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 teletraffic performance study of mobile LEO-Satellite cellular networks with gamma distributed call duration

In this paper, the authors develop an analytical model to study the performance of a mobile low earth orbiting (LEO) satellite cellular network. The model assumes that the call duration has a gamma distribution and considers the effect of system parameters such as the number of channels per cell, the number of channels reserved for the handoff, and the cell residence time, on the teletraffic performance of the system. The quality of service (QoS) measures studied in this paper include new call blocking probability, handoff failure probability, premature call-termination probability (CTP), and call dropping probability (CDP). Based on the causal central limit theorem, the authors use a two-parameter gamma distribution to approximate the distribution of the sum of the residence times in the cells. The analytical model presented in this paper may be used with any call-holding-time distribution. The analytical results are validated by a computer simulation.     

PCS分层系统切换性能分析

本文讨论了注册微小区PCS分层系统的切换性能,给出了强迫中断概率的通用解析解;分析了软切换对切换性能的影响,给出了它们的解析解;得出注册微小区PCS分层系统的切换性能要好于普通PCS分层系统,普通PCS分层系统的切换性能要好于单层PCS系统。     

Effects of the BS power and soft handoff on the outage and capacity in the forward link of an SIR-based power-controlled CDMA system

In the forward link of a signal-to-interference ratio (SIR)-based fast power-controlled CDMA system, the fraction of base station (BS) power allocated to a mobile station (MS) is considered as a key factor affecting the system performance. By using our proposed macrodiversity non-orthogonality factor, we establish a unified analytical model to characterize the distribution of the fraction of BS power allocated to an MS in either the non-soft handoff mode or the soft handoff mode. By using that distribution and limiting the maximum fraction of BS power available to an MS, a closed-form expression of the capacity at a certain outage probability is obtained. The effects of system parameters, such as the available Rake fingers, the soft handoff threshold, the unbalance of the BS power, and the power control error, on the capacity are investigated from the viewpoint of the limitation on the fraction of BS power allocated to an MS. Simulation results show that soft handoff does not always improve capacity and the capacity gain may result depending on the choice of the system parameters.     

Amulti-channel model for initial and handoff accesses in a mobile computing environment

The communication infrastructure of a mobile computing environment can be based on the structure of a cellular/microcellular telephone system or a PCS network. In such a system, the occurrence of handoffs cannot be avoided and when handoffs occur, wireless links held by mobile computers crossing cell boundaries may be forced to terminate. The probability that a handoff access request will result in forced termination has a significant effect on the performance of a mobile computing environment, as does the probability that an initial access request will be blocked. Although some research has been done on initial and handoff accesses in cellular/microcellular telephone systems and PCS networks, the analytical models used in this research are not appropriate for mobile computing, since unlike a telephone, a mobile computer may use several channels at once. In this paper, we develop an analytical model to study initial and handoff accesses in a mobile computing environment. The model is based on a multi-dimensional continuous time Markov chain. The accuracy of the model is verified by comparison with simulation results. We then use the model to find a practical approach to balancing the initial access blocking probability and avarage forced termination probability of a connection in a mobile computing network.This research was supported by the National Science Council, ROC, under grant NSC 85-2213-E-009-063.     

最大比宏分集下的CDMA系统反向容量分析

本文综合考虑快衰落,阴影衰落和路径衰减的影响,建立了最大比宏分集下CDMA系统反向信道分析模型并推导出了中断率(outage probability)表达式.数值结果表明,采用宏分集后反向容量随参与宏分集基站数的增加而大大提高了,而且移动台的反向性能和其位置有关,在小区边界处性能最佳.     

基于QoS优化的WCDMA上行接入控制算法

文章研究QoS优化下WCDMA系统的上行接入控制算法。从QoS分层结构,可度量参数出发,结合新用户呼叫到达率和用户切换到达率,提出WCDMA上行信号呼叫接入控制算法,考虑和平衡新接入用户阻塞率和已接入用户中断率。通过满足不同业务需要的QoS优化策略来控制上行允许接入用户,从而保证系统有稳定的通信质量。仿真分析结果表明,文中给出算法使系统阻塞率和掉话率下降,系统性能得到改善。     

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.     

分布式虚拟群小区中的接入控制

分布式多天线结构的虚拟群小区系统成为未来无线系统的一种解决方案。该系统具有灵活的组织结构,在增加频谱效率、提高通信质量上具有巨大潜力,并且能为各种新技术的采用提供广阔的发展空间。该文采用Markov链逼近的方法分析多天线情况下虚拟群小区系统的系统性能,对于基于干扰的接入控制策略下的系统阻塞率,切换失败率等给出了数值结果。研究结果表明分布式多天线系统有效地提高了上行系统性能。     

Inter-Carrier Interference Cancellation for OFDM Systems with Macrodiversity and Multiple Frequency Offsets

Macrodiversity provides improved coverage in wireless communications. It isused in single frequency networks (SFN), and in some cellular systems, whena mobile is in a soft handoff. The combination of OFDM and macrodiversity isvery attractive for the ability of OFDM to cope with delay spread resultingfrom macrodiversity. However, such system is very sensitive to poor frequencysynchronization among transmitters, which results in multiple frequencyoffsets. These cause inter-carrier interference (ICI), which severely degradesperformance. In this paper we quantify the degradation, providing expressionsfor the receiver SINR, and find the optimum receiver frequencysynchronization. Furthermore, we propose to mitigate ICI degradation usinglinear and decision feedback interference cancellation. In each case, weprovide a solution using both the decorrelating and the MMSE criteria.Simulation results show that, for macrodiversity OFDM, our approach resultsin significantly improved performance and robustness to frequency offsets.     

Performance analysis of a dynamic handoff scheme in wireless networks with heterogeneous call arrival processes

The Guard Channel Scheme (GCS) and Handoff Queueing Scheme (HQS) are the popular and practical strategies to prioritize handoff calls in wireless cellular networks. A key issue of giving handoff calls the higher priority is how to achieve a tradeoff among the handoff call blocking probability, new call blocking probability and handoff delay. This paper extends GCS and HQS and presents an efficient handoff scheme that dynamically manages the channels reserved for handoff calls depending on the current status of the handoff queue. A three-dimensional Markov model is developed to analyze the performance of this scheme and investigate the desirable performance tradeoff. The Poisson process and Markov-Modulated-Poisson-Process (MMPP) are used to model the arrival processes of new and handoff calls, respectively. The accuracy of this model is evaluated through the extensive comparison of the analytical results to those obtained from discrete-event simulation experiments. Performance measures in terms of the mean number of calls in the system, aggregate response time, aggregate call blocking probability, handoff call blocking probability, new call blocking probability and handoff delay are evaluated. The analytical model is used to investigate the effects of the number of channels originally reserved for handoff calls, the number of dynamic channels, and the ratio of the rate of handover calls to the aggregate arrival rate on the system performance.     

Load analysis of the soft handoff scheme in a CDMA cellular system

In a code-division multiple-access cellular system, the soft handoff with macrodiversity is known to increase the number of available channels in each cell from the interference-reduced radio environment. This capability, however, also incurs extra load on the system due to the increase in the number of channels occupied and reserved for macrodiversity. The objective is to evaluate the associated loads, thereby enabling effective channel management. We first differentiate two kinds of loads attributable to new and handoff calls, respectively. Further dividing handoff calls into two kinds, we derive a load balance equation from which three different loads at a cell are all exactly obtained in association with the size of the soft handoff region. Comparative analysis with the case of no soft handoff region, i.e., of hard handoff, shows the appropriate proportion of channels reserved for the macrodiversity in a given soft handoff region     

Mobility of queued call requests of a new call-queueing techniquefor cellular systems

A queueing algorithm for new call requests has been proposed to increase the system capacity of traditional fixed channel allocation (FCA) handoff channel reservation schemes. However, mobility of queued call requests was ignored in previous literature. In this paper, a detailed analysis on the effects of the mobility of queued call requests is included in the analytic model. This is important because we found that the mobiles with queued call requests are very likely to leave the cell before granted a free channel. Computer simulation is performed and the results are compared with the numerical ones. It is found that the mobility of the queued new call requests reduces the average queueing delay, but increases blocking probability generally     

Effects of Rician fading and branch correlation on alocal-mean-based macrodiversity cellular system

In a macrodiversity cellular system, switching radio links between base stations cannot be done instantaneously. Thus, branch selection is usually based on the measurement of the slowly varying local-mean power rather than the rapidly varying instantaneous signal power. We offer an exact mathematical model to analyze the performance of a local-mean-based macrodiversity cellular system in a shadowed-Rician (desired)/shadowed-Rayleigh (interfering) channel. We investigate the impact of both fading (Rician or Rayleigh) and shadowing in terms of cochannel interference (CCI) probability. We also present an analytical model to incorporate the effects of branch correlation on macrodiversity systems     

Handoff effect on packet reservation multiple access (PRMA) in microcellular system

Packet reservation multiple access (PRMA) has been proposed for third-generation wireless information networks by D.J. Goodman and S.X. Wei (see ibid., vol.40, p.170-6, 1991). The mobile-initiated handoff has been proposed to disperse the base station (BS) burden, which is increased by the small microcell radius. However, increased contentions, due to handoff, affect the overall performance of PRMA, even though these frequent handoffs do not burden the BS. We analyze the handoff effect on PRMA performance under a microcellular system, both voice-only and combined voice-and-data systems. A steady state voice-terminal model with handoff is proposed. System-equilibrium equations are derived using the proposed model. The system performance is evaluated, using a Markov analysis method, in terms of packet-dropping probability, data-packet delay, and throughput. The change of performance under handoff show the impact of handoff on PRMA.     

NACR: A New Adaptive Channel Reservation in Cellular Communication Systems

Future Personal Communication Networks (PCN) will employ microcells and picocells to support a higher capacity, thus increasing the frequency of handoff calls. Forced call terminations due to handoff call blocking are generally more objectionable than new call blocking. The proposed guard channel schemes for radio channel allocation in cellular networks reduce handoff call blocking probability at the expense of increases in new call blocking probability by giving resource access priority to handoff calls over new calls in call admission control. Under uniform traffic assumptions, it has been shown that a fixed number of guard channels leads to good performance results. In a more realistic system, non-uniform traffic conditions should be considered. In this case, the achieved call blocking probability may deviate significantly from the desired objective. In this paper, we propose a new adaptive guard channel scheme: New Adaptive Channel Reservation (NACR). In NACR, for a given period of time, a given number of channels are guarded in each cell for handoff traffic. An approximate analytical model of NACR is presented. Tabu search method has been implemented in order to optimize the grade of service. Discrete event simulations of NACR were run. The effectiveness of the proposed method is emphasized on a complex configuration.     

Effect of macrodiversity on average-error probabilities in a Ricianfading channel with correlated lognormal shadowing

The effect of correlated lognormal shadowing on the average probability of error performance of narrow-band mobile communication systems with micro- and macrodiversity reception in a Rician fading channel is studied by considering a constant correlation model for the shadowed signals at the base stations. The performance degradation due to correlated shadowing is illustrated by considering both coherent and differentially coherent binary phase-shift keying schemes. Numerical results presented show that when the base stations in a cellular system are very closely spaced, the effect of correlated shadowing on system performance cannot be ignored     

Analysis of soft handoff algorithm for multi‐cellular systems: A finite integral approach

In this paper the performance of soft handoff algorithm based on pilot signal strength measurements has been studied. In connection with the soft handoff it has been observed that the existing analytic expressions often involve integration in infinite limits. The expressions containing such kind of integrals have been meticulously transformed into finite range equations. Probability of outage, number of base stations (BS) in the active set, and number of active set updates are some of the performance metrics commonly considered in soft handoff. Accurate closed forms for all these performance indicators have been computed which, in turn, simplifies the calculation of several other metrics entailing them. Moreover, the underlying system model takes into account of more than two BS (specifically three) extending the traditional two BS model toward more complex but realistic characterization of soft handoff performance analysis framework. Copyright © 2009 John Wiley & Sons, Ltd.     

Performance analysis of cognitive radio networks with generalized call holding time distribution of secondary user

In cognitive radio networks (CRNs), spectrum handoff probability and expected number of spectrum handoffs are important parameters in the evaluation of network performance and design. This paper presents an analytical model for spectrum handoff probability and spectrum handoff rate for CRNs under general distribution of call holding time of secondary users (SUs). The standardized analytical forms of spectrum handoff probability and handoff rate of secondary network under negotiated scenario are derived for a complete service call duration. The effect of mobility parameters: departure rate of SUs (\(\upmu )\) and departure rate of spectrum holes (\(\uplambda )\) on spectrum handoff are also reported in this paper. Extensive results for all the proposed analytical models are obtained and presented in this paper. Analytical results show that exponential and Erlangian distribution functions are not suitable for call holding time of SU in the analysis of spectrum handoff in CRNs. Moreover, the superiority of lognormal distribution function ascertains its use for call holding time of SU in spectrum handoff estimation for better CRN performances. The Monte-Carlo simulation is also performed for spectrum handoff probability to validate the analytical model.     

Priority handoff analysis

Handoff will become an increasingly important issue in mobile phone communications, especially in microcellular systems with the increase of handoff traffic between cells. Many approaches have been proposed for handoff analysis. In a previous analysis, it was assumed that the probability density function (pdf) of the speeds of cell-crossing terminals is the same as the pdf of the speeds of all terminals. In contrast to this assumption, we argue that both pdf's are unequal. The pdf of the speeds of cell-crossing terminals is given by a biased sampling formula. This boundary condition is used in our study and improves the accuracy of performance measures in handoff analysis. In previous work, a priority handoff system which has dedicated channels for handoff calls and uses a blocked call cleared (BCC) policy has been analyzed. In this paper, we use a different approach in performance analysis. The proposed approach has low computational complexity and high accuracy.The author was a visiting scholar at WINLAB while this work was completed.