Technical data

256 Chapter 8
CDMA Over Air
Interpretation of CDMA Over Air Measurement Results
CDMA Over Air
Interpretation of CDMA Over Air Measurement Results
This section contains expected result values and possible causes of error if the
expected results are not met.
Frequency Error
The standards specify very tight Frequency Error performance. In the PCS bands,
the 0.05 parts-per-million specification translates to only 99 Hz at a carrier
frequency of 1980 MHz or 40 Hz at 850 MHz cellular frequencies. Frequency Error
shows up as uncorrelated power that adds to the noise floor or shows up in other
Walsh codes.
If a particular site loses its reference to GPS time, its reference signals will begin to
drift over time. Mobiles already using the site can remain on the air because they
derive their timing from the signals transmitted by the base station. However,
mobiles using other sites/sectors may be prevented from using the site because they
are confused by the error in frequency. This creates what is known as the “island cell
effect”. By itself, the cell is still functional. To the rest of the system, it's
inaccessible.
This island cell effect can be caused by a failure in the site's GPS receiver and
timebase distribution network. Using the test set's Internal GPS receiver provides an
independent time reference that will allow you to determine if this cell site is out of
sync with the rest of the network (“island cell effect”).
PN Offset
Verify the PN Offset is correct. If you are in Manual mode, the PN Offset will
display the value you entered. Make sure the Time Offset is small (less than 3 µs). If
the Time Offset is greater than expected, see the section on Time Offset below. If
you are in Auto mode, the test set will tune to the PN Offset with the least amount of
Time Offset. If an incorrect PN Offset is displayed, the Time Offset will likely be
very large.
Time Offset
The CDMA standards specify a maximum offset of 10 µs; generally 5 µs is a
recommended maximum. If the Time Offset is large enough, an “island cell” can
occur. The “island cell effect” was mentioned already when discussing Frequency
Error.
A phone moving outward toward the limit of its cell will need to acquire the