Direct Sequence Spread Spectrum Research Articles

A Robust Carrier Frequency Offset Estimation Algorithm in Burst Mode Multicarrier CDMA based Ad Hoc Networks

The future wireless communication systems demand very high data rates, anti-jamming ability and multiuser support. People want large amount of data to be continuously accessible in their personal devices. Direct Sequence (DS) spread spectrum based techniques such as Code Division Multiple Access (CDMA) fulfil these requirements but, at the same time, suffer from the Intersymbol Interference (ISI). Multicarrier CDMA (MC-CDMA) is an emerging technology to be used in mobile devices operating in an ad hoc setting due to its immunity towards ISI and having all the advantages of spread spectrum communication. One of the major problems with MC-CDMA is the high sensitivity towards carrier frequency offsets caused due to the inherent inaccuracy of crystal oscillators. This carrier frequency offset destroys the orthogonality of the subcarriers resulting in Intercarrier Interference (ICI). In this paper, we propose a computationally efficient algorithm based on Fast Fourier Transform (FFT) and biquadratic Lagrange interpolation. The FFT is based on the use of overlapping windows for each frame of the data instead of non-overlapping windows. This gives a coarse estimate of the frequency offset which is refined by the successive application of Lagrange quadratic interpolation to the samples in the vicinity of FFT peak. The proposed algorithm has been applied to the multiuser ad hoc network and simulated in Stanford University Interim (SUI) channels. It has been shown by simulations that the proposed algorithm provides better performance of almost 1~2 dB as compared to the well-known algorithms.

Non-coherent turbo coded frequency shift keying for reliable covert underwater acoustic communications

This paper presented efficient receiver structure of non-coherent turbo coded FSK signals for maintaining covertness and performance in underwater communication. In aspect to covertness, direct sequence spread spectrum method, which used to hide the transmitted signal by transmitting it at low power and, thus, making it difficult for an unintended listener to detect the signal in the presence of background noise. In aspect to performance improvement in the underwater acoustic environment with multipath and Doppler spreading, non-coherent FSK which is not required phase information, weighted multiband technology, and turbo equalization model with rate of 1/3 were suggested. In order to analyze the performance by applying four bands and spreading chips with 8 and 32, the experiment was conducted on a lake by moving in the range of 300m to 500m. Through the experimental results, we confirmed the performance was improved as increasing number of bands, turbo iterations, and spreading chips. Furthermore, weighted multiband technology which allocates lower weight to inferior bands based on preamble error rates were applied for some failed packets, all packets were decoded successfully. [This research was funded by Agency for Defense Development, South Korea, Grant No. UD200010DD.]

Long-range underwater acoustic communication of M-eyry spread spectrum with cyclic prefix

The Spread Spectrum is an effective transmission method for complex channels in underwater acoustic (UWA) communication because of the high correlation of pseudo-random noise (PN) sequence. In particular, the spread spectrum method is effective in an environment with a low signal-to-noise ratio. The bandwidth of UWA communication channel is limited due to the fact that sound wave attenuation in water is positively associated with frequency. The limited bandwidth directly leads to the low data rate of conventional direct-sequence spread spectrum UWA communication. M-ary spread spectrum (MSS) schemes using multiple orthogonal PN sequences were proposed as a solution to this problem. However, even in the MSS method, the performance may be limited by the delay spread characteristics in the multipath propagation channels. We propose a more robust communication method in the UWA channel by including Cyclic Prefix (CP) to achieve a more robust performance in the multipath channel. In the proposed method, a CP is inserted for every symbol. In the 160 km long-range UWA communication experiment conducted in the East Sea, the proposed method showed more reliable results than the conventional MSS method. [Work supported by the Agency for Defense Development, South Korea, Grant No. UD200010DD.]

Toward Physical Layer Security and Efficiency for SAGIN: A WFRFT-Based Parallel Complex-Valued Spectrum Spreading Approach

Space-air-ground integrated network (SAGIN), as an integration of interconnected space, air, and ground network segments, is expected to see prevalent usage as part of intelligent transportation systems (ITS), providing an enhanced service provision in terms of coverage, flexibility and reliability. However, restricted by the limited and unbalanced network resources, the efficiency and security of the underlying connectivities of SAGIN are of utmost concern for ITS applications. In this paper, a weighted fractional Fourier transform (WFRFT) based parallel complex spreading (PCS) approach is proposed to improve the communication efficiency and security of SAGIN at the physical (PHY-) layer. The concept of WFRFT along with the direct sequence spread spectrum technology establish the security kernel of the proposed scheme. The practicability of the complex-valued WFRFT-spreading architecture is verified by studying the correlation properties of the WFRFT-spreading signals. Taking advantages of the signal uniqueness of WFRFT, the proposed scheme is capable of providing more flexibility in signal characteristic control. Moreover, the complex-valued WFRFT-spreading processing makes the proposed scheme inherently robust against the large Doppler shift distortions in SAGIN. Simulation results demonstrate the superiority of the proposed WFRFT-PCS scheme in terms of communication efficiency and PHY-layer security. Finally, as a proof of concept, an all-digital FPGA prototype system is designed to show the practicability and the performance enhancement of the proposed scheme.

Efficient and lightweight protocol for anti-jamming communications in wireless body area networks

Wireless Body Area Networks (WBANs) have emerged as a new technology in healthcare. However, wireless interfaces make it more vulnerable to acute physical layer attacks. Indeed, jamming attacks on WBAN communications have become a significant challenge to address. This paper proposes a new security protocol against jamming attacks in healthcare and medical environments, combining Frequency-Hopping Spread Spectrum (FHSS) and Direct Sequence Spread Spectrum (DSSS), termed CFD-BD: Chaotic FH/DS Based Dissimulation. Our proposition is based on both the chaotic signal and dissimulation. The combination of these two techniques allows us to ensure an effective solution against jamming attacks and meet the challenges resulting from the constraints related to the limited resources of WBANs. The simulation results indicate that our proposal is lightweight in communication load, complexity, and storage.

Direct-sequence spread spectrum time division multiple access with direct detection for latency optimized passive optical network

To improve low-latency support of passive optical networks, direct-sequence spread spectrum time division multiple access implements bi-directional byte-interleaved transmission by encoding each bit of a bi-polar sequence with orthogonal chip pattern. Consequently, guard-interval between consecutive up-link bytes can be removed and latency caused by multi-point control protocol interaction can be reduced. However, direct-sequence spread spectrum requires bi-polar signaling, which is not possible with direct detection and several solutions exist, such as, biased and differential transmission. In this work, we investigate the performance of direct-sequence spread spectrum time division multiple access with direct detection and also analyze both up-link and down-link situations. Experimental results of transmission over a 20 km dispersion uncompensated link show: The 31-bits direct-sequence reduces required received signal power to −12 dBm and the performance is limited by dark-current. With differential transmission in up-link direction, modified duo-binary has 5 dB better performance than PAM4. Also, in up-link direction, differential transmission has 3 dB better performance than biased transmission.

An Interference-Tolerant Synchronization Scheme for Wireless Communication Systems Based on Direct Sequence Spread Spectrum

In this paper, an interference-tolerant three-level synchronization scheme for wireless communication systems based on Direct Sequence Spread Spectrum (DSSS) is presented. Although the proposed synchronization scheme is demonstrated in a mixed signal IC solution, it is exportable to fully digital realizations. The proposed technique synchronizes the received pseudo-random sequence (PRS) modulated signal with the receiver (RX) reference PRS by making use of its autocorrelation properties. The proposed scheme employs 3 levels that make the proposed scheme tolerant to strong in-band interferences. Experimental results for the prototype demonstrate the capabilities of the proposed approach to achieve over 50% of clock cycle tracking even in the presence of interferences with a power of 5dB higher than the desired signal power. Fabricated in a mainstream 40nm CMOS technology, the synchronization scheme employs 100MHz PRS at TX and RX; the proposed solution consumes 18.7mW power and around 0.087mm² silicon area.

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.

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.

Aggregated CDMA Crossbar With Hybrid ARQ for NoCs

Network-on-chips (NoCs) are the dominant interconnection technique in modern system-on-chips (SoCs). The medium access technique used in the physical layer of NoC routers profoundly impacts the performance and footprint of the router. Code division multiple access (CDMA) is a medium access technique widely deployed in various wireless communication systems, and it has recently been proposed as a prominent switching method for NoC routers. In a CDMA crossbar, several processing elements (PEs) can communicate simultaneously over a single communication channel by applying the direct-sequence spread spectrum technique to digital interconnects. However, existing CDMA switches are bit-wise architectures in which a binary data bit is spread and serially communicated on an exclusive digital channel while replicating this configuration to communicate multiple data bits, which increases the crossbar area and wiring density. In this work, we propose aggregated CDMA (ACDMA) to improve the area, throughput, and power consumption of existing CDMA NoC routers. ACDMA exploits the static nature and relative noise immunity of on-chip interconnects to aggregate transmission of multiple data bits into M-ary symbols on a single digital communication channel, which significantly reduces the wiring density and area overhead of the crossbar. Serial and parallel variants of the ACDMA crossbar offering a wide range of area-speed trade-offs are implemented in the ASIC <inline-formula> <tex-math notation="LaTeX">$65~nm$ </tex-math></inline-formula> standard cell technology. The implementation results show that the throughput-per-area (TPA) of the serial and parallel ACDMA crossbars is improved by 96.3&#x0025;, 18.2&#x0025;, and 118.6&#x0025;; and 400&#x0025;, 255.3&#x0025;, and 184.2&#x0025; compared to the serial and parallel counterparts of the Walsh Basis (WB), Standard Basis (SB), and Overloaded CDMA Interconnect (OCI) crossbars, respectively. A 65-node ACDMA NoC router is fully realized and compared to the state-of-the-art CDMA and CONNECT NoC routers under multiple synthetic workloads. Communication reliability of the ACDMA NoC router subject to noise is investigated and a hybrid ARQ approach is proposed to improve the interconnect reliability.

The case against alphabet awareness for detection of direct-sequence spread-spectrum signals

In this paper we investigate the detection of a direct-sequence spread-spectrum signal with QPSK coded chips, whose spreading sequence is unknown to the detector, and whose spreading code period is longer than the detector observation window. We propose a likelihood-ratio-test detector that takes advantage of knowledge of the signal alphabet, whose complexity grows linearly with the observation window length, and that makes use of multiple antennas. The proposed alphabet-aware detector outperforms multi-antenna alphabet-unaware detectors, especially when the SNR is high and the observation window is small. However, the performance advantage is not large, it diminishes further when the SNR is low, and it comes at the cost of higher computational complexity. In most cases, simpler alphabet-unaware detectors such as the energy detector provide comparable detection performance with less complexity. Due to the i.i.d. signal model this paper utilizes, the results in this work are also applicable to single-carrier unspread QPSK signals, with direct-sequence spread-spectrum signals as a specific case in which the SNR is low.

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.

Turbo Iterative DSSS Acquisition in Satellite High-Mobility Communications

Satellite high-mobility communications can provide global communication services for mobile terminals, which plays a vital role in today's world of wireless communications. Different from traditional terrestrial-satellite communications, high-mobility terminals impose more challenges on the direct sequence spread spectrum (DSSS) signal acquisition in satellite communications. On the one hand, the spread spectrum sequence used in satellite communication systems is relatively long, which inevitably increases the computational complexity. On the other hand, high Doppler frequency offsets and time-varying Doppler rates caused by high-mobility terminals make satellite DSSS signal acquisition become the bottleneck. To solve such a problem in high-mobility satellite communications, we propose a new acquisition algorithm based on the new factor graph where time-varying Doppler rate is modeled by the random walk model, named the turbo iterative acquisition algorithm. It consists of the constraint-detection loop and the Doppler-estimation loop. By means of soft messages passed between these loops, the negative influence of time-varying Doppler frequency offsets can be eliminated effectively. Simulation results show that, compared with traditional algorithms, the proposed algorithm improves acquisition performance more than 1.3 dB with low computational complexity, which demonstrates the superiority of the proposed algorithm in terms of acquisition performance and complexity.

Generalized Complementary Coded Scrambling Multiple Access for MIMO Communications

Spectrum efficiency is a critical performance metric in wireless communications due to an increasing demand for radio spectrum to support high data rate applications. Direct sequence spread spectrum technology enables a code division multiple access (CDMA) system a strong anti-interference capability but with a relatively low spectrum efficiency. To address this issue, complementary coded scrambling multiple access (CCSMA) (X. Liu <italic xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">et al</i> ., in 2018) was proposed to use three-dimensional (space-time-frequency) resources for performance enhancement. However, in a CCSMA system, if the number of multipath returns is larger than the number of transmit (Tx) antennas, a code reuse problem occurs to degrade bit error rate performance significantly. This work designs a new scrambling code, namely generalized complementary coded scrambling multiple access (GCCSMA) codes, to overcome the code reuse problem in CCSMA. GCCSMA uses complementary codes, scrambling modulation, and multiple-input-multiple-output (MIMO) techniques jointly, which works well even in multipath channels with more paths than the number of Tx antennas. The analysis and simulations are conducted to validate the effectiveness of the proposed GCCSMA system.

A Method for Underwater Wireless Data Transmission in a Hydroacoustic Channel under NLOS Conditions

Wireless data transmission in the hydroacoustic channel under non-line-of-sight (NLOS) propagation conditions, for example, during a wreck penetration, is difficult to implement reliably. This is mostly due to the multipath propagation, which causes a reduction in the quality of data reception. Therefore, in this work an attempt has been made to develop a reliable method of wireless underwater communication test it under the NLOS conditions. In our method, we used multiple frequency-shift keying (MFSK) modulation, sending a single bit on two carriers, and diversity combining. The method was tested in laboratory conditions which simulated underwater signal propagation during the penetration of the wreck. The propagation conditions were investigated by determining the impulse responses at selected measurement points using the correlation method. Additionally, for comparison, the data transmission quality was determined by the bit error rate (BER) under the same conditions using direct sequence spread spectrum (DSSS) and binary phase shift keying (BPSK) modulation. The obtained results confirmed the usefulness of the application of the developed method for wireless data transmission in a hydroacoustic channel under NLOS conditions.

Linear Feedback Shift Register and its Applications in Digital System Design

In digital system design, the Linear Feedback Shift Register (LFSR) is the queen of logic functions, and the design engineers can use LFSR in both hardware (HW) or software (SW) implementation. In this paper, LFSR will be discussed in its HW implementation via Hardware description language. In addition, the application of LFSR in of pseudorandom number generator (PRNG), direct sequence spread spectrum (DSSS), cyclic redundancy check (CRC) is also given. Keywords-- Digital system design, System on chip, ASIC digital design, Linear feedback shift register

Self-Synchronized DS/SS With High Spread Factors for Robust Millimeter-Wave Datalinks

This paper presents direct sequence spread spectrum (DS/SS) datalinks operating at 92-100 GHz with spreading factors up to 100K. Unlike traditional DS/SS datalinks which rely on preamble and cyclic dispreading for synchronization, the reported datalink uses a self-synchronous demodulation scheme, avoiding the large hardware complexity associated with exceptionally large spreading factors, and avoiding the difficulties of synchronizing the long de-spreading codes that are applicable to mm-wave systems. A demonstration link chipset is presented operating at 92-100 GHz with a spreading factor of 104K and a spread bandwidth of 6.0 GHz (baseband bandwidth of 56Kb/s) while consuming a total of 425mW.

Enhancing the Security of Chaotic Direct Sequence Spread Spectrum Communication Through WFRFT

In this letter, the weighted fractional Fourier transform (WFRFT) is performed on the chaotic sequence to enhance the security of the chaotic direct sequence spread spectrum (CD3S) communication, and a WFRFT-CD3S system that spreads the information bits through the WFRFT-chaotic sequence is proposed. In the proposed system, the transmitter scrambles the phase space trajectory of the chaotic sequence through WFRFT and make the distribution of the transmitted signal be Gaussian-like. The receiver synchronizes the WFRFT-chaotic sequence through bidirectional correlation search and demodulates the information through the synchronized local WFRFT-chaotic sequence. Simulations are implemented to evaluate the security performance, synchronization performance, and demodulation performance of the proposed system.

Compressive narrowband interference detection and parameter estimation in direct sequence spread spectrum communication

The Shannon-Nyquist sampling theorem that is based on narrowband interference (NBI) detection and parameter estimation methods in direct sequence spread spectrum (DSSS) communication is limited by the high sampling rate. Compressive sensing (CS) is adopted to address the problem. But it will change the signal nature, which leads to the unavailability of Shannon-Nyquist sampling theorem-based interference detection and parameter estimation methods. According to the different posterior probability distribution features of NBI, DSSS signal, and noise in compressed domain, a posterior probability model of whether NBI exists in the received signal is constructed by using the compressed measurements. The posterior probability that whether NBI exists in the received signal is employed as the feature parameters of NBI detection and parameter estimation. With the feature parameters detected, NBI detection can be achieved and the edge location of NBI components can be located. The relationship between the edge location of NBI components and the edge frequency of NBI is constructed, which will contribute to estimate the NBI edge frequency. The numerical simulation results demonstrate that the proposed method can effectively achieve NBI detection and parameter estimation in the compressed domain and it performs significantly better than the other existing methods.

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.