Sequence Spread Research Articles

Physical resilience to insider attacks in IoT networks: Independent cryptographically secure sequences for DSSS anti-jamming

Wireless communication is a key technology for the Internet of Things (IoT). Due to its open nature, the physical layer of wireless systems is a high-priority target for an adversary whose goal is to disrupt the normal behavior of the system. In particular, jamming attacks are one of the most straightforward and effective types of attacks: information flow of the system is stopped or severely disturbed. In this paper, we propose a method to improve the jamming resilience of IoT systems based on Direct-Sequence Spread-Spectrum (DSSS) techniques. Our proposal is inspired by the Moving Target Defense (MTD) paradigm. MTD strategies randomize components of a system, increasing the effort an attacker needs to compromise the system. We use state-of-the-art Cryptographically Secure Pseudo-Random Number Generators outputs as spreading sequences for DSSS. The sequences of the proposed system are generated in an ad-hoc, independent, and distributed way. We show probabilistically that the generated sequences have robust cross-correlation properties. We define a multi-user system model to evaluate the Bit-Error-Rate of our proposal in the presence of two types of jammers: a classical band-limited Gaussian noise jammer, and an insider smart jammer with knowledge of one spreading sequence used in the system. Our proposal proactively mitigates the insider jammer attack. We quantify the insider smart jammer resilience of a system implementing our proposal, as a function of the length of the spreading sequences and the jammer power.

Binary Golay Spreading Sequences and Reed-Muller Codes for Uplink Grant-Free NOMA

Non-orthogonal multiple access (NOMA) is an emerging technology for massive connectivity in machine-type communications (MTC). In code-domain NOMA, non-orthogonal spreading sequences are uniquely assigned to all devices, where active ones attempt a grant-free access to a system. In this paper, we study a set of user-specific, non-orthogonal, binary spreading sequences for uplink grant-free NOMA. Based on binary Golay complementary sequences, each spreading sequence provides the peak-to-average power ratio (PAPR) of at most 3 dB for multicarrier transmission. Exploiting the theoretical connection to Reed-Muller codes, we conduct a probabilistic analysis to search for a permutation set for Golay sequences, which presents theoretically bounded low coherence for the spreading matrix. Simulation results confirm that the maximum PAPR of transmitted multicarrier signals via the spreading sequences is significantly lower than those for random bipolar, Gaussian, and pseudo-random sequences. Thanks to the low coherence, the performance of compressed sensing (CS) based joint channel estimation and multiuser detection using the spreading sequences turns out to be superior or similar to those for random bipolar, Gaussian, pseudo-random, and Zadoff-Chu sequences. The binary Golay spreading sequences have only two phases regardless of the sequence length, which can make them suitable for low cost MTC devices.

Non-Orthogonal Golay-Based Spreading Sequences for Uplink Grant-Free Access

In this letter, new sets of non-orthogonal spreading sequences are constructed for uplink grant-free access of devices. Based on Golay complementary sequences and sets, the spreading sequences have theoretically bounded low peak-to-average power ratio (PAPR) for multicarrier transmission. Also, the corresponding spreading matrix has theoretically bounded low coherence, which guarantees the reliable performance of compressed sensing (CS) based joint channel estimation (CE) and multiuser detection (MUD). While the spreading sequence set accommodates up to 300% overloaded devices, a large number of distinct sequence sets can also be constructed for future applications. Simulation results demonstrate that the Golay-based sequences exhibit the PAPR of at most 4, which is lower than those for random bipolar, Gaussian, and Zadoff-Chu (ZC) sequences. We also observe that the Golay-based sequences provide the reliable performance for CS-based joint CE and MUD. Finally, the Golay-based sequences have fewer phases than ZC sequences, which can be more suitable for low cost devices in machine-type communications (MTC).

DFT-s-OFDM With Enhanced Time-Domain Spreading for B-IFDMA

Enhanced time-domain spreading is proposed for block-interleaved frequency division multiple access (B-IFDMA) with discrete Fourier transform spread orthogonal frequency division multiplexing (DFT-s-OFDM). By repeating each consecutive modulation symbol prior to the spreading, we show that the DFT of the spreading sequence becomes a window function, which compared to the existing method of repeating the whole block of modulation symbols, offers a signal with both lower cubic metric (CM) and block error rate (BLER). Modulation symbol-specific spreading sequences are further shown to improve the BLER. The new transmission scheme is supported by both time- and frequency domain despreaders, as well as by two channel estimation methods for B-IFDMA.

Construction of a two-dimensional-VWOOC based on time domain/frequency domain optical CDMA system

In this paper, a new two-dimensional variable weight optical orthogonal code (2D-VWOOC) for optical code division multiple access (OCDMA) system is constructed. It takes the C as the frequency hopping sequence and the strict variable weight optical orthogonal code (SVWOOC) as the time spreading sequence. The code can meet the different quality of service (QoS) requirements of different users. Under the same time spread sequence, the bit error rate (BER) of QCHC/SVWOOC is 3∼9 orders of magnitude lower than that of other similar 2D-VWOOCs. In Optisystem simulation, when the transmission rate of the system is 14 Gbits/s, the BER of users with code weight 4 is 7 orders of magnitude lower than that of users with code weight 2, and a clear and correct eye diagram can be obtained.

Digital blind despreading method for intermediate frequency short‐code DSSS signal

Direct sequence spread spectrum (DSSS) signals are widely used in various military and civilian communication systems owing to its useful properties, such as lower working signal-to-noise ratio, strong anti-jamming capability, and robustness to multi-path fading. In past few years, the estimation for spreading sequence in non-cooperative context has attracted lots of attention. Several methods are developed to realise a recovery of the sequence. Eigenvalue decomposition (EVD) is a classical method to obtain an effective estimation by extracting the main eigenvectors. The only obstacle to apply EVD method into practical systems is high complexity of computation when spreading sequence is long. Alternative algorithms such as neural network and clustering method have lower complexity, but an evident loss in performance. In this work, the authors propose to generalise the EVD algorithm to intermediate frequency (IF) short-code DSSS with an unknown carrier offset. The blind despreading process is designed to complete correlation demodulation. As a result, the computational afford is largely decreased due to the operation on IF real signal. To evaluate the performance, they derive the Crammer-Rao bound for spreading code estimation and compare the performance of several methods. Simulation demonstrates the superiority of the proposed method in estimation accuracy and runtime.

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.

Bayesian Receiver Design for Grant-Free NOMA With Message Passing Based Structured Signal Estimation

Grant-free non-orthogonal multiple access (NOMA) is promising to achieve low latency massive access in Internet of Things (IoT) applications. In grant-free NOMA, pilot signals are often used for user activity detection (UAD) and channel estimation (CE) prior to multiuser detection (MUD) of active users. However, the pilot overhead makes the communications inefficient for IoT devices with sporadic transmissions and short data packets, or when the channel coherence time is short. Hence, it is desirable to improve the efficiency by avoiding the use of pilot signals, which can also further achieve lower latency. This work focuses on Bayesian receiver design for grant-free low density signature orthogonal frequency division multiplexing (LDS-OFDM), where each user is allocated a unique low density spreading sequence. We propose to use the low density spreading sequences for active user detection, thereby avoiding the use of pilot signals. Firstly, the task of joint UAD, CE and MUD is formulated as a structured signal estimation problem. Then message passing based Bayesian approach is developed to solve the structured signal estimation problem. In particular, belief propagation (BP), expectation propagation (EP) and mean field (MF) message passing are used to develop efficient hybrid message passing algorithms to achieve trade-off between performance and complexity. Simulation results demonstrate the effectiveness of the proposed receiver for grant-free LDS-OFDM without the use of pilot signals.

A generalized linear threshold model for an improved description of the spreading dynamics.

Many spreading processes in our real-life can be considered as a complex contagion, and the linear threshold (LT) model is often applied as a very representative model for this mechanism. Despite its intensive usage, the LT model suffers several limitations in describing the time evolution of the spreading. First, the discrete-time step that captures the speed of the spreading is vaguely defined. Second, the synchronous updating rule makes the nodes infected in batches, which cannot take individual differences into account. Finally, the LT model is incompatible with existing models for the simple contagion. Here, we consider a generalized linear threshold (GLT) model for the continuous-time stochastic complex contagion process that can be efficiently implemented by the Gillespie algorithm. The time in this model has a clear mathematical definition, and the updating order is rigidly defined. We find that the traditional LT model systematically underestimates the spreading speed and the randomness in the spreading sequence order. We also show that the GLT model works seamlessly with the susceptible-infected or susceptible-infected-recovered model. One can easily combine them to model a hybrid spreading process in which simple contagion accumulates the critical mass for the complex contagion that leads to the global cascades. Overall, the GLT model we proposed can be a useful tool to study complex contagion, especially when studying the time evolution of the spreading.

Low-Computational Extended Orthogonal Matched Filter Structure for Multiuser Detection

This research provides a novel extended orthogonal matched filter (EOMF) structure that supports multiuser detection. In addition, this structure is designed to suppress increases in computational complexity and circuit scale. In coming years, the number of Internet of Things (IoT) applications that use wireless ad hoc networks is expected to increase. To realize a highly reliable wireless ad hoc network, it is essential to introduce a method for cancelling the interference from other users. This research adopts a method based on code division multiple access (CDMA). However, a CDMA-based method has difficulty with deteriorating performance due to the near-far problem and the increase in the amount of interference as the number of users increases. Another problem is that the spreading sequence of each user is unknown in a wireless ad hoc network. The EOMF effectively removes interference while solving the above problems by combining an OMF and an adaptive array antenna. In this research, an EOMF structure with multiuser detection and lower computational complexity than the conventional EOMF is proposed. By the derived formulas and numerical examples, the proposed structure reduces the computational complexity by approximately 75% compared to the conventional EOMF and obtains sufficient bit error ratio (BER) performance.

Последовательность и динамика распространения влияния хуннуских традиций с востока на запад (и с запада на восток?)

Вопрос о связях между центральнозиатскими хунну (сюнну) и восточноевропейскими гуннами, несмотря на большое количество посвященных этому работ, остается дискуссионным. В статье на основании археологических источников, рассматриваемых в историческом контексте, он решается положительно; устанавливается следующая последовательность его рассмотрения. Обосновывается понятие «хуннской триады» (определенные виды вооружения, украшений, бронзовые котлы), характеризующей ранжированную воинскую субкультуру хунну. Рассматриваются культурные особенности ближайшего окружения этносоциального объединения Хунну – Тува, Минусинская котловина, Горный Алтай (условно – «Ближний круг»). Приводятся археологические данные о распространении носителей этого культурного комплекса на территорию Западной Сибири (по Л.Н. Гумилеву), а именно саргатской культурно-исторической общности (условно – «Дальний круг»), ставшей местом исхода потомков хунну/гуннов при движении на Запад. Прослеживается сохранение элементов воинской субкультуры хунну в материалах восточноевропейских кочевников (по И.П. Засецкой), с заменой знаковых видов украшений – бронзовых изделий на изделия полихромного стиля. Впервые ставится вопрос о возможности «обратного» движения гуннов в сторону Центральной Азии, прослеживаемого по проявлениям прежних традиций, в первую очередь, по находкам полихромных изделий в ряде памятников Восточного Приуралья и Казахстана, Западной Сибири и Алтая

Estimating Symbol Duration of Long-Code Direct Sequence Spread Spectrum Signals at a Low Signal-to-Noise Ratio

Several existing spreading sequence estimation algorithms of long code direct-sequence spread spectrum (LC-DSSS) signals require prior knowledge of the symbol duration, but research on symbol duration estimation techniques of LC-DSSS signals are rare currently. In this paper, we proposed a method of estimating symbol duration for LC-DSSS signals. On the basis of the missing data model, a set of sample covariance matrices are constructed from the received signal with a set of window durations. Subsequently, the diagonals that contain noise component are removed from the sample covariance matrices to eliminate the effects of the noise and then squared Frobenius norm is performed on the sample covariance matrices to eliminate the effects of the long code. After analyzing the second-order statistical characteristic of the squared Frobenius norm of the sample covariance matrices, a symbol duration estimator of LC-DSSS signals is derived. Numerical experiments demonstrate that the proposed estimator provides satisfactory estimation performance of the symbol duration for LC-DSSS signals at low signal-to-noise ratio, even in multiple access interference scenario and multipath fading scenario. Compared to the existing estimators, the proposed estimator exhibits superior performance.

Developing a framework for fire investigations in informal settlements

Informal settlements regularly experience small and large fires. In South Africa, there were 5940 informal settlement fires during 2017, which is approximately 16 per day. Currently, post-fire investigations are often not being conducted, and close to 40% of all fire causes in informal settlements are labelled as “undetermined”. Conducting post-fire investigations can provide important information to develop better evidence-based responses for improving fire safety in these communities. It is acknowledged that due to the inherent nature of informal settlements high levels of certainty in fire-cause determination are often not possible. After ignition, the onset of flashover occurs rapidly, thus most structures are destroyed within minutes, and often multiple dwellings are involved in a single incident. This paper seeks to apply well-known forensic fire investigation principles (primarily using NFPA 921), to informal settlements to develop a framework and guidelines, in order to (a) identify the fire origin and the fire cause, (b) obtain data on human behaviour in fire, (c) understand the fire spread sequence, and (d) evaluate the effectiveness of suppression and response efforts. The proposed methodology was used to analyse the Imizamo Yethu fire that occurred in 2017. Additionally, the work highlights the challenges associated with conducting such investigations.

Delta-Sigma Modulation for Noise Cancellation in 5G-Compliant Network

The issue of accurate reception of signals in wireless communication systems is a constant concern due to the presence of noise/fading/interferences that may occur on the transmission path. Acknowledging that 5G requirements for these systems increase the standards related to high throughput and lowest bit error rate (BER), noise reduction must be performed to achieve reliable communication. Delta-sigma modulation (DSM) proved to be cost efficiency and reduced circuit complexity technique. Therefore, the performance of DSM is analyzed in a potential 5G-compliant system in Additive White Gaussian Noise, Nakagami-m and Weibull fading channels in the context of BER reduction. The main goal is to offer some landmarks related to expectations of DSM use in an OFDM-based communication system. The performance is evaluated for different sets of spreading sequences (Walsh-Hadamard and pseudo-noise) and based on the simulation results several conclusions are highlighted. Further improvement will be proposed for noise effects reduction and interference minimization in DSM-based 5G-compliant systems.

Improved Non-coherent Communication Systems Using Noise Reduction Chaotic ON–OFF Keying (NR-COOK) Techniques

The major drawback of the conventional chaotic ON–OFF keying (COOK) scheme is that the transmitted signal energy is not constant due to the non-periodic nature of the chaotic sequences. These chaotic sequences increase the amount of noise present in the received signal and degrade the system performance under multipath fading channel conditions. Moreover, in COOK scheme, the optimum threshold for symbol detection depends on the Signal to Noise Ratio (SNR) available at the detector input. In this paper, a noise reduction COOK scheme is proposed for improved performance. In this scheme, for transmitting binary bit ‘1’ $$\frac{\beta }{R}$$ distinct chaotic samples are generated and each sample is repeated R times to get a required spreading sequence length of β and no signal is being transmitted during binary bit ‘0’. For demodulation of the transmitted data, each of the R identical samples are averaged, correlated with itself and summed over the bit duration $$T_{b}$$ where $$T_{b} = \frac{\beta }{R}T_{c}$$ to recover the transmitted data. The averaging operation over R identical samples reduces the effect of noise present in the received signal and improves the performance of the system over noisy channels. Closed form expression for bit error rate (BER) of the proposed NR-COOK system is derived and analyzed under multipath fading channel and are validated through simulation results to demonstrate performance improvement over conventional non coherent schemes.

Quaternary DS-SSMA Communications Employing Generalized Detector over Rayleigh Fading Channels

This paper deals with an employment of the complex modulators and generalized receivers constructed based on the generalized approach to signal processing (GASP) in noise in quaternary direct-sequence spread-spectrum multiple-access (DS-SSMA) communication systems with complex signature sequences in flat Rayleigh fading. Owing to availability of large sets of complex spreading sequences possessing perfect correlation characteristics, the interest to use the complex spreading sequences in DS-SSMA communication systems has increased. The complex spreading sequences investigated in the present paper are based on the recently introduced orthogonal unified complex Hadamard transform (UCHN) sequences. In the present paper, the complex processing in modulators and generalized receivers is also employed with the purpose to take advantage of the correlation properties of complex signature sequences. We derive the average bit error rate (BER) for quarternary synchronous communications system based on the generalized approach to signal processing in noise first, and then the BER for quaternary asynchronous ones is evaluated using the characteristic function approach. We presented some results based on the Gaussian approximation method for asynchronous communication systems constructed on the basis of the generalized approach to signal processing in noise Computer modeling results demonstrate that the communication systems using UCHT spreading sequences perform generally better than the GOLD sequences and the 4-phase family A sequences. Comparative analysis between the asynchronous communication systems constructed on the basis of the generalized approach to signal processing in noise demonstrates superiority over the asynchronous communication systems employing the correlation receiver.

블라인드 DSSS 신호의 확산 수열 추정 알고리즘 성능 비교

블라인드 DSSS 신호의 확산 수열 추정 알고리즘 성능 비교

A New IDMA System Based on CSK Modulation for Multiuser Underwater Acoustic Communications

This work proposes a novel interleave-division multiple access (IDMA) system based on cyclic shift keying (CSK) modulation, named CSK-IDMA, for multiuser underwater acoustic (UWA) communications. The CSK modulation uses a circular cyclic shift of the spreading sequence to represent the information, which can be viewed as a special $M$ -ary spread spectrum modulation. Compared with conventional direct-sequence spread spectrum (DSSS) technique, CSK overcomes the spreading gain versus data rate limitations. The proposed scheme adopts the IDMA technique to reduce the cochannel interference. At the receiver, we employ passive time reversal (PTR) technique to compress the channels and improve the signal-to-interference-noise ratio (SINR) at first. In order to perfectly incorporate with the IDMA receive structure, we propose a soft CSK demodulation which can iteratively exchange extrinsic information with a soft channel decoder to form a turbo equalization. Simulation results show the proposed CSK-IDMA scheme can offer significantly improved performance. The data processing results from a lake experiment show that the proposed scheme can support 8 users and 10 users with zero error bits for 2.3 km transmission and 2.7 km transmission, respectively. The corresponding bit error rate (BER) for 16 users are $\text{8.4} \times \text{10}^{-4}$ and $\text{9.7} \times \text{10}^{-4}$ , respectively. The data rate is 62.5 bit/s per user within 4 kHz bandwidth and spreading sequences of 256 chips. To the best of our knowledge, this is the first work which can support more than 8 users for multiuser UWA communications with a high bandwidth efficiency.

Basal forebrain volume reliably predicts the cortical spread of Alzheimer's degeneration.

Alzheimer's disease neurodegeneration is thought to spread across anatomically and functionally connected brain regions. However, the precise sequence of spread remains ambiguous. The prevailing model used to guide in vivo human neuroimaging and non-human animal research assumes that Alzheimer's degeneration starts in the entorhinal cortices, before spreading to the temporoparietal cortex. Challenging this model, we previously provided evidence that in vivo markers of neurodegeneration within the nucleus basalis of Meynert (NbM), a subregion of the basal forebrain heavily populated by cortically projecting cholinergic neurons, precedes and predicts entorhinal degeneration. There have been few systematic attempts at directly comparing staging models using in vivo longitudinal biomarker data, and none to our knowledge testing if comparative evidence generalizes across independent samples. Here we addressed the sequence of pathological staging in Alzheimer's disease using two independent samples of the Alzheimer's Disease Neuroimaging Initiative (n1 = 284; n2 = 553) with harmonized CSF assays of amyloid-β and hyperphosphorylated tau (pTau), and longitudinal structural MRI data over 2 years. We derived measures of grey matter degeneration in a priori NbM and the entorhinal cortical regions of interest. To examine the spreading of degeneration, we used a predictive modelling strategy that tests whether baseline grey matter volume in a seed region accounts for longitudinal change in a target region. We demonstrated that predictive spread favoured the NbM→entorhinal over the entorhinal→NbM model. This evidence generalized across the independent samples. We also showed that CSF concentrations of pTau/amyloid-β moderated the observed predictive relationship, consistent with evidence in rodent models of an underlying trans-synaptic mechanism of pathophysiological spread. The moderating effect of CSF was robust to additional factors, including clinical diagnosis. We then applied our predictive modelling strategy to an exploratory whole-brain voxel-wise analysis to examine the spatial specificity of the NbM→entorhinal model. We found that smaller baseline NbM volumes predicted greater degeneration in localized regions of the entorhinal and perirhinal cortices. By contrast, smaller baseline entorhinal volumes predicted degeneration in the medial temporal cortex, recapitulating a prior influential staging model. Our findings suggest that degeneration of the basal forebrain cholinergic projection system is a robust and reliable upstream event of entorhinal and neocortical degeneration, calling into question a prevailing view of Alzheimer's disease pathogenesis.

Performance enhancement of LR WPAN spread spectrum system using chaotic spreading sequences

Performance enhancement using chaotic spreading sequences on Low Rate Wireless Personal Area Network is presented. Several one-dimensional chaotic maps are used for generating optimized chaotic sequences for spread spectrum applications. The system parameter of one-dimensional chaotic maps is varied to generate sample-based chaotic signals. Correlation properties analysis is used to generate optimized chaotic sequences. Spreading sequences with a high value of Golay figure of merit are chosen as optimized spreading sequences. Chaotic sequences with high Golay figure of merit factor are expected to have better synchronization and bit error performance. The LR-WPAN model is programmed for USRP (Universal Software Radio Peripheral) software defined radio platform using LabVIEW. The performance of chaotic spreading sequences is evaluated against Gold spreading sequences using the proposed system model. The performance evaluation is based on comparing bit error rate and error vector magnitude in multiple access scenario. The results show that optimized chaotic sequences have better performance than Gold codes.