Technical data

258 Chapter 8
CDMA Over Air
Interpretation of CDMA Over Air Measurement Results
CDMA Over Air
Poor Rho performance affects the base station capacity because the uncorrelated
power appears as interference to the mobiles. The added interference will require an
increase in the traffic channel level to overcome the interference. This may, in turn,
be seen as further interference. At some point, the site will have to shed calls in
order to supply the remaining calls with enough signal versus the interference in the
system.
If Estimated Rho is lower than expected, check the following:
• External interference may be degrading the CDMA signal. Use the spectrum
analyzer to verify that no spurious signals are present in the band of the
transmitter.
• Compression may be occurring in the base station power amplifier.
• There may be errors in the base station IQ modulator.
Carrier Feedthrough
A good Carrier Feedthrough level is less than -25 dB. The IS-97 standard does not
specify Carrier Feedthrough; however, this measurement provides an additional tool
to troubleshoot the base station's transmitter.
In the frequency domain, Carrier Feedthrough can show up as an uncorrelated
energy spike that can be seen on the spectrum analyzer by closely spanning into the
top of the CDMA signal. The effects of Carrier Feedthrough can also show up as
higher noise levels on the Code Domain Power screen. The inactive Walsh codes
will be pushing the -27 dB specification for noise. Carrier Feedthrough can be
caused by the lack of isolation across the mixer and cavity of the transmitter's I/Q
modulator.
Noise Floor
A good Noise Floor value is between -20 dB and -30 dB and depends on the quality
of the signal. If the Noise Floor is above these values (for example, -20 dB), the
modulator may be the source of the added noise. Another possibility is a problem
with the base station's channel card. The channel card generates the individual
Walsh codes.
Channel Power
If the Channel Power is lower than expected, verify you are in a location that meets
the criteria to provide valid measurements.
Inaccurate Channel Power (high or low) may indicate an incorrect power setting at
the base station.