Double-orthogonal coded space-time-frequency spreading CDMA scheme

In this paper, we propose double-orthogonal coded space-time-frequency spreading code-division multiple-access (DOC-STFS-CDMA) scheme for the downlink orthogonal frequency-division multiplexing (OFDM) systems employing multiple transmit and receive antennas. First, we introduce the so-called STFS-CDMA and the corresponding minimum mean-square error (MMSE) detection algorithms for three scenarios with different user information. Then, we propose a new class of spreading codes, called double-orthogonal code (DOC), to match the MMSE detection algorithms for STFS-CDMA system. With the special double orthogonality, DOC can effectively exploit space, time, and frequency diversity to enhance the performance of MMSE detectors, and provide the well-balanced signal-to-interference-and-noise ratio to different users. Finally, the performance gain of the proposed scheme over conventional schemes is shown by computer simulation.     

Orthogonal variable spreading codes for wideband CDMA

Orthogonal codes of variable lengths are essential for multirate services in the Third-Generation Partnership Project (3GPP). We propose an alternative to Walsh functions for variable spreading codes. There are three important points. First, we design an orthogonal code of constant length by mapping the points of a set of finite projective planes to chips of high/low amplitudes. Second, we develop variable spreading codes using multiple layers of recursion. Last, we compare and contrast the proposed variable spreading technique with Walsh functions in terms of ease of synchronization, blind-rate detection and wideband characteristics     

PN acquisition and tracking performance in DS/CDMA systems withsymbol-length spreading sequences

Direct sequence spread spectrum code-division multiple-access (DS/CDMA) is receiving increasing attention for cellular communications systems. When the users are synchronized and special symbol-length sequences, such as Gold codes, are used, the mutual interference can be substantially reduced relative to a system with very long or purely random spreading sequences. It is shown that this approach degrades the performance of the code phase acquisition and tracking, however. This effect prevents the system from acquiring and tracking long before the data detection is affected by the multiuser interference     

Performance enhancement of DS/CDMA system using chaotic complex spreading sequence

In this paper, a novel direct sequence/spread spectrum (DS/SS) communication system is proposed. This scheme exploits the two-dimensional complex valued chaotic Ikeda map as the spreading sequences. With this double spreading DS/SS system, the effect of multiple access interference can be mitigated by choosing the spreading sequences with appropriate cross-correlation properties. The performance of the system is assessed and demonstrated in a multiuser environment by means of computer simulation with additive white Gaussian noise, Rayleigh fading, and selective fading channel conditions. These studies reveal that the proposed system significantly outperforms the Gold code DS/SS-BPSK system in synchronous channel conditions. In asynchronous case, the improvement is substantial for low signal-to-noise ratios.     

A new spreading scheme for convolutionally coded CDMA communicationin a Rayleigh-fading channel

There has been increased interest in the use of direct-sequence code-division multiple-access (DS/CDMA) for wireless communication systems. We find that the asymptotic bit-error probability (BEP), P_b, of a convolutionally coded code-division multiple-access (CDMA) system in a frequency-selective Rayleigh-fading channel depends on the length of the shortest error event path and the product of symbol distances along that path. Based on this observation, we propose a new spreading scheme that maximizes the length of the shortest error event path. It is shown that the proposed scheme yields an improvement of 1.0-1.3 dB at P_b=10^-5 over the conventional convolutionally coded CDMA system, and even a higher improvement can be achieved as the required BEP is decreased     

Orthogonal on-off BPSK: a convenient signaling scheme for near-farresistant detection in DS/CDMA

This paper proposes a convenient signaling scheme-orthogonal on-off BPSK (O³BPSK)-for near-far (NF) resistant detection in asynchronous direct-sequence code-division multiple-access (DS/CDMA) systems (uplink). The temporally adjacent bits from different users in the received signals are decoupled by using the on-off signaling, and the original data rate is maintained with no increase in transmission rate by adopting an orthogonal structure. The detector at the receiver is a one-shot linear decorrelating detector, which depends upon neither hard decision nor specific channel coding. The application of O³ strategy to the differentially encoded BPSK (D-BPSK) sequences is also presented. Finally, some computer simulations are shown to confirm the theoretical analysis     

Hadamard扩谱及CDMA的研究

在MATLAB平台上,对直接序列扩频系统进行仿真分析.系统采用Hadamard序列作为扩频码,对系统在平坦衰落和瑞利衰落信道环境下的误码率进行分析,讨论了扩频、解扩过程.此外,还仿真比较了不同用户数和不同扩频码的系统,为分析设计直扩系统提供依据.仿真结果表明,本实验系统可在SNR为-1.6dB的传输条件下得到10-3量级的误码率,Hadamard序列可用作扩频序列,系统可用作检验各种正交码组的正交性.     

Collaborative space-time spreading scheme for higher than unity rate CDMA downlink

A novel scheme referred to as collaborative space-time spreading is proposed to provide a higher than unity rate with spatial diversity gain for the downlink of a CDMA system operating in a fading channel. This is achieved by performing collaborative coding and spreading over groups of T users, each of which is assigned a single orthogonal spreading sequence and followed by space-time encoding on the total composite signal of all groups. A user terminal simply despreads the incoming signal to recover the composite T-user group data and then extracts its intended data after space-time and collaborative decoding. Extensive comparisons with other relevant schemes are provided to show the superiority of this approach.     

A transmitter diversity scheme for wideband CDMA systems based onspace-time spreading

We present a transmit diversity technique for the downlink of (wideband) direct-sequence (DS) code division multiple access (CDMA) systems. The technique, called space-time spreading (STS), improves the downlink performance by using a small number of antenna elements at the base and one or more antennas at the handset, in conjunction with a novel spreading scheme that is inspired by space-time codes. It spreads each signal in a balanced way over the transmitter antenna elements to provide maximal path diversity at the receiver. In doing so, no extra spreading codes, transmit power or channel information are required at the transmitter and only minimal extra hardware complexity at both sides of the link. Both our analysis and simulation results show significant performance gains over conventional single-antenna systems and other open-loop transmit diversity techniques. Our approach is a practical way to increase the bit rate and/or improve the quality and range in the downlink of either mobile or fixed CDMA systems. A STS-based proposal for the case of two transmitter and single-receiver antennas has been accepted and will be included as an optional diversity mode in release A of the IS-2000 wideband CDMA standard     

Interleaving scheme for multicarrier CDMA system

In MultiCarrier Code-Division Multiple-Access (MC-CDMA) system,the received signals scattered in the frequency domain are combined to get frequency diversity gain.However,the frequency diversity gain is limited because of correlation between subcarriers.A novel interleaving scheme for MC-CDMA system is proposed in this paper.A circular shifting register is introduced into each subcarrier branch to decrease the correlation between subcarriers.By using interleaving,frequency diversity gain of system is increased.System structure and model with interleaver are discussed.In the case of multiple users,Parallel Interference Cancellation (PIC) technique is also introduced.Computer simulations demonstrate the performance of proposed scheme,and the performance comparison of MC-CDMA with interleaver and conventional MC-CDMA system is shown as well.     

Analysis of a simple successive interference cancellation scheme ina DS/CDMA system

Compensating for near/far effects is critical for satisfactory performance of DS/CDMA systems. So far, practical systems have used power control to overcome fading and near/far effects. Another approach, which has a fundamental potential in not only eliminating near/far effects but also in substantially raising the capacity, is multiuser detection and interference cancellation. Various optimal and suboptimal schemes have been investigated. Most of these schemes, however, get too complex even for relatively simple systems and rely on good channel estimates. For interference cancellation, estimation of channel parameters (viz. received amplitude and phase) is important. We analyze a simple successive interference cancellation scheme for coherent BPSK modulation, where the parameter estimation is done using the output of a linear correlator. We then extend the analysis for a noncoherent modulation scheme, namely M-ary orthogonal modulation. For the noncoherent case, the needed information on both the amplitude and phase is obtained from the correlator output. The performance of the IC scheme along with multipath diversity combining is studied     

Multicarrier CDMA systems using time-domain and frequency-domain spreading codes

For wideband code-division multiple-access systems, the paper introduces a multicarrier modulation scheme that performs the spreading simultaneously in the time and frequency domains. This scheme attains higher flexibility and spectrum efficiency because system parameters can be selected at will. The performance is compared with that of a single carrier RAKE system by calculating the probability of error over a frequency-selective Rayleigh fading channel. The proposed scheme outperforms the single carrier RAKE system if the system parameters are selected properly for given conditions, such as bandwidth and delay spread.     

基于正交变换的广义多载波系统自适应传输方案

针对基于正交变换的广义多载波系统(OT-GMC),提出一种正交变换模式和调制编码方式联合自适应的单载波混合传输方案。该方案以最大化频谱效率为优化准则,依据接收信号的信干噪比(SINR)自适应切换正交变换模式和调制编码方式。当正交变换采用恒等变换时,对子带数据进行独立的链路自适应,提高了频谱效率。当正交变换采用离散傅里叶变换时,发射信号具有较低的峰均比,提高了发射端的功放效率。该传输方案实质上获得了单载波混合传输系统内频谱效率和峰均比的最优化折中性能。仿真表明,多径信道下该混合传输方案的吞吐量性能优于单一传输方案。     

A scheme for throughput maximization in a dual-class CDMA system

This work focuses on the problem of efficient exploitation of the available bandwidth in order to provide high bit rates on the wireless link, as will be required in future wireless systems interfacing to broadband fixed networks. In particular, the uplink of a CDMA system with two user classes is considered. One of the classes consists of delay-intolerant users requiring support for a constant information bit rate, while the other consists of delay-tolerant users needing support for an information bit rate larger than a given value. It is assumed that when not transmitting information, synchronization contact is maintained with the base station at a given rate. The objective is to maximize the throughput of the delay tolerant users, while ensuring that the interference to other cells is as low as possible by minimizing the sum of all of the transmit powers used by the mobiles. Two transmission modes for the delay-tolerant users are considered. In the first mode, all of the users are allowed to transmit information when they wish. In the second mode, the transmissions of the delay-tolerant users are scheduled, so that only a limited number of them are transmitting information at any given time instant. It is shown that the second transmission mode, which is a time-scheduled scheme for the delay-tolerant users (with hybrid CDMA/TDMA as a special case), affords a better throughput while imposing the same average power requirements as conventional transmission. The results in this paper can be interpreted using results from previous work based on information theory     

Modified SUGAR/DS: a new CDMA scheme

The authors prove by analysis the possibility of gaining a minimum of 2 dB in signal-to-noise ratio by just, splitting the users to three or more groups and identifying each one by an orthogonal waveform (on top of his short Gold code) in a PSK/DS spread-spectrum network. User signals may arrive in a code-asynchronous fashion at the receiver, however, it is shown that by using the scheme the average code cross-correlation is minimal compared to the classic code division multiple access (CDMA) system. Both the chip-synchronous and asynchronous cases are investigated and the uniform and optimal cases of dividing the users into orthogonal groups are analyzed. The superior bit error and network data throughput results in the different fading and forward error correction environments make the system a strong candidate for competitive domestic applications     

多载波扩频系统中单音干扰抵消的新方法

利用离散傅立叶变换(DFT)计算的结果对干扰信号进行准确估计与重构,提出了一种有效去除多载波扩频系统中存在的单音干扰DFT变换域处理新方法.与基于变换域处理的"门限剔除"法相比,该方法可以更有效地去除干扰,同时保证有用信号的能量损失很小.研究与仿真结果表明,所提出的新方法具有较大的性能优势,尤其在极大功率干扰的场合中,采用"门限剔除"法失效的情况下,仍具有优异的性能.     

频率复用DS/CDMA系统中自适应盲多用户检测的一种方法

该文给出一种工作于强窄带干扰(NBI)多径衰落信道中DS/CDMA系统盲多用户检测接收机模型。它具有RAKE接收机的基本结构,在每一条并行支路中利用MMSE准则跟踪各路径信号。因此具有抗多径衰落,同时抑制NBI和多址接入干扰(MAI)的能力。模拟结果证明,它的性能优于未考虑NBI抑制的多用户检测接收机。     

Multi-carrier CDMA system using a code orthogonalising filter

Detection using a code orthogonalising filter in the reverse link of a multi-carrier code division multiple access (MC-CDMA) system is discussed. In addition, a scheme which can reduce hardware complexity is proposed. Simulation results show that the proposed MC-CDMA system has superiority in performance over an MC-CDMA system under a multi-user environment     

基于混沌扩频CDMA的伽利略卫星定位系统编码技术

在伽利略系统中编码技术日益重要。文章在将混沌扩频技术应用到伽利略系统中后,对系统的信道特性进行了探讨,特别是利用混沌扩频的优秀随机性和正交性对系统进行了信道仿真。仿真结果表明信道衰减小于-40 dB,满足CDMA2000协议规范,从而简化了传统CDMA系统PN码产生的复杂性,统一了信号源和信道编码的混沌序列的使用。这种将混沌扩频CDMA技术代替传统PN序列的方法在伽利略系统中的应用具有实用性和可行性。