Channel Estimation and Signal Combining for Multipoint Cellular Radio

To further increase spectral efficiency and reliability of future wireless communications systems, base station cooperation, or "coordinated multipoint (CoMP)", has become a major trend in physical layer research. In this paper, 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. We show that plain maximum ratio combining (MRC) is sufficient to collect a multitude of potentially weak signal contributions ("crumbs"), but the performance of the combiner is directly related to the quality of the channel estimation. Obviously, the channel estimator has to operate at very low SNR to make best use of CoMP, i.e., to benefit from many weak signal contributions. We show that practical estimators based on least squares approaches, which are reasonable solutions for a single cell set-up, fail in the CoMP scenario. Instead, estimators are required that implicitly de-weight ("pinch-off") unreliable signal contributions. Our analysis of the different channel estimators is supported by link-layer simulations to provide quantitative results, ranging from simple test channel models to various fully­ fletched LTE channel scenarios.