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Subchannelization is a part of adaptive modulation scheme in WiMAX. In WiMAX, Base station decides whether it should communicate with Subscriber station in a highly-robust modulation (i.e. BPSK- 1/2) or a relaxed modulation with higher throughput (i.e. 64 QAM 3/4) when channel is good (like LOS).
If channel is not good, the most possible robust modulation is BPSK 1/2. After this, if the quality of communication is still not good, and if both BS and SS support subchannelization, BS will start communication in sub-channelization mode, which means:
Divide the ofdm subcarriers (say,256) into number of subchannels (say, 16). Then don''t send anything on some of the sub-channels and transmit on only few sub-channels with more power. For example , if only one of the 16 subchannels is used, power in that sub-channel will be 16 times more (12 dB I guess). Therefore, it allows the BS and SS to talk in a worse channel condition.
Another reason for subchannelization is to share OFDM signal between different subscribers.