Equalizer Training (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. signal, the VSA uses an equalizer to correct for linear impairments in the signal path, such as multi-path. The VSA supports two different ways to initialize, or "train," the equalizer: Preamble Only and Preamble & Data. Switching between the two methods can help isolate problems contributing to increased EVM Error vector magnitude (EVM): A quality metric in digital communication systems. See the EVM metric in the Error Summary Table topic in each demodulator for more information on how EVM is calculated for that modulation format..
Preamble Only
The equalizer is trained by looking at the in the preamble of the OFDM burst. After this initialization, the equalizer coefficients are held constant while demodulating the rest of the burst.
Advantages of this method of equalization include the following:
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It models what a typical OFDM receiver would do, so the EVM measured using this method more accurately reflects the signal quality seen by a typical OFDM receiver.
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It complies with the description in the "Transmit modulation accuracy test" section (17.3.9.7) of the 802.11a standard.
The disadvantage of this method is that the measured EVM value may be higher for signals whose impairments change during the burst than it would be if the equalizer were trained over the entire burst.
Preamble & Data
The equalizer is trained by analyzing the entire OFDM burst, including the (contained in the preamble) and the Data symbols. This type of training generally gives a more accurate estimate of the true response of the transmission channel.
Advantages of this method of equalization include the following:
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The equalizer coefficients typically reflect the linear channel impairment with greater accuracy, as the data set used to train the equalizer is larger and is less affected by turn-on transient effects in the burst.
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The EVM is typically lower because the equalizer is less impacted by noise and some other forms of distortion. (EVM represents the error due to noise, non-linear distortion, spurious, and residual linear distortion.)
The disadvantage of this method is that it is less likely to accurately reflect the performance of a typical OFDM receiver. Because this type of equalizer computation is more complicated and therefore more expensive to implement, it is less likely to be used in practical receivers.
For details on using this feature, refer to the application note Equalization Techniques and OFDM Troubleshooting for Wireless LANs.
