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One major assumption in all orthogonal space-time block coding (O-STBC) schemes is that the channel remains static over the entire length of the codeword. However, time selective fading channels do exist, and in such case the conventional O-STBC detectors can suffer from a large error floor in the high signal-to-noise ratio (SNR) cases. This paper addresses such an issue by introducing a parallel interference cancellation (PIC) based detector for the [G.sub.i] coded systems (i = 3 and 4). Index Terms--Orthogonal space-time block coding (O-STBC), parallel interference cancellation (PIC), time selective fading channels, wireless communications.

Generalized likelihood ratio tests (GLRTs) are derived for the problem of detecting targets in hyperspectral images. These detectors are derived under the assumptions that the signals from the materials in the image mix linearly and that the noise in the system is Gaussian. It is also assumed that the abundances of the signals from the various materials in a pixel must sum to one. This constraint models the fact that the material abundances in a pixel are just the fraction of the pixel that they occupy. Under these assumptions, detectors are derived which outperform the detectors derived without the sum-to-one constraint.

A digital quadrature demodulator is proposed that gives the complex components of the received signal. The adoption of a modulated local oscillator (LO) with four periodic phase states synchronized with the sampling avoids the use of two mixing paths, simplifying the radio-frequency (RF) circuits and allowing an effective VLSI integration. This receiver minimizes the in-phase/quadrature (I/Q) mismatch and, with an adequate choice of the periodic modulating sequence, avoids the direct current (dc) offset of the direct conversion receivers. Index Terms--Digital quadrature demodulator, digital receiver, direct conversion receiver, low IF receivers, mismatch cancellation, VLSI integration.

An adaptive asynchronous code-division multiple-access (CDMA) multiuser detector is proposed that uses a recently derived extended Kalman filter based algorithm [5] to perform joint data detection and parameter tracking in frequency-selective Rayleigh fading channels. A receiver structure based on this adaptive multiuser detector is presented and its performance in terms of parameter tracking and bit error rate (BER) is investigated. The receiver is a form of an adaptive RAKE that exploits multipaths to achieve performance gain. Index Terms--Bit error rate (BER), code-division multiple access (CDMA), extended Kalman filter (EKF), frequency-selective Rayleigh fading.

In this paper, two simplified versions of the belief propagation algorithm for fast iterative decoding of low-density parity check codes on the additive white Gaussian noise channel are proposed. Both versions are implemented with real additions only, which greatly simplifies the decoding complexity of belief propagation in which products of probabilities have to be computed. Also, these two algorithms do not require any knowledge about the channel characteristics. Both algorithms yield a good performance-complexity tradeoff and can be efficiently implemented in software as well as in hardware, with possibly quantized received values. Index Terms - APP decoding, belief propagation, block codes, four-density parity check codes, iterative decoding.

The conventional signal demodulator of a chopper amplifier can be substituted by track-and-hold (T/H) and averaging functions. This arrangement provides offset cancellation without requiring low-pass filters and can ignore the residual offset generated by input spikes. A noise analysis shows that this T/H demodulator degrades the white noise signal-to-noise ratio (SNR), although degradation can be minimized by using maximum duty cycle and minimum amplifier bandwidth. Index Terms - Broadband white noise semiconductor, chopper amplifiers, offset cancellation, track and hold demodulator.

We consider the use of sigmoid functions for multistage detection in asynchronous code-division multiple-access (CDMA) systems. The sigmoid decision function for each stage of multistage detection is derived under the assumption that the residual noise which remains after the multiple-access interference (MAI) cancellation at a stage is Gaussian. It is suggested that the sigmoid function should be adjusted depending on the residual noise power at each stage. In addition, multilevel quantizers that best approximate the sigmoid function are designed. Computer simulation results demonstrate that multistage detectors employing these soft-decision functions perform considerably better than those with hard decision. Index Terms - Asynchronous CDMA, multiuser detection, sigmoid function.

Novel coherent detectors for DQPSK and [Pi]/4-DQPSK on Rayleigh fading channels are presented. The detectors are based on decision feedback and minimum mean-square error (MMSE) channel estimation. The proposed detectors may be used in both single carrier and multicarrier systems. Analytical results in the form of expressions for channel estimation error variance and bit error probability, both with and without diversity, are derived. The MMSE-based detector has a significantly lower irreducible bit error probability floor compared to a conventional differential detector. For most practical applications, the error floor of the MMSE-based detector is negligible. Simulated results are presented to verify the analysis and justify the approximations made. Index Terms - Channel estimation, coherent detection, DQPSK, multicarrier modulation, OFDM, Rayleigh fading.

We propose a novel detection scheme for continuous-phase modulation (CPM) signals transmitted over frequency-flat fading channels. Its most significant feature is that it operates without statistical information on the fading channel, and for this reason it is nicknamed 'blind detector.' Its error-rate performance is assessed with minimum shift keying (MSK) and Gaussian MSK (GMSK) schemes and compared with the performance of other decoders. Index Terms - Continuous-phase modulation, fading channels, maximum-likelihood detection.

An optical link system (OLS) is to be used in the data acquisition (DAQ) system for the Silicon Vertex Detector of the KEK B-factory experiment (BELLE collaboration). In this study we report the 10-bit, or channel (ch), OLS in which CMOS ICs are used as the transceiver circuitry for both transmitting and receiving, infrared LEDs are used as photo transmitting devices, and Si PIN photodiodes (PDs) are used as photo-receiving devices. We have obtained a 32 Mbps/ch data rate, at around 5 dB of dynamic range, with less than 8 nsec of jitters and an error rate of less than [10.sup.-9]. The scheme we proposed in this study has been confirmed assuming it will be used in the readout system.

In this paper, the performance of fully digital synchronization techniques for time-division multiple-access (TDMA) differentially encoded 16-QAM signal transmission, over cellular mobile radio channels, is considered. The preambleless feedforward nondecision-aided (FF-NDA) synchronization techniques used for symbol timing recovery, carrier phase recovery and amplitude gain control to overcome Rayleigh faded channel impairments, are described. Novel techniques for digital amplitude gain estimation are presented. The performance of the synchronization parameters' estimators at the receiver is evaluated by computer simulation over additive white Gaussian noise (AWGN) and slow Rayleigh fading environment. Statistical properties of the algorithms developed are evaluated in terms of the bias and variance of the estimated errors and are compared to the Cramer-Rao lower bound. Index Terms - Preambleless techniques, symbol timing recovery, synchronization techniques, TDMA transmission.

Several linear multiuser detectors for code-division multiple access (CDMA) systems can be characterized as an inverse of some form of correlation matrices. If the correlations change, the detectors must be redesigned. An ideal computation of the decorrelating or the linear minimum mean-squared-error (LMMSE) detector requires order [K.sup.3] flops, where K is the number of users. To alleviate the computational complexity, iterative decorrelating and LMMSE detectors are proposed. The iterative detectors use steepest descent (SD), conjugate gradient (CG), and preconditioned conjugate gradient (PCG) algorithms, and require order [K.sup.2] flops per iteration. Their main advantages are the reduced number of flops and their suitability to highly parallel implementations. The correlation coefficient computation can also be embedded into the CG algorithm, which is an advantage with time-varying signature waveforms. The performance of the iterative algorithms is studied via computer simulations. Index Terms - Code-division multiple access, iterative methods, linear multiuser detector.

Direct injection technology has been used with cryogenic GaAs/AlGaAs MODFETs and a n+-GaAs/AlGaAs/n+-GaAs semiconductor capacitor to develop an all-GaAs/AlGaAs hybrid readout circuit for an GaAs/AlGaAs background-limited quantum-well infrared photodetector focal plane array. Over 15,000 electrons per square micrometer can be stored in the capacitor. The device has no low-temperature degradation and loses around 100 fA in the subthreshold regime.

This paper examines the effect of timing errors on QPSK/FDMA signals regenerated by a SAW-based group demodulator. The presence of a timing offset not only degrades the performance of that channel, but also increases the amount of its interference into other channels. Time windowing of the input to the chirp Fourier transform is shown to reduce the impact of the timing errors, but at the cost of an increased output noise variance. Measurements are presented for the bit-error rate (BER) performance of a single channel with a fixed timing offset using a surface acoustic wave (SAW) group demodulator based on the convolve-multiply - convolve (CMC) configuration with Kaiser-Bessel windowing. Index Terms - Multicarrier demultiplexer demodulator (MCDD), on-board processing (OBP), satellite regenerative repeater.

This paper discusses the problem of detecting one out of M Gaussian correlated signals in impulsive noise, modeled as a compound-Gaussian, possibly correlated process. We first show that, for uncorrelated noise, the detection problem admits one and the same optimum - in the sense of attaining minimum error probability - solution, independent of the noise statistics: the optimum detector, in fact, amounts to an estimation block, aimed at measuring the short-time noise power spectral density (PSD), whose output is fed to a bank of estimators - correlators, each keyed to one of the M admissible waveforms and to the estimated noise PSD. We also give suggestions for realizing a suboptimum receiver, with reduced complexity, which again is canonical in its structure. As for the performance analysis, we focus on binary frequency-shift keying (BFSK) signaling: we provide numerical results for two particular channel correlations and for Cauchy-distributed noise. These results indicate that, like for the general Gauss-Gauss case, the performance depends on the energy contrast, as well as on the "time-bandwidth" product of the useful signal. Moreover, noise spikiness seems to negatively affect the performance, in the sense that heavier and heavier high-amplitude tails of the noise marginal distribution give rise to higher and higher error probabilities for fixed energy contrast and time-bandwidth signal product. Index Terms - Detection, fading channels, non-Gaussian noise, SIRP.

A blind adaptive interference suppression strategy for joint acquisition/demodulation provides a unique near-far resistant demodulator output that implicitly models the timing/amplitude of all transmissions. The blind technique exploits a prior knowledge of the signature sequence of the desired transmission to suppress the multiple-access interference. The adaptive technique may be implemented both as a centralized as well as a decentralized scheme in view of the similarities in their computations. Simulations of a least square process reveal the performance of the scheme.

A commercial n-well complementary metal oxide semiconductors (CMOS) foundry process is used to design and manufacture Schottky diodes for radio frequency detection. The SCMOS technology file of the public-domain graphics layout editor, MAGIC, has been modified for the implementation of these diodes. Current-voltage analysis reveals that only the n-type devices, having a barrier height of 0.78 eV, are suitable for rectifying purposes. The cut off frequency of such a detector circuit is 600 MHz.

Communication over a waveform channel corrupted by additive white Gaussian noise and by an unknown and arbitrary interference of bounded power is considered. For this channel, an upper bound is presented for the worst-case error probability of a communication system comprising a direct-sequence spread spectrum modulator and a nonlinear correlation receiver. It is shown that this bound is exponentially tight as the number of chips used in the modulator becomes large. This bound is evaluated for several detector nonlinearities. Numerical examples and comparisons to the performance of a pure Gaussian noise channel are also given. Index Terms - Spread spectrum, direct-sequence modulation, signal detection, robust detection, nonlinear detection.

The design of single-user decorrelating receivers employing finite-precision sequences for despreading is considered. The problem is formulated as a nonlinear bounded integer optimization problem which is shown to be network performance (NP)-hard. A branch-and-bound algorithm for finding the best finite-precision decorrelating sequence is described. Numerical examples demonstrate that the loss in performance between the optimum, infinite-precision, and the best finite-precision decorrelator is small even for large channel occupancies. Some sub-optimum algorithms are investigated which greatly reduce the computational complexity associated with finding good finite-precision decorrelator sequences.

This paper addresses new hop-timing (epoch) estimation schemes which employ a coarsely channelized preprocessor in order to suppress the frequency and phase dependence in random frequency-hopping (FH) signals. Coarse channelization implies a bank of filters, covering the total spread bandwidth of the FH signal, whose number is much smaller than the size of the candidate hop frequency set. Linear and nonlinear combinations of preprocessor outputs are explored and compared. It is found that post-processing, rather than the size of the filter bank alone, is the determining factor on estimation performance. Performance evaluation is presented via both extensive computer simulations and analytical lower bounds. Comparisons with existing optimal and suboptimal systems are also provided.

This paper studies the transient behavior of an adaptive near-far resistant receiver for direct-sequence (DS) code-division multiple-access (CDMA) known as the minimum mean-squared error (MMSE) receiver. This receiver structure is known to be near-far resistant and yet does not require the large amounts of side information that are typically required for other near-far resistant receivers. In fact, this receiver only requires code timing on the one desired signal. The MMSE receiver uses an adaptive filter which is operated in a manner similar to adaptive equalizers. Initially there is a training period where the filter locks onto the signal that is sending a known training sequence. After training, the system can then switch to a decision-directed mode and send actual data. This work examines the length of the training period needed as a function of the number of interfering users and the severity of the near-far problem. A standard least mean-square (LMS) algorithm is used to adapt the filter and so the trade-off between convergence and excess mean-squared error is studied. It is found that in almost all cases a step size near 1.0/(total input power) gives the best speed of convergence with a reasonable excess mean-squared error. Also, it is shown that the MMSE receiver can tolerate a 30-40 dB near-far problem without excessively long convergence time.

Pulse frequency modulation (PFM) provides robust long-distance communication and event-driven access to the communication channel. A PFM demodulator small and simple enough to equip every cell of a large analog neural network is analyzed. The circuit can demodulate PFM signals with pulse rates as low as 1 kHz using the switched-capacitor technique. Measurement results from integrated demodulators are presented.

We analyze the performance of combined binary Frequency and M-ary Differential Phase Shift Keying (2FSK/MDPSK) in the Satellite Mobile Channel (SMC). We use two detectors in which the frequency symbol is detected by a Limiter Discriminator Detector (LDD) and the phase symbol by a Differential Phase Detector (DPD). It is shown that this system is power and bandwidth efficient when compared with other systems with constant envelope (or nearly constant envelope) and noncoherent detection like L-ary DPSK (LDPSK), Gaussian Minimum Shift Keying (GMSK) and MSK.

In this paper, a new multiple differential detection (MDD) sequence estimator is described which uses a decision feedback for the demodulation of a GMSK signal. This technique is based upon a maximum-likelihood sequence estimation of the transmitted phases rather than on a symbol-by-symbol detection. An upper and a lower bound on the bit error probability of the described detector in the case of an AWGN channel and a two-ray Rayleigh fading channel are derived. The performance of the detection algorithm in a mobile radio communication system is obtained through computer simulation. Comparisons with the coherent detection algorithm show that the proposed detection algorithm is quite attractive both in an AWGN and in a multipath channel.

Star 16QAM is a modulation method that transmits 4 bits per symbol and has the advantage that it may be differentially encoded and detected. It is very robust to fast multiplicative Rayleigh fading and is suitable for mobile telephone systems and personal communication networks. The main contribution of this paper is the derivation and bit error probability simulation of the maximum likelihood differential detector using phase differences and amplitude ratios from L diversity branches for bit decisions. As a comparison, much simpler previously known post detection combining techniques are generalized for star 16QAM and optimized. The bit error probability is simulated for both diversity detectors on a multiplicative Rayleigh fading channel with additive white Gaussian noise. It is found that the bit error probability of the ML detector may also be obtained by the simple combining detector. This is also true for the error floor due to maximum Doppler frequency. The diversity gain is almost 8 dB, measured in signal to noise ratio per diversity branch, at a bit error probability of 1 percent. The diversity detector can sustain an almost 3 times larger Doppler frequency again at a bit error probability of 1 percent. We also show that star 16QAM offers, at most, 3 subchannels with different bit error probabilities.

This paper suggests a dual-channel PSK demodulator for LEO satellite DS/CDMA communications, whose performance is absolutely invariant to time-variant Doppler offset of a carrier and local oscillator instability. The demodulator does not require any preamble (pilot signal) for carrier recovery, differential encoder/decoder, and carrier recovery circuit such as PLL and Costas loop, thereby resulting in high transmission efficiency and system complexity reduction. In a CDMA channel with the demodulator, the transmitted CDMA signal is composed of two orthogonal linear polarized components, which are spread by different spreading codes, and only one of which is modulated by the data stream. At the receiver, one of the two components despread by the corresponding matched-filter is utilized as a pilot reference carrier for the demodulation. Thereby, the Doppler offset is completely eliminated from the received signal since both these components exhibit the same Doppler and local oscillator instability.

The problem of designing asymptotically optimum detectors for a weak signal sequence with random time delays in the presence of a white Gaussian noise is considered. The multidimensional probability distribution of the time delays is assumed to be known. As a result of asymptotic analysis of the log-likelihood ratio, the asymptotically optimum linear or quadratic detectors and their probability distributions and efficiencies are found. Index Terms - Detection of dependent random weak signals, log-likelihood ratio, limiting probability distribution.

In this correspondence, decoding that uses soft-decision information but with multiple low-complexity decoders are investigated. These decoders correct only errors and erasures. The structure of the receiver consists of a bank of z demodulators followed by errors- and erasures-correcting decoders operating in parallel. Each demodulator has a threshold for determining when to erase a given symbol. We assign a cost f([Theta]) to the noise for causing an erasure when the receiver uses a particular threshold [Theta] and a (larger) cost [Mathematical Expression Omitted] for causing an error. The goal in designing the receiver is to choose the thresholds to maximize the noise cost which is necessary to cause a decoding error. We demonstrate that the above formulation is solvable for many channels including the M-ary input-output channel, the additive channel with coherent demodulation, and an additive channel with orthogonal modulation and noncoherent demodulation. Then we show that the maximum worst case error-correcting capability of the parallel decoding algorithms is the same as the maximum worst case error-correcting capability of a correlation decoder with the same number of quantization regions. Index Terms - Soft-decision decoding, parallel decoding, concatenated codes, quantization, correlation decoding, communications gain.

This paper discussed four open-loop frequency acquisition techniques for suppressed-carrier BPSK signals. The techniques, which do not require accurate symbol timing nor a data preamble sequence, employ fast Fourier transforms (FFT's) on the Costas loop phase detector output to detect the Doppler frequency offset. Performance of the full-, half-, and staggered-symbol integration techniques as well as the low-pass filter technique are compared in terms of the error signal output SNR's. Then, the probability of detecting the frequency offset is computed for the special case when the frequency of the tone to be detected coincides with one of the FFT-bin center frequencies. It is shown that the performance of the integration techniques depends strongly on the symbol timing offset. On the other hand, the detection probability for the one-pole arm filter technique depends directly on the filter bandwidth. The staggered integration technique is shown to have comparable performance to the low-pass filter technique for low-to-medium symbol SNR's but the latter is superior at high symbol SNR's.

An adaptive receiver is considered for use in combating the near-far problem in direct-sequence code-division multiple-access (CDMA) communication networks. The focus of the paper is on the multiuser interference rejecting capability of the receiver. The receiver uses a chip matched filter followed by an adaptive equalizer structure to perform the despreading operation. This adaptive structure allows the receiver to adjust to the prevailing interference and noise environment. An analysis of the receiver giving the form of the equalizer coefficients and the mean squared error in steady state is given. From these results the capacity of a CDMA network using this receiver structure is obtained and compared with a system using conventional receivers. The receiver is shown to be immune to the near-far problem in the sense that the performance without any power control is nearly identical to the performance with perfect power control. The receiver is also shown to offer a two-fold increase in capacity relative to a conventional receiver with perfect power control.

Due to the scarcity of frequency allocations for emerging personal communications services, it has been proposed that spread spectrum (SS) signals be used in such services so that they can be overlaid on existing frequency band occupants. Such SS signals would be constrained in power so as not to interfere with pre-existing, narrowband users. In view of this application, suppression of strong narrowband signals that interfere with commercial SS communications systems is being investigated with renewed interest. Previous research has modeled narrowband users as sinusoidal or autoregressive signals, and consequently has advocated use of predictive filtering to suppress such signals. In this paper it is shown that when the narrowband interferer is in fact a digital communications signal these methods are less effective than techniques derived from multiuser communications. In particular, by modeling a narrowband user as a digital signal, optimal and asymptotically (low background noise) optimal linear algorithms for the recovery of the SS bit stream are developed. This technique represents not only a promising new approach to narrowband interference suppression, but also a new application for multiuser detection theory.

Exact and approximate statistical models (analytical and simulation), based on average- and maximun-likelihood ratio tests, are studied and compared for the purpose of establishing an accurate assessment of the performance of noncoherent SFH M-FSK intercept recievers. Both continuous and discontinuous phase M-FSK cases are considered with important dofferences between the two.

A multiuser communication system is considered where K users share a channel with multipath propagation by using code division for multiple access. Data modulation is carried out by binary phase shift keying and direct sequence spread spectrum signaling. The microcellular communication media is modeled as a frequency selective fading channel with multipath propagation. The multipath diversity of the received signals from the K users is exploited by a bank of K Rake correlators. Algorithms based on the maximum likelihood rule have been developed for estimating the complex channel coefficients as well as for detection of the desired data packets from the sufficient statistics provided by the Rake correlators. The performance of the resulting multiuser detector is evaluated analytically and via Monte Carlo simulations. The results indicate that the estimator of the channel coefficients has a variance close to the Cramer-Rao lower bound, and that the proposed multiuser detector is capable of eliminating the near-far effect as well as processing the signals propagated through multiple paths. Keywords - Multiuser detection, code division multiple access (CDMA), Rake, Gauss-Seidel iteration, Expectation Maximization (EM).

This paper deals with the use of the innovations approach for developing a general and practical MLSE algorithms for signals transmitted over Rayleigh fading channels. The proposed MLSE receiver can be implemented by a bank of FIR time-invariant filters followed by a Viterbi processor and is applicable to any practically modulated signal over either frequency-nonselective or selective, fast or slowly fading channels. Simulation results are presented that demonstrate the superiority of the innovations-based receiver over differential detection in the presence of fast fading. To further improve the error performance, the rived MLSE receiver is combined with diversity reception. The simulation also shows that the diversity technique results in a significant improvement of the error performance. A unified interpretation of MLSE of signals transmitted over both Gaussian noise channels in the presence of ISI and Rayleigh fading channels is obtained by using the innovations approach.

An innovative approach to the automatic monitoring of signal quality on digital transmission channels such as communication links is presented. A communication channel is said to be experiencing some form of stress when the channel (and hence the signal) is distorted from nominal conditions. The stress may be the result of a variety of sources such as transmitter and receiver equipment errors, ionospheric effects, tropospheric effects, and jamming. A new monitor, the DEGRAD (Digital Eye Gridded Receiver with Arbitrary Degradation) monitor, is developed that is better able to detect and identify channel degradations before a significant change in signal quality is noticed by the end user.