Asymptotically maximum likelihood detection for subcarrier mapping‐induced cyclostationary signatures in cognitive radio networks

In cognitive radio networks based on orthogonal frequency division multiplexing, the subcarrier mapping‐induced cyclostationary signatures are utilized for robust network coordination. However, the effect of network coordination depends on the reliable detection of such cyclostationary signatures. Moreover, doubly selective fading increases the difficulty of reliable detection. In this paper, we first derive an exact covariance matrix to characterize the statistics of the cyclostationary signatures induced by subcarrier mapping. Using the exact covariance matrix rather than its estimation adopted in conventional algorithms, we propose an asymptotically maximum likelihood detection algorithm. The detection performance of the proposed asymptotically maximum likelihood detection algorithm is analyzed and evaluated both in flat fading and doubly selective fading channels. Simulation results show that the proposed algorithm outperforms the conventional low‐complexity single‐cycle signature detection algorithm in terms of less observation period or smaller mapping set size. Copyright © 2011 John Wiley & Sons, Ltd.     

Cyclostationary Signatures in Practical Cognitive Radio Applications

We define a cyclostationary signature as a feature which may be intentionally embedded in a digital communications signal, detected through cyclostationary analysis and used as a unique identifier. The purpose of this paper is to demonstrate how cyclostationary signatures can be exploited to overcome a number of the challenges associated with network coordination in emerging cognitive radio applications and spectrum sharing regimes. In particular we show their uses for signal detection, network identification and rendezvous and discuss these in the context of dynamic spectrum access. We present a theoretical discussion followed by application-oriented examples of the cyclostationary signatures used in practical cognitive radio and dynamic spectrum usage scenarios. We focus on orthogonal frequency division multiplexing (OFDM) based systems and present an analysis of a transceiver implementation employing these techniques developed on a cognitive radio test platform.     

Multiuser detection in asynchronous CDMA frequency-selective fading channels

Multipath fading severely limits the performances of conventional code division multiple-access (CDMA) systems. Since every signal passes through an independent frequency-selective fading channel, even modest cross-correlations among signature sequences may induce severe near-far effects in a central multiuser receiver. This paper presents a systematic approach to the detection problem in CDMA frequency-selective fading channels and proposes a low complexity linear multiuser receiver, which eliminates fading induced near-far problem.We initially analyze an optimal multiuser detector, consisting of a bank of RAKE filters followed by a dynamic programming algorithm and evaluate its performance through error probability bounds. The concepts of error sequence decomposition and asymptotic multiuser efficiency, used to characterize the optimal receiver performance, are extended to multipath fading channels.The complexity of the optimal detector motivates the work on a near-far resistant, low complexity decorrelating multiuser detector, which exploits multipath diversity by using a multipath decorrelating filter followed by maximal-ratio combining. Analytic expressions for error probability and asymptotic multiuser efficiency of the suboptimal receiver are derived that include the effects of multipath fading, multiple-access interference and signature sequences correlation on the receiver's performance.The results indicate that multiuser detectors not only alleviate the near-far problem but approach single-user RAKE performance, while preserving the multipath diversity gain. In interference-limited scenarios multiuser receivers significantly outperform the RAKE receiver.This paper was presented in part at the Twenty-Sixth Annual Conference on Information Sciences and Systems, Princeton, NJ, March 1992 and MILCOM'92, San Diego, CA, October 1992. This work was performed while author was with the Department of Electrical and Computer Engineering, Northeastern University, Boston, USA.     

Cyclostationarity-based OFDM signal sensing over doubly-selective fading channels

In this paper, using cyclostationarity-based sensing method to detect the presence of Or-thogonal Frequency Division Multiplexing (OFDM) signal over doubly-selective fading channels is studied. By approximating the channel with Basis Expansion Model (BEM), we derive the second-order cyclostationary statistics of the received OFDM signal over doubly-selective fading channels. Theo-retical analysis indicates that new cyclostationary signatures produced by Doppler spread and mul-tipath delay can be further exploited in the detecting process. Simulation examples demonstrate that the sensing methods using channel-induced cyclostationary features provide substantial improvements on detection performance.     

基于OFDM频域内插导频的循环平稳特征频谱感知方法研究

本文研究了基于循环平稳特征的OFDM系统频谱感知算法,提出了一种基于内插导频的循环平稳特征感知算法,并在不同信道条件下对其进行了仿真分析,结果表明该算法不依赖于循环前缀的长度且在低信噪比衰落信道条件下具有良好的检测性能,衰落信道下非相关合并造成的性能损失可以通过适当的延长感知时间得到补偿。此外,对于一个实际的OFDM系统,该算法在导频相位伪随机变化时更具实用性。     

Interference cancelling equalizer (ICE) for mobile radiocommunication

This paper proposes an adaptive interference cancelling equalizer (ICE), which not only equalizes intersymbol interference (ISI), but also cancels cochannel interference (CCI) in the received signal in Rayleigh-fading environments, ICE is an adaptive multiuser detector for the frequency-selective fading environment commonly experienced by mobile communication channels. ICE employs a novel detection scheme: recursive least-squares maximum-likelihood sequence estimation (RLS-MLSE), which simultaneously estimates time-varying channel parameters and transmitted signal sequences. Diversity reception is used to enhance the signal detection performance of ICE. A computer simulation of a 40-kb/s QPSK time-division multiple-access (TDMA) cellular mobile radio system demonstrates the possibility of improving system capacity with ICE. Simulations of ICE with and without diversity are carried out under various fading conditions. For the maximum Doppler frequency of 40 Hz, ICE can attain an average bit-error rate (BER) of 10 ^-2 under a single CCI carrier-to-interference ratio (CIR) of ~14 dB. Moreover, ICE for two independent CCI signals can attain the average BER of 1.5×10^-2 with average CIR⩾-10 dB     

Primary network cognition with spatial diversity signature

The identification of primary networks improves efficiency in dynamic spectrum access. We propose a spatial diversity signature for the primary signal detection, which exploits the structure of space-time or space-frequency codeword based on Alamouti scheme. The signature pattern informs the secondary user which network transmits the primary signal and enables to maintain an experiential database for each primary network. The proposed scheme provides reliable detection and network identification performance over frequency-selective fading channels in low signal-to-noise ratio regimes, without sacrificing the data rate of primary system.     

Experimental detection of mobile satellite transmissions with cyclostationary features

One of the important functions of cognitive radio (CR) technology is spectrum sensing. The implementation of an efficient spectrum sensing function can be quite challenging because of various factors such as multi‐path fading, low signal‐to‐noise ratio of the radio communication services to be detected and the requirement to detect and analyze the signal in a short time. As a consequence, it is important to quantitatively assess the performance of spectrum sensing techniques in various scenarios. This paper investigates different digital signal processing techniques for spectrum sensing in the context of mobile satellite transmissions: power sensing, cyclostationary sensing, efficient cyclostationary sensing based on FFT accumulation method and strip spectral correlation algorithm. This paper presents experimental results on the cyclostationary properties of GSM Thuraya mobile satellite communications in various conditions both for the uplink and downlink channels. The receiver operating characteristics are computed, and the results are presented for different algorithms and different positions of the satellite terminals. The experimental results show that the cyclostationary‐feature‐based detection can be robust compared to energy‐based technique for low signal‐to‐noise ratio levels. Copyright © 2014 John Wiley & Sons, Ltd.     

Implementation of the Decorrelating Receiver for Asynchronous DS-CDMA Systems over Multipath Fading Channels

This paper presents a decorrelating detector for DS-CDMAasynchronous systems over multipath fading channels. The decorrelatingreceiver applies the inverse correlation matrix of signature waveformsto detect the received signal. A procedure for finding such an inversematrix is developed. This procedure employs the reciprocal basis,derived from the basis of normalized user signatures, to cancelsimultaneously both the multiuser and the intersymbolinterference. The proposed multiuser detector operates over a finitedetection window, and the only restriction imposed on the set of signaturesis that the signatures must be independent at the receiver end. If the channelis slowly variant, the detectorcan be very efficient for high bit rate transmissions. Theoreticalbounds for the multiuser interference are obtained. Simulationresults, in which both binary and non-binary sequences are used,reveal that the behaviour of the detector approaches the single userbound when an adequate set of signatures is employed. The reducedcomplexity of this detector indicates that it could be a good candidatefor practical implementation.     

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     

New Approach for Error Compensation in Coded V-BLAST OFDM Systems

In this paper, we investigate coded layered space-time architectures for frequency-selective fading multiple-input multiple-output orthogonal frequency-division multiplexing (OFDM) channels. By computing outage capacity formulas, we will show that the capacity of the vertical Bell Labs layered space-time (V-BLAST) architecture can closely approach the Shannon capacity in the frequency-selective OFDM environment. Motivated by the capacity analysis, we propose pragmatic approaches which preserve the optimality of the layered space-time concept. We present methods to prevent the error propagation from catastrophically affecting the signal detection in subsequent layers. First, we start with a comprehensive signal modeling which includes error propagation. We derive an improved signal detector and describe the optimal soft-bit log-likelihood ratio value-computation method by taking decision errors into account for soft-input channel decoding. Then, to further enhance the V-BLAST performance, we show that cancellation using decoded decisions from previous layers makes the decision errors almost completely disappear, so that the layered space-time architecture can approach the attainable channel capacity. Finally, simulations confirm that the proposed schemes show a significant performance improvement over the conventional methods     

A noncoherent tracking loop with diversity and multipathinterference cancellation for direct-sequence spread-spectrum systems

A noncoherent tracking loop with diversity reception and multipath interference cancellation is proposed for direct-sequence spread-spectrum systems on frequency-selective multipath fading channels. The basic idea of the loop is to reproduce the multipath interference and remove it from the received signal, and then a path diversity is embedded in the loop to improve the loop performance. The tracking error performance of the loop is evaluated by linear analyses and computer simulations. Numerical results show that the proposed tracking loop outperforms the traditional delay locked loop and performs very well under all considered fading conditions     

衰落信道盲接收条件下的调制分类

该文针对衰落信道下调制识别算法预处理要求高的问题,提出了一种基于循环平稳性检测的调制分类算法,提升了盲接收环境下的分类效果。算法以信号的循环频率为特征,结合循环平稳性检测,实现了OFDM, PSK, FSK等常见数字调制信号的区分。算法无需各种参数估计以及同步等预处理过程,理论推导及仿真结果证明,能够实现低信噪比衰落信道条件下信号的有效区分。该算法将有助于提升衰落信道中非合作接收条件下的信号分类能力。     

Blind Linear MMSE Receivers for MC-CDMA Systems

This paper studies blind constrained minimum output energy (CMOE)-based and subspace-based linear minimum mean-squared-error (LMMSE) detectors for multi-carrier code division multiple access (MC-CDMA) systems. By imposing quadratic weight constraint, the CMOE detector is made more robust against signature waveform mismatch, and a better performance over the standard CMOE detector is obtained. Because of separation of signal and noise subspaces, the more complicated subspace-based LMMSE detector has better performance than the CMOE detector. The recursive subspace tracking algorithms are also investigated for the subspace-based MMSE receiver. Numerical results show that the steady-state performance of the robust CMOE detector is close to the subspace-based MMSE method. The blind mode decision-directed LMMSE detection is studied where the blind detectors are used for initial adaptation. Numerical simulations illustrate that the blind mode decision-directed MMSE detection substantially improves the system performance when the frequency-selective channel is slowly-varying     

Multichannel joint detection of multicarrier 16-QAM DS/CDMA system for high-speed data transmission

We propose a multichannel joint detector which eliminates other user, multipath, and intercarrier interference through a decorrelating process in a multicarrier 16-QAM direct-sequence code-division multiple-access system. The performance of the proposed detector under a frequency-selective fading channel is analytically derived and compared to that of a conventional single-user detector. The symbol error ratio curve of the proposed detector does not show the error floor that is seen in the conventional detector, and the resultant performance is close to that of a multicarrier 16-QAM system without any interference.     

低信噪比下周期平稳信号的稳健检测算法

本文提出了一种新的对周期平稳信号进行检测以及对二阶周期循环频率进行估计的算法。该算法利用信号的递归性质构造高阶自相关矩阵,并通过利用周期平稳信号与自相关矩阵特征值和特征向量的关系,对其进行检测以及对循环频率进行估计。传统检测周期平稳信号的算法是通过计算其循环自相关函数或循环谱实现,相比传统算法而言,本算法由于利用到了信号更多的先验信息,因而在较低信噪比以及较低快拍数下对周期平稳信号均能有较好的检测性能。文中仿真实验表明,本文所提算法估计出的伪循环谱相比传统方法估计出的循环谱更为平滑,在相同快拍和信噪比条件下,检测概率均高于传统方法,特别在低信噪比下对检测概率的改善更为明显。      

Envelope-aided Viterbi receivers for GMSK signals with limiter-discriminator detection

Continuous phase modulation schemes, such as Gaussian minimum-shift keying (GMSK), are frequently used with limiter-discriminator (LD) detectors. This paper studies how the side information derived from the signal envelope can enhance the performance of a Viterbi algorithm (VA)-based receiver operating on the LD output of a GMSK scheme. By considering the joint probability density function of envelope and frequency, different approximations yield different novel metrics for VA, using the three-variables envelope and its derivative, and frequency error in different combinations. Simulation results confirm that such envelope-aided VA gives significant performance gains, and that envelope information complements the frequency information output by the LD detector in frequency-selective fading channels.     

Multi-user detector for multi-carrier CDMA systems

Blind multi-user detection is of special interest for multiple access interference (MAI) mitigation in code-division multiple-access (CDMA) systems since it is impractical to assume perfect knowledge of parameters such as spreading codes, time delays and amplitudes of all the users in a rapidly changing mobile environment. In [1], the blind decorrelating detector and the minimum mean square error (MMSE) detector were developed based on subspace estimation and derived in closed forms of the signal subspace components. To combat frequency-selective fading, the subspace approach to blind multi-user detection was extended to multi-carrier CDMA (MCCDMA) systems, e.g. in [2]. In this Letter, we develop further the algorithm presented in [1, 2], and show how the condition of the signal subspace can be improved by using not only the autocorrelation, but also the pseudo-autocorrelation of the received observations, leading to improved performance in blind multi-user detection.     

Adaptive joint multiuser detection and channel estimation in multipath fading CDMA channels

The problem of joint multiuser detection and channel estimation in frequency-selective Rayleigh fading CDMA channels is considered. First the optimal multiuser detector for such channels is derived, which is seen to have a computational complexity exponential in the product of the number of users and the length of the transmitted data sequence. Two suboptimal detectors are then developed and analyzed, both of which employ decorrelating filters at the front-ends to eliminate the multiple-access interference and the multipath interference. The symbol-by-symbol detector uses a Kalman filter and decision feedback to track the fading channel for diversity combining. The per-survivor sequence detector is in the form of the Viterbi algorithm with the trellis updates being computed by a bank of Kalman filters in the per-survivor fashion. Both suboptimal detectors require the knowledge of all waveforms of all users in the channel and the channel fading model parameters. Adaptive versions of these suboptimal detectors that require only the knowledge of the waveform of the user of interest are then developed. The adaptive receivers employ recursive-least-squares (RLS) minimum-mean-square-error (MMSE) filters at the front-end to mitigate the interference, and use a bank of linear predictors to track the fading channels. It is shown that the front-end RLS-MMSE filters can be implemented using systolic arrays to exploit massively parallel signal processing computation, and to achieve energy efficiency. Finally, the performance of the suboptimal detectors and their adaptive versions are assessed by simulations. This revised version was published online in July 2006 with corrections to the Cover Date.     

基于深度学习的多模OFDM索引调制检测器

本文提出了一种基于深度学习（Deep Learning, DL）的多模正交频分复用索引调制（Multi-Mode Orthogonal Frequency Division Multiplexing with Index Modulation, MM-OFDM-IM）检测器。在该检测器中包括两个子卷积神经网络（Sub-Convolutional Neural Network, SCNN）并行对MM-OFDM-IM信号的索引位和载波位进行检测,接收符号在经过迫零（Zero Force, ZF）均衡后再预处理生成二维矩阵,同时输入到子卷积网络中学习信号的内在特征。经过离线训练,该检测器可以实现MM-OFDM-IM符号的在线检测。仿真结果表明,该检测器在瑞利衰落信道条件下能以较低的计算复杂度获得近似最大似然（Maximum Likelihood, ML）检测性能。通过对已训练后的模型进行剪枝操作,能在保证检测误码率（Bit Error Rate, BER）的前提下大幅度减少模型的参数量,达到了性能与计算复杂度的有效平衡。