Nonlinear multiuser parameter estimation and tracking in CDMAsystems

This paper presents a joint channel coefficient and time-delay tracking technique for code-division multiple-access (CDMA) systems. Due to the highly nonlinear nature of time delay estimation, an iterative nonlinear filtering algorithm, called the “unscented filter” (UF), is employed. The UF can provide a better alternative to nonlinear filtering than the conventional extended Kalman filter (EKF) since it avoids errors associated with linearization. The Cramer-Rao lower bound is derived for the estimator, and computer simulations show that it provides a more viable means for tracking time-varying amplitudes and delays in CDMA communication systems than estimators based on the EKF     

A low‐complexity adaptive receiver for DS‐CDMA systems in unknown code delay environment

In direct‐sequence code division multiple access (DS/CDMA) multiuser communication systems in multipath channels, both intersymbol interference (ISI) and multiple‐access interference (MAI) must be considered. The multipath channel characterizes the propagation effects including the timing offset and delays, etc. Traditionally, we use the delay‐locked loop (DLL) code tracking loop to recover the timing delay. But DLL cannot work well in multipath environment. In this paper, we propose a low‐complexity adaptive receiver to suppress ISI/MAI and solve the timing offset problems without using conventional DLL code tracking loop. The proposed receiver employs an adaptive filter whose weights are adapted using a block least‐mean square error algorithm with fractional sampling. Simulations confirm the good performance, including learning curves and theoretical analysis of minimum mean‐square error, of the proposed receiver. Copyright © 2010 John Wiley & Sons, Ltd.     

Joint estimation of PN code delay and multipath using the extendedKalman filter

The problem of delay estimation in the presence of multipath is considered. It is shown that the extended Kalman filter (EKF) can be used to obtain joint estimates of time-of-arrival and multipath coefficients for deterministic signals when the channel can be modeled as a tapped-delay line. Simulation results are presented for the EKF joint estimator used for synchronization in a direct-sequence spread-spectrum system operating over a frequency-selective fading channel. A simplified model of the EKF joint estimator is considered for analysis purposes. The evolution in time of the tracking error probability density function and the nonlinear tracking error variance are examined through numerical solution of the Chapman-Kolmogorov equation. The nonlinear tracking error variance is compared to both the linear error variance estimate directly provided by the EKF and the Cramer-Rao lower bound     

An adaptive multiuser detector with joint amplitude and delayestimation

A multiuser detector is developed in which the delays and amplitudes of the incoming waveforms are estimated recursively. The algorithm is an extension of the symbol-by-symbol detector of Abend and Fritchman, originally derived for intersymbol interference (ISI) channels, to the multiuser application. In order to make the multiuser detector adaptive, the likelihoods in the symbol-by-symbol metric update are approximated using a set of extended Kalman filter (EKF) innovations. The EKF's provide, in addition to the likelihoods, joint estimates of the signal delays and amplitudes. The resulting algorithm is quite complex, due to the large number of possible composite symbols corresponding to the multiple users. However, it is shown that the likelihood computations and EKF updates can be expressed it terms of a set of cross-correlation functions, which need only be computed for a subset of the possible symbols. The cross-correlator outputs are updated at the bit rate, and thus the EKF and metric update computations need be performed only at a function of the spread-spectrum chip rate. A metric pruning technique is proposed that further reduces the number of EKF delay/amplitude estimators required. Finally, an important sampling simulation strategy is used to obtain bit-error rate estimates for the adaptive multiuser detector     

Adaptive symbol and parameter estimation in asynchronous multiuserCDMA detectors

Existing multiuser code-division multiple-access (CDMA) detectors either have to rely on strict power control or near-perfect parameter estimation for reliable operation. A novel adaptive multiuser CDMA detector structure is introduced. Using either an extended Kalman filter (EKF) or a recursive least squares (RLS) formulation, adaptive algorithms which jointly estimate the transmitted bits of each user and individual amplitudes and time delays may be derived. The proposed detectors work in a tracking mode after initial delay acquisition is accomplished using other techniques not discussed here. Through computer simulations, we show that the algorithms perform better than a bank of single-user (SU) receivers in terms of near-far resistance. Practical issues such as the selection of adaptation parameters are also discussed     

Joint detection and channel estimation algorithms for QS-CDMAsignals over time-varying channels

We consider a quasi-synchronous code-division multiple access (QS-CDMA) cellular system, where the code delay uncertainty at the base station is limited to a small number of chips. For such QS-CDMA systems, the need for code acquisition is eliminated, however, the residual code tracking and channel estimation problems still have to be solved. An extended Kalman filter (EKF) is employed to track the user delays and channel coefficients. By separating data detection, based on the QR decomposition combined with the M-algorithm (QRD-M) from the delay/channel estimation process, the computational complexity can be significantly reduced as the number of users increases. Simulations show that the EKF channel estimator performance is improved when the QRD-M algorithm is used instead of the MMSE detector or decorrelator for data decisions     

Exact and approximate maximum-likelihood parameter estimation forquasi-synchronous CDMA signals

In order to construct the optimal decorrelator for a quasi-synchronous (QS) code-division multiple-access receiver, it is necessary to estimate the user delays. However, in a QS system, it is shown that delay estimation is equivalent to estimating a parameter vector which is related linearly to the received signal. An exact maximum-likelihood (ML) solution using alternating maximization is first developed for joint estimation of the user delay vectors and amplitudes. Suboptimal recursive least squares (RLS) and least mean square algorithms are then presented which approximate the true ML solution. It is shown that the optimal decorrelator and demodulated symbols are also generated by the RLS algorithm. Simulation and analytical results are presented for the bit-error rate performance of the adaptive receiver, and the average squared error of the estimated delay vectors is compared with the Cramer-Rao bound     

Comparison of MC-CDMA with DS-CDMA using frequency domain and time domain RAKE receivers

In this paper a multicarrier CDMA (MC-CDMA) system with a soft decision differential phase shift keying (DPSK) frequency domain RAKE receiver is described. We compare a MC-CDMA system with a direct sequence CDMA system using RAKE receivers. In contrast with previous MC-CDMA systems, guard intervals are not used and the carriers are spaced at the reciprocal of the bit rate, optimising the usage of the bandwidth. In this way a comparison can be made between the multicarrier CDMA system described and a direct sequence (DS-CDMA) system with the same bandwidth. The results presented are received bit error rates from Monte Carlo simulations. The simulations are conducted in a multipath channel with Rayleigh fading and 300 Hz Doppler spectrum with additive white Gaussian noise. It is shown that the multicarrier CDMA matched filter receiver performs favourably compared to the direct sequence CDMA matched filter receiver for 1 -path fading. For a single user at a receive bit error rate of 1×10^–3 in the 4-path fading channel the multicarrier RAKE receiver requires no knowledge of the channel delay spread and performs 3 dB worse than the DS-CDMA RAKE receiver simulated. The performance of the MC-CDMA RAKE receiver for a single user increases with increasing channel dispersion. The performance of the DS-CDMA RAKE receiver for multiple user is superior to that of the MC-CDMA RAKE receiver.     

Performance comparison of a slotted ALOHA DS/SSMA network and amultichannel narrow-band slotted ALOHA network

Throughput, delay, and stability for two slotted ALOHA packet radio systems are compared. One system is a slotted direct-sequence spread-spectrum multiple-access (DS/SSMA) network where each user employs a newly chosen random signature sequence for each bit in a transmitted packet. The other system is a multiple-channel slotted narrow-band ALOHA network where each packet is transmitted over a randomly selected channel. Accurate packet success probabilities for the code-division multiple-access (CDMA) system are computed using an improved Gaussian approximation technique which accounts for bit-to-bit error dependencies. Average throughput and delay results are obtained for the multiple-channel slotted ALOHA system and CDMA systems with block error correction. The first exit time (FET) is computed for both systems and used as a measure of the network stability. The CDMA system is shown to have better performance than the multiple-channel ALOHA system in all three areas     

Adaptive fuzzy proportional integral power control for a cellularCDMA system with time delay

We investigate the use of fuzzy logic control techniques for adaptive power control in a direct-sequence code-division multiple-access (DS/CDMA) cellular system over the mobile fading radio channels with propagation time delay. A fuzzy PI (proportional-plus-integral) control whose input variables are the received power error and error change is introduced to determine each user transmitting power in order to simultaneously equalize all users' signal powers received at the base station and achieve better system stability and control performance. This control strategy can ensure long loop transmission delays without causing the system to become unstable. According to the well-known phase-plane method, the derivation of the fuzzy PI control has been carried out by analyzing the response areas, cross-over, and extreme points of the system step response with apriori knowledge of the dynamics of the CDMA mobile fading channels. For a long time-delay fading process, a methodology is developed to modify the rule base to contain the delay information for reducing the deadtime effects of the process. Moreover, the additional advantages of fuzzy PI control over conventional control theories are: increased robustness in spite of interference and the ability to handle the time-delay process whose parameters are not accurately known. Simulation results show that a good performance can be achieved both in RMS tracking error and traffic capacity by use of fuzzy PI power control, especially in poor interuser interference and long time delay conditions     

A Novel Detection Algorithm Efficient for Turbo Coded CDMA Signals in Detect and Forward Cooperative Channels

In this paper, a new detection algorithm is proposed for turbo coded Code Division Multiple Access (CDMA) signals in detect and forward cooperative channels. Use of user cooperation makes much improvement in the performance of CDMA systems. Due to the special structure of CDMA systems, cooperative schemes increase the sum and cutoff capacities of CDMA based wireless systems and improve the quality of user-partner link which enhances the overall performance of the system. In this paper, a new combining scheme is proposed that makes the receiver more robust against the decision errors in the partner link. This structure is simulated for punctured 1/2 rate 4 states turbo code in a channel with first order Markov time variation and different Rice factor variances. Through various simulations, it is shown when the channel estimates are available in the partner and receiver, the cooperation between users provides much diversity gain especially while using the new proposed combining algorithm.     

Generalized performance analysis of a delay diversity receiver in asynchronous CDMA channels

In this letter, the performance for the delay diversity receiver is analyzed in asynchronous code division multiple access (CDMA) channels. The outage probability and the bit error probability of the delay diversity receiver are accurately derived and compared with those of the conventional diversity receiver. From the analytical and numerical results, it is confirmed that the delay diversity receiver achieves a remarkable diversity gain with reasonable cost and complexity in asynchronous CDMA channels. Specifically, for roughly the same hardware complexity, the delay diversity receiver achieves nearly twice the diversity order of the conventional receiver.     

Performance of DS-CDMA with imperfect power control operating overa low earth orbiting satellite link

The analysis of both the performance and capacity of direct sequence CDMA in terrestrial cellular systems has been addressed in the technical literature. It has been suggested that CDMA be used as a multiple access method for satellite systems as well, in particular for multispot beam low Earth orbit satellites (LEOS). One is tempted to argue that since CDMA works well on terrestrial links, it will nominally work as well on satellite links. However, because there are fundamental differences in the characteristics of the two channels, such as larger time delays from the mobile to the base station and smaller multipath delay spreads on the satellite channels, the performance of CDMA on satellite links cannot always be accurately predicted from its performance on terrestrial channels. In the paper, the authors analytically derive the performance of a CDMA system which operates over a low Earth orbiting satellite channel. They incorporate such effects as imperfect power control and dual-order diversity to obtain the average probability of error of a single user     

A digital DS spread-spectrum receiver with joint channel andDoppler shift estimation

A digital spread-spectrum receiver design is presented for communication over multipath channels with severe Doppler shifts. The characteristics of the underwater channel relevant to spread-spectrum system design are discussed, and a channel model for short-range communications (less than 10 km) is defined. The receiver considered uses a digital coherent RAKE combiner, coupled with an extended Kalman filter (EKF)-based estimator for channel parameters and pseudonoise code delay. Receiver performance is evaluated by computing average bit-error rate (BER) versus iterations of the EKF joint estimator, using both fixed and time-varying channels. It is shown that the BER obtained using the EKF joint estimator closely tracks the optimum BER obtained when the channel, delay, and Doppler parameters are known exactly. Finally, the Cramer-Rao lower bound for time-invariant joint channel, delay, and Doppler estimation is derived, and compared with the ensemble averaged mean-squared error of the EKF estimator     

Performance of an adaptive successive serial-parallel CDMAcancellation scheme in flat Rayleigh fading channels

The performance of a successive concatenated cancellation scheme for code-division multiple-access (CDMA) uplink transmission in cellular mobile radio is presented. Both serial and parallel cancellation stages are employed. The serial cancellation stage is first used to obtain initial data estimates followed by the parallel cancellation stages to enhance the accuracy of the estimates. The performance of this scheme is evaluated via analysis and simulation. In our analysis, we develop a model to consider the impact of wrongly estimating the phase and amplitude of the channel impulse response on the successive concatenated cancellation scheme. Analysis and simulation results in flat Rayleigh fading asynchronous channels with both perfect and nonperfect channel estimation and with perfect ranking confirms the accuracy of our analytical model as well as the significant improvement in performance compared to the conventional single-user matched filter (MF) detection and the stand-alone parallel cancellation schemes. Analytical results also show that the concatenated scheme has the potential to reach the single-user performance bound for a wide range of user base size, up to K=120 users, with a processing gain of 127 using one serial and three parallel cancellation stages. Finally, we propose a method whereby the receiver adapts with the number of users in order to retain the bit error ratio (BER) performance while decreasing the processing delay     

长码CDMA系统上行链路多用户接收机设计

提出一种适用于长码CDMA移动通信系统上行链路的多用户接收机设计方案，包括多用户信道估计、多用户检测以及信道跟踪等算法。通过把用户传播时延的影响放入信道响应矩阵中，由信道估计间接得出各用户的传播时延。使用迭代运算的方式使得运算量分散，降低了对硬件平台的要求，使算法更为适用。增大观察窗的长度达到了更准确的用户信息比特判决，提高了接收机的性能。仿真结果表明文章提出的接收机设计方案具有很好的远近效应抑制能力，性能远高于传统的单用户Rake接收机。     

LMS algorithms for tracking slow Markov chains with applications to hidden Markov estimation and adaptive multiuser detection

This paper analyzes the tracking properties of the least mean squares (LMS) algorithm when the underlying parameter evolves according to a finite-state Markov chain with infrequent jumps. First, using perturbed Liapunov function methods, mean-square error estimates are obtained for the tracking error. Then using recent results on two-time-scale Markov chains, mean ordinary differential equation and diffusion approximation results are obtained. It is shown that a sequence of the centered tracking errors converges to an ordinary differential equation. Moreover, a suitably scaled sequence of the tracking errors converges weakly to a diffusion process. It is also shown that iterate averaging of the tracking algorithm results in optimal asymptotic convergence rate in an appropriate sense. Two application examples, analysis of the performance of an adaptive multiuser detection algorithm in a direct-sequence code-division multiple-access (DS/CDMA) system, and tracking analysis of the state of a hidden Markov model (HMM) with infrequent jumps, are presented.     

一种用于雷达量测的滤波算法

在雷达跟踪系统中,广泛使用混合坐标系下扩展卡尔曼滤波器（EKF）和转换卡尔曼滤波器（CMKF）两种算法,但是当目标距离较远时,这两种滤波器由于量测方程非线性的影响,误差较大,甚至导致滤波发射。本文计算了传统EKF滤波器引入的线性化误差,并在此基础上提出一种新的滤波算法（NDRKF）。计算机仿具结果表明,该算法减小了线性化误差的影响,较明显地改善了滤波顺的性能。     

Linear unbiased data estimation in mobile radio systems applyingCDMA

Data estimation in the uplink of a synchronous mobile radio system applying code-division multiple access (CDMA) is considered. In mobile radio systems applying CDMA, multipath propagation leads to intersymbol interference (ISI) and together with time variance, to cross interference between the signals of different users regardless of whether the user codes are chosen orthogonal or not. A linear unbiased data estimation algorithm is presented that eliminates both ISI and cross interference perfectly by jointly detecting the different user signals, leading to unbiased estimates of the transmitted data symbols. By theoretical analysis and simulation, the performance of the linear unbiased data estimation algorithm is examined under the assumption that the radio channel impulse responses are known at the receiver. The price to be paid for the interference elimination are SNR degradations, which are calculated for typical mobile radio situations in urban areas. The resulting average uncoded bit error probabilities lead to the conclusion that systems applying the linear unbiased data estimation algorithm are well suited for mobile radio applications     

A delay independent decorrelating detector for quasi-synchronousCDMA

The decorrelating detector is a near-far resistant linear joint detector for a code-division multiple-access (CDMA) system. It consists of a bank of matched filters followed by a decorrelating matrix. For proper operation, both the matched filter bank and the decorrelating matrix require knowledge of the delays of all users. The delays are due to the different propagation times from each user to the base station. Delay estimation is a weak link in this system since it is complex and prone to error. The proposed decorrelating detector does not require exact knowledge of the user delays, but instead requires that the delays be bounded to a fraction of a symbol interval. The delays are naturally bounded in this way in many microcell and picocell systems where the round trip propagation time is small compared to the symbol interval. The new delay independent decorrelating detector is shown to be near-far resistant and, through appropriate spreading code selection, to experience a modest 3 dB signal-to-noise ratio (SNR) loss relative to orthogonal access schemes. It is also shown to limit capacity to a maximum of 50% of the spreading gain when the delays are bounded by a single chip interval. The complexity is similar to the conventional correlating receiver which is far less complex than the joint detection schemes proposed to date