On the Directional Reciprocity of Uplink and Downlink Channels in Frequency Division Duplex Systems

The possibility of using the channel reciprocity between the uplink and downlink channels in Frequency Division Duplex (FDD) systems to improve the efficiency has been deeply investigated. Previous studies have come to different conclusions for the characterization of the dissimilarity in uplink and downlink channel properties. This paper analyzes the mismatch in directional properties of the uplink and downlink channels of FDD systems based on the power of their multipath clusters. At a system level, due to the limited directional resolution, the multipath components arriving at the base station are seen as clusters, rather than individual signal paths. This fact is used to describe the mismatch of directional properties between uplink and downlink. The contribution of this paper is the use of a spectral dissimilarity metric as a measure to characterize this mismatch; a detailed study of this dissimilarity metric is also presented. It is found that under favorable propagation conditions, for both actual channel measurement data and ray-tracing simulations, the directional and power properties of the downlink multipath clusters can be estimated from the uplink channel with high reliability. Therefore, directional-based beamforming transmission techniques for FDD systems will be able to benefit from such similarity in order to improve the system performance.