Cooperation Strategies for Two Co-located receivers, with no CSI at the transmitter

Abstract

We study the case of a single transmitter, which communicates to two co-located users, through an independent block Rayleigh fading channel. The co-location nature of the users allows cooperation, which increases the overall achievable rate, from the transmitter to both users. The transmitter is ignorant of the fading coefficients, while receivers have access to perfect channel state information (CSI). This gives rise to the broadcast approach used by the transmitter. The broadcast approach facilitates reliable transmission rates adapted to the actual channel conditions, designed to maximize average throughput. With the broadcast approach, users can decode partly the total message, with almost any fading realization. The better the channel quality, the more layers that can be decoded. Such an approach is useful when considering average rates, rather than outage vs. rate (outage never occurs). The cooperation between the users is performed over the co-location channel, modeled as separated additive white Gaussian channels (AWGN), with an average power constraint, and limited or unlimited bandwidth. New achievable rates when combining cooperation with the broadcasting approach are presented, where through simple change of power allocation, substantial gains are demonstrated. We consider both amplify-and-forward (AF) and Wyner-Ziv compress-and-forward (CF), as cooperation approaches, and also compare to decode-and-forward (DF). We extend these methods using the broadcast approach, and also to include separated processing of the layers, realized through multi-session cooperation. Further, novel closed form expressions for infinitely many multi-session AF and recursive expressions for the more complex multi-session CF are given. Numerical results for the various cooperation strategies demonstrate the efficiency of multi-session cooperation. Our results can be extended straightforwardly to a setting of a single transmitter sending common information for two users.