Guard Interval (802.11a/g/j/p OFDM)
See OFDM Standard Presets Table
0-1
The is the ratio of the Cyclic Prefix "CP 1) Contention period, or 2) Cyclic prefix" time to the inverse FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum. time "T(IFFT Inverse Fast Fourier Transform)." The guard interval is used to eliminate inter-symbol and inter-carrier interference. A copy of the last guard interval T(GI Guard interval) of the useful symbol period "T(IFFT)", termed Cyclic Prefix "CP", is used to collect multipath, while maintaining the orthogonality of the subcarriers. Each symbol is transmitted for a slightly longer time, extended symbol time T(s), than the active (or useful) symbol time T(IFFT). The extra time is the guard interval.
- 1/8: Sets the to 1/8 (see Guard Interval Time Calculation below)
- 1/4: Sets the to 1/4 (see Guard Interval Time Calculation below)
- Other: Enables you to enter values between 0 to 1.
The time period T(GI) is specified as a fraction (percentage) of the inverse FFT time period T(IFFT). For 802.11a, the only selection is a of 1/4 (1/8 is greyed). For HIPERLAN/2, both 1/4 and 1/8 are selections. The selection allows the input of a non-standard value between 0 and 1.
Guard Interval (TGI) Time Calculation
The following table shows calculated values for a HIPERLAN/2 OFDM Orthogonal Frequency Division Multiplexing: OFDM employs multiple overlapping radio frequency carriers, each operating at a carefully chosen frequency that is Orthogonal to the others, to produce a transmission scheme that supports higher bit rates due to parallel channel operation. OFDM is an alternative tranmission scheme to DSSS and FHSS. signal:
|
|
Guard Interval |
T(FFT) |
T(GI) |
|
|---|---|---|---|---|
|
|
1/4 |
3.2 µs |
0.8 µs |
|
|
|
1/8 |
3.2 µs |
0.4 µs |
|
|
|
where: |
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OFDM Frame and the Guard Interval
See Also
