PAPER A Preemptive Priority Handoff Scheme in Integrated Voice and Data Cellular Mobile Systems

Transcription

1 IEICE TRANS. COMMUN., VOL.E82 B, NO.10 OCTOBER PAPER A Preemptive Priority Handoff Scheme in Integrated Voice and Data Cellular Mobile Systems Bo LI, Qing-An ZENG, Kaiji MUKUMOTO, and Akira FUKUDA, Members SUMMARY In this paper, we propose a preemptive priority handoff scheme for integrated voice/data cellular mobile systems. In our scheme, calls are divided into three different classes: handoff voice calls, originating voice calls, and data calls. In each cell of the system there is a queue only for data calls. Priority is given to handoff voice calls over the other two kinds of calls. That is, the right to preempt the service of data is given to a handoff voice call if on arrival it finds no idle channels. The interrupted data call returns to the queue. The system is modeled by a two-dimensional Markov chain. We apply the Successive Over-Relaxation (SOR) method to obtain the equilibrium state probabilities. Blocking and forced termination probabilities for voice calls are obtained. Moreover, average queue length and average transmission delay of data calls are evaluated. The results are compared with another handoff scheme for integrated voice/data cellular mobile systems where some numbers of channels are reserved for voice handoff calls. It is shown that, when the data traffic is not very light, the new scheme can provide lower blocking probability for originating voice calls, lower forced termination probability for ongoing voice calls, and shorter average queue length and less average transmission delay for data calls. key words: mobile communication systems, integration of voice and data handoff scheme, preemptive priority 1. Introduction One of the central issues in the performance characterization of cellular mobile and personal communication systems (PCS) is the problem of handoff [1]. Handoff denotes the process of changing the channel (frequency, time slot, spreading code, or combination of them) associated with the current connection to maintain acceptable service quality or to provide better service. It is often initiated either by cell boundary crossing or deteriorated service quality in the current channel. With the penetration of PCS, the microcell and the hybrid cell (macro-, micro-, pico-) structures are exploited to support the drastically increased demand [2]. The smaller cell size and the variable propagation conditions in microcells introduce much more frequent handoffs than ever before [3]. Poorly designed handoff strategy will Manuscript received January 17, Manuscript revised April 30, The author is with Institute of Communication Engineering, Xidian University, Xi an, , China. The author is with 3rd Development Department, Mobile Communication Systems Division, NEC Corporation, Tokyo, Japan. The authors are with the Department of Electrical Engineering, Shizuoka University, Hamamatsu-shi, Japan. generate very heavy signaling traffic and worse service quality. The handoff process usually consists of two phases: the handoff initiation phase and the handoff execution phase. In the handoff initiation phase, the service quality is monitored in order to decide when to trigger the handoff. Various types of handoff initiating criteria have been proposed [4] [6]. In the handoff execution phase, allocation of new resources to the handoff call is performed. It should be noted that the focus of this paper is put on the handoff execution phase, and we assume that the handoff request detection and initiation procedures are perfect (i.e. all valid requests are detected and no invalid requests activate the handoff procedure). A handoff area is defined as the area in which the strength of the radio wave from the current base station received by a mobile is between the handoff threshold and the receiver threshold [1], [7]. When received signal strength falls below the handoff threshold, a handoff procedure is initiated. The handoff process must be completed within the handoff area. However, in our scheme, no queues are provided to handoff voice calls, so handoff area needs not be considered. In our scheme, if on arrival there are no channels available for a voice handoff request, the ongoing conversation is forced to terminate. Forced termination of ongoing voice calls is more undesirable than blocking of originating calls from the user s viewpoint. Therefore, several schemes giving priority to handoff requests have been proposed to reduce the forced termination probability. Traffic models and performance measures of systems adopting some priority schemes are discussed in [8] [19]. The simplest way of giving priority to handoff requests is to reserve a fixed number of channels for them. That is, when the number of free channels in a cell is less than or equal to a predefined value, originating calls will be blocked. The handoff calls can still gain access to the system until there are no available channels. This scheme is called priority reservation scheme or cut-off priority scheme. In [12], systems with queues only for handoff requests are studied. In [18], on the other hand, queues are allowed only for originating calls. Both the originating calls and handoff requests are allowed to be queued in the system studied in [8]. In [9], [13], handoff schemes with two-level priority reservation have been proposed. Cellular communication systems that sup-

2 1634 IEICE TRANS. COMMUN., VOL.E82 B, NO.10 OCTOBER 1999 port a mixture of platform types distinguished by different mobility characteristics are considered in [14] [17]. However, in all of the above studies, only cutoff priority is concerned. Moreover, they do not take multiple types of services into consideration. With the development of cellular mobile systems, data services must also be considered [20]. Thus, a handoff scheme in data mobile systems was studied in [10]. However, future cellular mobile systems will be required to accommodate multiple types of services, such as voice, data, and video. In order to meet the future demands, the handoff strategy needs to take different features of these services into account, i.e., the ideal handoff process is service-dependent. For example, voice transmission is very sensitive to interruption. On the other hand, transmission delay of data has not much impact on the data performance (the data is not delay-sensitive). Therefore, a successful handoff without interruption is very important to voice, but not critical to dada. In [19], a special two-dimensional model for integrated voice/data cellular mobile systems, which gave preemptive priority to voice calls, was proposed. However, in this model, originating voice calls and handoff requests were not distinguished. Since forced termination of ongoing voice calls is more annoying than blocking of originating voice calls from the user s viewpoint, much more priority should be given to handoff voice calls. In [11], a handoff scheme in integrated voice/data cellular systems with priority reservation for voice handoff requests was proposed. In this scheme, some numbers of channels were reserved for voice handoff requests. Queues are allowed for voice handoff requests and data channel requests (originating data calls and handoff data requests). Moreover, a data channel request in the queue can be transferred to another queue in an adjacent cell when it moves out of the cell before getting a channel. In this paper, we propose a preemptive priority handoff scheme for integrated voice/data cellular mobile systems. Calls are divided into three different classes: handoff voice calls, originating voice calls, and data calls (originating data calls, handoff data calls, and data calls transferred from other cells). Data calls are queued in a data queue with finite capacity if no channels are available on their arrival. In the paper, all the calls, no matter which class they belong to, can access to all the channel resources if they are in idle. That is to say, no guard channels are reserved. However, priority is given to handoff voice calls over the other two kinds of calls. Specifically, the right to preempt the service of data is given to handoff voice calls if on arrival they find no idle channels. The interrupted data call returns to the queue and waits for some channel to be available. In the next two sections, we describe the preemp- Fig. 1 System model. tive handoff scheme and the traffic model. Analysis of the scheme is given in Sect. 4. Numerical results are presented in Sect. 5. Finally, Sect. 6 concludes the paper. 2. Preemptive Handoff Scheme For simplicity, fixed channel assignment scheme is assumed in this paper. That is to say, a set of channels is permanently assigned to each cell. We consider a system with many homogeneous cells each having S channels, and focus our attention on a single cell. This cell is called the marked cell. As shown in Fig. 1, in the base station there is a queue Q with capacity M for data channel requests. There are no queues for voice calls (handoff and originating voice calls). For both voice mobile users and data mobile users, there is no handoff area. But there is a cell boundary between the two neighboring cells. The boundary is defined as the locus of points where the average received signal strength of the two neighboring cells is equal. When a mobile user holding a channel approaches from a neighboring cell toward the marked cell and the received signal strength of the current base station goes below that of the base station in the marked cell, a handoff request is generated in the marked cell. Priority is given to voice handoff requests over originating voice calls and data calls. Specifically, the right to preempt the service of data is given to handoff voice calls if on arrival they find no idle channels. The preempted data call returns to the queue, and waits for a channel to be available. A voice handoff request will be rejected by the system under either of the following two circumstances: one is that all the channels are occupied by already arrived voice calls, the other one is that the data queue is full (that is to say, in our scheme, data calls in service can not be preempted by voice handoff calls when the data queue is full). It is possible to consider another scheme, that is whether the queue is full or not, data calls in service can be preempted by handoff voice calls. However, there is almost no difference between these two schemes when the capacity of the queue is very large. As to a newly generated voice

3 LI et al: A PREEMPTIVE PRIORITY HANDOFF SCHEME 1635 call, it will be blocked if there are no available channels on its arrival. A data channel request is queued in Q when on arrival it finds no available channels. It will be blocked if the queue is full. Furthermore, a data channel request in the queue can be transferred to the queue of the target cell when it moves out of the cell before it gets a channel. 3. TrafficModel 3.1 Channel Holding Time Let the random variable T CV be the call holding time of a voice call. That is the time a voice call would continue if the call is not forced into termination. A call is forced into termination if a handoff is required and fails before call completion. The call holding time T CV of a voice call is assumed to have an exponential distribution with mean E[T CV ](= 1/µ CV ). Let the random variable T CD denote the data length (total transmission time needed), which is also assumed to have an exponential distribution with mean E[T CD ](= 1/µ CD ). Let the random variable T dwell be the dwell time of a mobile user (voice mobile user or data mobile user) in a cell. Again we assume that it has an exponential distribution with mean E[T dwell ](= 1/µ dwell ). The channel assigned to a voice call will be held until either the call is completed in the cell or the mobile user moves out of the cell before call completion. Therefore, the channel holding time of a voice call T V is equal to the smaller one between T dwell and T CV. Assuming independence of T dwell and T CV and using the memoryless property of the exponential pdf (probability density function), we see that the channel holding time T V of a voice mobile user is exponentially distributed with mean E[T V ]= 1 = E[min(T dwell,t CV )] µ V 1 = (1) µ CV + µ dwell As to a data call not preempted by a voice handoff request, the channel assigned to it will be held until the call is completed in the cell or the mobile data user moves out of the cell before call completion. Similar to the case of a voice call, its channel holding time T D is equal to the smaller one between T dwell and T CD.Thus T D is exponentially distributed with mean E[T D ]= 1 = E[min(T dwell,t CD )] µ D 1 = (2) µ CD + µ dwell 3.2 Arrival Process of Calls We assume that the arrival processes of originating voice calls and originating data calls in a cell are both Poisson, with rates λ OV and λ OD, respectively (see Fig. 1). If the call holding time of a mobile user is greater than the dwell time in the cell, a handoff request will be initiated in a neighboring cell. Since we assume an equilibrium homogeneous mobility pattern, the mean number of incoming users into a cell is equal to that of outgoing ones from the cell. Therefore, the arrival rate of handoff requests at the marked cell is equal to the departure rate of handoff calls from the cell. Let λ HV denote the arrival rate of handoff voice requests, and λ HD denote the arrival rate of handoff data requests. It is apparent from the above discussions that λ HV = E[C V ]µ dwell (3) where E[C V ] is the average number of voice calls holding channels in a cell. And λ HD = E[C D ]µ dwell (4) where E[C D ] is the average number of data calls holding channels in a cell. We assume that the arrival processes of both kinds of handoff requests are Poisson processes having the above rates. A data channel request in the queue of a cell is transferred to the queue of the target cell when it moves out of the cell before getting a channel. The arrival rate λ TD of transferred requests at the marked cell is given by λ TD = L D µ dwell (5) where L D is the average length of the data queue Q. Again we assume that the arrival process of transferred data requests is Poisson. 4. Performance Analysis 4.1 The System-State Probabilities We define the state of the marked cell by a two-tuple of non-negative integers v =(i, j), where i is the number of channels used by voice calls, j is the sum of the number of data calls holding channels and the number of data calls waiting in the queue. It is apparent from the above assumptions that (i, j) is a two-dimensional Markov chain. The state space V of the cell is given by V = {v 0 i i + j S 0 i S<i+ j S + M} (6) In our scheme, the underlying processes that drive the system are as follows:

4 1636 IEICE TRANS. COMMUN., VOL.E82 B, NO.10 OCTOBER The arrival of data calls at the marked cell. 2. The generation of originating voice calls in the marked cell. 3. The arrival of handoff voice calls at the marked cell. 4. The departure of voice calls from the marked cell. 5. The departure of data calls from the marked cell. In the following, we will describe the state transitions of the system based on the above driving processes. Data Call Arrival Define r 1 (v, w) as the rate of transition from state v = (i, j) to state w =(i, j + 1) caused by the arrival of a data call. If 0 i<sand i + j S 1, the call will be served immediately. If 0 i S and S i + j S+M 1, the data call will be buffered in the queue. In the above two cases, r 1 (v, w) =λ D = λ OD +λ HD +λ TD. Otherwise, the data call will be blocked by the system. Originating Voice Call Arrival Define r 2 (v, w) as the rate of transition from state v = (i, j) to state w =(i +1,j) caused by the arrival of an originating voice call. If 0 i i + j S 1, the originating voice call will be served immediately by the system, and r 2 (v, w) =λ OV. Otherwise, the call will be blocked by the system. Handoff Voice Call Arrival Define r 3 (v, w) as the rate of transition from state v = (i, j) to state w =(i +1,j) caused by the arrival of a handoff voice call. If 0 i i + j S 1, the handoff voice call will get a channel immediately. If 0 i S 1 and S i + j S + M 1, a data call in service will be preempted by the handoff voice request, and the handoff voice call will get the channel. The preempted data call returns to the queue. In the above two cases, r 3 (v, w) = λ HV. Otherwise, the handoff voice call will be blocked by the system. Voice Call Departure Define r 4 (v, w) as the rate of transition from state v = (i, j) to state w =(i 1,j) caused by the departure of a voice call from a channel. In this case, r 4 (v, w) =iµ V. Data Call Departure Define r 5 (v, w) as the rate of transition from state v = (i, j) to state w =(i, j 1) caused by the departure of a data call from the cell. If i + j S, the data call departures only from a channel, and r 5 (v, w) =jµ D.If i+j >S, the data call departures either from a channel or from the data queue, and r 5 (v, w) =(S i)µ D +(i+ j S)µ dwell. To find the statistical equilibrium state probabilities P (i, j) for the marked cell, we write the flow balance equations (7), (8) and (9) for the states. [λ V + λ D + u(i 1)iµ V + u(j 1)jµ D ]P (i, j) =(i +1)µ V P (i +1,j)+(j +1)µ D P (i, j +1) + u(i 1)λ V P (i 1,j)+u(j 1)λ D P (i, j 1) 0 i + j S 1, 0 i S 1 (7) [u(m 1)u(S i 1)λ HV + u(m 1)λ D + u(i 1)iµ V + u(j 1)jµ D ]P (i, j) = u(m 1)u(S i 1)(i +1)µ V P (i +1,j) +[(S i)µ D + µ dwell ]u(m 1)P (i, j +1) + u(i 1)λ V P (i 1,j)+u(j 1)λ D P (i, j 1) i + j = S, 0 i S (8) [u(s i 1)u(S + M i 1 j)λ HV + u(s + M i j 1)λ D + u(i 1)iµ V +(S 1)µ D +(j + i S)µ dwell ]P (i, j) = u(s i 1)u(S+M i 1 j)(i+1)µ V P (i+1,j) +[(S 1)µ D +(i + j +1 S)µ dwell ] u(s + M i j 1)P (i, j +1) + u(i 1)λ HV P (i 1,j)+λ D P (i, j 1) S<i+ j S + M, 0 i S (9) In the above flow balance equations, we assume that P (i, j) = 0 when (i, j) / V. We also define the unit step function as { 1 n 0 u(n) = (10) 0 n<0 and use the notation λ V = λ HV + λ OV (11) Corresponding to N T =(1+S) ( S 2 + M +1) states in the state space V, there are N T flow balance equations. However, note that one of these balance equations can be obtained from other N T 1 equations. Since the sum of all state probabilities must be equal to 1, we have S i=0 S+M i j=0 P (i, j) = 1 (12) as another independent equation. As an example, the state transition diagram, when S = 2 and M = 2, is given in Fig. 2. In order to obtain all the P (i, j)s from Eqs. (3) (5), (7) (9) and (12), we use the iteration method shown in Appendix. 4.2 Performance Measures Based on the P (i, j)s obtained, various performance characteristics can then be readily calculated. The blocking probability of originating voice calls is apparently S 1 B OV =1 i=0 S 1 i j=0 P (i, j) (13) The blocking probability of handoff voice calls is

5 LI et al: A PREEMPTIVE PRIORITY HANDOFF SCHEME 1637 Fig. 2 An example of state transition diagram with S =2,M =2. B HV = M j=0 S 1 P (S, j)+ P (i, S + M i) (14) i=0 When a voice user is assigned to a channel, subsequent cell boundary crossings while the call is in progress will necessitate further handoffs. The handoff requirement probability P H of a voice call is the probability that the call holding time exceeds the dwell time of the voice user in a cell, i.e. P H = Prob{T CV >T dwell } (15) Assuming that T CV and T dwell are independent, we can easily get µ dwell P H = (16) µ CV + µ dwell The forced termination probability P F, that a voice call accepted into the system is forced into termination during its lifetime, is an important measure of the system performance. It is important to distinguish between this probability and the blocking probability B HV of a single voice handoff attempt. The forced termination probability P F of a voice call can be expressed as P F = P H B HV [(1 B HV )P H ] l 1 l=1 B HV P H = 1 P H (1 B HV ) The average length of Q is L D = S S+M i i=0 j=s+1 i (17) (i + j S)P (i, j) (18) The blocking probability of data calls is S B D = P (i, S + M i) (19) i=0 E[C V ] denotes the average number of voice calls holding channels and we have E[C V ]= S i=0 S+M i j=0 ip (i, j) (20) The average number of data calls holding channels in a cell, denoted by E[C D ], is given by S S i S E[C D ]= jp(i, j)+ i=0 j=0 (S i)p (i, j) (21) S+M i i=0j=s+1 i Using Little s formula, average value of waiting time T W (random variable) of data channel requests in the queue is given by E[T W ]= L D λ D (1 B D ) (22) Average value of duration T S (random variable) of a data call in a cell is E[T S ]= E[C D]+L D (23) λ D (1 B D ) Let us define the random variable N h as the number of cells that a data mobile user must cross before the completion of the call. Then, we have E[T CD ] E[N h ]= (24) E[T S ] E[T W ] Therefore, from Eqs. (22), (23) and (24), we can get the average transmission delay E[T D ] of data calls E[T D ]=E[N h ]E[T W ]= L DE[T CD ] (25) E[C D ] 5. Numerical Results and Discussions In this section, some system characteristics are numerically studied. Moreover, the results are compared with those in the handoff scheme for integrated voice and data mobile cellular radio system with priority reservation for voice handoff requests [11]. In the scheme presented in [11], a system with many cells each having S channels is considered. In

6 1638 IEICE TRANS. COMMUN., VOL.E82 B, NO.10 OCTOBER 1999 Fig. 3 Comparison of forced termination probabilities for the three schemes versus γ. each cell, there are two queues Q V and Q D with capacities M V and M D for voice handoff requests and data calls, respectively. S S C channels are reserved for voice handoff requests. A voice handoff request is queued in Q V if on arrival it finds no idle channels. On the other hand, a data channel request is queued in Q D when on arrival it finds less than or equal to S S C available channels. Both channel requests are blocked if their own queues are full on their arrival. An originating voice call is blocked if on arrival it finds less than or equal to S S C available channels. No queue is assumed for originating voice calls. A voice handoff request in the queue is deleted from the queue when it passes through the handoff area before getting a new channel (i.e., forced termination) or its communication is completed before passing through the handoff area. A data channel request in the queue Q D can be transferred to the queue of the target cell when it moves out of the cell before it gets a channel. In our numerical examples, the parameters of the preemptive priority scheme are set as follows: E[T CV ] = 120.0s, E[T CD ]=60.0s, E[T dwell ]=30.0s, S = 10, M = 100. As for the priority reservation scheme, for comparison purposes, the capacity of the queue for handoff voice calls is set to be 0 and the capacity M D of the data queue is set to be 100. Other parameters in the scheme are set to be equal to those in the preemptive priority handoff scheme. Note that in the priority reservation scheme with S S C = 0, there are no guard channels provided to voice handoff requests. This system is nothing but a system without any priority given to voice handoff calls. Thus in the following, this system is called non-priority scheme. We define ρ = λ OD E[T CD ]+λ OV E[T CV ] as the traffic intensity of the originating calls in the marked cell. Moreover, γ is defined as the ratio of the data traffic intensity to the total traffic intensity, that is γ = λ ODE[T CD ] ρ λ OD E[T CD ] = (26) λ OD E[T CD ]+λ OV E[T CV ] Figure 3 shows the forced termination probabilities of voice handoff calls for the three schemes (preemptive priority scheme, priority reservation scheme, and nonpriority scheme) versus γ. Two cases are shown in the figure, one is the case where ρ =7.0, which means that the traffic load of the system is heavy; the other one is the case of moderate traffic load with ρ =3.0. In the preemptive priority scheme, with the increase of γ, the number of data calls holding channels increases, which makes it easier for a handoff voice call getting a channel by preempting a data call in service if on arrival it finds no idle channels. Therefore, with γ increasing, the forced termination probability in the preemptive priority scheme decreases much faster than that in the priority reservation scheme. When γ is greater than a certain value, the preemptive priority scheme can provide an improvement on the forced termination probability relative to the priority reservation scheme, which becomes much larger when γ increases. Since the forced termination of ongoing voice calls is very undesirable from the user s viewpoint, this improvement is of great importance for service quality of the whole cellular mobile system. However, with γ being small, the probability by which a handoff voice call can preempt a data call in service becomes small. Thus, it can be seen from Fig. 3 that the improvement on the forced termination probability of the voice handoff calls in the preemptive priority scheme is not large. Moreover, in the extreme case, where γ = 0, there is no difference between the preemptive priority scheme and the non-priority scheme. Therefore, the forced termination probabilities of the two schemes are the same. To cope with this problem,

7 LI et al: A PREEMPTIVE PRIORITY HANDOFF SCHEME 1639 Fig. 4 Comparison of the forced termination probabilities for the three schemes versus ρ. Fig. 5 Comparison of the blocking probabilities of the originating voice calls for the three schemes versus γ. some other priority schemes need to be adopted as will be mentioned in Sect. 6. Different from the other two schemes, it can be seen from Fig. 3 that the forced termination probability in the non-priority scheme slightly increases with the increase of γ. The reason can be explained as follows: In the non-priority scheme, since handoff voice calls have no priority over originating voice calls, the blocking probability B HV of handoff voice calls and the blocking probability B OV of originating voice calls are the same. From Eq. (17), we see that, the forced termination probability P F is a monotone increasing function of B HV. Since the system provides a queue only for data calls, the increase of γ results in the increase of the effective traffic intensity and thus the increase of B OV (= B HV ) (see Fig. 5). Therefore, with the increase of γ, the forced termination probability P F in non-priority scheme increases. Figure 4 shows the forced termination probabilities of voice calls for the three schemes versus ρ. Two cases are shown, one is for γ =0.5, and the other one is for γ = 0.1. When γ = 0.5, the forced termination probability in the preemptive priority scheme is always much smaller than that in the priority reservation scheme. That is, the effect of the preemption is much greater than that of the channel reservation for this case. However, performance of the preemptive priority scheme depends on γ. For the case of γ =0.1, the improvement on the forced termination probability is not large since the number of data calls in service is small. Figure 5 shows the blocking probabilities of originating voice calls for the three schemes versus γ. It can be seen that with the increase of γ, the blocking probabilities of originating voice calls in all of these three schemes increases. This is because, in these schemes, queues are provided only for data calls, so that the increase of γ causes the increase of effective traffic intensity. From the figure, we see that the blocking proba- bility in the preemptive priority scheme is smaller than that in the priority reservation scheme. This is because no guard channels are used in the preemptive priority scheme. When γ = 0, the blocking probability of originating voice calls in the preemptive priority scheme is the same as that in the non-priority scheme. When γ is between 0 and 1, the blocking probability of originating voice calls in the preemptive priority scheme is larger than that in the non-priority scheme. This is because, in the preemptive priority scheme, with the right to preempt the service of data calls given to handoff voice calls, larger number of channels will be occupied by handoff voice calls, which makes it more difficult for originating voice calls to get idle channels on their arrival. When γ is very close to 1, the blocking probability of originating voice calls is mainly determined by the data traffic intensity. Thus, the difference between the blocking probability of originating voice calls in the preemptive priority scheme and that in the nonpriority scheme becomes very small. Moreover, with the decrease of ρ, the preemption probability of data calls in service becomes smaller, so that the blocking probabilities of the two schemes become closer as we can see in the figure for the case of ρ =3.0. Figure 6 shows the average queue length of data calls for the three schemes versus γ. Comparing the priority reservation schemes, we see that the case of S S C = 0 (non-priority scheme) has the best performance. This is because that there are no guard channels in the non-priority scheme. The preemptive priority scheme has no guard channels as well. However, in the preemptive priority scheme, data calls preempted by voice handoff calls return to the data queue, which brings about the increase in the average queue length. Therefore, we see that the average queue length in the preemptive priority scheme is always larger than that in the non-priority scheme. Moreover, when γ is close to 1, the occurrence of data calls being preempted becomes

8 1640 IEICE TRANS. COMMUN., VOL.E82 B, NO.10 OCTOBER 1999 Fig. 6 Comparison of the average length of the data queue for the three schemes versus γ. is very small, we see that the difference between the average transmission delay of data calls in the preemptive priority scheme and that in the non-priority scheme slightly increases with the increase of γ. In this case, the arrival rate λ HV of the handoff voice calls is very large, but the number of data calls in service is very small. Therefore, in contrast to other cases, the increase of γ results in the increase of the number of data calls preempted and thus the increase in the average transmission delay of data calls. In summary, our scheme can provide great improvement on the forced termination probability of voice calls with only small increase in the blocking probability of originating voice calls and the average transmission delay for data calls relative to those in the non-priority scheme. Since the data is usually not delay-sensitive, we can say that, the preemptive priority scheme has great advantages over the non-priority scheme. When γ is larger than a certain value, all the performance characteristics (forced termination probability for voice calls, blocking probability for originating voice calls, average transmission delay, and average queue length) of the system are better than those in the priority reservation scheme. That is to say, a better quality of service for both voice calls and data calls can be obtained. 6. Conclusions Fig. 7 Comparison of the average transmission delay of data calls for the three schemes versus γ. small since the arrival rate of handoff voice calls is very small. Thus, the difference between the average queue length in the preemptive priority scheme and that in the non-priority scheme becomes very small. Figure 7 shows the average transmission delay of data calls for the three schemes versus γ. We can see that the average transmission delay of data calls in the preemptive priority scheme is larger than that in the non-priority scheme. This is because, in the preemptive priority scheme, data calls in service can be preempted by handoff voice calls and return to the data queue waiting for some channels to be available. Therefore, on the average, in the preemptive priority scheme, a data will spend more time waiting in the data queue. Furthermore, we see that when γ is close to 1, the average transmission delay of data calls in the preemptive priority scheme becomes very close to that in the nonpriority scheme. This is because with γ being close to 1, the preemption probability of a data call in service becomes very small. For the case of ρ =7.0, when γ In this paper, we proposed a preemptive priority handoff scheme for integrated voice/data cellular mobile systems. The calls generated are divided into three different classes: handoff voice calls, originating voice calls and data calls. Priority is given to handoff voice calls over the other two kinds of calls. Specifically, the right to preempt the service of data is given to handoff voice call if on arrival it finds no idle channels. Data calls are queued if no channels are available at the time of arrival. The interrupted data call returns to the data queue. Performance of the system is compared to that of another handoff scheme for integrated voice/data cellular mobile systems where cut-off priority for voice handoffs is considered. Comparison shows that, when the data traffic is not very light, our scheme can provide lower blocking probabilities of originating voice calls, lower forced termination probabilities of ongoing voice calls, shorter average queue length, and less average transmission delay for data calls. On the whole, the preemptive handoff scheme proposed in this paper not only decreases the forced termination probability of voice calls, but also improves the channel utilization. However, when the data traffic is light, the improvement on the forced termination probability is not large. Especially, when γ is very small, the performance of the preemptive priority scheme becomes worse than that of the priority reservation scheme. To cope with this problem, we can conceive to combine the pre-

9 LI et al: A PREEMPTIVE PRIORITY HANDOFF SCHEME 1641 emptive priority scheme with the priority reservation scheme. This combined system will be studied in the future work. As we know, the queuing handoff is another method to decrease the forced termination probability. However, in this paper, we did not consider this method. In the future work, we will add a queue for voice handoff requests. This will further decrease the forced termination probability for ongoing voice calls. Lastly, it should be noted that only fixed channel allocation scheme was considered in this paper. However, dynamic channel allocation scheme is also widely studied for modern mobile cellular systems. It is more suitable than the fixed allocation scheme when the system has non-uniform traffic distribution. To study how to adopt the preemptive priority handoff scheme into the system with dynamic channel allocation is also our future work. For example, it is an interesting problem to decide whether the preemptive priority scheme or the dynamic channel allocation scheme should be invoked first when there is no channel available for a handoff voice call. From the discussions in this paper, we can expect that even under the assumption of dynamic channel allocation scheme the basic idea of the preemptive priority scheme still works and better quality of service would be obtained. References [1] S. Tekinay and B. Jabbbari, Handover and channel assignment in mobile cellular networks, IEEE Commun. Mag., vol.29, no.11, pp.42 46, Nov [2] V.O.K. Li and X.X. Qiu, Personal communication systems (PCS), Proc. IEEE, vol.83, no.9, pp , Sept [3] Q.A. Zeng, K. Mukumoto, and A. Fukuda, Influence of cell radius, moving speed, and duration of calls on handoff rate in cellular mobile radio systems, Proc. Wireless 95, J-2, pp , June [4] A. Murase, I.C. Symington, and E. Green, Handover criterion for macro and microcellular systems, IEEE VTC-91, pp , [5] M.D. Austin and G.L. Stuber, Direction biased handoff algorithms for urban microcells, IEEE VTC-94, pp , [6] K.G. Cornett, Bit error rate estimation techniques for digital land mobile radios, IEEE VTC-91, pp , [7] S. Tekinay and B. Jabbari, A measurement-based prioritization scheme for handovers in mobile cellular networks, IEEE J. Sel. Areas Commun., vol.10, no.8, pp , Oct [8] Q.A. Zeng, K. Mukumoto, and A. Fukuda, Performance analysis of mobile cellular radio system with priority reservation handoff procedures, Proc. IEEE VTC-94, vol.3, pp , June [9] Q.A. Zeng, K. Mukumoto, and A. Fukuda, Performance analysis of mobile cellular radio systems with two-level priority reservation handoff procedure, IEICE Trans. Commun., vol.e80-b, no.4, pp , April [10] Q.A. Zeng, K. Mukumoto, and A. Fukuda, Performance analysis of a handoff scheme in data mobile cellular radio systems, IEICE Conf., B-5-193, March [11] Q.A. Zeng, K. Mukumoto, and A. Fukuda, Performance analysis of a handoff scheme in integrated voice/data mobile cellular radio systems, IEICE Conf., B-5-194, March [12] D. Hong and S.S. Rappaport, Traffic model and performance analysis for cellular mobile radiotelephone systems with prioritized and non-prioritized hand-off procedures, IEEE Trans., vol.vt-35, pp.77 92, [13] D. Hong and S.S. Rappaport, Priority oriented channel access for cellular systems serving vehicular and portable radio telephones, IEE Proc. I, CSV-136, no.5, pp , [14] S.S. Rappaport, The multiple-call hand-off problem in high-capacity communications systems, IEEE Trans. Veh. Technol., vol.40, no.3, pp , [15] S.S. Rappaport, Models for call hand-off schemes in cellular communication networks, in Third generation wireless information networks, eds. S. Nanda and D.J. Goodman, pp , Kluwer Academic Publishers, Boston, [16] S.S. Rappaport, Blocking, hand-off and traffic performance for cellular communication systems with mixed platforms, IEE Proc. I, CSV-140, no.5, pp , [17] C. Purzynski and S.S. Rappaport, Multiple call handoff problem with queued handoffs and mixed platform types, IEE Proc. I, CSV-142, no.1, pp.31 39, [18] R. Guerin, Queueing-blocking systems with two arrival stream and guard channels, IEEE Trans. Commun., vol.36, no.2, pp , Feb [19] F.-N. Parlidon, Two-dimensional traffic models for cellular mobile systems, IEEE Trans. Commun., vol.42, no.2/3/4, pp , Feb./March/April [20] D.J. Goodman, Trends in cellular and cordless communication, IEEE Commun. Mag., vol.29, no.6, pp.31 40, June [21] H. Akimaru and R.B. Cooper, Teletraffic Engineering, Ohm, Appendix In order to obtain all the state probabilities, we must solve the flow balance equations (7), (8), (9) and (12). If we assume that λ HV, λ HD and λ TD are constant, the flow balance equations can be solved as a set of linear simultaneous equations. However, actually λ HV, λ HD and λ TD are not constant, but dependent on the state probabilities P (i, j)s through Eqs. (3) (5). Thus we use the following iteration procedure to obtain all the state probabilities: Step 1. Select arbitrary initial (positive) values for λ HV, λ HD and λ TD. Step 2. Compute all the state probabilities P (i, j)s by using SOR method [21]. Step 3. Compute the average number of voice calls holding channels, the average number of data calls holding channels and the average number of data calls waiting in the queue by using Eqs. (20), (21) and (18). Step 4. Compute new λ HV, λ HD and λ TD using Eqs. (3) (5). If new λ HV oldλ HV <ε, new λ HD oldλ HD <εand new λ TD oldλ TD <ε, then stop the iteration. Otherwise, go to step 2 again. Where ε is a very small positive number to check the convergence.

10 1642 IEICE TRANS. COMMUN., VOL.E82 B, NO.10 OCTOBER 1999 Bo Li was born in Xi an, China, on November 18, He received the B.S. and M.S. degrees from Xidian University, China, in 1994 and 1997, all in communication engineering. From 1997 to 1998, as a research student, he did research work on mobile communication systems in Shizuoka University, Japan. Now he is with the institute of communication engineering of Xidian University. At the same time, he is also working towards Ph.D. degree in Xidian University. His research interests include handoffs, multiple access protocols, and mobile communication systems. He also has some interests in the study of mobile video communication systems. Qing-An Zeng received the B.S. degree from Chengdu Meteorological Institute, China, the M.S. and Ph.D. degrees from Shizuoka University, Japan, all in electrical engineering, in 1984, 1994 and 1997, respectively. From 1984 to 1990, he was with the State Meteorological Administration, China, as a researcher. In 1997, he joined NEC Corporation, Japan, where he has been engaged in the development of mobile radio communication systems. His research interests include mobile radio communication systems, wireless networks, channel allocation algorithms, handoffs, CDMA technology, and queueing theory. Dr. Zeng is a member of the IEEE. Akira Fukuda received the B.S., M.S., and Ph.D. degrees, all in electronic engineering, from the University of Tokyo, Tokyo, Japan, in 1967,1969, and 1972, respectively. In 1972, he joined the Department of Electrical Engineering, Shizuoka University, Hamamatsu, Japan, where he is now a professor. From 1983 to 1984, he was a visiting professor at the Department of Electrical and Computer Engineering, University of Massachusetts, Amherst, MA. His research interests include coding theory, modulation theory, queueing theory, mobile communication systems, and meteor burst communications. He has published about 100 papers in the above areas. He is the author of the two books Meteor Burst Communications (Corona, Tokyo) and Fundamentals of Communication Engineering (Morikita, Tokyo). He received the Yonezawa Memorial Prize in He was also honored from the Telecommunications Advancement Foundation in 1987 for his invention of EPA (Equilibrium Point Analysis). Dr. Fukuda is a member of the IEEE. Kaiji Mukumoto was born in Shizuoka, Japan, on March 28, He received the Ph.D. degree in electrical engineering from Shizuoka University, Hamamatsu, Japan in In 1974, he joined the Department of Electrical Engineering, Shizuoka University, Hamamatsu, Japan, as a technical official. His research interests include software modems, packet radio networks, queueing theory, and meteor burst communications.