A GLRT-based spread-spectrum receiver for joint channel estimationand interference suppression

The author considers the problem of demodulating a direct-sequence (DS) spread-spectrum signal in the presence of narrowband interference and multipath. A receiver is considered that is based on the generalized likelihood-ratio test (GLRT), in which the interferer is modeled as an Nth-order circular Gaussian autoregressive (AR) process and the multipath channel is represented by a tapped-delay line. The maximum-likelihood joint estimator for the channel coefficients and interferer AR parameters is then derived. Analytical expressions for bit-error rate are presented for GLRT receiver, under the assumption of perfect estimates of the channel and interferer parameters. The performance of the GLRT receiver is compared to that of a DS receiver using a transversal equalizer. It is shown that the GLRT receiver consistently outperforms the equalizer-based receiver by 2-3 dB. The performance of an adaptive GLRT receiver is evaluated where the recursive least-squares algorithm is used to jointly estimate the interferer and channel parameters     

宽带CDMA数字中频接收机

本文建立了宽带CDMA的阻塞模型,在此基础上,提出一种数字中接收机的实现方案,并对主要电路的性能要求作了分析,给出了关键器件设计与实现的方法。     

MLSE receiver for direct-sequence spread-spectrum systems on a multipath fading channel

To accommodate high-speed data transmissions, it may be necessary to substantially reduce the processing gain of a direct-sequence spread-spectrum (DSSS) system. As a result, intersymbol interference effects may become more severe. In this paper, we present a new structure for maximum-likelihood sequence estimation equalization of DSSS signals on a multipath fading channel that performs the function of despreading and equalization simultaneously. Analytical upper bounds are derived for the bit-error probability when random spreading sequences are used, and comparisons to simulation results show that the bounds are quite accurate. The results also show that significant performance improvement over the conventional RAKE receiver is obtained.     

An adaptive receiver of joint data and channel estimation for meteor burst communications

In view of the characteristics of the meteor burst channel, the variable rate data transmission should be adopted to improve the system average throughput, which results in channel tracing and equalization problems at the receiver. Although the joint data and channel estimation of maximum likelihood sequence detection performing the principle of per‐survivor processing (PSP) is considered as an optimal detection scheme, its great computational complexity is a major problem and can hardly agree with the decreasing of the meteor channel. Based on the estimation of the system parameters, an adaptive state reduction of the PSP (ASRP) algorithm with only a few states in the trellis diagram is employed, while these states are chosen by the time‐varying threshold according to the exponential decay of meteor channels. It is shown that, ASRP can make a good tradeoff between the performance and the computational complexity, and provides reliable data transmission for adaptive modulation and coding of the meteor burst communication system. Computer simulation results and performance analysis are also included to support our developments. Copyright © 2010 John Wiley & Sons, Ltd.     

A decision-directed spread-spectrum RAKE receiver for fast-fadingmobile channels

The operation of a decision-directed RAKE receiver is described that has the advantage of not requiring any bandwidth overhead for training bits. The performance is illustrated using bit error rate (BER) graphs and burst error length cumulative distribution functions (CDFs) obtained from Monte Carlo simulations. The receiver simulated operates over a fast-fading (100-Hz Doppler) mobile channel at a bit rate of 4.8 kbit/s using a 900-MHz carrier. It is shown that the receiver is not greatly affected by error propagation. However, the incurred lag on the channel impulse response estimate is the major limitation mechanism, and the resulting irreducible BER is quantified for various numbers of taps in our receiver design. The RAKE receiver performance is examined and compared using maximal-ratio, equal-gain, and differential phase combining techniques     

A fully digital noncoherent and coherent GMSK receiver architecturewith joint symbol timing error and frequency offset estimation

We propose a fully digital noncoherent and coherent Gaussian minimum shift keying (GMSK) receiver architecture with joint frequency offset compensation and symbol timing recovery. Carrier phase offset can be estimated if the coherent demodulation mode is adopted. The converted base-band complex signal is first frequency discriminated and then passed through a digital filter which performs a fast Fourier transform (FFT). The frequency offset can be estimated from the DC component of the FFT, and the symbol timing error can be estimated from the phase angle of the FFT at a specified frequency which is equal to an integral multiple of half the bit rate. These two estimated parameters are then used for frequency offset compensation and symbol timing recovery during a preamble period. Coarse carrier phase can be estimated by averaging sampled in-phase and quadrature-phase signals and finding its phase angle within the preamble period after carrier frequency offset is estimated and compensated. The bit error rate (BER) performance of this GMSK receiver architecture is assessed for an additive white Gaussian noise (AWGN) channel by computer simulation     

SAGE based iterative receiver for joint synchronization and channel estimation in uplink MIMO-OFDMA systems

A new Turbo iterative receiver structure is proposed for the uplink multiple-input multiple-output orthogonal frequency division multiple access （MIMO-OFDMA） systems. The space-alternating generalized expectation-maximization （SAGE） algorithm is naturally embedded in the framework of iterative receiver to perform synchronization and detection usin~ the Turbo detector outputs. In each iteration, the expectation step intends to remove the multiple access interference （MAI） caused by other asynchronous users, and the maximization step is utilized to estimate the required parameters （i.e., timing offset, carrier frequency offset, channel state information, etc.） sequentially for each user. Simulation results show that the proposed algorithm can approach the performance of ideal receiver closely, while the processing complexity is rather lower than the conventional detectors.     

Performance of a frequency-hopped differentially modulated spread-spectrum receiver in a Rayleigh fading channel

The performance of a spread-spectrum receiver previously described by the authors is analyzed in detail. Performance curves are given for a wide range of mobile radio channel conditions, including multipath distortion and correlated fading. The use of optimal filters to combat the former and space diversity to combat the latter are investigated. Some errors in the authors' earlier papers are corrected. The form of space diversity suggested has the unusual feature that it can be employed at the fixed station site only and provide diversity in both communication paths, i.e., to and from the mobile. Degradation due to fading and other aberrations is shown to be in the range 3-6 dB for typical mobile channel conditions. In a nonfading channel the receiver is suboptimum by about 3 dB.     

A DS-CDMA tracking mode receiver with joint channel/delayestimation and MMSE detection

A tracking mode receiver for asynchronous direct-sequence CDMA is presented based on the extended Kalman filter (EKF). The EKF jointly estimates the delays and multipath coefficients of the received CDMA waveform, and provides a modified minimum mean-square error (MMSE) estimate of the user data (MMSE-EKF). In order to obtain a practical algorithm, each user signal is tracked individually, with the remaining users modeled as colored Gaussian noise. However, the EKFs are coupled through the multiple access interference (MAI) covariance estimates. In order to obtain meaningful performance measures, approximate worst-case undesired user delays that minimize the desired user SNR and delay estimation Cramer-Rao bound are obtained. It is shown that such worst-case delays can be efficiently computed using the alternating maximization (A-M) algorithm. The resulting bit error rate (BER) performance of the MMSE-EKF tracking receiver is evaluated through a combination of simulation and analysis. The mean-time to lose lock (MTTLL) for a genie-aided EKF delay estimator is also obtained using the A-M computed delays     

Iterative receiver for joint detection and channel estimation in OFDM systems under mobile radio channels

In this paper, an iterative receiver for a joint data-detection and channel-estimation scheme is presented for orthogonal frequency-division multiplexing systems, which incorporates iterative decoding in the receiver. In the proposed scheme, a maximum a posteriori-based decoder and a channel estimator provide more reliable information on the coded bits for each other in an iterative manner. We first consider a practical implementation issue for the optimal minimum mean squared error two-dimensional (2-D) channel estimator as an essential element in the iterative receiver. To reduce the complexity of the 2-D estimator as suited to the iterative receiver, we focus on rigorously investigating how a separable estimator must be designed so that its structure may become asymptotically equivalent to that of the optimal 2-D estimator. Furthermore, we derive an analytical expression of the iterative process to evaluate a convergence performance as a function of the number of iterations and discuss its convergence property. Our simulation results demonstrate that the proposed iterative receiver achieves a near-ideal performance with only a few iterations under time-variant multipath fading channels.     

Asymptotic receiver structures for joint maximum likelihood timedelay estimation and channel identification using Gaussian signals

The problem of jointly estimating the relative time delay and the impulse response linking two received discrete-time Gaussian signals is addressed. Using two different methods, possible structures for the joint maximum-likelihood (ML) estimator are proposed, when the observation interval is long compared to both the delay to estimate and the correlation time, of the various random processes involved. These structures generalize the cross-correlation method with prefiltering that implements the ML estimation of pure time delays     

Iterative rake receiver with map channel estimation for ds-cdma systems

We propose an iterative rake receiver structure using an optimum semi-blind channel estimation algorithm for ds-cdma mobile communication systems. This receiver performs an iterative estimation of the channel according to the maximum a posteriori criterion, using the expectation-maximization algorithm. This estimation process requires a convenient representation of the discrete multipath fading channel based on the Karhunen-Loève orthogonal expansion theorem. The rake receiver uses pilot as well as unknown control and data symbols optimally for improving channel estimation quality. Moreover, it can take into account the coded structure of all unknown transmitted symbols when channel estimation quality is poor or unsatisfactory. The validity of the proposed method is highlighted by simulation results obtained for the FDD mode of the umts interface.     

Doubly iterative receiver for block transmissions with EM-based channel estimation

Cyclic-prefix code division multiple access (CP-CDMA), multicarrier CDMA (MC-CDMA) and single carrier cyclic-prefix (SCCP) transmission are some schemes that could support the increasing demand of future high data rate applications. The linear and nonlinear equalizers used to detect the transmitted signal are always far from the Maximum-Likelihood (ML) detection bound. The block iterative generalized decision feedback equalizer (BI-GDFE) is an iterative and effective interference cancelation scheme which could provide near-ML performance yet with very low complexity. In order to deploy this scheme, the channel state information (CSI) must be available at the receiver. In practice, this information has to be estimated by using pilot and data symbols. This paper investigates the problem of channel estimation using the Expectation Maximization (EM) algorithm. The BI-GDFE provides the soft information of the transmitted signals to the EM-based algorithm in the form a combination of hard decision and a coefficient so-called the input-decision correlation (IDC). The resultant receiver becomes a doubly iterative scheme. To evaluate the performance of the proposed estimation algorithm, the Cramér-Rao Lower Bound (CRLB) is also derived. Computer simulations show that the bit error rate (BER) performance of the proposed receiver for joint channel estimation and signal detection can reach the performance of the BI-GDFE with perfect CSI.     

Blind least-squares estimation of carrier phase, Doppler shift, andDoppler rate for m-PSK burst transmission

A novel blind algorithm is proposed for joint carrier phase/Doppler shift least-squares (LS) estimation for burst transmission of m-PSK modulated data. The algorithm can be extended to include Doppler rate estimation. The performance of the algorithm is contrasted against that of known algorithms, and it is shown to be advantageous particularly for short bursts and large frequency offsets. The signal-to-noise ratio penalty with respect to the ideal estimation case is shown to be below 1 dB for a frequency error as large as 0.1 of the symbol rate and bursts as short as 20 symbols     

直扩数字接收机中AD量化比特数的确定

量化比特数的确定是IF数字接收机设计的关键。从高斯分布函数的角度，得到了不同量化比特数和量化电平下直扩系统的输出信噪比计算公式；对存在窄带干扰情况下AD量化比特数的确定给出了定量的分析。计算机仿真结果表明：无干扰时取4bit量化即可获得较好的接收机性能；干信比为24dB时，量化比特数应至少为6bit。     

Blind RAKE receiver with joint diversity combining and codetracking for DS/SS communication

A RAKE receiver achieving joint blind multipath diversity combining and code tracking is proposed. An improved known modulus adaptive algorithm is exploited to perform multipath diversity combining and to support the modified code tracking in the blind mode. Computer simulation results have indicated very attractive behaviour of the proposed technique     

Data-efficient blind OFDM channel estimation using receiver diversity

We investigate non data-aided channel estimation for cyclically prefixed orthogonal frequency division multiplexing (OFDM) systems. By exploiting channel diversity using only two receive antennas, a blind deterministic algorithm is proposed. Identifiability conditions are derived that guarantee the perfect channel retrieval in the absence of noise. In the presence of noise, the proposed method has the desired property of being data efficient-only a single OFDM block is needed to achieve good estimation performance for a wide range of SNR values. The algorithm is also robust to input symbols as it does not have any restriction on the input symbols with regard to their constellation or their statistical properties. In addition, this diversity-based algorithm is computationally efficient, and its performance compares favorably to most existing blind algorithms.     

一种QPSK全数字化载波频偏估计算法

QPSK信号相干解调时的载波同步是设计QPSK全数字接收机的一个技术难点,它有两个方面,一是载波频偏估计,二是载波相位估计.本文对载波频偏估计算法进行研究,介绍了一种全数字化载波频偏估计器算法,对它进行数学分析,提出了改进的简化算法,通过计算机仿真对两种算法的性能做了比较.仿真结果表明,简化算法是可行的,并具有相对简单、更易于用DSP器件实现的优点,适合于工程实现.     

双通道接收的相位差估算方法

分析了双通道接收相位差的时域、频域计算方法，基于DFT的频域估算方法，充分利用了DFT、对信噪比的改善作用，克服了时域方法要求较高信噪比的缺点，能有效地抑制噪声，提高测量精度。但是离散的DFT输出谱限制了信噪比的改善，文中利用截断DFT的输出谱特性来估计信号频率，再用一次最大似然估计得到信号相位估计，减小运算量。分析和仿真表明文中方法可以得到接近最大似然法的测量精度。     

一种新的扩频信号估计方法

在直接序列扩频通信中,接收端需要保持伪随机码同步。提出了利用UKF算法和EKF算法联合对PRN码进行跟踪处理的联合估计算法,同时对信号的幅度参数进行联合估计。仿真结果表明,该算法在处理扩频信号的非线性估计问题上比传统的EKF算法估计精度更高,与UKF算法的估计精度相当,能够较好地保持PRN信号的同步性。同时其所消耗的总的时间比UKF算法更少,计算量更小。