Pilot Tracking (802.11a/g/j/p OFDM)

When demodulating an 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. burst, the equalizer response computed from the burst preamble is used to correct many imperfections in the OFDM signal. Because the equalizer response is not perfect, pilot tracking is used to correct for imperfections in the equalizer response and for imperfections that change over the length of the burst.

In each OFDM symbol, four of the subcarriers are dedicated to pilot signals in order to make the coherent detection robust against frequency offsets and phase noise. These pilot subcarriers occupy subcarriers -21, -7 , 7, and 21 and use BPSK Binary phase shift keying - A type of phase modulation using 2 distinct carrier phases to signal ones and zeros. modulation.

Pilot subcarriers transmit with a known data sequence. This information is used to determine the difference, or error, between an ideal signal and the actual received signal. Because the data is complex, the VSA calculates phase, amplitude, and timing error data. The error data can then be used to correct both pilot and data subcarrier imperfections, producing a more accurate demodulation.

The IEEE Institute of Electrical and Electronics Engineers. A US-based membership organisation that includes engineers, scientists, and students in electronics and related fields. The IEEE developed the 802 series wired and wireless LAN standards. Visit the IEEE at http://www.ieee.org 802.11a/g/j/p and HIPERLAN/2 specifications call for tracking and correcting only the phase of the pilot subcarriers. This is the 802.11a/g/j/p OFDM demodulation default, but phase tracking can be disabled. In addition, the VSA can optionally track and correct for amplitude and/or timing errors.

This feature is controlled by the pilot tracking check boxes (802.11a/g/j/p Demod Properties dialog box - Advanced tab:

Track Amplitude: Apply pilot subcarrier amplitude error correction to the pilot and data subcarriers.
Track Phase: Apply pilot subcarrier phase error correction to the pilot and data subcarriers.
Track Timing: Apply pilot subcarrier timing error correction to the pilot and data subcarriers.