Queueing priority channel assignment strategies for PCS hand-off and ...

704

IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 43, NO. 3, AUGUST 1994

Queueing Priority Channel Assignment Strategies for PCS Hand-Off and Initial Access Yi-Bing Lin, Seshadri Mohan, Member, IEEE, and Anthony Noerpel, Senior Member, IEEE

Abstract-The forced termination probability (the probability that a hand-off call is blocked) is an important criterion in the performance evaluation of personal communication service (PCS) networks. The forced termination of an ongoing call is considered less desirable than blocking the initial access of a new call. This paper proposes analytic and simulation models to study the performance of different channel assignment strategies for hand-off and initial access. We observe that giving priority to hand-off attempts over initial access attempts would dramatically improve the forced termination probability of the system without seriously degrading the number of failed initial access attempts. Some of our results are different from previously published results because our models capture features not considered in those studies.

I. INTRODUCTION personal communication service (PCS) network [4], &I is a digital communication network that provides low-power-high-quality wireless access for PCS subscribers to the public switched telephone network (PSTN). Low power is typically considered to be 100 mW or less of portable output power and high quality typically implies the use of a 32 Kb/s ADPCM voice coder. The service area of a PCS network is partitioned into several subareas or cells. This paper assumes a fixed or quasistatic channel assignment [2] where a group of channels (time slots, frequencies, spreading codes, or a combination of these) are assigned to each cell but the results are extensible to dynamic channel assignment schemes [3]. When a subscriber wishes to make or receive a phone call, the portable attempts to seize an available traffic channel for the connection. For some PCS radio systems, the portable launches an access request on a common signaling channel and is then directed to a traffic channel (DECT [7], or CT-2 Plus [15], [ 6 ] ) .In other PCS radio systems the access attempt is made directly on an available traffic channel (Bellcore WACS [l]). In the former case, there is a limited number of servers or transceivers in a port, and when a port is blocked there is no transceiver for the signaling channel since they are all used for existing calls. In both cases, there is usually no provision (either no channel, no protocol, or both) for a portable to signal the need for a traffic channel to a blocked port and therefore access attempts cannot be queued by the network. If there is no available traffic channel or common signaling channel then the call is blocked. If there is an available traffic Manuscript received November 30, 1993; March 7, 1994 The authors are with Bellcore, Morristown, NJ 07960. IEEE Log Number 9403208.

channel it is used to connect the call. The channel is released either when the call is completed or the portable (or the PCS subscriber) moves out of the cell. When a portable moves from one cell to another while a call is in progress, the call requires a new channel (in the new cell) to continue. This procedure of changing channels is called hand-off or automatic link transfer (ALT). If no channel is available in the new cell, then the call will be dropped or forced terminated. The forced termination probability is an important criterion in the performance evaluation of a PCS network. Forced termination of an ongoing call is considered less desirable than blocking of a new call attempt. Radio technologies for PCS typically support from 1 to 12 32-Kb/s servers per port or cell. CT-2 Plus at the low end supports one transceiver per port. However, several ports can be collocated thereby increasing the number of servers per cell. Bellcore’s WACS has nine servers per transceiver and DECT has 12 servers per port or cell. There are typically between 20 and 50 servers for cellular mobile technologies which have a 25 MHz frequency allocation. In order to fit into the FCC’s 10 MHz allocations for PCS in the 2 GHz emerging technologies band, most air interface specifications will have to be reengineered to have from four to eight-servers per port or microcell. In PCS networks it is expected that there will be more overlap of port coverage areas or microcells than that which exists for mobile networks with macrocells. Therefore, in most cases, there may be a “second best port” which a portable can successfully access either to complete a call or for an ALT, thus increasing the effective number of servers available to a portable in a cell. This effect is not studied in this paper. Three ALT strategies have been proposed for PCS networks: i) portable-controlled hand-off, ii) network controlled hand-off, and iii) portable assisted hand-off. Portable-controlled hand-off is the most popular technique and is employed by both the DECT and the WACS air interface protocols. In this method the portable is continuously monitoring the signal strength and quality from the accessed port and several ALT condidate ports. When some ALT criteria is met, the portable checks the best candidate port for an available traffic channel and launches an ALT request. Network-controlled hand-off is employed by CT-2 Plus. In this method, the port monitors the signal strength and quality from the portable and when these deteriorate below some threshold, the network arranges for a hand-off to another port. The network asks all the surrounding ports to monitor the signal from the portable and report the

0018-9545/94$04.000 1994 IEEE

705

LIN et a/.:HAND-OFF AND INITIAL ACCESS FOR PCS NETWORK

measurement results back to the network. The network then chooses a new port for the hand-off and informs both the portable (through the old port) and the new port. The hand-off is then effected. Portable assisted hand-off is a variant of network-controlled hand-off where the network asks the portable to measure the signals from surrounding ports and report those measurements back to the old port so that the network can make the determination as to where an ALT is required and to which port. This hand-off strategy is employed by the GSM mobile standard [14] but not by any candidate PCS radio system standards. In most proposed radio systems for PCS in the 2 GHz emerging technologies band, the ports are simple transponders or radio modems and all the intelligence resides in a radio port controller. It is presumed that the radio port controllers have the capacity to communicate with each other through the PCS network. Sever1 ALT-initial access channel assignment schemes are described below (see [16] for a survey).

&I

available?

channe-H

ongoing call

channel release

I

ri call blocked

the call is forced terminated

9 queue empty

noj---l

next the channel call in the is assigned waiting queue to the

The non-prioritized scheme (NPS): In this scheme, the port handles a hand-off call exactly the same as an originating call (Le., the hand-off call is blocked immediately if no channel is available; see the first flowchart in Fig. 1). This is the scheme employed by typFig. 1. Flow charts of hand-off queueing. ical radio technologies which have been proposed for PCS in the emerging technologies frequency allocane1 in the new cell is available for the ALT then that tion at 2 GHz. ALT actually occurs. If no channel is available after The guard channel scheme: This scheme is similar to the portable moves out of the hand-off area (Le., the NPS except that a number of channels or transceivers degradation interval expires), then the call is forced in each port are reserved for hand-off calls. terminated. In this scheme, when a channel is reThe first-come-first-out (FIFO) scheme: This scheme is leased, the port first checks if the waiting queue is based on the fact that adjacent cells in a PCS network empty. If not, the released channel is assigned to a overlay. Thus, there is a considerable area where a hand-off call in the queue (see the third flow chart in call can be handled by the ports in any of the adjacent Fig. 1). The “next” hand-off call to be served is secells. This area is called the hand-off area. The time lected based on the queueing policy. In the FIFO that a portable moves across the hand-off area is rescheme, the next hand-off call is selected in the firstferred as the degradation interval. The FIFO scheme in-first-out basis. is illustrated in the second flow chart in Fig. 1. For The measured-based priority scheme (MBPS): This scheme is similar to the FIFO scheme except for the portable controlled hand-off, when a portable with an queueing policy. MBPS uses a nonpreemptive dyongoing call enters a hand-off area, it checks if there namic priority policy. The priorities are defined by is a channel available on the new port. If not, this the power level that the portable receives from the scheme requires that there be a way for the portable to signal to the new port its desire for an ALT and the port of the new cell [ 171. The hand-off area can be hand-off call is buffered in a waiting queue, and the viewed as regions marked by different ranges of the channel on the old port is used until a new channel is power ratio. The network monitors the power levels available. For WACS, a physical channel exists for of a queued hand-off call dynamically. For our purposes, we may view a hand-off call as having a higher the portable to signal a blocked port of the ALT atpriority if its degradation interval is closer to expiratempt but the protocol is not currently specified so tion. This is determined by the network to be the rathat this channel can be used in this way. For DECT, if a port is blocked then no channel exists for the pordio link with the lowest received signal strength and the poorest quality as measured by the portable. This table to make such a request. For network controlled implies that a mechanism exists for the portable to hand-off systems, like CT-2 Plus, the new port can relay this information to the network over the failing always make such a request to the new port but the radio link between the portable and the old port. A protocol does not exist to inform the portable that it released channel is assigned to the queued hand-off is a hand-off candidate but that its hand-off is on hold call with the highest priority. subject to the availability of a transponder. If a chan-

1-

706


Hong and Rappaport [lo] proposed an analytic model for FIFO scheme assuming exponentially distributed degradation intervals. There are two major differences between Hong-Rappaport model and the model developed in this paper. First, their traffic model is based on a special portable movement pattern. This paper generalizes the traffic model to accommodate arbitrary portable movement patterns. Second, we follow the exponential portable residual times proposed by Wong [18] to derive the exact values for the forced termination probability. On the other hand, Hong-Rappaport model approaches the special portable movement pattern by a Poisson process, which provides approximate results. Tekinary and Jabbari [17] proposed a similar analytic model for the FIFO scheme assuming normally distributed degradation intervals. The system was modeled by a Markov chain. Gnedenko and Kovalenko [9] pointed out that a Markov chain cannot model a queueing system with timeout periods which are not exponentially distributed. Another problem of the model is that the authors considered the hand-off traffic as an independent input parameter. In reality, the hand-off traffic is affected by the portable mobility, the call holding time distribution, and the originating call traffic. Yoon and Kwan's study [19] made the same independent hand-off traffic assumption as Tekinary and Jabbari's study. Furthermore, they used finite buffer queues (i.e., M/M/C/K system) to model the ALT-initial access channel assignment schemes. The behavior of the finite buffer system is very different from the timeout systems, and their models may not adequately approximate the behavior of the ALT-initial access channel assignment schemes. This paper proposes analytic models to study NPS and FIFO schemes. These channel assignment schemes are studied under two conditions. We assume 50 servers per cell for the mobile application so as to compare our results with previously published results [17]. We also show results for 10 servers per port to reflect the PCS environment. A simulation model is developed to validate the analytic models, and is used to study MBPS. Our results are different from the previous studies [lo], [17] because our model captures features not considered in the previous models. 11. ANALYTICMODELS This section proposes analytic models for the ALTinitial access channel assignment schemes. We first describe the traffic model followed by the system models for NPS and the FIFO scheme. The guard-channel call-handling scheme can be directly extended from the FIFO queueing model as described in [lo], and is not studied in this paper. A . The TraBc Model Suppose that a portable moves across K cell boundaries during a call holding time tc assuming that the call is com-

pleted. That is, K is the number of hand-offs before the call is completed. The call is referred to as a K-hand-off call. Following the technique we developed in [ 111, the probability Pr[K = k] is derived assuming that the incoming calls to a portable are a Poisson process, and the time the portable resides in a cell has a general distribution. We assume that the time interval between two consecutive phone calls to a portable is sufficiently larger than the call holding time such that the busy line effect does not occur [12] (Le., there is no new phone call to a portable when it is in a conversation). Suppose that the portable resides in a cell Ro when a phone call arrived. During the phone call, the portable visits another K cells, and the portable resides in the j th cell for a time period tM, (0 < j IK). Let tmbe the time interval between the arrival of the phone call and the time when the portable moves out of Ro. Let tM, be independent and identically distributed random variables with the distribution F, (tM,)and the density function fm(tM,)with mean l/q. Let r,(t) be the density function of t,, and the call holding time tc be exponentially distributed with the density functionf,(t) = pe - p t . Since the incoming phone calls are a Poisson process, from the random observer property [8] of a Poisson process,

The probability of a K-hand-off call is

+ fMl +

Pr[K = k] = Pr[t,

+ tMk- < tc

* *

I

5

tm

F o r k 1 1, Pr[K = k]

From ( l ) , we have P a

nm

=

=

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(3)

LIN er ai.: HAND-OFF AND INITIAL ACCESS FOR PCS NETWORK

707

The Laplace-Stieltjes transform for the distribution F, ( t )

Assume homogeneous cells in the PCS network. Let A, be the originating call arrival rate. Then the hand-off call arrival rate is

If Fm(t) is an exponential distribution (as proposed in [ IS]), then

The probability p c that a call is completed (without being blocked or forced terminated) is

Let p f be the forced termination probability. Consider a K-hand-off call which is connected as a new call attempted. Let J be the number of portable moves when the call is forced terminated or successfully terminated, where J IK. The probability Pr [ J = j ( K = k] is O ~ j = k 1 1

((1

- Pf)k-'

Let pnc be the probability that a call attempt is not completed by either blocking or forced termination. Then pnc = 1 - p c . If F,,,(t)is exponentially distributed, then

j = k > l

The expected value of J for a k-hand-off call. For k = 0, E [ J ( K = 01 = 0. Fork = 1, E [ J I K = 13 = 1 . F o r k > 2,

(7)

Let p o be the blocking probability of the new call attempts. The expected number of hand-offs before a call terminates (either completes or is forced terminated) is (1 - p , ) E [ J ] where (from (5)-(7))

pnc = 1 -

1 -Po ~

VPf

1+-

If p >> 17, then a portable always completes the call before a move, and ( q p f ) / p = 0. Thus, pnc = p u (i.e., the incomplete calls are blocked new call attempts). If 17 >> p , then a call never completes, and is eventually blocked, and ( v p f ) l p + 03. Thus, pnc = 1. Hong and Rappaport [ 101 observed that pncis more sensitive to p o than p f Their observation is not true in general. Equation (10) indicates that as q / p (the mobility to the call completion rate ratio) increases, the impact ofpf onp,, increases, and the impact of p o decreases.

B. The System Model This section proposes analytic models for the nonprioritized scheme and the FIFO scheme. I ) The Nonprioritized Scheme: In the nonprioritized call handling, p f = p , . The channel occupancy time is the minimum of the call holding time (note that the call holding time for a hand-off call has the same distribution as an originating call because of the memoryless property of the exponential distribution) and the remaining portable residual time. In other words, the density function fc,,( t )

708


of channel occupancy time distribution is

= (p

+ q)e-(p+q)'.

The net traffic to the system is h, lang-B formula,

+

hh.

(1 1) From the Er-

Fig. 2. The state diagram for the experimental timeout model.

the waiting queue. The effective call traffic to a cell at s(n) is h, h h (and the process moves from s(n) to s(n 1) with this rate). Since a busy channel is released with the Po = (12) rate p + q , the process moves from s(n) to s(n - 1) (for ( h o + Ah)' 0 5 n 5 c) with the rate n ( p 7). i= 1 ( p q)'i! When the process is in s(c j ) wherej 1 0, all chanSince p o = pf in the nonprioritized hand-off scheme, (9) nels are busy, a n d j hand-off calls are in the waiting queue. When a call arrives at state s(c j ) , the call is dropped is rewritten as immediately if it is a new call. Otherwise the (hand-off) call is buffered in the waiting queue. Thus, the process moves from s(c j ) to s(c j + 1) with rate h h f o r j 2 The probability po can be obtained by assigning an initial 0. Since all channels are busy, the first call releases its value for Ah, and iterating (12)and (13)until the Ah value channel with rate c( p 17). Also, the maximum queueing times are exponentially distributed and the first hand-off converges. q ) . Thus, the 2) The FIFO Scheme: Our system model for the FIFO call leaves the queue with the rate j ( y s(c j ) to s(c j 1) with rate process moves from scheme is similar to the one proposed by Hong and Rapq ) f o r j > 0. paport [ 101.The model is a direct extension of the timeout c ( p q ) + j ( y Let anbe the steady state probability for s(n). Then model described in [9] that assumes unlimited call

(A, + (LL + V Y C !

+

+

+ + +

+

+

+

+

+

+

+ - - - + an + - - -

=

+

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sources. This model is appropriate when the expected number of portables in the cell is much larger than the number of channels. When the expected number of portables is small (but is still larger than the number of channels), other models must be considered [ 121, [ 131. Assume that the degradation interval is exponentially distributed with mean lly. Define the maximum queueing time of a hand-off call as the minimum of the degradation interval and the portable residual time. Thus, the maximum queueing time has the density function

We follow a conservative assumption in [lo],[17],and [19] that a hand-off call is blocked if it is not allocated a channel within the maximum queueing time. Note that the hand-off may not be blocked if the portable moves to another cell within the degradation interval. The impact of this assumption can be ignored if 7