Direct Sequence Spread Spectrum Communication Research Articles

Broadband interference suppression in DS-SS system with modified discrete chirp Fourier transform

This paper presents a method of broadband interference excision by using a modified discrete chirp Fourier transform to improve bit error rate (BER) performance of direct sequence spread spectrum communication systems. With a computational complexity in the order of O(Nlog2N), the proposed method is shown to be effective for both mono- and multi-component interferences within a large range of interference to signal ratios. Theoretical analysis and simulation results show that the proposed method can achieve a better BER performance in the presence of white noise and linear chirp-like interference.

Performance of Chaos-Based Digital Communication Systems in the Presence of a Pulsed-Noise Jammer

Chaos-based communication systems are known to offer advantages that are shared by spread-spectrum communications. One important aspect of the performance of any spread-spectrum communication system is the ability to resist jamming. In this paper, an analysis of the antijamming performance of a coherent chaos shift keying system under the influence of a common pulsed-noise jammer is presented. Two types of pulsed-noise jammers, slowly switching and fast switching jammers, are considered in our studies. The performance is evaluated in terms of the bit error rate (BER) under different levels of noise power, jamming power, spreading factor, and duty factor. The maximum (worst-case) BERs are also obtained analytically for the case where the jammer is slowly switching. Computer simulations are performed to verify the analytical results. Finally, the BERs are compared with those of a conventional direct-sequence spread-spectrum communication system.

Role of contemporary communications systems in the process of planning, organizing and realization of military operations

Fast progress in commercial communications systems development has made possibilities to fulfill communications needs in military organizations especially during the periods of peace. A reconnaissance system, based on specific military technology such as passive radar systems and acoustic sensors, available in commercial market, represents a possible realization of a simple and highly efficient system. Necessary communication with a higher command, also as actions that are made based on gathered information, could de supported through use of commercial communications systems employing adaptive modulation processes, direct sequence and freqyency hopping spread spectrum communications and code division multiple access. In this way organized systems completely fulfill communication needs of the Army of Serbia and Montenegro during peacetime, but the applicability of such systems expends partially or fully on cases of limited military interventions antiterrorist actions and nature disasters.

Analytical Description of Self-Interference in Direct Sequence Spread Spectrum Communications

A new analytical statistical model has been developed in order to describe the self-interference that results due to multipath propagation in a BPSK spread spectrum system. Motivation for this work was the fact that the well-known Gaussian approximation that has been extensively used in the past in order to describe such terms is not satisfactory, especially when the probability of error is smaller than 10−2 or 10−3. The existence of two paths in the radio channel has been assumed. The amplitude of each path has been assumed as Rayleigh distributed while the arrival time of the second path with respect to the arrival time of the first one has been assumed to be uniformly distributed. The validity of the developed model has been examined by comparing analytical results with results obtained by simulation using Monte Carlo techniques. Results taken from the Gaussian approximation were also compared with the simulation results. It was shown that the developed model describes with great accuracy the pre-mentioned undesirable term. On the contrary, the Gaussian approximation exhibits significant differences when compared to the simulation results. This is also evident by the use of a comparison parameter which is several orders of magnitude smaller for the developed model than for the Gaussian approximation (the smaller this parameter is the better the fit between two groups of data).

Performance evaluation of serial‐search strategies for the acquisition of DS/CDMA signals

AbstractSearch strategies for initial synchronization of direct sequence (DS) spread spectrum communication systems are compared, modelled and implemented using MATLAB then tested in the presence of additive white Gaussian noise (AWGN) and narrowband interference. The performance of these search strategies, namely, offset‐z‐search strategies and expanding alternate search strategies, in terms of the mean value of the search length is evaluated for different probabilities of detection. A truncated Gaussian probability phase distribution is assumed (as a priori information about the correct phase) and the performance of these search strategies is tested for different standard deviations (σ) of the phase.A novel search strategy, namely the offset‐z3, based on the offset z‐search strategy is introduced. Among all search strategies investigated the non‐uniform expanding alternate (NUEA) search strategy is found to perform best at high probability of detection. However, for a more peaked phase distribution, the offset‐z3‐search strategy is shown to offer better performance at low probability of detection. Outcomes of the simulated system support trends predicted by theory for the performance of different search strategies. Copyright © 2003 John Wiley & Sons, Ltd.

Nonstationary Interference Excision in Time-Frequency Domain Using Adaptive Hierarchical Lapped Orthogonal Transform for Direct Sequence Spread Spectrum Communications

An adaptive hierarchical lapped orthogonal transform (HLOT) exciser is proposed for tracking, localizing, and rejecting the non-stationary interference in direct sequence spread spectrum (DSSS) communications. The method is based on HLOT. It utilizes a fast dynamic programming algorithm to search for the best basis, which matches the interference structure best, in a library of lapped orthogonal bases. The adaptive HLOT differs from conventional block transform and the more advanced modulated lapped transform (MLT) in that the former produces arbitrary time-frequency tiling, which can be adapted to the signal structure, while the latter yields fixed tilings. The time-frequency tiling of the adaptive HLOT can be time varying, so it is also able to track the variations of the signal time-frequency structure. Simulation results show that the proposed exciser brings significant performance improvement in the presence of nonstationary time-localized interference with or without instantaneous frequency (IF) information compared with the existing block transform domain excisers. Also, the proposed exciser is effective in suppressing narrowband interference and combined narrowband and time-localized impulsive interference.

Low-complexity noncoherent PN code chip timing recovery with resistance to MAI for a bandlimited CDMA receiver

A noncoherent PN code tracking loop in a fully digital modem is investigated for bandlimited direct-sequence spread-spectrum communications. Its main features are low hardware complexity and good tracking performance. Analytical expressions of the error characteristics and steady-state timing jitter are derived and confirmed by means of computer simulation. The numerical results show that it achieves good steady-state performance. In addition, the proposed technique provides good resistance to multiple-access interference while operating in a code-division multiple-access environment.

HOS and GA based interference rejection in direct sequence spread spectrum communication systems

A new Higher Order Statistics (HOS) and Genetic Algorithm (GA)-based interference rejection filter is introduced. Compared with the adaptive filters based on second-order statistics and gradient algorithm, the HOS and GA-based filter can reject the interference more efficiently, is independent of uncorrelated Gaussian noise, tends to converge to the optimum solution and is much less sensitive to the choice of the step size parameter. Computer simulations show that the method can reject narrowband interference efficiently.

Non-stationary jammer excision in spread spectrum communications via discrete evolutionary and Hough transforms

Despite the advantages of direct sequence spread spectrum (DSSS) communications, intentional jamming or interference from other users reduces the robustness of the system to interference. Spreading the message by means of a pseudo-noise sequence, DSSS increases the bandwidth of the transmitted message making it thus difficult to track the message. Given the ease in tracking jammers in the time–frequency (TF) domain, TF-based excisers applied before despreading enhance the robustness to interferences of DSSS. In this paper, we propose an excision technique for multi-component chirp jammers with constant or time-varying amplitudes. Our procedure initially estimates the instantaneous frequency (IF) of each of the jammer components using a combination of the discrete evolutionary and the Hough transforms. These local linear IF estimates are efficiently obtained, and improved by recursively matching the instantaneous phase of each of the components. Using the estimated IFs, the received message is projected onto a semi-stationary domain where the desired jammer component appears as a low-frequency signal. Synthesis of the jammer components is performed by first processing the signal with well-understood procedures such as linear filtering or singular value decomposition, and then chirping the resulting signal. Subtracting the synthesized jammer from the received signal permits the correlator to eliminate the remaining jamming and channel noise interferences. Our procedure applies to multi-component chirp jammers with constant or time-varying amplitudes and instantaneous frequencies not parametrically modeled. A general statistical analysis of the process is developed based on the signal to interference and noise ratio and the probability of bit error. Performance of the proposed excisers is quantified by means of simulations of different cases, and the results used to verify the validity of our statistical analysis and to compare our results with those from existing projection methods.

Extension of the Capon's spectral estimator to time–frequency analysis and to the analysis of polynomial-phase signals

Incorporation of the linear time-varying filter and its Zadeh's generalized transfer function concepts to the derivation of the Capon's minimum-variance spectral estimator leads to a new, bilinear, cross-term suppressed and alias-free time-frequency representation (TFR) that has a higher resolution than the spectrogram with the same window width. Time-variant autocorrelation function of the nonstationary signal of interest is employed in this proposed TFR. By adopting an approximation for time-variant autocorrelation functions, we obtain another new, bilinear, parameterized TFR related to the spectrogram, the frequency resolution of which can be adjusted by varying its parameter for a fixed window width. We compare resolution and cross-term suppression properties of these proposed TFR's with other basic bilinear TFR's, via simulations on synthesized signals. Then, by incorporating polynomial-phase kernel functions to the Capon's estimator, we propose new bilinear signal representations for the analysis of constant-amplitude polynomial-phase signals and apply them to interference excision in direct-sequence spread-spectrum communications.

Design of piecewise maps for chaotic spread-spectrum communications using genetic programming

In this paper, we propose the use of genetic programming (GP) to design piecewise maps to generate chaotic spreading codes for direct sequence spread-spectrum communications. Using appropriate functional operators, we use GP to construct exploitable maps on the interval [0, 1]. The correlation performance of the maps is the objective function that is optimized using GP. Thus, the auto-correlation and cross-correlation performance of the maps are used as fitness measures to differentiate between potential solutions and a multi-objective GP is used to evolve maps. We are thus able to design specific maps under different number of users and show that it is better to design maps assuming large number of users as such maps work well for lower number of users. Further, in order to be able to design maps for use under different conditions of channel signal-to-noise ratio (SNR), we propose the use of the theoretical bit error rate performance of the maps as the objective function to be optimized by GP. Using this objective function, we can design maps for different conditions including channel SNR levels and number of users. We show that GP can design maps at degraded system conditions, which also work well at better system conditions. We compare the performance of the GP generated maps with other chaotic maps, as well as the traditionally used gold code through simulations. Finally, we apply GP to design maps in scenarios where mathematical design procedures cannot be applied. Using simulated channels under different channel conditions such as colored noise and multipath fading scenarios, we use GP to design maps whose performance is better than the other approaches. Thus, we show that GP can be used to design maps for different channel conditions and is therefore a promising means of solving the system design problem in communication systems.

Differentially coherent PN code acquisition based on a matched filter for chip-asynchronous DS/SS communications

A pseudonoise code acquisition technique based on a matched filter with differentially coherent detection is proposed for direct-sequence spread-spectrum communications in chip-asynchronous environments. Such a technique can be confidently exploited as an alternative to noncoherent detectors, which have been extensively investigated, because it can provide very attractive improvements over its noncoherent counterpart as well as overcome the problems caused by chip asynchronization without increasing hardware complexity. The results of statistical analyses show that the proposed technique certainly can achieve much lower error probabilities.

Cyclostationary model based multiple-tone interference rejection in DSSS communication systems

Multiple-tone interference is a kind of narrow band interference appears in Direct Sequence (DS) Spread Spectrum (SS) communication system frequently that degrades the system performance. Most open literature studies on narrowband interference rejection assumed stationary SS signal and interference. SS signal and interference are modeled as cyclostationary and based on cyclostationary model, an interference rejection algorithm is proposed. Through computer simulation, it is shown that the performance of the SS system can be improved considerably after adopting this algorithm.

A comparison of slow-frequency-hop and direct-sequence spread-spectrum packet communications over doubly selective fading channels

The performance is compared for slow-frequency-hop and direct-sequence spread-spectrum packet communication systems of similar complexity with doubly selective fading channels characterized by several different multipath delay profiles and Doppler spreads. The delay profiles are chosen to reflect experimental measurements of mobile-radio channels, and the Doppler spreads are chosen to represent the range of carrier frequencies and vehicle speeds encountered in common mobile communication systems.

Overcoming Wireless LAN Security Vulnerabilities

The IEEE 802.11b specification represents one of three wireless LAN standards developed by the Institute of Electrical and Electronic Engineers. The original standard, which was the 802.11 specification, defined wireless LANs using infrared, Frequency Hopping Spread Spectrum (FHSS), and Direct Sequence Spread Spectrum (DSSS) communications at data rates of 1 and 2 Mbps. The relatively low operating rate associated with the original IEEE 802.11 standard precluded its widespread adoption.

Cyclostationary model in DS spread spectrum communication system

During the past research, Direct Sequence (DS) Spread Spectrum (SS) signal is modeled as stationary in most cases. Cyclostationary model for SS signal is introduced in this letter. Compared with stationary model, cyclostationary model can preserve more information, and its advantages are shown through example.

Noncoherent sequential PN code acquisition using sliding correlation for chip-asynchronous direct-sequence spread-spectrum communications

Noncoherent sequential pseudonoise (PN) code acquisition using sliding correlation is proposed in this paper. Noncoherent detection and chip asynchronization should be taken care of to handle a severely noisy environment, while frequency offset and data modulation effects can be simultaneously dealt with in the proposed technique. To realize sequential detection, the cross-correlation sequences at the output end of the integrate/dump (LID) filter under out-of-lock conditions have to be modeled as either a Gaussian random sequence for chip-asynchronous applications or as their upper bound for chip-synchronous applications in order to avoid significantly high probabilities of false alarm caused by the conventional zero-sequence model. Meanwhile, the in-lock sequence also has to be modified by taking the frequency offset and chip-asynchronization effects into account in order to avoid the occurrence of high probabilities of miss. Extensive computer simulation results indicate that the proposed technique can achieve low probabilities of false alarm and miss and can outperform its fixed-sample-size (FSS) counterparts by roughly 2/spl sim/4 dB. This superiority, furthermore, increases with decreasing SNRs and/or decreasing desired error probabilities.

Direct-sequence spread-spectrum communications for multipath channels

Direct-sequence spread spectrum has been adopted for many current and future cellular CDMA communication systems, and it is also used widely for military communication networks and systems. One of the motivations for employing direct-sequence spread spectrum is its ability to combat fading due to multipath propagation. The use of direct-sequence spread spectrum to resolve multipath signals is discussed and illustrated. The role of a rake receiver is described, and tradeoffs in the selection of the chip rate for the spread-spectrum system are discussed.

The effect of automatic gain control on serial, matched-filter acquisition in direct-sequence packet radio communications

The performance of a noncoherent serial acquisition technique is evaluated for direct-sequence spread-spectrum packet communications. The acquisition technique that is considered uses threshold crossing of a matched-filter output to detect a fixed-length preamble at the start of each packet. The analysis accounts for frequency mismatch between the transmitter and the receiver due to oscillator inaccuracies and mobility-induced Doppler shifts. It also accounts for the effects of automatic gain control (AGC) in the receiver. The role of the AGC system in determining the acquisition performance is examined. In addition, selection of the optimal acquisition threshold is considered, and a simple method for selection of a good suboptimal threshold is presented. It is shown that use of this threshold results in performance close to that obtained with the optimal threshold over a wide range of channels.

Blind adaptive multiuser detection for DS/SSMA communications with generalized random spreading

A new spreading scheme and an accompanying blind adaptive receiver structure are proposed for direct-sequence spread-spectrum multiple-access communications in a slowly-varying, frequency-selective fading channel. Each user's spreading sequence is given by the Kronecker product of a long-period pseudonoise (PN) sequence, which is accurately modeled by a random sequence, and a short-length deterministic signature code. This spreading scheme bridges the gap between pure PN spreading and pure short-code spreading schemes. It is shown that with this spreading scheme, the channel response to the desired signal component is easily estimated without relying on the spectral decomposition of the signal correlation matrix. With the estimate of the channel response, the receiver suppresses interference based on the maximum signal-to-interference ratio criterion. The blind adaptive receiver requires only coarse timing information and a priori knowledge of the desired user's PN sequence for adaptation. Numerical results show that the adaptive receiver significantly suppresses interference by successfully estimating the channel response and the interference statistics with a low computational complexity. An extension to spatio-temporal processing using an array antenna is also discussed.