Wireless Network Design: Optimization Models and Solution ...

13 Optimization of Wireless Broadband (WiMAX) Systems 297
data bursts for different users occurring at the same time. Users on the cell
edge need more power to achieve the same performance of users in the cell
center. There is a need for a fair scheme of power distribution among users
to have optimal overall system performance and also ensure that calls of cell
edge users are not dropped. It is worthwhile to mention that the radio resource
allocation should also consider the channel fading. As the channels vary with
time, the resource allocations need to be adjusted accordingly. This leads to a
key topic: link adaptation. With link adaptation, one could dynamically adjust
the resource allocations, including user modulation and coding scheme, to
ensure near optimal system performance.
3. Scheduling optimization
Another area where optimization plays a key role is in data scheduling. For each
WiMAX BTS, there are multiple MS that request services with different QoS
requirements. QoS is one of the key features of mobile WiMAX. BTS need to
have a scheduling policy that can account for the various QoS requirements (priority,
data rate and latency). The packets could result from various applications,
for example, real time video, voice, web Browser, and file transfer (FTP). The
QoS for these applications are quite different. The scheduler needs to schedule
these packets differently so their QoS requirements can be met. Moreover, the
scheduling is the solution to an optimization problem with a constraint that the
physical channel can only handle certain throughput.
RF optimization is a generic procedure that needs be done for all wireless networks,
and it has been covered in chapter 5 of this book. This chapter will focus
on the optimizations associated with the radio resource allocation and scheduling.
The remainder of this chapter is organized as follows. Section 2 gives a brief
description of WiMAX physical layer and lower MAC layer that are directly
associated with radio resource allocation and scheduling. Section 3 describes
the resource allocation and the optimization procedure. Section 4 describes the
scheduling algorithm and some optimization consideration. The chapter is concluded
in Section 5 with an actual simulation example.
13.2 Brief Description of WiMAX
13.2.1 WiMAX Physical Layer
Mobile WiMAX employs OFDMA as the multiple access technology for its airinterface.
OFDM divides the bandwidth into multiple frequency sub-carriers. In an
OFDMA system, the input data stream is divided into several parallel sub-streams
of reduced data rate (thus increased symbol duration) and each sub-stream is modulated
and transmitted on a separate orthogonal sub-carrier. The increased symbol
duration improves the robustness of OFDM to delay spread. Furthermore, the introduction
of the cyclic prefix (CP) can completely eliminate Inter-Symbol Interference