Coordinated beamforming in multicell networks with Channel State Information exchange delays

Coordinated beamforming is an important technique for dealing with interference channels arising in multicell systems. However, the performance of coordinated beamforming suffers significantly from Channel State Information (CSI) feedback delay and back-haul delay caused by exchanging CSI through the back-haul link. Hence, designing a beamforming scheme that takes into consideration the different delays incurred by the local and interfering channel links is an important problem. In this paper, average Virtual SINR based criterion is used to design an optimal combination of Maximum Ratio Transmission (MRT) and Intercell Interference Cancellation (IIC) beamformers as a function of the feedback and back-haul delays. Although it is well known that MRT is close to optimality at low SNR and IIC better at high SNR, we demonstrate how the optimal solution is modified when taking feedback delays into account. In particular, our results suggest the egoistic strategy is much more robust with respect to delays, thus affecting the shape of the optimal beamformer and giving extra performance for a wide range of scenarios.