Two-Way Synchronization for Coordinated Multicell Retrodirective Downlink Beamforming

Coordinated multicell downlink transmission has recently been proposed as a technique that can enable spectrally efficient communication in cellular networks. By coordinating downlink transmissions, the base stations in a cellular system can transmit such that signals from multiple base stations arrive coherently at a mobile. One approach to coordinated multicell downlink transmission is to have the mobiles estimate the downlink channel state information (CSI) and feed the CSI back to the base stations for precoding. This paper proposes a different approach based on retrodirectivity and channel reciprocity. The primary advantage of this approach is that there is no need for CSI estimation or feedback by the mobiles. A tradeoff, however, is that the base stations must be synchronized to within a small fraction of a carrier period. A new two-way base station synchronization protocol is proposed to facilitate coordinated multicell coherent retrodirective downlink transmission techniques. An analysis of the statistical properties of the estimation errors in the two-way synchronization protocol and the resulting power gain of a multicell retrodirective downlink beamformer using this protocol is provided. Numerical examples are also presented characterizing the performance of multicell retrodirective downlink beamforming in a system using two-way base station synchronization. The numerical results demonstrate that near-ideal multicell downlink beamforming performance can be achieved with low synchronization overhead.