An information-theoretic look at MIMO energy-efficient communications

Abstract

One of the main objectives of this paper is to provide an information-theoretic answer on how to maximize energy- efficiency in MIMO (multiple input multiple output) sys- Tems. In static and fast fading channels, for which arbitrar- ily reliable communications are possible, it is shown that the best precoding scheme (which includes power alloca- Tion) is to transmit at very low power (Q ! 0). Whereas energy-efficiency is maximized in this regime, the latter also corresponds to communicating at very small transmission rates (R ! 0). In slow fading or quasi-static MIMO sys- Tems (where reliability cannot be ensured), based on the proposed information-theoretic performance measure, it is proven that energy-efficiency is maximized for a non-trivial precoding scheme; in particular, transmitting at zero power or saturating the transmit power constraint is suboptimal. The determination of the best precoding scheme is shown to be a new open problem. Based on this statement, the best precoding scheme is determined in several special but useful cases. As a second step, we show how to use the proposed energy-efficiency measure to analyze the impor- Tant case of distributed power allocation in MIMO multiple access channels. Simulations show the benefits brought by multiple antennas for saving energy while guaranteeing the system to reach a given transmission rate target. Copyright © 2009 ICST.