PN Offset Assignment

The assignment of the PN offset for each CDMA2000 channel and/or sector
utilizes the same rules that were and are used for IS-95 systems. In
CDMA2000, just as with IS-95 systems, the forward pilot channel carries no
data but it is used by the subscriber unit to acquire the system and assist in
the process of soft handoffs, synchronization, and channel estimation. A separate
forward pilot channel is transmitted for each sector of the cell site.The
forward pilot channel is uniquely identified by its PN offset, or rather, PN
short code that is used. The reverse pilot channel introduced in CDMA2000,
however, does not utilize the Pseudorandom Number (PN) offset.
The PN sequence has some 32,768 chips that, when divided by 64,
results in a total of 512 possible PN codes that are available for potential
use. The fact that there are 512 potential PN short codes to pick from
almost ensures that there will be no problems associated with the assignment
of these PN codes. However, there are some simple rules that must be
followed in order to ensure that there are no problems encountered with the
selection of the PN codes for the cell and its surrounding cell sites. It is suggested
that a reuse pattern be established for allocating the PN codes. The
rational behind establishment of a reuse pattern lies in the fact that it will
facilitate the operation of the network for maintenance and growth.
Table 13-23 shows what can be used for establishing the PN codes for
any cell site in the network. The method that should be used is to determine
whether you wish to have a 4, 7, 9, 19, and so on, reuse pattern for the PN
codes.

The suggested PN reuse pattern is a N
19 pattern for a new CDMA2000
system. If you are overlaying the CDMA system on to a cellular system, a N
14 pattern should be used when the analog system utilizes a N
7 voice
channel reuse pattern, or if a PN code scheme has been established for the
sector or site, then the same PN code should be used for that sector/cell.
Figure 13-2 is an example of a N
19 PN Code reuse pattern. Please
note that not all the codes have been utilized in the N
19 pattern. The
remaining codes should be left in reserve for use when there is a PN Code
problem that arises. In addition, a suggest PN_INC value of 6 is also recommended
for use.

The PN short code used by the pilot is an increment of 64 from the other
PN codes an offset value is defined. The Pilot_INC is the value that is used
to determine the amount of chips, or rather phase shift, one pilot has versed
another pilot. The method that is used for calculating the PN offset is
shown in Figure 3-1 of Chapter 3 and applies to CDMA2000 as well as
IS-95 systems.

Pilot_INC is valid from the range of 0 to 15. Pilot_INC is the PN
sequence offset index and is a multiple of 64 chips. The subscriber unit uses
the Pilot_INC to determine which are the valid pilots to be scanned. The method for calculating the Pilot _INC is the same as that used for IS-95 systems
and is a function of the distance between reusing sites.