  The underwater acoustic channel continues to present significant challenges
to efficient throughput performance of underwater acoustic sensor networks
(UASNs) in varying scenarios. As a result, cross-layer approaches that explore
joint PHY/MAC strategies are worthy of further exploration. We consider a
recent high-speed OFDM modem and propose a new cross-layer solution based on
modified CSMA/CA, for a canonical star network topology with few nodes (the
most common scenario in UASNs). Some innovations to an adaptive OFDM PHY link
are developed to jointly select the modulation, convolutional coding and
frequency diversity order (different transmission modes) for matching varying
channel conditions. Additionally, receiver logic that disambiguates the cause
of packet loss between a) that caused by channel vs. b) that due to collisions
is used to modify the ARQ/backoff logic for retransmissions with CSMA/CA random
access. Simulation results reveal that the cross-layer design can effectively
increase network throughput.
