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。     

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.     

Performance analysis of Doppler shift effects on OFDM‐based and MC‐CDMA‐based cognitive radios

The present development of high data rate wireless applications has led to extra bandwidth demands. However, finding a new spectrum bandwidth to accommodate these applications and services is a challenging task because of the scarcity of spectrum resources. In fact, the spectrum is utilized inefficiently for conventional spectrum allocation, so Federal Communications Commission has proposed dynamic spectrum access mechanism in cognitive radio, where unlicensed users can opportunistically borrow unused licensed spectrum, which is a challenge to obtain contiguous frequency spectrum block. This also has a significant impact on multicarrier transmission systems such as orthogonal frequency division multiplexing (OFDM) and multicarrier code division multiple access (MC‐CDMA). As a solution, this paper develops non‐contiguous OFDM (NC‐OFDM) and non‐contiguous MC‐CDMA (NC‐MC‐CDMA) cognitive system. The implementation of NC‐OFDM and NC‐MC‐CDMA systems provides high data rate via a large number of non‐contiguous subcarriers without interfering with the existing transmissions. The system performance evaluates NC‐OFDM and NC‐MC‐CDMA for mobile scenario where each propagation path will experience Doppler frequency shift because of the relative motion between the transmitter and receiver. The simulation results of this paper proved that NC‐OFDM system is a superior candidate than NC‐MC‐CDMA system considering the mobility for cognitive users. Copyright © 2013 John Wiley & Sons, Ltd.     

Performance analysis of MC‐CDMA by employing the MMSE‐PIC scheme in wireless communications

From the past decade, the multicarrier code division multiple access (MC‐CDMA) transmission schemes have placed major role in wireless communications. It is providing a secured wireless communication to the users with guaranteed performance. In many situations, the performance of the MC‐CDMA is restricted due to the interference caused by multiple access interference (MAI), which also influences the frameworks of CDMA. To overcome this issue, we concentrated on developing the efficient technique for data transmission with interference cancellation for downlink MC‐CDMA. In the proposed method, the interference cancellation procedure is done by using the regeneration and subtraction of MAI from the signal. The simulation results are evaluated using the MC‐CDMA system with different decision functions. Results proved that the proposed system is efficient in reducing the MAI along with an improved bit error rate (BER).     

Adaptive Resource Allocation for MC‐CDMA and OFDMA in Reconfigurable Radio Systems

This paper studies the uplink resource allocation for multiple radio access (MRA) in reconfigurable radio systems, where multiple‐input and multiple‐output (MIMO) multicarrier‐code division multiple access (MC‐CDMA) and MIMO orthogonal frequency‐division multiple access (OFDMA) networks coexist. By assuming multi‐radio user equipment with network‐guided operation, the optimal resource allocation for MRA is analyzed as a cross‐layer optimization framework with and without fairness consideration to maximize the uplink sum‐rate capacity. Numerical results reveal that parallel MRA, which uses MC‐CDMA and OFDMA networks concurrently, outperforms the performance of each MC‐CDMA and OFDMA network by exploiting the multiuser selection diversity.     

Blind zero forcing equalization of multichannel nonlinear CDMAsystems

Code division multiple access (CDMA) has emerged as a popular format for wireless communication systems. As a result of intersymbol interference (ISI) and nonlinearities, the performance of CDMA systems can suffer. In this paper, we propose a blind equalizer for CDMA systems with Volterra (nonlinear) channels. The equalizer requires multiple observations at the receiver, which are obtained through oversampling or an antenna array, and a knowledge of the code of the desired user. Zero forcing equalization is possible in the noise-free case. Simulations demonstrate the performance of the equalizer under a variety of operating conditions     

Comparison of coded orthogonal frequency division multiplexing and multicarrier code division multiple access systems for power line communications

Orthogonal frequency division multiplexing (OFDM) and multicarrier code division multiple access (MC‐CDMA) systems are comparatively evaluated for power line communications (PLC) in a frequency‐selective fading environment with additive coloured Gaussian noise which is used to model the actual in‐home power line channel. OFDM serves as a benchmark in order to measure the performance of various MC‐CDMA systems, since multicarrier modulation systems are considered the best candidate for this kind of channel. Both single‐user and multi‐user cases are taken into account, making use of the appropriate combiner schemes to take full advantage of each case. System efficiency is enhanced by the application of different coding techniques, a fact which shows that powerful coding can make the difference under such a hostile medium. The impact of block interleaving is investigated, while the simulation examines how different modulation schemes fair under the imposed channel conditions as well. The performance of the system is assessed by the commonly used bit error rate vs signal‐to‐noise ratio diagrams and there is also a comparison regarding throughput efficiency among all the tested systems. As stated in Section 4, a promising PLC application is attained. Copyright © 2004 John Wiley & Sons, Ltd.     

A Multicarrier CDMA System for Joint Time-Frequency Diversity Using Divided Spreading Sequence

One of the principal disadvantages of multicarrier modulation technique is the sensitivity to the frequency offset introduced by Doppler shift. This frequency offset introduces inter-carrier interference (ICI) among the multiplicity of carriers in the multicarrier modulated signal. However, Doppler spread induced by temporal channel variations can provides another means for diversity. In this paper, we propose a modified multicarrier code division multiple access (CDMA) system to exploit Doppler diversity as well as multipath diversity. The key work of our framework is a canonical time-frequency-based decomposition of the mobile wireless channel into series of independent fading channel. The decomposition naturally leads to a time-frequency generalization of the Rake receiver that exploits both multipath and Doppler diversity.     

CDMA／SDMA系统性能分析

空分多址接入技术（SDMA）可以通过信号不同的空间传播路径来区分用户，从而提高了蜂窝移动通信系统容量．而此技术又可以和其他多址方式相互兼容，比如频分多址，时分多址，码分多址．CDMA／SDMA系统就是码分多址和空分多址两种接入技术相结合的系统．该系统可通过时空编码实现的．文中分析此种系统的性能，并与多用户环境下的CDMA系统进行了比较．     

Blind Channel Estimation for Multi-Rate Multicarrier DS/CDMA Communications

Multi-rate multicarrier DS/CDMA is a potentially attractive multiple access method for future wireless communications networks that must support multimedia, and thus multi-rate, traffic. Several receiver structures exist for single-rate multicarrier systems, but little has been reported on multi-rate multicarrier systems. Considering that high-performance detection such as coherent demodulation needs the explicit knowledge of the channel, based on the finite-length chip waveform truncation, this paper proposes a subspace-based scheme for timing and channel estimation in multi-rate multicarrier DS/CDMA systems, which is applicable to both multicode and variable spreading factor systems. The performance of the proposed scheme for these two multi-rate systems is validated via numerical simulations. The effects of the finite-length chip waveform truncation on the performance of the proposed scheme is also analyzed theoretically.     

Adaptive Subcarrier Allocation in Synchronous Reverse Links of a Multicarrier CDMA System with Time and Frequency Spreading

Multicarrier code-division multiple access (CDMA) with time and frequency spreading has been recently considered as a candidate for fourth-generation (4G) wireless systems. This signaling scheme simultaneously utilizes code spreading in the time and frequency domains to simultaneously improve frequency diversity and minimize multiuser access interference. As a result, it is capable of outperforming multicarrier CDMA systems that employ 1-D spreading. In this paper, a novel adaptive subcarrier allocation algorithm is developed for multicarrier CDMA with time and frequency spreading to improve the overall bit error rate (BER) performance for all spreading configurations. This algorithm assigns users to subcarrier groups that provide favorable fading characteristics while simultaneously reducing the amount of interference caused to other users. The proposed algorithm is shown to provide a performance improvement, ranging from 1.5 dB with 2times16 (time times frequency) and spreading to 7 dB with 16times2 (time times frequency) spreading. The algorithm is also shown to maintain or improve the BER floor for each spreading configuration. It is concluded that at higher and lower levels of E_b/N_o, a higher frequency- and time-domain spreading should be, respectively, employed to improve BER performance. Furthermore, the E_b/N_o threshold level to switch between time and frequency spreading for the analyzed system is found to be 2.5 dB.     

Generalized frequency division multiplexing–based acoustic communication for underwater systems

The underwater (UW) acoustic channel poses multiple challenges like coloured ambient noise, frequency‐dependent attenuation, and doubly selective fading. The availability of a robust underwater communication mechanism can largely enhance the success of human effort in a multitude of applications, ranging from pollution surveillance to defence and search/rescue operations. In this work, generalized frequency division multiplexing (GFDM), a non‐orthogonal multicarrier scheme, which has recently been studied for terrestrial wireless fading channels, is developed and tested for signalling in UW acoustic communication. UW noise, attenuation, and doubly selective fading channels are modelled with appropriate statistics. The BER performance of proposed system is systematically evaluated under different channel conditions, starting from simple additive white Gaussian noise (AWGN) and Rayleigh fading channels to a horizontally configured UW channel. The performance is also compared with contemporary orthogonal frequency‐division multiplexing (OFDM)– and filter bank multicarrier (FBMC)–based systems.     

Performance of a Modified Multicarrier Direct Sequence CDMA System

In this paper, we present an improved multicarrier direct sequence (DS) code division multiple access (CDMA) scheme by modifying the system originally proposed by Kondo and Milstein [13]. In this modified system, different spreading sequences multiplied by a data sequence modulate different carriers. This is to prevent the multiple access capability from reducing when the fading characteristics of different carrier frequencies are highly correlated. We have derived a formula which determines the mean values of the relative received signal strength in a single carrier DS CDMA rake system and in a multicarrier DS-CDMA system. We present results on the comparison of the bit error rate (BER) performance of the two systems including the effect of correlation between fading characteristics of different frequencies under various multipath fading conditions. The results indicate that with 50 users the modified multicarrier DS CDMA system can achieve an uncoded irreducible BER of 1.7 × 10^??3 with an average received signal-to-noise ratio per bit of 10 dB, which is better than 3.0 × 10^??3 achieved by the single carrier DS CDMA rake system, and also show that if multicarrier CDMA system is used with respect to single carrier CDMA system, the SNR gain is up to 4.5 dB for the uncode BER of 10^??3 being achieved.     

A multiuser interference cancellation technique utilizingconvolutional codes and orthogonal multicarrier modulation for wirelessindoor communications

This paper suggests that multicarrier modulation reduces the complexity and the delay caused by the multiuser interference cancellation process utilizing convolutional codes. For spread spectrum multiple access, multiuser interference (interference due to signals from other users) limits the performance of the communication link. To remove this interference, a multiuser interference cancellation technique which utilizes orthogonal convolutional codes has been proposed for the uplink (mobiles to a base station) of the cellular code-division multiple-access (CDMA) systems. However, this technique requires large interleavers and huge memory, or artificial multipath diversity and a RAKE system to achieve sufficient coding gain if it is applied to wireless indoor communications and fading is slow compared to the data rate. To reduce the complexity of the canceller, multicarrier modulation is employed as it provides frequency diversity gain and coding gain without the interleavers or a RAKE system. This paper shows that multicarrier modulation reduces the complexity of the canceller and still provides sufficient coding gain in order to cancel the multiuser interference. A canceller with decoding in the initial decision and multicarrier modulation improves the capacity by a factor of 1.4 as compared with a canceller without decoding     

Dynamic resource scheduling (DRS): a multimedia QoS framework for W‐CDMA

Multi‐media support is an important feature of third generation (3G) wireless communication systems, and Quality of Service (QoS) is a crucial issue, as in any other networking environment. In this paper, the QoS issues in the wireless last‐mile is investigated for 3G systems based on Wideband‐Code division multiple access (W‐CDMA). Supporting multiple rates in the CDMA environment introduces the power assignment problem, which is coupled with the bandwidth and error QoS parameters. Also, multi‐media traffic flows should be classified and serviced in such a way to provision delay guarantees. In this paper, a new framework, namely dynamic resource scheduling (DRS), is described and extensively studied. In order to serve multi‐media services with different requirements, a family of nine algorithms has been developed within the DRS framework. These algorithms can be categorized with respect to single or prioritized queuing architectures, fixed or variable rate bandwidth and power allocation, and variable spreading gain or multi‐code spreading strategies. The paper presents the performance of the DRS algorithms in comparison with each other and with conventional scheduled‐CDMA (S‐CDMA) and proposed schemes in the W‐CDMA standard. The performance for error and throughput QoS provisioning and power control dynamics are explored; advantages, disadvantages and limitations of the algorithms are discussed. The DRS framework is concluded to be a promising QoS architecture, with a simple, flexible, scalable structure that can be configured according to a given traffic scenario. Copyright © 2004 John Wiley & Sons, Ltd.     

IDMA Based Anti-collision in RFID Networks

Even though object identification is its main objective, it is generally accepted that there are some technological challenges that should be addressed in order to exploit the full potential of RFID (Radio frequency identification). The anti-collision problem is one of them. Despite the different proposed methods, it still needs further study on how to improve identification ability. This paper presents a novel algorithm with IDMA (Interleave division multiple access) scheme to resolve the anti-collision problem for RFID. IDMA, which is one of the most promising technologies for high data rate wireless systems, can be regarded as a special case of CDMA (Code division multiple access). In contrast to CDMA, which separates tags by signatures or spreading codes, distinct interleavers are the only means to separate tags for IDMA. In this paper, a RFID based IDMA receiver is proposed and compared to classical systems. Simulation results demonstrate that this system provides an improvement in performance in terms of bit error rate in dense scenarios (large number of tags) and it offers an efficient alternative to systems with classical spreading sequence.     

二维大气无线光码分多址通信系统

在无线光通信系统中采用光码分多址(OCDMA)技术,可以充分利用其丰富带宽,提高系统性能。考虑背景光噪声、探测器噪声、多用户干扰和大气闪烁等影响因素,给出了二维无线光码分多址(2D-WOCDMA)系统模型,在该模型中采用了脉冲位置调制(PPM)和光纤布拉格光栅(FBG)编解码器。通过数值分析,详细讨论了该无线光通信系统的性能。结果表明,大气闪烁是影响二维无线光码分多址通信系统性能的重要因素,当其对数方差较大时,系统难以实现通信,需要采用诸如多用户检测、信道编码及阵列接收等技术提高系统的性能;二维无线光码分多址通信系统适合采用1550 nm波段;该通信系统还受背景光、雪崩光电二极管(APD)增益和调制消光比等因素的影响。     

Reconfigurable Transceivers for Wireless Broadband Access Schemes

Though code division multiple access (CDMA) and orthogonal frequency division multiple access (OFDMA) are potential candidates for future broadband wireless access schemes, the requirements of the users may not be completely supported by a single communication system. Therefore, for easy connectivity and compatibility with the existing communication systems and to support various quality-of-service (QoS) requirements, systems with dynamic reconfigurability are inevitable. This article gives an overview of CDMA, OFDMA, and other block-based transmissions, and proposes a reconfigurable transceiver whose blocks can be configured in order to support any of the above air interfaces. The reconfigurable transceiver allows low-cost implementation of multiple air interfaces in a single terminal and achieves optimal QoS based on the available wireless networks.     

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.     

Performance analysis of single carrier and multicarrier DS‐CDMA systems over generalized fading channels

Using a recently developed moment generating function‐based approach for the performance evaluation of digital communications over fading channels, we present a unified approach for the exact performance analysis of binary direct‐sequence code division multiple access (DS‐CDMA) systems operating over generalized frequency‐selective fading channels. The results are applicable to single carrier systems employing RAKE reception as well as to multicarrier DS‐CDMA systems with frequency diversity. Aside from simplifying previous results both analytically and computationally, the proposed approach also gives a solution for many situations which heretofore defied a simple form. Copyright © 2001 John Wiley & Sons, Ltd.