一种频选衰落信道下的Turbo多用户检测算法

联合MAP多用户检测与信道解码的迭代多用户检测(MUD)技术可显著提高宽带移动CDMA系统的容量和性能.在多径时变衰落的编码信道下,提出一种迭代实现干扰抑制、符号估计、信道解码的Turbo多用户检测算法.在每次迭代中,MUD自适应地实现干扰抑制并输出符号估计的软信息,软输入软输出的信道解码器使用LOG MAP方法实现信道解码并反馈符号估计的软信息作为下一次TurboMUD迭代的先验信息.仿真结果证实了该算法在频选衰落信道下经两次迭代就能逼近单用户编码CDMA系统的接收性能.     

数字移动通信中的抗多径衰落技术

移动信道的主要特征是由移动和多径传播所产生的多径衰落现象。在高速移动通信系统中，必须采用抗多径衰落技术，才能实现有效传输。本文首先概述了移动信道的数学模型及其典型形式，在此基础上，分别讨论了时分多址（ＴＤＭＡ）和码分多址（ＣＤＭＡ）系统的抗多径衰落方法──自适应信道均衡技术及多径分集接收技术，并简述了近年来较为活跃的多用户检测技术和自适应天线分集技术。     

A slotted multicode CDMA protocol for efficient integration of voice and data in cellular networks

This paper presents a novel radio channel structure based on slotted CDMA technology intended for carrying traffic with diverse bandwidth/QoS requirements in mobile environments, e.g. personal communication systems (PCS). The essence of our approach is a combination of flexible slotting with allocation of multiple codes to high‐bandwidth mobiles. As demonstrated by our performance studies, the proposed scheme efficiently integrates multiple traffic classes into a unified CDMA system. It is highly flexible and incurs low overheads for a wide range of realistic traffic conditions. Copyright © 2002 John Wiley & Sons, Ltd.     

Iterative near–far resistant channel estimation by using a linear minimum mean squared error detector

Channel estimation techniques for CDMA system need to combat multiple access interference (MAI) to improve the estimation performance. The linear MMSE detector has certain advantages with respect to the near–far problem and can be used to develop a channel estimation algorithm. In this paper, an efficient iterative method for near–far resistant single-user mobile radio channel estimation in slow fading multi-path direct sequence code division multiple access (DS-CDMA) channels is presented. Computer simulation results demonstrate that a significant performance improvement can be achieved with the proposed method especially under extreme near–far conditions.     

Performance analysis of a wireless multirate direct-sequence CDMAusing fast Walsh transform and decorrelating detection

We analyze the performance of a flexible multirate scheme for direct-sequence code division multiple-access (CDMA) mobile radio systems. The proposed scheme uses a variable processing gain serial pseudonoise modulation as a multirate strategy. To reduce the interference effects, the CDMA system utilizes the coherent fast Walsh transform transmission technique. The proposed scheme can be used in the reverse link (mobile-to-base station) of the upcoming third-generation wide-band CDMA system (has the feature of coherent reverse link). We analyze the system performance with and without using a decorrelating multiuser detector. The uncoded bit-error probability (BEP) with and without decorrelating detection on a multipath fading channel is derived analytically. In addition, the validity of the analysis results is demonstrated by computer simulations using the IMT-2000 vehicular multipath channel model. In order to make sure that the proposed processing techniques do not distort the soft values at the demodulator output, the proposed multirate scheme is also simulated in case of using turbo codes. The turbo-coded BEP is calculated for different user data rates and different number of decoding iterations     

Parallel Interference Cancellation in Multiuser CDMA Channel Estimation

Parallel interference cancellation (PIC) based channel parameter estimators for frequency selective fading channels are proposed for the uplink in code-division multiple-access (CDMA) mobile communication systems. The performance of PIC based algorithms depends heavily on the quality of the multiple-access interference estimates, which can be improved by using adaptive channel estimation filters. The performance of two adaptive complex channel coefficient estimation filters has been verified in a fading channel by computer simulations. According to the results, the PIC based adaptive channel estimators outperform clearly conventional, successive interference cancellation, and decorrelation based adaptive channel estimators. The PIC method is also used in delay tracking. By using the principles of sample-correlate-choose-largest (SCCL) delay trackers, a robust algorithm for multiuser delay tracking in fading channels is obtained.     

MISO-CDMA多径系统中空时分组码的译码方法

空时编码是实现宽带无线数据通信和下一代移动通信系统的一种极有潜力的技术。为有效的将空时分组码应用到多径衰落环境下的码分多址系统，以充分利用多个路径的信号能量，现提出了一种多径环境下空时分组译码的新方法。由于空时分组码译码与信道估计紧密相关，为此本文对多径信道估计以及信道估计误差对本方法产生的影响作研究。仿真结果表明，采用多路径译码方法可以明显提高系统的误码性能。     

阴影衰落信道下多波束卫星移动通信系统的动态信道分配策略

针对已有动态信道分配策略在阴影衰落环境中性能损失较大的问题,对卫星移动通信系统的阴影衰落信道模型进行分析,提出了一种动态信道分配策略。该策略结合用户的运动状态,通过在用户的预测运动轨迹上选取抽样点,将这些抽样点的平均干扰作为信道分配的指标,并采用链路质量约束避免了流量较大时的性能恶化。仿真结果表明提出的动态信道策略可在不增大阻塞率的前提下,在中低流量时将用户平均信噪比提高约0.5 dB。该策略可以应用于阴影衰落信道下的地面移动卫星通信系统,以提高用户的平均链路性能。     

MC‐CDMA MIMO systems with quasi‐orthogonal space–time block codes: Channel estimation and multiuser detection

This paper investigates blind channel estimation and multiuser detection for quasi‐synchronous multi‐carrier code‐division multiple‐access (MC‐CDMA) multiple‐input multiple‐output (MIMO) systems with quasi‐orthogonal space–time block codes (QO‐STBC). Subspace‐based blind channel estimation is proposed by considering a QO‐STBC scheme that involves four transmit antennas and multiple receive antennas. Based on the first‐order perturbation theory, the mean square error of the channel estimation is derived. With the estimated channel coefficients, we employ minimum output energy and eigenspace receivers for symbol detection. Using the QO‐STBC coding property, the weight analyses are performed to reduce the computational complexity of the system. In addition, the forward–backward averaging technique is presented to enhance the performance of multiuser detection. Numerical simulations are given to demonstrate the superiority of the proposed channel estimation methods and symbol detection techniques. Copyright © 2011 John Wiley & Sons, Ltd.     

多载波CDMA系统中的频域线性相位内插算法

多载波CDMA系统结合了正交频分复用（OFDM）技术和CDMA技术的优点,自1993年提出以来就受到广泛的重视。本文主要讨论了载波频偏对连续OFDM符号的影响,提出了一种新的频域线性相位内插算法来估计信道响应,以对抗载波频偏对连续OFDM符号引起的相位旋转。理论分析和仿真试验都证明我们提出的算法能有效减少载波频偏对系统性能的影响。这种新算法适用于使用导频信号块状分布的多载波CDMA系统或OFDM系统。     

Application of Rate Compatible Punctured Turbo Coded Hybrid ARQ to MC‐CDMA Mobile Radio

MC‐CDMA, a multicarrier (MC) modulation scheme based on code division multiple access (CDMA), is the most likely candidate for the next generation of mobile radio communications. The rate compatible punctured turbo (RCPT) coded hybrid automatic repeat request (HARQ) has been found to give improved throughput performance in a direct sequence (DS) CDMA system. However, the extent to which the RCPT HARQ improves the throughput performance of an MC‐CDMA system has not been fully understood. In this paper, we apply the RCPT HARQ to MC‐CDMA and evaluate by computer simulations its performance in a frequency selective Rayleigh fading channel. We found that the performance of RCPT HARQ MC‐CDMA is almost insensitive to channel characteristics. The performance can be drastically improved with receive diversity combined with space‐time transmit diversity. In addition, the comparison of RCPT HARQ MC‐CDMA, orthogonal frequency division multiplexing, and DS‐CDMA shows that under similar conditions the throughput of MC‐CDMA is the best in a frequency selective fading channel.     

移动通信中的信道估计技术

信道估计是移动通信系统需要解决的最核心的问题之一,信道估计的精度直接影响到移动通信系统的性能和容量。介绍了目前常用的几种基于导频序列的信道估计技术,分析了不同估计方式的优缺点;同时介绍了几种在时分复用导频辅助估计中能够减小多普勒频移对信道估计影响的插值技术,并对他们进行了比较分析。     

Uplink channel assignment with balanced load for code division multiple access cellular systems

In a code division multiple access (CDMA) wireless communication system, each mobile handset must be power controlled such that the power received at the base station is roughly the same. Otherwise, the interferences between mobile handsets will degrade the performance and increase the error rate. When a mobile handset uses channels from the neighbouring cells, it will raise its power to meet the threshold of signal strength. This will also increase the interference in the home cell. Therefore, we do not want a mobile handset to use channels from other cells blindly. In this paper, we propose an uplink channel assignment method based on the directed retry concept for CDMA cellular systems. The purpose is to achieve load balancing between neighbouring cells and at the same time controlling the interference levels at the base stations such that it will not affect the performance. Furthermore, priorities are given to handoff calls when assigning channels. Copyright © 2002 John Wiley & Sons, Ltd.     

Pseudonoise code tracking loop for a CDMA system with imperfect power control

In this paper, performance of a pseudonoise (PN) code tracking loop is analysed and simulated for a direct‐sequence/code‐division‐multiple‐access (DS/CDMA) system with imperfect power control in a multipath fading channel. A non‐coherent first‐order delay‐locked loop (DLL) is considered as a PN code tracking loop. Power control error is modelled as a log‐normally distributed random variable. The multipath fading channel is modelled as a two‐ray Rayleigh fading model that is typically applied to land mobile communication environments. The tracking performance of DLL is evaluated in terms of tracking jitter and mean‐time‐to‐lose‐lock (MTLL). From the simulation results, it is shown that for smaller discriminator offset, tracking jitter performance is improved while MTLL performance is degraded. It is shown that large power control error and heavy shadowing substantially degrade the PN tracking performance. Therefore, an accurate power control scheme is very essential to compensate for fading and shadowing for a DS/CDMA system. The extension of analysis to higher‐order loop models is very straightforward. The analysis in this paper can be applied to design the PN code tracking loop for a DS/CDMA system. Copyright © 2001 John Wiley & Sons, Ltd.     

非周期扩频的同步CDMA系统下行信道的分数阶间隔信道估计

使用正交扩频码的同步CDMA系统具有对抗同信道干扰的能力,但是多径传播导致信号失去正交性,互相关值增加。在这些情况下,需要对信道进行估计以提高接收机检测性能。以往的基于子空间的CDMA信道估计算法都是在chip速率采样的基础上得到的,其条件是接收信号满足Nyquist定理且信道是最小相位系统,这些条件在实际情况中是很难满足的。该文提出一种应用于非周期扩频的同步CDMA系统的下行信道的分数阶间隔信道估计的算法。计算机仿真表明了算法是有效的。     

2-D rake receiver based on space-time channel parameter estimation

2-D RAKE receiver is an efficient way to realize the space-time processing for CDMA systems with aperiodic spreading codes. The Direction Of Arrival (DOA) and the relative time delay of every user's multipath must be known to realize the 2-D RAKE receiver. In the third generation CDMA mobile communication system, auxiliary pilot channel is used in the uplink channels. The different user's Vector Channel Impulse Response (VCIR) can be estimated from the pilot channel easily. The VCIR contains spatial and temporal information. In this paper, by utilizing the known pulse shape function, a parameter matrix method used to estimate the Spatial Signature Vector (SSV) and the relative time delay is proposed in frequency domain. The DOA can be estimated from the SSV. By reconstructing the SSV and utilizing approximate Capon space filter, the performance of the 2-D RAKE receiver with uniform circular array can be improved with a little additional computation work.     

多波束移动卫星系统的自适应开环预编码方法

对于卫星移动通信系统,由于卫星与地面终端之间的相对运动以及星地间传输延迟,传统的基于理想信道信息的预编码方法是不适用的。针对这一问题,提出了一种基于开环信道估计的预编码方法。卫星端利用开环获取的部分信道信息实现多波束联合预编码,并导出了系统传输速率的闭合解析表达式。此外,为了克服强干扰环境下多波束预编码系统性能恶化问题,提出了一种自适应预编码传输方法。卫星发射机依据开环获得的慢时变用户位置信道信息和信道统计量信息,自适应地选择预编码方法或传统频率复用方法,实现最优的系统性能。理论分析和仿真结果表明,与传统的干扰抑制方法相比,所提方法能实现更优的系统性能,同时也克服了传统预编码方法的局限性。     

Effects of low noise amplifier non‐linearities on blind adaptive beamforming performance in CDMA wireless systems

We report a new approach to analyse the effects of low noise amplifier (LNA) non‐linear distortions in the code division multiple access (CDMA) wireless communication systems using spatio‐temporal analysis and Volterra series theory. For this purpose, the bit error rate (BER) performance of three blind algorithms is studied based on post correlated model of received signal, and a time‐varying multiple vector channel model which is an extended form of the Gaussian wide sense stationary uncorrelated scattering (GWSSUS) channel. By using the Volterra series theory, an analytical expression for amplitude modulation to phase modulation (AM–PM) conversion is determined as a phase statement of LNA compression. In this approach, by combining the analytical expression for AM–PM conversion and CDMA blind beamforming techniques, we evaluate the AM–PM distortion effects on BER performance of a CDMA system originated from multiple non‐linear LNA blocks. Simulation results show that conditions are found to minimize AM–PM conversion introduced by multiple non‐linear blocks in the system leading to low BER. Copyright © 2006 John Wiley & Sons, Ltd.     

Tree‐based channel assignment schemes for multi‐channel wireless sensor networks

Many sensor node platforms used for establishing wireless sensor networks (WSNs) can support multiple radio channels for wireless communication. Therefore, rather than using a single radio channel for whole network, multiple channels can be utilized in a sensor network simultaneously to decrease overall network interference, which may help increase the aggregate network throughput and decrease packet collisions and delays. This method, however, requires appropriate schemes to be used for assigning channels to nodes for multi‐channel communication in the network. Because data generated by sensor nodes are usually delivered to the sink node using routing trees, a tree‐based channel assignment scheme is a natural approach for assigning channels in a WSN. We present two fast tree‐based channel assignment schemes (called bottom up channel assignment and neighbor count‐based channel assignment) for multi‐channel WSNs. We also propose a new interference metric that is used by our algorithms in making decisions. We validated and evaluated our proposed schemes via extensive simulation experiments. Our simulation results show that our algorithms can decrease interference in a network, thereby increasing performance, and that our algorithms are good alternatives for static channel assignment in WSNs. Copyright © 2015 John Wiley & Sons, Ltd.     

Bit error rate performance of Haar wavelet based scale‐code division multiple access (HW/S‐CDMA) over the asynchronous AWGN channel

In this paper, we study a recently proposed multirate system, called wavelet based scale‐code division multiple access (W/S‐CDMA). W/S‐CDMA depends on the code, time and scale orthogonality introduced by pseudo‐noise (PN) sequences, and wavelets. In this system, the channel is partitioned into different scales, and each scale into time slots. In addition, the PN sequences are used in each scale to identify multiple users. In W/S‐CDMA, each user is assigned a specific scale and PN sequence, and transmits its successive information symbols with its PN sequence and the wavelets in that scale. More symbols are transmitted in finer scales. We analyse the bit error rate performance of Haar wavelet based S‐CDMA (HW/S‐CDMA) over an asynchronous additive white Gaussian noise (AWGN) channel by using a conventional detector for deterministic PN sequences. The performance of the system is compared to that of an equivalent multirate CDMA (MR‐CDMA) system for Gold and Kasami PN sequences. Results show that HW/S‐CDMA outperforms MR‐CDMA. In addition, because of its suitable format HW/S‐CDMA is also capable of employing the optimal PN sequence families with limited number of sequences such as Kasami, Bent, etc. repeatedly in different scales. Copyright © 2006 John Wiley & Sons, Ltd.