15.8 Location Area Planning

Location area (LA) planning in minimum cost plays an important role in cellular networks because of the trade-off caused by paging and registration signaling. The upper bound on the size of an LA is the service area of a mobile switching center (MSC). In that extreme case, the cost of paging is at its maximum, but no registration is needed. On the other hand, if each cell is an LA, the paging cost is minimal, but the registration cost is the largest. In general, the most important component of these costs is the load on the signaling resources. Between the extremes lie one or more partitions of the MSC service area that minimizes the total cost of paging and registration. In this section, a few approaches are discussed that address LA planning-related issues.

15.8.1 Two-Step Approach

This approach ^[77] deals with the planning of LAs in a personal communication services network (PCSN) to be overlaid on an existing wired network. Given the average speed of mobile terminals, the number of MSCs, their locations, call handling capacity of each MSC, handoff cost between adjacent cells, and call arrival rate, an important consideration in a PCSN is to identify the cells in every LA to be connected to the corresponding MSC in a cost-effective manner. While planning a location area, a two-step approach is presented, namely, optimization of total system recurring cost (subproblem I), and optimization of hybrid cost (subproblem II). The planning first determines the optimum number of cells in an LA from subproblem I. Then it finds out the exact LAs by assigning cells to the switches, while optimizing the hybrid cost which comprises the handoff cost and the cable cost, in subproblem II. The decomposition of the problem provides a practical way for designing LAs. As this approach toward LA planning takes into account both cost and network planning factors, this unique combination is of great interest to PCSN designers. It develops an optimum network-planning method for a wide range of call-to-mobility ratio (CMR) that minimizes the total system recurring cost, while still ensuring a good system performance. Approximate optimal results, with respect to cell-to-switch assignment, are achievable with a reasonable computational effort that supports the engineered plan of an existing PCSN.

In order to design a feasible PCSN, constraints such as traffic-handling capacity of MSCs and costs related to paging, registration, and cabling should be considered. Utilizing the available information of MTs and the network in a suitable manner, it is possible to devise a technique for planning of location areas in a PCSN that optimizes both system-recurring cost and hybrid cost.

This design is not restricted to any particular assumption on the mobility pattern of MTs or the mobility model either. Because the optimum LA size decreases significantly with the increase in CMR, as the corresponding hike in system cost is very high, design parameters at BS and MSCs cannot be specified until cell allocation is completed. Finally, channel assignment, which can further improve system performance in terms of QoS and improved carrier interference ratio, can only be determined once the architecture of the PCSN has been obtained.

15.8.2 LA Planning and Signaling Requirements

In 2G mobile systems, ^[78] LA planning does not generate significant problems because the number of generated LUs (as well as the amount of paging signaling) remains relatively low because of the low number of users. For 3G mobile telecommunications systems, several alternative location-tracking techniques have been used (e.g., based on the use of reporting centers or a dynamic location area management protocol). Nevertheless, in UMTS an approach similar to 2G location finding has been used, i.e., the system area is divided into LAs and a called user is located in two steps (determine the LA within which the user roams and perform paging within this LA). The main issue concerning location area planning in 3G mobile telecommunications systems is the amount of location finding related signaling load (paging signaling, location updating, and distributed database queries).

Because the size and shape of an LA affects the signaling requirements due to paging and LU, it is obvious that LA planning should minimize both, if possible. In order to provide a clear view of the relation between the LA planning and the above-mentioned parameters, we consider two extreme LA planning approaches:

The system area equals an LA. Whenever an MT is called, it is paged over the whole system coverage area, while no LUs are performed due to MU movements. In this case, paging signaling load can be enormous, especially during rush hour.
The cell area equals an LA. In this case, the location of an MT is determined with accuracy of a single cell area. The need for paging here is minimal; paging does not locate the MT, it just alerts the terminal for the incoming call. However, the number of LUs is expected to be enormous due to the small cell size and user mobility. A brief description of the LA planning methods under consideration follows:
- LA planning based on heuristic algorithms: This is a method to approximate the optimum LA planning as a set of cells. According to the example heuristic algorithm used in this chapter, cells are randomly selected to form LAs.
- LA planning based on area zones and highway topology: Area zones are defined according to geographical criteria (e.g., city center), and the approach considers the population distribution and the way that people move via city highways so as to determine the proper LA configuration.
- LA planning based on overlapping LA borders: This method can be considered as an attempt to improve the previous one by means of reducing the number of generated LUs. In this case, LAs have overlapping borders so as to avoid LUs due to MU movements around the LA borders.
- Time-dependent LA configuration: According to this scenario the network alters the LA configuration based on either some predefined timetable or monitoring of the number of LUs and the number of paging messages. The LA configuration here is selected so as to fit the time variable mobility and traffic conditions.
- LA planning based on MU grouping: This method considers the mobility behavior of each individual MU so as to minimize the number of LUs generated due to daily MU movements. To apply this method, MUs are grouped based on their mobility behavior (e.g., high mobility MUs) and different LA configuration is determined for each group.
- LA planning using simulated annealing. ^[79] This research focusing on LA management in wireless cellular networks has minimized the total paging and registration cost. This chapter finds an optimal method for determining the location areas. To that end, an appropriate objective function is defined with the addition of paging and registration costs. For that purpose, the available network information to formulate a realistic optimization problem is used. In reality, the load (i.e., cost) of paging and registration to the network varies from cell to cell. An algorithm based on simulated annealing for the solution of the resulting problem is used.

^[77]Bhattacharjee, P.S. et al., A practical approach for location area planning in a personal communication services network, Proc. MMT '98, 1998.

^[78]Markoulidakis, G.L. et al., Evaluation in LA planning in future mobile telecommunication systems, Wireless Networks, 1995.

^[79]Demirkol, I. et al., Location area planning in cellular networks using simulated annealing, Proc. INFOCOM, 2001.