  We consider an orthogonal frequency division multiple access (OFDMA)
scheduling system. A scheduling unit block consists of contiguous multiple
subcarriers. Users are scheduled based on their block average throughput in a
proportional fair way. The multiuser diversity gain increases with the degree
and dynamic range of channel fluctuations. %Lack of diversity in a limited
frequency selective channel may decrease the sum rate. However, a decrease of
the block average throughput in a too much selective channel may lessen the sum
rate as well. In this paper, we first study optimal channel selectivity in view
of maximizing the maximum of the block average throughput of an arbitrary user.
Based on this study, we then propose a method to determine a per-user optimal
cyclic delay when cyclic delay diversity (CDD) is used to enhance the sum rate
by increasing channel selectivity for a limited fluctuating channel. We show
that the proposed technique achieves better performance than a conventional
fixed cyclic delay scheme and that the throughput is very close to the optimal
sum rate possible with CDD.
