Modeling and performance analysis for soft handoff schemes in CDMA cellular systems

This paper investigates the features of a cellular geometry in code-division multiple-access (CDMA) systems with soft handoff and distinguishes controlling area of a cell from coverage area of a cell. Some important characteristics of the cellular configuration in soft handoff systems are used to propose a new design of efficient call admission control (CAC) in CDMA systems. Then, the paper constructs a continuous-time Markov chain (CTMC) model for CAC in CDMA with a soft handoff queue, obtains closed-form solutions, and thus develops loss formulas as performance indices such as the new blocking probability and the handoff dropping probability. In order to determine handoff traffic arrival rate, a fixed-point strategy is developed. Algorithms are also provided to stably compute loss probabilities and to determine the optimal number of guard channels. A new soft handoff scheme-eliminating pseudo handoff calls (EPHC)-is proposed to improve channel utilization efficiency based on mobility information. As an application of the loss formulas, the proposed modeling techniques are used to evaluate and compare the performance of conventional and proposed EPHC soft handoff schemes. Numerical results show the effectiveness of the proposed Markov chain models and the benefits of the new soft handoff scheme.     

Performance analysis of channel-borrowing handoff scheme based onuser mobility in CDMA cellular systems

Code-division multiple-access (CDMA) cellular systems use soft handoff. Although the capacity of CDMA systems is interference-limited in nature, channel shortages may occur because soft handoff uses several channels simultaneously. To cope with this problem, we propose an improved handoff method that borrows channels from stationary calls participating in soft handoff and allocates the borrowed channels to handoff requests by moving calls when a channel shortage occurs. Borrowing from stationary calls is possible because these calls do not undergo fast-fading and do not require receiver diversity. The proposed method is designed to avoid increased interference resulting from channel borrowing. The proposed channel-burrowing handoff scheme is analyzed in a situation involving both moving and stationary calls. A comparison is made between the performances of the typical IS-95-based handoff scheme and the proposed scheme. Numerical results show that the proposed scheme is better than the IS-95 scheme in view of the handoff refused probability, the handoff queuing delay, and total interference     

基于信道借用和信号预测的切换方法

随着业务需求的日益增长,蜂窝也越变越小,移动蜂窝通信环境中的切换成为日益重要的问题。在CDMA蜂窝系统中使用软切换,为了解决切换时的信道短缺问题,提出了许多解决方法。该文中提出了称为基于信道借用和信号预测的切换算法。当切换请求到达蜂窝时,如果没有空闲信道,就将从参与软切换的静止呼叫借用一信道,并将借用的信道分配给移动呼叫的切换请求.如果没有信道可借用,就将切换请求放入队列中,使用信号预测的方法来确定队列中的优先级。并将此切换方式的性能与其它切换方式进行了比较。     

Soft handoff algorithm with variable thresholds in CDMA cellularsystems

Traffic nonuniformity degrades the performance of CDMA cellular systems. The authors present a new algorithm called the variable threshold soft handoff (VTSH) which reduces the performance degradation due to traffic nonuniformity in CDMA cellular systems. Unlike the conventional fixed handoff thresholds, the proposed algorithm allows the handoff thresholds (T ADD and T DROP) to vary dynamically according to the traffic density of each cell. This algorithm has been implemented by means of computer simulation and the results show that VTSH improves the overall CDMA system performance in terms of outage probability. The VTSH algorithm can easily be applied to CDMA cellular systems without any modification     

Performance analysis of CDMA cellular networks with channel sub‐rating

This paper presents the analysis of CDMA cellular networks with channel sub‐rating. From users' point of view, our considered scheme gives higher priority to handoff calls over new calls by sub‐rating the existing connections when handoff calls find no idle channels upon their arrivals. Therefore, it is considered that the disadvantage of the soft handoff which needs more channels than the hard handoff is made up for. Handoff calls can also wait in a queue while they are in handoff areas if all channels are sub‐rated in the cell of interest. We mathematically model this scheme by applying queueing theory. Then, we analyse its performance to derive the blocking probabilities of the new and handoff calls, the probability that handoff calls leave the handoff area without getting new channels, the degradation ratio of the voice quality by sub‐rating, mean and coefficient of variation of the waiting time of handoff calls. In numerical results, the analytical results are compared with the simulation ones to validate our analytical approach. Moreover, we compare the sub‐rating scheme with full‐rating one with respect to some characteristic values to show the effect of sub‐rating. Copyright © 2005 John Wiley & Sons, Ltd.     

Design and Performance Analysis of a New Soft Handoff Scheme for CDMA Cellular Systems

In this paper, a new soft handoff scheme for CDMA cellular systems is proposed and investigated. It is pointed out that some handoff calls unnecessarily occupy multiple channels with little contribution to the performance of handoffs in IS95/CDMA2000-based handoff schemes or systems. To alleviate performance degradation due to channel resource shortage during soft handoff, a new concept of channel convertible set (CCS), which contains several types of handoff calls that unnecessarily occupy extra channels by considering the relative mobility of the calls in the handoff area is introduced. A new scheme that reallocates those extra channels in the CCS to new handoff calls when there is no available free channel in the system is proposed. Furthermore, according to the variation of the CCS, the proposed scheme dynamically adjusts the number of guard channels reserved exclusively for handoff. Then, the feasibility and implementation issues of the proposed scheme are discussed. To evaluate and compare performance indexes of different soft handoff schemes, continuous-time Markov chain models are constructed. Automated generation and solution of the underlying Markov chains are facilitated by stochastic reward net models, which are specified and solved by stochastic Petri net package. Numerical results show that this scheme can significantly decrease both the number of dropped handoff calls and the number of blocked calls without degrading the quality of communication service and the soft handoff process.     

Sensitivity analysis of handoff algorithms on CDMA forward link

We analyze the performance of different handoff algorithms on the forward link or downlink of a code-division multiple-access (CDMA) cellular system. Unlike the reverse link, soft handoff on the forward link requires additional resources such as CDMA codes and transmit power and also causes additional interference. If handoff requests can be processed and completed instantaneously, transmission from the base station with the best link to the user would achieve a significant fraction of the macrodiversity gain without utilizing additional resources. However, in practical systems, there is a nonzero handoff completion delay and soft handoff provides the required robustness to delays, although it comes at the expense of additional network resources. Thus, there is a tradeoff between the extent of soft handoff required and the handoff execution delay. We compare the performance of hard and soft handoff schemes and study their sensitivity to the delay in the execution of the handoff. Outage probability and the total average power required are used as performance metrics. We present an analytical framework to study this tradeoff and also discuss simulation results with field data. The results provide insights on the conditions under which soft handoff can be eliminated and on the effect of relevant handoff thresholds on the performance.     

A novel dynamic cell configuration scheme in next-generation situation-aware CDMA networks

To balance the time-varying traffic load between cells, caused by user mobility and diverse applications, it is crucial for next-generation code-division multiple-access (CDMA) cellular networks to configure cell coverage and capacity dynamically. In this paper, we show that pilot power allocation is highly coupled to other facets of radio resource management. We propose a novel dynamic cell configuration scheme for multimedia CDMA cellular networks, based on reinforcement-learning, which takes into account pilot, soft handoff, and maximum link power allocations, as well as call admission control mechanisms. Simulation results demonstrate the effectiveness of the proposed scheme in situation-aware CDMA networks.     

一种新的CDMA软切换算法及其对软切换性能的改善

定义了残余容损比(RCLR)的概念,提出了一种新的基于最大RCLR的无线移动CDMA小区系统的软切换判决算法,称之为RCLR算法。区别通常的最小路径损耗(MPOL)算法,RCLR算法不仅考虑了呼叫业务的路径损耗,而且兼顾了小区负载平衡。最后,通过分别在单层小区系统和分层小区系统中进行仿真,比较了RCLR算法和MPOL算法的软切换性能,仿真结果表明RCLR算法显著地改善了呼叫业务的中断概率和分层小区系统的层间溢出概率,减少了软切换次数。     

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     

Adaptive and predictive downlink resource management in next-generation CDMA networks

Guard channels have been proposed to minimize handoff call dropping when mobile hosts move from one cell to another. Code-division multiple-access (CDMA) systems are power- and interference-limited. Therefore, guard capacity in CDMA networks is soft, that is, a given capacity corresponds to variable number of connections. Thus, it is essential to adjust the guard capacity in response to changes in traffic conditions and user mobility. We propose two schemes for managing downlink CDMA radio resources: guard capacity adaptation based on dropping (GAD), and guard capacity adaptation based on prediction and dropping (GAPD). In both schemes, the guard capacity of a cell is dynamically adjusted so as to maintain the handoff dropping rate at a target level. In the second scheme, there is an additional, frequent adjustment component where guard capacity is adjusted based on soft handoff prediction. We show through extensive simulations that GAD and GAPD control the handoff dropping rate effectively under varying traffic conditions and system parameters. We also find that GAPD is more robust than GAD to temporal traffic variations and changes in control parameters.     

A Study of Handoff in CDMA Systems

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Characterization of soft handoff in CDMA systems

Many analytical approaches have been proposed for handoff analysis based on hard handoff in mobile communication systems. In code-division multiple-access (CDMA) systems with soft handoff, mobile stations (MSs) within a soft handoff region (SR) use multiple radio channels and receive their signals from multiple base stations (BSs) simultaneously. Therefore, the SRs should be considered for handoff analysis in CDMA systems. An analytical model for soft handoff in CDMA systems is developed by introducing an overlap region between adjacent cells and the handoff call attempt rate and the channel holding times are derived. Applying these results to a nonprioritized CDMA system, the effects of soft handoff and the mean cell residual time are investigated and compared with hard handoff     

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.     

CDMA系统基于多业务的软切换优化算法

文中利用剩余容量的概念,提出了CDMA系统中基于多业务的软切换优化算法,并对其进行了仿真.此算法综合考虑了传播衰耗和负载均衡两个因素,达到了各小区之间负载平衡和资源合理利用的目的.通过仿真结果可以看出,相对于传统的软切换算法,新算法降低了切换中断率,提高了系统性能,显示出较大的优越性,具有一定的价值.     

Radio resource management in multiple-chip-rate DS/CDMA systemssupporting multiclass services

This paper is concerned with radio resource management in multiple-chip-rate (MCR) DS/CDMA systems accommodating multiclass services with different information rates and quality requirements. Considering both power spectral density (PSD) over a radio frequency (RF) link and the effect of RF input filtering on the receiver in MCR-DS/CDMA systems, criteria for call admission are presented and the system performance is derived. The system performance in MCR-DS/CDMA systems is strongly affected by radio resource management. A minimum total-power-increment-based resource management scheme for an efficient resource management is proposed. The performance of this scheme is compared with that of a random-based resource management scheme in terms of the new call blocking probability, handoff call dropping probability, and normalized throughput. In addition, in order to reduce the handoff call dropping rate, reallocating subsystems assigned for communicating calls is proposed. The minimum total-power-increment-based resource management scheme yields better performance than the random-based resource management scheme for multiclass services     

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.     

基于RSS准则的CDMA软切换越区研究

硬切换技术中移动台在接收新基站信号之前中断与原基站的通信,通信中断的概率非常大。而软切换技术需要先建立与新基站之间的通信,等接收到原基站信号低于门限值时再切断与原基站的通信,故具有很高的可靠性。采用接收到的信号强度准则对软切换建立仿真模型,对两个小区之间的软切换进行了仿真,仿真结果表明,软切换的可靠性高于硬切换,采用软切换技术降低了系统的掉话率,提高了通信质量。然而,软切换也将使系统的复杂度和信道开销有所增加。     

Microcell engineering in CDMA cellular networks

The demand for cellular radio services is growing rapidly, and in heavily populated areas the need arises to shrink the cell sizes and “scale” the clustering pattern. The extension of the service into the PCN domain, mostly in-buildings and in pedestrian areas, further enhances this trend. The vision of the “third generation” cellular systems incorporates micro- and picocells for pedestrian use, with macrocells for roaming mobiles. Connectivity between all these cells, while maximizing total system capacity, is the main challenge facing the “third generation architects.” The CDMA cellular system, which shares the same frequency channel across the system (reuse pattern of one) and applies soft handoff between the cells, has already shown, both by analysis and by tests, to have full connectivity between the microcells and the overlaying macrocells without capacity degradation. The parameters involved in the engineering of a heterogeneous CDMA network are discussed in the paper. Factors that determine the size of the cell, the soft handoff zone, and the capacity of the cell clusters are analyzed, and engineering techniques for overlay-underlay cell clustering are outlined     

Design and Analysis of a Channel Assignment Scheme for CDMA/TDMA Mobile Networks

In this paper, a channel assignment scheme is proposed for use in CDMA/TDMA mobile networks carrying voice and data traffic. In each cell, three types of calls are assumed to compete for access to the limited number of available channels by the cell: new voice calls, handoff voice calls, and data calls. The scheme uses the movable boundary concept in both the code and time domains in order to guarantee the quality of service (QoS) requirements of each type. A traditional Markov analysis method is employed to evaluate the performance of the proposed scheme. Measures, namely, the new call blocking probability, the handoff call forced termination probability, the data call loss probability, the expected number of handoff and the handoff link maintenance probability are obtained from the analysis. The numerical results, which are validated by simulation, indicate that the scheme helps meet the QoS requirements of the different call types.