Mark R. Bell

Signal design for transmitter diversity wireless communication systems over Rayleigh fading channels

Transmitter diversity wireless communication systems over Rayleigh fading channels using pilot symbol assisted modulation (PSAM) are studied. Unlike conventional transmitter diversity systems with PSAM that estimate the superimposed fading process, we are able to estimate each individual fading process corresponding to the multiple transmitters by using appropriately designed pilot symbol sequences. With such sequences, special coded modulation schemes can then be designed to access the diversity provided by the multiple transmitters without having to use an interleaver or expand the signal bandwidth. The code matrix notion is introduced for the coded modulation scheme, and its design criteria are also established. In addition to the reduction in receiver complexity, simulation results are compared to, and shown to be superior to, that of an intentional frequency offset system over a wide range of system parameters.

Information theory and radar waveform design

The use of information theory to design waveforms for the measurement of extended radar targets exhibiting resonance phenomena is investigated. The target impulse response is introduced to model target scattering behavior. Two radar waveform design problems with constraints on waveform energy and duration are then solved. In the first, a deterministic target impulse response is used to design waveform/receiver-filter pairs for the optimal detection of extended targets in additive noise. In the second, a random target impulse response is used to design waveforms that maximize the mutual information between a target ensemble and the received signal in additive Gaussian noise. The two solutions are contrasted to show the difference between the characteristics of waveforms for extended target detection and information extraction. The optimal target detection solution places as much energy as possible in the largest target scattering mode under the imposed constraints on waveform duration and energy. The optimal information extraction solution distributes the energy among the target scattering modes in order to maximize the mutual information between the target ensemble and the received radar waveform. >

JEM modeling and measurement for radar target identification

The jet engine modulation (JEM) phenomenon, observed in radar returns from the rotating structure of jet engines, has been successfully exploited for aircraft target identification in a number of experimental radar systems. The authors develop a parametric model based on the periodic modulation of the scattered return, motivated by the potential reduction in time-on-target for reliable target identification provided by parametric models as well as by gaining insight into the JEM phenomenon. They compare the model with JEM measurements made with an experimental radar system and discuss the implications for JEM-based target identification systems. >

Asynchronous multicarrier DS-CDMA using mutually orthogonal complementary sets of sequences

The use of sets of multiple spreading sequences per user in multicarrier code-division multiple-access (CDMA) is investigated. Each user is assumed to have a distinct set of spreading sequences, with a different spreading sequence for each carrier in each users set. We show that when these sets of sequences are chosen to be the mutually orthogonal (MO) complementary sets of sequences, multiple-access interference is minimal on a nonfading channel. As a result of the autocorrelation sidelobe cancellation properties of the MO complementary sequences, it is possible to pack symbols more closely together on the nonfading channel, resulting in a higher data rate than in multicarrier CDMA using the same spreading sequence for each carrier. The resulting communication system scheme results in an easily parallelized receiver architecture that may be useful in nonfading coherent channels, such as the optical fiber channel or the Rician channel with a strong line-of-sight component. On the Rayleigh fading channel, the performance of the system is identical to that of multicarrier CDMA employing a single spreading sequence per user, with only a minimal increase in receiver complexity.

Diversity waveform sets for delay-Doppler imaging

The properties of the ambiguity function and the uncertainty relation of Fourier transforms assert fundamental limitations on the ability of any single radar waveform of constrained time-bandwidth product to distinguish two or more targets closely spaced in both time-delay (range) and Doppler-shift (radial velocity). These same mechanisms place fundamental limits on the ability radar imaging systems to distinguish separate scatterers in delay and Doppler. In this paper, the problem of using multiple waveform sets to make enhanced discrimination delay-Doppler measurements is considered. While small coded waveform sets for enhanced discrimination delay-only measurement are known (e.g., the Golay sequences), these waveforms do not have good Doppler discrimination properties. The problem of designing multiple waveform sets for enhanced discrimination delay-Doppler measurement is investigated, and the composite ambiguity function (CAF) is introduced as a tool to measure the delay-Doppler discrimination characteristics of these waveform sets. The problem of designing optimal coded waveform sets under a time-bandwidth product constraint is considered, and explicit optimal phase, frequency, and joint phase-frequency coded waveform sets having constant amplitude are presented. Algorithms for the construction of such waveform sets of arbitrary size and practical implementation issues are also presented.

Statistics of the scattering cross-section of a small number of random scatterers

Complex radar targets are often modeled as a number of individual scattering elements randomly distributed throughout the spatial region containing the target. While it is known that as the number of scatterers grows large the distribution of the scattered signal power or intensity is asymptotically exponential, this is not true for a small number of scatterers. The authors study the statistics of measured power or intensity, and hence scattering cross section, resulting from a small number of constant amplitude scatterers each having a random phase. They derive closed-form expressions for the probability density function (pdf) of the scattered signal intensity for one, two, and three scatterers having arbitrary amplitudes. For n>3 scatterers, they derive expressions for the pdf when the individual scatterers have identical constant amplitudes and independent random phases; these expressions are Gram-Charlier type expansions with weighting functions determined by the asymptotic form of the intensity pdf for a large number of scatterers n. The Kolmogorov-Smirnov goodness-of-fit test is used to show that the series expansions are a good fit to empirical pdfs computed using Monte-Carlo simulation of targets made up of a small number of constant amplitude scatterers with random phase. >

Frequency-coded waveforms for enhanced delay-Doppler resolution

In this paper, we propose techniques for the construction of frequency-coding sequences that give rise to frequency-coded waveforms having ambiguity functions with a clear area - containing no sidelobes - in a connected region surrounding the main lobe. These constructed sequences are called pushing sequences. First, two important properties of pushing sequences are investigated: the group D/sub 4/ dihedral symmetry property and the frequency omission property. Using the group D/sub 4/ dihedral symmetry property, we show how to construct additional pushing sequences from a given pushing sequence. Using the frequency omission property, we show how to construct pushing sequences of any length N and design proper frequency-coded waveforms that meet specific constraints in the frequency domain. Next, we use the Lempel T/sub 4/ construction of Costas sequences to construct pushing sequences with power 1. Finally, we show how to construct pushing sequences with any desired power using Lee codewords. Because these arbitrary-power pushing sequences constructed using Lee codewords do not have the Costas property, we derive expressions for the pattern of hits in the geometric array. Based on this, the general form of the positions and levels of all the sidelobe peaks are derived.

A novel approach for designing diversity radar waveforms that are orthogonal on both transmit and receive

In this paper, we present an approach to the design of orthogonal, Doppler tolerant waveforms for diversity waveform radar (e.g. MIMO radar). Previous work has given little consideration to the design of radar waveforms that remain orthogonal when they are received. Our research is focused on: (1) developing sets of waveforms that are orthogonal on both transmit and receive, and (2) ensuring that these waveforms are Doppler tolerant when properly processed. Our proposed solution achieves the above mentioned goals by incorporating direct sequence spread spectrum (DSSS) coding techniques on linear frequency modulated (LFM) signals. We call it Spread Spectrum Coded LFM (SSCL) signaling. Our transmitted LFM waveforms are rendered orthogonal with a unique spread spectrum code. At the receiver, the echo signal will be decoded using its spreading code. In this manner, transmitted orthogonal waveforms can be match filtered only with the intended received signals. From analytical expressions of the waveforms we have designed and from simulation results, we found that: (a) cross-ambiguity function of two LFM spread spectrum coded (orthogonal) waveforms is small for all delays and Dopplers (i.e. transmit and receive signals satisfy the orthogonality constraint), (b) The length and type of the spread spectrum code determines amount of suppression (i.e. complete orthogonal or near orthogonal of the received signal), (c) We can process the same received signal in two different ways; one method can provide LFM signal resolution and the other method can provide ultra high resolution.

Observing the Power Grid: Working Toward a More Intelligent, Efficient, and Reliable Smart Grid with Increasing User Visibility

As an essential fixture of modern society, electricity is often taken for granted. In many cases, it becomes invisible or is ignored until it is not there or it is time to pay the electric bill. Electricity has been around for well over a century, and the march of industry and progress have created the most complex interconnected system in existence. In a delicate balancing act, the amount of electricity generation must track the amount consumed to prevent the system from collapsing, as there is only very limited storage capacity on the grid. The balance is maintained via a combination of predictions and control systems distributed across the grid. How well this balance is maintained produces signals that propagate across the entire system from the large generators to the wall outlets that are ubiquitous across the world. Observing these signals can provide a great deal of insight into the current status and operation of the power grid and can be done cheaply and accurately from inside a home or office.

A coded modulation technique for cooperative diversity in wireless networks

We propose a novel coded modulation (CM) protocol for user cooperation diversity in wireless networks. Unlike most existing protocols, the proposed scheme does not involve repetition. This translates to significant gains in terms of achievable rates. Hence, it is suitable for higher spectral efficiencies too. Each user transmits its own bits, along with bits of cooperating users, in every allotted symbol at appropriate positions on a specially labelled signal constellation. The scheme achieves full diversity order in the number of cooperating relays and obtains additional coding gains compared to repetition protocols. We show gains ranging from 2 dB to 3 dB over repetition protocols for cases involving two and three cooperating users. In its simplest form, the proposed scheme has a complexity at the relay equal to that of repetition protocols. Complexity at the base station, however, is increased.