混沌扩频序列在多用户DS-CDMA系统中的应用

分析了混沌伪随机序列的自相关和互相关特性,采用Logistic序列作为扩频序列应用到多用户DS—CDMA系统中。分析了不同用户数下多用户干扰对系统BER的影响,并与M序列扩频系统进行了比较。计算机仿真表明,混沌伪随机序列具有优良的自相关和互相关特性,和传统的M序列相比,采用混沌扩频序列,可以大大降低系统的多用户干扰,使得系统的BER性能有大幅改善。     

Improving performance of DS-CDMA systems using chaotic complex Bernoulli spreading codes

The most important goal of spreading spectrum communication system is to protect communication signals against interference and exploitation of information by unintended listeners. In fact, low probability of detection and low probability of intercept are two important parameters to increase the performance of the system. In Direct Sequence Code Division Multiple Access (DS-CDMA) systems, these properties are achieved by multiplying the data information in spreading sequences. Chaotic sequences, with their particular properties, have numerous applications in constructing spreading codes. Using one-dimensional Bernoulli chaotic sequence as spreading code is proposed in literature previously. The main feature of this sequence is its negative auto-correlation at lag of 1, which with proper design, leads to increase in efficiency of the communication system based on these codes. On the other hand, employing the complex chaotic sequences as spreading sequence also has been discussed in several papers. In this paper, use of two-dimensional Bernoulli chaotic sequences is proposed as spreading codes. The performance of a multi-user synchronous and asynchronous DS-CDMA system will be evaluated by applying these sequences under Additive White Gaussian Noise (AWGN) and fading channel. Simulation results indicate improvement of the performance in comparison with conventional spreading codes like Gold codes as well as similar complex chaotic spreading sequences. Similar to one-dimensional Bernoulli chaotic sequences, the proposed sequences also have negative auto-correlation. Besides, construction of complex sequences with lower average cross-correlation is possible with the proposed method.     

改进型分段线性混沌序列用作DS-CDMA系统直扩码的分析

本文提出用改进型分段线性混沌映射来产生DS－CDMA系统的直扩码;分析了这类直扩码的特性以及两种量化函数对其特性的影响;计算机仿真结果表明,在适当的初值条件下这类序列具有周期长、平衡性好、自相关函数较尖锐、互关值较小以及线性复杂度高等特性,适合用作DS-CDMA系统直扩码.     

BER Performance of Bandlimited DS-CDMA Systems in Nakagami Fading

Accurate average bit-error rate analysis of asynchronous bandlimited binary direct-sequence code-division multiple-access (DS-CDMA) systems in Nakagami-m fading is studied. The fading is assumed to be flat and slow. The spectrum raised-cosine (SRC) and Beaulieu-Tan-Damen (BTD) pulse-shapes are employed. A new method which combines the characteristic function (CF) method with the improved Gaussian approximation (GA) is proposed for evaluating the BER of bandlimited DS-CDMA. A substantial computational complexity reduction is achieved. The well-known standard GA, Holtzman's simplified improved GA, and improved Holtzman's GA methods are also considered. The accuracies of the approximation methods are assessed using Monte Carlo simulation. For a system employing a deterministic sequence for the desired user and random sequences for the active interfering users, a CF method is employed to derive precise BER results. The new BTD pulse outperforms the SRC pulse in all situations     

A comparison of long versus short spreading sequences in codedasynchronous DS-CDMA systems

The performance of turbo-coded asynchronous direct sequence code division multiple access (DS-CDMA) using long and short spreading sequences is compared by both analysis and simulation. For coded systems with a conventional matched filter (MF) receiver, three analytical methods with different complexity are compared: the standard Gaussian approximation, the improved Gaussian approximation (IGA), and the density function approach. It is shown that while the standard Gaussian approximation is fairly accurate for the long sequences, it is too optimistic for the short sequences. For the short-sequence systems, the IGA gives an accurate estimate for the performance with much less complexity than the density function approach. The analysis shows that for either the additive white Gaussian noise (AWGN) channel or the flat Rayleigh fading channel and a MF receiver, there is a degradation in the average performance of the turbo-coded short-sequence systems compared to the long-sequence systems due to the fact that the cross-correlations are not time-varying. However, the short-sequence systems are amenable to the use of an interference suppression technique designed to minimize the mean square error. Such a minimum mean square error (MMSE) receiver in the turbo-coded system is shown to outperform the long-sequence system with the MF receiver, especially when there is a near-far problem, as previously observed in a convolutionally-coded system. Finally, similar results are obtained by computer simulations for the turbo-coded CDMA systems on a frequency-selective Rayleigh fading channel     

Performance of generalized multicarrier DS-CDMA over Nakagami-mfading channels

A class of generalized multicarrier direct sequence code-division multiple-access (MC DS-CDMA) schemes is defined and its performance is considered over multipath Nakagami-m fading channels. The spacing between two adjacent subcarriers of the generalized MC DS-CDMA is a variable, allowing us to gain insight into the effects of the spacing on the bit error rate (BER) performance of MC DS-CDMA systems. This generalized MC DS-CDMA scheme includes the subclasses of multitone DS-CDMA and orthogonal MC DS-CDMA as special cases. We present a unified analytical framework for determining the exact average BER of the generalized MC DS-CDMA system over generalized multipath Nakagami-m (1960) fading channels. The optimum spacing of the MC DS-CDMA system required for achieving the minimum BER is investigated and the BER performance of the system having optimum spacing is evaluated. The resultant BER is compared with that of both multitone DS-CDMA and orthogonal MC DS-CDMA     

Precise analysis of bit-error probability for asynchronous multicode DS-CDMA systems

A precise bit-error probability (BEP) analysis method is derived for a multicode direct-sequence code-division multiple-access (DS-CDMA) system in an additive white Gaussian noise channel. The method is applicable to a multicode DS-CDMA system with an arbitrary number of multiple code sequences, and any selection of multiple code sequences. The precise method gives results that discriminate the effect of the selection of different multiple code sequences on the BEP, whereas Gaussian approximations (GAs) do not. Thus, the new method can be used to select the best multicode set for a given system, a task that cannot be achieved using GAs. A two-step analytical procedure enables deriving an explicit, compact form for the CF of the receiver decision statistic in a DS-CDMA system with an arbitrary number of multiple code sequences, and for any selection of multiple code sequences.     

Quadriphase DS-CDMA with pulse shaping and the accuracy of the Gaussian approximation for matched filter receiver performance analysis

Probability of bit-error (P/sub e/) performance of asynchronous direct-sequence code-division multiple-access (DS-CDMA) systems is analyzed. In particular, the effects of pulse shaping, quadriphase (or direct-sequence quadriphase shift keying (DS-QPSK)) spreading, aperiodic spreading sequences and the coherent correlator or, equivalently, the matched filter (MF) receiver are considered. An exact P/sub e/ expression and several approximations: one using the characteristic function (CF) method, a simplified expression for the improved Gaussian approximation (IGA) and the simplified improved Gaussian approximation (SIGA) are derived. Two main results are presented. Under conditions typically satisfied in practice and even with a small number of interferers, the standard Gaussian approximation (SGA) for the multiple-access interference component of the MF statistic and for MF P/sub e/ performance is shown to be accurate. Moreover, the IGA is shown to reduce to the SGA for pulses with zero excess bandwidth. Second, the P/sub e/ performance of quadriphase DS-CDMA is shown to be superior or equivalent to that of biphase DS-CDMA. Numerical examples with Monte Carlo simulation are presented to illustrate P/sub e/ performance for square-root raised-cosine (Sqrt-RC) pulses and spreading factors of moderate to large values.     

基于IIR滤波器的最佳混沌扩频序列设计

在标准高斯近似条件下,以最小化系统平均多址干扰方差为准则,推导出了A-CDMA系统最佳扩频序列的部分自相关函数形式;并提出了通用的最佳混沌扩频序列设计方法,即利用数字IIR滤波器滤波混沌序列产生最佳混沌扩频序列。仿真结果表明,这种方法产生的序列性能明显优于纯随机序列和Gold序列。     

Bit-Error Probability of Asynchronous Multicode DS-CDMA Systems in Rayleigh Fading

A precise bit-error probability analysis method for multicode DS-CDMA systems in Rayleigh fading is derived. The proposed method applies to a multicode DS-CDMA system with an arbitrary number of multiple code sequences and any selection of multiple code sequences. The method gives accurate results for Rayleigh fading while requiring only a single numerical integration of an integrand involving the well-known Q-function. The solution discriminates the effect of the selection of different multiple code sequences on the bit-error probability, whereas Gaussian approximations do not. Numerical examples show that the differences between exact and approximate methods for a multicode DS-CDMA system operating over Rayleigh fading channels may be more significant than for a single code system in Rayleigh fading channels or for a multicode system in AWGN channels when the spreading factor is not significantly large compared to the number of multiple code sequences.     

A Unified Exact BER Performance Analysis of Asynchronous DS-CDMA Systems Using BPSK Modulation over Fading Channels

An asynchronous binary DS-CDMA system using random spreading sequences is considered when communicating over various fading channels. New closed-form expressions are derived for the conditional characteristic function (CF) of the multiple access interference. A unified analysis is provided for calculating the exact average bit error rate (BER) expressed in the form of a single numerical integration based on the CF approach. The numerical results obtained from our exact BER analysis are verified by our simulation results and are also compared to those obtained by the standard Gaussian approximation (SGA), confirming the accuracy of the SGA for most practical conditions, except for high signal-to-noise ratios (SNR) and for a low number of interferers.     

A Methodology for Bit Error Rate Prediction in Chaos-based Communication Systems

This paper is devoted to the derivation of an exact analytical expression of the bit error rate for chaos-based DS-CDMA systems. For the studied transmission system, we suppose that synchronization is achieved perfectly, coherent reception is considered, and an Additive White Gaussian Noise channel (AWGN) is assumed. In the first part of the paper, performance of a mono-user system with different chaotic sequences is evaluated and compared in terms of the error probability. This comparison is realized thanks to the probability density function of the bit energy of a chaotic sequence. The bit error rate can be easily derived by numerical integration. In some particular cases, for certain chaotic sequences with known probability density function of bit energy, we propose an analytical expression of the bit error. In the second part of the paper, the performance of a chaos-based DS-CDMA system is evaluated in the multi-user case. A general conclusion is that probability density function of chaos bit energy, for a given spreading factor, can give a clear idea about how to choose a “good” chaotic sequence for improving the performance of the chaos-based CDMA system.     

Achieving Single-User Performance in an FEC-Coded DS-CDMA System for Frequency Selective and Flat Fading Channels

This paper presents a reduced-complexity soft-input soft-output trellis/tree multiuser equalizer for an iterative DS-CDMA system undergoing Rayleigh frequency selective fading. The algorithm first expands the equalizer-trellis to an equivalent trellis/tree structure. Then it applies the M-algorithm to the equivalent structure twice, once to reduce the number of states in the trellis and the other to reduce the number of branches emanating from each state. To compute soft-information, the algorithm utilizes not only those fully-extended paths reaching the end of the trellis but also paths that are traversed and discarded in the pruned trellis. Through a simple update-and-discard procedure, reliable soft-information is extracted from discarded paths which enables an extremely large trellis to be successfully decoded with modest complexity. BER performance is presented for a convolutional-coded DS-CDMA system employing random spreading sequences. Our results demonstrate that the proposed algorithm is capable of achieving single-user performance with a much reduced complexity. The proposed algorithm can also be applied to reduce the complexity of multiuser detection where the transmission channel is frequency flat. Single-user performance can also be achieved with the proposed technique.     

Performance evaluation for band-limited DS-CDMA systems based on simplified improved Gaussian approximation

The standard Gaussian approximation (SGA) for error analysis of direct-sequence code-division multiple-access (DS-CDMA) systems is very optimistic in many cases. Improved Gaussian approximation (IGA) is a technique that produces accurate error probabilities, but is still computationally intensive. Simplified IGA (SIGA) has complexity similar to that of SGA and, at the same time, provides sufficient accuracy. In this paper, we consider SIGA for DS-CDMA systems employing random sequences in a band-limited scenario. The validity of IGA for band-limited systems is established in a rigorous mathematical sense. Then a key parameter in SIGA is derived via a frequency-domain approach. Applications to a number of typical chip waveforms, including the popular sinc and raised-cosine pulses, are investigated. Performance comparison with IGA-based lower and upper bounds shows that SIGA yields very accurate probability of error.     

A CDMA scheme based on perfect Gaussian integer sequences

Both m-sequences and Gold sequences have been applied as the pseudonoise (PN) code in the code division multiple access (CDMA) scheme to enable distinct users to simultaneously share the available capacity of a common channel, which is called direct sequence CDMA (DS-CDMA). However, this scheme does not provide the required ideal periodic cross-correlation function property among various members of sequences within the same set. This paper proposes a new perfect Gaussian integer sequence (PGIS)-based CDMA scheme called PGIS-CDMA; in this scheme, the PN code of DS-CDMA is replaced by a set of PGISs. The intrinsic orthogonal property of PGIS can be applied to a CDMA system to achieve perfect cochannel interference separability under the ideal synchronism; it can also be adapted at the receiver to obtain diversity gain and improve the carry-to-interference ratio. These two concepts are further developed to implement two PGIS-CDMA configurations, and they can outperform DS-CDMA on the basis of Gold sequences that possess a favorable cross-correlation property for the CDMA scheme.     

Received signal statistics in DS-CDMA channels with flat Rayleigh fading and fast closed-loop power control

The statistics of received signal power at the receiver in a cellular direct-sequence code-division multiple-access (DS-CDMA) system with flat Rayleigh fading and fast closed-loop power control mechanism are studied. A simple model for the fast closed-loop power control mechanism is proposed to ease the analysis. The probability density function (pdf) of the received signal power is derived by taking into consideration the updating frequency of power control mechanism, maximum allowable transmitted power, and Doppler frequency of the fading channel. The pdf derived can be used to study the performance of a reverse-link DS-CDMA system with noncoherent M-ary orthogonal modulation. The result shows that when optimum maximum allowable transmitted power is used, system capacity can be substantially increased for typical normalized Doppler frequency in cellular communication.     

广义Jacobi序列的自相关函数

相关函数是衡量序列密码安全性的重要指标。该文讨论了两类广义Jacobi序列的自相关特性,给出了它们的自相关函数的取值,结论表明:两类广义Jacobi序列都具有良好的自相关特性。     

On the performance of multicarrier RAKE systems

In this paper, we integrate the multicarrier signaling technique and the RAKE receiver to design a DS-CDMA system with the capability to increase the data rate, mitigate the effect of correlation among the various subcarriers, and suppress partial-band interference. In the proposed system, a data sequence is serial-to-parallel converted, and multicarrier DS-CDMA is used on each of the parallel data streams. The receiver provides a RAKE for each subcarrier, and the outputs of the RAKEs are combined by a maximal-ratio combiner. We employ a correlated subcarrier and frequency-selective fading channel model to derive the average probability of error of the system and compare the results with those of both a single carrier RAKE system and a multicarrier DS-CDMA system in a frequency selective Rayleigh fading channel with an exponential multipath intensity profile, when multiple access interference and partial-band interference are present     

On error probabilities of DS-CDMA systems with arbitrary chipwaveforms

The error probabilities of asynchronous DS-CDMA systems using random signature sequences depend on the number of users, the processing gain and the chip waveforms employed. The exact calculation of the error probabilities is computationally difficult and therefore approximations and bounds are more commonly used. In this article, the improved Gaussian approximation proposed by Holtzman (1992) is extended to include arbitrary chip waveforms. Comparisons to the exact calculation and the standard Gaussian approximation are also made to evaluate the accuracy of the improved Gaussian approximation     

2GHzDS—CDMA个人通信与数字微波通信系统电磁兼容优化设计

针对２ＧＨｚ－ＤＣ－ＣＤＭＡ个人通信系统与数字微波通信系统的电磁兼容问题提出了优化设计方案，一是优化ＤＳ－ＣＤＭＡ基站布局；二是优选高性能天线；本文给出的优化设计方案对于加快发展我国的ＤＳ－ＣＤＭＡ个人通信系统具有较强的理论指导意义，实施后会产生较大的经济效益。