Multi-Antenna Channel Hardening and its Implications for Rate Feedback and Scheduling


Bertrand M. Hochwald    Thomas L. Marzetta    Vahid Tarokh


Abstract: Wireless data traffic is expected to grow over the next few years and the technologies that will provide data services are still being debated. One possibility is to use multiple antennas at basestations and terminals to get very high spectral efficiencies in rich scattering environments. Such multiple-input multiple-output (MIMO) channels can then be used in conjunction with scheduling and rate-feedback algorithms to further increase channel throughput. This paper provides an analysis of the expected gains due to scheduling and bits needed for rate feedback.

Our analysis requires an accurate approximation of the distribution of the MIMO channel capacity. Because the exact distribution of the instantaneous channel capacity in a Rayleigh fading environment is difficult to analyze, we prove a central limit theorem for MIMO channels with a large number of antennas. While the growth in average (or ergodic) capacity of a MIMO channel with the number of antennas is well understood, it turns out that the variance of the instantaneous capacity can grow very slowly or even shrink as the number of antennas grows. We discuss implications of this ``channel-hardening" result for data and voice services, scheduling and rate feedback. We also briefly discuss the implications when shadow fading effects are included.

Status: To appear in IEEE Transactions on Information Theory, 2004.

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Bert Hochwald<hochwald@lucent.com>