User Guide

Technical Documentation
NHD–4
System Overview
PAMS
Page 3–27
Original 11/97
The forward link Short Code is the same for all base stations. However a specific
mask is AND’ed with the output of the code generator to create a unique short code.
Even though the specific mask does not change the PN pattern the code is
considered unique relative to system time. This means that each specific mask will
shift the PN code to a unique delay with respect to system time and in this way the
shifted PN code is considered unique.
Here is another way of saying the same thing: PN codes used are required to have
low auto–correlation properties–––a time shifted version of itself correlated with itself
looks like random noise. Therefore a time shifted version is unique. Short Code
and Long Codes are handled the same: they use time shifted versions to be unique.
An example of a mask is shown in Figure 26. The three–bit shift register in figure
25 has a three–bit mask circuit connected to it in Figure 26.
00 1
10 0
0
1
0
1
0
1
11
0
11 1
0
11
11
0
0
00
0
11
0
1
00
1
11
0
0
1
0
1
11
0
1
0
0
1
11
1
0
1
0
0
11
0
11
0
0
11
0
0
1
0
1
CDMA10.DRW
Figure 26. PN Code generator w/mask ckt.