Capacity of multi-antenna channels in the low-power regime

In emerging mobile systems users must operate very often in the low-power regime. Specifically, almost 40 % of geographical locations experience signal-to-noise ratios (SNR) below 0 dB. Despite its relevance, the multi-antenna low-power regime had not been analyzed in depth until the paper by S. Verdu (see IEEE Trans. on Inform. Theory, p.1319-43, June 2002), where the figure of merit is not the SNR, but rather the normalized energy per information bit, E_b/N₀. This paper expands these findings using a channel model that realistically describes the conditions found in typical wireless systems. The focus is on channels that are known to the receiver, but unknown to the transmitter.