On call admission control in DS/CDMA cellular networks

Analytical models are proposed for various direct sequence code-division multiple-access (DS/CDMA) call admission control schemes. Many mathematical call admission models for DS/CDMA cellular networks have been proposed. However, they have shortcomings. First, by ignoring the stochastic traffic load variation or call blocking effect, they failed to sufficiently characterize the second moment of other-cell interference. This leads to inaccurate analysis of a real network. Second, the optimal control parameters were often obtained through an exhaustive search which was very time consuming. Finally, the estimation of system capacity in previous models was obtained by using a simple one-slope path-loss propagation model. However, it is well known that a two-slope path loss propagation model is needed in a line-of-sight (LOS) microcell propagation environment. We propose an analytical model for call admission to overcome these drawbacks. In addition, we combine a modified linear programming technique with the built analytical model to find better call admission control schemes for a DS/CDMA cellular network     

Spectrum-efficient call control in a dual-mode TDMA cellular system

This study deals with a call control strategy in a dual-mode time-division multiple access (TDMA) cellular system, which provides services both to analog and digital calls. Since analog calls consume the frequency resource several times as much as digital calls, we consider a call control strategy of the threshold type that the number of active analog calls is restricted within a prespecified level. Given the arrival rates and the grade of service (GOS) for both types of calls in the cells, two nonlinear integer optimization problems are considered for a multicell system as well as for a single cell system. One problem is to find the threshold parameters for optimizing the relevant objective measures. The other is to obtain the minimum numbers of required channels in the cells satisfying the GOS of both types of calls. The solution methods for the two kinds of optimization problems are devised based on the properties of the objective function and the blocking probabilities of both call types. The efficiency of the proposed algorithms is verified by extensive computational experiments with realistic input data     

Adaptive channel reservation for call admission control to support prioritized soft handoff calls in a cellular CDMA system

This paper proposes a prioritized call admission control (CAC) model to support soft handoff calls with quality of service (QoS) assurances for both the uplink and downlink connections in a CDMA system. CAC is formulated as a combinatorial optimization problem in which the problem objective is to minimize the handoff forced termination rate. The model, which is based on the adaptive channel reservation (ACR) scheme for prioritized calls, adapts to changes in handoff traffic where the associated parameters (reserved channels, and new and handoff call arrival rates) can be varied. To solve the optimization model, iteration-based Lagrangean relaxation is applied by allocating a time budget. We express our achievements in terms of the problem formulation and performance improvement. Computational experiments demonstrate that the proposed ACR scheme outperforms other approaches when there are fewer rather than more channels, and it reduces the handoff call blocking rate more efficiently when the handoff traffic is heavily loaded. Moreover, the model can be adapted to any kind of reservation service.     

A call admission control scheme for packet data in CDMA cellular communications

In wireless cellular communication systems, call admission control (CAC) is to ensure satisfactory services for mobile users and maximize the utilization of the limited radio spectrum. In this paper, we propose a new CAC scheme for a code division multiple access (CDMA) wireless cellular network supporting heterogeneous self-similar data traffic. In addition to ensuring transmission accuracy at the bit level, the CAC scheme guarantees service requirements at both the call level and the packet level. The grade of service (GoS) at the call level and the quality of service (QoS) at the packet level are evaluated using the handoff call dropping probability and the packet transmission delay, respectively. The effective bandwidth approach for data traffic is applied to guarantee QoS requirements. Handoff probability and cell overload probability are derived via the traffic aggregation method. The two probabilities are used to determine the handoff call dropping probability, and the GoS requirement can be guaranteed on a per call basis. Numerical analysis and computer simulation results demonstrate that the proposed CAC scheme can meet both QoS and GoS requirements and achieve efficient resource utilization.     

多业务类分层结构CDMA系统功率控制研究

针对不同QoS要求的多业务类提供支持的分层结构CDMA系统的功率控制进行了研究，给出了该系统的一种功率控制算法。通过对反向链路运用HMRC方法，得到了一种支持多业务类的分层结构CDMA系统中反向链路CIR的简单分析方案和系统容量分析方法。     

Base station location in a cellular CDMA system

We consider a cellular CDMA system in which blocking is enforced when the relative interference exceeds a certain threshold level. This paper addresses a radio network design problem in such a CDMA system. Given the data of call‐traffic distributed over the service area and potential sites of base stations, the objective of the problem is to locate base stations so as to minimize the associated cost for establishing base stations while keeping the probability of blocking under control. We develop an efficient algorithm for solving the design problem. Computational experiments with real‐world data are conducted to show both the efficiency and the practicality of the proposed design method. This revised version was published online in August 2006 with corrections to the Cover Date.     

Distributed power control in CDMA cellular systems

In wireless cellular communication, it is essential to find effective means for power control of signals received from randomly dispersed users within one cell. Effective power control will heavily impact the system capacity. Distributed power control (DPC) is a natural choice for such purposes, because, unlike centralized power control, DPC does not require extensive computational power. Distributed power control should be able to adjust the power levels of each transmitted signal using only local measurements, so that, in a reasonable time, all users will maintain the desired signal-to-interference ratio. In this paper, we review different approaches for power control, focusing on CDMA systems. We also introduce state-space methods and linear quadratic power control (LQPC) to solve the power-control problem. A simulation environment was developed to compare LQPC with earlier approaches. The results show that LQPC is more effective, and is capable of computing the desired transmission power of each mobile station in fewer iterations, as well as being able to accommodate more users in the system     

On the capacity of a cellular CDMA system

It is shown that, particularly for terrestrial cellular telephony, the interference-suppression feature of CDMA (code division multiple access) can result in a many-fold increase in capacity over analog and even over competing digital techniques. A single-cell system, such as a hubbed satellite network, is addressed, and the basic expression for capacity is developed. The corresponding expressions for a multiple-cell system are derived. and the distribution on the number of users supportable per cell is determined. It is concluded that properly augmented and power-controlled multiple-cell CDMA promises a quantum increase in current cellular capacity     

Multipath propagation effects on a CDMA cellular system

The performance of the reverse link of a code division multiple access cellular system is evaluated. At the base station, the signal from each user is demodulated by a coherent BPSK RAKE receiver. Parameters for the model of the impulse response of the channel were taken from measurements of the digital cellular channel in Toronto. The signal-to-noise ratio (SNR) for a received signal is used to measure the performance of the reverse link. The variation in SNR of received signals at the base station should be as small as possible to reduce interference in the network. A power control scheme to lower the variation in SNR of the received signals is analyzed. The effects of lowering the bandwidth of the transmitted signal were also investigated     

Design issues in a CDMA cellular system with heterogeneous traffictypes

In this paper, the performance of a code-division multiple-access (CDMA) cellular system with various traffic types is examined. Traffic integration and resource management are two major issues in the design of the next generation of wireless networks with integrated services. To provide design guidelines for traffic integration, the impact of the line rate (actual transmission bit rate in the radio channel) selection on the system capacity is examined. Since in a CDMA system the signal from one specific user is spread over the entire available bandwidth, the actual bandwidth resource occupied by a user depends on the amount of the transmission power of the user. In this paper, we also address how to assign suitable power levels to the different traffic types. A method for optimizing the power assignment for multiple traffic types is developed     

Macrodiversity power control in hierarchical CDMA cellular systems

Hierarchical code division multiple access (CDMA) cellular systems, consisting of macrocells with underlying microcells, are studied. We seek power control schemes which will allow both hierarchical layers to share the same spectrum. For the reverse link, hierarchical maximal ratio combining (HMRC) is applied where each mobile station (MSs) is received and coherently combined by base stations (BSs) in both layers. For the forward link, selective transmit diversity (STD) is applied where each BS provides multiple transmit paths for MSs to choose. We show that both HMRC and STD are effective in hierarchical CDMA architectures. We conclude that hierarchical architectures are a viable solution for improving CDMA cellular system capacity, and a significant performance gain can be achieved without assigning disjoint spectrum between the layers, by utilizing macrodiversity schemes such as HMRC and STD     

An uplink power control based on truncated channel inversion fordata traffic in a cellular CDMA system

A power control scheme, called truncated channel inversion, in which a mobile suspends transmission when the wireless channel is in a bad condition in order to reduce out-cell interference is examined. It is found that capacity is improved and battery power consumption is reduced as a consequence of suspending transmission, but extra queuing delay is introduced. It is shown that the extra queuing delay depends on how fast the channel characteristics change and that the truncated scheme works well, even for moderate mobile velocities     

Centralised power control in CDMA cellular mobile systems

Efficient power control is of great importance in the design of high capacity cellular radio systems. Optimum power control scheme, in the sense that it minimises the outage probability, has been fully investigated for FDMA/TDMA cellular systems. The authors propose optimum power control and several centralised power control algorithms for CDMA cellular systems. Simulation results indicate that the optimum power control scheme outperforms the perfect average power control algorithm by ~1.9 dB under the IS-95 defined radio condition     

Forward-link performance of CDMA cellular system

The impact of the propagation path-loss exponent (γ) on the forward performance of a direct-sequence code-division multiple-access (DS-CDMA) cellular system is investigated. For this purpose, a simple analytical model based on the inverse power-of-distance γ law is developed. The problem of finding proper power-control factors is considered. According to γ, the proper power-control factors are estimated for optimum performance. With these factors, results show that the capacity is reduced by a half by changing γ from 4.5 to 2.5. For this range in γ, power control can approximately double the capacity compared to the case of no power control     

Integrating data traffic into a CDMA cellular voice system

Part one of this paper analyzes the effects of data traffic integration into a CDMA cellular voice system. The figure of merit used for the quality of service seen by the voice users is measured by the probability of blocking. The CDMA system under consideration is a power controlled, cellular architecture in which blocking occurs when the total interference level exceeds the background noise level by 10 dB [1]. It is shown that the introduction of data can be done at little or no increase in the probability of blocking on the voice users. In part two we propose and analyze a protocol which achieves the efficient integration of data by maximizing the utilization of the resources and minimizing the delay experienced by the voice users. The proposed protocol admits data traffic into the CDMA cellular system based on the current aggregate voice interference level, and allows for the efficient integration of voice and data without degrading the quality of service for the delay-critical voice traffic. A Markovian model for this protocol is developed, evaluated and compared to computer simulation results.     

Analysis of a multiple-cell direct-sequence CDMA cellular mobileradio system

The performance of a multiple-cell direct-sequence code division multiple-access cellular radio system is evaluated. Approximate expressions are obtained for the area-averaged bit error probability and the area-averaged outage probability for both the uplink and downlink channels. The analysis accounts for the effects of path loss, multipath fading, multiple-access interference, and background noise. Two types of differentially coherent receivers are considered: a multipath rejection receiver and a RAKE receiver with predetection selective combining. Macroscopic base station diversity techniques and uplink and downlink power control are also topics of discussion     

On optimal call admission control in cellular networks

Two important Quality-of-Service (QoS) measures for current cellular networks are the fractions of new and handoff “calls” that are blocked due to unavailability of “channels” (radio and/or computing resources). Based on these QoS measures, we derive optimal admission control policies for three problems: minimizing a linear objective function of the new and handoff call blocking probabilities (MINOBJ), minimizing the new call blocking probability with a hard constraint on the handoff call blocking probability (MINBLOCK) and minimizing the number of channels with hard constraints on both of the blocking probabilities (MINC). We show that the well-known Guard Channel policy is optimal for the MINOBJ problem, while a new Fractional Guard Channel policy is optimal for the MINBLOCK and MINC problems. The Guard Channel policy reserves a set of channels for handoff calls while the Fractional Guard Channel policy effectively reserves a non-integral number of guard channels for handoff calls by rejecting new calls with some probability that depends on the current channel occupancy. It is also shown that the Fractional policy results in significant savings (20-50\%) in the new call blocking probability for the MINBLOCK problem and provides some, though small, gains over the Guard Channel policy for the MINC problem. Further, we also develop computationally inexpensive algorithms for the determination of the parameters for the optimal policies. This revised version was published online in July 2006 with corrections to the Cover Date.     

Admission control for variable spreading gain CDMA cellular system with imperfect power control and shadowing

Variable spreading gain (VSG) CDMA is the prime transmission scheme for supporting multiple services for the next generation cellular systems. As different services have different quality of service and data rate requirements, admission control is essential in safeguarding the performance of different services in VSG‐CDMA cellular systems in harsh wireless environment. In this paper, we propose an admission control scheme for multiple services VSG‐CDMA cellular system in a practical multi‐cell environment where there exist adverse effects due to imperfect power control and shadowing. Rather than with a fixed bound as in previous studies, the admission bound in our scheme is based on the probability distribution of total intra‐cell power (P_in) and other‐cell interference power (I_other). This is used to estimate the blocking probability by utilizing a simple and accurate approximation that incorporates the random nature of P_in and I_other. Evaluation of the impact of power control error to the mean and standard deviation of P_in and I_other, we performed and compared the blocking probability with fixed admission bound scheme in an integrated voice/data environment. Our work has enabled the calculation of the blocking probability and capacity of different mobility users having different power control errors under shadowing. Copyright © 2006 John Wiley & Sons, Ltd.     

Open-loop power control error in cellular CDMA overlay systems

Adaptive power control has widely been used in DS/CDMA systems to overcome the so-called “near-far” problem. This paper studies the adaptive open-loop power control of a cellular CDMA system, which is overlaid in the downlink by a narrowband signal. The effects of downlink power allocation schemes to power control error in the presence of narrowband interference are analyzed. In order to get a minimum power control error in the CDMA overlay situations, an optimum downlink power allocation scheme is used, which works well for a wide range of signal to narrowband interference ratio     

An expert system approach for cellular CDMA

The demand for cordless communication has forced suppliers to exploit digital technology to satisfy the increasing demand for service. This demand has led to severe capacity limitations in many areas and spread spectrum techniques are being considered as one means to support these services. It is generally accepted that system control in the future will make use of intelligent processors. In this study we delineate an expert system to be used as an intelligent controller for a cell site working in a CDMA (Code Division Multiple Access) mobile cellular telecommunications system. An existing AI shell is used for program development and simulation and the results are discussed in terms of the benefits obtained by using an expert system approach in the highly dynamic environment of cellular mobile communications