User Guide

Technical Documentation
NHD–4
System Overview
PAMS
Page 3–31
Original 11/97
Orthogonal Functions
Two values are orthogonal if the result of exclusive–ORing them results in an equal
number of 1’s and 0’s.
Figure 30 uses the number 2 code, 0 1 0 1, in the Walsh code set 4 to “Orthogonally
Spread” some user input data. Each bit of user input data is exclusive–OR’ed with
the number 2 Walsh code that will result in TX Data shown in Figure 30.
Orthogonal Functions:
Two values are orthogonal if the result of
exclusive–or–ing them results in an equal
number of 1’s and 0’s
Orthogonal Spreading:
Note; Each Orthogonal Sequence in the
forward link will have 64 bits rather than
the 4 bits in this example.
User Input 1 0 0 1 1
–1
+1
???? ?
+1
–1
CDMA12.DRW
EXAMPLE: 1 1 1 1
0 1 0 1
1 0 1 0
Orthogonal
Sequence
0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
TX Data
Decoding using a Correct Orthogonal Function
RX Data 1 0 1 0 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0
Correct
Function
0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
1 1 1 1 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1
10 011
+1
–1
Decoding with Incorrect Orthogonal Function
RX Data 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1
Incorrect
Function
1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0 1 0
1 0 1 0 0 1 0 1 0 1 0 1 1 0 1 0 1 0 1 0
Figure 30. Orthogonal Functions.
When the number 2 Walsh code is exclusive OR’ed with what is now the “RX data”
each number 2 Walsh code yields the original user input data 4 times. The IS–95
CDMA standard uses a 64–bit Walsh code so the mobile cell phone has the
transmitted data repeated 64 times. When the data is repeated 64 times, your have
processing gain. Repeating the processing gain information: 10 log 64 equals 18
dB: another 3 dB is added because the data is modulated on two channels, I and Q
for a total of 21 dB. This is one of the reasons why IS–95 CDMA is so tolerant of
noise. That is to say a signal–to–noise ratio that would render an analog signal
useless works fine with CDMA.
BUT JUST HOW DOES AN ORTHOGONAL WALSH CODE SEPARATE
DIFFERENT USERS?
At first it would seem that broadcasting 25 to 30 code streams on one frequency
would create an “electronic tower of babel”. To explain how Walsh encoding works
a Walsh code set 2 that has 2 orthogonal Walsh codes will be used in Figure 31.