Frequency Tolerance

Frequency tolerance also needs to be tightly specified. Failure of a GPS receiver will
isolate a base station. The base station will still serve local handsets but will drift away
from the rest of the network and become invisible—an island cell that takes with it the
handsets it is presently supporting. Table 3.11 describes permitted frequency tolerance.
In IMT2000DS, a short code is used to bring the handset onto channel in terms of
time synchronization. This helps to relax the frequency reference in the handset (reducing
RF component count) but makes the long code acquisition process quite complex.
CDMA2000 is much simpler. All base stations share the same long code, but each
base station is offset by 64 chips from the next base station. There are 512 possible offsets.
When the handset is turned on, it should lock onto the long code with the shortest
PN offset, because this will be, by implication, the nearest base station in terms of
flight path.
The only disadvantage to this is that CDMA2000 timing errors need to be carefully
managed to maintain acquisition performance and prevent false acquisition. A timing
error in the offset higher than 3 μs can cause system performance degradation.
The orthogonality of Walsh codes and OVSF codes disappears if the codes are not timealigned.
Sources of timing errors can be within the application-specific integrated circuit
(ASIC); time adjustment parameters), and delay in baseband signal paths or Walsh code
intermodulation. The pilot to Walsh channel time tolerance is specified at <50 ns.

Phase errors between the receiver local oscillator and decorrelated Walsh channels
create IQ interference and Walsh code intermodulation. The phase tolerance must be
less than 2.86 degrees (50 milli-radians). The CDMA2000 handset uses the pilot channel
phase as a reference. If the pilot channel phase is not aligned with the traffic channels,
the traffic channels will not be demodulated! 105