Wireless Network Design: Optimization Models and Solution ...

13 Optimization of Wireless Broadband (WiMAX) Systems 299
uplink, one slot is a block of 24 sub-carriers by 3 symbols. Mobile WiMAX employs
a carrier permutation scheme that spreads the carriers of each slot over the whole
bandwidth to have better frequency diversity. Thus the sub-carriers of each slot are
spread over the whole band width.
For 10 MHz bandwidth systems, assuming 2:1 down link to uplink ratio, there
are about 390 slots for down link traffic after removing the overhead associate with
FCH and MAPs. The total slots for uplink is about 140, after removing the overheads
associated with ranging and feedback channels.
The down link slot is further split into two clusters, and each cluster contains 14
sub-carriers in frequency and 2 symbols in time, forming 28 points in the two dimensional
frequency-time space (Fig. 13.2). Among them, four points are the pilots
for channel estimation. The uplink slot is split into 6 tiles, and each tile occupies
4 sub-carriers in frequency and 3 symbols in time, forming 12 points in the twodimensional
frequency-time space, among them, 4 points are the pilots for channel
estimation (Fig. 13.3).
Fig. 13.2 Downlink PUSC Slot
WiMAX supports the following modulation coding schemes: 4QAM1/2, 4QAM3/4
16QAM1/2, 16QAM3/4, 64QAM1/2, 64QAM2/3, 64QAM3/4, and 64QAM5/6.
The first number indicates the modulation order and the last number indicates the
coding rate. The modulation coding scheme (MCS) can be dynamically adjusted
based on the signal strength, channel condition and the interference level. Hybrid