Troubleshooting (802.11a/g/j/p OFDM)

Demodulation fails to lock on signal

Signal not present.

Check connections

Carrier too far from center frequency.

Adjust Center frequency. The measurement hardware center frequency must be within 2 * Subcarrier Spacing (625 kHz kiloHertz: A radio frequency measurement (one kilohertz = one thousand cycles per second). for 802.11a or HIPERLAN/2) to lock properly.

Input is over-loaded or under ranged.

Adjust input Range (Input > Analog). 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. A frequency span of at least 18 MHz Megahertz: A unit of frequency equal to one million hertz or cycles per second. is typically required for 802.11a and HIPERLAN/2 signals.

Incorrect subcarrier spacing.

Set correct Subcarrier Spacing. The normal subcarrier spacing for 802.11a and HIPERLAN/2 signals is 312.5 kHz.

OFDM signal has inverted frequency spectrum

Check the Mirror Frequency Spectrum setting (MeasSetup > 802.11a/g/j/p Demod Properties > Advanced tab).

802.11a signal has incorrect SIGNAL symbol at start of burst

Override Automatic detection of modulation type and result length (MeasSetup > 802.11a/g/j/p Demod Properties > Format tab and Time tab).

HIPERLAN/2 signal is using 1/8 guard interval length instead of 1/4

If this is the case, the VSA will get a good Sync Correlation in the Syms/Errs trace, but the IQ Meas constellation and the EVM will be bad. Set the VSA to use 1/8 Guard Interval (MeasSetup > 802.11a/g/j/p Demod Properties > Format tab).

Wrong modulation format

Select correct modulation format (MeasSetup > IEEE 802.11a/g/j/p Demod Properties > Format tab).

Signal uses the 802.11g DSSS Direct sequence spread spectrum. The data transmission scheme (sometimes referred to as a "'modulation" scheme) used in 802.11b WLANs. DSSS uses a radio transmitter operating at a fixed centre frequency, but using a relatively broad range of frequencies, to spread data transmissions over a fixed range of the frequency band. 802.11a and 802.11g (when not operating in 802.11b mode) use Orthogonal Frequency Division Multiplexing (OFDM).-OFDM format

Use Preset to Standard to select IEEE 802.11g DSSS-OFDM format. Because a DSSS-OFDM signal has a much longer preamble than other OFDM signals, it is sometimes helpful to increase the Search Length.

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.

Both 802.11a and HIPERLAN/2 are pulsed signals. 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

802.11a signal has incorrect Short Training Sequence (Short Sync) in burst preamble.

Try setting the VSA to synchronize to the Channel Estimation Sequence (Long Sync) portion of the burst preamble (MeasSetup > IEEE 802.11a/g/j/p Demod Properties > Advanced tab).

VSA demodulator set incorrectly. Attempting to demodulate an 802.11b DSSS modulated signal.

Set the VSA demodulator to 802.11b/g mode. Select (MeasSetup > Measurement Type > Wireless Connectivity > IEEE 802.11b/g DSSS/CCK/PBCC).

EVM larger than expected

I/Q misaligned using I+jQ receiver.

The Error Vector Spectrum trace will shows as a "V" shape. Click Input > User Correction, select the Amplifier/Attenuator check box and adjust the delays for each input channel.

I/Q gain imbalance using I+jQ receiver.

1) Check that impedance is identical for both input channels.

2) Adjust the gains for each input channel (Click Input > User Correction > Amplifier/Attenuator check box).

Signal is transmitting incorrect pilot bits.

If the signal is transmitting incorrect pilot bits, the CPE level will be high, both in the Syms/Errs summary and on the Common Pilot Error trace. Turning off Pilot Track Timing, and possibly Pilot Track Phase, may help (MeasSetup > IEEE 802.11a/g/j/p Demod Properties > Advanced tab).

Signal has longer than usual transition region between symbols

Change Symbol Time Adjustment to be more negative (MeasSetup >IEEE 802.11a/g/j/p 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 > 802.11a/g/j/p 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 802.11a Short Training Sequence. 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 Track Amplitude (MeasSetup > IEEE 802.11a/g/j/p Demod Properties > Advanced tab).

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

802.11a signal has incorrect length portion of SIGNAL symbol at start of burst

Override Automatic detection of result length (MeasSetup > IEEE 802.11a/g/j/p 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.