct tradeoff between performance and feedback overhead. This paper provides analytical expressions that quantify the difference in spectral efficiency between both approaches for arbitrary numbers of antennas. Specifically, the characterization takes place in the realm of the low- and highpower regimes via expansions that are shown to have a wide range of validity. In addition, the possibility of adjusting the transmit power of each codeword individually is considered as an alternative to the separate control of their rates. Power allocation, however, turns out to be inferior to rate control within the context of this problem.