Qualcomm Incorporated (QCOM) Message Board

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Technology Stocks : Qualcomm Incorporated (QCOM)

QCOM 166.77

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3:59 PM EST

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To: Clarksterh who wrote (24602)	3/22/1999 2:32:00 AM
From: Ramus	Read Replies (1) of 152472

Clark, No need to disclaim with me sir!!! EIA/TIA/IS-95 states that when the private long code is negotiated it will then be applied to the forward and reverse links. In IS-95 systems the basestation uses 64 Walsh codes to provide 64 orthogonal code channels. Three are used for overhead, Pilot, synch and paging and the other 61 are used as data channels to service up to 61 mobiles(there can be other combos like up to 7 paging and only 55 mobiles...always a total of 64). It is this method, the application of Walsh coding, that allows the separation of code channels and thus the separation of the identity of the mobiles at a particular base station. To my knowledge, the only time shifting that occurs in the system is between adjacent basestations. For instance all basestations use Walsh 0 for the pilot channel. And in a particular network they might all be on the same channel frequency!! In order to separate basestations that are near each other a phase offset is introduced. The pilot channel transmitted by the basestation is used to provide frequency and phase information to the mobile. The pilot is transmitted with higher power than the other orthogonal channels. That way when the mobile detects the highest level Walsh 0 channel it knows its decoding the closest basestation. Once the mobile synchronizes to the basestation, the pilot channel is used as a coherent phase reference for that basestation. Also, if you look at a system diagram(either forward or reverse link) the long code is applied then the Walsh cover, then the spreading codes(partial transmitter forward/reverse). The point of all this is that the channels are separated by Walsh codes. The individual mobiles (ESN etc) are identified essentially by their long codes. But these long codes will only be seen in a receiver after despreading Whew! Now, having said all that..... you're right in your assumption that if we know the various codes and PN sequences we should be able to decode the data. But as Leland pointed out... it looks like we could be using essentially a public key encryption system in IS-95 CDMA. I have the feeling that it is very very secure. Walt

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