Troubleshooting (802.11n/ac/ax/be)

Demodulation fails to lock on signal

Signal not present.

Check connections.

Carrier too far from center frequency.

Adjust center frequency. The VSA center frequency must be within 2 * Subcarrier Spacing to lock properly.

Input is over-loaded or under ranged.

Adjust input range. Typically, the best 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. is achieved when the range is as small as possible while avoiding overloads.

Frequency span is too narrow.

Increase frequency span. The demodulator requires the following minimum frequency spans to demodulate correctly:

Incorrect subcarrier spacing.

Set correct subcarrier spacing. The normal subcarrier spacing for 802.11n/ac signals is 312.5 kHz kiloHertz: A radio frequency measurement (one kilohertz = one thousand cycles per second)..

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 has inverted frequency spectrum

Check the Mirror Frequency Spectrum setting (MeasSetup > IEEE 802.11n/ac/ax/be Demod Properties > Advanced tab).

Wrong modulation format.

Select correct modulation format.

Demod: "Not enough data.."

Error message when making an 802.11n/ac measurement using an 802.11n recording. The 802.11n recorded signal does not include the additional Search/Sync symbols required for 802.11ac signals.

Edit the 802.11n signal recording file contents and manually add two zero data symbols to the beginning and to the ending of the recording file. Choose an editing program for the signal format type; MATLAB for .mat files or an ASCI text editor for .txt or .csv files. . A typical 802.11n recording at an 80 MHz sample rate would require 80000000*0.000008 = 640 zeros at the beginning and end of the recording.

PULSE NOT FOUND error message

Search length too short.

Increase search length or adjust triggering to ensure that the leading and trailing edges of a pulse are fully within the search length.

Signal is not pulsed.

802.11n/ac uses a pulsed signal format. If the signal is not pulsed, try to use triggering to position the preamble just after the beginning of the search length. In this case, PULSE NOT FOUND will still be displayed, but demodulation may be successful anyway.

Search length not optimized.

Try the following method to determine Search Length when performing a Pulse Search:

SL = (2*MaxOn) + MaxOff

Where:

SL is search length in seconds.

MaxOn is the maximum on time of a pulse in seconds.

MaxOff is the maximum off time between pulses in seconds.

SYNC NOT FOUND error message

Signal is a legacy 802.11a signal.

Set the VSA to 802.11a/g/j/p OFDM mode.

VSA demodulator set incorrectly.

Set the VSA demodulator to 802.11n/ac mode. Select (MeasSetup > Measurement Type > Wireless Connectivity > IEEE 802.11n/ac).

EVM larger than expected

I/Q gain imbalance.

Check that impedance is identical for both input channels.

Signal has longer than usual transition region between symbols.

Change Symbol Time Adjustment to be more negative (MeasSetup > IEEE 802.11n/ac/ax/be Demod Properties > Advanced tab).

Noise or interference from adjacent signals.

Reduce the span to eliminate excess noise and interfering signals.

OFDM signal has inverted frequency spectrum.

Try inverting the frequency spectrum (MeasSetup > IEEE 802.11n Demod Properties > Advanced tab). It is possible for the VSA to appear to synchronize to a signal with inverted frequency content, because of symmetry properties of the 802.11n/ac Legacy Short Training Field (L-STF Non-HT Short Training field). In this case, the Sync Correlation value in the error summary table will be only about 5% less than expected, but the demodulation timing and channel estimation are incorrect and the EVM will be large.

EVM increasing from beginning of burst to end (visible on Error Vector Time trace)

Signal amplitude varies during the burst.

Turn on Pilot Amplitude Tracking (MeasSetup > IEEE 802.11n/ac/ax/be Demod Properties > Advanced tab).

EVM good at beginning of burst, then gets very large near the end of the burst

802.11n HT high throughput signal has incorrect length field in the High Throughput Signal Field (HT-SIG High-Throughput SIGNAL field) or Legacy Signal (L-SIG Non-HT SIGNAL field) symbol at start of burst.

Don't use SIG symbols to determine result length (MeasSetup > IEEE 802.11n/ac/ax/be Demod Properties > Time tab).

EVM good, but Sync Correlation coefficient not good

Center frequency not close enough to signal carrier frequency.

Adjust center frequency.

The trace data constellation symbol location shape, such as dots, is not properly displayed.

The display driver is not functioning correctly.

Contact your PC manufacturer to see if a newer version of the display driver is available. If not, try decreasing the hardware acceleration. See the Windows documentation for instructions.