IEEE Transactions on Wireless Communications,
vol. 4, no. 6, pp. 2726-2737, Nov. 2005.
Adaptive Resource Allocation in Multiuser OFDM Systems
with Proportional Rate Constraints
Jeff Andrews, and
Brian L. Evans
Department of Electrical and Computer Engineering,
Engineering Science Building,
The University of Texas at Austin,
Austin, TX 78712-1084 USA
Paper (IEEE Explore) -
Multiuser orthogonal frequency division multiplexing (MU-OFDM) is
a promising technique for achieving high downlink capacities in
future cellular and wireless LAN systems. The sum capacity of
MU-OFDM is maximized when each subchannel is assigned to the user
with the best channel-to-noise ratio for that subchannel, with
power subsequently distributed by water-filling. However, fairness
among the users cannot generally be achieved with such a scheme.
In this paper, we impose a set of proportional fairness
constraints to assure that each user can achieve a required data
rate, as in a system with quality of service guarantees. Since the
optimal solution to the constrained fairness problem is extremely
computationally complex to obtain, we propose a low-complexity
suboptimal algorithm that separates subchannel allocation and
power allocation. In the proposed algorithm, subchannel allocation
is first performed by assuming an equal power distribution. An
optimal power allocation algorithm then maximizes the sum capacity
while maintaining proportional fairness. The proposed algorithm is
shown to achieve about 95% of the optimal capacity in a
two-user system, while reducing the complexity from exponential to
linear in the number of subchannels. It is also shown that with
the proposed resource allocation algorithm, the sum capacity is
distributed more fairly and flexibly among users than the sum
capacity maximization method.
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