Abstract — Adaptive modulation schemes for fading channels are usually required to fulfill certain long-term average BER targets. However, for simplicity and mathematical tractability, these schemes are often designed by fixing the short-term instantaneous BER to the target value. In this paper, the analysis and design of variable-rate variable-power QAM schemes with average BER constraints are tackled for a MIMO beamforming system with MRC and imperfect CSI. The SISO system is considered as a special case of these more general results. Approximate closed-form policies are derived for continuous rate and power adaptation which are compared to fully discrete policies designed numerically. Closed-form expressions for the ASE of the proposed policies are derived and evaluated. Our results indicate that imperfect CSI and discrete rate and power constraints do not significantly degrade performance. I.

Abstract — Adaptive modulation schemes for fading channels are usually required to fulfill certain long-term average BER targets. However, for simplicity and mathematical tractability, these schemes are often designed by fixing the short-term instantaneous BER to the target value. In this paper, the analysis and design of variable-rate variable-power QAM schemes with average BER constraints are tackled for MIMO multiplexing with imperfect CSI. The SISO system is considered as a special case of these more general results. Approximate closed-form policies are derived for continuous rate and power adaptation which are compared to the fully discrete policies. Our results indicate that MIMO multiplexing is much more sensitive to imperfect CSI than MIMO beamforming due to the self-interference that arises from channel coupling. In particular, if interference between eigenchannels is large, MIMO multiplexing should utilize only one of its eigenchannels, in which case all multiplexing gain is lost. I.