  Due to the current trend towards smaller cells, an increasing number of users
of cellular networks reside at the edge between two cells; these users
typically receive poor service as a result of the relatively weak signal and
strong interference. Coordinated Multi-Point (CoMP) with Joint Transmission
(JT) is a cellular networking technique allowing multiple Base Stations (BSs)
to jointly transmit to a single user. This improves the users' reception
quality and facilitates better service to cell-edge users. We consider a
CoMP-enabled network, comprised of multiple BSs interconnected via a backhaul
network. We formulate the OFDMA Joint Scheduling (OJS) problem of determining a
subframe schedule and deciding if and how to use JT in order to maximize some
utility function. We show that the OJS problem is NP-hard. We develop optimal
and approximation algorithms for specific and general topologies, respectively.
We consider a time dimension and study a queueing model with packet arrivals in
which the service rates for each subframe are obtained by solving the OJS
problem. We prove that when the problem is formulated with a specific utility
function and solved optimally in each subframe, the resulting scheduling policy
is throughput-optimal. Via extensive simulations we show that the bulk of the
gains from CoMP with JT can be achieved with low capacity backhaul. Moreover,
our algorithms distribute the network resources evenly, increasing the
inter-cell users' throughput at only a slight cost to the intra-cell users.
This is the first step towards a rigorous, network-level understanding of the
impact of cross-layer scheduling algorithms on CoMP networks.
