By Dr. Ernest Simo
22.0 How are Long PN Code Offsets used in IS-95 Systems?
As with the Short PN codes, we can use the various offsets of the same Long PN sequence as different codes:
In principle, all 4.4 trillion offsets of the Long PN code can be treated as distinct codes since they are mutually isolated by a cross-correlation factor of -1/L, while maintaining a very good auto-correlation factor. However, if two adjacent offsets are used, a Multipath of the leading sequence (delayed by exactly one chip) would look identical to the lagging sequence.
In IS-95, a multiple of 64 chips separation (typically 256 chips) is recommended between adjacent Long PN offsets.
A 64-chip separation between adjacent offsets of the Long PN code would yield approximately 69 billion offsets. A 256-chip separation would yield about 242 / 28 = 234 = ……offsets.
- A subset is reserved and used as Access channels. They allow Mobiles to Register, Respond to a Page, Originate a call or perform Authentication. These channels are accessed on a contention basis.
- Another subset is used to isolate Mobiles in the Reverse Link. These offsets are assigned to mobiles on a dedicated basis. (Just like short PN offsets are assigned to the cells and sectors on a dedicated basis.)
© 2001 DR. ERNEST SIMO - SPACE2000