  The cooperative medium access control (CoopMAC) protocol in the presence of
randomly-distributed nodes and shadowing is considered. The nodes are assumed
to be distributed according to a homogeneous two-dimensional Poisson point
process. A new approach is proposed for helper selection and throughput
performance analysis which depends on the shadowing parameters as well as the
distribution of helpers. In the proposed protocol, the potential helpers are
divided into several tiers based on their distances from the source and
destination in a way that the lower the tier index, the higher its priority.
When there are several helpers of the same tier, the helper that is less
affected by shadowing is chosen for cooperation. Upper and lower bounds are
derived for the average cooperative throughput of the proposed CoopMAC
protocol. It is observed that the proposed scheme readily outperforms the
conventional CoopMAC protocol in having larger average throughput. It is also
seen that the cooperative throughput of the proposed scheme approaches the
upper bound when the density of nodes increases.
