Fast Frequency Hopping Spread Spectrum Research Articles

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.

METHOD OF SIGNAL PARAMETERS ESTIMATION OF RADIOSTATIONS WITH FAST FREQUENCY HOPPING SPREAD SPECTRUM

Modern special-purpose radios utilize ultra-short burst mode with frequency hopping spread spectrum. Such digital radios operate, as a rule, in the fast frequency hopping spread spectrum mode and use a considerable number of frequency channels with frequency separation between adjacent channels, which is significantly less than the spectrum width of the frequency element. The main modulation in such radios is frequency manipulation. The identification of signals with fast frequency hopping spread spectrum and frequency manipulation is performed by estimating the speed of frequency tuning, the speed of information transmission, the number of frequency channels and the step of the frequency grid and comparing the obtained values with the corresponding characteristics of known radio stations with fast frequency hopping spread spectrum. The time limits and the duration of the frequency elements are calculated by the envelope of the received signal. For this purpose, a suitable method has been developed, which consists in filtering the envelope by means of a moving average window for suppression of the noise component and threshold processing. The threshold value is selected by the given probability of its exceeding by noise sample. The proposed approach to determining the time limits of frequency elements does not require any a priori signal information. To determine the frequencies of the frequency elements periodogram estimates are used. An exponential extrapolation of the discrete power spectrum was used to increase the frequency resolution at a fixed window length of the fast Fourier transform. It is shown that in order to uniquely determine the step of the frequency grid and the diversity of frequency manipulation frequencies, it is necessary to analyze the number of frequency elements not less than the number of frequency channels. By analyzing the histogram of the differences of sorted frequency vector of the frequency elements, determine the frequency grid, frequency diversity and the multiplicity of frequency manipulation. An approximate estimate of the number of frequency channels is calculated as the ratio of the magnitude of the frequency vector of the frequency elements to the step of the frequency grid. As a rule, symbol transmission at frequency manipulation is carried out at orthogonal frequencies, so it can be roughly assumed that the symbolic speed is equal to the frequency diversity of the frequency manipulation. The baud rate can be set only after signal demodulation and bit stream analysis, which can determine the frame structure and the number of service and information bits. The proposed method provides the estimation of the values of the parameters necessary for the identification of radio stations of signal parameters with a relative error not exceeding 0.3%, with values of signal to noise ratio not lower than 5 dB.

Performance of M-ary turbo coded synchronous FHSS multiple access networks with noncoherent MFSK under Rayleigh fading channels

The performance of M-ary turbo coded synchronous, fast frequency-hopping spread spectrum multiple-access (FHSS-MA) networks with M-ary frequency shift keying (MFSK) and noncoherent detection is analyzed under Rayleigh fading. Results indicate that M-ary turbo codes dramatically enhance the performance of FHSS-MA networks using MFSK compared to binary turbo codes.

Performance of coherent fast frequency-hopped spread-spectrum receivers with partial-band noise jamming and AWGN

Performance analyses of coherent fast frequency-hopped spread-spectrum (FHSS) systems with the presence of partial-band noise jamming (PBNJ) and additive white gaussian noise (AWGN) are presented. Bit-error-rate expressions for the coherent maximum-likelihood (ML), linear-combination (LC), and hard-decision majority-vote (HDMV) receivers with binary phase-shift-keying are derived and validated by simulation results. Among the three proposed receivers, the coherent ML receiver gives the best performance under the worst-case PBNJ condition, followed by the coherent HDMV receiver and the coherent LC receiver. The BER results of the coherent ML receiver improve as the diversity level increases, although the incremental improvement in performance will diminish eventually when the diversity level is sufficiently large. The coherent LC receiver is incapable of providing diversity improvement under the worst-case PBNJ condition. For the coherent HDMV receiver it has been shown that there is an optimum diversity level under the worst-case PBNJ condition. The locus of the optimum diversity level for the coherent HDMV receiver is also presented, which allows one to obtain the optimum diversity level for the coherent HDMV receiver under different levels of signal-to-jamming ratio. It has been shown that the three coherent receivers significantly outperform the various existing noncoherent diversity-combining receivers under the worst-case PBNJ and AWGN conditions.

Experimental characterization of a low-cost fast frequency-hopping spread-spectrum system for wireless in-house optical communications

The design and experimental characterization of a wireless infrared fast frequency hopping spread spectrum (WIR-FFH-SS) system is presented. The use of spread spectrum techniques on wireless infrared indoor communications reduces the effect of interference present in the infrared channel. It also improves the system performance versus multipath propagation. The use of FH-SS techniques allow the use of code division multiple access (CDMA) strategies for sharing the channel. We have developed and tested a prototype with a bit rate of 1 Mb/s, but it can be easily raised to 4 Mb/s. We also present some results on a simplified synchronization structure for use with reduced-cost domestic equipment. These systems are suitable for different applications at low and medium rates.

Design of fast frequency-hopping spread-spectrum system for wireless infrared communications

The design and experimental characterisation of a wireless infrared fast frequency-hopping spread-spectrum (WIR-FFHSS) system is presented. The bit rate of this prototype is 512 kbit/s, but it can be easily increased to 1 Mbit/s. It is suitable for different applications at low and medium rates.

Fast hopping frequency synthesizer using difference of phase errors

Fast frequency hopped–spread spectrum (FFH-SS) communication is a system that can realize excellent interference immunity, disturbance immunity, and security. In the implementation of this system, there must be a synthesizer that can switch the frequency with high speed. From such a viewpoint, the authors previously proposed a fast hopping PLL synthesizer, based on the initial value presentation in which the phase difference obtained by the phase comparator at the same frequency output of the previous period is reproduced by utilizing the periodicity of the frequency hopping. It is shown that the hopping frequency can be raised to one-third the reference frequency. In this method, however, the phase error of the VCO output generated in frequency switching produces an error in the phase difference reproduced by the phase comparator, which results in a stepwise error in the output frequency. In order to deal with this problem, a circuit for the phase error compensation was proposed, but there remained problems such as the difficulty of high-frequency operation. This paper proposes a fast hopping frequency synthesizer that solves those problems. In the proposed method, the phase difference is not reproduced by the phase comparator, and there is no effect from phase error of the VCO output generated in frequency switching due to the use of the difference of the phase errors. Faster operation is realized, with the hopping frequency equal to one-half the reference frequency. This helps to reduce the number of bits in the A/D converter below that of the D/A converter. © 1999 Scripta Technica, Electron Comm Jpn Pt 1, 82(10): 10–19, 1999

Partial-band jammer suppression in FFH spread-spectrum system using FFT

This paper presents the performance evaluation of partial-band jammer suppression using fast Fourier transform (FFT) as an alternative in a fast frequency-hopped spread-spectrum (FFH/SS) communication system. The FFH/SS system employs a clipper to restrict the effect of a partial-band jammer before diversity combining. No additional side information of the jammer is required, and the analytical expression for the bit error rate (BER) is derived. It is shown that the performance of the FFT-based clipper receiver is better than that of the square-law linear combining receiver and comparable to that of the square-law clipper receiver.

FFT-based self-normalising receiver for FFH spread spectrum systems

Partial-band interference due to intentional and unintentional jamming in fast frequency-hopping spread-spectrum (FFH/SS) systems can be effectively suppressed by additional signal processing techniques. The authors present the performance analysis of a binary frequency-shift-keying SS system in conjunction with a fast Fourier transform (FFT) processor. The FFT-based self-normalising receiver, requiring no knowledge of signal and jamming levels, also enjoys the advantages provided by modern digital signal processing technology. Numerical results show that the receiver performance is better than that of the square-law linear combining receiver and comparable to that of the conventional self-normalising receiver as well as the clipper receiver.

Performance of a fast frequency-hopped noncoherent MFSK receiver with nonideal adaptive gain control

An error probability analysis is performed for an orthogonal noncoherent M-ary frequency-shift keying (MFSK) communication system employing fast frequency-hopped (FFH) spread spectrum with diversity. The signal is assumed to be transmitted through a frequency-nonselective slowly fading channel with partial-band noise interference. The partial-band interference is modeled as a Gaussian process. Both the information signal and the partial-band noise interference signal are assumed to be affected by channel fading; it is assumed that the two fading processes are independent and that channel fading need not necessarily affect the information signal and the interference signal in the same way. Each diversity reception is assumed to fade independently according to a Rician process. Adaptive gain control is employed to minimize partial-band interference effects, and the effect of inaccurate noise measurement on the ability of the adaptive gain control receiver to reject partial-band interference is examined. The effect of thermal noise is included in the analysis.

Design and analysis of a fast frequency-hopped DBPSK communication system. I. System description and hop timing tracking loop analysis

Design and performance analyses of a fast frequency-hopped (FFH) spread spectrum communication system employing differential binary phase shift keying (DBPSK) modulation and differentially coherent demodulation are presented. The receiver utilizes a hop timing tracking loop to lock the hop clock. Through this tracking loop, the differentially coherent demodulation of an FFH signal is made possible. The authors present the system model of an FFH/DBPSK system. A digital, full-time early-late gate timing error estimator is investigated. The tracking loop is shown to be a Markov chain, and various performance characteristics of the tracking loop are studied.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Design and analysis of a fast frequency-hopped DBPSK communication system. I. Error performance in AWGN plus partial-band noise jamming

The purpose of the paper is to present the design and performance analysis of a fast frequency-hopped (FFH) spread spectrum communication system employing differential binary phase-shifted-keying (DBPSK) modulation and differentially coherent demodulation. The receiver utilizes a hop time tracking loop to lock the hop clock. The authors present an analysis of the average bit error probability of the overall system when it is operating in additive white Gaussian noise plus partial-band noise jamming. The effect of channel phase distortion is also studied. >

Multiple hops/symbol fast frequency-hopping spread spectrum multiple access with coding for indoor radio

The paper is concerned with the bit error rate (BER) performance of multiple hops/ symbol fast-frequency-hopped spread-spectrum multiple-access (FFH-SSMA) systems operating through an indoor radio multipath Rayleigh fading channel. M-ary frequency-shift-keying (MFSK) modulation with noncoherent square-law envelope demodulation and Reed-Solomon (RS) coding is considered. Results obtained in this study show that an FFH-SSMA system is not sensitive to the changes in the number of paths. Employing RS coding with an increased number M of MFSK signals yields a significant improvement in BER performance. Assuming a given system bandwidth, using a single hop per symbol is the best solution when the multiaccess interference can be large. Using several hops per symbol is more advantageous when the multiaccess interference is small.

Error probability of fast frequency hopping spread spectrum with BFSK modulation in selective Rayleigh and selective Rician fading channels

The performance of noncoherent reception in fast frequency hopped spread-spectrum (FFH-SS) communication systems operating through noisy, fading multipath channels is investigated. Systems operating with binary frequency-shift keying (BFSK) modulation and noncoherent demodulation are examined under the assumption of very slow fading. These analyses demonstrate the frequency hopping benefits in selective channels. Expressions are derived for the bit error rate in the context of selective Rayleigh and selective Rician fading channels, as a function of channel and system parameters. >

Equal gain diversity improvement in fast frequency hopping spread spectrum multiple-access (FFH-SSMA) communications over Rayleigh fading channels

The authors present the performance analysis of an FFH-SSMA system with a binary frequency-shift keying modulation scheme and noncoherent demodulation, operating in a combined environment of Rayleigh selective fading, other users interference, and additive white Gaussian noise. Improvement due to the use of equal gain diversity (EGD) is studied in terms of bit-error rate (BER). Expressions of the BER are evaluated when a maximum-likelihood decision criterion is used to show the advantages of the use of frequency hopping selective Rayleigh fading channels. Results show the equivalence between frequency hopping and direct-sequence (DS) modulation in combating multipath fading. The performance, in terms of irreducible error probability, of the system under study and that of the DS-CPSK when using EGD are not significantly different.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>