Limits of channel estimation and signal combining for multipoint cellular radio (CoMP)

To further increase spectral efficiency and reliability of wireless communications systems, base station cooperation, or "coordinated multipoint (CoMP)", has become a major trend in physical layer research. We revisit, in a tutorial style, channel estimation algorithms for CoMP schemes in the uplink, i.e., when performing diversity combining using distributed antennas at the base station side. We focus on a single serving base station, and a geometric ring arrangement of a number of supporting base stations. Obviously, the channel estimator has to operate at very low SNR to make best use of CoMP by benefltting from many weak signal contributions. We show that conventional estimators based on least squares approaches, which are reasonable solutions for a single cell set-up, can cause severe degradation in the CoMP scenario. Rather, estimators are required that implicitly de-weight ("pinch-off") unreliable signal contributions. For those MMSE-type estimators, the challenge is practical parameter estimation of the auto - and cross-covariance.