CDMA-Based Analog Network Coding for Underwater Acoustic Sensor Networks

We propose ANC-CDMA, a CDMA-based analog network coding scheme for underwater acoustic sensor networks (UW-ASNs) that can significantly improve the underwater acoustic channel utilization. First, we analyze a unidirectional multihop network, in which two acoustic nodes separated by two hops are assigned the same code-division channel (i.e., spreading code) to transmit concurrently. The packets transmitted by the two nodes will collide at the intermediate (relay) node. However, we show that, by exploiting a priori information, i.e., the interfered packet previously received from one of the nodes, and applying joint channel estimation through pilot supervision along with a newly designed adaptive RAKE receiver, the relay node can cancel the interference before decoding the packet of interest. We then extend our analysis to a bidirectional network, in which two nodes first concurrently transmit their packets to the relay using the same spreading code. The relay amplifies and forwards the received interfered packet to the two nodes. We show that either node can still decode the packet of interest after equalizing the channel effects introduced during the propagation from the relay to itself and jointly estimating multipath affected channels and suppressing the self-interference signal before applying the adaptive RAKE receiver. The proposed ANC-CDMA scheme is implemented in a testbed based on Teledyne Benthos Telesonar SM-975 underwater modems and tested extensively in Lake LaSalle. Experiments and simulations demonstrate that for a 1-2 dB tradeoff in signal-to-noise ratio (SNR), the proposed scheme can significantly improve the channel utilization of a unidirectional and bidirectional networks by up to 50% and 100%, respectively, compared with conventional multiuser DS-CDMA scheme.