Performance Analysis of Hierarchically Combined Practical Beamforming Methods

Abstract

In literature various limited-feedback beamforming methods applied in co-located and distributed antenna systems have been proposed for enhancing wireless link spectral efficiency and reliability. We focus on practical limited-feedback methods and introduce a hierarchical feedback structure, whereby co-located/distributed transmit antennas are organized into two or more groups. To facilitate robust and flexible use of channel state information we apply independent feedback to antenna groups and between the groups. This structure provides additional implementation flexibility under practical constraints, particularly for coordinated multipoint (CoMP) systems. For the presented methods with hierarchical control structure, we compute closed-form expressions for the signal-to-noise power ratio gain, the fading figure and the average bit-error-probability. Although hierarchical structure leads in some cases to suboptimal methods, results show that performance loss against upper bound given by transmitter equal gain combining is negligible even when number of feedback bits is small. Thus, it is concluded that the hierarchical feedback that is robust against errors can be effectively used when antennas form natural groups like in e.g. CoMP transmission.