Worst-case design for optimal channel equalization in filterbank transceivers

A worst-case approach is adopted to tackle optimal channel equalization for multirate filterbank transceivers, which are widely used in data communication networks, such as discrete wavelet multitone (DWMT) systems in digital subscriber lines (DSL), orthogonal frequency multiplexing division (OFMD) in frequency division multiple access (FDMA) systems, applicable to direct-sequence/spread-spectrum (DS/SS) code division multiple access (CDMA) networks. It is assumed that the observation noise is nonwhite with bounded power-norm or root-mean-squared (RMS) value. Our objective is to design the optimal receiving filterbanks that not only achieve the zero-forcing (ZF) condition or channel equalization but also minimize the RMS error between the transmitted symbols and the received symbols in the presence of the worst-case nonwhite noise. All ZF receiving filterbanks will be parameterized, and optimal design for channel equalization will be converted into an equivalent optimal H/sub /spl infin// filtering problem for the augmented receiving filterbanks with RMS error preserved. Our main results cover computation of the optimal RMS error achievable for the worst-case noise and an explicit design algorithm for suboptimal channel equalizers with the performance index arbitrarily close to the optimal one. A simulation example is used to illustrate the proposed optimal channel equalization algorithm.     

Bit error performance evaluation of double-differential QPSK infaded channels characterized by Gaussian plus impulsive noise andDoppler effects

A Fourier-Bessel series-based analysis that allows real-time symbol-to-symbol error performance evaluation for mobile radio direct-sequence code division multiple-access (DS-CDMA) systems is described. The technique analyzes mobile radio communication aeronautic links and generates average bit error rate (BER) and bit-to-bit patterns. Various communication systems parameters (e.g., modulation scheme, data rate, signal-to-noise ratio, and receive speed) as well as multiaccess interference (MAI) environments (i.e., Rayleigh-faded channels with Doppler frequency shift) may be specified and permit performance comparison. Additive white Gaussian (AWG) and impulsive ϵ-mixture type of noise are also considered to be present for the asynchronous and quasi-synchronous DS-CDMA configurations, with both schemes characterized by the incorporation of double-differential QPSK (DDQPSK) modulation. The obtained results demonstrate that the proposed combination in terms of multiplexing and digital modulation may be a suitable alternative solution for air-to-air and/or air-to-ground aeronautic communications for optimized BER versus signal-to-noise ratio system performance and, hence, for “best” channel capacity versus receiver complexity     

Bit error rate performance of orthogonal frequency‐division multiplexing relaying systems with high power amplifiers and Doppler effects

This paper analyzes the bit error rate performance of orthogonal frequency‐division multiplexing systems in mobile multihop relaying channels. We considered the uplink scenario and quantified the effects of mobile channel impairments such as Doppler shift due to user mobility per hop, high power amplifier distortions when amplifying the transmitted/relayed orthogonal frequency‐division multiplexing symbol per hop, and the cumulative effects of these impairments on multihop relaying channels. It was shown that the resulting intercarrier interference due to the cumulative effects of the phase noise generated by these impairments per hop becomes very significant in a multihop relaying communication system and severely degrades the bit error rate performance of the system. Simulation results agree well with and validate the analysis. Copyright © 2011 John Wiley & Sons, Ltd.     

Research on the system error performance of coherent orthogonal frequency division multiplexing system with M-distribution in satellite-to-ground laser communication

In order to alleviate the influence of atmospheric turbulence on the performance of satellite-to-ground laser communication system,based on the M-distribution atmospheric channel model,a multi-carrier coherent orthogonal frequency division multiplexing (OFDM) modulation system was proposed for uplink and downlink in the satellite-to-ground laser communication.The closed-form expression of bit error rate (BER) of coherent OFDM modulation system was derived.The relationship between the zenith angle,receiving aperture,signal-to-noise ratio (SNR),optimal beam divergence angle,and optimal transmission radius and the BER were studied under weak,and strong atmosphere turbulence,and compared with binary coherent differential phase shift keying (DPSK) modulation.Both the theory and the simulation results show that compared with coherent DPSK modulation,the bit error performance of the coherent OFDM modulation system in the satellite-to-ground laser communication system is better.     

Comparison of coded OFDM systems with coherent and incoherent detection

Orthogonal frequency division multiplexing (ofdm) is used for transmitting high data rates over frequency-selective radio channels in order to simplify equalization. No equalization at all is required if a differential modulation scheme is used. In this paper we compare the performance of convolutionally coded ofdm systems with coherent and with differential modulation in terms of bit error rate.     

Effects of Carrier Frequency Offset and Channel Estimation Errors on the Performance of MIMO-OFDM Systems in Spatially Correlated Channels

As an effective technique for combating multipath fading and for high-bit-rate transmission over wireless channels, orthogonal frequency division multiplexing (OFDM) is extensively used in high-rate wireless communication systems, such as, the wireless local area network (WLAN) and the digital television terrestrial broadcasting (DTTB) systems, to support high performance bandwidth-efficient multimedia services. Multiple antennas and transmit or receive diversity, multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM), can be used to improve error performance and capacity of wireless systems. In this paper, we consider the effects of carrier frequency offset and channel estimation errors on the performance of MIMO-OFDM systems in spatially correlated channels. Theoretical calculations and computer simulations are done to analyze the performance degradation of MIMO-OFDM systems in spatially correlated channels due to carrier frequency offset and channel estimation errors, and the theoretical and simulated results match well.     

Simplified channel estimation for OFDM systems with multipletransmit antennas

Multiple transmit-and-receive antennas can be used in orthogonal frequency division multiplexing (OFDM) systems to improve communication quality and capacity. In this paper, we present two techniques to improve the performance and reduce the complexity of channel parameter estimation: optimum training-sequence design and simplified channel estimation. The optimal training sequences not only simplify the initial channel estimation, but also attain the best estimation performance. The simplified channel estimation significantly reduces the complexity of the channel estimation at the expense of a negligible performance degradation. The effectiveness of the new techniques is demonstrated through the simulation of an OFDM system with two-transmit and two-receive antennas. The space-time coding with 240 information bits per codeword is used for transmit diversity. From the simulation, the required signal-to-noise ratio is only about 9 dB for a 10% word error rate for a channel with the typical urban- or hilly-terrain delay profile and a 40-Hz Doppler frequency     

Optimal detection of random phase signal with Doppler frequency offset in Chirp-BOK multi-antenna system

When there are Doppler frequency deviation and phase noise in channel,the MIMO-OFDM system with coherent detection requires complicated and accurate frequency offset estimation and Doppler compensation.Based on this,an optimal random phase detection was proposed in multi-antenna Chirp-BOK system,which improved the system capacity by space division multiplexing in the condition of anti-interference and anti-frequency deviation.The equivalent MIMO channel coefficient matrix of multi-antenna Chirp-BOK system was derived as a real matrix,so as to reduce the complexity of channel estimation and avoid to be affected by random phase.A two plus two wireless transceiver system was built on the NI USRP-2942R platform.Simulation and experiment results show that the MIMO-Chirp-BOK system inherits the advantages of low complexity,insensitivity to phase shift and frequency offset of Chirp-BOK communication system.It achieves multiplied communication capacity by increasing the number of antennas.     

使用PRONY方法的OFDM频偏估计

对OFDM频偏的盲估计算法进行了研究,采用PRONY方法,利用对OFDM码元的过取样来构成估计所需的空间。着重对多径环境下的性能进行了仿真,证明该方法在多普勒频移下有较好的性能,并进一步分析了估计性能与过采样倍数的关系。     

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.     

An equalization technique for orthogonal frequency-divisionmultiplexing systems in time-variant multipath channels

A loss of subchannel orthogonality due to time-variant multipath channels in orthogonal frequency division multiplexing (OFDM) systems leads to interchannel interference (ICI) which increases the error floor in proportion to the Doppler frequency. A simple frequency-domain equalization technique which can compensate for the effect of ICI in a multipath fading channel is proposed. In this technique, the equalization of the received OFDM signal is achieved by using the assumption that the channel impulse response (CIR) varies in a linear fashion during a block period and by compensating for the ICI terms that significantly affect the bit-error rate (BER) performance     

超宽带射频接收机的研制

超宽带(UWB)短距离无线通信技术是当前国内外研究的热点,直接序列超宽带(DS-UWB)方案和多带－正交频分复用(MB-OFDM UWB)方案是两个主要候选方案,其中多带-正交频分复用方案是较受重视的方案.本文针对MB-OFDM UWB系统,提出了一种双载波-正交频分复用(DC-OFDM UWB)的射频解决方案.该方案采用了两个相邻的子载波实现宽带通信,两个子载波是在中频部分采用合路/分路的方式,以降低硬件实现难度和系统的复杂度.设计了UWB射频接收机中的低噪声放大器(LNA)、频率合成器和解调器等关键部件,并建立了DC-OFDM UWB接收机实验演示平台.测试结果表明,研制的射频接收机满足FCC规定的射频指标要求,该方案也适用于其它的宽带通信系统中.     

A Comparison Study Between Pre-Equalization Schemes for Single and Multi Carrier Modulation Systems

In this paper, frequency domain pre-equalization (Pre-FDE) is developed for broadband cyclic prefix-code division multiple access (CP-CDMA) as a single carrier transmission scheme, and for multi carrier-CDMA (MC-CDMA) and orthogonal frequency division multiplexing (OFDM) as multi carrier transmission schemes. A comparison study is held between these schemes and the traditional equalization schemes. Experimental results show that pre-equalization improves significantly the performance of the single and multi carrier communication systems with a very low complexity at the receiver. The comparative study between MIMO pre-equalization for single carrier systems and for multi carrier systems shows that MIMO pre-equalization for single carrier systems outperforms that for multi carrier systems in terms of bit error rate (BER) performance since single carrier transmission has more frequency diversity than multi carrier transmission in the uncoded case.     

Robust ICI Self-cancellation OFDM Receiver with Dynamic Phase and Amplitude Estimations

Orthogonal frequency division multiplexing (OFDM) has been applied in the current wireless local-area networks and digital video broadcasting since it is robust against the frequency-selective channels. However, there is still a crucial intercarrier-interference (ICI) problem due to Doppler effect, local frequency drift and sampling clock offset, associated with OFDM systems. Recently ICI self-cancellation schemes have been proposed to significantly reduce the ICI and empirically they greatly outperform the convolutional coding schemes adopted by the IEEE 802.11 standard. However, all existing ICI self-cancellation receivers are still sensitive to the phase and amplitude ambiguities due to the phase offset, the local oscillator frequency drift and the multipath reflections. Therefore, in this paper, a novel receiver design integrating the ICI self-cancellation with a proposed dynamic phase and amplitude estimation mechanism is introduced, which can well solve the ambiguity problem. The Monte Carlo simulation results show that our phase and amplitude estimators can greatly decrease the error probability for the final symbol detection in the ICI self-cancellation OFDM receivers.     

Downlink transmission of broadband OFCDM Systems-part II: effect of Doppler shift

The orthogonal frequency and code division multiplexing (OFCDM) system with 2-D spreading (time- and frequency-domain spreading) is becoming a promising candidate for future broadband wireless communication systems. OFCDM is more attractive than orthogonal frequency-division multiplexing (OFDM) both by introducing frequency-domain spreading for frequency diversity provision and time-domain spreading for flexible data rate provision. To provide high-speed mobile services, multicode transmission is employed in conjunction with OFCDM. In a Gaussian or flat-fading channel, multicode channels are orthogonal. However, in a realistic wireless channel, the orthogonality no longer maintains. Thus, multicode interference (MCI) is caused. This paper focuses on the investigation of the effect of Doppler shift on the downlink transmission of high-speed mobile OFCDM systems. A practical channel estimation algorithm based on a code-multiplexed pilot channel is employed to track the variations of fading channels. Hybrid MCI cancellation and minimum mean-square error (MMSE) detection proposed by the authors is employed as an efficient way to eliminate the MCI in the frequency domain. The system performance is analytically studied with imperfect channel estimation to show how it is affected by parameters such as the window size in the channel estimation, Doppler shift, the number of stages of the hybrid detection, the power ratio of pilot to data channels, spreading factor, and so on.     

Lagrange/Vandermonde MUI eliminating user codes forquasi-synchronous CDMA in unknown multipath

A family of codes for low-complexity quasi-synchronous code division multiple access (CDMA) systems is developed in order to eliminate multiuser interference (MUI) completely in the presence of unknown and even rapidly varying multipath. Judiciously designed precomputable symbol-periodic user codes, which we term Lagrange or Vandermonde, and the corresponding linear receivers offer a generalization of orthogonal frequency division multiplexing (OFDM), which are especially valuable when deep-fading, carrier frequency errors, and Doppler effects are present. The flexibility inherent to the designed transceivers is exploited to derive transmission strategies that cope with major impairments of wireless CDMA channels. The symbol-periodic code design is also generalized to include the class of aperiodic spreading and orthogonal multirate codes for variable bit rate users. Performance analysis and simulations results illustrate the advantages of the proposed scheme over competing alternatives     

CO-OFDM系统中基于卡尔曼滤波对相位噪声补偿算法的研究

基于卡尔曼滤波提出了两种相干光正交频分复用(CO-OFDM)系统的相位噪声补偿算法,这两种算法在发射端的时域均插入导频,并在接收端对导频进行卡尔曼滤波,最后利用插值算法补全全部子载波的相位噪声.仿真结果表明,基于最小均方误差(MMSE)准则的判决反馈算法在相位噪声比率为10-1时,系统误码率约为10-4,并且出现了错误平层,而基于卡尔曼滤波所提出的两种相位噪声算法在大相位噪声的情况下仍然具有较好性能且能有效地降低错误平层,因而所提出的相位噪声补偿算法能改善CO-OFDM系统的性能.     

基于相干光正交频分复用的WDM传输系统及其性能分析

相干光正交频分复用由于其良好的传输性能成为近年来光传输领域的研究热点,波分复用技术可以在光纤中通过增加并行波长的数量来提高系统的容量,将CO-OFDM和WDM技术结合,可以构造出高速率、大容量、低成本的光传输网络。文章首先对基于CO-OFDM的WDM传输系统的理论模型和基本原理进行了研究,然后对基于CO-OFDM的100Gb/s×32-信道WDM传输系统进行了仿真分析。并研究了该系统的传输性能。结果表明：在没有任何光纤的色散及非线性补偿的情况下,当信号速率为3.2 Tb/s时,系统的Q因子高于16.0 dB,在标准单模光纤中的传输距离可达1500km。     

QAM-OFDM雷达通信共享信号长时间相参积累算法

QAM-OFDM(Quadrature Amplitude Modulation-Orthogonal Frequency Division Multiplexing)雷达通信共享信号因携带随机通信信息,其脉压旁瓣的随机性较大,类似噪声的影响。针对该问题,采用基于Keystone变换的长时间相参积累算法抑制其旁瓣。在共享信号模型的基础上,分析了其脉压旁瓣受随机通信信息的影响以及采用长时间相参积累抑制其旁瓣的可行性,然后采用Keystone变换校正其长时间相参积累产生的距离单元走动,并进行多普勒模糊补偿处理。理论分析和仿真结果表明,该方法使得回波能量积累集中,能有效实现共享信号脉压旁瓣的抑制。