Implementation and Performance Analysis of a Non-codebook Based MU-MIMO System with Dynamic Precoding for TDD LTE-Advanced

This paper presents a non-codebook based Multiple User Multiple Input Multiple Output (MU-MIMO) test-bed system for Time Division Duplex (TDD) Long Term Evolution-Advanced (LTE-A). The system was implemented for verifying the performance of a dynamic precoding procedure that selects the most appropriate precoder among a given set of precoding techniques. Using two parameters, the propagation path gain and the condition number of the channel matrix, the proposed MU-MIMO system can adaptively switch the precoding software to guarantee that the upper bound Bit Error Rate (BER) is maintained with a minimum computational burden. This paper also presents a novel procedure for accurately and simply calibrating the multiple Radio Frequency (RF) paths of the multiple antennas and RF transceivers of the MU-MIMO system. From various experimental measurements obtained from the implemented test-bed system, the upper bound BER, which was arbitrarily set to 10(-3) in this paper, can be maintained with the simplest precoder, Zero Forcing (ZF), unless the propagation path gain becomes less than 0.25. It was also found in the experimental tests that, as the distance between 2 handsets becomes shorter than 1 meter, which causes the condition number of the channel matrix to be larger than 17, the precoder should be switched from Lattice Reduction (LR) to Tomlinson-Harashima Precoding (THP) when the minimum distance between base-station and each handset maintained is set to at least 6 meters.