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Spreading Codes

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.

Goals

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 are idle.

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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