OFDMA Sub-Channels and Permutation Zones

The sub-carriers are divided into groups known as sub-channels

• Sub-carriers may be adjacent or distributed in a sub-channel
• Sub-carriers are assigned to sub-channels to ensure frequency diversity and interference diversity.

Sub-channel Usage Schemes

• PUSC – Partial Usage of Sub-Channels

-       Mandatory mode for sending preambles and allocation messages and all the uplink messages

-       Sub-carriers are divided between cells (N=3) and then grouped into sub-channels

-       Goal: Reduce RF interference

• FUSC - Full Usage of Sub-Channels

-       Optional and used in the downlink only.

-       All sub-carriers are available in every cell (N=1)

-       Goal: Maximize throughput

• AMC – Adaptive Modulation and Coding

-       Adjacent sub-carriers are grouped into sub-channels.

-       Mobile devices provide feedback on channel conditions so the BS can adjust coding and modulation to match channel conditions.

In Mobile WiMAX, flexible sub-channel reuse is facilitated by sub-channel segmentation and

permutation zones. A Permutation Zone is a number of contiguous OFDMA symbols in the downlink

or uplink that use the same permutation or mapping sequence between the sub-channels and the

sub-carriers. A downlink frame may contain more than one permutation zone.

Zone Partitioning makes use of sub-carrier characteristics

• Normal region contains frequency diverse sub-channels. Time scheduling is used to support voice service.
• Band AMC region makes use of adjacent sub-channels and both time and frequency scheduling is available.
• Broadcast region uses frequency-diverse sub-channels in a simulcast mode. This concept is borrowed from DVB/DAB system.
• Preamble is a data-free symbol at the beginning of the frame for rough frequency synchronization in the receiver.