Spread Spectrum Research Articles

A high dynamic weak spread spectrum signal acquisition strategy based on the transfer model

To solve the problem of low Earth orbit (Leo) satellite constellation link spread spectrum signal acquisition in the high dynamic weak environment, this paper proposes a acquisition strategy based on transfer model. In this paper, we analyze the dynamic characteristics of link signal. An improved method of parallel code phase search acquisition based on transfer model acquisition strategy is introduced, the improvement of the proposed acquisition strategy to the traditional acquisition algorithm is illustrated through theoretical analysis. Simulation results verify the superiority of the acquisition strategy based on the transfer model for high dynamic weak signal acquisition.

Application of Augmented Spread Spectrum Time Domain Reflectometry for Detection and Localization of Soft Faults on a Coaxial Cable

Aviation cables are an essential part of aircraft, which transmits signals and power. Aviation cable faults often occur due to drastic changes in temperature and humidity and the effects of vibration. Obvious cable faults, such as open circuits and short circuits, can be detected during maintenance. However, some soft faults like the loose connection of the connector, damage of the insulation layer, and improper cable bending radius cannot be detected. Spread spectrum time domain reflectometry (SSTDR) is a diagnostic method for cable faults; however, the amplitude of the reflected signal is low when the soft fault occurs. SSTDR is only sensitive to apparent faults, but it is insufficient to detect soft faults. This article proposes an aviation cable fault detection and location method, augmented spread spectrum time domain reflectometry (ASSTDR), to detect soft faults effectively. First, the SSTDR results are decomposed into some intrinsic mode functions (IMFs) using the improved variational mode decomposition and selects the IMF with the highest kurtosis to analyze. Second, short-time Fourier transform (STFT) is applied to deal with the selected IMF to enhance the amplitude characteristics. Then, reassign spectrogram is computed to improve the resolution of STFT time-frequency distribution images to locate the location of soft faults. Finally, the effectiveness of ASSTDR is verified by experiments with different fault types and fault degrees. The comparative experiments demonstrate that the method can accurately detect soft faults.

A Chirp Spread Spectrum Modulation Scheme for Robust Power Line Communication

This paper proposes the use of a LoRa like chirp spread spectrum physical layer as the basis for a new Power Line Communication modulation scheme suited for low-bandwidth communication. It is shown that robust communication can be established even in channels exhibiting extreme multipath interference, impulsive noise and low SNR (−40 dB), with synchronisation requirements relaxed compared to conventional LoRa. ATP-EMTP simulations using frequency dependent line and transformer models, and simulations using artificial Rayleigh channels demonstrate the effectiveness of the new scheme in providing load data from LV feeders back to the MV primary substation. Our results demonstrate error-free communication at SNRs of −40 dB, and can be further improved by trading-off data rate. We further present experimental results based on a Field Programmable Gate Array hardware implementation which match the simulated performance.

Recent Advances in LoRa: A Comprehensive Survey

The vast demand for diverse applications raises new networking challenges, which have encouraged the development of a new paradigm of Internet of Things (IoT), e.g., LoRa. LoRa is a proprietary spread spectrum modulation technique that provides a solution for long-range and ultra-low power-consumption transmission. Due to promising prospects of LoRa, significant effort has been made on this compelling technology since its emergence. In this article, we provide a comprehensive survey of LoRa from a systematic perspective: LoRa analysis, communication, security, and its enabled applications. First, we summarize works focusing on analyzing the performance of LoRa networks. Then, we review studies enhancing the performance of LoRa networks in communication. Afterward, we analyze the security vulnerabilities and countermeasures. Finally, we survey the various LoRa-enabled applications. We also present comparisons of existing methods, together with insightful observations and inspiring future research directions.

A Novel Time-Interval Based Modulation for Large-Scale, Low-Power, Wide-Area-Networks

Wireless communication over long distances has become the bottleneck for battery-powered, large-scale deployments. Low-power protocols like Zigbee and Bluetooth Low Energy have limited communication range, whereas long-range communication strategies like cellular and satellite networks are power-hungry. Technologies that use narrow-band communication like LoRa, SigFox, and NB-IoT have low spectral efficiency, leading to scalability issues. The goal of this work is to develop a communication framework that is energy efficient, long-range, and scalable. We propose, design, and prototype WiChronos, a communication paradigm that encodes information in the time interval between two narrowband symbols to drastically reduce the energy consumption in a wide area network with large number of senders. We leverage the low data-rate and relaxed latency requirements of such applications to achieve the desired features identified above. We design and implement chirp spread spectrum transmitter and receiver using off-the-shelf components to send the narrowband symbols. Based on our prototype, WiChronos achieves an impressive 60% improvement in battery life compared to state-of-the-art LPWAN technologies in transmission of payloads less than 10 bytes at experimentally verified distances of over 4 km. We also show that more than 1,000 WiChronos senders can co-exist with less than 5% collision probability under low traffic conditions.

Chirp-spreaded OFDM-MFSK with differentially encoded phase for applications in low-power wide-area networks

For the unlicensed frequency bands, long-range wide-area networks (LoRaWAN) is a widespread solution for low-power wide-area networks. The physical layer of LoRaWAN is long range (LoRa) which uses a combination of chirp spread spectrum (CSS) and M-ary frequency-shift keying (M-FSK) for transmission. LoRa is known to be highly robust and well-suited for low-complexity implementation, but the weak aspect is its low spectral efficiency. Orthogonal frequency division multiplexing M-FSK (OFDM-MFSK) is a technique which combines M-FSK with OFDM. The bandwidth is divided into several parallel M-FSK transmissions, i.e., more than one sub-carrier is active at a time. This leads to an increased peak-to-average power ratio of the transmit signal. On the other hand, it allows to combine OFDM-MFSK with differential phase-shift keying (DPSK) to increase the data rate and thus the spectral efficiency. The reception of OFDM-MFSK as well as the DPSK demodulation can be done non-coherently. Therefore, they hold many similar design principles as LoRa. We propose a combination of OFDM-MFSK with differentially encoded phase and CSS. Simulation results show, that LoRa and OFDM-MFSK with CSS show similar performance with respect to power and bandwidth efficiency. Adding DPSK leads to an improved spectral as well as power efficiency.

RSSI Analysis on CSS Modulation in the 433 MHz Frequency Band Using Lora in Flood Sensor

Long Range (LoRa) low power is a transmission technology capable of connecting multiple sensors to the Internet of Things in the future. LoRa uses Chirp Spread Spectrum (CSS) frequency modulation which promises to achieve remote connectivity at very low power. More specifically, this paper focuses on the analysis of Receive Signal Strength Indicator (RSSI) on CSS modulation on flood sensors. This paper presents a detailed model of using LoRa on flood sensors, showing that LoRa is capable of transmitting sensor data over long distances with perfect RSSI values. The results showed the potential for a close communication range with a narrow band network at a frequency of 433 MHz and an average RSSI of -50 dB at a maximum distance of 600 meters without obstruction. Although LoRa communication uses CSS modulation and has good RSSI values, in long-distance communication with data in the form of video or large sensor data, this toughness is not sufficient due to long distances and large bandwidth usage, while in this study LoRa type Ai Thinker Ra- 02 SX 1278 can only be set up to 125 KHz according to the datasheet. The disadvantages of using large sensor data over long distances and a larger footprint make CSS vulnerable to other, possibly larger, resources.

2.4 GHz LoRa Wireless Technology for Internet of Things: An Experimental Study

LoRa is a new low data-rate modulation technology for wireless communication, which uses spread spectrum communications to increase transmission distance. At present, sub-GHz LoRa is mainly used to build LoRaWAN and is applied to the monitoring and data collection of the Internet of Things (IoT). Focusing on the latest 2.4 GHz LoRa technology, this study introduces the characteristics of 2.4 GHz LoRa in the physical layer and experimentally analyzes the basic performance of communication, energy consumption, and ranging. Meanwhile, for the deficiencies of time on air in the evaluation of effective data rate, this study proposes the concepts of time on PHY and time of transmission (ToT). Then, the ToT duration of 2.4 GHz LoRa under different parameter configurations is calculated. Finally, combined with the physical-layer characteristics of 2.4 GHz LoRa, four potential networking topologies of the 2.4 GHz LoRa are listed, and their characteristics and application scenarios are analyzed. Compared with sub-GHz LoRa, 2.4 GHz LoRa is more applicable for building self-organized networks and worldwide IoT solutions.

Principle and Analysis of Anti-interference and Anti-fading Techniques in Mobile Communication

In recent years, with the rapid development of mobile communication, anti-interference and anti-fading has always been an important research topic in mobile communication. There is a lot of interference in mobile channel, and the signal will fade because of various reasons. Designers must consider these factors in the design, development and production of mobile communication systems, so the corresponding anti-interference and anti-fading measures should be studied to ensure that the interference level does not exceed the threshold compared with useful signals during the operation of the system, or ensure that the transmission error rate is within a certain range. This paper introduces diversity technique, error correcting code, spread spectrum, adaptive equalization, digital modulation technique and so on. The advantages and disadvantages of different diversity techniques are analyzed, and their performances in common digital mobile communication systems are compared. Different types of error correcting codes are introduced, and their principles are analyzed. Then, other anti - interference and anti - fading techniques and their principles are briefly introduced. Finally, this paper analyzes the development direction of anti-interference and anti-fading techniques in the future mobile communication.

An expert algorithm for spectrum sensing and signal detection in NOMA-enabled 5G networks

The Spectrum sensing is an essential function in the various types of fifth-generation (5G) networks and beyond, which can address some of the fundamental challenges for 5G, including spectral efficiency and the reliable connectivity of internet of things (IoT) devices. In this article, we aim to present a real-time and expert spectrum sensing algorithm for 5G cellular networks. The novel method exploits the cyclostationary nature of the spread spectrum signal without any prior knowledge of its structure. In the proposed algorithm, the multi-carrier code division multiple access (MC-CDMA) technique will be utilized by the users. The multidimensional spectrum sensing method introduces a novel correlation measurement based on the algebraic characteristics of pseudo-noise (PN) sequences. The existence of periodic peaks in the new correlation function presents new modelling of the emergence of the spread-spectrum signal. Our basis for this is to use the cyclostatic property of the pseudo-noise code and obtain the distance between the two main peaks in the received signal correlation. According to the simulation results, the proposed algorithm exhibits excellent performance in signal detection and exact estimation of PN code length in low SNRs and multipath fading environments.

Spread Spectrum Modulation with Grassmannian Constellations for Mobile Multiple Access Underwater Acoustic Channels.

The objective of this study is to evaluate Grassmannian constellations combined with a spread spectrum multiple access scheme for underwater acoustic mobile multiple access communication systems. These communication systems enable the coordination of a fleet of Autonomous Underwater Vehicles (AUVs) from a surface or bottom control unit, e.g., a boat. Due to its robustness against phase rotation, the demodulator of Grassmannian constellations uses non-coherent detection, and the main advantage of such modulation lies in the spectrum efficiency gain with respect to conventional differential modulation. The communication system under study in this paper consists of (i), at the transmitter side, a Grassmannian modulation used in an orthogonal spread spectrum multiple access scheme called Multiuser Hyperbolic Frequency Modulation (MU-HFM) and (ii), at the receiver side, a non-coherent array decoder. The modulation and demodulation are presented as well as the considered spreading sequences. Finally, performances of the proposed transmission scheme are evaluated over replayed underwater acoustic channel responses collected at sea by a multi-sensor acoustic acquisition system.

Examining the Performance of MIL-STD-188-110D Waveform 0 Against FBMC-SS Over Skywave HF Channels

This paper provides a comprehensive performance comparison between a current robust military waveform; namely, MIL-STD-188-110D, Waveform 0, and a filter bank multicarrier spread-spectrum (FBMC-SS) waveform proposed for communications through ionospheric/skywave HF channels. Waveform 0 is effectively a direct sequence spread spectrum waveform that uses Walsh multi-codes to enhance the information transmission rate. It may thus be referred to as Walsh-DSSS. FBMC-SS, on the other hand, makes use of filter banks to provide excellent performance when the received signal is subject to partial band interference. Successful application of FBMC-SS for communications across skywave HF channels has been previously demonstrated, both theoretically and through experimental work. However, very little has been done to contrast FBMC-SS against Walsh-DSSS. The goal of this paper is to first add new features to FBMC-SS to bring it on par with Walsh-DSSS. These features include: (i), introduction of multi-codes that achieve a comparable (or better) data rate to Walsh-DSSS; and (ii), addition of a scrambling step applied to the multi-codes to make the receiver detection robust against widely spread multipaths. With this established, in the second part of the paper, we examine the performance of the developed FBMC-SS against Walsh-DSSS when both are applied for communications across skywave HF channels. The two waveforms are compared both through a theoretical study and through experimental works across several skywave channels ranging from hundreds to thousands of kilometers.

Space Spread-Spectrum Strategy for MMC to Reduce the Conducted EMI

A modular multilevel converter (MMC) is widely used due to its various advantages. However, it causes serious electromagnetic interference (EMI) problems. Therefore, it is necessary to suppress the conducted EMI of MMC. A novel spread-spectrum (SS) strategy, called space SS (SSS), is proposed in this article. The core idea of the SSS strategy is to make the carrier frequency of each submodule different. The principle and formula derivation of the SSS strategy are elaborated. SSS is a highly free strategy because carrier frequency can be distributed arbitrarily with space position. Several methods for determining carrier frequency distribution are proposed in this article. These methods not only achieve EMI suppression, but also take into account less circulating current, higher switching frequency, and lower switching loss. Total harmonic distortion (THD) and switching loss variation under the SSS strategy are also studied in the article. Experiments are carried out to validate the effectiveness of the SSS strategy. The SSS strategy can reduce the EMI by more than 12 dB and all obvious spikes in the spectrum are eliminated effectively. Besides, it is easy to implement and suitable for practical systems with quantities of submodules.

Combating Stealthy Thermal Covert Channel Attack With Its Thermal Signal Transmitted in Direct Sequence Spread Spectrum

Many-core systems are susceptible to attacks launched by thermal covert channel (TCC) attacks. Detection of TCC attacks often relies on the use of threshold-based approaches or variants, and a countermeasure to thwart the channel can be applied only after an attack is deemed to be present. In this article, we describe a direct sequence spread spectrum (DSSS)-based TCC, where its thermal data are modulated by a pseudo-random bit sequence. Unfortunately, such DSSS-based TCC has an extremely low signal strength that the signal is nearly indistinguishable from the noise and thus cannot be detected by any existing threshold-based detection methods. To combat this stealthy TCC, we propose a novel detection scheme that lets the received signal pass through a differential filter where irrelevant frequency components occupied mainly by the noise gets eliminated and the filtered signal is next compared against a threshold for successful detection. Experimental results show that the DSSS-based TCC can effectively survive detection by the existing detection methods with its BER as low as 4%. In contrast, with the proposed detection and countermeasure applied, the detection accuracy jumps to 89%, and the BER of the DSSS-based TCC soars to 50%, which indicates that the TCC is practically shut down.

A Novel Scheme for Discrete and Secure LoRa Communications.

In this paper, we present a new LoRa transceiver scheme to ensure discrete communications secure from potential eavesdroppers by leveraging a simple and elegant spread spectrum philosophy. The scheme modifies both preamble and payload waveforms by adapting a current state-of-the-art LoRa synchronization front-end. This scheme can also be seen as a self-jamming approach. Furthermore, we introduce a new payload demodulation method that avoids the adverse effects of the traditional cross-correlation solution that would otherwise be used. Our simulation results show that the self-jamming scheme exhibits very good symbol error rate (SER) performance with a loss of just 0.5 dB for a frequency spread factor of up to 10.

Design of Noncoherent and Coherent Receivers for Chirp Spread Spectrum Systems

LoRaWAN is a prominent communication standard to enable reliable low-power, long-range communications for the Internet of Things (IoT). The modulation technique used in LoRaWAN, commonly known as LoRa modulation, is based on the principle of chirp spread spectrum (CSS). The main objective of this article is to design a practical, low-complexity coherent receiver for a recently proposed CSS-based modulation scheme, called phase-shift keying CSS (PSK-CSS), that embeds extra information bits in the starting phases of conventional CSS symbols. To this end, a novel method is proposed to perform coarse timing and frequency synchronization that makes use of a preamble consisting of both up and down chirps and the shape of the pulse shaping and/or receive (matched) filters. Furthermore, to enable fine synchronization, timing, and phase loop filters are designed as simple first-order and second-order phase-locked loop (PLL) circuits with dynamic gain control. A natural outcome of our design is a practical noncoherent receiver that can be used for the conventional LoRa/CSS system and has better performance and/or lower computational complexity as compared to other existing designs. Extensive simulation results are presented to demonstrate the excellent performance and merits of the proposed design. In particular, the bit-error-rate (BER) performance of the higher-rate PSK-CSS system obtained with the proposed coherent receiver is only 0.25 dB worse than that obtained with the ideal co-coherent receiver, whereas it enjoys about 0.75-dB power gain over the noncoherent detection performance of the conventional CSS system. Thanks to the extra information bits carried by the phases of CSS symbols, the PSK-CSS system implemented with quadrature phase-shift keying (QPSK) delivers additional 23.44% and 18.75% data rate as compared to the conventional CSS system for the spreading factors (SFs) of 8 and 10, respectively.

Optimization of LoRa SF Allocation Based on Deep Reinforcement Learning

LoRa is an IoT communication technology that realizes ultra-long-distance transmission through spread spectrum modulation. However, its ultra-long-distance transmission also sacrifices the corresponding rate, and data conflicts are prone to occur when the number of nodes is large. In this article, we investigate various types of data collisions in LoRa wireless work, most of which are affected by Spreading Factor (SF) assignment. At present, the distribution of the SF for LoRa in the industry is mostly based on Min-airtime and Min-distance. In the case of a large number of nodes, the data collision between nodes will increase sharply. This paper proposes a SF redistribution scheme under limited network resources, in order to improve the terminal capacity of the LoRa gateway. First, the problem of minimizing the data collision rate without expanding gateway or network resources is presented. Specifically, the reallocation of SF with increasing number of terminals is studied. Finally, considering the randomness of the data sent by the terminal, SF redistribution schemes based on deep reinforcement learning (DRL) are developed. The simulation results show that the collision rate of the proposed SF redistribution scheme is nearly 30% lower than Min-airtime and Min-distance, and its total energy consumption is close to Min-distance. Therefore, the proposed SF redistribution scheme can effectively improve the gateway capacity of LoRa wireless network.

Blind Estimation Method of the Pseudo-Noise sequence of DS-SS Signals Based on Improved Propagator

The problem of pseudo-noise (PN) sequence estimation of direct sequence spread spectrum (DS-SS) signals is studied in this paper. A new estimation method for PN sequence is given based on improved propagator. The algorithm could estimate the spread spectrum sequence and the signal out-of-step time by using the signal subspace. Compared with the eigenvalue decomposition (EVD) method, the estimation accuracy of the out-of-step time is improved and the amount of calculation is effectively reduced. Computational complexity and performance comparisons are made between the new method and the existing algorithm. Simulation results show that the method can provide a good estimation, even the received signal is below the noise level.

MIMO spread spectrum GFDM communication systems with index modulation

SummaryIn the fourth‐generation communication system (4G), orthogonal frequency division multiplexing (OFDM) is widely used. However, due to the cyclic prefix (CP) being added, the low spectrum efficiency and high out‐of‐band radiation (OOB) cause OFDM not to be suitable for the beyond fifth‐generation (B5G) communication system. Therefore, to meet the rigorous demands in B5G for the Internet of Things (IoT) and massive machine‐type communications (MMTC), generalized frequency division multiplexing (GFDM) technology has received significant attention. On the other hand, index modulation (IM) can provide flexible settings of different parameters to achieve the ideal system goals. Therefore, this paper considers the combination of spread spectrum (SS) and IM under the framework of GFDM, called SS‐IM‐GFDM. In this scheme, the information bits are jointly conveyed by the indices of spreading codes and the conventional M‐ary modulated symbols, which increase the spectrum efficiency and improve the bit error rate performance. Furthermore, a low‐complexity maximum likelihood (ML) detector is proposed to recover the transmitted signal effectively at the receiver. Finally, the space‐time block code (STBC) is employed to construct the multi‐input multi‐output SS‐GFDM‐IM (MIMO SS‐GFDM‐IM) system to improve the system's reliability. The simulation results show that the proposed scheme is superior to the existing systems.

Millimeter-Wave Multi-Channel Backscatter Communication and Ranging with an FMCW Radar.

A multi-channel backscatter communication and radar sensing system is proposed and demonstrated in this paper. Frequency modulated continuous wave (FMCW) radar ranging is integrated with simultaneous uplink data transmission from a self-packaged active radio frequency (RF) tag. A novel package solution is proposed for the RF tag. With the proposed package, the RF tag can transmit a 32-QAM signal up to 2.5 Gbps and QPSK signal up to 8 Gbps. For a multi-tag scenario, we proposed using spread spectrum code to separate the data from each tag. In this case, tags can be placed at arbitrary locations without adjacent channel interference. Proof-of-concept simulations and measurements are demonstrated. A 625 Mbps data rate is achieved in a dual-tag scenario for two tags.