mohatta2015.pdf

8 INTRODUCTION
Figure 1.7
QPSK.
transmitter
s(m)
titter
ΐ
m
► Ι ^
V T
^
x(r)
channel
*0
:
T L-1
1
A
-Θ-
+
»l^l
» ( X )
^
-
■
—► +
Î
iM
40
ft,1
Figure 1.8
System block diagram showing notation.
We will use a complex, baseband equivalent of the system. A radio signal can
be written as the sum of cosine component and a sine component, i.e.,
x(t) = u r (t)y/2cos{2nf c t) -Ui(t)\/2sm(2nf c t), (1.15)
where f c is the carrier frequency in Hertz (cycles per second). The two components
are orthogonal (occupy different signal dimensions) under normal assumptions. The
\pl is included so that the power is the average of uf.(t) + uf(t). We can rewrite
(1.15) as
Re{u(t)V2exp(j2nf c t)}, (1.16)