The design of a PN offset plan for CDMA is comparable to that of a signalling channel frequency plan in analog. The consequences of poor offset planning include the following:
Active Set Pilot Interference –
This phenomenon would occur in the active area and involve the active search window (SRCH_WIN_A). The interfering signal would need to be strong enough to be processed as an active finger (except in the less likely case where the timing was perfectly coincident with a true active finger).
Neighbor Set Pilot Falsing –
A neighbor set pilot may falsely appear strong enough for the mobile station (MS) to promote the pilot to the candidate set and recommend to the base station (BS) to perform a soft handoff ‘add’ via the Pilot Strength Measurement Message (PSMM). This falsing would occur in the neighbor area and involve the neighbor search window (SRCH_WIN_N). The falsing signal strength would need to meet the T_ADD threshold criteria.
The probability for interference or falsing is dependent upon two factors: timing and strength. Time differentials can be translated into geographic regions and have as their threshold the search window size. A detailed discussion of this topic will be found later
within this chapter. If a signal falls outside of a search window, its energy becomes nothing more than uncorrelated interference.
Note that the term active area is meant to refer to the area in which a signal may be (or is intended to be) actively demodulated. The term neighbor area refers to the area in which a signal will be sought as a candidate. In geographic terms, the neighbor area greatly expands the region where problems may occur since we search for a neighbor signal in many areas outside of the active area.
The use of large or generous neighbor lists along with the technique of merging neighbor lists when in soft/softer handoff creates further expansion. Mitigating this expansion of the geographic space in which falsing may occur is the heightened signal strength threshold at which interference may occur (a T_ADD of -14dB versus a finger-locking threshold of approximately -24dB).
Incorrect BS Identification –
A signal may travel far enough to be incorrectly identified by the BS when it translates the MS reported phase into a PILOT_PN offset index.