OFDM系统中PSAM M-QAM的导引符号功率的优化

该文分析了多径衰落信道下OFDM系统中导引符号辅助相干解调(PASM)M-QAM的导引符号功率对解调性能的影响,优化了导引符号与数据符号功率比,计算了不精确信道估计造成性能的损失。分析和仿真结果表明,最优导引符号功率与数据符号的功率之比由信噪比、多普勒频率和内插滤波器系数等因素决定。当采用最优的比值时,为达到相同的误比特率,所需的发射总功率最低。     

相干OFDM系统中导引符号功率优化

在瑞利衰落信道下，本文研究了导引符号功率对相干OFDM系统性能的影响，计算了由于不精确信道估计引起的系统性能损失，优化分析了导引符号与数据符号功率比，并给出完整的最优功率分配数学表达式。理论分析和仿真结果给出了各种参数对最优功率比的影响，同时理论结果为系统设计提供了依据。     

限幅OFDM的一种信道估计优化方法

在OFDM 系统中,限幅常用于抑制信号的峰平比,而限幅会引入额外的限幅噪声,以致干扰数据和导引符号,恶化传输性能.本文针对限幅OFDM系统,提出一种基于导引符号的限幅噪声消除算法.该算法在插入导引符号之前,滤除位于导引位置的限幅噪声,从而改善信道估计性能.在此基础上,本文还提出了利用迭代算法以进一步提高信道估计精确度.本文对所提出的限幅噪声消除算法及迭代算法进行了仿真验证,结果表明,新算法能够明显改善信道估计,优化传输性能.     

TDS-OFDM系统的功率调整技术研究

针对频域双PN序列填充的TDS-OFDM系统,提出了通过调整训练序列与帧体数据功率比值来降低系统误符号率的方法,并通过调整两者的功率比值对系统在多径衰落信道下的误符号率性能进行仿真.仿真结果表明选择合适的功率比值会带来信噪比的增益.     

增益导引折射率反导引大模场光纤激光器特性分析

为了研究增益导引折射率反导引光纤激光器的功率分布及输出特性,根据此类光纤的结构原理和特点,建立了端面抽运的增益导引折射率反导引光纤激光器的基模光速率方程,推导了避免激发高阶模的增益阈值判决条件,并运用弦切法和Runge-Kutta法数值求解了该速率方程,分析了光纤长度、腔镜反射率等参量对基模输出功率的影响.采用芯径为1...     

多径衰落信道中导引符号辅助的二维扩频相干解调的性能分析

在多径衰落信道中,本文分析了导引辅助的二维扩频相干解调的误码率性能.提出了信道的时频二维相关区域的概念,给出了信道时频二维信道相关特性的计算方法.结果表明,分析与仿真的误码率相近;当二维扩频的扩频增益对应的时频二维区域大于信道的时频二维相关区域时,再增大扩频增益会导致误码率性能变差.     

MIMO-OFDM系统定时同步算法

应用MIMO—OFDM无线通信系统的空间信号资源，提出了基于单个前导符号的MIMO--OFDM系统帧定时和符号定时同步的分集算法，以克服高速无线多径信道中深衰落对MIMO—OFDM系统定时同步性能的影响，给出了具体的帧定时、符号定时同步的分集算法以及在高速无线多径信道COST207模型下帧定时和符号定时同步的仿真结果。     

一种适用于多径衰落信道的OFDM系统同步算法

该文针对多径衰落信道的具体特点,提出了一种适用于无线衰落信道环境下的OFDM符号同步方法。此种方法利用前一帧估计的信道最大时延来调整相关窗长度,在最大程度上避开了多径的影响;并利用本帧信道估计得到的多径信息进一步完成符号精细同步。仿真证明,该方法比传统的方法有较大的性能改善,比较适合多径衰落信道环境下的符号定时同步。     

基于Simulink的OFDM系统仿真分析

无线衰落信道使得通信系统在传输过程中出现符号间干扰和信道间干扰,严重影响通信质量。正交频率分割复用(OFDM)是一种多载波数字调制技术,他在无线多径衰落信道下却具有较好的传输特性,对其抗多径衰落的原理进行了研究。在Simulink下建立了系统的仿真模型,并对所建模型进行了分析。同时在两径和六径瑞利衰落信道以及系统中有无交织下对其进行仿真,给出了仿真结果。     

反折射率增益导引光纤放大器增益特性分析

为了能够更好地研究反折射率增益导引光纤的工作特性及掺镱反折射率增益导引光纤放大器模型,分析其对信号光的放大特性,利用信号光饱和参量S方法,求解速率方程,针对100μm芯径特征尺寸,进行了理论分析及仿真计算。结果表明,在特定的掺杂离子浓度和抽运功率下,得到信号光增益与光纤长度的相互关系,并得到最佳光纤长度、掺杂离子浓度和抽运功率的数值曲线,与普通的光纤放大器相比,其放大特性有很大的提高。     

Effect of imperfect channel estimation on transmit diversity in CDMA systems

In this paper, the effect of imperfect channel estimation on the transmit diversity based on space-time block coding for the downlink of a direct-sequence code-division multiple-access system is studied. Two transmit antenna and one receiving antennas are employed. However, the results of this paper can be extended to the system with more receiving antennas. Each channel is modeled as frequency-selective Rayleigh fading and the pair of channels corresponding to two transmit antennas are mutually independent. Both spatial diversity gain and multipath diversity gain are obtained in the system. The system performance is evaluated in terms of bit-error rate under the perfect and imperfect channel estimation. A pilot-assisted channel-estimation scheme with one common spreading code sequence is exploited. It is shown that the inaccurate channel estimates suffering from multiple access and multipath interference significantly degrade the system performance and can be effectively improved by use of a simple low-pass filter. The investigation of the power ratio of pilot to data channels illustrates that the base station should dynamically adjust the transmit power of the pilot channel according to the varying system configurations in order to achieve the best performance.     

Experiments on pilot symbol-assisted coherent multistageinterference canceller for DS-CDMA mobile radio

A three-stage coherent multistage interference canceller (COMSIC) employing pilot symbol-assisted (PSA) channel estimation for replica generation of multiple access interference (MAI) is implemented and its performance in the presence of frequency selective multipath fading is experimentally evaluated by a multipath fading simulator. A fast transmission power control (TPC) method suitable for COMSIC is also proposed, in which the signal-to-interference plus background noise power ratio (SIR) at the matched filter (MF) based RAKE receiver is measured to achieve a short TPC delay and the target signal-to-interference ratio value is compensated by an outer loop so that the measured block error rate (BLER) is equal to the prescribed target value. The experimental results show that as expected the COMSIC satisfactorily reduces the MAI even when the number of active users is equal to the spreading factor in a multipath fading environment, and thus, improves the bit error rate (BER) performance in a multiuser environment. The results also show that the proposed fast TPC method with a two-slot delay associated with COMSIC works satisfactorily and the combination of COMSIC and fast TPC significantly decreases the transmission power of a mobile station (required transmission power of a mobile station with COMSIC at the average BER of 10^-3 is decreased by approximately 2.0 (3.0) dB compared with the MF-based RAKE receiver with (without) antenna diversity reception). This extends the cell coverage, battery life, and increases the system capacity in the reverse link     

Frequency estimation in slowly fading multipath channels

This paper concerns the estimation of a frequency offset of a known (pilot) signal propagated through a slowly fading multipath channel, such that channel parameters are considered to he constant over the observation interval. We derive a maximum-likelihood (ML) frequency estimation algorithm for additive Gaussian noise and path amplitudes having complex Gaussian distribution when covariance matrices of the fading and noise are known; we consider in detail the algorithm for the white noise and Rayleigh fading, in particular, for independent fading of path amplitudes and pilot signals with diagonal autocorrelation matrices. For the latter scenario, we also derive an ML frequency estimator when the power delay profile is unknown, but the noise variance and bounds for the path amplitude variances are specified; in particular, this algorithm can be used when path delays and amplitude variances are unknown. Finally, we consider frequency estimators which do not use a priori information about the noise variance; these algorithms are also operable without timing synchronization. All the frequency estimators exploit the multipath diversity by combining periodograms of multipath signal components and searching for the maximum of the combined statistic. For implementation of the algorithms, we use a fast Fourier transform-based coarse search and fine dichotomous search. We perform simulations to compare the algorithms. The simulation results demonstrate high accuracy performance of the proposed frequency estimators in wide signal-to-noise ratio and frequency acquisition range.     

Model-based pilot and data power adaptation in psam with periodic delayed feedback

We consider the optimum design of pilot-symbolassisted modulation (PSAM) schemes with feedback. The received signal is periodically fed back to the transmitter through a noiseless delayed link and the time-varying channel is modeled as a Gauss-Markov process. We optimize a lower bound on the channel capacity which incorporates the PSAM parameters and Kalman-based channel estimation and prediction. The parameters available for the capacity optimization are the data power adaptation strategy, pilot spacing and pilot power ratio, subject to an average power constraint. Compared to the optimized open-loop PSAM (i.e., the case where no feedback is provided from the receiver), our results show that even in the presence of feedback delay, the optimized power adaptation provides higher information rates at low signal-to-noise ratios (SNR) in mediumrate fading channels. However, in fast fading channels, even the presence of modest feedback delay dissipates the advantages of power adaptation.     

Convolutional coding for BPSK with pilot tone over Rayleigh fading channels

To improve the performance of digital communication systems over fading channels, several pilot signal techniques have been proposed in the literature. The authors investigate the use of binary convolutional coding for such systems and derives an upper bound on the bit error probability (BEP), the optimum power split ratio between data and pilot signals, and the channel cutoff rate.<>     

Downlink performance analysis of a BPSK-based WCDMA usingconventional RAKE receivers with channel estimation

We consider the downlink of a Universal Mobile Telecommunication System terrestrial radio access-wideband code division multiple access (UTRA-WCDMA) system and we investigate the performance of the conventional RAKE receiver. A multipath slowly Rayleigh fading channel is assumed. For the purpose of channel tap weight estimation, a common control physical channel, that is either serial or parallel multiplexed with the dedicated physical channels, is used. The receiver sensitivity to imperfect knowledge of the path delays, to the number of pilot symbols, and to the power ratio of pilot to data channels is also investigated. The system performance is evaluated by means of bit-error rates (BERs) derived using quadratic forms and characteristic functions for a BPSK modulation. The mean-squared estimation error (MSEE) of the channel tap weights is also computed and compared to the classical Cramer-Rao lower bound (CRLB). The mutual interference between pilot and data symbols is taken into account     

一种DS-CDMA无线反向链路空间分集方案研究

本文利用自适应天线阵列分离出多径信号,利用RAKE消除多径衰落。导频位符号辅助(PSA)信道估计用来对RAKE的输入端进行加权,天线阵列的加权系数的自适应更新是采用RLS算法而得。结果表明,该方案具有很强的抗多径干扰和多址干扰的能力。     

Pilot-aided chip-interleaved DS-CDMA transmission over time-varying channels

Time-varying multipath fading associated with the wireless link limits the capacity of a wireless system. In order to adapt to this adverse radio environment efficiently, we investigate the use of a pilot-aided fade-resistant transmission scheme for the uplink of a chip-interleaved code-division multiple-access (CDMA) system. We analyze the tradeoff between the number of diversity branches and the channel estimation error. We derive the optimum ratio of pilot signal energy to information signal energy. Our numerical study indicates that the proposed system is capable of outperforming the conventional CDMA system depending on the transmitter energy and channel condition.     

An optimum interference mitigating combining scheme for the CDMA downlink with the same complexity as MRC

This letter presents a new optimum interference mitigating combining (OIMC) scheme for the code-division multiple-access downlink RAKE receiver. The OIMC scheme optimizes the RAKE weights and maximizes the signal-to-interference-plus-noise ratio (SINR) at the RAKE combiner output. Unlike other interference mitigation schemes, the new scheme does not need to estimate the interference or data correlation matrix (and its inverse) of the received signal to render a reliable and low complexity receiver. The OIMC scheme mitigates interference by inversely proportionately weighting the finger output by its associated interference power, while simultaneously mitigating multipath fading. The interference power is found to be directly related to the finger's associated multipath channel gain, rendering the OIMC scheme with the same order of complexity as a maximal ratio combining (MRC) scheme. Under realistic channel conditions, simulation results show that the proposed OIMC scheme always outperforms MRC with a gain of up to more than 1 dB.