Reliable implicit feedback generation in unsynchronized CoMP transmission

Coordinated multipoint (CoMP) transmission is a key technique in 4G long term evolution (LTE) systems to enable broadband communication and coverage for user equipments (UEs) at the cell edge. However, as the transmit signals will reach the UE at several and varying delays, unsynchronized CoMP transmission might experience large timing offsets (TOs), imposing a significant performance loss. In this paper we focus on the impact of implicit feedback generation in closed-loop CoMP transmission operating in joint processing mode. To enable the expected advantages, reliable channel state information is crucial for practical realization of closed-loop CoMP transmission. We show that a spacing in terms of subcarriers between the reference signals in the orthogonal frequency division multiplexing symbols imposes a phase offset on the transmit covariance matrix in presence of TO, affecting the feedback generation and limiting the expected performance. We provide a solution to overcome this limitation by estimating and compensating this phase offset. We show analytically and numerically that keeping the phase offset close to zero, reliable feedback generation can be guaranteed and, as a result, near optimum performance is achieved in closed-loop CoMP transmission in unsynchronized networks with TO present.