Performance of DS-CDMA receivers with MRC in nakagami-m fading channels with arbitrary fading parameters

The receivers that combine spatial antenna diversity with temporal multipath diversity are known as two-dimensional (2-D) RAKE receivers. In this paper, we consider the outage probability and the bit error rate performance of a coherent binary phase shift keying 2-D RAKE receiver in the context of an asynchronous direct sequence (DS)-code division multiple access (CDMA) system operating in a Nakagami-m fading channel with real and arbitrary fading parameters. The closed-form expressions derived for the two wireless performance measures are easily evaluated numerically and enable the link designer to examine the effects of system parameters, such as the number of receive antennas, RAKE fingers per antenna, and asynchronous CDMA users in the cell, as well as channel conditions, such as the amount of fading in the combined paths and the multipath intensity profile of the channel on the link performance. In addition, the diversity loss due to correlated fading among the spatially separated RAKE fingers is quantified.     

Performance of antenna diversity multiuser receivers in CDMA channels with imperfect fading estimation

The performance of antenna diversity coherent and differentially coherent linear multiuser receivers is analyzed in frequency-nonselective Rayleigh fading CDMA channels with memory. The estimates of the complex fading processes are utilized for maximal-ratio combining and carrier recovery of the coherent multiuser receiver. To analyze the impact of channel estimation errors on the receiver performance, error probability is assessed directly in terms of the fading rate and the number of active users, showing the penalty imposed by imperfect channel estimation as well as the fading-induced error probability floor. The impact of fading dynamics on the differentially coherent decorrelating receiver with equal-gain combining is quantified. While performance of multiuser receivers at lower SNR is determined by both the fading dynamics and the number of active CDMA users, performance at higher SNR is given by an error probability floor which is due to fading only and has the same value as in a single-user case. The comparison of the two receiver structures indicates that the coherent decorrelating receiver with diversity reception may be preferable to the differentially coherent one in nonselective fading CDMA channels with memory.     

Effects of Nakagami-Fading Parameters and Power Control Error on Performance of DS-CDMA Cellular Systems with Adaptive Beamforming

It is well known that power control error (PCE) is a critical issue in CDMA cellular systems. In this paper, the bit error rate (BER) of a direct sequence-code division multiple access (DS-CDMA) receiver with imperfect power control, adaptive beamforming, and voice activity is derived in frequency-selective Nakagami fading channels. We discuss the effects of PCE, Nakagami-m fading parameter, and channel’s multipath intensity profile as average signal strength and rate of average power decay and their effects on the BER performance of DS-CDMA cellular systems. In this paper, the RAKE receiver consists of three stages. In the first stage, with conjugate gradient adaptive beamforming algorithm, the desired users’ signal in an arbitrary path is passed and the inter-path interference is canceled in other paths in each RAKE finger. Also in this stage, the multiple access interference (MAI) from other users is reduced. Thus, the matched filter (MF) can be used for the MAI reduction in each RAKE finger in the second stage. In the third stage, the output signals from the MFs are combined according to the conventional maximal ratio combining principle and then are fed into the decision circuit for the desired user. How the Nakagami fading parameters, power control imperfections, or the number of resolvable paths affect the reverse link capacity of the system is discussed in detail. Analytical and simulation results are also given for systems with different processing gains and number of BSs in the cell-selection process with various Nakagami fading parameters.     

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     

Performance analysis of a DS/CDMA system with noncoherent M-aryorthogonal modulation in Nakagami fading

The probability of error performance of a direct sequence code-division multiple-access (DS/CDMA) system employing noncoherent M-ary orthogonal signaling in a Nakagami multipath fading channel is analyzed. A RAKE receiver structure with square-law demodulation is used at the receiver. The multiple-access interference are modeled as Gaussian and expressions derived for the exact probability of error. The performance is also evaluated in terms of the number of users that can be supported by the system at a given probability of error. The effect of correlated fading on system performance is also investigated by considering two correlation models, which can be characterized by a single correlation coefficient ρ. In the first model, the correlation coefficient between any two diversity branches is constant. In the second model, it is assumed that the correlation coefficient between any two diversity branches decreases exponentially as the separation between them increases. For both models, it is found that the presence of correlation deteriorates system performance. The use of larger signal alphabets than binary modulation in conjunction with diversity reception provides a considerable performance improvement even in the presence of correlated fading     

Average block error probability in the reverse link of a packetDS/CDMA system under Rayleigh fading channel conditions

Analytic expressions have been derived for the probability density functions of the total signal-to-interference ratio when considering mixed-type interference sources in the reverse link of a DS/CDMA packet radio system with Rayleigh fading. Four different scenarios were studied, and the corresponding expressions are presented for each situation. The users are divided into two groups: those that apply a closed-loop power control and those that only use an open-loop power control. By making use of these expressions, the mean block error probability is derived as a function of the number of total interfering users in each group     

Multicarrier CDMA with adaptive frequency hopping for mobile radiosystems

A modified multicarrier (MC) direct-sequence code-division multiple-access (DS-CDMA) system has been proposed for use over slow multipath fading channels with frequency selectivity in the reverse link transmission of a cellular network. Instead of transmitting data substreams uniformly through subchannels, data substreams hop over subchannels with the hopping patterns adaptively adjusted to the channel fading characteristics. The problem of determining the optimal hopping pattern is formulated as a multiobjective optimization problem, for which an efficient algorithm, based on the water-filling (WF) principle, is designed to solve the problem practically. Simulation results show that the performance in terms of the average bit-error probability (BEP) (over all users) is better than that of single carrier RAKE receiver systems, conventional MC CDMA systems applying moderate error protection, or diversity systems with different combining techniques     

IS-95反向信道在Nakagami(m)多径衰落信道下的性能分析

IS-95是北美制定的基于CDMA技术的无线空中接口标准,本文给出了IS-95反向信道调制和解调过程的数学模型。一般认为Nakagami(m)分布较好地描述了多径衰落信道,本文分析了IS-95反向信道在Nakagami(m)衰落信道下的误码率,结果可以用来综合分析系统和信道的不同参数对系统性能的影响。     

Coded modulation for a coherent DS-CDMA system employing an MMSE receiver in a fading channel

Previous results have shown that high rate codes tend to yield a lower average bit-error rate than low rate codes when employing a minimum mean-square error (MMSE) receiver for a direct-sequence code-division multiple-access (CDMA) system in either an additive white Gaussian noise channel or a flat Rayleigh fading channel. we consider the use of larger signal constellations with both trellis-coded modulation and bit-interleaved coded modulation (BICM) to determine if further gains can be achieved in either the Rayleigh or Ricean fading channel. The average bit-error probability is derived for both coding schemes using the general Ricean fading channel model, based upon the common assumptions of infinite interleaving, perfect channel state information, and optimal MMSE receiver coefficients. New bounds are presented for BICM with 8-PSK and 16-QAM symbols, which take advantage of the symmetries inherent in the signal constellations with Gray code mapping. In addition, simulation results are presented which show the important effect a finite interleaving delay constraint has on the comparison of various codes. The results show that there are cases when coded modulation does yield a significant improvement in performance for a CDMA system using an MMSE receiver, compared to standard convolutional coding. However, the best coding strategy depends upon several factors, including the nature of the fading process (Rayleigh or Ricean), the operating signal-to-noise ratio, the interleaving delay constraint, the time-variability of the channel, the number of users in the system, and the severity of the near-far problem.     

Optimal reception, performance bound, and cutoff rate analysis ofreferences-assisted coherent CDMA communications with applications

Coherent spread-spectrum communications with transmitted reference-based, also called pilot-based, channel estimation is considered for code-division multiple-access (CDMA) communications over fading channels. Both noncontiguous time-division multiplexed reference symbol-based and continuous code-division multiplexed channel-based schemes are described. Assuming mean square error or optimal robust channel estimation, we derive the optimal receiver structure with a maximum-likelihood convolutional decoder. In addition, the Bhattacharyya bound and the cutoff rate of the reference-assisted coherent communications are derived. These analytical results are used for evaluating system performance and for selecting parameters such as coding rate and the data to reference power ratio to optimize system performance. Simulation results are given showing that the reverse link performance in a CDMA system can be significantly improved by using the reference-assisted coherent communication instead of noncoherent reception of orthogonally coded signal     

Linear multipath-decorrelating receivers for CDMAfrequency-selective fading channels

Analytical expressions for the error probability of linear multipath-decorrelating receivers with coherent and differentially coherent reception are derived. Both multi-user receivers have superior performance compared to the RAKE receiver and eliminate error probability floor caused by multiple-access interference on a code-division multiple access (CDMA) reverse link. Results also emphasize benefits of using coherent multi-user reception with maximal-ratio combining in frequency-selective fading channels     

基于智能天线阵接收的蜂窝CDMA网络性能分析

本文分析了在频率选择性信道中基于智能天线阵接民的蜂窝CDMA网络上行链路的性能,分析基于多个小区的异步蜂窝CDMA系统,系统采用BPSK调制,并采用智能天线阵接收的RAKE合并技术,通过分析最终得到了在该环境下的闭合的CDMA系统误码率公式,结果表明基于智能天线阵接收的蜂窝CDMA性能要比不采用的好得多,并给出了误码率与系统用户数,小区数和衰落模型之间的关系,结果对于分析蜂窝CDMA系统容量具有一定的指导意义。     

An efficient receiver scheme for downlink ZP-CDMA

A signal transmitted through a wireless channel may be severely distorted by intersymbol interference (ISI) and multiple access interference (MAI). In this paper, we propose an efficient CDMA receiver based on frequency domain equalization (FDE) with a regularized zero forcing (RZF) equalizer and parallel interference cancellation with a unit clipper decision function (CPIC) to combat both the ISI and the MAI. We call this receiver the FDE-RZF-CPIC receiver. This receiver is suitable for downlink zero padding CDMA cellular systems. The effects of the decision function, the channel estimation, the number of cancelled users, and the user loading on the performance of the proposed receiver are discussed in the paper. The bit error rate (BER) of the data received by the proposed receiver is evaluated by computer simulations. The experimental results show that the proposed receiver provides a good performance, even with a large number of interfering users. At a BER of 10^?3, the performance gain of the proposed receiver is about 2 dB over the RAKE receiver with a clipper decision function and PIC in the half-loaded case (eight users) and is much larger in the full-loaded case (16 users).     

Optimal power allocation in DS-CDMA with adaptive SIC technique

This paper proposes an optimal power allocation in direct sequence-code division multiple access (DS-CDMA) system. The objective is to minimize total transmit power, while simultaneously meeting the certain sum channel capacity (data transmission rate) and outage probability constraints on Rayleigh fading channel. Then a weighted correlator with an adaptive successive interference cancelation (SIC) scheme is developed using neural network (NN) for an improvement in receiver performance. A closed mathematical form of joint probability of error (JPOE) is derived. This determines the number of active users’ interfering effect that needs to be canceled in order to achieve a desired bit error rate (BER) value. Mathematical analysis shows that better receiver performance can be achieved through large change in weight up-gradation (w) for the strong users with a particular change in learning rate (η). Simulation results in terms of sum capacity as well as weak user’s (users with poor channel gain) capacity, outage probability and BER performance duly support the effectiveness of the proposed scheme over the existing works.     

Decision feedback equalization for CDMA in indoor wirelesscommunications

Commercial interest in Code Division Multiple Access (CDMA) systems has risen dramatically in the last few years. It yields a potential increase in capacity over other access schemes, because it provides protection against interference, multipath, fading, and jamming. Recently, several interference cancellation schemes for CDMA have been proposed but they require information about all interfering active users or some channel parameters. The authors present an adaptive fractionally spaced decision feedback equalizer (DFE) for a CDMA system in an indoor wireless Rayleigh fading environment. This system only uses information about the desired user's spreading code and a training sequence. An analysis on the optimum performance of the DFE receiver shows the advantages of this system over others in terms of capacity improvements. A simulation of this system is also presented to study the convergence properties and implementation considerations of the DFE receiver. Effects on the performance because of sudden birth and death of users in the CDMA system and bit error rate performance of the DFE receiver is also presented     

Performance of multicarrier CDMA with DPSK modulation anddifferential detection in fading multipath channels

Multicarrier (MC) direct sequence (DS) code division multiple access (CDMA) with differential phase-shift keying (DPSK) modulation and differential detection is proposed. Transmitted data bits are differentially encoded after serial-to-parallel conversion to a number of parallel streams. On each branch, encoded bits are direct sequence spread spectrum (SS) modulated and transmitted using different carriers. The system is analyzed with a differential detector in static Rayleigh fading multipath channel, in fast Rayleigh fading multipath channel and for variable overlapping between carrier spectra in static fading channel. Closed-form expressions are derived for the error probability and evaluated for many cases. The performance is compared to that of a system using phase-shift keying (PSK) with conventional matched filter (CMF) coherent receiver. For static fading channel, the error probability performance of the differential detector is close to that of CMF receiver. For fast fading, the performance degrades slightly with increasing fading rate. Finally; successive carriers of the system are allowed to overlap with various overlapping percentages. The condition of a single path can be achieved by increasing both the number of carriers and the separation between successive carriers. Also, for each number of carriers, there exists an optimum overlapping percentage at which the system performance is optimized. The performance of the proposed DPSK with differential detection system is close to that of PSK with CMF receiver, but the former is simpler to implement     

DS/CDMAM-ary orthogonal signalling in a shadowed Rayleigh channel for mobile communications

In this paper a modified RAKE receiver is studied for a frequency selective mobile radio channel. The reverse link (Mobile to base station) is analysed, assuming lognormal shadowing and Rayleigh fading andK asynchronous users, withM orthogonal sequences per user. The analysis is based on the consideration of the quadrature components of the signal and noise, taking advantage of the multipath effects. The performance evaluation is carried out in terms of both the bit error rate and outage probability in order to qualify completely the proposed receiver. The positive results assure the possibility of applying this system in a microcellular mobile radio environment.     

Multicarrier CDMA for cellular overlay systems

A code-division multiple access (CDMA) cellular overlay system is investigated, employing the idea of multicarrier CDMA, which has previously received significant attention as an alternative to traditional single-carrier CDMA. Overlay is pursued here as a means of long-term transition from narrowband cellular to CDMA cellular. A major result of this paper is the demonstration that the use of multicarrier CDMA in a fading channel is particularly beneficial to the narrowband system, as the CDMA users can reduce their transmitted powers as a result of diversity. Another significant conclusion is that the use of transmitter notching in the CDMA system in order to avoid active narrowband users outperforms a strategy in which a narrowband user is avoided by simply dropping the particular carrier which overlays it. Finally, results on the use of the minimum mean-squared error (MMSE) receiver in a fading channel are extended for use in the overlay scenario     

Selection diversity for wireless communications in Nakagami-fadingwith arbitrary parameters

By means of analytical and numerical methods, the probability of error and the outage probability of a selection diversity RAKE receiver system employing direct sequence/code division multiple access (DS/CDMA) is derived. A noise-limited propagation environment is modeled as a Nakagami (1960)-fading channel with arbitrary fading parameters and unequal mean power at the receiver. New analytical expressions are derived for the average probability of error and outage probability. Binary detection schemes are considered including binary phase-shift keying (PSK) and frequency-shift keying (FSK). Both coherent and noncoherent detection is considered as well as identical and arbitrary fading. It is shown that the effect of arbitrary fading on system performance is significant and may not be ignored     

Selective partial PIC for wireless CDMA communications

This paper deals with a cancellation multiuser detector for CDMA communication systems. The proposed receiver, defined as selective partial parallel interference cancellation (SP‐PIC), is supposed to be used at the end of an up‐link channel characterized by multipath fading phenomena. The SP‐PIC main feature is to perform a weighted selective cancellation of the co‐channel interfering signals according to the received power level. With respect to other approaches, the proposed detector exhibits an improved bit error rate (BER) and a low computational complexity, linear with the number of users. Copyright © 2003 John Wiley & Sons, Ltd.