On the Effective Capacities of Multiple-Antenna Gaussian Channels

The concept of effective capacity offers a novel methodology to investigate the impact that design decisions at the physical layer may have on system performance at the link layer. Assuming a constant flow of incoming data, the effective capacity characterizes the maximum arrival rate that a wireless system can support as a function of its service requirements. Service requirements in this framework are defined in terms of the asymptotic decay-rate of buffer occupancy. This article studies the effective capacity of a class of multiple-antenna wireless systems subject to Rayleigh flat fading. The effective capacity of the multi-antenna Gaussian channel is characterized, and system performance is evaluated in the low signal-to-noise ratio regime. Additional to the power gain of the multiple receive antenna system, we show that there is a statistical gain associated with a multiple transmit antenna system. When the number of transmit and/or receive antennas becomes large, the effective capacity of the system is bounded away from zero, even under very stringent service constraints. This phenomena, which results from channel-hardening, suggests that a multiple-antenna configuration is especially beneficial to delay-sensitive traffic.