Improved Closed-Loop Communication in the Presence of Feedback Delay and Error

Abstract

The closed-loop transmit diversity technique is used to improve the performance of the downlink channel in MIMO communication systems. The WCDMA standard endorsed by 3GPP adopts two modes of downlink closed-loop schemes based on partial channel state information. The information is fed back from the mobile unit to the base station through a low-rate uncoded feedback bit stream. Previously it was addressed the efficient reconstruction of the beamforming weight in the presence of feedback error, with the constraint of a constant transmit power. In this article, the issue of feedback delay is also considered. Using joint source-channel coding techniques, a reconstruction algorithm is introduced to improve the performance of mode 1 of 3GPP in the presence of feedback error and delay, by taking advantage of the redundancy available in the bitstream of channel state information. We also introduce the novel concept of blind antenna verification. It can substitute the conventional antenna weight verification process without the need to any training data. The performance is examined within a simulated 3GPP framework. It is demonstrated that the proposed algorithms have substantial gain over the conventional method for low, moderate and high mobile speeds. The proposed approaches are applicable to other feedback schemes as well.