Performance of optimum wavelet waveform for DS‐CDMA chip waveform over QS‐AWGN channel

In this paper, we search for a better chip waveform based on orthogonal wavelets for direct sequence‐code division multiple access (DS‐CDMA) signals to improve the probability of error (P_e) performance with minimal signal bandwidth variations. First, we derive the P_e expression over a quasi‐synchronous additive white Gaussian noise channel for DS‐CDMA signals, which use various pulse shaping waveforms including orthogonal wavelets as chip waveforms. It is observed that this expression depends on the chip waveform. Then, we design an optimum wavelet by using the relationship between wavelets and filter coefficients to reduce the probability of error. The DS‐CDMA system using the optimum wavelet waveform results in a lower probability of error than those using the conventional chip waveforms such as raised cosine, half‐sine and rectangular waveforms. Especially, the P_e of the optimum wavelet‐based scheme becomes significantly better than those of the conventional chip waveforms‐based schemes under the heavy loading that is the case for commercial wireless systems. When the systems work with full load (i.e. the number of users equals the processing gain), the optimum wavelet‐based system results in 0.5, 2.1 and 4 dB better SNR values than those of the raised cosine, half‐sine and rectangular‐based systems, respectively, for a P_e value of 10^?3. Copyright © 2005 John Wiley & Sons, Ltd.     

CDMA通信中多址序列相关性的研究及仿真

扩频序列设计和选择是码分多址(CDMA) 扩频通信的关键技术之一,扩频码序列的设计就是构造不同结构的具有良好相关性的伪随机序列来满足CDMA 系统的要求.现主要研究了应用于CDMA通信中的m序列、Gold序列和Kasami序列的原理、性能和构造方法,且基于MATLAB软件M语言编程以上序列的实现和相关性分析,并比较了其各自特点分析了其在不同环境下的适用性.     

Performance of wavelet based Multi‐Chip Rate DS‐CDMA signals over multi‐path Nakagami‐m fading channels

Multi‐Chip Rate/Direct Sequence‐Code Division Multiple Access (MCR/DS‐CDMA) technique using scaled chip waveforms has been designed as an alternative to multi‐data rate DS‐CDMA techniques having constant chip rates. In this work, the probability of error (P_e) expression for MCR/DS‐CDMA signals is derived over multi‐path Nakagami‐m fading channels to investigate the effects of chip waveforms on it. This paper also proposes the use of orthogonal wavelets as chip waveforms of MCR/DS‐CDMA signals over the considered channel. For numerical calculations, Daubechies‐22 (D₂₂) wavelet is used because its side lobes are 40 dB below its main lobe in frequency domain. D₂₂ is compared with a Square Root Raised Cosine (SRRC) chip waveform. In the numerical calculations, only first four scales of the chip waveforms relating to four different chip/data rates are considered. The results for the P_e performance and the capacity (the number of user per Hertz for a same P_e level) show that D₂₂ significantly outperforms the SRRC chip waveform at all data rates, due to low cross correlations among different scales of orthogonal wavelets. Besides, by increasing the number of scales, the advantage of the use of orthogonal wavelets will increase furthermore. Copyright © 2007 John Wiley & Sons, Ltd.     

Spreading‐code‐dependent bit error rate and capacity analysis for finite asynchronous CDMA systems

Most CDMA systems serve only finite number of active users. Therefore, using Gaussian approximation to evaluate its performance is inappropriate. In such finite CDMA systems, the selection of spreading codes is of great importance because its performance is very much spreading‐code‐dependent. In this paper, a new algorithm based on multiple variable Bernoulli process is introduced to evaluate bit error rate due to co‐channel interference and thus the capacity of a CDMA system. The algorithm can also be used for large‐sized CDMA systems. The results show that small Kasami and Gold‐like codes offer higher capacity than the others do. Copyright © 1999 John Wiley & Sons, Ltd.     

m序列复合码的一种快速相关算法

在直扩系统中,PN码的快速捕获十分重要。由于m序列的优选对有限,故在实际应用中采用复合码作为PN 码。常用的复合码主要有Gold序列和Kasami小集合序列。本文提出了一种利用快速沃尔什变换(FWT)实现对复合码进行快速相关的算法。这种算法适用于由两个m序列模2加形成的复合码,并原则上可以推广到Kasami大集合序列的捕获上。分析表明,我们提出的算法与传统相关算法相比,降低了算法的时间复杂度。     

Adaptive reference code‐based MAI cancellation for DS‐CDMA

A computationally efficient and practically deployable adaptive reference code‐based multiple access interference (MAI) cancellation scheme, in which the conventional transmitter/receiver architecture is minimally modified only at the receiver (and/or transmitter) end, is proposed for direct sequence code division multiple access (DS‐CDMA) communication. Upon numerical and theoretical analyses, the proposed communications system is seen to always outperform the existing conventional communications system. The theoretical analyses and results as presented are generally useful and applicable to any situation wherein IS95 pseudo noise (PN) codes are employed towards multiple access. Copyright © 2009 John Wiley & Sons, Ltd.     

Performance of SCHT Sequences in Asynchronous CDMA System

In this letter, the performance of a new set of spreading sequences for a direct sequence (DS) code division multiple access (CDMA) system is examined. These new sequences are derived from sequency-ordered complex Hadamard transform (SCHT). Various correlation properties are presented and the performance of the new sequence is analyzed by comparing it with the other well-known existing spreading sequences, such as UCHT, WHT, Gold, small set of Kasami and m-sequences. According to the simulation results, it is shown that the SCHT sequences provide smaller mean square cross correlation values and offer better average bit error rate for an asynchronous CDMA system over the multipath fading channel.     

PN Sequence Generation from 2‐D Array of Shift Registers

Toward code division multiple access (CDMA) communications and data protection, we propose and analyze pseudorandom noise (PN) sequences generated from a 2‐dimensional array structure of shift‐registers. For any positive integers m and n, we construct PN sequences of period 2^mn–1 using an m×n array of registers and show that we can generate all shifted PN sequences as required by IS‐95x with the proper linear combination of available sequences.     

On the comparisons of system performance and capacity of asynchronous STBC MC‐CDMA multirate access schemes

Three multirate access schemes, multicode, variable spreading gain (VSG), and spectral overlaid multiple‐symbol‐rate (MSR), for asynchronous space‐time block coded (STBC) multicarrier code division multiple access (MC‐CDMA) systems are proposed. The three possible spectral overlaid configurations for MSR systems are also investigated. The expressions to evaluate the multiple access interferences, bit error rate (BER) performances, and system capacities of a antenna STBC MC‐CDMA using the three multirate access schemes are obtained. Transmit power allocation is adjusted according to the service rates and the number of active users in each service class to maintain the link quality and to improve the system capacity. Our numerical results show that systems with multicode access scheme using orthogonal Gold spreading codes and with VSG access scheme have similar system performance and capacity, and both perform in general better than systems with MSR access scheme of any spectrum configurations. In case when non‐orthogonal Gold codes are used, multicode access scheme shows degradation in the system capacity as compared to VSG, due to the presence of larger self‐interference (SI) among the codes used by each user. The achievable capacities for the three spectral overlaid configurations of MSR multirate systems are also compared. Copyright © 2007 John Wiley & Sons, Ltd.     

ZCZ‐CDMA and OFDMA using M‐QAM for broadband wireless communications

This paper considers direct‐sequence code‐division multiple‐access with zero‐correlation zone sequences (ZCZ‐CDMA) and orthogonal frequency‐division multiple‐access (OFDMA) schemes using M‐ary QAM signaling for broadband wireless communications. Their system structures, complexities and performances in both AWGN and multipath frequency‐selective fading channels are evaluated and compared. For ZCZ‐CDMA, joint suppression of the multipath fading interference and multiple‐access interference can be achieved with a reduced family‐size of the spreading sequences. For OFDMA, analytical and simulation results indicate that it has the same performance as ZCZ‐CDMA in fast time‐varying multipath fading channels. In time‐invariant or slowly time‐varying channels, where the channel information can be made available to transmitters, OFDMA outperforms ZCZ‐CDMA, offers a higher capacity and is more flexible for system reconfiguration with a comparable computational complexity. Copyright © 2004 John Wiley & Sons, Ltd.     

Pseudonoise code tracking loop for a CDMA system with imperfect power control

In this paper, performance of a pseudonoise (PN) code tracking loop is analysed and simulated for a direct‐sequence/code‐division‐multiple‐access (DS/CDMA) system with imperfect power control in a multipath fading channel. A non‐coherent first‐order delay‐locked loop (DLL) is considered as a PN code tracking loop. Power control error is modelled as a log‐normally distributed random variable. The multipath fading channel is modelled as a two‐ray Rayleigh fading model that is typically applied to land mobile communication environments. The tracking performance of DLL is evaluated in terms of tracking jitter and mean‐time‐to‐lose‐lock (MTLL). From the simulation results, it is shown that for smaller discriminator offset, tracking jitter performance is improved while MTLL performance is degraded. It is shown that large power control error and heavy shadowing substantially degrade the PN tracking performance. Therefore, an accurate power control scheme is very essential to compensate for fading and shadowing for a DS/CDMA system. The extension of analysis to higher‐order loop models is very straightforward. The analysis in this paper can be applied to design the PN code tracking loop for a DS/CDMA system. Copyright © 2001 John Wiley & Sons, Ltd.     

Correlation-aided distributed sample acquisition scheme for fast and robust code synchronization in cellular DS-CDMA systems

We introduce a correlation-aided distributed sample acquisition (CDSA)-based intercell synchronous direct-sequence code-division multiple-access (DS/CDMA) system for enhancing the robustness of the previously proposed distributed sample acquisition (DSA)-based system. The CDSA system incorporates the state symbol correlation process in the existing DSA process to maintain the fast acquisition performance even under very poor channel environment. For its realization, each CDSA-based mobile station stores in its memory a period of the possible state symbol sequences, which are determined by the long-period scrambling PN sequences used in the system and the sampling time parameters for the state samples. In the first stage of the synchronization procedure, the mobile station (MS) attempts to acquire the psuedo noise (PN) sequence by taking the comparison-correction based synchronization approach of the original DSA scheme. However, if it cannot acquire the synchronization until it collects a predetermined number of state symbols (e.g., a period of the state symbol sequence), then the MS determines the PN sequence timing by correlating the received symbol sequence with each shift of the prestored state symbol sequences. Performance analysis and simulation results reveal strong robustness of the CDSA-based system in low-signal-to-noise ratio, fading, and large frequency-offset channels.     

Variable-length code construction for incoherent optical CDMA systems

The purpose of this study is to investigate the multirate transmission in fiber-optic code-division multiple-access (CDMA) networks. In this article, we propose a variable-length code construction for any existing optical orthogonal code to implement a multirate optical CDMA system (called as the multirate code system). For comparison, a multirate system where the lower-rate user sends each symbol twice is implemented and is called as the repeat code system. The repetition as an error-detection code in an ARQ scheme in the repeat code system is also investigated. Moreover, a parallel approach for the optical CDMA systems, which is proposed by Marić et al., is also compared with other systems proposed in this study. Theoretical analysis shows that the bit error probability of the proposed multirate code system is smaller than other systems, especially when the number of lower-rate users is large. Moreover, if there is at least one lower-rate user in the system, the multirate code system accommodates more users than other systems when the error probability of system is set below 10⁻⁹.     

KM序列用作RAKE型CDMA系统扩频码的分析

本文提出将KM 序列用作RAKE型CDMA系统的扩频码.基于一个RAKE接收机模型,分析了CDMA系统性能并提出了适于作RAKE型CDMA系统扩频码的PN序列的特性.接着,分析了KM 序列作RAKE型CDMA系统扩频码的优劣.最后,运用计算机仿真,比较了m序列和KM 序列的互相关特性以及在相干接收机和RAKE接收机下,这两种序列和Gold序列对应的系统平均误码率,从而证实了KM 序列作为扩频码的可行性.     

混沌扩谱CDMA的系统模型及其性能分析

混沌序列在具有很好随机性的同时,一些序列还具有KroneckerDelta自相关和近似正交的互相关特点,因此被认为是实现直序扩谱CDMA系统较理想的扩谱序列之一,成为近年来研究的热点。文中从异步DS-CDMA系统的模型入手,分析影响CDMA系统性能的关键问题,说明混沌扩谱CDMA系统的性能优点。为了实现混沌扩谱CDMA系统的数值仿真与性能验证,以切比雪夫映射产生的混沌序列为例进行混沌扩谱CDMA系统的性能分析,并与基于Gold和Kasami两种序列的传统DS-CDMA系统进行比较。     

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.     

An algebraic approach to generate super‐set of perfect complementary codes for interference‐free CDMA

This paper introduces an algebraic approach to generate the super‐set of perfect complementary (PC) codes suitable for new generation CDMA applications, characterized by isotropic multiple access interference (MAI) free and multipath interference (MI) free properties. The code design methodology proposed in this paper takes into account major impairing factors existing in real applications, such as MAI, MI, asynchronous transmissions, and random signs in consecutive symbols, such that a CDMA system using the generated codes can insure a truly interference‐free operation. Two important facts will be revealed by the analysis given in this paper. First, implementation of an interference‐free CDMA will never be possible unless using complementary code sets, such as the PC code sets generated in this paper. In other words, all traditional spreading codes working on an one‐code‐per‐user basis are not useful for implementation of an MAI‐free and MI‐free CDMA system. Second, to enable the interference‐free CDMA operation, the flock size of the PC codes should be made equal to the set size of the codes, implying that a PC code set can support as many users as the flock size of the code set. A systematic search has been carried out to generate the super‐set of various PC codes with the help of carefully selected seed codes belonging to distinct sub‐sets. This paper will also propose an implementation scheme based on multi‐carrier CDMA architecture and its performance is compared by simulations with the ones using traditional spreading codes. Copyright © 2006 John Wiley & Sons, Ltd.     

KM序列用作RAKE型CDMA系统扩频码的分析

本文提出将KM M序列用作RAKE型CDMA系统的扩频码 .基于一个RAKE接收机模型 ,分析了CDMA系统性能并提出了适于作RAKE型CDMA系统扩频码的PN序列的特性 .接着 ,分析了KM M序列作RAKE型CDMA系统扩频码的优劣 .最后 ,运用计算机仿真 ,比较了m序列和KM M序列的互相关特性以及在相干接收机和RAKE接收机下 ,这两种序列和Gold序列对应的系统平均误码率 ,从而证实了KM M序列作为扩频码的可行性     

A ZCZ‐CDMA system with BFSK modulation

A quasi‐timing synchronous code division multiple access (CDMA) system called ZCZ‐CDMA, which uses a set of sequences with a zero‐correlation zone called ZCZ code as a spreading code, is useful for short‐range wireless communications because of its excellent properties such as co‐channel interference‐free performance, simplified hardware design, and low transmit power as well as fast frame synchronization capability. In this paper, a ZCZ‐CDMA system with binary frequency‐shift keying (BFSK) modulation called BFSK‐ZCZ‐CDMA is proposed. This system is characterized by using a pair of balanced ZCZ codes for spreading and transmitting the two spread components over the respective keying carrier frequencies. Its bit error rate performance, compared with those of existing BPSK‐ZCZ‐CDMA, ASK‐ZCZ‐CDMA, and CDMA systems using the other spreading codes, is evaluated in theory and simulation. The bit error rate performance of the three ZCZ‐CDMA systems over additive white Gaussian noise and Rayleigh fading channels are formulated. It is proved that BFSK‐ZCZ‐CDMA is much more robust in anti‐fading performance and low transmit power in such an environment that fading distributions on the keying frequencies are independent mutually. Fading versus frequency characteristics are also investigated. Copyright © 2012 John Wiley & Sons, Ltd.     

A Time‐Varying Modified MMSE Detector for Multirate CDMA Signals in Fast Rayleigh Fading Channels

In this paper, we propose a time‐varying modified minimum mean‐squared error (MMSE) detector for the detection of higher data rate signals in a multirate asynchronous code‐division multiple‐access (CDMA) system which is signaled in a fast Rayleigh fading channel. The interference viewed by a higher data rate symbol will be periodic due to the presence of a lower data rate symbol which spans multiple higher data rate symbols. The detection is carried out on the basis of a modified MMSE criterion which incorporates differential detection and the ratio of channel coefficients in two consecutive observation intervals inherently compensating the fast variation of the channel due to fading. The numerical results obtained by the MMSE detector with time‐varying detection show around 3 dB (M=2) and 6 dB (M=4) performance improvement at a BER of 10^?3 in the AWGN channel, while introducing more computational complexity than the MMSE detector without time‐varying detection. At a higher Eb/N0, the proposed scheme can achieve a BER of approximately 10^?3 in the presence of fast channel variation which is an improvement over other schemes.