Wireless Network Design: Optimization Models and Solution ...

15 Major Trends in Cellular Networks and Corresponding Optimization Issues 355
Thus if we take the network operator’s profit as a performance metric, there
are two main optimization spaces which are somewhat overlapping. One space is
first-time access to voice and Internet for low income populations. Another space
is optimization in the context of broadband wireless Internet. Although the general
problems are intractable, they can be decomposed into optimization sub-problems
that are easier to solve. The profit metric in many cases is correlated with a higher
service penetration rate, i.e., more widespread access to mobile communications
with ensuing socioeconomic benefits. For example, in its Global Mobile Tax Review
2006-2007, the GSMA found that “penetration boosts economic activity. In
developing countries a 10% increase in penetration leads to a 1.2% increase in the
annual growth rate in GDP.” [25]
This chapter focuses on some of the optimization issues, approaches and techniques
that could be part of a toolbox to tackle the general problem. Section 2 reviews
the current situation in wireless networks, and Section 3 provides an overview
of some solution approaches. Sections 4 through 8 describe in more detail some selected
approaches. Section 9 concludes the chapter.
15.2 Current Situation
The two main factors impacting an operator’s profit are the costs and revenues. Cost
is more precisely total cost of ownership (TCO) consisting of Capital Expenses
(CAPEX) and Operating Expenses (OPEX). CAPEX are one-time expenses and
include items such as cost to acquire the spectrum, and cost to buy and deploy
the network infrastructure. OPEX are recurring expenses and correspond to items
such as staff salary, rental of facilities, and electricity bills. Revenues mostly stem
from providing wireless voice and data access services. They could also come from
commercial scale/grade products such as bundled voice, web hosting, VPNs, mail,
conference and messaging facilities offered to government and corporate clients.
Many, if not most operators, have to deal with multiple generations of equipment
and technologies from different vendors that coexist and interoperate, and this
makes the network costly to operate and manage. Capacity expansion and growth
becomes more and more complex and costly as each new wave of expansion has
to take into account the previous systems. On top of that, current network architectures
and technologies still carry the legacy from the voice centric, circuit switched
networks. That is, they are well suited to carry voice traffic, which is low volume,
and incur the limitations of older technologies. A good example is the backhauling
technology, based on T1/E1 trunks, used to transport the traffic between the base
stations and the core network. Since the T1/E1 are circuit switched, they are not
well optimized for the packet traffic of Internet access as there can be no efficiency
gain from statistical multiplexing. Furthermore, the network operators commonly
lease the T1/E1, and the leasing fees are proportional to the bandwidth. Thus, it
does not scale well with the envisioned broadband data traffic which is orders of
magnitude higher than voice. Clearly, cost of mobile backhauling is an issue and