Multiple-symbol differential detection of GMSK for mobilecommunications

A new multiple differential detection (MDD) sequence estimator is described which uses a decision feedback for the demodulation of a GMSK signal. This technique is based upon a maximum-likelihood sequence estimation of the transmitted phases rather than on a symbol-by-symbol detection. An upper and a lower bound on the bit error probability of the described detector in the case of an AWGN channel and a two-ray Rayleigh fading channel are derived. The performance of the detection algorithm in a mobile radio communication system is obtained through computer simulation. Comparisons with the coherent detection algorithm show that the proposed detection algorithm is quite attractive both in an AWGN and in a multipath channel     

A simple modulation scheme for error performance improvement oflow-deviation digital FM

The paper proposes a simple digital modulation scheme named LF (lowpass filtered)-π/2 TFSK. Since it is essentially a constant envelope modulation scheme, it can be applied to severely band-limited channels such as land mobile and satellite mobile radio channels. Its error performance is superior to that of MSK modulation in band-limited channels at the cost of a slightly wider bandwidth (approximately 1.06 times) than GMSK. The required E_b/N₀ with 2-bit differential detection at the error rate of 10^-5 is 2.3 dB smaller than that of GMSK in an additive Gaussian channel and 3 dB smaller at the average error rate of 10^-4 in a fast Rayleigh fading channel. The above result, obtained by an exact error rate analysis, was confirmed by laboratory experiments at a frequency of 880 MHz. The spectral characteristics were analyzed through computer simulations and the results were found to agree with those obtained from a hardware implementation     

An adaptive truncated MLSE receiver for Japanese personal digitalcellular

This paper describes a dual-mode Japanese personal digital cellular receiver that uses an adaptive truncated symbol-spaced maximum-likelihood sequence-estimation (MLSE) equalizer in one mode and a tangent type differential detector in the other. The receiver employs a channel estimation and symbol synchronization procedure that uses the known phase shifts between successive symbols in the synchronization word. Per-survivor processing is used to track the channel variations and carrier frequency offset. Simulation results are presented for multipath Rayleigh fading channels having various delay profiles. Comparisons between the regular symbol-spaced truncated MLSE equalizer and a fractionally spaced truncated MLSE equalizer are also furnished     

Performance comparison of space diversity and equalizationtechniques for indoor radio systems

By means of analytical and numerical methods, we derive the bit error rate (BER) of π/4-DQPSK systems in frequency-selective fading channels. For a theoretical analysis of the system, a simplified two-ray channel model has been used. However, both Rayleigh and lognormal distributions for the ray envelope have been considered. The system performance in the presence of antenna diversity and in combination with a new nonlinear equalizer has been evaluated. In particular, it is seen that in flat fading environments, space diversity may improve the performance by more than 10 dB at a BER=10^-3. However, for channels with a large delay spread, nonlinear intersymbol interference (ISI) is the predominant disturbance, and the performance can only be enhanced by the nonlinear equalizer     

Detection performance of cooperative spectrum sensing with hard decision fusion in fading channels

In this paper, we investigate the detection performance of cooperative spectrum sensing (CSS) using energy detector in several fading scenarios. The fading environments comprise relatively less-studied Hoyt and Weibull channels in addition to the conventional Rayleigh, Rician, Nakagami-m and log-normal shadowing channels. We have presented an analytical framework for evaluating different probabilities related to spectrum sensing, i.e. missed detection, false alarm and total error due to both of them, for all the fading/shadowing models mentioned. The major theoretical contribution is, however, the derivation of closed-form expressions for probability of detection. Based on our developed framework, we present performance results of CSS under various hard decision fusion strategies such as OR rule, AND rule and Majority rule. Effects of sensing channel signal-to-noise ratio, detection threshold, fusion rules, number of cooperating cognitive radios (CRs) and fading/shadowing parameters on the sensing performance have been illustrated. The performance improvement achieved with CSS over a single CR-based sensing is depicted in terms of total error probability. Further, an optimal threshold that minimises total error probability has been indicated for all the fading/shadowing channels.     

A robust noncoherent equalizer receiver for fading channels with short memory .I. Basic structure

Traditional equalizers are very sensitive to carrier frequency offsets between the transmitter and receiver. Coherent receivers with frequency estimation algorithms can remove the offset to prevent the equalizer breakdown, but with a penalty in receiver complexity. On the other hand, noncoherent receivers such as differential detectors are inherently robust to the frequency offsets but cannot employ standard equalization techniques due to their nonlinear front-end. We introduce a simple noncoherent equalizer receiver structure for fading channel environments with short memory (up to two-bit intervals). The receiver consists of a whitened matched filter followed by a differential detector and a maximum likelihood sequence estimation (MLSE) equalizer. We examine the performance of this noncoherent equalizer by both analysis and simulation. It is shown that despite the simplicity, this receiver structure is capable of significant performance improvement as compared to an ordinary differential detector while operating with receiver frequency offsets two orders of magnitude greater than a traditional MLSE equalizer. This structure offers an attractive solution for high-bit-rate cordless transmission systems such as Digital Enhanced Cordless Telecommunications (DECT) that use simple noncoherent receivers whose performance can be constrained by channel dispersion. Using DECT as a case study, we show that the equalizer's performance limits are caused by the receiver nonlinearity and can be improved by adaptation of this nonlinearity to channel conditions.     

Performance Comparison of Advanced Receiver Structures for Dispersive DECT Channels

A range of new advanced differential receivers using a decision feedbackequaliser are introduced and evaluated in order to obtain robust performancewith relatively low complexity in dispersive Rayleigh fading DECT channels.In particular a simple multi-differential detector is proposed which cansignificantly extend the range of acceptable delay spread to around 50%of the 868 ns bit period.     

自适应均衡技术在宽带微波传输中的应用

在微波视距通信中通常采用二径模型来描述信道,简要介绍了二径模型的频率响应、损伤参数以及多径时延与天线高度、通信距离的关系。自适应均衡器是对抗多径造成的频率选择性衰落的一种重要手段,简要介绍了自适应均衡器的原理以及设计思路,采用4种典型信道下均衡前后解调器的误码性能对自适应均衡器的抗二径损伤的效果进行评估。测试结果表明,自适应均衡器能有效地对抗二径造成的频率选择性衰落,该均衡器能满足工程使用要求。     

Error performance of multiple-symbol differential detection of PSKsignals transmitted over correlated Rayleigh fading channels

The error performance of multiple-symbol differential detection of uncoded QPSK signals transmitted over correlated Rayleigh fading channels is studied. The optimal detector is presented, along with an exact expression for the corresponding pairwise error event probability. It is shown that multiple-symbol differential detection is a very effective strategy for eliminating the irreducible error floor associated with a conventional differential detector. In all of the cases investigated, a detector with an observation interval as small as two symbols is sufficient for this purpose. It is also found that the error performance of a multiple-symbol differential detector is not sensitive to the mismatch between the decoding metric and the channel fading statistics     

Further results on differential detection of GMSK signals

We evaluate the performance, in terms of bit-error rate, of 1- and 2-bit differential detectors for Gaussian minimum-shift keying (GMSK) signals by using a very fast numerical technique which makes use of the saddle point approximation. Moreover, the performance of each detector is optimized by using a closed-form expression of the optimum threshold in the slicer     

Performance of multicarrier CDMA with DPSK modulation anddifferential detection in fading multipath channels

Multicarrier (MC) direct sequence (DS) code division multiple access (CDMA) with differential phase-shift keying (DPSK) modulation and differential detection is proposed. Transmitted data bits are differentially encoded after serial-to-parallel conversion to a number of parallel streams. On each branch, encoded bits are direct sequence spread spectrum (SS) modulated and transmitted using different carriers. The system is analyzed with a differential detector in static Rayleigh fading multipath channel, in fast Rayleigh fading multipath channel and for variable overlapping between carrier spectra in static fading channel. Closed-form expressions are derived for the error probability and evaluated for many cases. The performance is compared to that of a system using phase-shift keying (PSK) with conventional matched filter (CMF) coherent receiver. For static fading channel, the error probability performance of the differential detector is close to that of CMF receiver. For fast fading, the performance degrades slightly with increasing fading rate. Finally; successive carriers of the system are allowed to overlap with various overlapping percentages. The condition of a single path can be achieved by increasing both the number of carriers and the separation between successive carriers. Also, for each number of carriers, there exists an optimum overlapping percentage at which the system performance is optimized. The performance of the proposed DPSK with differential detection system is close to that of PSK with CMF receiver, but the former is simpler to implement     

Nonlinear equalization of multipath fading channels withnoncoherent demodulation

A nonlinear decision-based adaptive equalizer compatible with differentially coherent phase shift keying (PSK) is proposed for frequency-selective fading channels. This equalization scheme is appropriate whenever conventional equalizers are not capable of tracking phase variations in selective fading channels. The received signal is first converted to a baseband signal and then sent through a differential detector. A nonlinear processor before the equalizer generates the needed nonlinear terms that are weighted and summed in the equalizer. Nonlinear intersymbol interference at the output of the differential detector is dealt with by minimizing an error signal between the output of the equalizer and the detected data. The adaptation algorithm can be any algorithm currently used for conventional equalizers. Our simulation results confirm that for channels with spectral nulls, equalization is achieved successfully with the proposed scheme, whereas, linear equalizers, either with coherent or noncoherent detection, fail     

Optimum multiuser noncoherent DPSK detection in generalized diversity Rayleigh-fading channels

The jointly optimum multiuser noncoherent detector for differential phase-shift keying (DPSK) modulation over the generalized diversity Rayleigh-fading (GDRF) channel is derived and analyzed. The GDRF channel includes time/frequency/receiver antenna diversity and allows fading correlations between the various diversity branches of each user. Noncoherent detection here refers to the case where the receiver has neither knowledge of the instantaneous phases nor of the envelopes of the users' channels. Upper and lower bounds on the bit-error probability of the optimum detector are derived for a given user. For fast fading, when the fading coefficients vary from one symbol interval to the next (but are still essentially constant over one symbol interval), the detector asymptotically (for high signal-to-noise ratios (SNRs)) reaches an error floor, which is bounded from below and above for different fast fading scenarios. For slow fading, when the channel is constant for at least two consecutive symbol intervals, the upper bound is shown to converge asymptotically to the lower bound. Thus, the asymptotic efficiency of optimum multiuser DPSK detection can be determined and is found to be positive. In contrast to coherent detection, however, it is smaller than unity in general. Since the asymptotic efficiency is independent of the interfering users' signal strengths, the optimum detector is near-far resistant. While optimum multiuser detection is exponentially complex in the number of users, its performance provides the benchmark for suboptimal detectors. In particular, it is seen that the previously suggested post-decorrelative detectors can be far from satisfactory.     

FH-MFSK multiple-access communications systems performance in thefactory environment

A frequency-hopped (FH), M-ary frequency-shift-keyed (MFSK), spread-spectrum (SS) communication system operating over the factory radio channel is described. The performance of the system for Rayleigh, Rician, and log-normal multipath fading for factory environments is investigated. The statistics of these channels, based on recent channel modeling studies, are used to evaluate the performance of the FH-MFSK system. A quadrature rule is employed to calculate the channel error probabilities. The average bit error rate (BER) is formulated and is evaluated approximately using Stirling's formula. The numbers of simultaneous users in terms of the number of hopped frequencies, number of MFSK chips, receiver threshold, signal-to-noise ratio, and channel statistics at a fixed BER for Rayleigh, Rician, and lognormal fading channels are determined     

平坦瑞利衰落信道中多用户检测器的性能分析

在采用传统信号检测方式的CDMA系统中,多址干扰限制了系统的容量和性能.本文分析了在精确估计信道的情况下平坦瑞利衰落信道中的判决反馈和线性解相关两种多用户检测器的性能,主要分析了二者的误码率性能和抗远近效应的能力,并首次分析了衰落信道中的渐近多用户效率(AME).结果表明,判决反馈多用户检测器的渐近多用户效率优于线性解相关检测器.     

Blind detection of CPM signals transmitted over frequency-flatfading channels

We propose a novel detection scheme for continuous-phase modulation (CPM) signals transmitted over frequency-flat fading channels. Its most significant feature is that it operates without statistical information on the fading channel, and for this reason it is nicknamed “blind detector.” Its error-rate performance is assessed with minimum shift keying (MSK) and Gaussian MSK (GMSK) schemes and compared with the performance of other decoders     

Rayleigh衰落信道中的CDMA多用户分集接收性能

本文针对频率选择性Ｒａｙｌｅｉｇｈ衰落信道，设计了一种ＣＤＭＡ多用户检测器分集结构，通过该结构，高斯信道多用户检测算法可以有效地应用到Ｒａｙｌｅｉｇｈ衰落信道中，仿真结果表明，分集合并实现的多用户中以显著改善衰落信道的误码性能，其处理多址干扰能力并不因分集而受到影响，另外，本文还对不同的合并以及不同的检测地作了性能上的比较。     

Analysis of GMSK with differential detection in land mobile radio channels

The effect of intersymbol interference (ISI) of Gaussian filtered minimum shift keying (GMSK) on the probability of error of one bit differential detection is analyzed theoretically in fast Rayleigh fading characterizing land mobile radio channels. A closed-form expression is derived for the probability of error as a function of the fading rate, IF filter bandwidth, signal-to-noise ratio (SNR) and ISI. Numerical results are presented for cases of practical interest to researchers and designers of cellular land mobile radio systems.     

Performance of differential chaos-shift-keying digital communication systems over a multipath fading channel with delay spread

The performance of the noncoherent differential chaos-shift-keying (DCSK) communication system over a multipath fading channel with delay spread is evaluated. Analytical expressions of the bit error rates are derived under the assumption of an independent Rayleigh fading two-ray channel model. Analytical and simulated results are presented and compared. The multipath performance of the DCSK system is also compared with that of the coherent CSK system as well as conventional generic waveform communication schemes.     

Quasi-maximum-likelihood multiple-symbol differential detection for time-varying Rayleigh fading channel

The maximum-likelihood multiple-symbol differential detector (ML-MSDD) has better bit-error-rate performance than many other detectors for differential modulation. Unfortunately, the computational complexity of ML-MSDD quickly becomes prohibitive as the observation window size grows. While low-complexity MSDD algorithms for the time-invariant Rayleigh fading channel have been considered before, there is a need for low-complexity MSDD algorithms for general time-varying Rayleigh fading channels. A polynomial-time complexity approach called semi-definite relaxation (SDR) is employed to achieve differential detection with near maximum-likelihood (ML) performance. The proposed SDR quasi-maximum-likelihood (QML) multiple-symbol differential detection (SDR-QML-MSDD) is efficient in that its complexity is polynomial in the observation window size, even in the worst case, while it exhibits almost the same performance as ML-MSDD does.