  The performance of multiple-input multiple-output wireless systems is
investigated in the presence of statistical queueing constraints. Queuing
constraints are imposed as limitations on buffer violation probabilities. The
performance under such constraints is captured through the effective capacity
formulation. A detailed analysis of the effective capacity is carried out in
the low-power, wideband, and high--signal-to-noise ratio (SNR) regimes. In the
low-power analysis, expressions for the first and second derivatives of the
effective capacity with respect to SNR at SNR= 0 are obtained under various
assumptions on the degree of channel state information at the transmitter.
Transmission strategies that are optimal in the sense of achieving the first
and second derivatives are identified. It is shown that while the first
derivative does not get affected by the presence of queueing constraints, the
second derivative gets smaller as the constraints become more stringent.
Through the energy efficiency analysis, this is shown to imply that the minimum
bit energy requirements do not change with more strict limitations but the
wideband slope diminishes. Similar results are obtained in the wideband regime
if rich multipath fading is being experienced. On the other hand, sparse
multipath fading with bounded number of degrees of freedom is shown to increase
the minimum bit energy requirements in the presence of queueing constraints.
Following the low-SNR study, the impact of buffer limitations on the high-SNR
performance is quantified by analyzing the high-SNR slope and the power offset
in Rayleigh fading channels. Finally, numerical results are provided to
illustrate the theoretical findings.
