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。     

Multicarrier CDMA techniques for future wideband wireless networks

In this paper, multicarrier techniques are considered in the context of the future wideband wireless networks. An overview of the different access schemes based on a combination of code division and multicarrier techniques, such as Multi-Carrier Code Division Multiple Access (MC-CDMA), Multi-Carrier Direct Sequence CDMA (MC-DS-CDMA) and Multi-Tone CDMA (MT-CDMA) is presented first. Afterwards, MC-CDMA systems are considered for the downlink of high rate cellular networks. The performance of different mono-user detection techniques are first analytically estimated and then compared with the help of Monte Carlo simulations for a Rayleigh channel. Simulation results on the performance of mono-user and multi-user detection techniques are also provided for Bad Urban (BU) Hilly Terrain (HT) and Vehicular UMTS channels. With regard to the channel coding, convolutional codes and turbo-codes are considered. Among others, a new linear detection technique named Global Minimum Mean Square Error Algorithm (GMMSE) is compared to multi-user Parallel Interference Cancellation detection technique. It is shown that GMMSE offers good performance especially for non-full load systems. Thus, the efficiency of MC-CDMA as a very promising multiple access and robust modulation scheme is successfully demonstrated for the downlink of the future wideband mobile networks.     

MC-CDMA系统存在频偏时的性能分析

多载波码分多址(MC-CDMA)是把正交频分复用(OFDM)和码分多址(CDMA) 结合起来的新一代移动通信系统.详细构建了一个MC-CDMA 系统的数学模型,并给出了载频偏移和多径信道存在时的矩阵-向量表示模型.在此系统模型的基础上,分析了频偏对MC-CDMA系统多用户检测器性能的影响.MATLAB仿真显示了MC-CDMA系统对频偏极为敏感,当经历多径Rayleigh信道时,系统只能忍受较小的频偏.     

一种新的MC-CDMA系统同步算法

多载波CDMA(MC-CDMA)技术结合了正交频分复用(Orthogonal Frequency Division Multiplexing,OFDM)和CDMA技术的特点,在无线衰落信道中传输高速数据时具有突出的优势.整个MC-CDMA系统中同步性能起着非常重要的作用.本文在分析现有各种同步算法性能的基础上提出了一种新的MC-CDMA同步算法,理论分析和仿真表明这种新算法具有更好的同步性能且具有更低的系统复杂度.     

多载波CDMA系统仿真与性能分析

多载波CDMA是OFDM和CDMA结合的综合技术,兼有OFDM和CDMA的优点,可以在无线信道中传送高速数据流,在频谱效率、频率分集、抗多径干扰等方面都能获得较大好处。首先介绍了OFDM的基本原理,给出MC-CDMA的系统模型,然后通过仿真,比较了OFDM系统和MC-CDMA系统的误码性能。结果表明,MC-CDMA系统性能优于OFDM系统,说明了OFDM和CDMA技术结合的必要性。     

无线宽带数据传输中多载波CDMA系统的性能

在分析同步多载波CDMA系统基带发射、接收模型和多径衰落信道模型的基础上,提出一种新颖的误码率分析方法。对系统在瑞利(Rayleigh)衰落信道中的误码性能进行了理论研究,得到了BPSK调制条件下的系统误码率表达式。数值分析结果表明:多载波CDMA在无线宽带数据传输中性能大大优于CDMA,具有良好的应用前景。     

Channel access and interference issues in multi-code DS-CDMA wireless packet (ATM) networks

Multi-Code Direct-Sequence Code-Division-Multiple-Access (MC-CDMA) has been proposed as a flexible multiple access scheme for wireless packet networks that support a large variety of mobiles with different and even time-varying rates. Using MC-CDMA, traffic streams with significantly different transmission rates can be easily integrated into a unified architecture, with all the transmissions occupying the same bandwidth and having the same spread spectrum processing gain. In this paper, we address medium-access and interference issues in MC-CDMA wireless packet networks. For medium access, we propose and study Multi-Code CDMA (MC-CDMA) with Distributed-Queueing Request Update Multiple Access (DQRUMA) to form a unified bandwidth-on-demand fair-sharing platform for multi-rate wireless services. DQRUMA is an efficient demand-assignment multiple access protocol for wireless access and scheduling. Pseudo-Noise (PN) codes (primary codes) and optimal power levels are allocated to the mobiles on a slot-by-slot basis, and a Maximum Capacity Power Allocation (MCPA) criterion exploits the sub-code concatenation property of the MC-CDMA transmission. Simulation results show that the system provides close to ideal-access performance for multi-rate mobiles, both with homogeneous traffic characteristics and with a mix of heterogeneous traffic characteristics. Finally, we analyze the effects of MC-CDMA intercell interference on the reverse link (i.e., mobile to cell site) and investigate interference reduction by using the Maximum Capacity Power Allocation (MCPA) criterion. Our results show significant reduction in reverse-link MC-CDMA intercell interference is possible using the MCPA criterion.     

一种基于快跳频技术的多载波CDMA系统

为进一步提高多载波CDMA系统的频带效率与抗干扰能力,提出了一种跳频多址技术结合多载波调制的传输方案,利用跳频技术将部分子载波传输与随机交织引入到多载波传输系统中,给出了系统实现的模型,并对该传输方案的系统带宽和误码性能进行了分析。理论分析与仿真结果表明：与以往的多载波CDMA系统相比,所建议系统的结构简单,性能优异,消除了已有系统中峰均功率比高和远近效应等问题,具有较强的抗多址干扰与窄带干扰的能力,并能有效地抗信道中突发错误的能力。     

Comparison of maximum-likelihood-based detection for two multirateaccess schemes for CDMA signals

Future wireless systems will need to accommodate information sources with different data rates. Direct-sequence code-division multiple-access (DS/CDMA) is a multiple access technique that is well suited to provide multirate access. Thus, in this paper, multirate communication systems are considered for the transmission of DS/CDMA wireless signals. Performance for maximum-likelihood-based detection is studied in the context of two multirate access methodologies: multicode access, where high data rate users multiplex their information streams onto multiple codes; and variable spreading length access where signature sequences of different lengths are assigned to users with different data rates. Various maximum-likelihood-based detection schemes for the variable spreading length system are considered as they can achieve near-optimal performance and thus provide reference points for comparison with suboptimal schemes. In addition, asymptotic multiuser performance measures are calculated and bounded to compare performance of the two systems     

On Multicarrier Detection and Coherent CDMA Mobile Receiver Radio Systems with Joint Antenna Diversity

The combination of code division multiple access(CDMA) and multicarrier (MC) transmission techniques,termed MC-CDMA, is considered a promising alternative toconventional DS (direct sequence)-CDMA. For this reason, recent research activities haveconcentrated on the application of MC-CDMA to mobileradio systems. In this paper an MC-CDMA concept which iswell suited for mobile radio applications is described. The described MC-CDMA concept overcomesdisadvantages of previously proposed concepts. InMC-CDMA mobile radio systems, signal reception isimpaired by time-varying multipath propagation. Theimpairments can be reduced by applying diversitytechniques. Coherent receiver antenna diversity (CRAD)is especially attractive because only the signalprocessing at the receivers must be modified. In thiscommunication, the application of CRAD in combination withjoint detection (JD) techniques to the more criticaluplink of MC-CDMA mobile radio systems is investigated.It is explained that the deployment of JD techniques for CRAD is an effective countermeasure againstthe influence of the mobile radio channel on the systemperformance. Four JD techniques for CRAD which areapplicable to MC-CDMA are presented. Their performances are studied in bad urban, typical urban, andtypical macrocellular environments. It is shown thatMC-CDMA allows a favorable performance compared to otherCDMA concepts.     

Design of spread spectrum multicode CDMA transport architecture formultimedia services

We investigate a new application of the well-known spread spectrum code division multiple access (SS-CDMA) techniques to multimedia services related to the development of the next-generation wireless mobile networks interconnecting with a wireline ATM-based broadband network. Such services allow users to share novel multimedia applications without any geographical restrictions. However, since the mobile radio channel has a fixed limited bandwidth, the traditional SS-CDMA system may not be sufficient to accommodate the variable bit rate (VBR) multimedia services requested by multiple mobile users simultaneously. Moreover, the traffic load at the base station can change dynamically due to the time-varying throughput requirement of these requested multimedia services. To tackle this difficulty, a multicode CDMA (MC-CDMA) technique is proposed to provide multirate multimedia services by varying the number of spreading codes assigned to each user in order to meet its throughput requirement. In MC-CDMA, a spreading code can be used to transmit information at a basic bit rate. Users (video or data) who need higher transmission rates can use multiple codes in parallel. Meanwhile, the maximum available number of codes in the MC-CDMA system is still limited. Hence, a cost-effective dynamic code allocation scheme has then been proposed to dynamically assign an appropriate number of codes to each user for achieving the maximum resource utilization for multiuser multimedia services via the mobile radio channel. Finally, a number of real multimedia titles generated from the well-known MacroMind Director are conducted to evaluate performance     

Multimedia transmission in MC-CDMA using adaptive subcarrier power allocation and CFO compensation

Multicarrier code division multiple access (MC-CDMA) system is one of the most effective techniques in fourth-generation (4G) wireless technology, due to its high data rate, high spectral efficiency and resistance to multipath fading. However, MC-CDMA systems are greatly deteriorated by carrier frequency offset (CFO) which is due to Doppler shift and oscillator instabilities. It leads to loss of orthogonality among the subcarriers and causes intercarrier interference (ICI). Water filling algorithm (WFA) is an efficient resource allocation algorithm to solve the power utilisation problems among the subcarriers in time-dispersive channels. The conventional WFA fails to consider the effect of CFO. To perform subcarrier power allocation with reduced CFO and to improve the capacity of MC-CDMA system, residual CFO compensated adaptive subcarrier power allocation algorithm is proposed in this paper. The proposed technique allocates power only to subcarriers with high channel to noise power ratio. The performance of the proposed method is evaluated using random binary data and image as source inputs. Simulation results depict that the bit error rate performance and ICI reduction capability of the proposed modified WFA offered superior performance in both power allocation and image compression for high-quality multimedia transmission in the presence of CFO and imperfect channel state information conditions.     

On the Suboptimality of Iterative Decoding for Turbo-Like and LDPC Codes With Cycles in Their Graph Representation

Performance of Multicarrier CDMA With Successive Interference Cancellation in a Multipath Fading Channel A high-capacity, low-complexity, and robust system design for a successive interference cancellation (SIC) system is developed and analyzed. Multicarrier code-division multiple access (MC-CDMA) is used to suppress multipath and to overcome the multipath channel estimation problem in single-carrier SIC systems. In addition, an optimal power control algorithm for MC-CDMA with SIC is derived, allowing analytical bit-error rate expressions to be found for an uncoded system. Low-rate forward error-correcting codes are then added to the system to achieve robustness. It is found that the capacity of the coded system approaches the additive white Gaussian noise capacity for SIC, even in a fading multipath channel with channel estimation error. This indicates that MC-CDMA is very attractive for systems employing SIC.     

High-performance MC-CDMA via carrier interferometry codes

This paper introduces the principles of interferometry to multicarrier code division multiple access (MC-CDMA). Specifically, we propose the use of MC-CDMA with novel carrier interferometry (CI) complex spreading codes. The CI/MC-CDMA method, applied to mobile wireless communication systems, offers enhanced performance and flexibility relative to MC-CDMA with conventional spreading codes. Specifically, assuming a frequency selective Rayleigh-fading channel, CI/MC-CDMAs performance matches that of orthogonal MC-CDMA using Hadamard-Walsh codes up to the MC-CDMA N user limit; and, CI/MC-CDMA provides the added flexibility of going beyond N users, adding up to N-1 additional users with pseudo orthogonal positioning. When compared to MC-CDMA schemes capable of supporting greater than N users, CI/MC-CDMAs performance exceeds that of MC-CDMA. Additionally, this new system is analyzed in the presence of phase jitters and frequency offsets and is shown to be robust to both cases     

Joint frequency-domain differential detection and equalization for DS-CDMA signal transmissions in a frequency-selective fading channel

It has been revealed that direct sequence code-division multiple access (DS-CDMA) can achieve a good bit-error rate (BER) performance, comparable to multicarrier CDMA (MC-CDMA), by using coherent frequency-domain equalization (FDE) instead of coherent Rake combining. However, coherent FDE requires accurate channel estimation. Pilot-assisted channel estimation is a practical solution, but its accuracy is sensitive to the Doppler spread. In this paper, a frequency-domain differential encoding and detection scheme is proposed for a DS-CDMA mobile radio. Joint frequency-domain differential detection and equalization (FDDDE) based on minimum mean-square error (MMSE) criterion is presented, where a simple decision feedback filter is used to provide a reliable reference signal for MMSE-FDDDE. Also presented is an approximate BER analysis. It is confirmed by both approximate BER analysis and computer simulation that MMSE-FDDDE provides good BER performance close to the coherent MMSE-FDE and shows high robustness against the Doppler spread; it outperforms coherent MMSE-FDE for large Doppler spreads. The proposed MMSE-FDDDE can also be applied to MC-CDMA. A performance comparison between uncoded DS- and MC-CDMA shows that DS-CDMA with MMSE-FDDDE achieves better BER performance than MC-CDMA with MMSE-FDDDE for small spreading factors.     

Multiuser MIMO OFDM Based TDD/TDMA for Next Generation Wireless Communication Systems

Multiple-input multiple-output (MIMO) wireless technology in combination with orthogonal frequency division multiplexing (MIMO OFDM) is an attractive air-interface solution for next-generation wireless local area networks (WLANs), wireless metropolitan area networks (WMANs), and fourth-generation mobile cellular wireless systems. In this paper, one multiuser MIMO OFDM systems with TDD/TDMA was proposed for next-generation wireless mobile communications, i.e., TDD/TDMA 4G, which can avoid or alleviate the specific limitations of existing techniques designed for multiuser MIMO OFDM systems in broadband wireless mobile channel scenarios, i.e., bad performance and extreme complexity of multiuser detectors for rank-deficient multiuser MIMO OFDM systems with CDMA as access modes, extreme challenges of spatial MIMO channel estimators in rank-deficient MIMO OFDM systems, and exponential growth complexity of optimal sub-carrier allocations for OFDMA-based MIMO OFDM systems. Furthermore, inspired from the Steiner channel estimation method in multi-user CDMA uplink wireless channels, we proposed a new design scheme of training sequence in time domain to conduct channel estimation. Training sequences of different transmit antennas can be simply obtained by truncating the circular extension of one basic training sequence, and the pilot matrix assembled by these training sequences is one circular matrix with good reversibility. A novel eigenmode transmission was also given in this paper, and data symbols encoded by space–time codes can be steered to these eigenmodes similar to MIMO wireless communication systems with single-carrier transmission. At the same time,, an improved water-filling scheme was also described for determining the optimal transmit powers for orthogonal eigenmodes. The classical water-filling strategy is firstly adopted to determine the optimal power allocation and correspondent bit numbers for every eigenmode, followed by a residual power reallocation to further determine the additional bit numbers carried by every eigenmode. Compared with classical water-filling schemes, it can also obtain larger throughputs via residual power allocation. At last, three typical implementation schemes of multiuser MIMO OFDM with TDMA, CDMA and OFDMA, i.e., TDD/TDMA 4G, VSF-OFCDM and FuTURE B3G TDD, were tested by numerical simulations. Results indicated that the proposed multiuser MIMO OFDM system schemes with TDD/TDMA, i.e., TDD/TDMA 4G, can achieve comparable system performance and throughputs with low complexity and radio resource overhead to that of DoCoMo MIMO VSF-OFCDM and FuTURE B3G TDD.     

MIMO Frequency Hopping Spread Spectrum Multi-Carrier Multiple Access: A Novel Uplink System for B3G Cellular Networks

In this paper, we propose and give the performance of a novel uplink system based on the combination of multi-carrier (MC), code division multiple access (CDMA) and multiple input multiple output (MIMO) techniques. First, we describe the interests of spread-spectrum multi-carrier multiple access (SS-MC-MA) scheme for uplink, especially compared to MC-CDMA. Classically, with SS-MC-MA, each user spreads its data symbols on a specific subset of adjacent or multiplexed subcarriers, to facilitate the channel estimation and reduce complexity at the reception. In order to compensate for the lack of frequency diversity of SS-MC-MA with adjacent subcarriers, we first combine it with an orthogonal space-time block code (STBC) and demonstrate the resulting spatial diversity gain. Then, we propose to allocate the subsets to the different users by applying a frequency hopping pattern (FH). In that case, each user benefits from the frequency diversity linked to the total bandwidth as with the multiplexed subcarriers solution, while keeping the advantages of the adjacent subcarriers solution. The gain provided by the use of the frequency hopping is stressed on. Finally, the performance of this scheme is evaluated over realistic MIMO channel with channel turbo coding for systems offering asymptotic spectrum efficiency of 1, 2, 3 and 4.5 bit/s/Hz. Thus, the efficiency of the novel proposed STBC FH SS-MC-MA system as a very promising multiple access and modulation scheme for the uplink of the future wideband wireless networks is successfully demonstrated.     

Diffused Channel Estimation and Reception for Cyclic Prefix-free MC-CDMA Arrayed-MIMO Communication Systems

An arrayed-MIMO communication system, which employs antenna arrays at both ends of the wireless link is proposed to leverage upon spatial information such as directions-of-arrival to achieve an improvement in performance. This is in contrast with conventional MIMO systems, which typically assume multiple independent antenna elements at the transmitters and receivers. This paper focuses on an arrayed-MIMO communication system operating over a frequency selective fading channel and employs MC-CDMA as the modulation technique. However, in a departure from conventional MC-CDMA systems, cyclic prefixes or guard intervals are not used for the MC-CDMA system employed here so that valuable bandwidth is not wasted on cyclic prefixes or guard intervals. Localized scattering is assumed to occur for each multipath; hence the wireless channel is modelled as a diffused vector channel. A robust blind estimation method is presented to estimate the parameters of the spatially diffused channel, followed by reception based on these parameters. The feasibility of the proposed system is supported by simulation results.     

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.     

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.