Wireless Network Design: Optimization Models and Solution ...

15 Major Trends in Cellular Networks and Corresponding Optimization Issues 361
• Reduction of OPEX and CAPEX: Flattening of the architecture and reduction of
the number of network elements. Maintenance and operation of fewer elements
will translate to lower cost
• Improved spectral efficiency through advanced techniques like Orthogonal Frequency
Division Multiple Access (OFDMA), higher order modulation, channel
dependent scheduling and time/frequency diversity
• Scalability of the frequency band also contributes to efficient usage of the spectrum,
by enabling to choose the frequency bandwidth that best fits the available
spectrum
• Maximized reuse of the infrastructure. LTE provides a packet core common to
the various wireless access, as well as built-in mechanisms for inter-working
and handover between these access systems. In particular, included are “legacy”
access systems such as 2G and 3G. A common core will also help to reduce the
OPEX and CAPEX.
In addition, an integral part of LTE design is Self Organizing Networks (SON).
SON encompasses Self Configuration, Self Optimization, and Self Healing. The
target of SON is to minimize human intervention in the tasks of configuration (e.g.,
when a new element such a as base station is put in service), optimization (e.g.,
when network element configuration parameters are readjusted during network operation
to account for imperfections in the network planning and traffic evolution)
and healing (outage detection and compensation). Besides its direct OPEX lowering
benefit, SON’s indirect benefit is to minimize the chance of human errors and the
associated indirect costs to the network operator’s image. However, SON’s target is
very ambitious, and it remains to be seen how much automation can be achieved,
for example in the area of optimization. The next major step in evolution is LTE-
Advanced, which aims at peak data rates around 1Gb/sec in the downlink, and 500
Mbps in the uplink, by using advanced techniques such as carrier aggregation, and
coordinated multi-point transmission reception [6].
Another example is WIMAX 802.16. WIMAX, initially developed by IEEE as
a wireless substitution to fixed broadband access (802.16-2004), was enhanced to
be able to handle mobile users (802.16-e). The various versions of WIMAX are following
the same evolutionary trend as the GSM family towards higher data rates,
lower round trip time, improved spectral efficiency, frequency scalability and reduced
OPEX and CAPEX [38]. IEEE 802.16m is the next major milestone in the
WiMAX standards evolution beyond IEEE 802.16e-2005 [39].
15.5 Backhauling and Transport
Mobile backhauling is traditionally based on TDM E1/T1 lines, each providing 1.5
or 2.0 Mbps capacity. With mobile broadband such as LTE, a speed higher than 100
Mbps is required on the backhaul transport. Such speed can in principle be achieved
by bundling a large number of E1/T1 lines, but as the leasing costs are proportional
to the number of lines, that does not scale well. Technologies that can achieve speeds