Analysis of compressive receivers for the optimal interception offrequency-hopped waveforms

Establishes that the compressive receiver is a practical interceptor of high performance. Given a signal of a particular duration, a compressive receiver can estimate simultaneously all frequency components within a set wide band. This processing is similar to a parallel bank of narrowband filters, which is the optimal detector of frequency-hopped signals. Furthermore, hop frequency is estimated to yield performance equal to the parallel filter configuration. The authors assume interference to be stationary, colored Gaussian noise, and present a model of the compressive receiver that contains all its salient features. Low energy coherence detection is achieved by taking the compressive receiver output as an observation and applying likelihood ratio theory at small signal-to-noise ratios. For small signals, this approach guarantees the largest probability of correct detection for a given probability of false alarm, and thus provides a reference, to which simplified or ad hoc schemes can be compared. Since the low energy coherence detector has an unwieldy structure, a simplified suboptimal detector structure is developed that consists of a simple filter, followed by a sampler and a square-envelope detector. Several candidates for the filter's response are presented. The performance of the low energy coherence detector based on compressive receiver observations is compared to the optimal filter-bank detector based on direct observations, thus showing the exact loss incurred when a compressive receiver is used. The performance of various simplified schemes, based on compressive receiver observations, is analyzed     

The University of Illinois and German Architectural Education

Abstract

In our review of the literature concerning sustainable architecture, we find a remarkably diverse constellation of ideas that defy simple categorization. But rather than lament the apparent inability to standardize a singular approach to degraded environmental and social conditions, we celebrate pluralism as a means to contest technological and scientific certainty. At the same time, we reject epistemological and moral relativism. These twin points of departure lead us to propose a research agenda for an architecture of reflective engagement that is sympathetic to the pragmatist tradition.     

Network Synchronization of an Orthogonal CDMA Satellite Communication System

This paper provides the network synchronization of an orthogonal CDMA geostationary satellite system for fixed service communications. It includes the synchronization procedures, the system architecture and the performance evaluation. The main objective is to provide network wide synchronization of all uplink orthogonal CDMA transmissions. This is achieved in steps; first by providing coarse synchronization using the uplink random access channel and then fine sync using innovative tracking control mechanisms. The uplink access channel receiver utilize a parallel/serial search method for rapid code acquisition, while the code tracking of the uplink orthogonal CDMA traffic channel is based on a delay feedback early-late gate in which the sych control resides in the receiver. The proposed system is designed to minimize the onboard complexity and satisfy the performance requirements. As shown in the performance section, the requirement that all uplink transmissions are synchronized to a reference time within 10% of the chip length can be achieved. In addition, the system analysis determines the design parameters values which optimize performance.     

Direct-Sequence Spread-Spectrum Multiple-Access Communications Over Nonselective and Frequency-Selective Rician Fading Channels

An accurate approximation is obtained for the average probability of error in an asynchronous binary direct-sequence spreadspectrum multiple-access communications system operating over nonselective and frequency-selective Rician fading channels. The approximation is based on the integration of the characteristic function of the multiple-access interference which now consists of specular and scatter components. For nonselective fading, the amount of computation required to evaluate this approximation grows linearly with the productKN, whereKis the number of simultaneous transmitters andNis the number of chips per bit. For frequency-selective fading, the computational effort grows linearly with the product KN². The resulting probability of error is also compared with an approximation based on the signal-to-noise ratio. Numerical results are presented for specific chip waveforms and signature sequences.     

Broadcast capability of direct sequence and hybrid spread spectrum

Two forms of spread-spectrum signaling, namely direct-sequence and hybrid (direct-sequence/frequency-hopped) are shown to provide high broadcast capability, especially when used in conjunction with forward-error-control coding schemes. The broadcast capability is defined as the maximum number of simultaneous distinct messages that can be transmitted to distant receivers from a single transmitter at a given bit-error rate. The quantity provides a useful measure of the capacity of hub-to-mobile or satellite-to-earth-station links of communication networks. When bursty data or voice traffic is dominant in such networks, the above forms of spread-spectrum CDMA provide a viable alternative to FDMA or TDMA. Ways of multiplexing the direct-sequence and hybrid signals are presented that use, respectively, distinct carriers, distinct pairs of orthogonal carriers, and only two orthogonal carriers for broadcasting the different messages. Systems with chip-synchronous signals and systems with random delays between the signals are considered. The average error probability of all systems is evaluated using the characteristic-function and Gaussian-approximation techniques. Besides the uncoded systems, systems using Reed-Solomon and convolutional codes are analyzed. A comparison of the broadcast capability of the different schemes is presented     

Coherent Hybrid DS-SFH Spread-Spectrum Multiple-Access Communications

The performance of synchronous and asynchronous hybrid direct-sequence/slow-frequency-hopped spread-spectrum multiple-access communications over additive white Gaussian noise channels is examined. Systems employing binary or quaternary phase-shift-keying modulation with coherent demodulation are investigated. Both deterministic and random signature sequences and frequency-hopping patterns are considered and several possible assignments for them are discussed. It is shown that the multiple-access capability of hybrid spread-spectrum is superior to that of pure frequency-hopped spread-spectrum, and inferior to that of pure direct-sequence spread-spectrum for systems with identical bandwidth expansion which employ the same data modulation and demodulation scheme and random hopping patterns and signature sequences.     

Wideband TDOA/FDOA processing using summation of short-time CAF's

A commonly used method of calculating TDOA and FDOA is the cross ambiguity function (CAF). The narrowband model assumes that the Doppler is constant over the bandwidth of the signal. When time companding, caused by relative motion, becomes large enough, the CAF performance is degraded. One method of compensation is to resample the signal, but this can be computationally intensive, especially if there is a wide range of time companding. A second method is to reduce the time the CAF is computed over, but the processing gain is reduced. If the short-term CAFs are recombined after the time companding has been compensated, much of the processing gain can be recovered. In this paper, three novel methods of recombining short-time CAFs are developed and analyzed. The first method is to estimate the TDOA and FDOA for each short-time CAF. The second method is to compensate and noncoherently add the CAFs together. This improves the peak detection over the first method but not the TDOA and FDOA accuracy. The third method is to compensate for both time and phase and then coherently add the short-time CAFs together. This last method improves the FDOA accuracy over the first two methods. The effects of sampled data and frequency dual are analyzed. Simulation results are provided to verify analytical results     

Space-time iterative receivers for narrowband multichannel networks

The iterative MMSE multiuser detection paradigm is applied to the suppression of cochannel interference in the coded narrowband (multicell) uplink. The equivalent of multiple chips per bit (necessary for MMSE multiuser demodulation) is generated via multisensor reception, the array responses serving as user signatures. This receiver's robustness to overloading allows its sensor count to be much lower than the typical number of other-cell cochannel interferers. A permutation transmit diversity technique that exploits channel time-selectivity is proposed in order to enhance the interuser separation afforded by multisensor reception.     

Effect of worst case multiple partial-band noise and tone jammerson coded FH/SSMA systems

The author characterizes and evaluates the effect of simultaneous multiple partial-band noise or tone jammers and other user interference on a single communication link employing frequency-hopped spread-spectrum (FH/SS) signaling, M-ary frequency-shift keying (FSK) modulation with noncoherent demodulation, and Reed-Solomon coding. For the symbol error probability of these systems, the author derives exact expressions in the absence of multiple-access interference and tight upper bounds in the presence of other-user interference. Although the analytical methods are valid for any number of multiple jammers, the numerical study is restricted to the cases of two and three-partial-band noise and tone jammers. For fixed values of the spectral densities of noise jammers, or the energies per symbol of tone jammers, the worst-case fraction of the band that each jammer should use in order to maximize the error probability of the FH/SS or FH/SSMA system is evaluated. For the range of the signal-to-jammer power ratios examined, multiple-noise or multiple-tone jammers appear to have no advantage over single-tone jammers of equivalent spectral density or energy per symbol, but achieve approximately the same worst-case performance by jamming smaller fractions of the band     

One-step memory nonlinearities for signal detection anddiscrimination from correlated observations

Detectors based on one-step memory nonlinearities are introduced. Problems of discrimination between two arbitrary stationary m-dependent or mixing sequences of observations and problems of detecting a weak signal in additive stationary m-dependent or mixing noise are considered in this context. For each problem, the nonlinearity is optimized for performance criteria such as the generalized signal-to-noise ratio (SNR) and the efficacy, and is obtained as the solution to an appropriate linear integral equation. The schemes considered can be made robust to statistical uncertainties determined by two-alternating capacity classes, for the second-order joint PDFs (probability distribution functions) of the observations, and by bounds on the correlation coefficients of time shifts of the observation sequence for the third- and fourth-order joint PDFs. Evaluation of the performance of the schemes via simulation reveals significant gains over that of detectors employing memoryless nonlinearities or the independent and identically distributed nonlinearity