Wireless Network Design: Optimization Models and Solution ...

296 Hang Jin and Li Guo
formance in non-line-of-sight environments, and multiple-input-multiple-output to
enhance the spectrum efficiency. Scalable OFDMA (SOFDMA) is introduced in the
IEEE 802.16e Amendment to support scalable channel bandwidths from 1.25MHz
to 20 MHz. Mobile WiMAX systems offer scalability in both radio access technology
and network architecture, thus providing a great deal of flexibility in network
deployment options and service offerings. Mobile WiMAX supports up to 63 Mbps
downlink and 28 Mbps uplink data rate; end-to-end IP based QoS; ultimate scalability
from 1.25MHz to 20MHz, robust operation with mobility and fading environments
[1, 2, 11].
As a new broadband wireless access system, OFDMA-based WiMax system has
a number of distinct features, including flexible subchannelization, adaptive modulation
and coding, space-time coding and spatial multiplexing, dynamic packetswitch
based air interface, and flexible network deployment such as fractional frequency
re-use. All those features impose a requirement of careful optimization of
the critical parameters and schemes in the system for obtaining superior performance.
The optimization of mobile WiMAX systems is broadly categorized into the
following three areas:
1. RF optimization
The RF optimization is the key step in WiMAX networks deployment planning.
The purpose of RF optimization is to provide reliable and ubiquitous RF coverage
for the whole market. RF optimization consists of frequency re-use planning, site
location selections, BTS height selection, and BTS antenna orientation selection
(azimuth and down tilt selection). The optimization is an iterative procedure,
which needs the interactions between optimization and design of each individual
factor listed above.
2. Radio resource allocation optimization
Radio resource allocation plays an important role in determining the overall system
performances. Given a network configuration, the radio resources consist
of:
a. BTS transmitter power
b. Total frequency bandwidth allocated to a BTS
c. The total time duration ratio between downlink subframe and uplink subframe
in TDD system, and the time duration of slot allocated to each user or service
connection.
The later two can be visualized as an optimization in a two-dimensional
frequency-time space. In the case of mobile WiMAX, the minimum resource
allocation unit is called a slot, which is a block of about 15 kHz in frequency
and 200us in time in the frequency-time space. WiMAX support 5ms frame,
and with 2:1 down link to up link ratio; there are about 420 slots for down link,
and 175 slots for uplink. These down link and uplink slots need to distributed
among hundreds users in an optimal way that the overall system performance
metrics (capacity and latency) are optimized.
BTS transmitter power varies from a few watts (micro/pico cells) to 10–20
watts (Macro cells). Mobile WiMAX employs OFDMA and there are multiple