多径衰落信号循环平稳特性研究及其应用

利用信号的循环平稳特性可有效解决通信中的很多问题。在通信双方未知干扰信号先验知识时,信干噪比（SJNR, Signal-to-Jamming-Plus-Noise Ratio）可以描述人为有意干扰和噪声对通信信道的综合影响。分析典型人为音调干扰、加性高斯白噪声及多径衰落信号的循环平稳特性,得出结论：可利用通信信号、干扰及噪声三者不同的循环平稳特性进行SJNR估计,表征信道质量,为通信中的信道质量判决提供可靠依据。     

一种快速FH-OFDM通信帧格式及其验证

针对通信中跳频与正交频分复用两种技术结合存在同步困难的技术难点,提出一种快速跳频OFDM无线宽带通信帧格式,并设计了一种占用带宽为10 MHz、跳频速率为2 kHz、基带数据传输速率为14.324 Mbit/s的快速跳频OFDM通信收发系统.该帧格式使用加长循环前缀的方式提供高精度载波同步依据,将伪随机序列隐藏到频域数据中以便利用其完成精确定时同步、采样时钟同步和整数倍频偏估计,并且插入了梳状导频以便接收端进行信道估计.实验结果表明,接收机星座图可以很好地收敛,在信噪比为10 dB情况下,误码率低于10-8.本帧格式可较好地满足视频等数据的无线通信要求.     

Transmitter precoding in synchronous multiuser communications

A synchronous multiuser system operating in an additive white Gaussian noise channel, with or without multipath fading, is considered. It is shown that when either a conventional single user receiver or the RAKE receiver is employed, both multiple access and intersymbol interference can be eliminated by means of a suitable transmitter precoding scheme. Transmitter precoding represents a linear transformation of transmitted signals, such that the mean squared errors at all receivers are minimized. Precoding, with both conventional single user receiver and with the RAKE receiver, results in near-far resistant performance and outperforms considerably the respective schemes without precoding. The crucial assumption, in the multipath case, is that the transmitter knows the multipath characteristics of all channels and that channel dynamics are sufficiently slow so that multipath profiles remain essentially constant over the block of precoded bits     

BER performance enhancement for secure wireless communication systems based on DCSK-MIMO techniques under Rayleigh fading channel

There has been a growing interest in the use of chaotic techniques for enabling secure communication in recent years. This need has been motivated by the emergence of a number of wireless services which require the channel to provide very low bit error rates (BER) along with information security. As more and more information is transacted over wireless media, there has been increasing criminal activity directed against such systems. This paper investigates the feasibility of using chaotic communications over Multiple-Input-Multiple-Output (MIMO) channels. We have studied the performance of differential chaos shift keying (DCSK) with 2×2 Alamouti scheme and 2×1 Alamouti scheme for different chaotic maps over additive white Gaussian noise (AWGN) and channels disturbed by Rayleigh fading. Both the inherently wideband DCSK modulation and the space-time block code (STBC) are techniques that can mitigate the effect of multipath fading. The use of these schemes allows us to enhance security without degrading the BER performance for Rayleigh fading channels. We have employed an exact method to analyze the performances of DCSK communication system over fading channel. Our simulations indicate that the combination of the STC and tent map provides the best BER performance in addition to security when compared to the choice of other maps. Hence, this study shows that the use of these schemes can allow the user to enhance security without degrading the BER performance while communicating over these channels.     

An adaptive correlator receiver for direct-sequence spread-spectrumcommunication

This paper presents a novel receiver for direct sequence spread-spectrum signals over channels containing interference and multipath. The receiver employs an adaptive correlator that jointly detects the transmitted data, removes interference, and compensates for multipath. The optimum correlation vector is derived by determining the Wiener vector that minimizes the mean squared error (MSE) between the transmitted data bit and the correlator output. For an additive white Gaussian noise (AWGN) channel, the optimal correlation vector is the spreading sequence used by the transmitter. For interference and multipath channels, the optimal correlation vector will suppress the interference and combine the multipath while optimizing the detection of the transmitted data bit. The paper presents analytical and simulation results which illustrate the bit-error rate (BER) performance of the receiver in multipath and narrowband interference. Additionally, simulation results are presented illustrating the convergence performance of the receiver when the tap weights are adjusted using either the least mean square (LMS) or recursive least squares (RLS) adaptive algorithms     

Performance evaluation and implementation of convolution coded OFDM modem in wireless underwater acoustic communication

High‐data transmission rate and reliable communication in underwater acoustic channel is challenging because of limited bandwidth, multipath propagation, and Doppler shift, which results in poor bit error performance. Under this constraint, this paper explains the simulation results of underwater wireless acoustic data transmission system by using quadrature phase shift keying modulation with convolution encoder at the transmitter and proposed Viterbi decoder at the receiver. The decoder algorithm in comparison with MATLAB inbuilt function shows asserting improved results. This paper evaluates the performance of convolution coded orthogonal frequency division multiplexing modem, which is studied over typical underwater channel through an extensive computer simulation and a semirealistic experimentation. The performance of convolution coded orthogonal frequency division multiplexing system is measured in time domain plots, bit error rate curves, and number of bit errors per frame over additive white Gaussian noise and Rayleigh channel conditions.     

A Novel Pulse Design Based on Hermite Functions for UWB Communications

1IntroductionUltra-Wide Band(UWB)technology is foreseen as apossible solution for future short-range high-data-ratewireless communications.Unlike the conventional carrier-based wireless communication systems,a carrier-freeUWB radio system usually transmit…     

SER Computation in M-QAM Systems with Phase Noise

This paper presents an exact closed-form expression for the symbol error rate of the square and rectangular quadrature amplitude modulation (QAM) constellations, under the assumption that the transmitted and/or received signals are corrupted by the phase noise fluctuation. Phase noise is one of the most important radio frequency (RF) imperfections which usually comes from the local oscillator (LO) at the transmitter and/or receiver. In this paper, the additive white Gaussian noise (AWGN) channel is assumed. Although AWGN is a simple channel, but our exact analysis of the symbol error rate in this paper can be led to the precise study of the real communication systems in the fading channel. Hence, an exact closed-form solution for the symbol error rate is derived here as a finite summation of the two-dimensional Q-function and verified by the simulation.     

Comparison of MC-CDMA with DS-CDMA using frequency domain and time domain RAKE receivers

In this paper a multicarrier CDMA (MC-CDMA) system with a soft decision differential phase shift keying (DPSK) frequency domain RAKE receiver is described. We compare a MC-CDMA system with a direct sequence CDMA system using RAKE receivers. In contrast with previous MC-CDMA systems, guard intervals are not used and the carriers are spaced at the reciprocal of the bit rate, optimising the usage of the bandwidth. In this way a comparison can be made between the multicarrier CDMA system described and a direct sequence (DS-CDMA) system with the same bandwidth. The results presented are received bit error rates from Monte Carlo simulations. The simulations are conducted in a multipath channel with Rayleigh fading and 300 Hz Doppler spectrum with additive white Gaussian noise. It is shown that the multicarrier CDMA matched filter receiver performs favourably compared to the direct sequence CDMA matched filter receiver for 1 -path fading. For a single user at a receive bit error rate of 1×10^–3 in the 4-path fading channel the multicarrier RAKE receiver requires no knowledge of the channel delay spread and performs 3 dB worse than the DS-CDMA RAKE receiver simulated. The performance of the MC-CDMA RAKE receiver for a single user increases with increasing channel dispersion. The performance of the DS-CDMA RAKE receiver for multiple user is superior to that of the MC-CDMA RAKE receiver.     

Error rate performance of digital FM mobile radio withpostdetection diversity

An analysis of bit error rate (BER) in a binary digital FM system with postdetection diversity is presented. Expressions for the average BER due to additive white Gaussian noise (AWGN), random FM noise and delay-spread in the multipath channel are derived for reception using differential demodulation (DD), and frequency demodulation (FD) assuming independent fading signals. Calculated results for MSK show that the BER performance is strongly dependent on the RMS-delay/bit-duration ratio and that the delay-spectrum shape is of no importance when the receiver predetection filter product is optimized for the effect of AWGN. The effect of fading correlation on the diversity improvement is also analyzed for a two-branch case with multiplicative Rayleigh fading signals. Expressions for the average BER due to AWGN and random FM noise are derived. Calculated results are shown for the average BER due to random FM noise assuming a horizontally spaced antenna system at a mobile station. It is shown that the use of small antenna spacings leads to a diversity improvement greater than that obtainable for the case of independent AWGN     

On cracking direct‐sequence spread‐spectrum systems

Secure transmission of information over hostile wireless environments is desired by both military and civilian parties. Direct‐sequence spread‐spectrum (DS‐SS) is such a covert technique resistant to interference, interception, and multipath fading. Identifying spread‐spectrum signals or cracking DS‐SS systems by an unintended receiver (or eavesdropper) without a priori knowledge is a challenging problem. To address this problem, we first search for the start position of data symbols in the spread signal (for symbol synchronization); our method is based on maximizing the spectral norm of a sample covariance matrix, which achieves smaller estimation error than the existing method of maximizing the Frobenius norm. After synchronization, we remove a spread sequence by a cross‐correlation based method, and identify the spread sequence by a matched filter. The proposed identification method is less expensive and more accurate than the existing methods. We also propose a zigzag searching method to identify a generator polynomial that reduces memory requirement and is capable of correcting polarity errors existing in the previous methods. In addition, we analyze the bit error performance of our proposed method. The simulation results agree well with our analytical results, indicating the accuracy of our analysis in additive white Gaussian noise (AWGN) channel. By simulation, we also demonstrate the performance improvement of our proposed schemes over the existing methods. Copyright © 2009 John Wiley & Sons, Ltd.     

非线性功放条件下的恒包络OFDM系统性能

正交频分复用技术具有较高的频谱效率和好的抗频率选择性衰落性能,被广泛应用于无线通信系统中。但是OFDM信号具有较高的峰均比,当系统采用非线性功放时,OFDM系统需要较大的输入功率回退以避免信号失真。为了研究非线性功放条件下的系统性能,分析了恒包络OFDM系统的实现方式,并以误码率为指标,在加性高斯白噪声信道和多径衰落信道场景下对比了恒包络OFDM系统与采用传统迭代限幅滤波PAPR抑制技术的OFDM系统的性能。     

On the performance of ultra-wide-band signals in Gaussian noise anddense multipath

An ultra-wide-band (UWB) signal is characterized by a radiated spectrum with a very wide bandwidth around a relatively low center frequency. In this paper, we study the reduced fading margin property of UWB signals. To evaluate the fading margin, we compare the performance of UWB signals in an environment with only additive white Gaussian noise (AWGN) versus the performance of UWB signals in a dense multipath environment with AWGN. The assumption here is that the presence of multipath causes a small increase in the signal-to-noise ratio required to achieve reasonable levels of bit error rate. A numerical example confirms this assumption, more specifically, the example shows that to achieve a bit error rate equal to 10^-5, we require about 13.5 dB in the AWGN case and about 15 dB in the multipath case, resulting in a fading margin of just 1.5 dB. This small fading margin can be understood by the ability of the UWB signal to resolve the dense multipath     

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     

New strides towards success

This paper presents a mathematical analysis of the joint effects of additive white Gaussian noise and gated noise in an M-ary Quadrature Amplitude Modulation communication system subject to Nakagami fading. This channel model is suitable to describe scenarios in which the Nakagami channel is affected by noisy signals originated from switching electronic circuits present in industrial environments, for example. Novel and exact analytic expressions for the bit error probability of the system are derived and computer simulations corroborate the analytic results.     

Prediction/cancellation techniques for fading broadcastingchannels. I. PSK signals

An approach for reducing the effects of multiplicative noise, such as fading, on the performance of mobile broadcasting transmission systems is introduced. The proposed method is based on a noise prediction/cancellation technique and is applied to phase-shift keyed (PSK) signals. By exploiting the statistical characteristics of the multipath fading signal and its strong correlation properties, a novel sequential receiver structure is derived that uses a detection algorithm which identifies the most probable transmitted sequence in the maximum-likelihood detection sense. The algorithm has been obtained for both Rayleigh and Rician fading channels. The proposed detection technique has been evaluated for various QPSK schemes operated in channels corrupted by a combination of additive white Gaussian noise (AWGN) and fading. The computer simulation results obtained show that by using a relatively simple receiver with a prediction algorithm of second order, considerable error floor reductions are achieved by the proposed receivers. As a direct result of these error floor reductions substantial bit error rate (BER) improvements of more than three orders of magnitude over conventionally detected schemes are obtained     

Design and Experimental Results for a Direct-Sequence Spread-Spectrum Radio Using Differential Phase-Shift Keying Modulation for Indoor, Wireless Communications

We report on our design and measurements that have been made for a direct-sequence spread-spectrum radio using differential phase-shift keying modulation for a wireless PBX. We describe the design and implementation of a transmitter and a receiver using a surface acoustic wave (SAW) filter matching the spread-spectrum code of a user. The receiver performance is within 1 dB of the theoretical performance of a differential phase-shift keying (DPSK) receiver in the presence of additive white Gaussian noise. We also show receiver performance in a multipath fading indoor environment with multipath fade notches of up to 50 dB depth. The indoor channel multipath fading can be overcome by using an equal gain diversity combiner which is suitable when DPSK modulation is used. We confirm that the indoor mean power level attenuation follows the inverse fourth power of the distance. Also, we investigate the multiple-access capability of the system by introducing an interfering transmitter with a different spread-spectrum code sequence.     

Discrete-time chaotic systems synchronization performance under additive noise

In recent decades many articles have discussed the possibilities of chaos applied in communications. However, the vast majority consider in practical terms the ideal channel condition, which is clearly a restringing condition. Some papers show that when there is an additive noise, the synchronization error often disrupts communication. In this work, we present results of a comparison between synchronization error due to additive Gaussian noise when the transmitter and receiver are implemented by single or coupled maps.     

Data-aided linear prediction receiver for coherent DPSK and CPMtransmitted over Rayleigh flat-fading channels

In this paper, a new data-aided linear prediction receiver for coherent differentially encoded phase-shift keying (DPSK) and coherent continuous phase modulation (CPM) over Rayleigh flat-fading channels is presented, This receiver uses the previously detected symbols to estimate the carrier-phase reference and predict the channel gain continuously and therefore makes the optimal coherent detection of DPSK and CPM. The receiver has a simple structure and can be implemented easily. This is due partly to the fact that the linear predictors used for channel estimation do not depend on the autocorrelation function of the fading process. Simulation results on the bit error performance of QDPSK and minimum-shift keying (MSK) with the new receiver are given for both the additive white Gaussian noise (AWGN) and the Rayleigh flat-fading channels. The results show that the proposed receiver provides almost the same bit error rate (BER) performance as the ideal coherent receiver in an AWGN channel, is very robust against large carrier frequency offset between transmitter and receiver, and can provide a reasonably good BER performance in a fast Rayleigh fading channel. Finally, a multisample receiver is discussed and its error rate performance is evaluated by means of computer simulations. The results show that the multisample receiver provides good BER performance for higher fading rate     

Postdetection selection diversity reception with correlated,unequal average power Rayleigh fading signals for π/4-shift QDPSKmobile radio

The diversity gain degradations due to fading correlation and unequal average power are investigated for practical, two-branch postdetection selection diversity reception. The average bit error rate (BER) of π/4-shift QDPSK is theoretically analyzed taking into account additive white Gaussian noise (AWGN), cochannel interference, and multipath channel delay spread; exact diversity gain degradations are calculated. Simple and useful approximate expressions for the gain degradations are also presented