In an OFDM system, a high-data-rate sequence of symbols is split into multiple parallel low-data rate-sequences, each of which is used to modulate an orthogonal tone, or subcarrier.
WiMAX has three classes of subcarriers:
1. Data subcarriers are used for carrying data symbols.
2. Pilot subcarriers are used for carrying pilot symbols. The pilot symbols are known a priori and can be used for channel estimation and channel tracking.
3. Null subcarriers have no power allocated to them, including the DC subcarrier and the guard subcarriers toward the edge. The DC subcarrier is not modulated, to prevent any saturation effects or excess power draw at the amplifier. No power is allocated to the guard subcarrier toward the edge of the spectrum in order to fit the spectrum, of the OFDM symbol within the allocated bandwidth and thus reduce the interference between adjacent channels
2. Pilot subcarriers are used for carrying pilot symbols. The pilot symbols are known a priori and can be used for channel estimation and channel tracking.
3. Null subcarriers have no power allocated to them, including the DC subcarrier and the guard subcarriers toward the edge. The DC subcarrier is not modulated, to prevent any saturation effects or excess power draw at the amplifier. No power is allocated to the guard subcarrier toward the edge of the spectrum in order to fit the spectrum, of the OFDM symbol within the allocated bandwidth and thus reduce the interference between adjacent channels
Above figure shows a typical frequency domain representation of an IEEE 802.16e-2005 OFDM symbol containing the data subcarriers, pilot subcarriers, and null subcarriers.
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