Technical data

252 Chapter 8
CDMA Over Air
Metrics Provided by CDMA Over Air Measurements
CDMA Over Air
Metrics Provided by CDMA Over Air Measurements
This section contains descriptions of the individual CDMA transmit parameters in
the metric display of the screen. For expected result values and possible causes of
error if the expected results are not met, see “Interpretation of CDMA Over Air
Measurement Results” on page 256.
Frequency Error
Frequency error is the frequency difference between your transmitter's actual center
frequency and the frequency (or channel) you entered.
PN Offset
The PN Offset is a “short code” sequence that provides a unique identifier for each
sector of each cell site. The PN Offsets are applied to the I and Q signals before
modulation. PN Offsets are offset in time by 52.08 µs and they repeat every 26.666
ms. This yields 512 unique short code sequences (0-511). The mobile phone needs
the PN Offset to decode information in the Sync and Paging channels, which are
transmitted by the base station.
Time Offset
The Time Offset compares the PN Offset timing with the overall system time. This
measurement checks the start of the PN offset in comparison to either the GPS
signal or the Even Second clock signal. For example, PN Offset 0 should repeat
exactly on the rising edge of the Even Second clock. PN Offset 1 should repeat
52.08 µs after the rising edge of the Even Second clock, and so forth. Any error in
time from that event is reported as a Time Offset.
IS-95 specifies that the base station timing must be within +/-10 µs of its assigned
value. Many base station manufactures specify that the timing must be within +/-3
µs. For example, if you are parked near the transmit antenna, the propagation delay
is less than 1 µs. The measured value of delay should be within the recommended
limit, which is greater than -10 µs and less than +10 µs. As you move away from the
transmit antenna, propagation delay needs to be considered—about 1 µs for every
300 meters (1000 feet). So, if you are parked 800 meters (½ mile) from the transmit
antenna, the additional propagation delay would be approximately 2.5 µs. The
measured value of delay should be greater than -7.5 µs and less than +12.5 µs.