  In this paper we study opportunistic spectrum access (OSA) policies in a
multiuser multichannel random access cognitive radio network, where users
perform channel probing and switching in order to obtain better channel
condition or higher instantaneous transmission quality. However, unlikely many
prior works in this area, including those on channel probing and switching
policies for a single user to exploit spectral diversity, and on probing and
access policies for multiple users over a single channel to exploit temporal
and multiuser diversity, in this study we consider the collective switching of
multiple users over multiple channels. In addition, we consider finite
arrivals, i.e., users are not assumed to always have data to send and demand
for channel follow a certain arrival process. Under such a scenario, the users'
ability to opportunistically exploit temporal diversity (the temporal variation
in channel quality over a single channel) and spectral diversity (quality
variation across multiple channels at a given time) is greatly affected by the
level of congestion in the system. We investigate the optimal decision process
in this case, and evaluate the extent to which congestion affects potential
gains from opportunistic dynamic channel switching.
