Capacity considerations do not say much about the spreading
codes, except that they should have low cross correlations.
Essentially they should look like Gaussian noise to all but
the intended receiver. They should also have low, ideally
zero, autocorrelation between non-adjacent bits of the sequence.
Other system considerations, however, dictate many additional
properties of the codes.
1. Timing in the subscriber stations (mobiles) is
to be established, at least in part, by synchronizing with
the code radiated by the base stations. The goal is to eliminate
any need for accurate timekeeping in the mobiles when they
2. The mobiles identify base stations, at least in part, by
correlating with a priori known base station spreading codes.
3. The process of synchronization in the mobiles should be
rapid enough that the placement of a call from a "cold
start" takes no more than a few seconds.
4. Access to base stations by mobiles, should not require
any prearrangement. That is, it should not be necessary for
the base station to have a database of authorized users in
order to establish radio communications. The base station,
once physical layer access has been achieved, may choose to
deny service for administrative reasons, such as
non-payment of the bill, but communication through the air
interface should be possible no matter what. This is important
for things like 911 emergency access.
5. In the CDMA Forward Link the fact that multiple channels
are being radiated by each base station can be used to advantage
to decrease mutual interference.
6. The acquisition search rate for Reverse CDMA Channel signals
in the base stations can be speeded if the mobiles can pre-correct
their timing so that their signal arrives at the base station
as close to system time as possible.
Forward CDMA Channel Spreading
Reverse CDMA Channel Spreading
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