Direct Sequence Spread Spectrum Systems Research Articles

The Effect of a Selective Receiver on CDMA

The quest for higher spectral efficiency in wireless communications is creating a new impetus for selective and sensitive base-station receivers. One consequence of high selectivity is phase dispersion, which critically affects the operation of a phase-modulated communications system. If phase equalization is not used, increasing selectivity will eventually compromise the performance of a receiver. These performance limitations are examined in a coherent and noncoherent direct-sequence spread-spectrum system

Limitations in the Code Acquisition Performance for Adaptive Filter Versus Correlator

In this letter, we compare the code acquisition performance of the adaptive filter to that of the correlator in direct sequence spread spectrum (DS/SS) systems. The test variables used in the code acquisition are statistically analyzed for both schemes, and then, the obtained results are used in comparing the detection and false-alarm probabilities. Analysis and simulation results show that the correlator outperforms the adaptive filter under most acquisition environments

A New Adaptive Code-Acquisition Algorithm Using Parallel Subfilter Structure

This paper presents a new adaptation algorithm using parallel subfilter structure, which enables fast-code acquisition for direct-sequence spread-spectrum systems. The proposed algorithm adapts the filter, based on a concept of dividing it into several subfilters, thus reducing the adaptation time. The detection and false-alarm probabilities are derived analytically. Simulation results show that the adaptive code-acquisition scheme, using the proposed algorithm, improves the mean-acquisition-time performance by 4-5 and 3-4 dB on additive white Gaussian noise and multipath Rayleigh fading channels for 64 taps, respectively

The Bank of Matched Filters for an Optical ZCZ Code Using a Sylvester Type Hadamard Matrix

The optical ZCZ code is a set of pairs of binary and bi-phase sequences with zero correlation zone. An optical M-ary direct sequence spread spectrum (M-ary/DS-SS) system using this code can detect a desired sequence without interference of undesired sequences. However, the bank of matched filters in a receiver circuit may fall into large scale. In this paper, we propose the compact construction of a bank of matched filters for an M-ary/DS-SS system using an optical ZCZ code. This filter bank can decrease the number of 2-input adders from O(N2) to O(N) and delay circuits from O(N2) to O(N log2 N), respectively, and is implemented on a field programmable gate array (FPGA) corresponding to 400,000 logic gates.

Performance Analysis of a Decision-Feedback Coherent Code Tracking Loop for Pilot-Symbol-Aided DS/SS Systems

In this paper, the performance of a decision-feedback coherent code tracking loop for pilot-symbol-aided (PSA) direct-sequence spread-spectrum systems, considering the effect of both symbol and channel estimation errors, is analyzed. An analytical closed-form expression on the steady-state jitter variance in additive white Gaussian noise (AWGN) environments using the linear analysis model is first derived. Then, the upper and lower bounds of the jitter variance in fading environments are obtained based on the analysis of the loop bandwidth variation according to the input signal level. Finally, a second-order coherent tracking loop is designed, and its performance in both AWGN and fading environments is evaluated, along with verification by computer simulations. It is demonstrated that analytical results are very close to the simulation results in AWGN environments and that the jitter variance in fading environments is located between analytical upper and lower bounds with its position depending on the fading rate.

A Rapid Code Acquisition Scheme for DS/SS Systems

In this paper, we propose a rapid code acquisition scheme for direct-sequence spread-spectrum (DS/SS) system basing on the improved seed accumulation of sequential estimation (ISASE). This method solves the problem that SASE scheme cannot work in the presence of carrier-frequency offset. The mean acquisition time of the ISASE scheme is derived by the generating function flow graph technique, and the tolerance of carrier-frequency offset is obtained in a closed-form expression. The performance of the proposed acquisition scheme is analytically evaluated in terms of the mean acquisition time. Analysis in theory and numerical results show that the proposed scheme yields improvement of the mean acquisition time over the RASE scheme by a factor of 2---15 times for the PN} period more than 255 in normal operating condition. For the important parameter-accumulating number m, we recommend to choose from 5 to 20 through the tradeoff of the tolerance of carrier-frequency offset and acquisition speed.

Statistical approach to the code acquisition problem in direct-sequence spread-spectrum communication systems

Statistical approach to the code acquisition problem in direct-sequence spread-spectrum communication systems

Performance of multicarrier DS/SSMA over fast Rayleigh fading channels with Doppler diversity

The significant loss in performance of multicarrier direct-sequence spread-spectrum multiple-access systems over fast-fading channels is investigated. First, the channel model for individual subchannels is obtained, using a canonical decomposition to a wide-sense stationary (WSS), uncorrelated scattering channel. Next, a receiver structure, which features both Doppler diversity and frequency diversity, is presented and analyzed. It is found that large Doppler spreads cause the average magnitude of the desired signals to diminish dramatically. Expressions for the bit-error rate are derived for both uncoded and coded systems by applying the standard Gaussian approximation. Numerical results, which show that Doppler diversity is preferable over frequency diversity in fast-fading channels, are provided for different combinations of the diversity orders. Numerical results, which show that the intersymbol and intersubchannel interference can be effectively suppressed, and the multiple-access interference tends to degenerate into a WSS process with the statistical periodicity destroyed by the fast-fading channels, are also provided.

Performance and Capacity of Direct-Sequence Spread Spectrum Overlay in the Microwave Landing System Band

We examine the performance of a direct-sequence spread spectrum (DS-SS) code-division multiple-access (CDMA) system used in a spectral overlay mode in the Microwave Landing System (MLS) band, from 5091–5150 MHz. The purpose of the overlay system is to enable increased spectral efficiency (higher data throughput), for future potential aeronautical datalink communications, particularly for short-range systems such as for airport surface, and terminal areas. We estimate the performance degradation incurred by both systems for a range of relative transmit powers, several DS-SS bandwidths and data rates, and example MLS and DS-SS system parameters, including the number of DS-SS users. We also briefly examine the use of multicarrier (MC) DS-SS signaling techniques. Our study uses both analysis and computer simulations, and complements prior work done for the use of DS-SS in the Instrument Landing System Glideslope band, where the ILS signal is an analog signal, in contrast to the (analog and) digital MLS signal. We provide examples that explore both MLS and DS-SS system parameter values appropriate for the successful application of overlay.

An adaptive double-dwell PN code acquisition system in DS-CDMA communications

An adaptive double-dwell acquisition system for pseudo-noise (PN) sequences is presented for direct-sequence spread-spectrum (DS-SS) systems. Since existing acquisition systems have a fixed threshold value, they are unable to adapt to varying mobile communication environments resulting in a high false alarm rate or low detection probability. Accordingly, this study presents an adaptively varying threshold scheme that uses a constant false alarm rate (CFAR) algorithm. The performance of the proposed system is compared to the conventional double-dwell system based on deriving formulas for the detection probability, false alarm rate, and mean acquisition time of the proposed system. The results confirm that the proposed system can produce a highly improved performance over the conventional system.

Comprehensive evaluation of chip interleaving effect on turbo‐coded DS‐SS in a Rayleigh fading channel with antenna diversity reception

AbstractIn this paper, we present a comprehensive performance evaluation of chip interleaving effect on turbo coded direct sequence spread spectrum (DS‐SS) system in a frequency non‐selective Rayleigh fading channel with antenna diversity reception. At the transmitter, chip interleaver scrambles the SF (spreading factor) chips associated with a data symbol and transforms the transmission channel into a highly time‐selective or highly memoryless channel at the chip level. The use of chip interleaving is equivalent to using SF‐antenna diversity reception with correlated fading among the branches and with reduced average received signal power per antenna by a factor of SF. We theoretically analyze how chip interleaving alters the received signal statistics. Then, the effect of the various parameters, viz. interleaver size, interleaving depth, information sequence length, spreading factor and the fading maximum Doppler frequency, are also evaluated. It is found that the bit error rate (BER) performance improves with increasing spreading factor and increasing frame length. Chip interleaving is found to be effective in the presence of receive antenna diversity as well. Copyright © 2005 John Wiley & Sons, Ltd.

Detection performance of chaotic spreading LPI waveforms

Low probability of intercept (LPI) performance of a direct-sequence (DS) spread-spectrum (SS) system with chaotic spreading sequences is investigated in this paper. Several intercept receivers, including energy detectors, synchronous and asynchronous, coherent and noncoherent structures, which are typically used to detect binary DS SS signals, are examined here to detect the presence of chaotic DS SS signals. A simple detection approach using a binary correlating function to detect nonbinary chaotic sequences is proposed. The expressions of detection probabilities of chaotic spreading signals using those intercept receivers are derived. Comparisons between systems using chaotic and binary sequences are given in terms of the LPI performance, and the performance improvement with chaotic spreading sequences is observed.

A direct-sequence spread-spectrum communication system for integrated sensor microsystems

Some of the most important challenges in health-care technologies have been identified to be development of noninvasive systems and miniaturization. In developing the core technologies, progress is required in pushing the limits of miniaturization, minimizing the costs and power consumption of microsystems components, developing mobile/wireless communication infrastructures and computing technologies that are reliable. The implementation of such miniaturized systems has become feasible by the advent of system-on-chip technology, which enables us to integrate most of the components of a system on to a single chip. One of the most important tasks in such a system is to convey information reliably on a multiple-access-based environment. When considering the design of telecommunication system for such a network, the receiver is the key performance critical block. The paper describes the application environment, the choice of the communication protocol, the implementation of the transmitter and receiver circuitry, and research work carried out on studying the impact of input data characteristics and internal data path complexity on area and power performance of the receiver. We provide results using a test data recorded from a pH sensor. The results demonstrate satisfying functionality, area, and power constraints even when a degree of programmability is incorporated in the system.

Optimal Signature Sets for Oversaturated Quasi-Scalable Direct-Sequence Spread-Spectrum Systems

Signature sets that maximize the sum capacity of an oversaturated code-division multiple-access (CDMA) system are derived, under the constraint that the signature sets have to be quasi-scalable. It turns out that the optimal signature set is composed of orthogonal signature sets, with at most one incomplete orthogonal set. As compared to the Gaussian multiple-access channel (GMAC), the loss in spectral efficiency for this multiple-orthogonal CDMA (OCDMA) system remains very low.

Frequency shift keyed narrowband interference rejection: optimal exponential weighting factor for the RLS algorithm

Previous work has shown that co-channel narrowband interference can limit the performance of direct sequence spread spectrum (DSSS) and high frequency (HF) systems. Narrowband interference (NBI) can be single tone, chirped or frequency shift keyed (FSK) in nature and numerous techniques for its removal have been proposed. Linear adaptive prediction filters based on autoregressive modelling have been suggested owing to their ability to perform in a non-stationary environment. In the FSK narrowband interference case, adaptive filters are susceptible to excess residual errors owing to instantaneous frequency step changes and the finite convergence time required for the filter to adapt to a new interference frequency. The signal degradation owing to this type of interference becomes greater in high SNR regimes and has been found to be a function of the frequency parameters of the FSK interference signal. This paper discusses the convergence and frequency tracking properties of the recursive least squares (RLS) adaptive lattice filter using a posteriori estimation errors in the presence of FSK narrowband interference. An optimal exponential weighting factor that balances convergence time and steady state error is derived for this case of NBI. Results are compared to those of the previously proposed fast converging minimum frequency error (FCMFE) RLS lattice filter.

Optimal design of an all-digital chip timing recovery loop for direct-sequence spread-spectrum systems

Noncoherent digital delay lock loops (DDLL) are suited for chip timing synchronisation in band-limited direct-sequence spread-spectrum (DSSS) demodulators. The diffusion approximation and the singular perturbation method are used in this paper to calculate the mean time to lose lock (MTLL) of the DDLL. Loop bandwidth optimisation for first order loop with severe Doppler is presented. A simple design rule for the loop bandwidth and a systematic approach for the loop threshold calculation are presented. Copyright © 2005 AEIT.

Convergence of a ML Parameter-Estimation Algorithm for DS/SS Systems in Time-Varying Channels With Strong Interference

An unbiased, maximum-likelihood (ML), channel parameter-estimation algorithm for direct-sequence spread-spectrum systems with strong interference is discussed in this paper. The algorithm includes correcting terms to the extended Kalman filter (EKF) based on the gradient of the negative log-likelihood function of the output of a conventional matched filter. By an asymptotic analysis, the algorithm is shown to determine the actual parameters. A complete implementation of the algorithm is given, and its transient behavior is examined by computer simulations. Results show the ML algorithm, albeit optimal in the sense of unbiased parameter estimation, is less robust than the modified EKF described in the first reference.

Performance Analysis of Noncoherent Digital Delay Locked Loops for Direct Sequence Spread Spectrum Systems With Doppler Shift and Quantized Adaptation

The noncoherent second-order digital delay locked loop (DLL) with the presence of Doppler shift is investigated. The loop performance such as lock-in range, transient response, mean time-to-lose lock (MTLL), and mean square tracking error (MSE) are analyzed. The analysis is unique in two respects. First, MTLL and MSE are evaluated more accurately than by previous methods. Second, the impact of quantized adaptation on the loop performance is examined for digital DLL that was neglected previously. Numerical results show that: 1) the quantized adaptation may significantly alter the loop behaviors, including the lock-in range, transient responses, MTLL, and MSE; and 2) the traditional analysis based on Gaussian approximation may result in a large error in performance evaluation when quantized adaptation is taken into account.

Comments on "A noncoherent tracking loop with diversity and multipath interference cancellation for direct-sequence spread-spectrum systems"

For original paper see IEEE Trans. Commun., vol.46, p.1516-24 (1998 November). In the original paper, the authors presented a noncoherent tracking loop with diversity and multipath interference cancellation to improve the loop performance on frequency-selective fading channels. However, the effects of imperfections in multipath interference cancellation have been dropped out in the statistical analyses, and only evaluated by using computer simulations. The present authors show that such effects can change the error characteristic, at least in the mean sense, and thus make the analyses of jitter performance improper.

Experimental characterization of a direct-sequence spread-spectrum optical wireless system based on pulse-conformation techniques for in-house communications

In this paper, the design and experimental testing of a direct-sequence spread spectrum (DSSS) system for wireless infrared LAN is presented. This method improves the wireless IR communications performances against multipath propagation and narrowband interference. We have implemented a IR-DSSS prototype with 100 MHz code frequency and 6 Mbps bit rate. We have also studied the effects of both Gaussian and sinusoidal signal conformation over wireless optical DSSS systems. These results show that these techniques reduce the adjacent-channel interference without loss of process gain or substantial reductions of BER vs. S/N performances. We also present some results about a simplified synchronization structure to be used on reduced-cost domestic equipment. These systems are suitable for different applications at low and medium rates, especially for in-house.