Wireless Network Design: Optimization Models and Solution ...

13 Optimization of Wireless Broadband (WiMAX) Systems 309
from BS to MS, and MS varies its output power accordingly. The closed loop power
control is implemented as follows:
1. BS computes the CINR of the uplink traffic
2. If the CINR is higher than that required for the MCS assigned, BS sends a power
control command to MS, asking MS to reduce its output power
3. If the CINR is lower than that required for the MCS assigned, BS sends a power
control command to MS, asking MS to increase its output power.
4. MS receives the power control command, and reduces or increases its output
power accordingly.
Compared to the open loop power control, closed loop power control is slower
and requires additional overhead (the power control commands are sent down with
power control IE (information element) in the down link broadcast channel, and
has a total of 3 bytes data). While WiMAX supports both TDD and FDD duplex,
the current system profile (profile 1.0) supports TDD only. In the case of TDD,
open loop power control based on the down link signal variation can eliminate most
of the uplink channel variation by utilizing the reciprocal property of the channel.
Closed loop power control is used infrequently (once every 20 50 frames), and for
the purpose of ’calibrating’ the open loop power control.
13.3.4 Link Adaptation
The complexity of the radio resource allocation results mainly from the channel
fading. In the case of mobility, the movement of MS not only causes the variation
of the path loss, but also degrades the channel condition, a so-called Doppler effect.
Channel fading and Doppler effects cause selective fading along both frequency
and time. These effects are addressed in detail in Chapter 3. The communication
link between BS and MS needs to be adapted to the variation of the channel.
Moreover, in the case of MIMO, the mode which is selected, MIMO-A or
MIMO-B, depends on the signal quality and the MIMO channel condition. For
example, MIMO-B operation requires certain de-correlation between two data
streams, and if there is a strong correlation between the channels seen by different
antennas, even when the signal qualities of each data stream is very high (high
CINR), MIMO-B can not be supported due to the high MIMO co-channel interferences.
To optimize the systems throughput, the link adaptation technique adjusts the
MCS and MIMO mode based on the channel conditions through
1. MCS adjustment
2. Down and up link powers adaptation
3. MIMO mode selection
4. Modifying the number of channels assigned