IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 55, NO. 5, SEPTEMBER 2006 1603 Design and Performance Analysis of a New Soft Handoff Scheme for CDMA Cellular Systems Xiaomin Ma, Member, IEEE, Yun Liu, Student Member, IEEE, and Kishor S. Trivedi, Fellow, IEEE Abstract—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 chan- nels 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 unnecessar- ily 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 con- structed. 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. Index Terms—Code-division multiple access (CDMA), perfor- mance, soft handoff, stochastic reward net (SRN). I. I NTRODUCTION A N important feature of cellular code-division multiple- access (CDMA) systems is soft handoff, which provides seamless connections of mobile calls between cells [9]. The soft handoff process is initiated and ended based on the received signal strength of a mobile station (MS). Comparing with hard handoff, which is used in frequency-division multiple-access (FDMA) and time-division multiple-access (TDMA) systems, there are many advantages of using soft handoff in CDMA systems along with power control, selection diversity, and rake receiver [1], [10], [11], [13], [14]. Although the capacity of CDMA systems is interference-limited in nature [6], channel shortages may occur, and the utilization efficiency of traffic channels may decrease because soft handoff Manuscript received June 15, 2004; revised June 15, 2005 and November 6, 2005. This work was supported by Motorola Fellowships and by AFOSR MURI under Grant F49620-1-0327. The review of this paper was coordinated by Prof. T. Hou. X. Ma is with the Department of Engineering and Physics, Oral Roberts University, Tulsa, OK 74171 USA (e-mail: mx@ee.duke.edu). Y. Liu and K. S. Trivedi are with CACC, Department of ECE, Duke Univer- sity, Durham, NC 27708 USA (e-mail: liu@ee.duke.edu; kst@ee.duke.edu). Digital Object Identifier 10.1109/TVT.2006.877489 occupies multiple channels simultaneously. In urban CDMA cellular systems, the handoff area takes about 30%–50% [21] of the entire cell area. Therefore, the handoff process, in which each MS in the handoff area occupies two or more channels, would cause degradation in radio frequency (RF) capacity and an increase in call dropping probability in the downlink. Some techniques have been introduced to cope with these problems by adjusting soft handoff parameters to optimize performance [3]–[5], setting prioritized queues for handoff [16], using adaptive channel reservation [15], and eliminating pseudohandoff calls [2]. Lee and Cho [12] proposed channel borrowing from a stationary handoff call to reduce the chances of handoff dropping. However, those schemes only concentrate on improving a particular aspect of the performance in the CDMA system. In this paper, we first analyze the conventional soft handoff scheme in CDMA cellular systems, such as IS95/CDMA2000 [17], [18]. We observe that there exist soft handoff calls that move away from the target cell to the neighboring cell or are in stationary status, which unnecessarily occupy multiple chan- nels. The proposed scheme discriminates such pseudohandoff calls from real handoff calls by measuring and estimating their relative mobility, thus setting up a channel convertible set (CCS) [28]. When there are free channels, the handoff process operates in the same way as that in conventional IS95/CDMA2000 cellular systems. However, when all chan- nels are occupied and a handoff request occurs, a weaker (or noncontrolling) channel used by a call in the CCS is converted to a new handoff request. Meanwhile, the number of guard channels reserved exclusively for soft handoff calls can be dynamically adjusted in terms of the size of the CCS so that more channel resources are saved for new calls. With our soft handoff scheme, the system channel shortage due to multiple- channel occupation by soft handoff calls is remarkably allevi- ated. Consequently, the performance of the CDMA system is improved without sacrificing the quality of service. This paper is organized as follows: In Section II, some fundamental concepts and features of soft handoff in CDMA are described, and a relative mobility estimation method is illustrated and discussed. The proposed handoff scheme and its implementation issues are presented in Section III. To evaluate the new handoff scheme, stochastic reward nets (SRNs) are developed in Section IV. In Section V, the performance of the proposed scheme is analyzed and compared with that of the IS95/CDMA2000 conventional soft handoff scheme and channel-borrowing handoff scheme [12]. Finally, conclusions are made in Section VI. 0018-9545/$20.00 © 2006 IEEE