Monday, November 2, 2009

Sub Channelization (Sub-carriers)

Sub channelization is the dividing of communication channels into smaller sub-parts. The air interface portions of the WiMAX system divide a wide radio channel into several sub-carriers. A sub-carrier is a modulation signal that is imposed on another carrier that can be used to independently transfer information from other sub-carriers located on the radio channel. The WiMAX system sub-carrier signal types include a pilot sub-carrier (reference signal), guard sub-carrier (interference protection) and data sub-carrier (user information).

A pilot sub-carrier is a reference signal that serves as a control signal for use in the reception of other sub-carrier signals. A guard sub-carrier is one or more sub-carriers on a communication channel that is not used (null channel). The guard sub-carrier is dedicated to the protection of a communication channel from interference due to radio signal energy or time overlap of signals. Data sub-carriers are transmission channels that carry user information or data. Sub-carrier signals are referenced from the center of the radio channel. The sub-carrier that is located at the center of the radio channel is called the DC sub-carrier.

The number of sub-carriers in the WirelessMAN-OFDM system is 256. Of these, 55 are reserved as guard bands and 8 sub-carriers are used to transfer reference pilot signals. This allows up to 192 sub-carriers to be used for data transfer.

For a 20 MHz WiMAX OFDMA radio channel, there may be 2048 subcarriers. Of these, approximately 70% can be used as data carriers, 25% are reserved to protect from interference (guard bands) and approximately 15% are used as reference signals (pilot channels).

Figure 1 depicts how the WirelessMAN-OFDM system divides a wide radio channel into several independent orthogonal channels with smaller bandwidth. The sub-channel in the center of the RF channel is called the DC subcarrier. Some of the sub-carriers are used as reference pilot channels and some are reserved for guard bands.

Figure 1: WiMax OFDMA Sub Channelization
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