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     

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.     

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 “soft 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     

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     

Performance analysis of macrodiversity, voice/data CDMA systems

A performance analysis is presented for macrodiversity integrated voice/data CDMA systems. Macrodiversity with maximal ratio combining (MMRC) is ideal for such voice/data systems since it allows robust resource sharing between the two users while removing uncertainties in estimating the system capacity. Our analytical model allows the systems to dynamically allocate additional channels to data users to increase throughput when the necessary resource is available. The data users are characterized by arrival rates and average data size, and the resulting data user quality of service (QoS) performances are evaluated by using a simple birth-and-death Markov process. Our analytical results are fully verified by computer simulation. We show how various system QoS measures such as blocking and outage probabilities can be obtained and used in call admission control (CAC) decisions.     

Antenna gain against interference in CDMA macrodiversity systems

In a multiantenna system, there is a potential antenna gain against interference, in addition to the diversity gain achieved against fading. It is well known that in order to attain most of the diversity gain (against fading), the antenna elements should be placed apart with a distance (many times) greater than the wavelength=[speed of light]/[frequency] of the carrier. The results presented in this paper indicate that in order to attain most of the antenna gain (against interference) in the reverse-link of finite-bandwidth interference-limited CDMA systems, the interantenna distance should be (many times) greater than a new parameter which is defined as the chiplength=[speed of light]/[chip rate] of the spreading code.     

Substream-based soft handoff in CDMA cellular networks

We present a new soft handoff scheme that enhances the reliability during soft handoff by increasing the signal distance (Euclidean and/or Hamming) in forward link code division multiple access cellular networks. Each base station participating in soft handoff sends a disjoint subset of the main data stream (called sub-stream) and the mobile receiver reassembles the sub-streams and restores the main data stream. This approach can reduce the data rate per base station by a factor of the number of participating base stations and thereby can increase the signal distance as opposed to the diversity gain. It is shown that the proposed soft handoff scheme is promising for high data rate applications which are the major interests in the next generation cellular networks.     

Performance of wideband CDMA systems supporting multimedia traffic

We evaluate the performance of wideband CDMA cellular systems providing different classes of multimedia traffic and supporting user mobility. A Markovian teletraffic model of the user dynamics is developed. Constraints are imposed in the model which account for the multiple access interference among active users. Results are shown in terms of call blocking probabilities and average number of active connections. The methodology proposed is a useful tool for the design and planning of third generation cellular systems     

Improving idle handoff in CDMA mobile systems

In this letter, we have developed an efficient idle handoff technique, starting from field-test results obtained with the method used in present CDMA mobile phones, where the handoff is performed when a new pilot rises over the current one by a predefined constant. Unlike the conventional method, the proposed method decides the handoff based on a variable threshold, the value of which is automatically determined depending on the pilot strength received from the home base. It is shown that the proposed method reduces unnecessary idle handoffs and keeps the mobile tracking the pilot received with sufficient strength     

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     

Performance evaluation of wireless multimedia CDMA networks usingadaptive transmission control

When multimedia information including speech, image, data etc. are transmitted, the difference in information rates, required quality, and traffic performance should be taken into account. A wireless spread spectrum system can achieve a flexible balance of these differences because of inherent asynchronous code-division multiple-access (CDMA) capability. In order to accomplish wireless multimedia communication, three parameters are investigated: the transmission power, packet length, and time slot. We propose and investigate two methods to accomplish wireless multimedia communication by using these parameters: (1) a method based on control of both transmission power and transmission packet length and (2) a method based on control of both transmission power and adaptive time slot. We derive the optimal transmission packet length and transmission power in (1) and derive optimal adaptive time slot and transmission power in (2) by using traffic of measurement and nonlinear programming. By the comparison, the two proposed methods are compared to the minislotted alternating priority (MSAP). Numerical results demonstrate that the proposed methods, when the priority of each medium is graded, show better performance than other systems in terms of average throughput and delay time of all media     

Vertical handoff in integrated CDMA and WLAN systems

Future wireless networks are expected to consist of different types of wireless networks such as code division multiple access (CDMA) networks and public wireless local area networks (WLANs). The integrated network will require vertical handoffs between different networks. In this letter, we propose vertical hard- and soft-handoff algorithms and evaluate their performance in commercial wireless networks.     

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     

Performance evaluation of a joint CDMA/NC-PRMA protocol forwireless multimedia communications

A joint code division multiple access and noncollision packet reservation multiple access (CDMA/NC-PRMA) technique is proposed and investigated as an uplink protocol for the third-generation (3G) mobile systems. Being the underlying time division multiple access (TDMA) architecture of the CDMA transmissions, NC-PRMA enables the base station (BS) to have a centralized control over the slot allocation policy. In order to reduce the multiple access interference (MAI) variation in a CDMA transmission, two different slot assignment schemes, referred to as load-balancing (LB) and power-grouping (PG) schemes, are proposed and evaluated. Simulation results show that considerable improvement can be achieved over the joint CDMA/PRMA scheme, in which the MAI variation is reduced by way of a dynamic permission probability for contending terminals. Especially when an imperfect power control mechanism is considered, the proposed PG assignment scheme achieves significant performance advantages     

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。     

CDMA soft handoff analysis in the presence of power control errorand shadowing correlation

Soft handoff has a special importance to power controlled code division multiple access (CDMA) systems. We introduce a new analytical model for CDMA soft handoff with emphasis on interference statistics at a system level. The relationship between soft handoff and power control is further explored by integrating power control error and shadowing correlation into the model. A new forward link model is also devised to study the effects of soft handoff on forward link performance. It is observed that the performance degradation due to power control error increases with increase in soft handoff region and higher shadowing correlation lowers the interference contribution from nonhandoff mobile stations (MS). It is also found that, unlike the reverse link, the forward performance depends on both given soft handoff setting and system load     

软切换参数对CDMA1X性能的影响及优化

分析并仿真得到了在CDMA1X系统中,不同业务负载情况下的软切换开销因子与静态和动态软切换门限,以及导频信道功率比例等参数的关系,并在此基础上提出了相应的网络优化措施。仿真采用蒙特卡罗"快照"法,并在进行传播损耗计算时加入了阴影衰落的影响。     

Power control for wireless multimedia CDMA systems

In a wireless multimedia communication system where different types of traffic such as speech, data, and video have different data rates, good power control for individual traffic can ensure quality of service (QoS) and achieve a better throughput at the same time. The authors propose a closed-form power control function based on a parameter called the traffic exponent that simplifies the power control processing and improves the overall CDMA system performance     

QoS guarantee and power distribution for soft handoff connections in cellular CDMA downlinks

A two-phase power distribution scheme for supporting quality-of-service (QoS) and best effort traffic is proposed. We first formulate the power distribution for QoS traffic as an optimization problem so that the number of simultaneously transmitting connections is maximized. Optimum power distribution is difficult to implement in practice due to both the computational complexity and the requirement for global information about the mobile station (MS) locations, connection channel conditions, and traffic load in the system. We then propose a heuristic scheme of power distribution for soft handoff (SHO) connections. The full scheme includes an initial power distribution (IPD) and a power distribution adjustment (PDA). IPD allocates BS power resource based on the channel condition of each individual connection, while PDA further coordinates the power distribution between neighboring base stations (BSs) in order to accommodate more connections. The proposed power distribution scheme can achieve a capacity close to that of the optimum power distribution, while providing much higher transmission throughput for best effort data traffic. The proposed power distribution scheme can be applied to existing SHO schemes for efficient BS power resource usage. The scheme does not require global information, and its implementation can be further simplified by performing IPD only with slight performance degradation.