Troubleshooting (802.11b/g DSSS/CCK/PBCC)

This topic contains information to help troubleshoot measurements when using 802.11b/g 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)./CCK complementary code keying/PBCC packet binary convolutional code Demodulation.

PROBLEM	POSSIBLE CAUSE	SOLUTION
Demodulation fails to lock on signal or	Signal not present.	Check connections
SYNC NOT FOUND" error message.	Carrier too far from center frequency.	Adjust center frequency. The VSA center frequency must be within 2.5 MHz Megahertz: A unit of frequency equal to one million hertz or cycles per second. to lock-on. For good 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. measurements, set the center frequency to within 550 kHz kiloHertz: A radio frequency measurement (one kilohertz = one thousand cycles per second)..

	Input is over-loaded or under ranged.	Adjust input range. Typically, the best EVM 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 19.8 MHz is typically required for 802.11b/g DSSS signals.

	Signal has inverted frequency spectrum	Check the Mirror Frequency Spectrum setting (MeasSetup > IEEE 802.11b/g Demod Properties > Advanced tab).

	Signal has incorrect PLCP Physical layer convergence protocol Header at start of burst	Override the auto detection of modulation type and result length (MeasSetup > IEEE 802.11b/g Demod Properties > Format tab and Time tab).

	Wrong modulation format	Select correct modulation format.

	Input signal scrambler disabled	Disable the VSA's descrambler (MeasSetup > IEEE 802.11b/g Demod Properties > Advanced tab).

	Maximum result length shorter than preamble length	Increase maximum result length (MeasSetup > IEEE 802.11b/g Demod Properties > Time tab).

	Signal is a 802.11a/g/j/p 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. modulated signal	Switch the VSA to 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 802.11a/g/j/p OFDM mode.

VSA locks, but data bits incorrect	Signal has inverted frequency spectrum	Check the Mirror Frequency Spectrum setting (MeasSetup > IEEE 802.11b/g Demod Properties > Advanced tab). Note that the burst Preamble and Header are usually BPSK Binary phase shift keying - A type of phase modulation using 2 distinct carrier phases to signal ones and zeros. modulated, so would be unaffected by an inverted frequency spectrum. But the data portion of the burst is generally QPSK Quadrature phase shift keying modulated and the detected data bits will be incorrect if the frequency spectrum is inverted.

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.	IEEE 802.11b/g DSSS/CCK/PBCC uses 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.

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 > 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) Click Input > User Correction then select the Amplifier/Attenuator check box and adjust the gains for each input channel.

	Noise or interference from adjacent signals.	Reduce the span to eliminate excess noise and interfering signals.

EVM good at beginning of burst, then gets very large near the end of the burst	Signal has incorrect Length segment of PLCP Header at start of burst	Override the auto detection of result length (MeasSetup > IEEE 802.11b/g Demod Properties > Time tab).
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.