**
On the performance of a two-user MIMO downlink system in heavy traffic
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S. Bhardwaj,
R. J. Williams, and A. S. Acampora
A MIMO downlink system in which data is transmitted to two users over
a
common wireless channel is considered.
The channel is assumed to be fixed for all transmissions over the
period
of interest and the ratio of anticipated average arrival rates for
the
two users, also known as the relative traffic rate, is the system
design
parameter.
A packet-based traffic model is considered where data for each user
is
queued at the transmit end.
A queueing analogue for this system leads to a coupled queueing
system
for which a simple policy is known to be throughput-optimal under
Markovian assumptions.
Since an exact expression for the performance is not available, as a
measure of performance (in heavy traffic), a diffusion approximation
is
established.
This diffusion process is a two-dimensional semimartingale reflecting
Brownian motion living in the positive quadrant of two-dimensional
space.

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This paper appears in IEEE Transactions on
Information Theory, Vol. 53 (2007), 1851-1859.

* Last updated: April 27, 2007*