Symbol error rate performance of nonlinear OFDM receiver with peak value threshold over frequency selective fading channel

Nonlinear blanking and clipping methods are widely used in Orthogonal Frequency Division Multiplexing (OFDM) receiver to mitigate impulse interference. To quantitatively analyze the reliability performance of nonlinear OFDM receivers with pulse blanking and clipping based on peak value threshold, the symbol error rate (SER) performance of nonlinear OFDM receiver over frequency selective Rayleigh and Ricean fading channels is presented. Firstly, the analytical expressions of instantaneous output signal-to-interference and noise ratio (SINR) for nonlinear OFDM receivers with regular method, peak value blanking and peak value clipping are derived. Then, the SER performance of nonlinear OFDM receiver over frequency selective Rayleigh and Ricean fading channels is given based on the SINR expressions. Finally, simulation results are demonstrated to show good agreement with theoretical results. It has been observed that the peak value blanking method has achieved the best SER performance, and the inter-carrier interference based on the peak value blanking and peak value clipping will lead to error floor.     

An ADPCM Receiver for NTSC Composite Video Signals

An ADPCM codec for carrying one broadcast quality NTSC color TV channel at a bit rate of 42.9 Mb/s has been proposed. The system uses 3 fsc sampling, adaptive intrafield contour prediction, adaptive quantization., 4/8-bit dual length coding, and horizontal blanking interval suppression techniques. The receiver of.the video codec is designed and implemented in ECL for recovery of the original signal. The receiver accepts.a 42.9 Mb/s serial data stream with a synchronous clock from the transmitter. The receiver detects the line synchronization code, demultiplexes the audio signal and video signal, and generates the horizontal blanking patterns which have been removed at the transmitter side. The 4/8-bit dual length code is decoded and fed to the ADPCM reconstruction loop to obtain the reconstructed active video signal. The generated horizontal blanking pattern is multiplexed with the reconstructed video=signal and sent through a D/A converter to form the reconstructed analog NTSC composite video signal.     

降低可见光OFDM系统限幅失真的部分线性压扩变换研究

针对可见光正交频分复用（OFDM）系统发光二极管（LED）限幅失真严重的问题,提出一种部分线性压扩变换方法。输出光功率约束较大或较小时,在发射端对直流光正交频分复用（DCO-OFDM）双极性信号中的正信号或负信号进行线性压缩变换,在接收端对压扩信号进行扩展变换可以有效地减小限幅噪声。输入信号功率一定时,在有效信噪比最大原则下对部分线性压扩变换系数和直流偏置进行优化,系统能够获得最佳性能。仿真实验表明,随着约束光功率增大,系统性能越差,且压缩系数的最优值也越小。随着QAM调制阶数的增加,部分线性压扩变换获得的性能增益越小。      

A linear MMSE receiver for multipath asynchronous random-CDMA with chip pulse shaping

This paper studies the design and implementation of a linear minimum mean-square error (LMMSE) receiver in asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems that employ long-code pseudonoise (PN) sequences and operate in multipath environments. The receiver is shown to be capable of multiple-access interference (MAI) suppression and multipath diversity combining without the knowledge of other users' signature sequences. It maximizes output signal-to-noise ratio (SNR) with the aid of a new chip filter which exploits the cyclostationarity of the received signal and combines all paths of the desired user that fall within its supported time span. The performance of the LMMSE receiver is compared with that of the coherent selective RAKE receiver. The achieved gain is on the order of 0.6-1.8 dB in dense multipath environments of current narrow-band settings and nonuniform power distribution scenarios of next-generation CDMA systems. An example of adaptive implementation of the LMMSE receiver is presented and accompanied by complexity analysis, training curves, and quantitative performance comparisons illustrating the convergence rate and steady-state performance of the adaptive algorithms.     

New stopping criteria for iterative decoding of LDPC codes in H‐ARQ systems

Under severely unreliable channel, decoding of error‐correcting codes frequently fails, which requires a lot of computational complexity, especially, in the iterative decoding algorithm. In hybrid automatic repeat request systems, most of computation power is wasted on failed decoding if a codeword is retransmitted many times. Therefore, early stopping of iterative decoding needs to be adopted. In this paper, we propose a new stopping algorithm of iterative belief propagation decoding for low‐density parity‐check codes, which is effective on both high and low signal‐to‐noise ratio ranges and scalable to variable code rate and length. The proposed stopping algorithm combines several good stopping criteria. Each criterion is extremely simple and will not be a burden to the overall system. With the proposed stopping algorithm, it is shown via numerical analysis that the decoding complexity of hybrid automatic repeat request system with adaptive modulation and coding scheme can be fairly reduced. Copyright © 2012 John Wiley & Sons, Ltd.     

基于工频畸变技术的抄表系统

随着抄表系统和低压电力线通信技术的日益发展,利用低压电力线作为媒介的抄表方式得到了广泛关注。但由于我国低压配电网的干扰强,该方式的应用效果并不理想。为使抄表系统既节约成本,又满足通信距离的需求,设计了以双向工频通信技术作为通信手段的抄表系统。用二进制小渡变换进行信号检测,实验结果证实该方法有效。根据DL／T645—1997规约,设计出有中继功能的通信协议。     

Investigation of the distortion factor for Bussgang noise cancellation iterative receivers

Orthogonal frequency‐division multiplexing (OFDM) is considered an efficient modulation approach in the field of wireless systems. However, the significant high peak to average power ratios (PAPR) are one of the prominant shortcomings of the OFDM systems leading to severe signal distortions and acute power inefficiency. Iterative receivers, which use Bussgang algorithm approach along with clipping at the transmitter, have been presented in the literature. This methodology is majorly receiver based in order to compensate the effects of the clipping noise that was added at the transmitter. These receivers are used to analyse the distortions of clipping noise directly subtracting from the signals that are received in order to get the original signal. We demonstrate in this study a detailed and technical investigation of the Bussgang noise cancellation (BNC) receiver along with some significant results. Simulation results are presented to show that the BNC iterative receiver does not estimate the accurate clipping noise, and the performance may be improved if at the receiver end, it is somehow capable to analyze the precise clipping noise for each OFDM signal.     

Performance of coded DS‐CDMA system using reduced rank MMSE receiver in Rayleigh fading channels

This paper examines the performance of a reduced rank minimum mean square error (MMSE) receiver‐based direct sequence code division multiple access (DS‐CDMA) system. For such system, when a large processing gain is employed, substantial time is consumed in computing the filter tap weights. Many schemes for reducing the complexity of the MMSE have been proposed in recent years. In this paper, computational complexity reduction of the MMSE receiver is achieved by using the K‐mean classification algorithm. The performance of the uncoded and coded systems are investigated for the full rank MMSE receiver and reduced rank MMSE receiver and results are compared in terms of bit error rate at different loading levels in both AWGN and fading channels. A system with the matched filter (MF) receiver is also presented for the purpose of comparison and an analytical pair‐wise error bound for the coded system is derived. In the adaptive implementation of the receivers, results show that good performance is achieved for the reduced rank receiver when compared to the full rank receiver in both coded and uncoded systems, while in the optimum implementation of the tap weights, the reduced dimension receiver performance experiences degradation when compared to the full rank scheme. Over the band‐limited channels considered, results for the reduced rank receiver also reiterate the fact that higher code rates tend to yield lower BER than that of low rate codes. Copyright © 2006 John Wiley & Sons, Ltd.     

Low‐Power‐Adaptive MC‐CDMA Receiver Architecture

This paper proposes a novel concept of adjusting the hardware size in a multi‐carrier code division multiple access (MC‐CDMA) receiver in real time as per the channel parameters such as delay spread, signal‐to‐noise ratio, transmission rate, and Doppler frequency. The fast Fourier transform (FFT) or inverse FFT (IFFT) size in orthogonal frequency division multiplexing (OFDM)/MC‐CDMA transceivers varies from 1024 points to 16 points. Two low‐power reconfigurable radix‐4 256‐point FFT processor architectures are proposed that can also be dynamically configured as 64‐point and 16‐point as per the channel parameters to prove the concept. By tailoring the clock of the higher FFT stages for longer FFTs and switching to shorter FFTs from longer FFTs, significant power saving is achieved. In addition, two 256 sub‐carrier MC‐CDMA receiver architectures are proposed which can also be configured for 64 sub‐carriers in real time to prove the feasibility of the concept over the whole receiver.     

Multi-dimensional adaptive transmission technique for shortto-medium reach optical fiber communication system

According to the features of short-to-medium reach optical fiber communication systems,three high performance adaptive modulation and coding schemes were investigated.In the first one,a new dimension,forward error correction (FEC),was introduced to the traditional bit and power loading (BPL) scheme,and the three-dimensional adaptive modulation and coding were achieved.The modulation format and FEC code were allocated based on look-up table (LUT).The proposed algorithm has lower complexity and higher data rate compared to the BPL scheme.The second one was also based on LUT method where one similar data rate with the BPL scheme was achieved using partitioned precoding,but peak-to-average power ratio was reduced up to two dB,and the power efficiency was improved.In the last scheme,probabilistic shaping QAM was adopted as the modulation format,and shaping gain and almost indefinitely fine modulation granularity were achieved at the expense of certain complexity.With adaptive partitioned precoding,without decreasing data rate the number of PS-QAM was limited to reduce the complexity.The experimental results demonstrate that the proposed scheme outperforms the BPL scheme in terms of data rate and receiver sensitivity.     

Performance of the deliberate clipping with adaptive symbolselection for strictly band-limited OFDM systems

The performance of the strictly band-limited OFDM systems with deliberate clipping is examined in terms of the peak-to-average power ratio (PAPR) and the resultant bit error performance. The clipping is performed on the OFDM signals sampled at the Nyquist rate, followed by the ideal low-pass filter, Since the low-pass filter considerably enlarges the PAPR, there is a severe limitation in PAPR reduction capability. Thus, in order to achieve further reduction of the PAPR, the application of the adaptive symbol selection scheme is also considered. It is shown that the significant PAPR reduction with moderate complexity can be achieved by the combination of the clipping and the adaptive symbol selection. The price to be paid for PAPR reduction by this scheme is its performance degradation. The paper theoretically analyzes the bit error rate performance of the OFDM system with the Nyquist-rate clipping combined with the adaptive symbol selection, and considers the use of the forward error correction for compensation of the degradation. It is shown that even though the clipping scheme causes severe loss in required signal-to-noise ratio, the use of a powerful channel coding scheme such as turbo codes significantly alleviates the bit error rate performance degradation     

宽频单相智能电能表的设计

基于智能电表具有宽频接收、数据的保护、报警功能、断电的显示、负荷的控制、防窃电功能、远程抄表系统等,且具有自动修正误差和自诊断的能力,为使普通电能表实现智能化,本文提出了一种基于ATMEGA16单片机的测试系统设计方案,并完成系统的硬件改进设计,实现数据的传输、费率的计算、数据的显示、报警、远程抄表控制,以使电表实现智能化。     

Adaptive blind receivers for MC‐CDMA communication systems

In this paper, we propose three adaptive blind algorithms for multiuser multicarrier code division multiple‐access systems in multipath fading channels. The proposed adaptive blind receivers are based on the property of the discreteness of the input data symbol and are updated in every symbol interval. We also use the concept that the variance of the output signal approaches to the variance of the desired signal to get the cost function. The three proposed receiver structures are the traditional finite impulse response (FIR) structure, the despreading (DES) filter structure and the generalized sidelobe canceller (GSC) structure. The advantage of the FIR filter is that the length of the filter weights does not have to be the same length as the spreading code. For the DES filter, the combination of the adaptive weight and the despreading code has the simplest structure than the other two proposed receiver structures. The constrained GSC filter is superior to the other two proposed receiver structures in the environments dominated by multiple‐access interference. By this constraint, the blind GSC filter can guarantee to converge to the desired solution. Simulation results are given to show the effectiveness and comparison of the proposed adaptive blind receivers. Copyright © 2012 John Wiley & Sons, Ltd.     

LED clipping distortion compensation in optical wireless communication via multiple transmit LEDs

A method to reduce signal clipping distortion in indoor optical wireless communication systems based on orthogonal frequency division multiplexing (OFDM) modulation is presented in this paper. Compensating the resulting clipping distortion due to LED operational constrains is achieved through the use of iterative signal clipping (ISC) and multiple light emitting diodes (LED) at the transmitter. ISC technique is based on iterative clipping of the time domain OFDM signal and transmission from a multiple LED transmitter. Transmit LEDs are synchronized, located close to each other, and placed to emit light in the same direction. Hence, the channel path gains from each LED to the receiver photo diode are similar. The received signals from the different LEDs add coherently at the receiver. Reported results demonstrate that the effect of distortion due to clipping is eliminated or significantly reduced based on the considered number of LEDs.     

Performance analysis of coded space‐time adaptive detection in DS/CDMA systems over Rayleigh fading channels

In this paper, we study the use of channel coding in a direct‐sequence code‐division multiple‐access (DS‐CDMA) system that employs space‐time adaptive minimum‐mean square‐error (MMSE) interference suppression over Rayleigh fading channels. It is shown that the employment of adaptive antenna arrays at the receiver can assist in attenuating multiuser interference and at the same time speeds‐up the convergence rate of the adaptive receiver. In this work, we assess the accuracy of the theoretical results developed for the uncoded and convolutionally coded space‐time multiuser detector when applied to the adaptive case. It is found that the use of antenna arrays brings the receiver performance very close to its multiuser counterpart. Using performance error bounds, we show that a user‐capacity gain of approximately 200% can easily be achieved for the space‐time adaptive detector when used with a rate 1/2 convolutional code (CC) and a practical channel interleaver. This capacity gain is only 10% less than the gain achieved for the more complicated multiuser‐based receiver. Finally, we perform a comparison between convolutional and turbo coding where we find that the latter outperforms the former at all practical bit‐error rates (BER). Copyright © 2006 John Wiley & Sons, Ltd.     

PAPR reduction for OFDM‐based visible light communication systems using proposed hybrid technique

Orthogonal Frequency Division Multiplexing (OFDM) is utilized with visible light communication (VLC) systems to decrease the impacts of inter‐symbol interference and to achieve communication with high speed of data transmission and huge bandwidth. In any case, the main problem in OFDM‐based VLC systems is high peak‐to‐average power ratios (PAPRs). This paper proposes a hybrid PAPR reduction technique based on signal transformation combined with clipping. The Hadamard transform is used in the proposed technique to reduce the PAPR without affecting the bit error rate (BER) of the VLC systems. The optimum clipping threshold at which the PAPR is reduced simultaneously with the improvement the BER of the VLC systems is also determined. The performance of the proposed system is assessed in terms of complementary cumulative distribution function and the BER. The obtained results demonstrate that the proposed procedure can simultaneously decrease the PAPR and achieve good BER performance compared to the OFDM‐based VLC system.     

A null despreader for interference suppression in GPS

The Global Positioning System (GPS) utilizes low‐power spread‐spectrum signals and thus is vulnerable to various types of high‐power interference sources. It requires at least four satellites for estimating three‐dimensional user positions and the receiver clock bias. In this paper, we propose a blind adaptive GPS receiver that is based on a new despreader and the one‐stage constant modulus (CM) array. The despreader consists of a conventional GPS despreader and a so‐called null despreader, which together modify the received signal so that the CM array can extract the GPS signal of interest. The beamformer not only rejects jammers and extracts the GPS signal of interest without explicit direction‐of‐arrival (DOA) information of any of the signals but also it has a low computational complexity compared with conventional techniques, such as minimum‐variance distortionless‐response (MVDR) beamforming. As a conventional despreader can recover only one GPS signal, multiple despreaders are usually required for separating multiple GPS signals. We also explore an extension of the proposed null despreader to detect multiple GPS signals. Computer simulation examples are presented to illustrate the performance of the receiver for different types of jammer signals. Copyright © 2010 John Wiley & Sons, Ltd.     

基于噪声功率估计的抗窄带干扰接收机自适应PN码捕获技术

在存在强窄带干扰条件下,正确实现伪码捕获是抗干扰接收机的关键技术之一。变换域干扰检测与PN码捕获判决都需要估计接收机背景噪声功率,本文在变换域谱线幅度平方服从指数分布假设下,利用中位数估计理论对存在窄带干扰接收信号的背景噪声功率进行估计,实现干扰检测门限和PN码捕获判决门限的自适应设置。仿真结果表明,文中设计的检测装置可实现强窄带干扰、大动态范围接收机PN码的正确捕获。     

Adaptive Channel‐Matched Extended Alamouti Space‐Time Code Exploiting Partial Feedback

Since the publication of Alamouti's famous space‐time block code, various quasi‐orthogonal space‐time block codes (QSTBC) for multi‐input multi‐output (MIMO) fading channels for more than two transmit antennas have been proposed. It has been shown that these codes cannot achieve full diversity at full rate. In this paper, we present a simple feedback scheme for rich scattering (flat Rayleigh fading) MIMO channels that improves the coding gain and diversity of a QSTBC for 2ⁿ (n = 3, 4,…) transmit antennas. The relevant channel state information is sent back from the receiver to the transmitter quantized to one or two bits per code block. In this way, signal transmission with an improved coding gain and diversity near to the maximum diversity order is achieved. Such high diversity can be exploited with either a maximum‐likelihood receiver or low‐complexity zero‐forcing receiver.     

一种自适应直扩超宽带(DS-UWB)接收机在高速无线室内传输环境中的性能

该文提出了一种基于可变长训练序列的自适应直扩超宽带(DS-UWB)接收机。该接收机充分利用自适应NLMS算法在收敛性、复杂度及稳定性方面的综合优势,以较小的训练开销实现了自适应接收机的功能。理论分析及计算机仿真实验表明,在误码性能相当的前提下,所提出的自适应接收机比传统的Rake接收机具有更为简单的结构,同时与现有的自适应接收机相比,其训练序列的开销可节省60%以上,因而适用于低功率、低成本以及简易实用的室内超宽带应用场合。