Multihop-enabled orthogonalization in Distributed MIMO Networks

It has recently been proposed to orthogonalize the data streams of n interfering source-destination pairs by employing an intermediate stage of non-cooperating coherent amplify-and-forward relays. This distributed zero-forcing scheme achieves a spatial multiplexing gain n/2 and does not require multi-antenna nodes. In this paper we generalize the concept from a single to multiple relay stages. Due to the concatenation of relay stages, the gain allocation problem becomes nonlinear. We show that the zero-forcing relay gain coefficients are given by the common roots of a set of polynomials, and we identify the set of network configurations for which such roots exist. In general, there exist multiple common roots. Based on experimental results we study the impact of the root selection and the number of relay stages on achievable rates. Finally, we propose a new scheme to achieve spatial orthogonalization in a single-hop interference network. Our proposal is based on the observation, that the mathematical model of the multihop problem is identical with the one of a single-hop network if we allow signals to be bounced forth and back between sources and destinations.