User Guide

PAMS
Technical Documentation
NHD–4
System Overview
Page 3–20
Original 11/97
Quadrature Phase Shift Keying – QPSK
Forward link transmissions from the Base Station (BS) to the Mobile Subscriber
(MS) use QPSK modulation. QPSK is the sum of Two Binary Shift Keyed (BPSK)
signals. Figure 19 shows how a BPSK signal is made up.
D1 (on)
D3 and D4
(off)
Carrier
input
0 deg
D2 (on)
Binary 1
0 deg
Carrier
output
++ ++
–– ––
Binary Phase Shift Keying
Binary 0
D1 (off)
D2 (off)
Carrier
input
0 deg
D3/D4
(on)
180 deg
Carrier
output
++
––
––
++
Binary
input
BPSK
output
Time
10 1 10
Degrees
Radians
0
TT
180
TT
0
TT
180
TT
0
TT
Binary input Output phase
Logic 0 180 deg
Logic 1 0 deg
D1
T1 T2
Modulator
output
D3
D2
D4
Reference
carrier
input
Binary input
DAMPS_4
Figure 19. BPSK Modulator
A
B
C
Before starting any explanation about phase modulation a convention needs to be
established that will carry on throughout this study guide. Digital signals are
generally generated by use of a modulator that generates a sine and a cosine
channel and scales each channel by a factor that ranges from –1 to +1. What
the last sentence means is that the values of Data Channels are –1 and +1, not
0 and 1. A logic one will be “plus one” and a logic zero will be “minus one”.
In drawing ”B” of Figure 19, diodes D1 and D2 are forward biased into conduction
with a logic one. Transformer’s T1 and T2 are connected together in an in–phase
condition. In this case the output carrier’s signal would have the same phase as the
input.
In drawing “C” of Figure 19, diodes D3 and D4 are forwarded biased into conduction
with a logic zero. The output of T1 is cross connected to the input of T2 which will
result in the output being 180 degrees out of phase with the input signal.