Dianne E. Egnor

A multiple access differential frequency hopping system

Military communication systems require waveforms that are resilient in the presence of jamming signals (i.e. are antijam (AJ)), that have a low probability of detection (LPD) by unintended receivers, that have a low probability of intercept (LPI) by hostile receivers, and that operate well in the presence of many authorized users. Current spread spectrum communication techniques such as standard frequency hopping (FH) and direct sequence spread spectrum (DSSS) do not provide signals that simultaneously exhibit AJ, LPD, and LPI properties, allow conferencing, and are easily implemented. A newly proposed multiple access differential frequency hopping (DFH) communications system demonstrates improved AJ and LPD performance when compared to existing spread spectrum techniques. In addition, it successfully detects and decodes multiple (interfering) users in an architecture that is ad hoc and is easily implemented. Preliminary analysis and simulation results demonstrating the improved performance are provided.

A performance comparison of differential frequency hopping and fast frequency hopping

This paper discusses the multiple access performance of the spread spectrum modulation called differential frequency hopping (DFH) and its associated signal processing algorithms, and compares DFH performance to the performance of fast frequency hopping (FFH) multiple access systems. To fairly compare FFH and DFH, it is necessary to carefully choose the parameters for each system, using the concept of efficiency. The single-user efficiency provides a fair way to set parameters when comparing FFH and DFH systems when there is no multiuser interference (MUI) present. We develop the multi-user efficiencies for the two waveforms, as a mechanism to determine comparable system parameters for FFH and DFH multiple-access systems. It is shown that, for the same bits/symbol, the data rate of a single-user DFH is larger than the FFH data rate if the hop dwell time is the same DFH is shown to be significantly more MUI-tolerant than the comparable FFH and FFH systems with interference cancellation, particularly at low and mid-level SNRs. Similarly, the theoretical same-efficiency DFH outperforms the non-optimal FFH at most SNRs and outperforms optimal FFH with collision resolution at SNRs above 7 to 11 dB, depending on the number of simultaneous users. The theoretical and simulation results consistently indicate that DFH is more MUI-tolerant than FFH for the same efficiency, which is desirable in ad hoc networks with multiple users.

Orthogonal and projected orthogonal matched filter detection

This paper considers the genetic problem of detecting in the presence of additive noise, which one from a set of known signals has been received. In place of the classical matched filter (MF) receiver we propose a modified receiver. When the transmitted signals are linearly independent this receiver is referred to as an orthogonal matched filter (OMF) receiver, and when the transmitted signals are linearly dependent it is referred to as a projected orthogonal matched filter (POMF) receiver. Two equivalent representations of the receiver are developed with different implications in terms of implementation. In the first, the demodulator consists of a MF demodulator followed by an optimal whitening transformation on a space formed by the transmitted signals, that optimally decorrelates the MF outputs prior to detection. In the second, the demodulator consists of a bank of correlators with correlating signals that are projections of a set of orthogonal signals, and are closest in a least-squares sense to the transmitted signals. We provide simulation results that suggest that in certain cases the OMF and POMF receivers can significantly increase the probability of correct detection over the MF receiver in non-Gaussian noise with only a minor impact on performance in Gaussian noise.

Underwater acoustic single- and multi-user differential frequency hopping communications

Differential frequency hopping (DFH) is a fast frequency hopping, digital signaling technology that achieves the desirable performance features of non-interfering spread spectrum operation, spectral re-use, fading mitigation, and interference resistance. Therefore, DFH coding provides the critical capability for multiple users to seamlessly communicate in the bandwidth-limited acoustic channel. In previous work, DFH coding has been shown to be superior to other coding schemes in additive Gaussian white noise and Rayleigh-fading environments when considering the joint constraints of multiple user access, detectability mitigation, and the presence of jamming. In this paper, we describe the auto-synchronizing single-user DFH decoder we have developed for a single hydrophone receiver. We present the performance of this decoder on multi-user simulated data and on multi-user data collected at sea during the Rescheduled Acoustic Communications Experiment (RACE08).We use the Sonar Simulation Toolset (SST) to produce the simulated data for soft through hard bottom compositions to provide a range of multipath severity to gain insight into DFH performance across environments. Based on these initial results, the DFH waveform shows considerable promise for computationally minimal, high reliability communications among uncoordinated users in an underwater acoustic channel.

Improved multipath robustness of DFH modulation in the underwater acoustic channel

We characterize performance improvement of differential frequency hopping modulation under two techniques that mitigate the multipath effects of the underwater acoustic channel: blind adaptive equalization and beamforming. We report results on data collected at-sea during the RACE08, SPACE08 and WHOI09 experiments, and show that the bit-error rate improves with the application of these two techniques. Future improvements may combine joint single-element, blind equalization and beamforming (multi-element) approaches to leverage their respective strengths.

Differential frequency hopping performance in Doppler‐inducing underwater acoustic communication channels.

The relationships between the parameters of Doppler spread‐inducing underwater environments and the bit‐error performance of differential frequency hopping (DFH) modulation in the underwater acoustic channel are characterized. Wind speed determines the nature of the effect that the water surface imposes on the acoustic DFH wavefoms propagating underwater. Low wind speeds result in an essentially flat, low‐absorption sea surface. In this regime, strong surface reflections and little frequency spreading make intersymbol interference (ISI) the dominant effect on the received waveforms. At high wind speeds, the higher density of air bubbles in the surface layer absorbs almost all energy incident on the surface, resulting in no surface reflections reaching the receiver. In this regime, the surface has little effect on the received signal, either in the form of ISI or frequency spreading. The intermediate ranges of wind speed, with a mix of ISI and surface‐induced Doppler spread, pose the most challenging condi...

Two‐stage diver tracking in a high‐clutter harbor environment.

One approach to detecting swimmers with active sonar is to deploy a larger number of relatively simple nodes and network them together. Instead of using a complex multi‐element phased array with electronic beamforming, this system uses air‐backed parabolic reflectors, each with an omni‐directional transducer. To avoid performance degrading acoustic interactions, the available operating frequency band is managed at the channel level. The physical configuration of this sonar system presents challenges for tracking through and across beams, and between nodes. Swimmers can exhibit low target strength and the acoustic clutter fields can be dense and highly dynamic. To detect and track swimmers in this environment, we employ a windowed Hough‐transform (HT) tracker at the beam level. The HT has received wide use for track initiation. However, because of the processing gain required to continually track a weak target in such a significant clutter field, the HT is used in this case to maintain as well as to initia...

Multichannel combination investigations for differential frequency hopping transmissions in shallow water.

Underwater acoustic communication requires waveforms that are robust to the signal distortions caused by the acoustic channel. Many waveforms used for this purpose require the transmission of training symbols that span the intersymbol interference to “learn” and compensate for these channel effects. These waveforms also require exacting coordination between the transmitters to avoid multiple access interference. Differential frequency hopping (DFH) is a fast frequency hopping, digital signaling technology that requires minimal information at the transmitter to communicate in the underwater channel. DFH has the desirable performance features of noninterfering spread spectrum operation, spectral reuse, and fading and interference resistance. This presentation describes the baseline autosynchronizing, single‐user DFH decoder for a multiple hydrophone receiver and investigates two processing techniques incorporated for shallow‐water multiuser applications: fading mitigation and multiuser interference mitigati...

Modeling differential frequency hopping communication in the underwater acoustic channel.

Differential frequency hopping (DFH) is a fast frequency hopping digital modulation scheme with proven multiple‐access and jamming robustness properties in typical wireless channels. Characterizing the capabilities of DFH modulation in the more challenging underwater acoustic channel requires careful analyses that rely on both computer simulations and data collected at sea. The Sonar Simulation Toolkit (SST) is used to model challenging underwater environments and simulate the propagation of DFH waveforms in the corresponding underwater acoustic channels. Our simulations provide baseline performance results that can be used to assess the capabilities of DFH modulation and guide future algorithmic improvements to the receiver. In particular, our simulations show that incorporating equalization techniques into the DFH baseline receiver leads to improved decoding performance in challenging environments characterized by long channel impulse responses, which are known to cause inter‐symbol interference in the ...