The Effect of Imperfect Feedback on Opportunistic Beamforming Schemes

In multiantenna systems the optimization of linear spatial precoding is severely hampered by the amount of feedback. An efficient solution consists in the opportunistic beamforming (OB), which randomly generates the precoding and schedules the users according to the corresponding signal-to-noise ratio. Performance of OB can be enhanced by matching the generation of the beamforming to the users spatial covariance as in the recently proposed Cluster-eigenbeamforming (Cluster-EB). In this paper we show that the benefits of Cluster-EB further improve in practical systems when the transmitter has imperfect knowledge of the instantaneous feedback. Cluster-EB capitalizes on the knowledge of the spatial covariance to reduce the sensitivity with respect to the feedback degradation, thus preserving the systems performance. Analytical derivations and simulation results validate the analysis.