Code Division Multiple Access Cellular Networks Research Articles

Call admission control in wideband CDMA cellular networks by using fuzzy logic

In this paper, a novel call admission control (CAC) scheme using fuzzy logic is proposed for the reverse link transmission in wideband code division multiple access (CDMA) cellular communications. The fuzzy CAC scheme first estimates the effective bandwidths of the call request from a mobile station (MS) and its mobility information, then makes a decision to accept or reject the connection request based on the estimation and system resource availability. Numerical results are given to demonstrate the effectiveness of the proposed fuzzy CAC scheme in terms of new call blocking probability/handoff call dropping probability, outage probability, and resource utilization.

Performance Evaluation of Hierarchical Cellular CDMA Networks With Soft Handoff Queueing

We consider hierarchical cellular code-division multiple-access networks supporting soft handoff, where users with different mobility are assigned to different layers, i.e., microcells in the lower layer are used to carry slow users, whereas macrocells in the upper layer are for fast users, and handoff queues are provided for handoff calls that cannot obtain the required channel immediately, so that forced termination probability can be reduced. According to whether handoff queues are provided in microcells and/or macrocells, four different call admission control schemes are proposed and studied. We derive the mathematical model of the considered system with multidimensional birth-death process and utilize Gauss-Seidel iterative method to find the steady-state probability distribution and thus the performance measures of interest: new call blocking probability, handoff failure probability, and forced termination probability. Analytical results show that providing handoff queues in both microcells and macrocells can achieve largest performance improvement. Furthermore, handoff queue size greater than a threshold is shown to have little effect on performance measures of interest. Last but not least, the studied two-tier system is compared with a one-tier counterpart. It is shown that the two-tier system performs better in terms of average number of handoffs per fast call.

Effect of Power Control Imperfections on the Reverse Link of Cellular CDMA Networks Under Multipath Fading

In this paper, we present an analytical approach for the evaluation of the impact of power control errors on the reverse link of a multicell direct sequence code-division multiple access (CDMA) system with fast power control under multipath fading. Unlike many previous papers, the joint effect of multipath fading and fast power control on interference statistics is explicitly accounted for and mobiles are assumed to connect to a base station according to a minimum attenuation criterion. Both the average bit error rate (BER) and the outage probabilities that a user experiences are estimated. The results have been used to evaluate the system capacity from two points of view. First, the maximum capacity supported by the system in order to maintain an average BER below a prescribed level has been calculated. Second, the maximum capacity possible to ensure that the outage probability does not exceed a set limit is analyzed. Capacity is shown to be significantly affected by the imperfections of power control. Our results can be used to quantify the relative capacity loss due to fast power control errors in a cellular CDMA network affected by slow fading, multipath fading, and cochannel interference.

An adaptive resource reservation strategy for handoff in wireless CDMA cellular networks

A threshold-based adaptive resource reservation scheme for soft handoff calls in wireless code division multiple access (CDMA) cellular networks is proposed. The scheme gives handoff calls a higher admission priority than new calls, and is designed to adaptively adjust the reservation-request time threshold according to the varying traffic load. The individual reservation requests form a common reservation pool, and handoff calls are served on a first-come, first-served basis. With the proposed scheme, a minimum grade of service (GoS) in terms of new call blocking and handoff call dropping probabilities can be achieved. Simulation results are given to demonstrate the improvement in the GoS performance in comparison with other schemes.

Dynamic Fair Scheduling With QoS Constraints in Multimedia Wideband CDMA Cellular Networks

A class of dynamic fair scheduling schemes based on the generalized processor sharing (GPS) fair service discipline, under the generic name code-division GPS (CDGPS), is proposed for a wideband direct-sequence code-division multiple-access (CDMA) cellular network to support multimedia traffic. The CDGPS scheduler makes use of both the traffic characteristics in the link layer and the adaptivity of the wideband CDMA physical layer to perform fair scheduling on a time-slot by time-slot basis, by using a dynamic rate-scheduling approach rather than the conventional time-scheduling approach. Soft uplink capacity is characterized for designing an efficient CDGPS resource allocation procedure. A credit-based CDGPS (C-CDGPS) scheme is proposed to further improve the utilization of the soft capacity by trading off the short-term fairness. Theoretical analysis shows that, with the C-CDGPS scheme, tight delay bounds can be provided to delay-sensitive traffic, and short-term unfairness can be bounded so that long-term weighted fairness for all users can still be satisfied. Simulation results show that bounded delays, increased throughput, and long-term fairness can be achieved for both homogeneous and heterogeneous traffic.

Forward-link soft-handoff in CDMA with multiple-antenna selection and fast joint power control

We consider forward-link soft-handoff with multiple antenna selection and fast joint power control at high data rates in a cellular code-division multiple-access network, where signals are directed to a mobile station (MS) from antennas located at the same or different base stations. The total power transmitted to any mobile is divided among the active antennas selected according to the momentary channel conditions so as to maximize the signal-interference ratio at each MS. Multiple-antenna selection is used to mitigate the effects of both short- and long-term fading, and achieve the best soft-handoff with respect to system capacity and complexity. To achieve capacity gains with soft-handoff, we derive optimum handoff thresholds corresponding to the optimum handoff region in different cell environments. Numerical results demonstrate that under high Doppler spread and large handoff-delay conditions, the proposed soft-handoff employing two transmit antennas and the optimum handoff threshold achieves a significant gain in microcell environments, but not in macrocell environments.

Performance analysis of soft handoff in CDMA cellular networks

A unique feature of code division multiple access (CDMA) systems is the use of soft handoff between cells. Soft handoff, in general, increases the system capacity because while the link between a mobile and one base station is poor, it might be better between the same mobile and some other base station. Hence, the user may transmit at a lower power in a soft handoff situation. Teletraffic analysis of soft handoff is complex because one cannot separate transmission issues from traffic issues. Many papers in the literature have independently analyzed the effect of soft capacity and soft handoff on network performance. Some papers have analyzed the effect of soft handoff on soft capacity but there has been no proper teletraffic analysis that includes both soft capacity and soft handoff. This paper proposes a traffic model for a DS-CDMA cellular network that includes both soft capacity and soft handoff. Network performance is then computed in terms of call blocking.

A framework for analyzing the user membership in cellular CDMA networks

A theoretical framework is developed to analyze the user membership in cellular networks. The main interest is on code-division multiple-access (CDMA) networks, although the validity of the proposed approach is not restricted to these networks. The membership of a user terminal to a certain base station (BS) is established if the local mean attenuation to this BS is the lowest one among all surrounding BSs. The estimate of local mean attenuation on the radio links from a mobile terminal to each of the surrounding BSs is carried out through proper filtering of the sequences of received signal strength measurements taken on broadcast signals at regular time intervals. In CDMA networks, pilot channels can be considered, while broadcast control channels can be generically considered in narrow-band systems. A decision is made each time on whether the terminal is to be assigned to its "current" BS or if it would be better served by a different BS. Two performance measures are considered: the probability that the best BS is not selected and the average number of membership switchings per time unit. A channel model with log-normal shadowing and fast fading superimposed is considered. Analytical expressions for the above-mentioned performance measures are derived in a general system scenario. From numerical results, it is seen that a proper choice of filter parameters is required to achieve the lowest values for both performance measures. Moreover, these values are not negligible when the mobile terminal is close to a boundary cell region. The obtained results are considered to be of interest in handover and power control algorithms.

High data-rate packet communications for cellular networks using CDMA: algorithms and performance

The advantages of code division multiple access (CDMA) for cellular voice have become well known, and IS-95-based systems have now been widely deployed. Attention is now focused on higher data-rate packet services for cellular systems. Although many packet multiple access schemes have been studied over the years, researchers have often studied single cell performance and ignored reuse. Moreover, direct sequence spread spectrum (DSSS) has been considered unsuitable for high data-rate packet multiple access since spreading limits the permitted data rates, DSSS requires large overhead (preambles) for acquisition and requires closed-loop power control. In this paper, we describe a scheme for high data-rate packet service using CDMA that addresses all of the above problems and has been standardized in Revision B of IS-95. A low rate fundamental code channel is maintained that eliminates the need for long preamble and provides closed-loop power control. Reuse is managed by the infrastructure through a "burst-level" admission control based on load and interference-level measurements at the base stations and mobiles. We report on the feasibility of such a burst-mode packet data service for cellular CDMA networks. The focus is not only on the performance of high data-rate users, but also on the impact on voice users sharing the CDMA band. We propose a multitiered performance analysis methodology consisting of a mix of static simulations, dynamic simulations at different time scales, and analytic methods to address the various feasibility issues: impact on coverage; capacity; power control; and effectiveness of burst admission algorithms. Based on the current study, we can conclude that the proposed approach is well suited for third-generation wideband CDMA systems being considered for standardization throughout the world.

On the teletraffic capacity of CDMA cellular networks

The aim of this paper is to contribute to the understanding of the teletraffic behavior of code-division multiple-access (CDMA) cellular networks. In particular, we examine a technique to assess the reverse link traffic capacity and its sensitivity to various propagation and system parameters. We begin by discussing methods of characterizing interference from other users in the network. These methods are extremely important in the development of the traffic models. We begin with a review of several existing approaches to the problem of handling other-cell interference before presenting a novel characterization of the interference in the form of an analytic expression for the interference distribution function in the deterministic propagation environment. We then look at extending the capacity analyses that assume a fixed and equal number of users in every cell to handle the random nature of call arrivals and departures. The simplest way to do this is by modeling each cell of the network as an independent M/G/x/spl infin/ queue. This allows us to replace the deterministic number of users in each cell by an independent Poisson random variable for each cell. The resulting compound Poisson sums have some very nice properties that allow us to calculate an outage probability by analyzing a single random sum. This leads to a very efficient technique for assessing the reverse link traffic capacity of CDMA cellular networks.

Effective bandwidth-based admission control for multiservice CDMA cellular networks

We develop product form traffic models for single- and multiple-cell code-division multiple-access (CDMA) networks with multiple classes of mobile subscribers. The key feature of this development is the specification of a flexible call admission control procedure that details the numbers of mobiles of each class in each cell that the system operator should allow in order to maintain an acceptable quality of service. Effective bandwidth techniques from the analysis of statistical multiplexing at an asynchronous transfer mode (ATM) based broadband integrated services digital network (ISDN) link are used to give performance guarantees that overcome the variability in interference levels characteristic of CDMA cellular networks. The result is an admissible region bounded by a finite number of hyperplanes and a simple and efficient call admission policy. The CDMA mobile network, operating within the admissible region described, has a very similar form to a circuit-switched network operating with fixed routing. This similarity allows the existing traffic modeling techniques and network management strategies for general loss networks to be applied to CDMA mobile cellular networks. In particular, with standard assumptions on the call arrival processes and holding times, the stationary state distribution has a product form on the truncated state space defined by the call admission strategy.

Performance analysis of a double-dwell serial search technique for cellular CDMA networks in the case of multiple pilot signals

The performance of a double-dwell serial search technique is analyzed for cellular code-division multiple-access (CDMA) networks in the case of multiple pilot signals. A general expression for the mean acquisition time is obtained by considering the multiple H/sub 1/ regions formed by the pilot signals coming from different base stations. The statistics of the demodulator output in code acquisition systems are discussed in detail for synchronous cellular CDMA networks with a focus on the nonstationarity of the interference in the forward link. The probabilities of detection and false alarm are then derived for frequency-selective Rayleigh fading environments in the case where the interference can be approximated by a stationary Gaussian noise process. Numerical evaluations are performed to examine the effects of decision thresholds, postdetection integration, fading rate, and so on, with emphasis on the case where the mobile station is located around the cell boundary.

Subscriber location in CDMA cellular networks

Subscriber radio location techniques are investigated for code-division multiple-access (CDMA) cellular networks. Two methods are considered for radio location: measured times of arrival (ToA) and angles of arrival (AoA). The ToA measurements are obtained from the code tracking loop in the CDMA receiver, and the AoA measurements at a base station (BS) are assumed to be made with an antenna array. The performance of the two methods is evaluated for both ranging and two-dimensional (2-D) location, while varying the propagation conditions and the number of BS's used for the location estimate.

Fixed and adaptive blocking thresholds in CDMA cellular networks

A number of papers have shown that, in a code-division multiple access cellular network, bursty user mobility causes remarkable negative effects on link availability, and hence on cellular network capacity. This article shows that a possible provision for contrasting such negative effects is the introduction of a blocking threshold which produces an increase in the link availability and a decrease in the dropping probability at the expense of the appearance of a non-null blocking probability. In order to maximize the capacity of the cellular network, the blocking threshold has to be selected so that a proper balancing of the blocking probability, dropping probability, and link availability is achieved. In particular, the article presents a simple self-adaptive mechanism for selecting the blocking threshold, which achieves good performance and has the fundamental advantage of not requiring any a priori knowledge of the system parameters.

Performance analysis of cellular CDMA networks over frequency-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.

Effects of user mobility on the capacity of a CDMA cellular network

AbstractIn the recent technical literature on cellular networks Code Division Multiple Access (CDMA) has received a large attention as a promising radio interface access technique. Many studies. are devoted to the capacity evaluation of the radio interface adopting CDMA, but seemingly none of them accounts for the variability of the user spatial density due to user mobility. This should be a concern especially in a microcellular environment, where it cannot be relied upon a significant spatial average over the radio coverage area of a Base Station. The major aim of this work is a preliminary assessment of the effects of “bursty” user mobility on the capacity of the CDMA radio interface. To this end. we introduce a user mobility model apt to describe large fluctuations of the number of users in a radio cell area. A reference model of a CDMA network is used to evaluate the effects of user mobility on the capacity for a wide range of the model parameters, by means of simulations accounting for shadowing, call attempt process, voice activity and antenna directivity and assuming perfect power control. We show that user mobility can remarkably affect the CDMA capacity, mainly because of the resulting highly bursty behaviour of self‐noise. A simple traffic control scheme is devised to increase the capacity of the CDMA network, under a joint constraint on link availability, call blocking and call dropping probabilities. It is pointed out that the concepts introduced in this paper are independent of the specific CDMA implementation, i.e. they hold for any asynchronous CDMA based cellular network.

Effects of imperfect power control and user mobility on a CDMA cellular network

Code-division multiple-access (CDMA) is one of the major candidate access techniques for third generation systems. A great deal of effort has been devoted to the study of the capacity it can support. This paper presents analytical derivations which allow the determination of the link availability in the presence of user mobility and power control imperfections in a CDMA network; moreover, it provides the guidelines which permit the implementation of a simple and flexible simulation tool which is independent of the specific CDMA implementations. As a matter of fact, the reported concepts can be applied to any asynchronous CDMA system, i.e., they hold both for the American Standard IS-95 and for the European Community Standard developed in the framework of the RACE CODIT Project.

Performance analysis of soft handoff in CDMA cellular networks

The code-division multiple-access (CDMA) scheme has been considered as one possible choice of the future standards for cellular networks because of its various advantages. Since there can be only one carrier frequency being used in CDMA systems, a handoff scheme with diversity, a so-called handoff, was proposed for higher communication quality and capacity. A mathematical model is developed to analyze the soft handoff process. Markov's concept is applied to describe the system's steady state statistical behavior. System performance such as blocking probability, handoff refused probability, and channel efficiency are also determined. It is concluded that the larger the area the soft handoff region is, the better users in the cellular network will feel.

Cellular CDMA networks impaired by Rayleigh fading: system performance with power control

The effect of power control on the performance of cellular code division multiple access (CDMA) networks is investigated. The bit error rate (BER) of a mobile unit at any given distance from the base station is obtained for both the forward link and the reverse link, taking into account Rayleigh fading and UHF propagation attenuation, To this end, a simple multiple-cell-system model is developed. Our work shows the dependence of performance gain on the different user spatial distributions under power control. Also, it is shown that power control in the forward link of CDMA networks may lead to a dismal performance degradation for close-in users. To alleviate this problem, the adjusted power control scheme is revisited. Two different performance criteria are proposed for this purpose, and the optimal threshold values corresponding to these criteria are computed. Results indicate that, although both threshold criteria are acceptable, the second criterion is more recommendable from a BER performance point of view, because it circumvents the so-called service "hole" problem which may occur when one uses the first criterion. The analysis for both the forward and reverse links leads to closed-form signal-to-interference ratio (SIR) results, which not only facilitate physical insight into the impact of key system parameters on the system performance of CDMA networks with power control, but may also eliminate the need for cumbersome Monte-Carlo-type simulations. It is shown that fourth-order power control (n=4) leads to at least a threefold enhancement in capacity for BER=10/sup -3/.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>