Handoff Management in Cdma Systems With a Mixture of ... - CiteSeerX

HANDOFF MANAGEMENT IN CDMA SYSTEMS WITH A MIXTURE OF LOW RATE AND HIGH RATE TRAFFICS Duk Kyung Kim and Dan Keun Sung

Dept. of EE, Korea Advanced Institute of Science and Technology, Taejon, 305-701, KOREA [email protected], [email protected] Abstract - Hando is an important issue in mobile communication systems. However, most previous studies were based on TDMA systems considering voice and data trac with the same bit rate. Hando management for high rate trac is required in multi-media CDMA systems. Hando management scheme for high rate trac is proposed by using guard channels and Reservation-on-Demand queue control. The effects of the number of channels, the required number of basic rate channels, the number of guard channels, and the mean residual time of overlap region are also investigated. I. Introduction

Hando is an important issue in mobile communication systems and many analytical approaches have been proposed for hando analysis in these systems. Previous studies [1{ 3], however, were based primarily on hard hando in TDMA systems. Unlike TDMA systems, CDMA systems can provide soft hando [4]. Mobile stations within a soft hando region utilize multiple radio channels and receive their signals from multiple base stations simultaneously. Hando problems [5{8] assumed that both voice and data have the same bit rate, and that they require homogeneous channels for handos. Epstein and Schwartz [9] considered narrowband and wideband traf cs but assumed wideband trac does not require handos. Future mobile communication systems will be required to accommodate multi-media services with dierent bit rates and dierent quality-of-service's (QoSs). As one of candidate systems, a multi-code (MC) CDMA system [10] splits a data signal into a multiple of basic rate and spreads each basic rate stream with a dierent spreading code. The IMT (International Mobile Telecommunication)-2000, which will be commercially implemented in the near future, is expected to accommodate a wide variety of services such as video phone, fax, and voice. Therefore hando management of this multirate trac is required in this system. High rate trac suers from more blockings than low rate trac in a multimedia trac environment. In the case of the high rate trac, it is desirable that hando trac may be controlled not to suer from severe hando failures compared with low rate trac.

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Hando management may be classi ed into two methods: a guard channel method and a queue method. The guard channel method may waste a resource under low trac load, while the queue method may not be suitable for real-time trac due to the inevitable delay problem. We consider two dierent types of real-time trac: rate-1 trac and rate-m trac. The rate-1 trac requires a basic rate channel and the rate-m trac requires m basic rate channels. Ch guard channels are allocated for hando calls of both rate-1 and rate-m tracs. Three hando management cases are considered according to the queue control method for rate-m hando calls. Queue can be used for real-time traf c in CDMA systems because the old link can be continued to be served until the mobile station leaves out soft hando region. The above three cases are compared, and the eects of the number of channels C , the required number of basic rate channels m, the number of guard channels Ch , the queue size Q, and the mean residual time of overlap region are also investigated. Section II describes soft hando in a square cell placement and section III proposes a hando management scheme. Section IV analyzes the system performance using a birth-death model and section V shows some numerical examples. Finally, section VI draws conclusions. II. Soft Handoff

Since communicating mobile stations move from a cell to its neighboring cell, hando is essential for seamless communications. CDMA systems can support soft hando, which is a make-before-break method. When the pilot signal from a new basestation (BS) is stronger than the threshold value T ADD, a new link to the BS is established while maintaining the existing link. In this case the call is said to be in soft hando. It is assumed that a mobile station can be in soft hando with two BSs with strong signals. If the pilot signal from a third BS becomes strong than either of the two strong pilot signals, another hando occurs and the network drops the weakest link. When the pilot signal from either the old BS or the new BS weakens to below T DROP, the bad connection is released and only a single good connection is

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Radio Resource (C) Basestation (BS)

λ h1

Reserved Channel (Ch )

Overlap region Outer boundary

Reservation -on-Demand

λ hm

Inner boundary

λ n1

Cell boundary

λ nm

Fig. 2. Hando management scheme (Case I).

Fig. 1. Illustration of various boundaries and overlap regions.

maintained after that time. Soft hando regions may vary according to hando-related parameters such as T ADD and T DROP, and hando is also aected by radio propagation characteristics and the required Eb =Io value. Fig. 1 illustrates various boundaries and overlap regions in a square cell structure. For geometrical simplicity it is assumed that one cell area is divided into three regions in the analysis of soft hando. These regions are 1) the inner cell region, 2) the soft hando region (SR), and 3) the outer cell region. These regions are bounded by an inner boundary and an outer boundary. The region bounded by a cell boundary is called an ordinary cell. Even when a mobile station is in soft hando, another hando occurs if the pilot signal from a third BS becomes stronger than one of the original pilot signals. Thus, we here introduce an overlap region, which is the region between two overlapping adjacent outer cells. In this cell structure the SR is subdivided into four overlap regions. For a hexagonal cell structure the SR consists of six overlap regions. III. Handoff Management Scheme

channel resources. Ch guard channels are here commonly accessed by hando calls of both rate-1 and rate-m tracs. Queue is used by rate-m hando calls to give priority over rate-1 hando calls. In CDMA systems, the queue can be used for real-time trac because the old link can be continued to be served until the mobile station leaves out the outer cell. because the delay up to the residual time in an overlap region is allowable. As a queue control, a Reservation-on-Demand scheme is proposed which reserves up to m idle channels when the queue is not empty. We classify the hando management scheme into the following three cases according to the queue control method:  Case I : Queue/guard method with Reservation-onDemand queue control.  Case II : Queue/guard method without Reservation-onDemand queue control.  Case III : Guard channel method without queue. Fig. 2 shows case I. New calls are blocked if the number of communicating channels exceeds C , Ch after arrivals. Ch guard channels are accessed commonly by rate-1 and rate-m hando calls. rate-1 hando calls are blocked if there is no idle channel on their arrivals but rate-m calls are queued if the number of communicating channels exceeds C after arrival. If queued, the hando calls do not wait until the number of the idle channels reaches m but they reserve idle channels up to m. Thus the other arrivals are blocked or failed after the queueing.

Two dierent types of real-time tracs are considered: rate-1 trac and rate-m trac. Without a special control rate-m trac suers from more blockings and hando failures than rate-1 trac. But the hando failures of rate-m IV. Performance Analysis trac are desired not to be more severe than for rate-1 trafThe system performance is now analyzed for case I. The c. Therefore, hando trac management may be required for rate-m trac. A guard channel method is suitable for real- performance analyses for cases II and III are similar to the time hando calls. However, the required number of guard analysis for case I, and thus these analyses are omitted in this channels should be minimized because this method may waste paper. New rate-1 calls and new rate-m calls are assumed 0-7803-4323-9/98/$5.00 copyright 1998 IEEE

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to originate from each cell as Poisson processes with rates The blocking probabilities of rate-1 calls (Pb1 ) and rate-m n1 and nm , respectively. In addition, the hando attempts calls (Pbm ) are given by X of rate-1 calls and rate-m calls follow Poisson processes with Pb1 = p(i; j; q); (2) arrival rates h1 and hm , respectively. The hando attempts s2 b1 are a function of terminal mobility, new call arrivals, soft

b1 = f(i; j; q)jC , Ch  i + mj  C; 0  q  Qg hando, and other factors. The call holding times of rateX P = p(i; j; q); (3) bm 1 calls and rate-m calls are exponentially distributed with s2 bm means of 1=c1 and 1=cm, respectively. The residual times of

bm = f(i; j; q)jC , Ch , m < i + mj  C; 0  q  Qg rate-i calls in the outer cell and in the overlap region are also exponentially distributed with means of 1=O;i and 1=V;i , The hando failure probability of rate-1 calls (Pf 1 ) is exrespectively. pressed as The system performance can be analyzed by the birthX Pf 1 = p(i; j; q); (4) death process. The state is de ned as s2 h1

h1 = f(i; j; q)jC = i + mj; 0  q  Qg:

s = (i; j; q);

where i denotes the number of rate-1 calls in the system, j Let Pfull denote the probability that the queue is full, then the number of communicating rate-m calls, and q the number Pfull is obtained by of rate-m calls in queue. The detailed state transitions are X Pfull = p(i; j; q); full = f(i; j; q)jq = Qg: given by (5) s2 full n1 +h1 (i + 1; j; q ) (i; j; q) ,,,,,! h1 (i + 1; j; q ) (i; j; q) ,,! nm +hm (i; j + 1; q ) (i; j; q) ,,,,,,! hm (i; j; q) ,,, ! (i; j; q + 1)

(i; j; 0) ,i,! (i , 1; j; 0) 1 (i; j; 0) ,,! (i; j , 1; 0) im (i; j; q) ,j ,,,,, ! (i; j; q , 1) m +qq (i; j; q) ,i,! (i , 1; j + 1; q , 1) 1

(i; j; q) ,i,! (i , 1; j; q) 1

The hando failure probability of rate-m calls (Pfm ) is given ; i + mj < C , Ch by ; C , Ch  i + mj < C X  qq p(i; j; q)   V m  +  + Pfull Pfm = ; i + mj  C , Ch , m cm Vm (6) for all s hm (1 , Pfull ) ; C , Ch , m < i + mj  C The hando calls that encountered a full queue can be sup&0q