A multi-rate modulation scheme suitable for code-division multiple-access (CDMA) communications systems is introduced. Balanced incomplete block designs (BIBD) are used to control the peak-to-average power (PAV) ratio and produce a signal whose amplitude is within the linear region of a power amplifier a larger proportion of the time than is the amplitude of a signal without a controlled PAV ratio. The PAV ratio and its effects on the power amplifier behavior are examined. The performances of the proposed scheme in the additive white Gaussian noise (AWGN) channel and in a multi-user environment are investigated.

A multirate modulation scheme with controlled peak-to-average power (PAV) ratio and suitable for code division multiple access techniques is introduced. The scheme utilizes codes drawn from a fixed set of Walsh-Hadamard codes but other code families can also be used. The strategy employs balanced incomplete block designs to define the multirate data symbols, with the objective being to reduce depletion of the code pool while controlling the PAV ratio. Existing multirate schemes are reviewed. Code depletion and PAV ratio characteristics of the proposed scheme are compared to existing schemes. Symbol error performance of the proposed scheme in an additive white Gaussian noise channel is examined. It is concluded that the proposed scheme, unlike the two other existing schemes that are examined, strikes a good balance between PAV ratio, code depletion, and error performance.

A way to allow variable data rates in CDMA is by varying the spreading factor in accordance with the data rate requested by the user. A detection scheme suitable for multirate transmission is required in addition to one that combats multiple access interference of other users and intersymbol interference from multipath. In this letter, an energy-add multiuser detection method is combined with a scheme called Cholesky-iterative detection to cope with these challenges. It allows the users to be estimated on a symbol-by-symbol basis, making receiver complexity independent of data package length. Simulation results correspond closely to a single-user lower bound. The results also show that the bit-error probability performances of the various users for the Cholesky-iterative detector are closely clustered, while those of a previously published scheme, decorrelation feedback detection, are more widely spread.

In synchronous code division multiple access (CDMA) systems, both multiple access interference (MAI) and intersymbol interference (ISI) arise. MAI is present due to the CDMA format while ISI is present due to channel multipath. One approach to overcome these problems is to detect the entire package of data for all users at once. This implies working with matrices with dimensions proportional to the product of the data package length and the number of active users. In this paper, we present an alternative approach. This alternative consists of a type of multistage detector that overcomes the MAI and ISI problems; in addition, it has complexity proportional to the number of CDMA users, is computationally efficient, and is suitable for pipeline implementation which allows fast data processing. Simulations are compared to a derived single user bound for the average bit error probability and the results prove the validity of the proposed detection method.

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 satellite-to-land-mobile communications system is analyzed with respect to fading/multipath effects in Rayleigh or Rayleigh faded multiple-access environment with an unfaded, Rayleigh, and Rician faded desired user. Numerical results employing Gold codes of lengths 31 and 127 for all three fading cases of the desired user are presented in terms of tight lower and upper bounds of the bit error probability as well as user capacity. The latter performance measure is represented by the minimum required SNR for a specified acceptable bit error probability versus the number off users in the system. Conclusions on the fading/multipath effect on the number of simultaneous users of the satellite-to-land-mobile communications system are drawn from the numerical results.>

A method for evaluating the topological quality of networks for distributed sensor applications is presented. The criteria for evaluation are network survivability and delay. Nodal connectivity is used to characterize survivability, and mean path length is used to characterize delay. To calculate these two quantities, an algorithm is developed to find k shortest node-disjoint paths between a pair of nodes when each link has unit distance. The effects of nodal losses in a multiple-node satellite network consisting of a Walker low-orbit sphere and a geosynchronous constellation are examined. The example demonstrates how topological analysis on the basis of connectivity and mean path length may be used to detect, and subsequently to address, potential flaws in a network design. The results show that the geosynchronous/low-orbit link assignment protocol should be a primary concern in the design of this network. They also show that the nodal degree of a failed node, and the distribution of links between the Walker sphere and the geosynchronous constellation, are the fundamental determinants of mean path length.>

An approach for calculating upper and lower bounds for the probability of error for asynchronous multiple-access spread-spectrum communication systems using deterministic codes is presented. The techniques is then generalized to include multiple-tone jamming. The approach utilizes the cumulative distribution function of individual interference terms. The computational complexity of the technique is calculated to the polynomial-like. Results showing the multiple-access performance of gold codes of lengths 31 and 127 in the presence of jamming are shown. The fact that this computational technique gives upper and lower bounds is rigorously proved using the Riemann-Stieltjes integral.>

Serial minimum-shift keyed (MSK) modulation, a technique for generating and detecting MSK using series filtering, is ideally suited for high data rate applications provided the required conversion and matched filters can be closely approximated. Lowpass implementations of these filters as parallel inphase- and quadrature-mixer structures are characterized in this paper in terms of signal-to-noise ratio (SNR) degradation from ideal and envelope deviation. Several hardware implementation techniques utilizing microwave devices or lumped elements are presented. Optimization of parameter values results in realizations whose SNR degradation is less than 0.5 dB at error probabilities of 10-6.

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.

The use of the chirped z-transform (CZT) for processing various communication signals has increased recently with the advent of charge-coupled devices (CCDs). In this paper a CCD/CZT oriented detection scheme for a Frequency-Time Hopping (FTH) spread spectrum signal is presented. An important aspect of the proposed scheme is the correlation operation performed in the frequency domain between the received signal and its stored replica. The signal is assumed to be corrupted by additive white Gaussian noise and narrowband interference described in terms of signal-to-noise ratios SNRGand SNRNB, respectively. The correlation detection results are presented as a function of SNRGwith SNRNBand duty-cycle d of the narrowband noise as the parameters. The probability of symbol detection PDfor the Gaussian case is derived based upon a double threshold detector and the possibilities of determining PDfor the narrowband case are discussed.

This paper describes the use of equalization in conjunction with channel filtering to improve QPSK transmission subject to both intersymbol interference (ISI) and interchannel interference (ICI). Performance bounds are computed using the nonclassical Gaussquadrature rule (GQR) method. The signal-to-noise ratio (SNR) gain due to linear equalization over nonequalization is thereby obtained and presented. The performance of a linear equalizer thus obtained is compared with the Viterbi algorithm sequence estimator (VASE). In the absence of bounds for the VASE receiver under the channel conditions considered, simulation results are used to make the comparison. With a possible difference in the accuracies of the performance thus obtained it is shown that the VASE provides improved performance over the linear equalizer under the channel conditions considered.

AnM-ary, large time-bandwidth product signal design technique is described which is useful for signaling in channels characterized by narrow-band interference and/or wide-band noise impulses. Diversity is also provided, thereby making it a useful signaling technique for dispersive channels. The motivations for choosing this signal format are explained, and design procedures, with two candidate signal designs, are given. The performance of a fast-fast-Fourier-transform-based receiver, which employs an excision algorithm for reducing the effects of narrow-band interference, is investigated through computer simulation.

This concise paper shows that a band-limited bivariatenormal time series is a useful model for the envelope of scintillating radio signals traversing the ionosphere. A software model is presented whose generated time series is compared to an actual block of scintillation data. A comparison of computer-generated and actual scintillation data shows that the fade-duration distribution of the simulated data fits that of the actual data closely through proper choice of model parameters.

The operation of a hybrid tracking (HT) phase-locked loop (PLL) in radio-frequency interference (RFI) is considered. Results from a perturbation analysis for the loop phase error in continuous-wave (CW) RFI are found to compare favorably with results obtained by digital computer simulation. These results, together with simulation results for wide-band (in comparison to the loop bandwidth) RFI, indicate that loop structures which track both on the signal carrier and modulation components are advantageous in RFI plus Gaussian noise backgrounds.

This paper reports the results of some theoretical and experimental studies of the extraction of signals from convecting and decaying fluid dynamic turbulent noise employing an analog correlation receiver. In addition, it presents an extension of the theory of correlator analysis for finite observation times to the case of a linear time-varying delay. Detailed study of the mean and variance of the output signal from a correlation receiver permits a prediction of the gain in signal-to-noise ratio in the receiver and its dependence upon various fluid dynamic fluctuation, signal, and correlator parameters. The gain in signal-to-noise ratio for periodic and Gaussian random signals in extraction from turbulent noise is found to depend upon 1) a ratio of correlation lengths involving several statistical properties of the turbulence obtained from a model of the space-time correlations, 2) a ratio of a characteristic signal frequency to a characteristic frequency of the turbulence and 3) a time bandwidth product associated with the effective finite averaging time of the correlator and a characteristic frequency of the convolution of the signal and noise spectra. Experimental studies in a subsonic wind tunnel employing acoustic signals are in good agreement with theoretical predictions of the gain in signal-to-noise ratio. Some receiver design considerations emerge from the analysis. Of special interest are the influence of linear scanning and ranging and the finite lifetime of the turbulence on the required averaging process.