Symbol Time Adjustment (802.16 OFDMA)

Default: -3.125% (See OFDMA Standard Presets Table.)

Symbol Time Adjustment adjusts the "useful symbol time period" (TFFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum.) within the "OFDMA extended symbol time period" (TS). Symbol Time Adjustment shifts the start of the TFFT period earlier in the TS time period.The amount of TFFT shift is specified as a percentage of the TFFT length.

The extended OFDMA symbol time period (TS) consists of a guard interval (TGI Guard interval) plus a "useful symbol time period" (TFFT). Within the TS, the VSA performs the demodulation and data analysis on only one TFFT time period. The Symbol Time Adjustment parameter moves the TFFT within the TS.

Typically, the TGI time period is ignored and only the TFFT time period is used. The Symbol Time Adjustment parameter causes the demodulation start time to begin earlier within each extended symbol time. The minimum Symbol Time Adjustment value is equal to " - (guard interval)*100", the maximum value is "0" (full guard interval).

The Symbol Time Adjustment parameter is reset to the default value whenever the Guard Interval is changed. The default value is -3.125% if the Guard Interval is 1/16 or greater. The default value is equal to TGI*100/2 (this is half the guard interval) when the Guard Interval is less than 1/16.

$image\OFDMASymTimeStruct_wmf.jpg$

Moving the TFFT period away from the ending transition time (TTR) into the guard interval (TGI) may help avoid intersymbol interference. Make sure that the TFFT period is not shifted so much that it includes corrupt data from the transition time at the beginning of the symbol-time.