The performance of OFDM modulation degrades in a Doppler spread channel due to intercarrier interference (ICI). Multirate sampling theory and a detection scheme, called sequential interference cancellation (SIC), are applied to a standard OFDM system to reduce ICI in. The increased sampling rate (upsampling) causes less ICI for the edge subcarriers. The SIC detection scheme recovers the data from the outer carriers first and then detection moves from the outer carriers to the inner carriers to lower the error floor. In this paper, SIC detection with modified time-domain interpolation estimation is presented and its performance is compared to the existing time-domain and low-pass interpolation channel estimations via computer simulation. The results show that the modified method performs better than existing estimation schemes for BPSK and QPSK modulation.

We consider GMSK for use in frequency division multiple access (FDMA) satellite communication systems where only limited system bandwidth is available. The demodulator employs maximum likelihood sequence estimation (MLSE) of the GMSK signal using a simplified Viterbi processor. For the case of GMSK having a time-bandwidth product of BT=1/4, a two matched filter approximation gives very good performance, while, for BT=1/6, a two filter approximation is very similar to a four filter approximation. For a convolutional code with rate 1/2 and constraint length 7, simulation shows that soft output Viterbi algorithm (SOVA) decoding using a four-filter demodulator for BT=1/6 and BT=1/8, results in a performance improvement of 2.1 dB and 2.3 dB, respectively, in signal-to-noise ratio (SNR), compared to hard decision Viterbi algorithm (HOVA) decoding, at a bit error probability (BEP) of 10/sup -5/. When compared to a BPSK system that employs a similar soft decision decoder, the BEP power penalty for these schemes is 2.3 dB and 4.1 dB respectively. The spectral efficiency, relative the 95% RF power containment bandwidth, is 1.88 bits per sec per Hz (bps/Hz) and 2.1 bps/Hz of bandwidth for BT=1/6 and 1/8 GMSK respectively, and only 0.35 bps/Hz for BPSK. As ratios, the spectral efficiency improvement factor is 5.37 and 6.0 respectively.

In the IEEE 802.11 WLAN standard users transmit and receive within a common frequency band to achieve frequency re-use, but this creates capacity limitations. In this paper we investigate using a single array of sensors at the access node to provide spatial re-use, and hence, increased capacity. Multiple independent beamformers are established at the base station. Specifically, parallel generalized sidelobe cancellers are implemented. The impact of a mobile user traversing an accurately scaled indoor trajectory is of particular interest. System performance is characterized in terms of time varying link SNR and the resulting link availability. Beamformer and angle-of-arrival processor update rates are varied to determine degradation thresholds.

This paper addresses Gaussian minimum-shift keyed (GMSK) receiver performance analysis for limiter discriminator detection with cochannel interference resulting from a combined cellular voice and packet radio network. A specific example is cellular digital packet data (CDPD), which shares spectrum with cellular voice users; thus, the CDPD receiver will see both narrow-band frequency modulation (NBFM) voice and GMSK data interference. Previous studies investigated the performance of a GMSK receiver with a single GMSK interferer and multipath delay. In this paper, we extend earlier work to multiple Rayleigh fading (Doppler spread) interferers having NBFM (f/sub dm/=12 kHz) analog modulation and/or GMSK modulation. Probability of bit-error expressions are derived for the frequency modulation (FM) and GMSK interferer cases assuming medium-to-high signal-to-noise ratio regions. For scenarios of from one to six NBFM interferers, having the same total power, a corresponding Gaussian approximation is within 1.5 dB. Simulation results are also presented for various combinations of FM and GMSK interference. FM interference is only slightly worse than GMSK interference. Delay spread representing an urban scenario resulted in only a slight increase in the error floor.

We first derive the joint optimal maximum-likelihood (ML) estimator of the carrier phase, both Doppler shift and Doppler rate, and the spreading code delay for a short burst direct sequence/spread spectrum (DS/SS) transmission in the absence of the data modulation. The typical burst duration is three data bit periods (60 ms). The performance of the joint estimator is analytically derived separately for high and low carrier-to-noise ratio (CNR) values. A suboptimal ML estimator based on the method of averaged periodogram is proposed for the data modulation present case, allowing the joint estimation of the data bit values. Then the above parameters are assumed correctly estimated and a segmentation approach is adopted, deriving the optimal joint ML estimator for the bit synchronization epoch and data. Simulations show the joint estimators perform reliable down to a CNR of approximately 30 dB/spl middot/Hz.

A direct-sequence spread-spectrum communication system using an adaptive antenna array to null interference is examined. The system model is a modification of Compton's (1978) system, using a digital implementation and a directional constraint. Computer simulations are used to compare the behavior of the directionally constrained model with Compton's model with emphasis on spread-spectrum code synchronization. In the directed system, nulling of a tone jammer occurs without respect to direct-sequence code acquisition, and nulling of an off-beam desired signal occurs very slowly. Theoretical results are obtained by using the system covariance matrix, and incorporating the effects of spread-spectrum filtering to compute the system response as a function of time. The simulated results indicate that incomplete directional knowledge may be exploited to enhance signal quality and speed up code acquisition.>

Estimating the frequencies of sinusoidal signals from noisy observations is discussed. In particular it is assumed that the sinusoids are phase coupled. The biperiodogram is used to estimate the frequencies of the phase coupled sinusoids. The mean-square convergence of (modified) biperiodogram frequency estimates is proven. The asymptotic variances of the biperiodogram estimates are shown to be of the same order as the corresponding Cramer-Rao bounds (CRB); however, the asymptotic CRB is not achieved. For the case of equal amplitude sinusoids the asymptotic efficiency of the estimates is 3/8. The finite sample size properties of the biperiodogram estimates are studied through simulations and compared with corresponding periodogram frequency estimates. For small sample size (e.g. N=64) experimental results indicate that better estimates are obtained with the biperiodogram.>

The effect of a secondary path signal on the detectability of the symbol rate spectral line of a binary phase-shift keyed (BPSK) signal by a delay and multiply receiver is investigated. The measure of detectability used is the rate line signal-to-noise ratio. The parameters that define the secondary path signal are initially modeled as deterministic quantities; the phase of the secondary path signal is then taken to be a random variable independent and uniform on (0, 2 pi ). It is found that, with deterministic path parameters, the presence of a secondary path can seriously affect the detectability of BPSK rate lines. When the phase is taken to be uniform on (0, 2 pi ) over the receiver observation time, then the delay and multiply receiver performs as if no multipath is present. In this case, a noncoherent power addition of the primary and secondary path signals occurs.>

Expressions are found for the probability of error of a slow frequency-hopped spread-spectrum (FHSS) code-division multiple-access (CDMA) system using noncoherent M-ary frequency-shift-keyed (FSK) data modification in the presence of slow-nonselective Rayleigh or single-term Rician fading. The analysis is general enough for the consideration of the near/far problem under the specified channel conditions. Comparisons are made between the error expressions developed here and previously published upper bounds. It is found, under certain conditions, that the previous upper bounds on the probability of error may exceed the true probability of error by an order of magnitude.>

The mean, variance and normalized mean-squared error of the modified biperiodogram are calculated for a finite sample of the observed signal. It is assumed that an overall record of length N is available, which can be divided into K subrecords, each of length L, and data windowing, can be used. The normalized mean-squared error is used to obtain the best selection of L and K given the overall record length N, and a specified window type. A numerical example is given for a process created by exciting a finite impulse response filter with non-zero skewness zero mean white noise.>

A three-layer neural network is trained to perform spectral estimation. Excellent performance is found through computer simulations. When compared with an equivalent length radix-2 fast Fourier transform, the neural network has the following advantages: (1) the neural network can suppress the signal magnitude in sidelobes without widening the mainlobe; (2) the neural network completes its computation in two stages regardless of the number of input signal samples; (3) the number of input signal samples does not need to be an integer power of two; and (4) the neural network still generates valid results even if there are 25% broken interconnects uniformly distributed between the output layer and the hidden layer or between the hidden layer and the input layer. An application example of using a three-layer neural network for moving target detection is also included.>

Spread-spectrum signals that use spreading codes chosen to give a low probability of chip rate detection are examined. Pulse shaping and pseudorandom amplitude weighting are used to defeat the interceptor who uses a nonlinear circuit to generate a tone at the chip rate of the signal. The case of low input signal-to-noise ratio (SNR) is considered. Under this assumption, the detector output spectrum is found, and the output SNR is then used as a measure of detectability. The effect of signal pulse shape, i.e., band-limiting, on performance is studied. It is shown that a combination of band-limiting and correctly chosen amplitude weightings can, ideally, defeat any such circuit. The performance of a fourth-power envelope detector is analyzed with a combination of signal pulse shape and amplitude weighting. The amplitude distributions are binary, Gaussian, and quantized Gaussian.>

A method for finding the nonstationary moments of a random binary pulse train with arbitrary pulse shape is described and used to find the joint moments of the pulse train with a delayed version of itself. These moments are used in the analysis of nonlinear synchronization circuits. The method uses the moment generating function and a recursive relationship for moments and cumulants. Example derivations are given for moments up to eighth order.>

The detectability of the symbol rate of a digitally modulated signal by a delay and multiply receiver is investigated. Two signal pulse types are considered: Nyquist pulses and rectangular pulses. Since many digital transmission systems use rectangular pulse shapes, symbol rate-line detectability is considered for receivers optimized for rectangular pulses. For a receiver configuration that is reasonable for rectangular pulses, it is found that with a Nyquist pulse parameter alpha =0.25, the rate-line signal-to-noise ratio is down about 20 dB from that for a rectangular pulse signal.>

A baseband signal processor for 1 Gbit/s pulse quaternary modulation (PQM), a pulse modulation format, has been developed and tested. A coupled stripline network is implemented for signal formatting, along with a frequency/phase tracking pulse demultiplexer which reduces predetection bandwidth and interpulse interference. Measured BER performance is presented along with computer simulation predictions.