By Dr. Ernest Simo
19.0 How is the Short PN Code is Used in
IS-95 Systems
Shifted versions of the same PN sequence exhibit very good (but not excellent) cross-correlation properties, (-1/L). So, we can use the various offsets of the same sequence as different codes:
- In principle, all 32,768 offsets of the Short PN code can be respectively treated as different codes since they are mutually isolated by a cross-correlation factor of -1/L, while maintaining an auto-correlation factor of 1.
- 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 64-chip separation is recommended between Short PN offsets. A one-chip distance is [3•10exp(8)/1.2288•10exp(6)] or 244m. So, a 64-chip separation represents a minimum multipath separation distance of 15,616m.
- A 64-chip offset separation on the Short PN code yields 512 offsets that will be used to isolate cells and sectors.
Let us assign the Short PN offsets PN(t-
A) and PN(t- B) to cell A and cell B respectively. The signal from cell A to Mobile A is of the form di(t)• PN(t- A). Likewise the signal from cell B to Mobile B is of the form dj(t)• PN(t- B).The received signal at Mobile A is the composite signal from cell A and cell B. (Both cells are on the same 1.25 MHz Frequency band.). Likewise, the received signal at Mobile B is the same composite signal from cell A and cell B. The composite signal received by each Mobile can be represented as:
A) + dj(t)• PN(t- B)Here, we have ignored the propagation delay which can be readily resolved by the synchronization process at the Mobiles.
© 2001 DR. ERNEST SIMO - SPACE2000