  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.
