一种用于CDMA系统的信道估计与检测方案

在DS/CDMA系统中,多用户检测技术可以减小多址干扰和远近效应,但一般情况下,多用户检测总是假设接收机对信道特性已知。本文提出一种慢衰落信道中的有效的联合信道估计和干扰抵消方法,这种方法需要很少的导频符号来进行信道估计,而且实现简单,可以用在较快衰落信道中。     

Performance of quasisynchronous scale time code division multiple access for AWGN channel

We analyze bit error rate performance of a recently proposed multiple access system called scale time code division multiple access (STCDMA) for quasisynchronous communication over an AWGN channel. STCDMA depends on code, time and scale orthogonality introduced by spreading sequences and wavelets. Wavelets are employed as an orthogonal set of symbols for signaling, and their orthogonality over scale and time is exploited. The channel is partitioned into different scales, and each scale into different time slots. Each user is assigned a specific scale, time slot, and spreading code. Information symbols of each user are encoded by the Haar wavelet in its scale and time slot, and then they are spread by its spreading code. Complex-valued Hadamard sequences are used as spreading sequences and conventional detector (i.e. matched filter) is used at the receiver. Results show that the performance of STCDMA gets much better than that of CDMA over the quasisynchronous AWGN channel as the number of scales increases.     

Achievable throughput enhancement based on modified carrier interferometry for MIMO/OFDM

In MIMO systems, channel estimation is important to distinguish the transmitted signals from multiple transmit antennas. When MIMO systems are introduced in cellular systems, we have to measure the received power from all the connectable base station (BS), as well as to distinguish all the channel state information (CSI) for the combination of transmitter and receiver antenna elements. One of the most typical channel estimation schemes for MIMO in a cellular system is to employ a code division multiplexing (CDM) scheme in which a unique spreading code is assigned to distinguish both BS and MS antenna elements. However, by increasing the number of transmit antenna elements, large spreading codes and pilot symbols are required to estimate an accurate CSI. To reduce this problem, in this paper, we propose a high time resolution carrier interferometry (HTRCI) for MIMO/OFDM to achieve an accurate CSI without increasing the number of pilot symbols.     

Blind Capon-like adaptive ST-BC MIMO-CDMA receiver based on constant modulus criterion

In this paper, we present a new Capon-like blind receiver based on linearly constrained constant modulus (LCCM) criterion for the multiple-input multiple-output (MIMO) antennas system along with space–time block code (ST-BC) using direct-sequence code division multiple access (CDMA) modulation technique. A time-varying channel model with generalized sidelobe canceller (GSC) associated with the recursive least squares (RLS) algorithm is implemented to reduce the complexity of receiver design. In our derived algorithm, the parameter of constant modulus, α, relating to the desired user power is updated adaptively via stochastic gradient algorithm to track user?s amplitude variation. Also we prove theoretically that in the two-branch filter bank receiver design the weight vector of one branch can be updated simply using the other one, which has been obtained with our proposed CM-GSC-RLS algorithm, with simple pre-calculated transform. Hence computation complexity of the proposed adaptive blind receiver can be further reduced significantly. Via intense simulations it reveals that our proposed scheme has robust performance against the user?s acquisition inaccuracies comparing with current available algorithms.     

An adaptive least squares channel estimation based on conjugate gradient approach for multipath fading CDMA systems

Multiple access interference (MAI) is the main factor affecting the performance of channel estimation techniques for code division multiple access (CDMA) systems. Although, several multi-user channel estimation algorithms have been proposed to mitigate MAI, these algorithms require high computational complexities. In this paper, we address the problem of iterative least squares (LS) mobile channel estimation at high channel efficiency that requires a short training sequence along with the spreading sequences. We employ an efficient iterative method based on conjugate gradient (CG) algorithm to reduce the computational complexity of the estimation method. Computer simulations illustrate that the proposed method performs almost identical to the exact LS estimate for reasonable training lengths.     

Robust adaptive spread-spectrum receiver with neural netpreprocessing in non-Gaussian noise

Multiuser communications channels based on code division multiple access (CDMA) technique exhibit non-Gaussian statistics due to the presence of highly structured multiple access interference (MAI) and impulsive ambient noise. Linear adaptive interference suppression techniques are attractive for mitigating MAI under Gaussian noise. However, the Gaussian noise hypothesis has been found inadequate in many wireless channels characterized by impulsive disturbance. Linear finite impulse response (FIR) filters adapted with linear algorithms are limited by their structural formulation as a simple linear combiner with a hyperplanar decision boundary, which are extremely vulnerable to impulsive interference. This raises the issues of devising robust reception algorithms accounting at the design stage the non-Gaussian behavior of the interference. We propose a multiuser receiver that involves an adaptive nonlinear preprocessing front-end based on a multilayer perceptron neural network, which acts as a mechanism to reduce the influence of impulsive noise followed by a postprocessing stage using linear adaptive filters for MAI suppression. Theoretical arguments supported by promising simulation results suggest that the proposed receiver, which combines the relative merits of both nonlinear and linear signal processing, presents an effective approach for joint suppression of MAI and non-Gaussian ambient noise.     

具有多网络自适应选择的无线宽带视频传输系统

针对移动信道带宽不稳定以及Wi-Fi热点覆盖有限等网络问题,提出了一种可自适应选择第二代移动通信网络(CDMA1X)、第三代移动通信网络(3G-EVDO)及无线局域网(WLAN)的多网络自适应无线视频传输的系统架构,并给出了多网络自适应选择的算法。该算法主要包括多网络的自适应接入、无线异构网络的动态切换以及无线同构网络的自适应选择传输。该方案可实现无线视频的高质量可靠传输。     

New theoretical bounds on the aperiodic correlation functions of binary sequences

In order to reduce or eliminate the multiple access interference in code division multiple access (CDMA) systems, we need to design a set of spreading sequences with good autocorrelation functions (ACF) and crosscorrelation functions (CCF). The importance of the spreading codes to CDMA systems cannot be overemphasized, for the type of the code used, its length, and its chip rate set bounds on the capability of the system that can be changed only by changing the code. Several new lower bounds which are stronger than the well-known Sarwate bounds, Welch bounds and Levenshtein bounds for binary sequence set with respect to the spreading sequence length, family size, maximum aperiodic autocorrelation sidelobe and maximum aperiodic crosscorrelation value are established.     

Multiple level orthogonal codes and their application in MC-CDMA systems

This paper proposes the use of multiple level orthogonal (MLO) codes in multicarrier CDMA (MC-CDMA) systems. It is extremely challenging to improve system error probability performance of MC-CDMA systems with binary spreading codes in multi-user conditions, since attainable-diversity performance is severely degraged by multi-user interference (MUI) in frequency-selective fading channel conditions, even with the use of optimum multi-user detection (MUD) methods. MLO codes are shown to improve system error probability performance in heavily-loaded or fully-loaded systems, in comparison to binary codes. Some widely used MLO code generation methods are summarized, and a new generation method is also provided. The performance advantage of MLO codes over binary codes is analyzed by treating the spreading process in MC-CDMA as a coding process and via analysis of pair-wise sequence error probability. Rules for choosing desirable MLO codes for multi-user MC-CDMA are also given. Numerical results show that MLO codes can provide a substantial performance improvement in fully-loaded systems. For example, for a K = 4 user system with spreading gain L = 4, our system can obtain a diversity order of 3, whereas the binary code system diversity order is only slightly larger than 1. The MLO code application provides a new way to compensate for multi-user interference (MUI) and makes MC-CDMA more attractive for future high data-rate transmission systems.     

Dynamic hybrid automatic repeat request (DHARQ) for WiMAX – Mobile multihop relay using adaptive power control

Wireless multihop network is currently attracting much attention as a new wireless broadband access technology due to numerous benefits. This work proposes a power control scheme for WiMAX multihop relay system. In contrast to existing power control and optimization approaches, our proposed method uses an adaptive Channel Quality Measurement for a relay station to reduce interferences to other mobile station (MS) or relay station (RS) within the same cell and hence increase the number of hops per link and consequently maximize the spatial reuse. The proposed power control is applied to a new dynamic HARQ algorithm for adaptive channel quality enhancement. Simulation results have indicated that the proposed approach achieves superior BER/PER performance enhancement in comparison to previous related works.     

Multiuser detection algorithm based on belief propagation in multiple-input multiple-output systems

In MIMO (multiple-input, multiple-output) systems, signals from differenttransmitting antennas interfere at each receiving antenna and multiuser detection (MUD)algorithms may be adopted to improve the system performance. This paper proposes anovel multiuser detection algorithm in MIMO systems based on the idea of "beliefpropagation" which has achieved great accomplishment in decoding of low-densityparity-check codes. The proposed algorithm has a low computation complexityproportional to the square of transmitting/receiving antenna number. Simulation resultsshow that under low signal-to-noise ratio (SNR) circumstances, the proposed algorithmoutperforms the traditional linear minimum mean square error (MMSE) detector while itencounters a "floor' of bit error rate under high SNR circumstances. So the proposedalgorithm is applicable to MIMO systems with channel coding and decoding. Although inthis paper the proposed algorithm is derived in MIMO systems, obviously it can be appliedto ordinary code-division m     

基于强跟踪滤波器的盲自适应多用户检测

盲自适应多用户检测是码分多址(CDMA)系统中抑制多址干扰的一项重要技术.为克服传统的基于最小化输出能量准则的盲自适应多用户检测技术收敛速度慢的这一缺点,利用一种新近提出的多用户接收器的典范表示方式,构造了多用户检测系统的状态空间模型,再利用强跟踪滤波器(STF)理论,设计了一种新的盲自适应多用户检测方法.仿真结果表明,在异步CDMA系统中,与基于LMS算法、RLS算法及Kalman算法的盲自适应多用户检测相比, 方法具有收敛速度快速稳定, 跟踪性能好等优点.因此,基于强跟踪滤波器的盲自适应多用户检测是有效的.     

Performance evaluation of cyclic prefix CDMA systems with frequency domain interference cancellation

Direct-sequence code division multiple access (DS/CDMA) systems transmitting over multipath channels suffer from intersymbol interference (ISI) and multiple access interference (MAI). Recently, DS/CDMA with frequency domain equalization (FDE) has attracted much attention for its ability to obtain an excellent performance even in strong frequency selective fading channels. However, the presence of residual interference after the FDE degrades the orthogonality among the spreading codes and hence the BER performance deteriorates. Parallel interference cancellation (PIC) is an effective method to suppress the MAI and improve the CDMA system capacity. In this paper, we propose three interference cancellation architectures for downlink cyclic prefix CDMA systems. The common thread between these three architectures is their implementation in frequency domain. A comparison between such architectures is presented. Simulation results show that the combination of PIC and FDE provides an efficient solution to suppress the MAI in downlink CDMA systems over frequency selective fading channels.     

A modified Elman neural network-based power controller in mobile communications systems

It has been well recognized that power control is an important technique to combat with the harmful near-far effect as well as increase the maximum user capacity of direct sequence code division multiple access (DS/CDMA) cellular systems. In this paper, we propose a modified Elman neural network (MENN)-based power control scheme, which can regulate the received power level at the base station. Unlike the conventional “bang–bang” and fuzzy logic power control, our MENN-based controller first identifies the inverse dynamical characteristics of mobile channel by adaptive on-line learning. The inverse channel model is then employed for power regulation to reduce large overshoots and shorten long rise time. Simulations show that the fluctuation of controlled received power levels can be smoothed with small channel tracking errors. Acknowledgments The authors would like to thank the anonymous reviewers for their insightful comments and constructive suggestions that have improved this paper.     

WCDMA系统中的差分最小均方误差多用户检测算法

宽带码分多址(Wideband code division multiple access,WCDMA)系统中现有的自适应多用户检测算法需要训练比特,不能适应快衰落信道。本文根据WCDMA系统中特有的二级扩频体制的特点,提出基于差分最小均方误差(Differential minimum mean square error,DMMSE)准则的自适应多用户检测算法。该算法根据相邻接收符号幅度变化的比率变化自适应调整横向滤波器的权系数,从而不需要通过训练比特开销跟踪信道状态信息的方式来抑制多址干扰。仿真结果表明,在存在多普勒频移的传播环境下,DMMSE算法的误码性能优于现有的自适应最小均方误差(Minimum mean square error,MMSE)算法。     

Evaluation of channel adaptive access point systemwith Fano decoding

The popularity of wireless networks is increasing due to their mobility, flexibility, and scalability. High error rate over wireless channels leads current transmission control protocol congestion algorithms to be maintained. Finding approaches at lower layers to overcome this is very efficient and powerful. However, Fano decoding is an efficient error correction mechanism, especially in wireless networks, due to its capability of being adaptive to the channel. In this paper, Stop-and-wait with hybrid automatic repeat request using Fano decoding is adopted at the link layer. Consequently, we propose novel analytical and numerical studies for the distribution of packets in the system of wireless access point or base station. Moreover, the capacity of a sufficient access point system is evaluated numerically.     

Reconfigurable method of smart antenna beam shaping in indoor fading environment using adaptive algorithms: An exhaustive experimental study

Adaptive algorithms for beam shaping of a phased array antenna and multiple‐input multiple‐output (MIMO) system gaining importance in today's advanced wireless networks to mitigate interference effects and distortion in the receiving signal due to multipath, small scale, and large scale fading effects. This article deals with the development of reconfigurable field programmable gate array (FPGA)‐based hardware for smart antenna system to explore parameter dependencies, drawbacks, and relative performance comparison of popular adaptive beamforming and interference suppression algorithms. These are least mean square, recursive least squares (RLS), and sample matrix inversion (SMI) used in real‐time under laboratory environment where the existing wireless channel between transmitters and receivers is linear time‐varying in nature due to presence of secondary sources giving rise to small‐scale fading. For this at first, we propose a novel received signal strength indicator‐based procedure to measure the radiation pattern of the antenna under an echoic indoor environment on a reconfigurable and portable FPGA system named wireless open‐access research platform (WARP), controllable by generic programming codes over a user‐friendly MATLAB interface. For better performance, the SMI algorithm was modified to increase block size rather than block shifting in general SMI. Later a comparative study was performed under varying conditions to observe the best utilization of three adaptive algorithms in beam shaping. In all cases, SMI performs the best with less beam shaping error and faster convergence, validating its use in a real‐time fading environment.     

A self-learning call admission control scheme for CDMA cellular networks

In the present paper, a call admission control scheme that can learn from the network environment and user behavior is developed for code division multiple access (CDMA) cellular networks that handle both voice and data services. The idea is built upon a novel learning control architecture with only a single module instead of two or three modules in adaptive critic designs (ACDs). The use of adaptive critic approach for call admission control in wireless cellular networks is new. The call admission controller can perform learning in real-time as well as in offline environments and the controller improves its performance as it gains more experience. Another important contribution in the present work is the choice of utility function for the present self-learning control approach which makes the present learning process much more efficient than existing learning control methods. The performance of our algorithm will be shown through computer simulation and compared with existing algorithms.     

A CDMA/TDD approach for wireless mesh networks

Wide band mesh or star oriented networks have recently become a subject of greater interest. Providing wideband multimedia access for a variety of applications has led to the inception of mesh networks. Classic access techniques such as FDMA and TDMA have been the norm for such networks. CDMA maximum transmitter power is much less than TDMA and FDMA counter parts, which is an important asset for mobile operation. In this paper we introduce a code division multiple access/time division duplex technique CDMA/TDD for such networks. The CDMA approach is an almost play and plug technology for wireless access, making it amenable for implementation by the mesh network service station, SS. Further it inherently allows mesh network service stations to use a combination of turbo coding and dynamic parallel orthogonal transmission to improve network efficiency. We outline briefly the new transmitter and receiver structures then evaluate the efficiency, delay and delay jitter. By analysis we show the advantages over classic counter parts with respect to the total network efficiency achievable especially for larger number of hops.     

Performance of multi-processing gain QS-CDMA over AWGN and Rayleigh fading channels

In this paper, we analyze the bit error rate performance of multi-processing gain quasi-synchronous (QS) code division multiple access (CDMA). Analysis is carried out for deterministic spreading sequences over additive white Gaussian noise (AWGN) and frequency selective slowly Rayleigh fading channels. We also find the bit error rate performance of multi-processing gain QS-CDMA for random spreading sequences and compare to that of deterministic spreading sequences. Results show that deterministic Gold codes outperform random codes considerably and perform differently for different rates while random codes give almost the same performance for all rates.