Maximum-likelihood diversity combining in partial-band noise

Maximum-likelihood diversity combining is investigated for an FFH/MFSK spread spectrum system in partial-band noise (PBN). The structure of maximum-likelihood diversity reception in PBN plus white Gaussian noise is derived. It is shown that signal-to-noise ratio and the noise variance at each hop have to be known to implement this optimum diversity combiner. Several suboptimum diversity combining schemes are also considered. The performance of the optimum combining scheme is evaluated. It is shown that adaptive gain control diversity combining actually achieves the optimum performance when interference is not very weak     

宽带MFSK/DFH系统抗部分频带噪声干扰性能分析

本文提出一种宽带MFSK/DFH系统模型,与常规差分跳频系统相比,在跳频点数一定条件下,通过增加多进制编码器复杂度,提高编码增益的方法提高系统抗干扰性能。对基于FFT的宽带MFSK/DFH系统接收机抗干扰性能进行了理论分析和计算机仿真验证。结果表明:在相同条件下,提出方法通过增加多进制编码器约束长度提高了系统抗干扰能力,实现方法简单有效,具有一定的实际应用价值。      

Worst-case partial-band noise jamming of Rician fading channels

Analyses of worst-case partial-band noise jamming of binary digital modulation are presented. Both noncoherent detection and differentially coherent detection over slowly varying, frequency-nonselective Rician fading channels are considered. The analysis bridges the gap between earlier works by Viterbi and Jacobs (1975) and by Omura (1981) for the limiting cases of Gaussian and Rayleigh channels, respectively. The results complement those by Crepeau (1990) for noncoherent binary transmission over the Nakagami fading channel. However, the Rician channel model has more physical significance since the parameter k is the ratio of scattered and specular powers. It is shown that a partial-band (partial-time) jamming strategy is optimum for all Rician channels with k<1 if the signal-to-jamming power ratio is sufficiently high. On the other hand, a full-band jamming strategy is the optimal strategy for k⩾1. Results for the Gaussian and Rayleigh channels can be derived from the present analysis in the limit as k=0 and k→∞, respectively     

Error correcting codes for fast frequency-hopped MFSK spread-spectrum satellite communications under worst case jamming

Spread-spectrum techniques such as frequency hopping (FH) have been used to reduce the vulnerability of satellite communications to jamming. In such a system, error correcting (EC) codes play an essential role. Thus a knowledge of the performance of various EC codes is necessary in order to choose an EC code and related system parameters in the design of an anti-jam system, this paper examines the performance of various types of error correcting codes under worst case partial band noise jamming and worst case multitone jamming using fast frequency-hopped, non-coherent M-ary frequency-shift keying (NCMFSK) with optimum diversity. A comprehensive study including convolutional codes, binary and non-binary block codes and concatenated codes has been conducted. This paper presents bit error rate (BER) performances of various error correcting codes. New candidate codes with superior anti-jam performance are identified.     

Performance of robust metrics with convolutional coding anddiversity in FHSS systems under partial-band noise jamming

The performance of robust metrics (metrics that can be computed from the outputs of the matched filters only) with convolutional coding and diversity under worst-case partial-band noise jamming is analyzed. Both binary and dual-k convolutional codes employing these metrics with diversity are compared via Union-Chernoff bounds. The performances of metrics considered in the literature that assume perfect side-information are given for comparison purposes. It is found that there exist very good robust metrics that provide performance comparable to metrics using perfect side-information. Among the robust metrics considered, the self-normalized metric offers the best performance and achieves performance practically identical to that of the square-law-combining metric with perfect side-information for M=8     

Multiple-dwell and sequential code acquisition with diversity forFFH/MFSK spread-spectrum systems under band multitone jamming

Pseudonoise code acquisition is investigated for constant hopping rate fast frequency-hopped (FFH)/M-ary frequency-shift keying systems under the effects of white Gaussian noise and band multitone jamming. In particular, serial search acquisition systems based on the traditional multiple-dwell test (up to three dwells) and three novel sequential tests are analyzed and compared. Analytical results show the following in a heavily jammed environment: (1) the inherent time diversity in an FFH system can significantly improve the acquisition performance of the multiple-dwell test and (2) the novel sequential tests can significantly outperform the multiple-dwell test. Our analytical results are verified by computer simulations     

Combined tone and noise jamming against coded FH/MFSK ECCM radios

The authors conjecture that a proper combination of partial-band tone jamming (PBTJ) and full-band noise jamming (FBNJ) under a given total jamming power constraint may be more effective than PBTJ alone, not only for the case with a low (E_s/N_J), but also for the case with high E_s/N_J, since the FBNJ can corrupt the jamming state information (JSI). Assuming this combination of PBTJ and FBNJ jamming, they consider three cases of receiver processing-the hard decision (HD) metric without JSI, the HD metric with perfect JSI, and the maximum-likelihood (ML) metric using Viterbi's ratio threshold (VRT) to generate a 1-b symbol decision quality indicator. System performance is evaluated in terms of the Chernoff bound on the probability of symbol error. From extensive numerical analysis the authors conclude that, for the case of the HD metric without JSI, PBTJ-only jamming is the worst form of jamming, as expected, since the receiver does not use JSI at all; for the other cases, a combination of PBTJ and FBNJ is the worst, with the worst ratio of PBTJ power to FBNJ power a function of the values of M and E_s/N_J     

Diversity and coding for FH/MFSK systems with fading and jamming.II. Selective diversity

For pt.1 see ibid., vol.COM-35, p.1329-41 (1987). A performance evaluation is presented for selective diversity with feedback for frequency-hopping M-ary frequency-shift-keyed systems operating over Rayleigh faded channels in the presence of partial-band noise and partial-band tone jamming. The behavior of uncoded and coded systems is studied. For coded systems, the performance is evaluated for hard-decision receivers without channel state information and soft-decision receivers with perfect jammer state information. The results demonstrate that the performance of uncoded FH/MFSK with selective diversity is unacceptable. However, this diversity technique can offer definite improvements for coded FH/MFSK systems. Specifically, the effectiveness of selective diversity signaling depends on the provision of a feedback channel between the transmitter and receiver to provide the transmitter with the fading gains of the independently faded channels. To obtain an improvement from the selective diversity signaling scheme described here, there must be multiple independently faded channels between the transmitter and receiver. If not, the performance of the selective diversity signaling scheme will be identical to the performance of FH/MFSK without diversity     

卷积码差分跳频系统抗部分频带干扰的性能

基于对无编码差分跳频系统抗部分频带干扰性能的研究,将卷积码引入差分跳频系统,研究了在有精确干扰状态信息的情况下,采用无迭代译码和迭代译码时相对于无编码系统的性能改善.同时,在无法得到精确干扰状态信息的情况下,提出了一种迭代干扰状态估计及译码算法.理论分析结果表明:在有精确干扰状态信息的情况下,采用卷积纠错编码和无迭代译码,对可用频率数为8的差分跳频系统,当比特误码率(BER)为10~(-6)时,性能改善约为2.5dB;而采用迭代译码,当BER为10~(-10)时,相对于无迭代译码,性能可进一步改善6dB.仿真结果则验证了迭代估计及译码算法的正确性,采用该算法可使编码系统在无精确干扰状态信息的情况下,仍能保持良好的抗干扰能力.     

短波差分跳频系统抗部分频带干扰性能分析

对接收端采用非相干平方律能量检测器及维特比译码器的短波差分跳频通信系统,在无衰落信道和频率非选择性慢衰落信道下抗部分频带干扰的性能进行了理论分析.若信道存在衰落,则假设每跳所经历的衰落过程是相互独立的,且服从瑞利分布.部分频带干扰被模拟为加性高斯噪声,分析中考虑了背景热噪声.结果表明:差分跳频相邻两跳频率之间的相关性为系统提供了良好的抗部分频带干扰的能力.若忽略背景热噪声,则当信干比为20dB时,在无衰落信道下最坏情况比特误码率可达10-5,而在频率非选择性瑞利慢衰落信道下最坏情况比特误码率可达10-3.     

Effect of worst case multiple partial-band noise and tone jammerson coded FH/SSMA systems

The author characterizes and evaluates the effect of simultaneous multiple partial-band noise or tone jammers and other user interference on a single communication link employing frequency-hopped spread-spectrum (FH/SS) signaling, M-ary frequency-shift keying (FSK) modulation with noncoherent demodulation, and Reed-Solomon coding. For the symbol error probability of these systems, the author derives exact expressions in the absence of multiple-access interference and tight upper bounds in the presence of other-user interference. Although the analytical methods are valid for any number of multiple jammers, the numerical study is restricted to the cases of two and three-partial-band noise and tone jammers. For fixed values of the spectral densities of noise jammers, or the energies per symbol of tone jammers, the worst-case fraction of the band that each jammer should use in order to maximize the error probability of the FH/SS or FH/SSMA system is evaluated. For the range of the signal-to-jammer power ratios examined, multiple-noise or multiple-tone jammers appear to have no advantage over single-tone jammers of equivalent spectral density or energy per symbol, but achieve approximately the same worst-case performance by jamming smaller fractions of the band     

Performance of FFH/BFSK product combining receiver over aRician-fading channel with partial-band jamming

A theoretical performance evaluation for an FFH/BFSK spread spectrum product combining receiver over a Rician-fading channel in the presence of partial-band jamming and AWGN is presented. The BER of this receiver is better than that of a linear combining receiver for a strong direct signal, and improvement is significant when diversity is increased     

Rejection of partial-band noise jamming with FFH/BFSK productcombining receiver over Nakagami-fading channel

The bit error rate performance of a noncoherent fast frequency-hopping binary frequency shift keying system equipped with a product combining receiver is evaluated in the presence of partial-band noise jamming and additive white Gaussian noise over a Nakagami-faded channel. Using the derived output characteristic function, numerical results are generated for different fading conditions     

Maximum likelihood combining for noncoherent and differentially coherent frequency-hopping multiple-access systems

The frequency-hopping multiple-access (FHMA) approach to digital mobile radio provides diversity gain against frequency selective fading. Optimum combining methods, in the maximum likelihood sense, are derived for FHMA systems with either noncoherent or differentially coherent detection. In the optimum combining structure, the detector outputs are first passed through a nonlinearity and then a linear combiner.     

Effects of fading and partial-band noise jamming on a fast FH/BFSKacquisition receiver with noise-normalization combination

We present the performance analysis of a fast frequency-hopped (FH) binary orthogonal frequency-shift keying acquisition receiver for communication against adverse environments. The receiver employs noncoherent, noise-normalized, matched-filtered (MF) correlation detection for rapid acquisition in the search mode. Our analysis includes four types of communication environments, namely additive white Gaussian noise (AWGN) channel, AWGN channel with partial-band noise jamming, fading channels, and fading channels with partial-band noise jamming. The considered fading channels include Nakagami-m, Rician, and Rayleigh amplitude models. Based on Beaulieu's (see ibid., vol.38, no.9, p.1463, 1990) convergent series approach, efficient analytical formulas are developed for performance evaluation. Example performance results for various environments are presented in terms of two acquisition probabilities, namely the detection probability and the false alarm probability of the noise-normalized MF detector. It is analytically shown that with a short MF correlation length and with a sufficiently large ratio of signal power to noise power the fast FH diversity combining yields noticeable performance improvement for environments with strong fading. When the MF correlation is lengthened, this improvement tends to fade away and the diversity combining results in performance loss     

Iterative demodulation and decoding of frequency-hopped PSK in partial-band jamming

This paper considers the problem of developing and utilizing side information in a frequency-hopped communication system using phase-shift keying (PSK) and operating in an environment with partial-band jamming. Two aspects of side information are studied. The first deals with estimating the unknown random carrier phase that varies from hop to hop. The second aspect is the detection of jamming signals. We use a serially concatenated convolutional code structure with differential M-ary PSK as the inner code. The iterative receiver uses an expanded trellis in the inner decoder to resolve the phase ambiguity and is augmented by a ratio-threshold test for detecting jammer energy. Performance is compared for different dwell interval lengths and both log-APP and max-log-APP decoding algorithms. This paper also considers the effect of different thresholds on the false alarm and detection probabilities of the ratio-threshold test.     

Performance of MFSK signals with postdetection square-law diversity combining in arbitrarily correlated Nakagami-m fading channels

This letter presents an analysis of the error probability for noncoherent orthogonal multiple frequency-shift keying (MFSK) signals with postdetection square-law combining (SLC) when the signals transmitted over additive white Gaussian noise (AWGN) and slow frequency-nonselective arbitrarily correlated Nakagami-m fading channels. New exact expressions in a onefold integral for the probability of error of MFSK signals with postdetection square-law diversity combining operating in AWGN channel as well as in arbitrarily correlated Nakagami-m fading channels are derived. The effects of arbitrarily values of fading severity parameter m and the arbitrarily correlation between the L diversity channels are considered. The derived expressions can be easily computed, and hence, can be usefully exploited in the performance evaluation of digital mobile radio systems.     

A novel hybrid ARQ scheme using turbo codes and diversity combining

A novel HARQ scheme using turbo codes is proposed. The method jointly utilizes diversity combining, partial retransmission, and power scaling. Computer simulations and density evolution analysis show that the new method outperforms equal gain diversity combining and soft information combining techniques for a wide E_b/N₀$E_b/N_0$ range.     

Vector similarity-based detection scheme for multi-antenna FH/MFSK systems in the presence of follower jamming

Follower partial band jamming has been recognised as an efficient strategy to degrade the performance of frequency hopping (FH) systems. To alleviate this detrimental effect, a novel detection scheme for slow FH/Mary frequency shift keying systems, which use two receive antennas in a quasi-static flat fading channel, is proposed. Specifically, using the principle of vector similarity, an area-based vector similarity metric is formulated to give an estimate of the unknown spatial correlation of the received jamming components at the two receive antennas. The jamming signal can then be removed in the symbol detection process. The improved performance of the new scheme is verified by using an analysis under a jamming dominant environment as well as simulated bit error rate results.     

Performance study of suboptimum maximum-likelihood receivers for FFH/MFSK systems with multitone jamming over fading channels

We derive two suboptimum maximum-likelihood (ML) receivers for fast frequency-hopped M-ary frequency-shift-keying (FSK) spread-spectrum (SS) communication systems. These two receiver structures attempt to countermeasure the effects of the worst case multitone jamming (MTJ) and additive white Gaussian noise over Rayleigh- and Rician-fading channels, respectively. In addition, analytical bit-error-rate (BER) expressions for the two proposed suboptimum structures are derived and validated by simulation results. Performance comparisons among various receivers show that the proposed suboptimum receivers significantly outperform the other existing receivers over fading channels. The optimum diversity level of the suboptimum ML receiver for the Rayleigh-fading case is found to be higher than that of the Rician-fading case. In addition, the proposed suboptimum ML receivers with optimum diversity levels can effectively remove the effect of MTJ, even under very low signal-to-jamming ratio conditions.