Designing efficient medium access control protocols for underwater acoustic sensor networks (UW-ASNs) is a major challenge because of the spatial and temporal interference uncertainty caused by asynchronous transmissions and by the low propagation speed of sound. To address these challenges, in this article we propose a new approach for distributed underwater medium access based on lightweight and asynchronous distributed algorithms that optimize the access probability profile over a series of time slots based on a new statistical physical interference model. The latter is based on measuring the level of interference at multiple instants of time in each time slot in order to capture the effects of temporal uncertainty and of unaligned interference. At each measurement instant, the statistical properties of time-varying interference are represented by a Gamma probability distribution. The model is validated through extensive channel measurement experiments conducted with an underwater acoustic testbed in Lake LaSalle. Based on this model, we formulate the problem of queue-aware stochastic channel access. The objective is to maximize the sum throughput of a set of concurrent and mutually interfering source-destination pairs by letting the transmitters adjust their own transmission probability profiles, without collaborating with each other, over a series of time slots based on a statistical characterization of interference obtained through past observations. We propose an iterative distributed solution algorithm for this problem based on a best-response strategy. At each iteration, each node individually solves a non-convex optimization problem of logarithmic complexity. The performance of the proposed distributed algorithm is evaluated by comparing it with two alternative distributed schemes and with the global optimum obtained through a newly-developed centralized globally optimal solution algorithm. Results indicate that by jointly taking the queueing and multi-slot optimization into consideration considerable improvement in terms of sum-throughput can be achieved by the proposed distributed algorithm.

In this paper, we propose a waveform design that is robust to noncoherent detection for quadrature phase-shift keying (QPSK) and quadrature amplitude modulation (QAM) schemes. Our proposed complex constellation waveforms are based on the quaternary complex Hadamard matrices. The advantage of the proposed waveforms is that they have the largest noncoherent minimum distance in the complex domain and meet the maximum Hamming distance in their equivalent binary domain.

It was shown by Guruswami and Rudra that Reed-Solomon codes can be list decoded to recover from phased burst errors (i.e. errors occurring within fixed regular intervals) up to the information-theoretic limit and, in particular, beyond the Guruswami-Sudan bound. In this paper, we present evidence that the algorithm developed by Guruswami and Rudra can also give improvement for more "irregular" burst errors. We develop a low-complexity multiplicity assignment scheme for soft decoding of Reed-Solomon (RS) codes. Specifically, we present simulation results where such soft decoding of RS codes outperforms the existing soft decision decoding algorithms of Koetter and Vardy as well as the algorithm of Das and Vardy on Gilbert-Elliott channels (under QAM and BPSK modulations) for channels that are more bursty. We also present a theoretical result that shows that for certain Gilbert-Elliott channels, with high probability of errors, the output list size for list decoding RS codes is one.

In this paper, we propose a fast decoder algorithm for uniquely decodable (errorless) code sets for overloaded synchronous optical code-division multiple-access (O-CDMA) systems. The proposed decoder is designed in a such a way that the users can uniquely recover the information bits with a very simple decoder, which uses only a few comparisons. Compared to maximum-likelihood (ML) decoder, which has a high computational complexity for even moderate code lengths, the proposed decoder has much lower computational complexity. Simulation results in terms of bit error rate (BER) demonstrate that the performance of the proposed decoder for a given BER requires only 1 - 2 dB higher signal-to-noise ratio (SNR) than the ML decoder.

In this paper, we consider the problem of recursively designing uniquely decodable ternary code sets for highly overloaded synchronous code-division multiple-access (CDMA) systems. The proposed code set achieves larger number of users than any other known state-of-the-art ternary codes that offer low-complexity decoders in the noisy transmission. Moreover, we propose a simple decoder that uses only a few comparisons and can allow the user to uniquely recover the information bits. Compared to maximum likelihood (ML) decoder, which has a high computational complexity for even moderate code length, the proposed decoder has much lower computational complexity. We also derived the computational complexity of the proposed recursive decoder analytically. Simulation results show that the performance of the proposed decoder is almost as good as the ML decoder.

We focus on a multiway relay channel (MWRC) network where two or more users simultaneously exchange information with each other through the help of a relay node. We propose for the first time to apply ternary uniquely decodable (UD) code sets that we have developed to allow each user to uniquely recover the information bits from the noisy channel environment. One of the key features of the proposed scheme is that it utilizes a very simple decoding algorithm, which requires only a few logical comparisons. Simulation results in terms of bit error rate (BER) demonstrate that the performance of the proposed decoder is almost as good as the maximum‐likelihood (ML) decoder. In addition to that through simulations, we show that the proposed scheme can significantly improve the sum‐rate capacity, which in turn can potentially improve overall throughput, as it needs only two time slots (TSs) to exchange information compared to the conventional methods.

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.

We propose a new secure underwater acoustic communication scheme designed to let a user (Alice) transmit a confidential message to another user (Bob) in the presence of an eavesdropper (Eve). A typical approach in conventional wireless physical-layer security is to rely on a friendly jammer to jam Eve through artificial noise (AN). Instead, for the first time, we propose a secure underwater communication scheme that relies on cooperative friendly jamming through CDMA-based analog network coding (ANC). The cooperative friendly jammer transmits information using the same spreading code used in the legitimate Alice-Bob link. The information transmitted by the cooperative jammer is known a priori to Bob, but not to Eve. Although the jammer's packet will also interfere at Bob, we show that after jointly estimating the two multipath-affected channels, Bob can suppress the interfering packet and decode Alice's packet, while Eve cannot. We also formulate the problem of joint optimal selection of friendly jammer and power allocation (for Alice and the jammer) that minimize Eve's capability of intercepting the signal while guaranteeing a predefined level of quality of service (QoS) for Bob. The proposed scheme is implemented in a testbed based on Teledyne Benthos Telesonar SM-975 underwater modems and tested extensively in Lake LaSalle at the University at Buffalo. Experiments and simulations demonstrate that, for a given energy budget, the proposed scheme can guarantee much higher bit error rate (BER) at Eve, while creating minimal BER disturbance at Bob, compared to the AN-aided approach.