Interference suppression for DS/CDMA

A direct-sequence spread spectrum (DS/SS) receiver for suppressing multiple-access interference in direct-sequence code-division multiple-access (DS/CDMA) communication systems is introduced. The proposed receiver does not require knowledge of other users' spreading codes, timing, or phase information. Moreover, the receiver allows the number of taps to be chosen independently of the processing gain and, hence, is easily applicable to CDMA systems employing either a small or a large processing gain. Performance analysis, including average probability of error and signal-to-noise ratio, is provided, and results are presented for systems varying from lightly loaded (for example, eight user/255 chip) to heavily loaded (for example, 50 user/200 chip). Performance results indicate that the proposed receiver outperforms the linear correlation receiver and, in many cases, it does so by a considerable margin     

MISO CDMA transmission with simplified receiver for wirelesscommunication handsets

The next-generation wireless personal and mobile communication systems are expected to accommodate not only high-quality voice services, but also a broad range of other multirate services. Of the various multiaccess techniques, wide-band code-division multiple access (CDMA) has been regarded as an important part of the third-generation wireless communication systems because of its high frequency utilization efficiency and suitability for handling multimedia and multirate services. In this paper, we consider a system with a simplified receiver structure for direct-sequence CDMA (DS/CDMA) wireless communication handsets, in which improved performance is demonstrated when compared to a conventional DS/CDMA system with a RAKE receiver at the mobile station. We arrive at this system by finding the optimal solution to a general multiple-input single-output (MISO) DS/CDMA smart antenna system. We find that this solution reduces to a pre-RAKE with space transmit diversity system under the assumption that a simple one-finger matched filter is used at the receiver. This system combines the advantages of pre-RAKE diversity and transmit antenna diversity. It is shown that significant system performance and capacity improvements are possible. The numerical results also reveal that this system is not too sensitive to channel estimation errors     

A Sequential Algorithm for Joint Parameter Estimation and Multiuser Detection in DS/CDMA Systems with Multipath Propagation

A direct sequence (DS) spread spectrum code division multiple access (CDMA) communication system is considered where several users transmit data symbols over different multipath channels. The main objective of this work is the proposal of a sequential algorithm for joint parameter estimation and multiuser data detection. The computationally prohibitive maximization of the log-likelihood function is replaced by a sequential scheme using the multistage detector for recovering the data symbols and the expectation-maximization algorithm for estimating the channel parameters. The performance of the resulting multiuser receiver is evaluated via Monte Carlo simulations using both synthetic as well as measured channel impulse responses. A comparison of the performance with analytical expressions for the single-user single-path case shows that the proposed system is capable of eliminating the near-far effect existing in conventional DS/CDMA systems.     

Distributed sample acquisition-based fast cell search in inter-cellasynchronous DS/CDMA systems

We introduce a fast fell-search scheme based on the distributed sample acquisition (DSA) technique for inter-cell asynchronous DS/CDMA systems. The proposed DSA scheme enables the long-code acquisition extremely fast even without employing any passive matched filter which used to dominate synchronization circuit complexity in asynchronous DS/CDMA systems. In the DSA-based asynchronous DS/CDMA system, each base station spreads its DQPSK-modulated long-code generator information with an assigned short-period igniter sequence, and broadcasts it as a common pilot signal. A mobile station first identifies and acquires the igniter sequence of the cell group currently located, then detects the conveyed long-code generator information by despreading the acquired igniter sequence, thereby identifying and synchronizing the long-code of the current cell. The mean acquisition time of this proposed DSA-based inter-cell asynchronous DS/CDMA system, evaluated under the assumption that each of 512 cell-specific long-codes is a 10-ms segment (38400-chips for the chip rates of 3.84 Mchips/s) of complex Gold codes of period 2¹⁸-1 and the igniter sequence (or, group code) set is composed of 7 complex orthogonal Gold codes of period 256, turned out even shorter than that of the inter-cell synchronous DS/CDMA systems employing the conventional serial search method. Furthermore, simulation results showed that the DSA scheme employing a passive matched filter is much superior in terms of acquisition time and robustness, to the 3GPP W-CDMA synchronization scheme having comparable complexity     

随机M进制正交码混合DS—SFH CDMA扩频通信系统性能分析

本文研究了随机ＭＦ是制正交码混合ＤＳ－ＳＦＨ扩频码分多址信号经过多径瑞利频率非选择性衰落信道，在非相干ＲＡＫＥ接收机中采用最大输出信噪比选择或最大输出选择接收时系统的性能；给出了两种分集接收情况下差错概率表示式。数值计算研究了分集数，Ｍ值，多用户干扰对系统性能的影响，并且对这两种分集接收情况下系统性能进行了比较。     

PERFORMANCE OF MULTIUSER DETECTION FOR CDMA SYSTEM IN A FLAT RAYLEIGH FADING ENVIRONMENT

In a CDMA communication network using conventional signal detection, system capacity is limited and its performance is degraded by the multi-access interference (MAI). Multiuser detection, which makes use of cross-correlation information between spread spectrum codes, can reduce or eliminate the MAI in a Gaussian channel so as to mitigate the near-far effects and increase the system capacity. This paper extends the multiuser detector to a flat Rayleigh fading CDMA environment, and discusses the bit error rate of typical multiuser detection algorithms in such a environment by combining theoretical analysis and computer simulation. It is shown that multiuser detection is superior to conventional detections in the flat Rayleigh fading channels.     

DS-CDMA移动通信系统的捕获技术

PN (PseudoNoise)码捕获技术是DS CDMA (DirectSequence CodeDivisionMultipleAccess)蜂窝移动通信系统的关键技术。文中提出了一种基于能量窗的捕获技术 ,有效地利用了CDMA系统固有的多径分集能力 ,捕获对象是多径信号 ,研究结果表明 :对有效多径成分进行捕获有比单径捕获和跟踪有更好的性能。     

On the spectral efficiency of wideband CDMA systems

The reverse-link spectral efficiency is evaluated for a direct-sequence (DS) code division multiple access (CDMA) cellular communication system that employs rapid closed-loop power control and coding with interleaving on channels that exhibit doubly selective Rician fading. The focus of the paper is the effect of the chip rate of the DS spread-spectrum signal on the spectral efficiency of the system. Performance is considered for systems with different numbers of demodulators in the RAKE receiver and for systems both with and without antenna diversity. Channels with different delay spectra and Doppler spreads are used to examine the performance of the system in various operating conditions. The implications of the results for the design of wideband CDMA are discussed     

A Toeplitz displacement method for blind multipath estimation forlong code DS/CDMA signals

The problem of blind channel identification for direct-sequence/code-division multiple-access (DS/CDMA) multiuser systems is explored. For wideband DS/CDMA signals, multipath distortion is well modeled by a finite-impulse response filter. In this work, a blind channel identification technique based on second-order statistics is investigated. The method exploits knowledge of the spreading code of the user of interest via matched filtering, as well as properties of spreading codes. The current scheme focuses on a method appropriate for randomized long sequence DS/CDMA. This access scheme poses special challenges as the spreading codes are time varying. An analytical approximation of the mean-squared error is derived using perturbation techniques. The performance of the algorithm is studied via simulation and through the mean-squared error approximation, which is observed to be tight     

DS/CDMA closed-loop power control with adaptive algorithm

Closed-loop power control in the reverse link of a DS/CDMA system is analysed. The transmitter/receiver based on IS-95 and the radio propagation channel under mobile communication environments are modelled and a new power control algorithm is proposed. This algorithm utilises the burst error characteristics of the channel and can be directly applied to the current IS-95 system. The new power control algorithm, with respect to the SIR (signal to interference ratio), increases the service quality, and finally, enhances the system capacity     

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.     

On the chip rate of CDMA systems with doubly selective fading andrake reception

The bit error probability for a direct-sequence code division multiple access (CDMA) system is determined as a function of the chip rate for communications over doubly selective fading channels. The effect the chip rate has on the performance of a CDMA system is considered for both a correlation receiver and a rake receiver. Several selective fading channels are employed as examples to illustrate that the choice of the best chip rate depends upon the characteristics of the channel and the number of taps in the rake receiver. The tradeoff between the number of taps and the bit error probability is investigated for different chip rates. Additionally, the effects of the Doppler spread on the performance of the system and the choice of chip rate are considered. The purpose of this paper is to examine some of the fundamental issues that influence performance of CDMA systems when employed for mobile cellular telephony and personal communication systems on channels with doubly selective fading     

Channel estimation and interference suppression for uplink CDMA mobile communication systems

Wireless communications for mobile telephone and data transmission is currently undergoing very rapid development. Code division multiple‐access (CDMA) implemented with direct sequence spread spectrum signaling is among the most promising multiplexing technologies for cellular telecommunications services. In this paper, jointly period inserted pilot symbols assisted recursive (PIPSAR) channel estimation and interference suppression techniques are proposed for uplink CDMA mobile communication systems. The uplink CDMA mobile communication system model is described in the form of space‐time domain through antenna array and multipath expression. Interference suppression is achieved by using adaptive minimum mean squared error (MMSE) digital filters that span several successive received chip oversampling vectors of a symbol interval. PIPSAR techniques are used to estimate channel parameters. The correlation between the successive periods is considered to further improve the performance of the proposed scheme. Analysis and simulations are used to evaluate the performance of the proposed scheme. Copyright © 2004 John Wiley & Sons, Ltd.     

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.     

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     

Error probability analysis for CDMA systems with closed-loop powercontrol

Fast closed-loop-power-control (CLPC) plays a key role in direct-sequence (DS) code division multiple access (CDMA) systems, aiming at achieving acceptable carrier-to-interference (C/I) ratios in all active links. Assuming a terrestrial mobile access, where the round-trip delay may be smaller than the channel time-coherence, fast CLPC can compensate for fading and reduce the error burstiness, as well. This paper provides analytical expressions for the bit error probability given: the CLPC algorithm specified in terms of loop delay and updating rate, the propagation power delay-profile and the terminal speed. A binary PSK DS spread spectrum radio interface, with RAKE processing of multipath or diversity, is considered. Our link-level analysis, being conditioned to a specified average value of C/I, is focused on the inverse update algorithm, which is aimed at keeping the received power at a constant level. Applications of the derived method, aimed at getting insight about CLPC performance in 3rd generation radio interfaces, is also provided and discussed     

平坦瑞利衰落环境下CDMA系统的多用户检测性能

在采用传统信号检测方式的CDMA无线通信网中,多址干扰限制了系统的容量和性能。多用户信号检测利用扩频码互相关信息在高斯信道中可以减小或消除多址干扰,缓解远近效应,增加系统容量。本文将多用户检测器直接用于平坦瑞利衰落的CDMA环境中,用分析与仿真相结合的方法讨论了典型多用户算法的误码性能。结果表明,平坦瑞利衰落信道中多用户检测算法的性能仍然优于传统检测算法。     

Performance analysis of cellular CDMA networks overfrequency-selective fading channel

An effect of multipath fading on the performance of a cellular code-division multiple-access (CDMA) system is analyzed in this paper. A wide-sense stationary uncorrelated scattering (WSSUS) channel model and the coherent binary phase-shift keying (BPSK) with asynchronous direct-sequence (DS) spreading signal are assumed in the analysis. The average error probability for both the forward link and reverse link of a cellular CDMA system over a frequency-selective fading channel using a conventional correlation-type receiver and RAKE receiver are derived. The impact of imperfect power control and channel capacity of a cellular CDMA system is also investigated. The closed forms of average error probability derived in the paper can save a lot of computation time to analyze the performance and channel capacity of a cellular CDMA system. The analytical results show that the performance and maximum transmission rate of cellular CDMA systems degrade with an increase in the number of simultaneous users and the number of interfering cells. The signal-to-interface ratio (SIR) for the reverse link derived in this paper can directly describe the interrelationships among a number of paths, number of users, number of interfering cells, fading factors, and maximum variation of a received unfaded signal     

Decorrelating detectors for quasi-synchronous CDMA

To date, two forms of spread spectrum code division multiple access (CDMA) have been considered in the literature. Either synchronous CDMA, where all users are both chip and bit synchronized relative to each other, or asynchronous CDMA, where all users are neither chip nor bit synchronized relative to each other. Between these two extremes lies quasi-synchronous CDMA (QS-CDMA) where users are not chip synchronized but are approximately bit synchronized. Quasi-synchronous CDMA arises in microcell systems where the combined propagation time and delay spread produces a variation in the round trip delay time limited to a few chips.Detection of QS-CDMA at the base station is considered. It is shown that memoryless detection can be achieved by appending blank chips to the user's spreading codes.Two decorrelating detectors are proposed and analyzed. The first uses a matched filter bank to produce a discrete time signal, and the second uses a single oversampling integrate-and-dump filter to produce a discrete time signal. For the first detector, each matched filter is synchronized to its respective user, and for the second detector, the integrate-and-dump filter is synchronized to the base station transmissions. The performance of the two detectors is shown to be approximately equal provided the integrate-and-dump filter is sampled at least two times the chip rate.     

Throughput performance of slotted DS/CDMA ALOHA with packetcombining over generalised fading channels

A time diversity automatic repeat-request (ARQ) scheme is investigated for a slotted narrowband DS/CDMA wireless data network over a Nakagami fading channel. Numerical results reveal that the proposed adaptive retransmission diversity with packet combining provides a considerable advantage over the conventional slotted DS/CDMA ALOHA at the expense of a slight increase in implementation complexity