Showing papers by "Milica Stojanovic published in 2018"

The SEANet Project: Toward a Programmable Internet of Underwater Things

Abstract: Wirelessly networked systems of underwater devices are becoming the basis of many commercial, scientific, and military applications. In spite of increased attention in the last few years, underwater wireless networking technology still suffers from major limitations, including severe hardware dependence. In this paper, we introduce the SEANet Project, an NSF-funded effort that aims at developing a new generation of programmable platforms and a networking testbed to enable the vision of a programmable Internet of Underwater Things (IoUT). SEANet will be based on new software-defined platforms based on an open architecture to enable the flexibility to define, add, update, and swap new components in both hardware and software. SEANet is designed to support data rates at least one order of magnitude higher than existing commercial platforms over short and moderate range links. Moreover, the SEANet project will explore the design of new custom-designed ultra-wide band Microelectromechanical systems (MEMS) transducers that allow operating over much wider acoustic bandwidth (i.e., 0.01.-2 MHz) than possible with bulk piezoelectric transducers. We present a set of preliminary experiments showing that SEANet can outperform existing software-defined radio SDR-based acoustic modems based on a commercial off-the-shelf (COTS) SDR platforms. We also demonstrate the real-time reconfiguration capability of SEANet and preliminary performance of the MEMS transducers.

On the Average Achievable Rate of QPSK and DQPSK OFDM Over Rapidly Fading Channels

Abstract: Motivated by recent experimental observations, where differentially coherent detection outperforms coherent detection on certain underwater acoustic channels, we revisit the performance of these two methods with respect to the achievable rate in orthogonal frequency-division multiplexing systems. Our comparison is based on a class of time-varying channels, where coherent quadrature phase shift keying (QPSK) detection requires channel estimation through a set of designated pilot symbols, while differentially coherent QPSK (DQPSK) enjoys an almost pilot-free detection. We show that given the cost of pilot overhead and channel estimation errors, the average rate achievable with DQPSK can be higher than that of QPSK. We use analytical results to identify channels for which this is the case, and expand the results to larger constellation sizes such as 8PSK, 16PSK, and 16QAM. In addition to acoustic channels, wireless radio channels with unusually long delay spread or high Doppler spread, such as those found in unmanned aerial systems and high-speed trains, may also benefit from using differentially coherent detection. In general, differentially coherent detection is favored in rapidly time-varying channels which require large pilot overhead and/or whose bit error rate performance is dominated by channel estimation errors. Our analysis includes the results for array receivers and soft detectors, showing that the favorable parameter range for differentially coherent detection expands significantly in both cases.

Mobile Acoustic Communications: Real Data Analysis of Partial FFT Demodulation with Coherent Detection

Abstract: We address the use of orthogonal frequency division multiplexing (OFDM) for high-rate communication over a mobile acoustic channel. To counteract the frequency offset and time-variability of the broadband mobile channel, we employ a dedicated method for synchronization and partial FFT (P-FFT) demodulation, cast into the framework of multichannel diversity combining. Unlike conventional receivers, where the signal is demodulated using a single FFT operating over the full OFDM block interval, P-FFT employs multiple FFT operations to demodulate the signal over several partial intervals. The partial demodulator outputs are subsequently combined, and the combined signal is fed to a second stage, where refined channel estimation and data detection take place. We investigate both coherent and differentially coherent detection, and test refined channel estimation with least squares (LS) and matching pursuit (MP) algorithms. Partial interval demodulation offers an additional degree of freedom by allowing for suppression of time-variation before inter-carrier interference has been created in the process of demodulation. The result is an improved quality of data detection, which we demonstrate using real data recorded during the 2010 Mobile Acoustic Communications Experiment MACE’10. Results of experimental data processing show excellent performance with up to 2048 QPSK-modulated carriers operating in the 10.5 kHz -15.5 kHz acoustic band over varying distance (3-7 km) and speeds up to 1.5 m/s.

Exploitation of Spatial Coherence for Reducing the Complexity of Acoustic OFDM Systems

Abstract: This paper addresses the question as to how to exploitspatial coherence between receiving elements in multichannel multi- carrier acoustic communication systems in order to reduce the signal processing complexity without compromising the performance. To answer this question, an adaptive pre-combining method is proposed. Without requiring any a priori knowledge about the spatial distribution of received signals, the method exploits spatial coherence between receive channels by linearly combining them into fewer output channels so as to reduce the number of subsequent channel estimators. The algorithm learns the spatial coherence pattern recursively over the carriers, thus effectively achieving broadband beamforming. The reduced-complexity pre-combining method relies on differentially coherent detection which keeps the receiver complexity at a minimum and requires a very low pilot overhead. Using the experimental data transmitted over a 3–7 km shallow water channel in the $10. 5-15.5 \mathrm {k}\mathrm {H}\mathrm {z}$ acoustic band, we study the system performance in terms of data detection mean squared error (MSE) and show that the receiver equipped with the proposed reduced- complexity pre-combining scheme requires three times fewer channel estimators while achieving the same MSE performance as the full- complexity receiver.

Relay for Data: An Underwater Race

Abstract: We show that unlike radio frequency channels, amplify-and-forward relaying can increase the data rate in non-fading underwater acoustic channels. For frequency-selective Rayleigh fading underwater acoustic channels, we focus on QPSK modulation and show that the data rate can be increased by using either decode-and-forward or amplify-and-forward relaying, with the former having a significantly higher performance. This fact indicates that rather than increase the transmit power to increase the data rate, a more power-efficient method involves increasing the number of relays and decreasing the path length between them. We also investigate power allocation methods and analyze their effect on the achievable rate. We show that distributing the power uniformly across a frequency band can match the performance of the rate-maximizing power allocation for QPSK modulation.