Equalizer Training (802.11n/ac/ax/be)
When demodulating the 802.11n/ac/ax/be signal, the VSA uses an equalizer to correct for linear impairments in the signal path, such as multi-path. The VSA supports two different methods to initialize, or "train", the equalizer: Preamble Only and Preamble, Pilots & 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
When is selected, the channel estimation sequences in the preamble of the 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 are used for equalization. L-LTF Non-HT Long Training field is used to equalize the SIG symbols (L-SIG Non-HT SIGNAL field, HT-SIG High-Throughput SIGNAL field, or VHT very high throughput (VHT) physical (PHY) layer protocol data unit (PPDU):A PPDU transmitted using the TXVECTOR FORMAT parameter equal to VHT.-SIG). HT high throughput-LTF Long Training field (VHT-LTF for 802.11ac) is used to compute the equalizer coefficients for the Data symbols (and these equalizer coefficients are shown in the Ch Frequency Response trace).
The advantage of this method of equalization is that it models what a typical OFDM receiver would do when receiving transmissions from a base station. Thus the EVM measured using this method will more accurately reflect the signal quality seen by a typical OFDM receiver.
The disadvantage of this method of equalization 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, Pilots & Data
When is selected, the Data symbols are also used to equalize the signal. This type of training generally gives a more accurate estimate of the true response of the transmission channel than Preamble Only.
This is a two-pass equalization. The signal is first equalized using as described in Preamble Only and demodulated. Then the equalizer coefficients calculated from HT-LTF (VHT-LTF for 802.11ac) are updated by using the demodulated Data symbols. Finally, the signal is demodulated again using the updated equalizer coefficients. These updated coefficients are shown in the Ch Frequency Response trace.
Advantages of this method of equalization include the following:
- The equalizer coefficients typically reflect the linear channel impairment with greater accuracy than , as the data set used to train the equalizer is larger and is less affected by turn-on transient effects in the burst.
- The EVM is typically lower than the EVM using because the equalizer is less impacted by noise and 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. This type of equalizer computation is less likely to be used in practical receivers because it is more complicated and therefore more expensive to implement.
When Equalizer Training is set to Preamble, Pilots & Data, Channel Estimation parameters are not editable and the interpolation filter is internally set to None/Linear, regardless of the current user setting.
See Also
