| Coordinated Multi-Point (CoMP) transmission is an infrastructural enhancement under consideration for next generation wireless networks. In this work, the capacity gain achieved through CoMP transmission is studied in various models of wireless networks that have practical significance. The capacity gain is analyzed through the degrees of freedom (DoF) criterion. The DoF available for communication provides an analytically tractable way to characterize the capacity of interference channels. The considered channel model has K transmitter/receiver pairs, and each receiver is interested in one unique message from a set of K independent messages. Each message can be available at more than one transmitter. The maximum number of transmitters at which each message can be available, is defined as the cooperation order M. For fully connected interference channels, it is shown that the asymptotic per user DoF, as K goes to infinity, remains at 1/2 as M is increased from 1 to 2. Furthermore, the same negative result is shown to hold for all M > 1 for any message assignment that satisfies a local cooperation constraint. On the other hand, when the assumption of full connectivity is relaxed to local connectivity, and each transmitter is connected only to its own receiver as well as L neighboring receivers, it is shown that local cooperation is optimal. The asymptotic per user DoF is shown to be at least max{(1/2),(2M )/(2M+L)} for locally connected channels, and is shown to be (2M)/(2M+1) for the special case of Wyner's asymmetric model where L=1. An interesting feature of the proposed achievability scheme is that it relies on simple zero-forcing transmit beams and does not require symbol extensions. Also, to achieve the optimal per user DoF for Wyner's model, messages are assigned to transmitters in an asymmetric fashion unlike traditional assignments where message i has to be available at transmitter i. It is also worth noting that some receivers have to be inactive, and fractional reuse is needed to achieve equal DoF for all users. The obtained results for locally connected channels are then extended to each of the following scenarios. First, a multiple-antenna transmitters setting is studied to highlight the comparison between dedicating multiple antennas to each message and sharing multiple antennas between messages. Second, an average transmit set size constraint is considered, where instead of imposing a constraint on the number of transmitters carrying each message, the considered constraint is a backhaul load constraint that limits the number of messages that can be delivered from a centralized controller to the base station transmitters. Third, an interference channel with block erasures is studied, where long-term fluctuations (shadow fading) in the wireless channel can lead to any link being erased with probability p. For each value of p, our goal is to find a fixed assignment of messages to transmitters that maximizes the average per user DoF. |
