How PN Codes are used in IS-95 CDMA Systems
By Dr. Ernest Simo
2.0 Using the Long PN Code
The Long PN Code Offsets
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 chip 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.)
PN Sequences - Part 2
By Dr. Ernest Simo
© 1999 DR. ERNEST SIMO - SPACE2000