Wireless Network Design: Optimization Models and Solution ...

13 Optimization of Wireless Broadband (WiMAX) Systems 303
of the signal to fluctuate over time. In the case of mobility, the fluctuation can be
very severe. For 5 ms frame, the signal level can swing as much as 10dB from
frame to frame. So, the resource allocation needs be dynamical with time, and
reacts very fast to account for the channel fading.
3. A dynamical mix of various traffic types
The applications and services that are running in wireless broadband networks
are very diverse, and have wide range of the requirements. Common internet
traffic models and requirements are given in Table 13.1.
Table 13.1 The Common Internet Traffic Models and Their Requirements
Class Application Protocol Bandwidth Latency Jitter
1 Multiplayer rtPS Real time ≈ 50 kbps Low < 25 msec N/A
Interactive Gaming polling service
2 VoIP and ertPS Extended real time 32-64 kbps < 160 msec < 50 msec
Video Conference polling service
3 Streaming Media rtPS Real time 5 kbps to 2 Mbps N/A < 100 msec
polling service
4 Web Browsing and nrtPS rtPS Non-real time 10 kbps to 2 Mbps N/A N/A
Instant Messaging polling service
5 Media Content Downloads BE Best effort > 2 Mbps N/A N/A
For typical WiMAX networks, the traffic is mixed with the various types described
above. The resource allocation needs to account for the requirements of
these individual requirements for various services and applications. The radio resource
allocation consists of the following steps:
1. MCS selection
2. MIMO mode selection
3. Power allocation (power control)
4. Link adaptation
Each of these topics will be discussed next.
13.3.1 Modulation Coding Scheme Selection
There are a total of 8 different MCS supported in WiMAX (Table 13.2). Downlink
supports MCS 0-7, and for uplink it is mandatory to support MCS 0-3 and optional
to supports MCS 4-7. Different MCS requires different signal quality, i.e. CINR
(carrier to interference-noise ratio).
For uplink, BS receives the uplink signals form all MSs, and from the uplink
signals, BS can compute the CINR for each MS. WiMAX specifies that the CINR
shall be computed only from the pilots embedded in the traffic.