基于扩频与跳频的多带OFDM-UWB多址技术

根据多带OFDM-UWB通信系统的特点,提出一种基于扩频与跳频的多址技术。该多址技术将多载波码分多址和跳频多址相结合,构成一种新颖的多址技术-正交频分多址(OFDMA),实现多载波码分多址技术和跳频多址技术的优势互补,此接收机的复杂性明显小于常规时频码的多用户接收机。仿真结果表明,基于OFDMA多址技术方案的UWB系统在系统误比特率性能上优于常规时频码方案约5dB。     

多载波CDMA技术简介

CDMA技术能提供高容量、软切换,可望用于多媒体业务,但其性能受到多址干扰和符号间干扰的影响;正交频分复用对频率选择性衰落不敏感,可以用于无线环境中高速数据的传送,综合两者的优点,所形成的多载波CDMA具有很强的竞争力,可望用于第三代移动通信系统。本文介绍了三种多载波CDMA技术,即MC-CDMA、多载波DS-CDMA和多音CDMA,并简单介绍了它们的发射和接收技术。     

基于OFDM-MSFH系统的全部载波接入方法的研究

本文根据MB-OFDM-UWB系统的特点,提出了将OFDM技术和MSFH(多级跳频)技术结合的全部载波多址接入方案.该多址方案有二级构成,其中前一级使用OFDM,设计时参考了Multi-band OFDM Physical Proposal for IEEE 802.15 TaskGroup3a中的物理层建议,后一级采用跳频模块,并将基于余数域的多级跳频图案应用到该多址方案中,不仅大大减小了多用户碰撞的概率,而且有利于超宽带信号的生成.理论分析和仿真结果表明,基于OFDM和多级跳频结构的全部载波多址接入方案在系统容量、误比特率性能、抗干扰等方面具有诸多的优势.在8用户情况下,当系统误比特率为10~(-2)时,基于该结构的全载波方案比TH-PPM多址方案改善信噪比5dB左右.     

点对多点微波通信系统跳频同步的设计实现

跳频通信具有很强的抗干扰、抗衰落、抗截获能力,兼有能多址组网应用等诸多优点。跳频同步是跳频通信的技术关键,也是跳频通信系统开发的难点,只有实现了快速精确的同步,才能正确提取传递信息,发挥出跳频通信的优点。分析了点对多点微波传输及时分多址通信跳频同步的特点,结合工程中的时分多址跳频抗干扰通信技术,设计了一种基于TOD的同步方案,并利用FPGA技术实现,给出了应用过程中的跳频图案及其同步过程的详细设计。     

多载波CDMA的三种方案及其特性比较

多载波扩频码分多址系统是一种消除符号间干扰的有效技术。它结合了OFDM和CDMA的优点,在高速数据传输时,其抗符号间干扰的性能明显优于传统的单载波CDMA系统。本文首先分析了三种多载波CDMA方案的基本原理,然后对它们的系统特性进行了比较。     

基于小波分组调制的多码多载波跳频通信系统分析

介绍了一种将多码调制、小波分组调制以及跳频技术相结合的高效通信方式——小波分组多码多载波跳频通信系统.推导了系统在Nakagami-m衰落信道下的误码性能,给出了用户数、小波分组类型、小波分组滤波器长度以及跳频点数与系统性能之间的关系.理论分析与数值计算表明:增加小波分组滤波器长度可以有效降低系统误码率,但不同小波分组类型对系统性能的影响不大;跳频技术的引入不仅增加了系统的安全性,同时也可有效降低多址干扰对系统性能的影响,提高了系统的可靠性.     

CDMA多用户传输系统物理层仿真

为了研究CDMA系统抗多址干扰和多径干扰能力的目的,采用同一信道下比较调制对系统性能的影响,以及相同的调制方法下比较不同信道系统的性能的方法,做了对CDMA多用户传输系统的仿真实验,获得CDMA通信系统抗多址干扰和抗多径干扰能力与系统扩频码正交性能之间的关系,得到CDMA系统优化性能的结论,并通过仿真实例验证各种传输模型的特点。     

基于FPGA和Si4463的跳频语音通信系统设计与实现

跳频通信具有抗干扰、抗截获以及多址组网等优点,在民用通信系统中的应用越来越广泛.基于FPGA和Si4463,设计并实现了跳频语音通信系统.首先对跳频通信系统抗干扰性能进行了建模仿真和分析,得到抗全频带干扰和抗跟踪式干扰性能.提出一种简化的基于TOD(Time of day)的跳频同步方案,使用本地计数器代替精确时间产生模块(例如GPS模块),降低系统实施复杂性,节约硬件资源和成本.最后设计并实现了基于FPGA和通用射频收发芯片Si4463的跳频语音通信系统,为复杂环境下民用语音通信提供了可行的方案.     

OFDM跳频通信系统设计

跳频通信具有较强的抗干扰、抗衰落、抗截获能力,多址组网能力也较强,在民用和军事通信中都得到了广泛地应用。OFDM调制方式具有较高的频谱效率和较强的抗衰落能力,应用于跳频通信可提高其性能。本文给出了MASK-OFDM跳频系统框图和设计参数,并通过仿真分析了系统性能。     

Rayleigh衰落信道下相位误差对相干跳频多址接入MRC分集接收系统的影响

该文研究一种新型混合通信系统,即基于多载波BPSK调制的相干跳频多址接入(FH/CDMA)系统,接收端采用最大比合并(MRC)分集接收。由于采用相位调制,接收端载波相位恢复的精确度直接影响系统性能。假设载波同步通过一阶锁相环电路(PLL)完成,由于载波恢复环路中存在高斯噪声,当系统达到同步稳态时不可避免地会产生稳态相位误差。该文采用Beaulieu级数方法和特征函数方法,得到理想相干和部分相干(存在稳态相位误差)时FH/CDMA系统误码率性能表达式。仿真结果表明,相对于单用户系统,多用户系统误码率受到相位误差影响较小,误码率性能降低的主要因素是多用户干扰。MRC分集接收能提高系统抵抗相位误差和信道衰落的能力。     

Interference Cancellation with Space Diversity for Downlink MC-CDMA Systems

Modern wireless communications require an efficient spectrum usage and high channel capacity and throughput. Multiple-input and multiple-output (MIMO), Linear equalizers, multi-user detection and multicarrier code-division multiple access (MC-CDMA) are possible solutions to achieve spectral efficiency, high channel capacity, eliminate multiple access interference (MAI), eliminate Inter symbol interference (ISI) and robustness against frequency selective fading. In this paper, we combine all these techniques and investigate BER performance. We propose a low complexity receiver structure for Single-input Multiple-output (SIMO) downlink MC-CDMA systems. It employs an interference cancellation scheme to suppress the interference caused by the multipath fading channel. Also, the proposed scheme is developed for MIMO MC-CDMA system. The performance analysis of Downlink MIMO MC-CDMA systems with V-BLAST over frequency selective fading channel is investigated under various number of transmit and receive antennas. The simulation results show proposed SIMO equalization with parallel interference cancellation scheme is effective in reducing the ISI and the MAI. It improves the performance significantly and the simulation results show that MIMO MC-CDMA with V-BLAST multi-user detection provides high data rate and the BER significant improvement.     

Multicarrier FH codes for multicarrier FH-CDMA wireless systems

A multicarrier (multihopping) scheme for multilevel frequency-shift keying/frequency-hopping code-division multiple access (MFSK/FH-CDMA) systems is proposed. In order to improve the frequency diversity capability for the proposed multicarrier MFSK/FH-CDMA system, several hopping frequencies per time slot are employed and transmitted in parallel. The frequency band is partitioned into N (N/spl les/P, P is a prime number) disjoint subbands on which N hop carrier frequencies are simultaneously transmitted. In previous work on multicarrier systems, it has been assumed that the hopping frequencies of N disjoint subbands are identical. We consider the FH patterns without this assumption. These FH patterns have cross correlation no greater than one in order to minimize mutual interference. In addition, we analyze the performance of the system in order to show the improvement of user capacity. Our results show that the proposed scheme performs better than the conventional one in both nonfading and fading channels.     

Efficient wireless transmission scheme based on the recent DST-MC-CDMA

The capacity of a multicarrier code-division multiple access (MC-CDMA) system is limited by a multiple access interference (MAI) from other users. In this paper, we propose a MAI cancellation scheme to transmit images over a recent discrete sine transform (DST) based MC-CDMA (DST-MC-CDMA) system. In the proposed scheme the minimum mean square error (MMSE) equalizer is used to provide the initial estimate of users’ data and the parallel interference cancellation (PIC) scheme is then used to regenerate and cancel the MAI from the desired user. The proposed scheme is called MMSE-PIC. Simulation results in multi-path fading channel confirm the excellent performance of the proposed scheme as compared to MMSE equalization method. It is also found that the best suitable tentative decision for the proposed scheme is the null zone decision or the clipper decision. We also conduct experiments to show the performance of the proposed scheme with a real image transmission over the DST-MC-CDMA system. Results show that the proposed scheme provides significant image quality improvement as compared to the existing schemes. The average peak signal to noise ratio improvement achieved by the proposed scheme over the conventional MMSE equalizer at a SNR = 30 dB is about 6.5 dB for different number of users.     

Multiple-Access Interference Suppression for MC-CDMA by Frequency-Domain Oversampling

Multiple-Access Interference Suppression for MC-CDMA by Frequency-Domain Oversampling A technique for interference suppression in multicarrier code-division multiple-access (MC-CDMA) systems is proposed which exploits the structural differences in signals that arrive at the receiver with Doppler shifts or carrier offsets. Oversampling the received signal in the frequency domain and properly combining the samples provides the interference suppression. Frequency-domain oversampling is accomplished by using a time extension of the conventional MC-CDMA signal or unconsumed portions of the cyclic prefix. Furthermore, a receiver structure is introduced that despreads and combines groups of samples so that a linear minimum mean-squared error solution for combining the groups is easily found. This combining scheme increases the signal-to-interference ratio experienced by the desired user in the MC-CDMA system. In addition, the receiver performs as well in severe near–far scenarios when there is sufficient frequency separation between the signals of the desired user and an interferer. Numerical results show that the proposed receiver significantly outperforms the conventional MC-CDMA receiver in many channel realizations.     

A Fuzzy MOE Receiver for Uplink MC-CDMA Systems with Carrier Frequency Offset over Multipath Fading Channels

A novel fuzzy minimum output energy (MOE) detector is proposed for uplink multicarrier CDMA (MC-CDMA) systems with carrier frequency offset (CFO) over multipath fading channels. The proposed receiver involves the following stages. First, the fuzzy CFO constrained MOE detector after coarse CFO estimation is proposed to suppress multiple access interference and combat the degradation problem of the conventional MOE detector caused by the CFO effect. Next, using the signal subspace projection technique, the proposed detector can further reduce the enhanced noise due to the fuzzy CFO constrained detector. Finally, the output data obtained from these detectors are coherently combined to offer multipath diversity gain in accordance with the maximum ratio combining criterion. Furthermore, the proposed single input single output (SISO) robust detector can be easily extended for a multiple input multiple output (MIMO) MC-CDMA system with a high rate of performance. Simulation results show that the proposed SISO detector, which offers a similar performance as the optimal detector, can provide robustness against CFO and outperform the conventional detectors. The proposed MIMO detector with spatial multiplexing gain also exhibits excellent performance.     

A joint cross-layer transmission design with time–frequency coded cooperation HARQ for underground coal mine MC-CDMA WSNs

In this paper, multicarrier code division multiple access (MC-CDMA) modulation is adapted to constitute wireless sensors to improve the monitoring performance of wireless sensor networks (WSNs) for underground coal mine. A subcarrier phase compensation algorithm based on selective mapping (SLM) is proposed to reduce the relatively high PAPR of MC-CDMA signal. To further improve the monitoring performance of the underground MC-CDMA WSNs, a joint cross-layer transmission with time–frequency coded cooperation hybrid automatic repeat request (HARQ) is also proposed. The proposed cross-layer transmission combines time–frequency coded cooperation of physical layer with HARQ of media access control (MAC) layer. In the proposed transmission, the cooperative sensor utilizes time–frequency coded cooperation method to retransmit the monitoring information of source sensor at each retransmission time to obtain the coding gain and spatial diversity gain. Simulation results show that the proposed joint cross-layer transmission for underground coal mine MC-CDMA WSNs based on SLM phase compensation has significantly reduced the PAPR of MC-CDMA signal and improved the monitor performance of the coal mine MC-CDMA WSNs.     

An efficient digital implementation of multicarrier CDMA system based on generalized DFT filter Banks

In this paper, an efficient digital implementation of multicarrier transmission scheme based on generalized discrete Fourier transform (DFT) filter banks is presented for multicarrier code-division multiple-access (MC-CDMA) systems. Generalized DFT filter banks has been traditionally discussed for very high-speed digital subscriber lines (VDSL) wireline systems, received interest also for wireless applications. The design of the generalized DFT modulated filter banks is investigated and its fast implementation is derived in the time-domain for arbitrary integer sampling rate. Through the utilization of the generalized DFT modulated filter banks, one or more subcarriers, even noncontiguous subcarriers can be easily supported by system in both uplink and downlink, which facilitates the users to access the network in a frequency-division multiple-access manner. Simulation results show that the overall bit-error-rate performance of the proposed multicarrier transmission scheme well approaches that of a single-carrier system due to the negligible intercarrier interference introduced by an appropriate design of the generalized DFT modulated filter banks.     

Exact analysis for downlink MC-CDMA in Rayleigh fading channels

In this letter, downlink multicarrier code-division multiple access (MC-CDMA) systems with maximal ratio combining (MRC) in frequency-selective Rayleigh fading channels is considered. An exact bit error rate (BER) is provided based on the characteristic function (CF) and residue calculation method. The BER performance is evaluated in the time domain instead of the frequency domain. This method does not require any assumption regarding the statistical or spectral behavior of multiple access interference (MAI).     

Performance of group ordered successive interference cancellation for multiuser detection in GSTBC SFH/MC DS‐CDMA system

In this paper, we present a novel group space‐time block coding slow frequency‐hopping multicarrier direct‐sequence code division multiple access (GSTBC SFH/MC DS‐CDMA) system over frequency selective fading channels. The proposed scheme greatly improves the bandwidth‐efficient through assigning the users employing the same frequency‐hopping (FH) pattern. Moreover, the users employing the same FH pattern are assigned into different virtual groups, in which the users are assigned to the different spreading codes, while the users are assigned to the same spreading code in the same virtual group. Then, a novel group detection scheme that we denote by group ordered successive interference cancellation (GOSIC) is presented to suppress the interference between the different virtual groups. Our proposed scheme consists of ordering group at the receiver side in order to maximize the overall system performance, and carrying BLAST‐STBC (LSTBC) detection for the users in the same virtual group. We define and derive the optimal group order based on the post group signal to interference plus noise ratio (PGSINR). We also propose another suboptimal group order in order to overcome the complexity issues. Finally, we compare the performance of our proposed GOSIC with conventional group successive interference cancellation (GSIC), conventional STBC multiuser detection (MUD), and LSTBC‐MUD, and show that significant improvement is introduced. Finally, it is shown that the proposed scheme is robust to the imperfect channel estimation. Copyright © 2006 John Wiley & Sons, Ltd.     

瑞利衰落信道中MC-CDMA系统的载波频率偏移性能

多载波CDMA（MC－CDMA）技术结合了OFDM和CDMA技术的优点,在无线衰落信道中传输高速数据时具有突出的性能。但由于MC－CDMA采用了多载波技术,载波频率偏移对系统性能的影响非常大。本文对瑞利衰落信道下存在载波频率偏移的MC－CDMA系统性能进行了理论分析并对误码率进行了详细讨论。