Reliable feedback generation in unsynchronized joint-processing CoMP transmission networks

Coordinated multipoint (CoMP) transmission is a key technique in 4G long term evolution (LTE) systems to provide the ubiquitous broadband communication with cell edge free experience at the user equipment (UE). However, as the transmit signals will reach the UE at several and varying delays, practical CoMP networks might experience large timing offsets (TOs), imposing a significant performance loss. In this paper we focus on implicit feedback generation for practical CoMP joint processing (JP)-joint transmission (JT) networks suffering from TOs. We show that conventional implicit feedback schemes prohibit the expected gain in the downlink, getting outperformed by open-loop transmission schemes and call the benefits of channel state information (CSI) feedback for CoMP JP-JT networks into question. Analytically we prove that the mapping of CSI-reference signal (RS) on different subcarriers imposes multiple phase offsets on the covariance matrix, leading to improper precoding vector selection, while imbalanced TOs among the transmitters limit coherent signal demodulation at the UE. With numerical results we show that estimating and compensating the phase offsets reliable precoding vector selection and phase feedback for coherent signal demodulation can be obtained and as a result considerable improvements for practical CoMP JP-JT networks can be achieved in presence of varying TOs.