User Guide

Technical Documentation
NHD–4
System Overview
PAMS
Page 3–33
Original 11/97
Here is how the “bipolar” addition works:
Voice data 1 0 1 1
bipolar Walsh code –1–1 +1+1 –1–1 –1–1
Walsh encoded data 0 0 1 1 0 0 0 0
The voice data is added to both bipolar Walsh code numbers. The example is for
User A.
If the two Walsh encoded voice data channels are added together the result is a
data stream that varies between +2 and –2. Walsh code decoding will show that
both user data streams are contained in this waveform and further more they do not
interfere with each other.
111
0
+1
0
Original User A Voice Data
+1
0
0110
Original User B Voice Data
+1
–1
+2
–2
User A + B Walsh DataUser A + B Walsh Data
CDMA14.DRW
+1
–1
+2
–2
X =
+2
+1
–1
–2
+1
–1
00
=
+2
+1
–1
–2
1
Multiply summed data with desired Walsh
code then find the area under the
resultant curve.
–2
–1
+1
+2
–1
+1
+2
+1
–1
–2
X==
–1
Multiply summed data with desired Walsh
code then find the area under the
resultant curve.
Figure 32. Walsh Decoding Example
To see how user data is recovered from the summed signal lets extract the first bit of
each users’ data. First remember that each user bit is XOR’ed with two Walsh code
bits in this example. Taking the first two summed data bits, multiply them with
desired Walsh code. For User A this results in a wave form that starts at zero for
the first bit period and goes to +2 in the second bit period, 0 X –1 = 0 and –2 X –1 =
+2. The next step requires a little calculus, very little to figure “the area under the
curve”. Since the waveform is a square wave its not to hard. Add the two resultant
bits and divide by the number of bit periods, (0 + 2) / 2 = 1. User A’s first data bit is
“1”.