Frequency Hopping Spread Spectrum System Research Articles

Photonics based frequency hopping spread spectrum system for secure terahertz communications.

Terahertz (THz) spectrum (100 GHz-10 THz) is considered the next frontier in the design of high-speed wireless communication systems. While the high-power THz sources have commercially become available, it increases the possibility of developing THz jammers to disrupt the THz communication link. Therefore, the development of novel anti-jamming solutions is the need of the hour. In this work, we present the photonics-based THz communication system and demonstrate the frequency hopping spread spectrum (FHSS) technique which acts against the single/multi-tone jamming attack in the frequency window of 110 GHz-170 GHz. By tuning the output wavelength of the distributed feedback lasers, the THz carrier frequencies are swept back and forth within the scanning window. The frequency tuning range was measured for different scanning rates of the laser which decreases rapidly with the increase in the scanning rate. Next, we demonstrate the THz FHSS technique in a real-time communication system by transmitting a 6 Gbps NRZ signal in both wireless and THz-fiber-based links within the link distance of 1.75 m. We experimentally found that the measured bit error rate in the THz FHSS system is the time average of the measured BER for individual carrier frequencies within the hopping frequency window. By combining with the forward error correction codes and by using the tunable filter in the receiver, we believe that the proposed technique will offer a novel and compact solution against the single/multi-tone jammer for high-bit rate THz communications.

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

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

Radio Frequency Fingerprinting for Frequency Hopping Emitter Identification

In a frequency hopping spread spectrum (FHSS) network, the hopping pattern plays an important role in user authentication at the physical layer. However, recently, it has been possible to trace the hopping pattern through a blind estimation method for frequency hopping (FH) signals. If the hopping pattern can be reproduced, the attacker can imitate the FH signal and send the fake data to the FHSS system. To prevent this situation, a non-replicable authentication system that targets the physical layer of an FHSS network is required. In this study, a radio frequency fingerprinting-based emitter identification method targeting FH signals was proposed. A signal fingerprint (SF) was extracted and transformed into a spectrogram representing the time–frequency behavior of the SF. This spectrogram was trained on a deep inception network-based classifier, and an ensemble approach utilizing the multimodality of the SFs was applied. A detection algorithm was applied to the output vectors of the ensemble classifier for attacker detection. The results showed that the SF spectrogram can be effectively utilized to identify the emitter with 97% accuracy, and the output vectors of the classifier can be effectively utilized to detect the attacker with an area under the receiver operating characteristic curve of 0.99.

Design of a spurious-free RF frequency synthesizer for fast-settling receivers

A tunable reference clock frequency topology is presented as a spur reduction application for frequency synthesizers of fast frequency hopping spread spectrum systems. The method was verified by measurements on a designed hardware operating at L-band frequencies. This spur reduction method is based on optimizing the reference clock frequency of synthesizers to mitigate spurs. By using the spur reduction method, the power of spurious signals was reduced up to 57 dB. The performance of the spur reduction method was also analyzed at different loop-filter configurations. Smaller lock time was obtained by enlarging the bandwidth of the loop filter up to 150 kHz. The required power response of the spurious signals specified in telecommunication standards was achieved even though the loop filter bandwidth was enlarged.

Antijamming Improvement for Frequency Hopping Using Noise-Jammer Power Estimator

In frequency-hopping spread-spectrum (FHSS) systems, jammer detection and mitigation are important but difficult. Each slot of the FHSS experiences frequency-selective fading and unequal transceiver-frequency gains that hinder the detection of jammed slots and result in a poor bit-error rate (BER). To increase BER performance, we first propose a noise-jammer power estimator (NJPE) that estimates noise and jammer powers regardless of different channel gains, and derived its normalized Cramér–Rao bound (NCRB). Second, we developed a jammer detector based on gamma distribution, and designed a restoration method combining all nonjammed slots. Computer simulations verified the derived NCRB of the proposed NJPE by normalized mean squared error (NMSE), and showed that the jammer-detection probability of the proposed jammer detector was better than that of conventional detectors. The BER performance of the proposed method was also shown to be better than that of conventional methods.

Interceptor complexity analysis for mixed BPSK–QPSK modulated frequency hopping spread spectrum systems

In this paper, we propose a method of mixing modulation schemes to make it very difficult to intercept information from frequency hopping signals and assess analytically the difficulty of information interception using the interceptor computational complexity as a metric. In particular, we consider, to illustrate the effectiveness of mixing modulation schemes for secure communications, a method of randomly mixing BPSK and QPSK modulations in frequency hopping systems and compare it with conventional frequency hopping systems using BPSK only or QPSK only modulation scheme by deriving simple to evaluate upper and lower bounds on the interceptor complexity.

Serial search code acquisition of FHSS system

Frequency hopping spread spectrum (FHSS) communication systems offer significant performance advantages in view of their low probability of intercept, improved performance in multipath fading environments and their ability to avoid interference by hopping into low interference frequency channels. For the transmitted sequence to be correctly received and demodulated, the frequency hop sequence used at the receiver should be similar to that employed in the transmitter. Code acquisition in frequency hopping attempts to address this problem by providing a frequency hop pattern at the receiver that is nearly identical to that used at the transmitter. Code acquisition brings the alignment between the transmitter and the receiver hop pattern to at least one hop period. In this paper, the serial search code acquisition, which is the first step of synchronization, is studied for frequency hopping spread spectrum (HFSS) system over Rayleigh fading channel. Simulation results show the effect of fading channel on the code acquisition.

New Extension Constructions of Optimal Frequency-Hopping Sequence Sets

In this paper, a general framework of constructing the optimal frequency-hopping sequence (FHS) sets is presented based on the designated direct product. Under this framework, we obtain infinitely many new optimal FHS sets by combining some one-coincide (OC) sequence sets that are newly constructed in this paper with some known optimal FHS sets. Our constructions are also based on extension method. However, our constructions give new and flexible parameters due to the free choice of OC sequence set. As a result, our constructions allow a great flexibility of choosing parameters of FHS sets for a given frequency-hopping spread spectrum system.

Study of Code Acquisition of FHSS System Over Rician Channel

Abstract Frequency hopping is used in different communications systems for its robustness by providing frequency diversity against jamming and interfering signals. Successful detection and demodulation of a frequency hopping signal is dependent on proper tuning to transmit frequency and time synchronization of the burst. The sequence of hop frequencies is generally determined by a Pseudo-Noise (PN) sequence and time synchronization is achieved using synchronization preambles in the transmit burst. Successful acquisition of the hop frequency sequence could be achieved when at least a single burst’s data is successfully decoded at the receiver. This paper studies the serial search code acquisition which is the first step of synchronization for frequency hopping spread spectrum (FHSS) system over Rician fading channel. Simulation results show the effect of fading channel on the code acquisition.

A new family of optimal FHS sets with composite lengths

In frequency-hopping spread spectrum systems, frequency-hopping sequences (FHSs) are widely used to alleviate the interferences caused by the hits of frequencies. In this paper, based on the Zeng–Cai–Tang–Yang cyclotomy and the Chinese Remainder Theorem, a family of FHS sets with composite lengths is constructed and the Hamming correlations of the new FHS sets are derived by some basic properties of cyclotomic numbers. The results show that the proposed FHS sets are optimal with respect to the Peng–Fan bound. Furthermore, it generalizes some earlier cyclotomic constructions of optimal FHS sets and produces optimal FHS sets with new parameters which are not covered in the literature.

New Extension Interleaved Constructions of Optimal Frequency Hopping Sequence Sets With Low Hit Zone

In this paper, a designated interleaved structure of constructing optimal frequency-hopping sequence (FHS) sets with low hit zone (LHZ) is presented based on the Cartesian product. By the general structure, we obtain infinitely many optimal LHZ FHS sets with new and flexible parameters by combining the optimal LHZ FHS sets with one-coincidence sequence sets. Moreover, our constructions remove the constraint requiring that the extension factor is co-prime with the length of the original FHSs. In this paper, most of the extension constructions suffer from this constraint. As a result, our constructions allow great flexibility of choosing parameters of the LHZ FHS sets for a given quasi-synchronization frequency-hopping spread spectrum system.

Performance evaluation and mathematical analysis of direct sequence and frequency hopping spread spectrum systems under wideband interference

This paper presents performance evaluation and comparison analysis of Direct Sequence Spread Spectrum (DSSS) and Frequency Hopping Spread Spectrum (FHSS) systems. The evaluation and analysis are done based on the systems performance against wideband interferences. The interferences are signals with similar spectrum characteristic to the transmitted signals of DSSS and FHSS systems. Bit Error Ratio (BER) is used as evaluation parameter to assess the performance of both systems. Simulation and mathematical analysis are performed to test and verify the performance of both systems. Mathematical analysis also verifies that increasing Spreading Frequency on certain conditions will reduce the BER. This research also points out that FHSS system has a better performance compared to DSSS system indicated by smaller BER.

주파수 도약 확산 스펙트럼 시스템에서 CFAR 검출기를 이용한 항재밍 영향 성능 분석

주파수 도약 확산 스펙트럼 시스템에서 CFAR 검출기를 이용한 항재밍 영향 성능 분석

New Design of Optimal Frequency-Hopping Sequence Based on Cyclotomy

In wireless communication systems, frequency-hopping spread spectrum and direct sequence spread spectrum are two main spread coding technologies. Frequency-hopping sequence (FHS) plays an important role in frequency-hopping spread spectrum systems. In this paper, a new class of FHS based on Cyclotomy is constructed. New FHS has new parameters not covered in the literature, and is optimal according to the Lempel-Greenberger bound.

New Families of Optimal Frequency Hopping Sequence Sets

Frequency hopping sequences (FHSs) are employed to mitigate the interferences caused by the hits of frequencies in frequency hopping spread spectrum systems. In this paper, we present combinatorial constructions for FHS sets, including direct constructions by using cyclotomic classes, recursive constructions based on cyclic difference matrices, merging blocks, and discrete logarithm. By these constructions, a number of series of new FHS sets are then produced. These FHS sets are optimal with respect to the Peng–Fan bounds.

Fast carrier recovery in FHSS systems with DDS based Costas loop

Owing to fast change of carrier frequency, the usageof coherent demodulation methods in Frequency-Hopping SpreadSpectrum (FHSS) communication systems is limited by thecoherent state acquisition performed by the local oscillator. Toget around this difficulty, it is needed to implement the circuitwithin the FHSS system that is able to rapidly generate acoherent carrier. The work presented in this paper proposes arealization of Costas loop based on a combination of DirectDigital Synthesis (DDS), (Temperature-Compensated) Voltage-Controlled Oscillator ((TC)VCXO), and Phase-Locked Loop(PLL), which in particular enables a short period (less than 100μs) required for a local oscillator to reach the coherent state. Theimplemented model is thoroughly described with measurementsresults also provided.

Blind Hopping Phase Estimator in Frequency-Hopped FM and BFSK Systems

A blind hopping phase estimator is proposed for the demodulation of received signals in frequency-hopping spread spectrum systems. The received signals are assumed to be bandwidth limited with a shaping filter, modulated as frequency modulation (FM) or binary frequency shift keying (BFSK), and hopped by predetermined random frequency sequences. In the demodulation procedure in this paper, the hopping frequency tracking is accomplished by choosing a frequency component with maximum amplitude after taking a discrete Fourier transform, and the hopping phase estimator performs the conjugated product of two consecutive signals and moving-average filtering. The probability density function and Cramer-Rao low bound (CRLB) of the proposed estimator are evaluated. The proposed scheme not only is very simple to implement but also performs close to the CRLB in demodulating hopped FM/BFSK signals.

OFDM Synthetic Aperture Radar Imaging With Sufficient Cyclic Prefix

The existing linear frequency modulated (LFM) (or step frequency) and random noise synthetic aperture radar (SAR) systems may correspond to the frequency hopping (FH) and direct sequence (DS) spread spectrum systems in the past second and third generation wireless communications. Similar to the current and future wireless communications generations, in this paper, we propose OFDM SAR imaging, where a sufficient cyclic prefix (CP) is added to each OFDM pulse. The sufficient CP insertion converts an inter-symbol interference (ISI) channel from multipaths into multiple ISI-free subchannels as the key in a wireless communications system, and analogously, it provides an inter-range-cell interference (IRCI) free (high range resolution) SAR image in a SAR system. The sufficient CP insertion along with our newly proposed SAR imaging algorithm particularly for the OFDM signals also differentiates this paper from all the existing studies in the literature on OFDM radar signal processing. Simulation results are presented to illustrate the high range resolution performance of our proposed CP based OFDM SAR imaging algorithm.

도약 FM/BFSK 시스템에서 블라인드 도약 위상 추정기

본 논문에서는 주파수 도약 대역확산 시스템에서 도약에 대한 사전 정보 없이 수신신호를 복조 할 수 있는 도약 위상 추정기를 제안하였다. 복조 과정은 첫째 샘플링 된 수신 신호를 DFT(Discrete Fourier Transform)를 한 후 도약 대역 내에 존재하는 신호 중 가장 큰 크기를 갖는 주파수 성분을 선택한다. 둘째 검출된 신호는 차곱 과정과 다운샘플링을 통하여 기저대역신호로 변환된 후 도약위상 추정기에서 도약 주파수에 의해서 발생된 도약 위상을 추정한다. 제안된 위상 추정기의 수학적 모델에 대한 성능 분석을 위해 확률밀도함수 및 분산 성능을 구하고 모의실험 결과와 비교하여 잘 일치함을 확인하였다. We proposed a hopping phase estimator to demodulate the received signals without any hopping information in frequency hopping spread spectrum systems. The demodulation process in this paper is as follows: hopped frequency tracking is accomplished by choosing a frequency component with maximum amplitude after taking discrete Fourier transform and a hopping frequency estimator which estimates the phase generated by hopped frequency is established through difference product and down-sampling. We obtained the probability density function and variance performance of the proposed estimator and confirmed that the analysis and the simulation results were agreed with each other.

New Families of Optimal Frequency-Hopping Sequences of Composite Lengths

Frequency-hopping sequences (FHSs) are employed to mitigate the interferences caused by the hits of frequencies in frequency-hopping spread spectrum systems. In this paper, we present two new constructions for FHS sets. We first give a new construction for FHS sets of length nN for two positive integers n and N with gcd(n, N) = 1. We then present another construction for FHS sets of length (q - 1)N, where q is a prime power satisfying gcd(q - 1, N) = 1. By these two constructions, we obtain infinitely many new optimal FHS sets with respect to the Peng-Fan bound as well as new optimal FHSs with respect to the Lempel-Greenberger bound, which have length nN or n(q -1)N. As a result, a great deal of flexibility may be provided in the choice of FHS sets for a given frequency-hopping spread spectrum system.