Cooperative diversity through virtual arrays in multihop networks

We propose a multihop cellular network architecture which takes advantage of cooperation among users to induce a diversity gain. First, mobile terminals (MT), willing to cooperate, share their data during a time slot reserved to MT-to-MT links, and then, in a successive time slot, they send their data to the base station (BS) through a virtual array of antennas, constituted by the antennas of the cooperating users. We derive the coding strategy, for such a virtual array, that maximizes the sum of the rates from the MTs to the BS, under the constraint of a given total available power. We assume at the beginning that the channels from the MT to the BS are perfectly known. This allows us to derive, for each MT, a closed form expression for the optimal power allocation, as a function of frequency. Then, we remove this assumption and we use a first order perturbation analysis to compute the loss resulting from imperfect channel knowledge.