704 IEEE TRANSACTIONS ON VEHICULAR TECHNOLOGY, VOL. 43, NO. 3, AUGUST 1994 Queueing Priority Channel PCS Hand-Off and Initial Access Assignment Strategies for Yi-Bing Lin, Seshadri Mohan, Member, IEEE, and Anthony Noerpel, Senior Member, IEEE Abstract-The forced termination probability (the probabil- ity 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 strat- egies 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 previ- ously 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 sub- scribers 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 im- plies the use of a 32 Kb/s ADPCM voice coder. The ser- vice area of a PCS network is partitioned into several subareas or cells. This paper assumes a fixed or quasi- static channel assignment [2] where a group of channels (time slots, frequencies, spreading codes, or a combina- tion 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 por- table 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 chan- nel 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 re- leased 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 at- tempt. 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, sev- eral 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 al- location. 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. There- fore, in most cases, there may be a “second best port” which a portable can successfully access either to com- plete 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-con- trolled 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 avail- able 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 be- low 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$04.00 0 1994 IEEE