Impact of Antenna Configuration on Feedback Generation for Non-Ideal CoMP Transmission

Coordinated multipoint (CoMP) transmission is a promised candidate in long term evolution advanced (LTEA) systems to assure coverage and broadband communication for cell edge user equipments (UEs). However, the presence of timing offsets (TOs) between the received signals from multiple transmitters might limit the performance of CoMP transmission. The promised advantages of closed-loop CoMP transmission vanish for the system configuration with four transmit antennas, showing significantly poorer performance than the transmitter equipped with two antennas for large TOs. The reason is that the transmit covariance matrix used for implicit feedback generation is affected by a phase shift caused by TO after orthogonal frequency division multiplexing demodulation, leading to improper precoding matrix index (PMI) selection and therefore limiting the beamforming gain at the cell edge UE. On the other hand the system configuration with two transmit antennas is robust against that phase shift, despite the TO. Keeping the phase shift in systems with four transmit antennas low, reliable PMI selection can be guaranteed and as a result the promised CoMP transmission benefits can be achieved in a practical realization with TO present.