Wireless Network Design: Optimization Models and Solution ...

102 Eli Olinick
carrying capacity. The planning process for FDMA/TDMA systems was typically
addressed in two phases. The first phase consisted of solving a tower location problem,
optimizing the location of radio towers (also called base stations or Node Bs) to
provide coverage for the area being served, and the second involved assigning frequencies
to the selected tower locations (cells) [3]. Sherali et al. [53] and Mathar and
Niessen [42] apply mathematical programming techniques to solve tower location
problems, and Murphey et al. [46] give an extensive survey of the graph coloring
problems that arise during the frequency assignment phase. A model that solves
both problems jointly is given in Kalvenes et al. [33]. However, as noted in the seminal
paper by Amaldi et al. [7], the use of code division multiple access (CDMA)
schemes in the current generation (3G) networks gives rise to a fundamentally different
network design problem.
CDMA schemes enable service providers to use all of their licensed frequencies
in each cell, and so the models described in this chapter do not have a frequency assignment
component. Instead, capacity in a CDMA-based system is determined by
a signal-to-interference ratio (SIR). Every active connection between a mobile device
and a tower in a CDMA system can potentially interfere with every other active
mobile-tower connection. Thus, CDMA systems use admission control policies to
ensure that new connections are only allowed if they will not cause too much interference
with currently active connections. Since the assignment of mobiles to towers
(subscriber assignment) determines how much interference each connection causes
with every other connection, the most efficient CDMA network designs are found
by simultaneously solving the tower location and subscriber assignment problems
[7].
Calls from mobiles in cellular networks are typically relayed via radio links to
cell sites (towers). Each tower is connected to a mobile telephone switching office
(MTSO). MTSOs are switching offices in cellular networks that perform various
operational functions such as handoffs (determining when to switch a mobile device’s
connection from one tower to another to improve reception) [56] and keeping
track of billing information [44]. The MTSOs are connected (usually by fiber-optic
cable) to form a backbone network for the CDMA system which allows for routing
traffic between cells in the network as well as to and from public switched
telephone network (PSTN) gateways to provide cellular users with access to long
distance networks and other services. This chapter develops a comprehensive optimization
model for CDMA network design including the selection of base stations
and MTSO locations, the assignment of mobiles to base stations, and the design
of a backbone network connecting the base stations, MTSOs, and the PSTN gateways.
This model takes as input a set of candidate tower and MTSO locations with
corresponding costs, costs for provisioning links between the candidate tower and
MTSO locations as well as costs for linking the MTSOs to PSTN gateway nodes.
Demand for traffic is modeled by test points in the service area. Test points are
described in [7] as centroids of demand where given amounts of traffic must be
satisfied at a given minimum SIR, SIRmin [7, 39]. Based on these data, the model
can be used to determine the selection of radio towers, MTSO locations, backbone
network topology, and the service capacity of the resulting radio network. A typical