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 down-link 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.