Multiway Physical-Layer Network Coding via Uniquely Decodable Codes:

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.

2-D DOA Estimation of Coherent Wideband Signals with AV Basis:

We develop a two-dimensional (2-D) direction-ofarrival (DOA) estimation scheme for coherent wideband source signals using coherent signal subspace method based auxiliaryvector (CSSM-AV) basis. Computation of the basis is carried out by a modified version of the orthogonal CSSM-AV filtering algorithm. The proposed method reconstructs the signal subspace using a cross-correlation matrix after which the modified CSSMAV algorithm is employed to estimate the azimuth and elevation angles. Then, successive orthogonal maximum cross-correlation auxiliary vectors are calculated to form a basis for the scannerextended signal subspace. This technique is very efficient in reducing the algorithm complexity. Since it does not require that the eigenvectors be determined in order to find the signal subspace and yields a superior resolution performance for closely spaced sources even when the number of samples is low.

Specifically, the complexity of the proposed 2-D DOA estimation algorithm compared to the CSSM algorithm is more favorable when the number of signals arriving on the antenna element is much less than the number of antenna elements. Performance evaluation shows that the proposed method outperforms competing methods such as CSSM, TOPS and WAVES algorithms in terms of estimation error, probability of resolution and number of sample support for a given SNR in scenarios in which many sources are present in the system, the array size is large, and the number of samples is small.

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 multi-hop 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 bi-directional 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 1-2dB tradeoff in signal-to-noise ratio (SNR) the proposed scheme can significantly improve the channel utilization of a unidirectional and bi-directional networks by up to 50% and 100%, respectively, compared to conventional multi-user DS-CDMA scheme.

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-75 underwater modems and tested extensively in Lake LaSalle. 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.

Development of a Reconfigurable Underwater Networking Testbed:

The underwater acoustic networking testbed was designed to bridge the gap between experimentation and theoretical developments in underwater communications and networking. The objective of the project is to provide the research community with a versatile and shared reconfigurable platform to enable experimental evaluation of underwater communications and networking protocols.

We then discuss integration of the testbed with a channel emulator that allows performing laboratory controlled experiments where the user is able to configure transmission distance, channel effects, and underwater communication parameters. Emulation results for several different scenarios as well as actual experimental results for transmission of custom defined acoustic waveform are demonstrated at the end. This platform therefore allows playing, processing, and recording custom-defined acoustic waveforms to support reconfigurable physical layer experimentation with arbitrary transmission schemes

We propose a new low-cost distributed networked localization and time synchronization framework for underwater acoustic sensor network testbeds. The proposal is based on decoupling the two problems and solving first the time synchronization then localization using the same set of messages, i.e., with no additional overhead. A coarse, followed by a fine-grained localization algorithms are adopted to accurately estimate the location of an unknown node. 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 in terms of root mean square error (RMSE) demonstrate that the proposed scheme can achieve a high accuracy for a given energy budget, i.e., for a given number of message exchanges.

Modeling Underwater Acoustic Channels in Short-range Shallow Water Environments:

Statistical modeling of radio frequency (RF) wireless communication in air has been well studied, and widely accepted channel models are available. Because of the high complexity of the underwater acoustic channel it is very difficult to come up with a single statistical channel model that can be used to capture different underwater channel environments. Moreover, conducting underwater experiments and collecting data is very costly. Accordingly, there are only a few joint venture underwater experiments conducted to collect underwater data, which are not readily available. In addition to that, only very limited work has been done to study the characteristics of the underwater acoustic channel in shallow water environments.

We analyze the statistical channel properties of short to very short-range shallow water communication environments based on real channel measurements taken in a water-tank, a swimming pool, very shallow and shallow lakes. More specifically, we estimate the channel impulse response (CIR), the probability density function (PDF) of channel fading and fit to Rayleigh, Nakagami, Weibull, Rician and Beta distributions. We compare the 'goodness-of-fit' of these distributions based on the Kullback-Leibler (KL) divergence criteria. From our experimental results, we confirm that the shallow water acoustic channel is highly time-varying and does not necessarily follow a Rayleigh distribution. Instead, we observe that in very-shallow water lake environments the channel fading exhibits close-to Weibull or Rician distribution. On the other hand, in shallow water lake the channel fading behavior is better captured by a Beta distribution.

The proposed MAC protocol is implemented and tested on a real underwater acoustic testbed using SM-75 acoustic modems by Teledyne Benthos. Simulations illustrate an improvement in network lifetime. Additionally, simulations demonstrate that the proposed scheduling scheme with urgency factor achieves a throughput increase of 28% and improves the reliability by up to 25% as compared to the scheduling scheme that neither use MDP nor optimization. Furthermore, testbed experiments show an improvement in throughput by up to 10% along with an improvement in reliability.