User Guide

Technical Documentation
NHP–4
System Overview
PAMS
Page 3–9
Issue 1 04/99
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 4 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 4. 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” 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”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.