Troubleshooting (802.16 OFDMA)

This topic contains information to help troubleshoot measurements when using IEEE Institute of Electrical and Electronics Engineers. A US-based membership organisation that includes engineers, scientists, and students in electronics and related fields. The IEEE developed the 802 series wired and wireless LAN standards. Visit the IEEE at http://www.ieee.org 802.16 OFDMA Demodulation.

Additional Information

It is recommended to read the 89600 VSA Version Differences and Measurement Tips topic. These topics contains information that will help with understanding the VSA setup configuration and measurement limitations for IEEE 802.16 OFDMA signal analysis.

IEEE 802.16 OFDMA Troubleshooting Table

PROBLEM	POSSIBLE CAUSE	SOLUTION
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 carrier lock range to lock properly, see Setting the Center Frequency.
	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 IEEE 802.16 OFDMA signals.
	OFDMA signal has inverted frequency spectrum	Check the Mirror Frequency Spectrum setting (MeasSetup > IEEE 802.16 OFDMA Demod Properties > Advanced tab).
	802.16 signal has incorrect FCH symbol at start of burst	Check the Data Sub-carrier Modulation setting (MeasSetup > IEEE 802.16 OFDMA Demod Properties > Format tab). If it is set to Auto Detect, the signal may be too noisy for auto-detection to work reliably. The FCH or Manual Override settings may be used to work around this problem. If it is set to FCH, the FCH symbol may be incorrect. Switch to auto-detection. If it is set to Manual Override, it may be specifying the wrong modulation type. Specify the correct modulation type or switch to auto-detection.
	Wrong modulation format used for the data subcarriers.	Select Manual Data Tone Modulation mode from the Advanced tab, and make sure the correct modulation format is selected for the defined bursts (use the Edit Downlink/Uplink Zone Definition dialog). When “Auto” mode isselected for Downlink analysis, the DIUC mappings defined on the “Burst Profiles” tab must be configured to match the current signal.
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-OFDMA 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.
P802.16-2004/Cor1/D2 signal shows incorrect DL-PUSC subchannel mapping (either a DUT signal source or SignalStudio OFDMA signal source).	The 89601 interprets the Cor1/D2 DL Down Link (forward link: from base station to cell phone)-PUSC partial usage of subchannels: Some of the subchannels are allocated to the transmitter. subchannel mapping as defined in clause 8.4.6.1.2.1.1 item #4. Some other implementations (SignalStudio OFDMA as an example) may implement DL-PUSC subchannel mapping as defined in Cor1/D3 instead, but keep the PRBS Pseudo-Random Binary Sequence definition as defined in Cor1/D2, and call this a "Cor1/D2" signal.	When using the “P802.16-2004 (Cor1/D2)” standard mode, clear the “Use Cor1/D2 DL-PUSC cluster renumbering” checkbox on the Advanced tab. This will select usage of the Cor1/D3 subchannel mapping for DL-PUSC, but will continue to use the Cor1/D2 modulation PRBS definition
Unexpectedly high Error Vector Spectrum results for highest and lowest subcarriers of a DL-AMC measurement	VSA DL-AMC Adaptive Modulation and Coding measurement algorithm is including a portion of the channel response of the multi-channel filter.	Disable the multi-channel filter (Clear: Use multi-channel filter on the "Advanced" tab of theOFDMA Demod Properties dialog). The subcarriers at the high and low ends of the spectrum in a DL-AMC zone extend above and below the subcarriers used in the preamble, and therefore the channel estimation algorithms in the demodulator cannot completely correct for the frequency response of the multi-channel filter on these subcarriers.
Non-default Spans result in larger EVM/RCE for muticarrier OFDMA signals.	The VSAs Multi-channel filter performance is optimized for spans equal to NBWBWRatio/1.28 (where N=2 or 4). Other spans may result in slightly elevated RCE and EVM in a multi-carrier measurement due to aliasing of adjacent channel power into the analysis channel.	Only use the VSA default standard Span bandwidths.
RCE (EVM) larger than expected	I/Q misaligned using I+jQ receiver.	The "Error Vector Spectrum" trace appears as a "V" shape. Click Utilities > Calibration > External Amplitude tab, select the Calibration Constants check box and adjust the delays for each input channel.
	Input hardware range is not optimized for high peak-to-average signals.	Adjust the Mixer Level to reduce distortion.
	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 > 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 OFDMA 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 OFDMA Demod Properties > Advanced tab).
	Noise or interference from adjacent signals.	Reduce the span to eliminate excess noise and interfering signals.
	OFDMA signal has inverted frequency spectrum	Try inverting the frequency spectrum (MeasSetup > IEEE 802.16 OFDMA 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 OFDMA Demod Properties > Advanced tab).
RCE (EVM) good, but Sync Correlation coefficient not good	Center frequency not close enough to signal carrier frequency	Adjust center frequency.
High Error Vector Spectrum (or Time) subcarriers present, but RCE on the Syms/Errs summary table seems normal.	Active subchannels are present, but not defined in the zone definition, and "Include inactive subchannels" is selected. This will show subcarriers belonging to inactive subchannels in the trace results, but does not use them in computing RCE or DataRCE. Since inactive subchannels are assumed to contain noise, they will appear to have high EVM if they actually contain transmitted power.	Either define all active bursts in the zone definition or clear "Include inactive subchannels".
DL-MAP is decoded correctly, but UL-MAP is not.	The DIUC mapping for the UL Up Link (reverse link: from cell phone to base station)-MAP burst is set to an incorrect burst profile.	Update the DIUC mapping on the “Burst Profiles” tab to match the current signal. The coding type, rate and modulation format for the DIUC used by the UL-MAP data burst must match for successful UL-MAP decoding.
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.
IQ Mismatch impairments are not estimated accurately	The wrong data modulation format was auto-detected.	See description of limits on IQ Gain Imbalance estimation, IQ Gain Imbalance topic.
MIMO Multiple Input, Multiple Output: A physical layer (PHY) configuration in which both transmitter and receiver use multiple antennas. Eq Chan Freq Resp Trace shows exaggerated spurs at edges of the trace, where no spurs exist in the Chan Freq Resp trace.	Interaction of the multi-channel filter and extrapolation of the pilot-based channel estimation.	Disable the Multi-channel filter, Clear the .Use Multi-channel filter Advanced property.
Recalling a setup file or MapFile saved with the 2nd zone specified as active recalls to the first zone specified as active.	This occurs when the IEEE 802.16 OFDMA Demod Properties dialog box is open during the file Recall.	Close the IEEE 802.16 OFDMA Demod Properties dialog box prior to Recalling a sSetup file, or re-specify the desired zone active after recalling the MapFile or Setup file.
Incorrectly scaled constellation	Zone Boosting is not used by the waveform	Zone Boosting compensation for DL-PUSC zones is performed automatically. If a DL-PUSC signal does not use all subchannel groups (one or more of the Subchannel Group Bitmask bits is "Cleared") and Zone Boosting is not used by the waveform, then a mis-scaled constellation and high EVM will result. You can correct the measurement impairments by configuring the VSA to use Pilot Amplitude Tracking (MeasSetup > DemodProperties.> Advanced tab > Select Track Amplitude).