Troubleshooting Custom OFDM Measurements
This topic contains information to help you troubleshoot measurements when using Custom 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. Demodulation. You should also review Making a Custom OFDM Measurement for additional measurement information.
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PROBLEM |
POSSIBLE CAUSE |
SOLUTION |
|---|---|---|
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Signal not present. |
Check connections |
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Carrier too far from center frequency. |
Adjust center frequency. The measurement hardware center frequency must be within a few subcarriers of the signal center frequency to lock properly. (The subcarrier spacing is equal to the OFDM System Sample Frequency divided by the FFT Fast Fourier Transform: A mathematical operation performed on a time-domain signal to yield the individual spectral components that constitute the signal. See Spectrum. Length.) |
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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. |
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Frequency span is too narrow. |
Increase frequency span. The span should be wide enough to contain all subcarriers being analyzed. |
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Incorrect OFDM System Sample Frequency. |
Set correct OFDM System Sample Frequency as specified in the signal standard being demodulated. |
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OFDM signal has inverted frequency spectrum |
Check the Mirror Frequency Spectrum setting (). |
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Wrong modulation format |
Set the correct modulation format in the Resource Modulation array, or set the Resource Modulation array values to zero so that the analyzer auto-detects the modulation format. |
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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. |
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Signal is not pulsed. |
If the signal is not pulsed, try to use triggering to position the preamble just after the beginning of the search length. Disable pulse search (). |
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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 check box). |
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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 ( tab). |
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Signal has longer than usual transition region between symbols |
Change Symbol Time Adjustment to be more negative ( tab). |
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Noise or interference from adjacent signals. |
Reduce the span to eliminate excess noise and interfering signals. |
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OFDM signal has inverted frequency spectrum |
Try inverting the frequency spectrum (). It is possible for the VSA to appear to synchronize to a signal with inverted frequency content, because of symmetry properties of the preamble of some signal types (for example 802.11a can sometimes show this). |
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Signal amplitude varies during the burst. |
Turn on Pilot Track Amplitude (). |
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Center frequency not close enough to signal carrier frequency |
Adjust center frequency. |
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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. |
See Also
Making a Custom OFDM Measurement
