  The Opportunistic Spectrum Access (OSA) model has been developed for the
secondary users (SUs) to exploit the stochastic dynamics of licensed channels
for file transfer in an opportunistic manner. Common approaches to design
channel sensing strategies for throughput-oriented applications tend to
maximize the long-term throughput, with the hope that it provides reduced file
transfer time as well. In this paper, we show that this is not correct in
general, especially for small files. Unlike prior delay-related works that
seldom consider the heterogeneous channel rate and bursty incoming packets, our
work explicitly considers minimizing the file transfer time of a single file
consisting of multiple packets in a set of heterogeneous channels. We formulate
a mathematical framework for the static policy, and extend to dynamic policy by
mapping our file transfer problem to the stochastic shortest path problem. We
analyze the performance of our proposed static optimal and dynamic optimal
policies over the policy that maximizes long-term throughput. We then propose a
heuristic policy that takes into account the performance-complexity tradeoff
and an extension to online implementation with unknown channel parameters, and
also present the regret bound for our online algorithm. We also present
numerical simulations that reflect our analytical results.
