CDMA Mobile Radio System Research Articles

Power control in CDMA networks based on soft computing methods

Power control is a fundamental procedure for CDMA mobile radio communication systems. In multiservice CDMA systems, power control should be used to minimise the transmission power of each connection, in order to limit the multiple access interference, while obtaining the desirable SIR levels. This paper starts from a transmitted-power allocation algorithm (TPAA) that considers a set of uplink transmissions, which should be supported by the system. In the sequel, the TPAA algorithm is used for training an Elman neural network, which, due to its internal characteristics, is applicable in the time critical context of power control. Simulations and numerical results are analysed for obtaining a solid basis for employing our scheme in the power control of CDMA systems.

Joint Detection and Diversity Techniques in CDMA Mobile Radio Systems

CDMA mobile radio systems suffer from intersymbol interference (ISI) and multiple access interference (MAI) which can be combated by using joint detection (JD) techniques. Furthermore, the time variation of the radio channels leads to degradations of the receiver performance due to fading. These degradations can be reduced by applying diversity techniques. Three suboptimum detection techniques based on matched filters (MF), zero forcing (ZF) and minimum mean square-error (MMSE) equalization are considered. For further improvements, switched and equal gain diversity techniques are employed to combat fading. The performance is depicted in terms of the average bit error probability versus the average SNR per bit in a single cell environment showing an appreciable improvement over the non diversity situation. Theoretical results for the SNR at the front end of the receiver and the BER for ideal channel are obtained and compared with the simulation results.

Novel SIR-estimation-based power control in a CDMA mobile radio system under multipath environment

A novel decisive algorithm based on signal-to-interference power ratio estimation has been proposed to control the power transmission of the mobiles in a code-division multiple-access mobile radio system under multipath environment. A simple second-order equation, together with three different sets of measurement and power-control periods, is first used to predict the power gains of the channels. Based on the predicted power gains, five decision rules have been proposed to determine the power-control commands for the mobiles. The novel algorithm with the best prediction method and decision rule has been observed to accomplish a lower outage probability when compared with the conventional power-control algorithm with optimum power-control threshold.

Joint transmission: advantageous downlink conceptfor CDMA mobile radiosystems using time division duplexing

A novel downlink transmission scheme for time-slotted CDMA employing time division duplexing (TDD) is proposed. It utilises the knowledge of the channel impulse responses at the BS transmitter in such a way that at the receivers of the MSs channel estimators are no longer required and the computational expense of data detection is dramatically reduced.

Outage statistics in CDMA mobile radio systems

The statistics of the outage events in the reverse link of a direct sequence code-division multiple access (DS/CDMA) wireless system are investigated taking into account the effects of correlated Rician fading. The outage probability and the average fade duration are derived by assuming a constant multipath intensity profile, which includes a direct-path component in the first path. Simulations have been carried out which validate the analytical results.

Intelligent closed-loop power control algorithmin CDMA mobile radio system

A novel decisive algorithm has been developed for controlling the power transmission of the mobiles in a CDMA mobile radio system. The new algorithm has been found to achieve a lower outage probability when compared with the conventional power control algorithm with optimum power control threshold.

Adaptive multiuser detection and beamforming for interference suppression in CDMA mobile radio systems

This paper considers the problem of interference suppression in direct-sequence code-division multiple-access (DS-CDMA) systems over fading channels. An adaptive array receiver is presented which integrates multiuser detection, beamforming, and RAKE reception to mitigate cochannel interference and fading. The adaptive multiuser detector is formulated using a blind constrained energy minimization criterion and adaptation is carried out using a novel algorithm based on set-membership parameter estimation theory. The proposed detector overcomes the shortcomings of conventional LMS- and RLS-type algorithms, namely, that of slow convergence and large computational load, respectively. This is especially the case when strong interferers are present or when the number of adaptive weights is relatively large. DS-CDMA systems can have a relatively large number of spatially distributed interferers. Thus beamforming is based on direction-of-arrival (DOA) estimates provided by an approximate maximum-likelihood estimator (DOA-MLE). Unlike previous approaches, the DOA-MLE exploits the structure of the DS-CDMA signaling scheme resulting in robust performance and simple implementation in the presence of angle spreading. The overall method is suitable for real-time implementation and can substantially improve the interference suppression capabilities of a CDMA system.

Smart Antennas with Two-Dimensional Array Configurations for Performance Enhancement of a Joint Detection CDMA Mobile Radio System

Smart antennas for base stations of cellular mobile radio systems offer the potential of system performance enhancement by taking advantage of the directionally inhomogeneous signal reception at the receiver. In this paper, two-dimensional array configurations employed at the uplink receiver of a joint detection CDMA (JD-CDMA) mobile radio system are investigated. This smart antenna concept can be split up into a novel channel estimator and data detector which incorporate explicitely the information of the direction-of-arrival (DOA) of signals emerging from users assigned to the considered base station. Proceeding from channel models that model the directional inhomogeneity of the mobile radio channel with single DOAs, the link level performance of a JD-CDMA mobile radio system using this smart antenna concept is evaluated for the rural propagation environment. The performance evaluation is based on Monte Carlo simulations of data transmission and average bit error rates versus the average signal to noise ratio per net information bit are presented for different array configurations. Although these results should be considered as upper bounds for the link level performance, they reveal the advantages of implementing two-dimensional array configurations at the uplink receiver of a JD-CDMA mobile radio system.

A Generalized View on Multicarrier CDMA Mobile Radio Systems with Joint Detection (Part II)

This communication presents a generalized view on multicarrier code division multiple access (MC-CDMA) concepts for mobile radio applications. A comprehensive introduction to CDMA and the arising multiple access interference (MAI) will be presented. It will be explained why joint detection (JD) is a mandatory prerequisite for hybrid frequency and time divided code division multiple access (F/T/CDMA) which is a viable basis for third generation mobile radio systems. The benefits of MC-CDMA over other CDMA concepts will be discussed in an illustrative manner and a flexible MC-CDMA concept will be introduced and explained in detail. The signal generation in MC-CDMA will be described in continuous-time and discrete-time form revealing the close relationship between the presented flexible MC-CDMA and the well known joint detection CDMA (JD-CDMA) concept. The discrete-time transmission model which results from the continuous-time representation will be introduced to facilitate digital signal processing. This transmission model takes coherent receiver antenna diversity (CRAD) explicitly into account. Four viable JD techniques suitable for the discussed flexible MC-CDMA concept will be presented. Their structure and mode of operation will be explained in detail and their uplink performance will be studied in a variety of transmission conditions.

A Generalized View on Multicarrier CDMA Mobile Radio Systems with Joint Detection (Part I)

A Generalized View on Multicarrier CDMA Mobile Radio Systems with Joint Detection (Part I)

Computation of the outage probability in rice fading radio channels

AbstractMethods for the evaluation of the outage probability in mobile radio systems have been widely investigated, but analytical results have been published only for Rayleigh fading radio channels. For the case of Rice fading, only approximated results have been found. In this paper, an exact expression of the outage probability for a subscriber of a CDMA cellular mobile radio system, with the radio channel impaired by Rice fading and log‐normal shadowing superimposed is given.

Engineering of a CDMA radio uplink for a mobile radio demonstration system

A CDMA mobile radio demonstration system has been created by a joint industry/university project in the UK. The paper describes the engineering of the reverse link (from mobile to base station). The link hardware is described. The main features of interest are the power control mechanism and the all-digital rake receiver. Results presented illustrate the BER performance over a multipath channel simulator and the power control mechanism performance.

Results on turbo-codes for speech transmission in a joint detection CDMA mobile radio system with coherent receiver antenna diversity

Turbo-codes which are applicable to speech transmission in digital mobile radio systems are treated. Three turbo-codes of different complexity are presented. The proposed turbo-codes are suitable for the application to speech transmission in the joint detection code-division multiple access (JD-CDMA) mobile radio system with coherent receiver antenna diversity (CRAD) which are described concisely. The performance of the designed turbo-codes in terms of bit and frame error rates are shown in the case of additive white Gaussian noise (AWGN) channels, flat Rayleigh fading channels, and in the uplink of the aforementioned JD-CDMA mobile radio system.

Adaptive fuzzy power control for CDMA mobile radio systems

Introduces a new application of fuzzy-logic control (FLC) to power control in a DS/CDMA cellular system over mobile fading radio channels. The conventional feedback power control algorithms allow the base station to send a power command to raise/lower each user transmitting signal power level by a fixed power step and then keep the received powers almost equal. The fixed-step approach is actually an integral control whose power increment is determined according to the bang-bang-like control policy. However, this control scheme suffers from several disadvantages. To tackle these difficulties, a fuzzy proportional-plus-integral (PI) control, whose input variables are the received power error and error change, is introduced to determine each user's transmitting power in order to maintain simultaneously all users' signal power received at the base station nearly equal and to achieve better system stability and control performance. The derivation of the fuzzy PI control has been carried out by analyzing both the closed-loop steady state behavior and transient response of the system with a priori knowledge of the dynamics of the CDMA mobile fading channels. In fuzzy control, linguistic descriptions of actions in controlling a process are represented as fuzzy rules. This fuzzy-rule base is used by an inference mechanism in conjunction with some knowledge of the states of process in order to determine control actions. These actions would lead to the fast rise time, minimum overshoot, and small root-mean-squared (rms) tracking error. Additional advantages of fuzzy PI control over conventional control theories are discussed.

Analysis of a direct-sequence CDMA mobile radio system with reduced set of code sequences

A code-division multiple-access scheme that requires a reduced set of code sequences, in which the size of the set is much less than the number of users in the system, is proposed for mobile radio communications. A direct-sequence spread-spectrum signaling is adopted to exploit the property of high time resolution and combat fading effects through multipath diversity reception. Theoretical performance of the proposed system is analyzed by deriving the collision-free path distribution and determining the proper size of the set without reducing the effect of multipath diversity. In connection with the proposed system, two combining schemes are investigated to characterize the effects of the primary and secondary user interferences and evaluate the behavior of the error rate when compared to the original CDMA system.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the detection of bandlimited direct-sequence spread-spectrum signals transmitted via fading multipath channels

Recent investigations suggest bandlimited direct-sequence spread-spectrum transmission for use in CDMA mobile radio systems. In this paper, transmission systems with signal bandwidths not exceeding the chip rate are considered. First, the optimum noncoherent single-user receiver and various suboptimum approximations thereof are derived. These receivers utilize the fact that the received signals can be sampled at chip rate. This results in receiver concepts, which are well suited for fully digital implementations. Then, it is shown that the main result on the structure of the optimum single-user receiver is directly applicable to optimum noncoherent multiuser joint detection and interference cancellation. In this paper, noncoherent detection is considered. By this, we mean that only the channel statistics and not the channel itself is known to the receiver. It is shown that due to this less restrictive assumption, more powerful channel coding and interleaving schemes can be employed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

An investigation of space-path hybrid diversity scheme for base station reception in CDMA mobile radio

In CDMA mobile radio communication systems, degradation of instantaneous signal to interference power ratio (C/I), which causes impairment of frequency utilization factor, will be significant in multipath fading environments. In this paper, a hybrid diversity scheme for fading reduction combining effects of space diversity and path diversity (represented by the RAKE method) is investigated. A quantitative evaluation of fading reduction effects of the hybrid diversity is performed, comparing them with those of space diversity only and path diversity only. The hybrid diversity scheme is promising in environments where the delay spread of a transmission path is 1 /spl mu/s or less.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Optimum and suboptimum channel estimation for the uplink of CDMA mobile radio systems with joint detection

AbstractIn the present paper, channel estimation in the uplink of CDMA mobile radio systems with burst transmission applying joint detection is considered. The different radio channels are estimated at the receiver using known midamble codes. Either maximum‐likelihood, i.e. optimum, or suboptimum channel estimation can be realized at the receiver. Several methods for constructing code sets for the simultaneously active users are proposed. It is shown that the structure of the channel estimator for a multi‐user synchronous CDMA mobile radio system is the same as for a single‐user system when the mid amble codes of the users are segments derived from a single basic code. Furthermore, it is shown that the channel estimator can be implemented as a correlator when the midamble codes are constructed from a periodic basic code. Finally, the effect of noisy channel estimation, i.e. the presence of channel estimation errors, on the detected data is examined.

Simulation of the uplink of JD-CDMA mobile radio systems with coherent receiver antenna diversity

Due to time variant multipath propagation, both intersymbol interference and multiple access interference occur at CDMA receivers. These degrading effects can be combatted by joint detection (JD) techniques. In order to reduce the performance impairments resulting from time variance, coherent receiver antenna diversity (CRAD) can be used. In the present paper, a system model of CDMA mobile radio systems using various JD techniques in combination with CRAD shall be considered. This system model is an evolution of the pan-European GSM and takes important real world aspects such as imperfect channel estimation, nonlinear amplification and D/A and A/D conversion into account. The viability of JD with CRAD shall be demonstrated by bit error rate simulations of this system model. It is shown that by using JD with two receiver antennas in bad urban areas,Eb/N0 < 8 dB per antenna is sufficient for a bit error rate of 10−2, andEb/N0 < 11 dB per antenna is required for a bit error rate of 10−3.

Low Cost Channel Estimation in the Uplink Receiver of CDMA Mobile Radio Systems

Low Cost Channel Estimation in the Uplink Receiver of CDMA Mobile Radio Systems