Crest factors of complementary-sequence-based multicode MC-CDMA signals

The crest factor properties of binary phase-shift keying modulated two- and four-code assisted multicarrier code-division multiple-access (MC-CDMA) employing complementary-sequence-based spreading sequences are characterized. More specifically, a complementary-sequence pair, a complementary-sequence-based subcomplementary code pair, and a Sivaswamy complementary code set are studied. It was found that the corresponding crest factors are bounded by 3 dB, which corresponds to the crest factor of a single sine wave. This low crest factor resulted in a lower power loss and a lower out-of-band power spectrum due to clipping when the time-domain signal was subjected to a typical nonlinear power amplifier, in comparison to those of Walsh code and orthogonal Gold code-spread MC-CDMA signals.     

Hopfield multiuser detection of asynchronous MC-CDMA signals inmultipath fading channels

Multiuser reception using Hopfield neural network (HNN) for asynchronous multicarrier code-division multiple access (MC-CDMA) in a multipath fading channel is investigated. We have shown that by the appropriate choice of HNN parameters from the channel state information, the HNN can collectively resolve the multipath fading effects and the multiple-access interference in the system     

Bandwidth utilization efficiency enhancement for OFDM‐based WSN

Normally IEEE 802.16 (WiMAX) is used for mainly downlink traffic applications. However in the upper tier of 2‐tier (WiMAX‐WiFi) wireless sensor network, the uplink bandwidth faces bottlenecks for high throughput. In this paper, a solution has been proposed for this limitation of uplink bandwidth allocation through the use of queuing theoretic performance modeling. A Markov‐modulated Poisson process traffic model has been formed for orthogonal frequency division multiple access‐based transmission along with discrete time Markov chain system model for queuing. A downlink traffic pattern has been defined for wireless sensor network nodes. Analytical methods are used to estimate the performance parameters like throughput, delay, and probability of packet drop for resource allocation. An algorithm is formulated to find out minimum resource requirement for downlink and to transfer rest of the resources to uplink bandwidth allocation, for throughput enhancement. Uplink frame utilization is determined through another discrete time Markov chain model for adaptive triggering between the proposed maximum and the normal downlink to uplink ratio operations, for efficient distribution of bandwidth resources. Algorithm and simulation results prove outstanding improvement in the uplink throughput around 50%, without degrading the downlink throughput.     

Bandwidth, SAR, and efficiency of internal mobile phone antennas

This paper presents a thorough investigation into the effects of several phone chassis-related parameters-length, width, thickness, and distance between the head and phone-on the bandwidth, efficiency, and specific absorption rate (SAR) characteristics of internal mobile phone antennas. The studied antenna-chassis combinations are located beside an anatomical head model in a position of actual handset use. The effect of the user's hand is also studied with two different hand models. The main part of the study is based on FDTD simulations, but also experimental results, which support the computationally obtained conclusions, are given. The presented analysis provides novel and useful information for future design of mobile handset antennas. The results show the general trends of bandwidth, SAR, and efficiency with different chassis parameters. The results also reveal a connection between these three performance parameters: an increase in SARs and a decrease in radiation efficiency occur compared to the general trend when the bandwidth reaches its maximum. This happens when the resonant frequency of the chassis equals that of the antenna.     

多载波码分多址系统的多用户检测

正交多载波码分多址（MC—CDMA）是近年来无线通信研究的热点。本文介绍了MC—CDMA系统的多用户检测技术。分析了与直接序列（DS—CDMA）系统的对偶性关系，并通过将两种系统的多用户检测算法分类比较，指出将DS—CDMA的多用户检测算法引入到MC—CDMA中的可行性，讨论了MC—CDMA多用户检测的发展方向。     

Bandwidth extension of speech signals: a catalyst for the introduction of wideband speech coding?

The restricted audio quality of today's telephone networks is mainly due to the narrowband (NB) limitation to the frequency range from about 300 Hz to 3.4 kHz. Meanwhile, codecs for wideband (WB) telephony (50 Hz to 7 kHz) exist with significantly improved speech intelligibility and naturalness. However, the broad introduction of wideband speech coding requires strong efforts of both network operators and their customers because many elements of the networks (i.e., terminals and network nodes) have to be modified. An intermediate step to overcome the narrowband limitation can be achieved by applying artificial bandwidth extension (BWE) in the receiver. In this article we review the basic principles of bandwidth extension, and discuss several application scenarios in which both wideband coding and BWE complement each other. The introduction of BWE methods in terminals and networks may help to speed up the introduction of true wideband speech coding in the near future.     

一种基于MC—CDMA的测距算法

多裁波CDMA技术在无线衰落信道中传输高速数据时具有突出的优势.同时伴随着无线电网络的发展,各种基于测距定位的业务也越来越多的涌现出来.给出一种快速测距方法,利用MC-CDMA信号扩频码的频域相关性,进行精确测距,具有一定的创新性.仿真表明,该算法的抗噪声性能好,在非常低的信噪比下,具有良好的测距精度,且能通过过采样进一步提高精度,具有很好的应用前景.     

Adaptive carrier interferometry MC-CDMA

An adaptive carrier interferometry (CI) scheme is proposed for multicarrier code-division multiple-access (MC-CDMA) systems where it is assumed that there is a feedback channel between the receiver and the transmitter. By exploiting the additional degree of freedom in selecting the amplitudes of the subcarriers in accordance with the channel condition, the proposed scheme attains a significant performance gain over the conventional CI-MC-CDMA systems in which a constant amplitude is set for all the carriers. Two adaptation strategies, namely 1) local adaptation and 2) global adaptation, are considered for estimating the appropriate subcarrier amplitudes at the receiver in the proposed systems. Both single-user adaptation, where the other users do not adapt, and multiuser adaptation, in which all users adapt, are investigated. A further advantage of the proposed scheme is that it eliminates the peak-to-average power ratio problem present in the conventional CI-MC-CDMA systems.     

MC-CDMA在无线局域网中的应用

OFDM技术的应用极大促进了WLAN的发展.本文介绍了MC-CDMA技术的基本原理和优点,提出把MC-CDMA技术用于WLAN的可行性,并通过仿真结果的比较说明了MC-CDMA的应用将一步提高WLAN的性能.     

Multi-Rate Access Schemes for MC-CDMA

In order to accommodate different types of traffic in future wirelesscommunications, it is necessary to consider a system, which can operatesatisfactorily at multiple transmission rates. Due to its capability to copewith the hostile frequency selective fading, that limits transmission rate,and its suitability to handle multi-rate data, multi-carrier CDMA (MC-CDMA)has recently drawn considerable attention as a suitable candidate forsupporting multimedia services in wireless communications. Multi-rate accessschemes where users are able to transmit at different data rates on MC-CDMAsystems are presented in this paper. Four multi-rate access schemes: uncodedfixed spreading length (UFSL), coded fixed spreading length (CFSL), multi-codefixed spreading length (MFSL) and variable spreading length (VSL) schemes areproposed. With these schemes, different traffic such as voice, video and highrate data can be transmitted seamlessly through one MC-CDMA infrastructure.A chip-level minimum mean square error combining (MMSEC) technique is employedfor joint energy combining and interference cancellation purpose. Theperformance of these schemes with MMSEC is compared by both theoreticalderivations and simulation results under frequency selective Rayleigh fadingchannels.     

一种改进的MC-CDMA系统结构

MC-CDMA在频域扩频时实际上是一种频率分集,对于频率分集重数的要求通常小于子载波的数目N,故扩频因子可以小于子载波数目。改进了这种MC-CDMA系统的结构,通过在频域引入交织器,使交织后频率分集各分量经历独立的哀落,从而提高了抗哀落能力。最后给出了用系统仿真软件SPW仿真的结果。     

Performance Evaluation and ParameterOptimization of MC-CDMA

The performance of multicarrier systems depends on the propagation channel behavior. The latter is subject to time and/or frequency selectivity. The designer has to select properly the guard interval and the number of carriers for a given system bandwidth to combat the channel dispersiveness. In this paper, we investigate the sensitivity of downlink multicarrier code-division multiple-access (MC-CDMA) performance to these parameters in different environments. We derive closed-form expressions of the useful and the different interference powers after maximum ratio combining and despreading. The optimum parameters correspond to the minimum of the signal-to-noise-ratio degradation. It turns out that the derivations in the paper of Steendam and Moeneclaey ( IEEE Trans. Commun., 1999) that are restricted to the orthogonal frequency-division multiplexing (OFDM) system are a particular case of our results. Numerical evaluation of the analytical expressions reveals that the optimum parameters of the MC-CDMA and its corresponding OFDM system are similar and depend in the same way on the channel characteristics. We show in this paper that the load of the MC-CDMA system only has small influence on the optimization of the parameters. Therefore, from the point of view of parameter optimization, single-user transmission is sufficient to find the optimum parameters. Furthermore, as the optimum parameters mainly depend on the channel characteristics, similar conclusions can be drawn for the uplink transmission     

Enhancing the efficiency of coherent processing of chaotic signals during the transmission of binary messages using surrogate signals

A method has been proposed for reducing the influence of noise in observations of chaotic carrier during the binary message transmission using the technology of surrogate data generated with due regard for maintaining the signal attractor in phase space. The impact of the correlation of observation noise and its difference from the Gaussian noise on the probability of correct separation of one message bit is analyzed.     

Parallel interference cancellation efficiency for noisy signals in O-CDMA system

The aim of this paper is to evaluate the robustness of Parallel Interference Cancellation (Pic) to noise contribution for an optical Code Division Multiple Access system. The theoretical expression of thePic error probability is developed in the case of white additive Gaussian noise. From theoretical analysis, we show that, even with noise contribution, thePic receiver outperforms the Conventional Correlation Receiver (Ccr). Moreover, the results highlight that, for a given performance, thePic receiver relaxes not only the constraint on the code length, but also the Signal to Noise Ratio compared toCcr. Particularly, this is proofed in access network context, i.e. 30 users withBer lt; 10^?9.     

MC-CDMA系统中的帧同步

简要介绍了多载波码分多址(MC-CDMA)系统,重点讨论MC-CDMA的帧同步问题.在介绍循环前缀(CP)同步算法的基础上,对基于CP的最大似然估计(ML)的似然函数进行了改进,改进方案大大降低了系统的复杂度,仿真表明性能优异,具有较高的实用价值.     

改进的多载波CDMA系统性能分析

多载波CDMA技术能够提供大的系统容量、对抗瑞利衰落和消除符号间干扰等优点,成为具有发展前景的调制技术之一。本文基于各用户服务指标(QoS)的不同,提出了一种自适应子载波分配的改进的多载波CDMA系统。通过在衰落信道下的仿真,结果表明:采用本文提出的自适应子载波分配算法的改进MC—CDMA系统较之传统的MC-CDMA系统的误码率性能有了较大程度的改善。     

Increase of efficiency of spread spectrum radio signals application in communication systems

Communication systems that provide high jamming protection of the radio exchange based on the spread spectrum signals are the object of research. Finding the optimal mode and parameters of the radiosignal shaping is the research subject. The estimation of the influence of pseudorandom sequence properties, modulation type and structure of the time-frequency matrix on the power and structural security of the spread spectrum radio signals was carried out.     

Transmission Performance of MC-CDMA Hybrid Fiber-Radio Systems

This paper reports an analysis of the transmission performance of a hybrid broadband access system consisting of a fiber optic feeder and a millimeter-wave radio channel in the last mile. It is also assumed that the system adopts the multi-carrier code division multiple access (MC-CDMA) technique, that allows effective radio resource utilization and does not require a strong equalization. The analysis reveals that the impact of non-linear effects, mostly due to the non-linear optical source and secondarily due to non-ideal RF power amplifier characteristics, could prevent the good performance of the system as a whole, and indicates which are the good operating conditions for the systems.     

MC-CDMA上行链路盲多用户检测技术研究

文中提出一种在异步多径瑞利衰落MC-CDMA系统上行链路中改进型的仿射投影算法。利用盲信号空间分离的方法,将期望用户的信道信息估计出来,再将其结果作为参数带入到改进型仿射投影滤波器中。在权向量更新的过程中,抵抗多址干扰（MAI）的影响,将期望用户的信息可靠的恢复。计算机仿真显示,在考虑远近效应时,该盲算法的性能与非盲的最小均方误差接收器的性能接近。