A Hierarchical AF Protocol for Distributed Orthogonalization in Multiuser Relay Networks

We consider distributed orthogonalization of multiple source-destination terminal pairs through coherent amplify-and-forward (AF) relaying. When an excess number of relays are available in the network, additional gains of diversity and sum rates are known to be attained through efficient gain-allocation schemes. The common requirement of these schemes is global dissemination of local channel state information (CSI) of each relay. Consequently, as the number of relays grows, the CSI dissemination overhead may diminish the aforementioned gains in practice. We propose a novel hierarchical relaying protocol where relays with different amounts of CSI knowledge can coexist within the same network. In the lower level of hierarchy, each relay employs only local CSI to compute its gain factor and, hence, is cost-free in terms of CSI dissemination overhead. However, the relays in the higher level of hierarchy have further channel information that allows them to determine the gain factors enabling distributed orthogonalization of multiple pairs. We study sufficient conditions to achieve full spatial multiplexing, the required channel knowledge per hierarchical level, the corresponding relay gain allocations, and power-allocation strategies among hierarchical levels. We finally identify relay selection for different hierarchical levels as an efficient means to recover outage (diversity) and sum-rate gains. It is shown that with drastically reduced CSI dissemination overhead, the hierarchical protocol combined with relay selection approaches the performance of conventional multiuser relaying that requires global CSI at all relay nodes.