Direct Sequence Spread Spectrum Systems Research Articles

Detection of Signals with Chaotic Varying Forms and Low Intercept Probability

Low probability of intercept performance of direct-sequence spread-spectrum system with chaotic spreading sequences is investigated. The energy detectors, synchronous and asynchronous, coherent and non-coherent structures are studied here to detect the presence of chaotic direct-sequence spread-spectrum signals. Simple detection approach using a binary correlation function to detect nonbinary chaotic sequences is proposed. The expressions of detection probabilities of chaotic spreading signals are derived. Comparisons between systems using chaotic and binary sequences are given in terms of the low probability of intercept performance, and the performance improvement with chaotic spreading sequences is observed.

Intelligent Search Strategy for Doppler Frequency Based on Fuzzy Logic in DSSS Signal Acquisition

This study focuses on Doppler frequency search strategy for signal acquisition in direct sequence spread spectrum (DSSS) systems. According to DSSS signal accumulation energy vibration characteristics, the whole two-dimension search region can be divided into non-vicinity region, transition region and vicinity region. If only one search strategy is used for the whole region, it is difficult to balance the frequency search times and the frequency estimation accuracy. To solve the above problem, this study proposes an intelligent Doppler frequency search (IDFS) strategy. A fuzzy logic (FL) controller with two sets of fuzzy parameters is utilized to adjust the frequency search step size in the non-vicinity region and vicinity region, respectively. In the non-vicinity region, by jointly considering signal amplitude and code phase to design the fuzzy parameters of the FL controller, the IDFS strategy can reduce the frequency search times while reducing frequency false detection caused by abnormal change of signal energy. In the vicinity region, the fuzzy parameters are designed based on our previous work which aims to achieve higher frequency estimation accuracy. The effectiveness of the IDFS strategy is evaluated through Monte Carlo simulations. Simulation results show that the proposed intelligent search strategy can reduce the frequency search times while ensuring frequency estimation accuracy compared to other existing search strategies. Moreover, the proposed search strategy can reduce false detection in the non-vicinity region.

Fast acquisition method using modified PCA with a sparse factor for burst DS spread-spectrum transmission

To improve the acquisition speed and inbound capacity of the ground station in a burst direct-sequence (DS) spread-spectrum transmission system, an acquisition method based on a modified parallel code-phase acquisition (PCA) scheme is proposed. By taking advantage of the sparsity of the acquisition result with PCA in the time domain, we introduce the sparse factor to handle the signals via sparsification and apply the sparse recovery algorithm to search and estimate the acquisition result. The computational complexity, mean acquisition time, and relationship between the inbound capacity and acquisition performance are provided. We theoretically analyse the effect of the sparse factor on the acquisition performance. The estimation errors verify our analysis, and simulations show that the acquisition time of our proposed method outperforms that of advanced PCA by 1.2–4.0 times; additionally, the inbound capacity increases by 6.2–36.7%.

Gaussian-Distributed Spread-Spectrum for Covert Communications.

Covert communication techniques play a crucial role in military and commercial applications to maintain the privacy and security of wireless transmissions from prying eyes. These techniques ensure that adversaries cannot detect or exploit the existence of such transmissions. Covert communications, also known as low probability of detection (LPD) communication, are instrumental in preventing attacks such as eavesdropping, jamming, or interference that could compromise the confidentiality, integrity, and availability of wireless communication. Direct-sequence spread-spectrum (DSSS) is a widely used covert communication scheme that expands the bandwidth to mitigate interference and hostile detection effects, reducing the signal power spectral density (PSD) to a low level. However, DSSS signals possess cyclostationary random properties that an adversary can exploit using cyclic spectral analysis to extract useful features from the transmitted signal. These features can then be used to detect and analyse the signal, making it more susceptible to electronic attacks such as jamming. To overcome this problem, a method to randomise the transmitted signal and reduce its cyclic features is proposed in this paper. This method produces a signal with a probability density function (PDF) similar to thermal noise, which masks the signal constellation to appear as thermal white noise to unintended receivers. This proposed scheme, called Gaussian distributed spread-spectrum (GDSS), is designed such that the receiver does not need to know any information about the thermal white noise used to mask the transmit signal to recover the message. The paper presents the details of the proposed scheme and investigates its performance in comparison to the standard DSSS system. This study used three detectors, namely, a high-order moments based detector, a modulation stripping detector, and a spectral correlation detector, to evaluate the detectability of the proposed scheme. The detectors were applied to noisy signals, and the results revealed that the moment-based detector failed to detect the GDSS signal with a spreading factor, N = 256 at all signal-to-noise ratios (SNRs), whereas it could detect the DSSS signals up to an SNR of -12 dB. The results obtained using the modulation stripping detector showed no significant phase distribution convergence for the GDSS signals, similar to the noise-only case, whereas the DSSS signals generated a phase distribution with a distinct shape, indicating the presence of a valid signal. Additionally, the spectral correlation detector applied to the GDSS signal at an SNR of -12 dB showed no identifiable peaks on the spectrum, providing further evidence of the effectiveness of the GDSS scheme and making it a favourable choice for covert communication applications. A semi-analytical calculation of the bit error rate is also presented for the uncoded system. The investigation results show that the GDSS scheme can generate a noise-like signal with reduced identifiable features, making it a superior solution for covert communication. However, achieving this comes at a cost of approximately 2 dB on the signal-to-noise ratio.

High performance bit‐activation code index modulation method

AbstractWith the increasing demand of applications for the spread spectrum technique, especially the demand for data transmission rates and spectral efficiency, the advantages of the traditional direct sequence spread spectrum (DSSS) system are limited. Therefore, multi‐ary spread spectrum (M‐ary) technology, parallel combinatory spread spectrum (PCSS) technology, and code index modulation (CIM) technology have been proposed. Although these three new technologies can improve the data rate, they all face the problem of the large consumption of pseudo‐code resources. In order to solve the problem of pseudo‐code resources, a bit‐activation code index modulation (BA‐CIM) method is proposed. At the transmitter, considering the good correlation among multiple pseudo‐codes, the corresponding pseudo‐code activation principle is established, and the corresponding spreading pseudo‐code is activated by using the status of each bit of the index data according to the pseudo‐code activation principle. Then, multicode superposition processing is carried out to spread the modulation data. At the receiver, the corresponding activation pseudo‐code is obtained using the maximum peak‐to‐average ratio (MPAR) and secondary peak‐to‐average ratio (SPAR) judgement mechanisms to decode the multibit index data. Compared with existing methods, the proposed BA‐CIM method can not only achieve a better bit error rate performance but also use the least pseudo‐code resources. Moreover, BA‐CIM has the best comprehensive performance improvement and is far superior to other methods. This research can provide technical support for the application of efficient spread spectrum communication.

Efficiently All-Digital Code Tracking for Band-Limited DSSS Systems

In this letter, we have studied the all-digital code tracking problem for band-limited direct sequence spread spectrum (DSSS) systems. Band-limited DSSS systems have many advantages over rectangular-pulse DSSS systems. However, the current all-digital code tracking methods for band-limited DSSS systems have more computational complexity than the latter. Therefore, we propose an efficiently all-digital code tracking method for band-limited DSSS systems where we implement despreading in the frequency domain with others and take advantage of the narrow time-width characteristic of the signal autocorrelation to achieve a lower complexity and constant tracking performance, i.e., the constant steady-state time-delay error variance at the same time. We also give a closed-form expression of the steady-state time-delay error variance of the proposed method in this letter, which can be used to analyze the performance of the proposed method theoretically and design proper band-limited DSSS systems. The theoretical analysis and simulation results show that the proposed method has lower computational complexity and constant tracking performance.

A Superresolution Multipath Estimation Algorithm for DSSS Systems

In direct sequence spread spectrum systems, the multipath interference is known to be one of the most important factors in degrading the measurement accuracy. To resolve this problem, we propose a superresolution multipath estimation method based on waveform replacement and adaptive-filtering. In this method, the continuous pseudorandom noise code autocorrelation waveform of the matched filtering output is first replaced by a designed triangular pulse waveform. Subsequently, a reiterative minimum mean-square error adaptive filtering approach is applied to realize superresolution estimation of multipath parameters. The proposed method can achieve accurate multipath estimation even in complex multipath scenarios containing short-delay multipaths and make a good balance between the computational complexity and the estimation performance of multipath parameters. Simulation results verify the effectiveness of the proposed method.

Efficient LDPC-Coded CCSK Links for Robust High Data Rates GNSS

Global navigation satellite system links may require increased data rates to accommodate future features and needs (e.g., precise positioning, authentication, reduction of time-to-first-fix data). A particular form of $M$-ary orthogonal modulation designed for direct-sequence spread-spectrum (DSSS) systems, the cyclic code-shift keying (CCSK) modulation, has been proposed for this purpose. This modulation inherently allows noncoherent processing at receiver side and has the potential to improve the energy efficiency of the data link with respect to classical DSSS/BPSK signals. In this article, $q$-ary ($q\in \lbrace 2,M\rbrace$) low-density parity-check (LDPC)-based channel coding for $M$-ary CCSK is analyzed, both in terms of robustness and computational complexity. $(q=M)$-ary LDPC-coded CCSK is compared to a bit-interleaved binary LDPC-coded CCSK strategy. Though both solutions provide very reliable links for practical decoding algorithms, it is shown that adequately designed bit-interleaved binary LDPC-coded CCSK signals can offer the additional flexibility inherent to bit-interleaved coded modulation (BICM) while remaining competitive from the point of view of both error rate performance and computational complexity. The latter can be adjusted through the use of incomplete iterative demapping schedules. The optimization of this performance/complexity tradeoff is discussed.

Novel Algorithm for Blind Estimation of Scramblers in DSSS Systems

In this paper, we propose a novel algorithm for blind estimation of a linear scrambler, which is a synchronous scrambler or a self-synchronous scrambler, in direct sequence spread spectrum systems and analyze its estimation performance. We first examine the statistics considering the linearity of the scrambled data and the repetition property of the spreading code. Based on this, we then propose an improved estimation algorithm for the feedback polynomial of the linear scrambler constituting a search process and a verification process of feedback polynomial candidates to determine the correct feedback polynomial of the scrambler. To validate the proposed algorithm, we show through computer simulations that the proposed algorithm outperforms the conventional algorithm in terms of estimation performance.

UNJUK KERJA KODE ORTHOGONAL M-SEQUENCE DALAM SISTEM DIRECT SEQUENCE SPREAD SPECTRUM MELALUI KANAL MULTIPATH FA

Multipath and multi-user interference are the main challenges in wireless communication systems. This has become a driving force for the development of data transmission technology that is focused to overcome these problems, one of which is spread spectrum. Spread spectrum is applied to CDMA technology which has the limitations of MAI, so a spreading code is required with specifications that have a very low cross-correlation value as in the orthogonal m-sequence code. In this study, simulate the orthogonal m-sequence code with a length of 8 chips on a multipath fading channel using the Direct Sequence Spread Spectrum system with aims of assessing the performance of the orthogonal m-sequence code on a multipath fading channel, based on the effect of changes in the number of multipath components. The research method used is a Monte Carlo simulation on simulink with the output data in the form of a graph of the comparison value of BER with Eb/No. The results show that on the flat fading channel with the number of components multipath 1, the BER value of the simulation results is in accordance with the BER theory so that the simulation is valid and can be applied to frequency selective fading channels. In the frequency selective fading channel with the number of multipath components 4,6, and 8, the best performance was shown in the multipath 4 component with the smallest BER value, this was influenced by the ACF and CCF values so that the multipath 4 component had the lowest ISI and interference between users.

Code index modulation method with multidimensional invisible data mapping

With the continuous improvement of the application requirements of Direct Sequence Spread Spectrum (DSSS), especially the high requirements of data transmission rate, the advantages of traditional DSSS system are limited, so the index modulation DSSS technology appears. However, at present, there are still some key problems such as high-order data rate, high bit error rate and high synchronisation complexity. Therefore, in view of the above bottleneck problems, the code index modulation method with multidimensional invisible data (MID-CIM) is proposed. On the basis of deeply mining the hidden features of DSSS communication mechanism, the index order and shift order are established, and the two-dimensional invisible data index transmission rules are established using a bit reversed and pseudo code (PN) set mapping mechanism, and then the three-dimensional invisible data index transmission rules are established by using the cyclic displacement and pseudo code offset mechanism. The MID-CIM method not only ensures the effective transmission of one-dimensional data, but also uses the transmission of low-speed one-dimensional data to index transmit high-speed two-dimensional and three-dimensional invisible data. This research can provide the technical basis for the efficient application of DSSS system, and also provide theoretical support and technical support for the new development and application of a communication system based on the spread spectrum system.

A New Scheme for Spreading & De-spreading in the Direct Sequence Spread Spectrum Mechanism

Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS) techniques are widely used to implement code-division multiple access (CDMA) in wireless communication systems. Both DSSS and FHSS systems help reducing the effects of interference on the transmitted information making it robust against channel impairments. DSSS uses a signal bandwidth that is much broader than the information signal bandwidth. Traditionally, the wide band signal is generated by multiplying the narrowband information signal with a binary code, often designated as a spreading code, to generate the wideband signal that is transmitted. The original information signal can be recreated at the receiver by multiplying the received wideband signal by the same binary code (now designated as a de-spreading code) used to generate the wideband transmitted signal. To extract the original information signal, the spreading and de-spreading codes must be in synchronism at the receiver and amplitude match with each other. A new modification for the direct sequence spread spectrum is proposed in this paper. The mechanism introduced in this approach implicates generating the wideband signal by circularly shifting the spreading code (PN) by n places, where n represents the value of the current byte of information signal. The yielded signal is modulated using BPSK modulator before transmitting it. The original information signal is extracted at the receiver by correlating the received signal (which is actually the original spread sequence circularly shifted by n places) with a locally generated replica of the spreading code. The position of the maximum value of the cross-correlation vector represents the value of the information signal byte. The proposed configuration has been implemented using Simulink simulator and the obtained results show that its performance is identical with the conventional DSSS.

Acquisition of Time and Doppler Shift in WFRFT Encrypted Chaotic Direct Sequence Spread Spectrum System

Weighted fractional Fourier transform encrypted chaotic direct sequence spread spectrum (WFRFT-CD3S) signal is aperiodic and complex Gaussian distributed. The aperiodicity and complex Gaussian distribution are conducive to the covert transmission of information but also make it difficult to acquire the WFRFT-CD3S signal. An acquisition method implemented in two steps is proposed to acquire the Doppler-shifted WFRFT-CD3S signal. First, a bidirectional correlation search method is proposed to align the received signal with the local spreading sequence in the time domain. The correlation peak attenuation caused by the Doppler frequency shift is eliminated by differential. Second, the accumulated phase angle caused by the Doppler frequency shift is obtained by the differential correlation value between the aligned received signal and the local spreading sequence. Then, the Doppler frequency shift is estimated by the accumulated phase angle. The detection probability, the false alarm probability, and the root mean square error (RMSE) of the Doppler shift estimation are analyzed theoretically and simulated. The theoretical results are verified by simulation. Theoretical results and simulations show that the detection probability is higher than 0.999, the false alarm probability is less than 0.001, and the RMSE of Doppler shift estimation is lower than 63 Hz when the signal-to-noise ratio is higher than -7.3 dB.

Blind Estimation of Self-Synchronous Scrambler Using Orthogonal Complement Space in DSSS Systems

In a non-cooperative context, a receiver has to estimate the communication parameters without any prior knowledge of the transmitter, which is highly demanding. Estimating a self-synchronous scrambler is even more challenging because the scrambling sequence of the self-synchronous scrambler is affected by the input sequence. This paper proposes an improved algorithm for blind estimation of a self-synchronous scrambler using the orthogonal property without any bias condition of the received signals in direct sequence spread spectrum (DSSS) systems. We first examine the linear relation of a scrambling sequence using the repetitive property of the spreading code used in a DSSS system. Using the obtained linear relation and the basis of the orthogonal complement space, we then acquire the feedback polynomial candidates of the scrambler. Finally, by calculating the greatest common divisor polynomial of the feedback polynomial candidates, we estimate the correct feedback polynomial. Through computer simulations, we verify that the proposed method achieves superior estimation performance compared to the existing method. Furthermore, we show that the proposed method has practically acceptable computational complexity. For these reasons, it is expected that the proposed method can be applied to blind estimation of a self-synchronous scrambler in a practical non-cooperative system.

DETECTION OF THE DSSS SIGNAL WITH VARIABLE PN CODES USING THE ANN

- Direct sequence spread spectrum systems appeared and are used to protect the transmitted data DSSS Systems might be one of the solutions for reliable and secured communications. Also, it is one of the approaches used by signals for transmitting bandwidth larger compared to the satisfied frequency related to the original information. The communication systems of SS were vital to suppress interference, complicating the detection and processing of secure communications, the technology of spread spectrum (DSSS) has been initially created for military applications. In a traditional DSSS system, the PN code is the primary key to make the receiver recover the transmitted data. In this paper, by using the MATLAB R2020a is used to simulate the proposed system, it is considered that the transmitter sends data bits and wants to protect the sent data by making each bit send with a PN code consisting of 127 bits randomly without informing the receiver of that. here the artificial neural network (ANN) was used as a tool to find the PN code for each initial value of 7 flip-flops. so, the receiver could detect the transmitted data with BER =0.

Optimization of Spreading Code Using Modified Differential Evolution for Wireless Communication

Spread spectrum linked to optimization techniques play a very important role in today’s world. Optimization gives the best one among all the solutions and the spread spectrum is used to minimize distortion, hence optimized spreading code is very important in wireless communication. This study shows a comparison of two recent modified techniques of differential evolution (DE) with the help of engineering design problems and application of that modified DE, considering the better one, in wireless communication to optimize logistic map based spreading code. A comparative study of properties of both optimized and non-optimized spreading code is also discussed in this paper. The performance of the optimized spreading code is also evaluated with the help of bit error rate (BER) by applying it in static and dynamic direct sequence spread spectrum system (DSSS). The performance of the proposed technique (optimized dynamic logistic map code based DSSS) displays better results than non-optimized spreading code and orthogonal spreading code considering BER. Static optimization is improved by 40% than non-optimized static, dynamic optimization is improved by 70% than static optimization but orthogonal is showing 33% improvement comparing to static optimization and dynamic optimization is improved by 56% than orthogonal one. Therefore, the proposed method can be efficiently applied in the wireless communication system.

Double-differential coded M-ary direct sequence spread spectrum for mobile underwater acoustic communication system

This paper presents a double-differential coded M-ary direct sequence spread spectrum (DD-MSS) underwater acoustic communication system to overcome problems of both low signal-to-noise ratio and severe Doppler effect. The system is very efficient in the challenging band-limited underwater acoustic channel and has many applications, particularly in which the transmitter and the receiver are mobile. Given that the bit error rate (BER) performance is unaffected, this scheme results in a significant improvement in the data rate compared to direct sequence spread spectrum (DSSS) communication systems. The MSS system uses M pseudo-random codes to represent k bits of information and offers advantages of a higher spectral efficiency and a lower energy consumption than the DSSS system. Both simulations and experiments are performed, and the results are discussed in the paper for the performance evaluation of the proposed scheme. The results demonstrate a better BER performance using the DD-MSS in a mobile environment.

Low-power DSSS transmitter and its VLSI implementation

An interesting area of application in wireless data communication is direct-sequence spread spectrum (DSSS). Spread spectrum communication techniques make the signals more robust against interference and jamming. These are based on a concept that narrowband signal is scrambled before transmission in such a way that the signals occupy a much larger part of the radio frequency spectrum. As the digital and the analogue system components are required on the same substrate in today’s mixed-signal chips, the DSSS transmitter system is proposed to be implemented in field-programmable gate array (FPGA)–based platforms and application-specific integrated circuits (ASICs). With a low-power very large-scale integration (VLSI) architecture, sophisticated processing of wide-bandwidth DSSS systems can be exploited in FPGAs/ASICs. In this article, binary pseudo-noise (PN) sequences are generated using a low-power linear feedback shift register (LFSR) in order to spread transmit signals extensively. The proposed low-power design of LFSR and DSSS transmitter with implementation results is illustrated in this paper. Dynamic power dissipation of the proposed DSSS transmitter is reduced up to 15% and 15.6% when compared to the conventional LFSR and the Gold code–based systems respectively. The proposed hardware is implemented in 180-nm technology and operates at 15.36-MHz frequency.

Low Complexity Robust Data Demodulation for GNSS.

In this article, we provide closed-form approximations of log-likelihood ratio (LLR) values for direct sequence spread spectrum (DS-SS) systems over three particular scenarios, which are commonly found in the Global Navigation Satellite System (GNSS) environment. Those scenarios are the open sky with smooth variation of the signal-to-noise ratio (SNR), the additive Gaussian interference, and pulsed jamming. In most of the current communications systems, block-wise estimators are considered. However, for some applications such as GNSSs, symbol-wise estimators are available due to the low data rate. Usually, the noise variance is considered either perfectly known or available through symbol-wise estimators, leading to possible mismatched demodulation, which could induce errors in the decoding process. In this contribution, we first derive two closed-form expressions for LLRs in additive white Gaussian and Laplacian noise channels, under noise uncertainty, based on conjugate priors. Then, assuming those cases where the statistical knowledge about the estimation error is characterized by a noise variance following an inverse log-normal distribution, we derive the corresponding closed-form LLR approximations. The relevance of the proposed expressions is investigated in the context of the GPS L1C signal where the clock and ephemeris data (CED) are encoded with low-density parity-check (LDPC) codes. Then, the CED is iteratively decoded based on the belief propagation (BP) algorithm. Simulation results show significant frame error rate (FER) improvement compared to classical approaches not accounting for such uncertainty.

Blind Estimation of Self-Synchronous Scrambler in DSSS Systems

Self-synchronous scramblers are more difficult to blindly estimate than synchronous scramblers because their input sequence affects the state of the scrambler's linear feedback shift register. There has been much research on the estimation of synchronous scramblers and relatively little on the estimation of self-synchronous scramblers. For non-cooperative contexts, this paper proposes a method for the estimation of a self-synchronous scrambler in direct sequence spread spectrum systems. We blindly estimate the self-synchronous scrambling parameter, the feedback polynomial, using the repeated patterns by a spreading code, the linearity between the bits in a scrambling sequence, and the coidentity of the self-synchronous scrambled sequence and the linear feedback shift register states. To validate the proposed method, we show the estimation performance in terms of the computational complexity, required minimum scrambled sequence length, execution time, and detection probability.