Spread Spectrum Communication System Research Articles

The pupil outdoes the master: Imperfect demonstration-assisted trust region jamming policy optimization against frequency-hopping spread spectrum

Jamming decision-making is a pivotal component of modern electromagnetic warfare, wherein recent years have witnessed the extensive application of deep reinforcement learning techniques to enhance the autonomy and intelligence of wireless communication jamming decisions. However, existing researches heavily rely on manually designed customized jamming reward functions, leading to significant consumption of human and computational resources. To this end, under the premise of obviating designing task-customized reward functions, we propose a jamming policy optimization method that learns from imperfect demonstrations to effectively address the complex and high-dimensional jamming resource allocation problem against frequency hopping spread spectrum (FHSS) communication systems. To achieve this, a policy network is meticulously architected to consecutively ascertain jamming schemes for each jamming node, facilitating the construction of the dynamic transition within the Markov decision process. Subsequently, anchored in the dual-trust region concept, we design policy improvement and policy adversarial imitation phases. During the policy improvement phase, the trust region policy optimization method is utilized to refine the policy, while the policy adversarial imitation phase employs adversarial training to guide policy exploration using information embedded in demonstrations. Extensive simulation results indicate that our proposed method can approximate the optimal jamming performance trained under customized reward functions, even with rough binary reward settings, and also significantly surpass demonstration performance.

Simulation study of spread spectrum communication on satellite

Abstract All the basic principles and simulation models of the PMF-FFT capture algorithm, overlap-transform interference suppression, digital matched filter capture algorithm, delayed phase-locked loop tracking algorithm and Costas ring carrier synchronization in the starborne spread spectrum communication system are elaborated in detail. The waveforms and spectral variations in each stage of spread spectrum communication are simulated and verified using the Simulink tool. The results show that the model can achieve interference suppression, fast capture tracking of pseudocodes, and accurate synchronization of carriers at -30 dB SNR.

Sidelobes Reduction Technique for Binary Codes

Pulse compression techniques enable the radar designers to transmit long duration pulse to cater the need of high power at the transmitter and achieve the range resolution of short duration pulse at the radar receiver simultaneously. Due to easy generation and processing, binary codes are widely used as pulse compression codes in radars, sonars and spread-spectrum communication systems. But binary codes have high unwanted pulse compression sidelobes at the output of pulse compression receivers. In this paper, optimum binary codes reported in the literatures are overlaid on Discrete frequency sequences to reduce unwanted peaks sidelobes of optimum binary codes. The proposed approach not only reduced peak sidelobes of optimum binary codes but also increased the pulse compression ratio. In this paper, optimum discrete frequency sequences for overlaying with binary codes are optimized using the Modified Simulated Annealing Algorithm (MSAA). A Discrete Frequency Coded binary signal has a complex signal structure which will be difficult to detect and analyze by enemy Electronics Support Measures (ESM). In this paper, a unified adaptive radar model is also proposed to cater the needs of today's radar signal design requirements. The proposed model will help to improve range resolution, the detection capability of radar, reduce the peak sidelobes significantly, and also improve the ESM capability of radar systems.

Accumulation and Elimination: A Hard Decision-Based Multi-User Interference Cancellation Method in Satellite Communication System

With the increasing number of users in the Medium-Orbit (MEO) satellite communication system, multi-access interference (MAI) has become an important factor that restricts the reliability and capacity of the system. Additionally, the low carrier-power-to-noise-density ratio (C/N0) resulting from long-distance transmission poses a significant concern. The parallel interference cancellation (PIC) algorithm, utilized within the paradigm of multi-user detection (MUD), exhibits the capability to effectively mitigate the impact of MAI within the same system. Simultaneously, coherent accumulation serves as a means to substantially enhance the correct detection probability (Pcd) at low C/N0. In this study, a signal acquisition method for multi-user spread spectrum satellite receivers is proposed, which employs interference cancellation and coherent accumulation as its core mechanisms. Furthermore, we introduce a power estimation method based on the outcomes of signal acquisition, which can be integrated into the signal reconstruction module of PIC. Finally, we implement the aforementioned algorithms in both simulation and hardware platforms. Remarkably, we observe that when the interference-to-signal ratio (ISR) caused by MAI equals 20 dB, the improved algorithm attains a maximum Pcd of 0.95 within the high signal-to-noise ratio (SNR) region, closely approaching the theoretical limit for the bit error rate (BER). The experimental results prove the effectiveness and feasibility of the acquisition algorithm. In summary, the enhanced algorithm holds vast potential for widespread implementation in multi-user spread spectrum communication systems.

The line-of-sight peak detection and tracking of underwater acoustic DSSS communications in the doubly spread channel

In the underwater acoustic (UWA) channel, the line-of-sight (LOS) path has the characteristics of stable magnitude and continuous phase, which is seldom affected by the surface/bottom reflection. Therefore, in the direct sequence spread spectrum (DSSS) communication systems, the receiver usually extracts the LOS path to recover the information bits and achieve the robust communication. However, the UWA channel is typically the doubly spread channel, i.e., multipath fast fading and non-uniform Doppler shifts, which seriously degrades the performance of extracting the LOS path by the conventional maximum energy method. Therefore, this paper proposes a LOS peak detection and tracking algorithm to extract the LOS path in the UWA DSSS communications. In the proposed algorithm, the candidate correlation peaks after de-spreading are firstly selected by the adaptive peak detection threshold. Then, according to the transmitter/receiver motion relationship, the Delay-Doppler joint state equation is established, and thus the nearest neighbor method adopting the Kalman filter is proposed to track the LOS path. Numerical simulation results demonstrate that the proposed algorithm has better processing performance in the doubly spread channels compared with the conventional method. Finally, the performance of the proposed algorithm is confirmed by the shallow water horizontal channel and deep sea vertical channel experiments.

Design of spread spectrum communication system based on multipath signal synthesis

AbstractA low‐complexity anti‐multipath interference spread spectrum communication system is proposed to address shortcomings in traditional anti‐multipath technology, such as complex structures and high resource consumption. The system utilizes a fast Fourier transform to capture pseudo‐noise code signals and simultaneously detect multipath signals. Moreover, the signal‐to‐noise ratio (SNR) of each path signal is calculated, and the multipath signal is synthesized through the designed optimal SNR synthesis method to improve the SNR of the demodulated signal to reduce the bit error rate. MATLAB is used to perform functional simulations of the system to verify its feasibility. The simulation results show that the system can successfully detect and synthesize multipath signals and achieve lower communication error rates than traditional code division multiple access (CDMA) systems.

Short-Block-Length Low-Density Parity-Check Codes-Based Underwater Acoustic Spread-Spectrum Communication System

Low-density parity-check (LDPC) codes are commonly used in communication systems to improve the system performance, but LDPC codes takes too long for decoding, making communication inefficient and unsuitable for short-frame data transmission methods. In underwater acoustic channels, spread-spectrum communication becomes an effective way to realize long-distance communication. This paper combines short-block LDPC codes with a direct sequence spread spectrum and soft spread spectrum in underwater acoustic communication, addressing the problem of the inapplicability of conventional LDPC codes. The applicability of the proposed method in this paper is verified through simulation tests and pool experiments. The results indicate that the proposed communication system achieves lower bit error rates compared to the classical coding methods used in underwater acoustic spread-spectrum communication systems under the same channel conditions.

Analytical Technique Leveraging Processing Gain for Evaluating the Anti-Jamming Potential of Underwater Acoustic Direct Sequence Spread Spectrum Communication Systems

This study proposes an analytical technique underpinned by processing gain to evaluate the anti-jamming potential of an underwater acoustic direct-sequence spread-spectrum (DSSS) communication system that employs a short-period pseudo-noise (PN) sequence. The processing gain comes from the symmetry of the coding, which provides a mechanism for separating desired signals from unwanted ones, and the apparent randomness of the coding, which suppresses interference and noise in the system. The robustness of such a system against wideband interference, partial-band jamming, and single-frequency interference is emulated. Outcomes suggest that, in comparison to a standard binary phase shift keying (BPSK) system, the DSSS system’s ability to resist wideband interference is limited, with only a marginal increase in immunity performance of approximately 0.5 dB. Contrarily, it suppresses partial-band jamming effectively, with the suppression level dependent on the interference bandwidth and its relative position concerning the signal carrier frequency. The influence of single-frequency interference on system performance depends similarly on its relative location relative to the signal carrier frequency. In all situations where the interference frequency offset is an integer multiple of the bit bandwidth, the system exhibits the worst performance when the frequency offset equals the bit bandwidth. Upon comparing resistance levels to identical power interferences targeted at the signal carrier frequency, our system demonstrates optimal resilience to single-frequency interference. In concordance with the empirical findings, the simulated results substantiate both the effectiveness and practicability of the proposed analytical method based on processing gain. Subsequently, this study contributes a novel perspective for evaluating the anti-jamming potential of DSSS systems.

Software-Defined Radio Implementation and Performance Evaluation of Frequency-Modulated Antipodal Chaos Shift Keying Communication System

This paper is devoted to software-defined radio (SDR) implementation of frequency modulated antipodal chaos shift keying (FM-ACSK) transceiver and presents results of prototype testing in real conditions. This novel and perspective class of spread-spectrum communication systems employs chaotic synchronization for the acquisition and tracking of the analog chaotic spreading code and does not need resource-demanding cross-correlation. The main motivation of the given work is to assess the performance of FM-ACSK in real conditions and demonstrate that chaotic synchronization can be considered an efficient spread-spectrum demodulation method. The work focuses on the real-time implementation aspects of the modulation-demodulation algorithms, forward error correction (FEC) and symbol timing synchronization approach in MATLAB Simulink. The performance of the presented prototype is assessed via extensive testing, which includes measurement of bit error ratio (BER) in single-user and multi-user scenarios, estimation of carrier frequency offset (CFO) impact and image transmission over-the-air between two independent sites and comparison with classical frequency hopping spread spectrum (FHSS). The paper shows that the presented class of the spread spectrum communication systems demonstrates good performance in low signal-to-noise ratio (SNR) conditions and in terms of BER significantly outperforms the classic spread-spectrum modulation schemes which employ correlation-based detection.

Iterative double-differential direct-sequence spread spectrum reception in underwater acoustic channel with time-varying Doppler shifts.

Conventional double differential phase-shift keying modulation amplifies the phase noise and performs poorly under the time-varying direct-sequence spread-spectrum (DSSS) communication system. Therefore, the authors propose an iterative reception for DSSS communication in time-varying underwater acoustic channels. First, bit-interleaved coded modulation with iterative decoding integrated with multi-symbol differential detection is used. Second, this paper uses cross correlation method to estimate and track the Doppler shift of each symbol. Based on Doppler estimates, a dynamic linear prediction model is proposed to estimate and track the channel phase variation. Third, an algorithm for adaptive selection of reference signals is utilized to recover the magnitude attenuation of correlation peaks. Numerical simulation results demonstrate that the proposed reception achieves around 9 dB gain compared to conventional differential decision reception under constant acceleration of 0.14 m/s2. During the acoustic communication experiment in Songhua Lake, the proposed reception was tested by using a moving source at a speed of 1-6 knots at 2-m depth and the farthest distance between the transceivers is 2.8 km. The proposed reception achieves only one frame error from a total of 205 frames collected in the lake experiment, and it also achieves error-free communications over 96 frames during a 10 km depth deep-sea experiment.

Airborne Acoustic Chirp Spread Spectrum Communication System for User Identification in Indoor Localization

In this contribution, we propose a reliable, longrange, communication system based on chirp spread spectrum techniques for aerial ultrasonic channels, to be employed for user identification in ultrasonic indoor localization systems. We adopt binary orthogonal keying, where the binary information, modulates the shape of the chirp instantaneous frequency, mapping zeros onto down-chirps and ones onto up-chirps. In order to reduce the demodulation computational costs, we exploit the band-pass sampling theorem and develop an ad-hoc algorithm to under-sample the received signal. The incoming packet detection and frame synchronization are guaranteed by employing a characteristic chirp signal, typically utilized for localization purposes, as a pilot for the transmitted identification packet. A simple, yet efficient, pilot detection scheme enables live features by constantly listening to the channel, triggering the demodulator only when the pilot is detected. The performance of our system has been evaluated in low power and ultra-low power real-world experiments in a warehouse-like environment. It is demonstrated that our system is capable of error-free communication up to a range of 22 m in the low power setup, where the transmitter had a transmission power of 80 mW, corresponding to a sound pressure level of approximately 37 dBSPL at 1 m distance. In the ultra-low power mode, with a transmission power to 15 mW, corresponding to a sound pressure level of approximately 29 dBSPL at 1 m, the system achieved 99% success rate up to 16 m. Moreover, the capability of the system to be robust against the Doppler effect and in a multi-user environment has been tested via simulations.

Deep learning-based M-ary spread spectrum communication system in shallow water acoustic channel

A deep learning-based M-ary spread spectrum (SS) underwater acoustic (UWA) communication system is proposed. The long short-term memory (LSTM) model is used as the receiver of the system, enabling the direct demodulation of received signals without de-carrier and de-spreading operations. LSTM architecture-based neural network models are fed with the time-domain waveform of each symbol during the training process to learn the source information carried by each symbol. Simulation results show that the deep learning-based M-ary SS UWA communication system outperforms the conventional system under the low signal-to-noise ratio and complex shallow water acoustic channel. For optimal performance, different LSTM models can be selected according to the simulation conditions. Furthermore, a water tank experiment simulating the complex shallow water channel was conducted and the results show that experimental equipment, different training sample acquisition methods, and LSTM models can affect the performance of the system.

Space–time receiver for spread spectrum communication systems with beam tracking

A method based on beam tracking in an underwater acoustic (UWA) time-varying channel was proposed to improve the performance of UWA spread spectrum communication systems using arrays. In the ocean, the acoustic signal reaches the receiver through various paths, and the signal on each path has its own propagation delay and incidence angle. To effectively use the time and space characteristics of the received signal in a typical time-varying or all types of UWA channels of mobile communication scenarios, the multiple signal classification (MUSIC) algorithm constantly updates the beam orientation. In addition, the receiving signal beamforming and maximum ratio combination can solve the severe inter-symbol interference in a sea channel and significantly improve the signal-to-noise ratio of the received signal; the UWA spread spectrum communication system is more reliable. Through computer simulation and an actual pool experiment, a space–time receiver for an UWA spread spectrum communication system with beam tracking was successfully implemented.

Time-coding spread-spectrum reconfigurable intelligent surface for secure wireless communication: theory and experiment.

We present a time-coding reconfigurable intelligent surface (RIS) enabled spread-spectrum secure wireless communication system. The time-coding spread-spectrum RIS is realized by altering the RIS between the two states of a perfect electrical conductor (PEC) and perfect magnetic conductor (PMC), according to a pseudo-random time sequence, respectively. This spectrum spreading function of the time-coding RIS can be simply applied to the secure wireless communication systems, providing a low-cost, easy-implementation encrypting architecture with the two key features of interception prevention and interference suppression. These two features together with the overall spread-spectrum secure wireless communication system are demonstrated by both theoretical analysis and experimental results using a designed 8 × 8 time-coding RIS.

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.

Performance Simulation of Underwater Acoustic Spread Spectrum Communication Based on LDPC Code

As we all know, the underwater acoustic channel is a time-varying, space-changing, and frequency-varying fading channel with limited bandwidth and severe noise interference. This paper studies and analyzes the main factors restricting the transmission of underwater acoustic channels, and establishes an underwater acoustic multipath time-varying fading channel. The channel coding adopts Quasi-Cyslic Low-Density Parity-Check Codes (QC-LDPC), combined with spread spectrum technology and high-order modulation technology, and utilizes Log-likelihood Ratios Belief Propagation (LLR-BP) is decoded, the simulation results show that LDPC codes have stronger error correction capabilities in underwater acoustic spread spectrum communication systems, ensuring that the system can operate reliably under low SNR conditions.

Performance of orthogonal stochastic code signals in the presence of delayed interference

This article provides analytical expressions for performance evaluation of hybrid FH-DSSS system in delayed interference and fluctuation noise presence. Delayed interference is quite common in environment with multiple obstacles, occurring as consequence of signal reflection and multipath propagation (fig. 1). Introduction describes known methods of mitigating this interference many of whom include either use of direct sequence spreading or some variation on hybrid sequence spreading including pseudo random frequency hopping and periodical signal ensemble change. To improve on those methods considered dynamic program method for orthogonal stochastic spread spectrum code generation based on Viterby algorithm [6]. Section 1 of the paper describes FH-DSSS signal, it’s delayed interference, equation corresponding to their sum on the receiver input and noncoherent correlator response to it. By using Gaussian approximation, expressions derived for equivalent signal/noise level estimation and corresponding average probability of error at the output of the correlation receiver. By including actual signal statistic, like code length and cross-correlation value, final expressions (8)–(15) allow for accurate estimation of correlation receiver response to signal sequences with specific spread spectrum codes, as well as calculation for probability of error for M-ary spread spectrum communication system in delayed interference presence. Accuracy of the Gaussian approximation is examined in section 2 of the paper based on comparisons between analytical results and simulation modeling. Numerical results obtained, which showcase dependence of system performance on various signal characteristics, such as correlation properties, spreading code length, signal ensemble size and frequency hop delay time. Plotted graph on fig. 2 shows advantage from using M-ary orthogonal signals over binary in delayed interference presence for code sequences with length N = 32, 64, 128. Graph on fig. 3 shows gains from frequency hopping when interference delay equals to approximately half of signal information symbol. In the end performance gains from using hybrid FH-DSSS system under electronic suppression by delayed interference is also evaluated. As shown on fig. 4,a system performance significaly depend on value of delay between received signal and it’s delayed copy.

Synchronization in DSSS system

AbstractDirect sequence spread spectrum (DSSS) communication systems offer huge performance focal points in perspective on their low probability of block, improved performance in multipath fading situations and their capacity to stay away from interference by spreading the signal over a wide bandwidth subsequently conveying the power. For the transmitted sequence to be effectively received and demodulated, the spreading sequence utilized at the receiver ought to be like that utilized in the transmitter. This paper uses MATLAB Simulink to show a technique for synchronizing the code clock at the receiver with the code clock at the transmitter. This fine arrangement procedure is known as code tracking.

Modulation Parameter Estimation of LFM Interference for Direct Sequence Spread Spectrum Communication System in Alpha-Stable Noise

The linear frequency modulation (LFM) interference is one of the typical broadband interferences in direct sequence spread spectrum (DSSS) communication system. In this article, a novel modulation parameter estimation method of LFM interference is proposed for the DSSS communication system in alpha-stable noise. To accurately estimate the modulation parameters, the alpha-stable noise should be eliminated first. Thus, we formulate a new generalized extended linear chirplet transform to suppress the alpha-stable noise, for a robust time-frequency, transformation of LFM interference is realized. Then, using the Radon transform, the maximum value after transformation and the chirp rate according to the angle related to the maximum value are estimated. In addition, a generalized Fourier transform is introduced to estimate the initial frequency of the LFM interference. For the performance analysis, the Cramer–Rao lower bounds of the estimated chirp rate and the initial frequency of the LFM interference in the presence of alpha-stable noise are derived. Moreover, the asymptotic properties of the modulation parameter estimator are analyzed. Simulation results demonstrate that the performance of the proposed parameter estimation method significantly outperforms existing methods, especially in a low SNR regime.

Generation of Orthogonal Sequence Sets for M-ary Spread Spectrum Communications with OFDM Modulation

The number of orthogonal sequences is a key point of M-ary spread spectrum (MSS) communication system since more sequences mean higher data rate. When orthogonal frequency division multiplexing (OFDM) modulation is used into MSS system, the additional low peak-to-average power ratio (PAPR) property is required due to high power efficiency. Based on the segmentation-shift operation, multiple orthogonal sequence sets with the same size and sequence length are generated in this paper. By combining these orthogonal sequence sets, the number of the generated orthogonal sequences is very large. For any two orthogonal sequence sets, there exists good cross-correlation performance between them at zero shift. In addition to large set size and good correlation property, the generated multiple orthogonal sequence sets possess low PAPR, which is very suitable for OFDM modulation. The generation algorithm of multiple orthogonal sequence sets is provided in this paper, and the corresponding simulation results show the effectiveness of the provide method.