Troubleshooting (802.16 OFDM)

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 RCE Relative Constellation Error is the RMS level of the Error Vector Magnitude, averaged over all subcarriers and all detected OFDM symbols. (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 1.2 * the nominal BW is required for 802.16 OFDM signals.

OFDM signal has inverted frequency spectrum

Check the Mirror Frequency Spectrum setting (MeasSetup > IEEE 802.16 OFDM Demod Properties > Advanced tab).

802.16 signal has incorrect FCH symbol at start of burst

Check the Data Sub-carrier Modulation selection (MeasSetup > IEEE 802.16 OFDM Demod Properties > Format tab).

• If it is set to Auto Detect, the signal may be too noisy for auto-detection to work reliably. The From FCH or Manual settings may be used to work around this problem.

• If it is set to From FCH, the FCH symbol may be incorrect. Switch to auto-detection.

• If it is set to Manual, it may be specifying the wrong modulation type. Specify the correct modulation type or switch to auto-detection.

Wrong modulation format

Select correct modulation format

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.

802.16-OFDM TDD Time Division Duplex: A duplexing technique dividing a radio channel in time to allow downlink operation during part of the frame period and uplink operation in the remainder of the frame period. mode is assumed by default. If the signal is not pulsed, turn off Pulse Search.

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.

RCE (EVM) larger than expected

I/Q misaligned using I+jQ receiver.

The Error Vector Spectrum trace will shows as a "V" shape. Click Utilities > Calibration > External Amplitude tab, select the Calibration Constants check box and adjust the delays for each input channel.

89611 with ESA H70 hardware need adjusting.

Adjust the Tuner Maximum Mixer Level for the ESA.

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 Utilities > Calibration > External Amplitude tab > Calibration Constants 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 Timing Tracking and possibly Pilot Phase Tracking may help (MeasSetup > IEEE 802.16 OFDM Demod Properties > Advanced tab).

Signal has longer than usual transition region between symbols

Change Symbol Timing Adjustment to be more negative (MeasSetup > IEEE 802.16 OFDM 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 802.16 OFDM Demod Properties > Advanced tab). It is possible for the VSA to appear to synchronize to a signal with inverted frequency content.

RCE (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 802.16 OFDM Demod Properties > Advanced tab).

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

802.16 signal has incorrect length portion of SIGNAL symbol at start of burst

Override the auto detection of Result Length (MeasSetup > IEEE 802.16 OFDM Demod Properties > Time tab).

RCE (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.