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Wire and Wireless Communication Applications Chap. 8
as a function of the communication link parameters, such as transmitted EIRP, space loss,
receiver antenna gain, and the effective input-noise temperature of the receiving system. The
formula that relates these system link parameters to the CNR at the detector input is called the
link budget.
The communication system may be described by the block diagram shown in Fig. 8–24.
In this model, the receiving system from the receiving-antenna output to the detector input is
modeled by one linear block that represents the cascaded stages in the receiving system, such as
a transmission line, a low-noise amplifier (LNA), a down converter, and an IF amplifier. This
linear block describes the overall available power gain and effective input-noise temperature of
these cascaded devices, as described in the preceding section and modeled in Fig. 8–20.
As shown in Fig. 8–24, the CNR at the ideal amplifier input (with gain G a ) is identical
to that at the detector input, because the ideal amplifier adds no excess noise and it amplifies
the signal and noise equally well in a bandwidth of B hertz (the IF bandwidth). Thus, denoting
this simply by CNR, we get
a C N b ! a C N b = a C
Rx N b Det
(8–38)
where these carrier-to-noise power ratios are indicated on the figure.
Cosmic noise
n 1 (t)
m (t)
Transmitter
P Tx
Transmit
antenna
G AT
P EIRP
Free space gain
G FS
+
+
s(t)+n 1 (t)
Receive
antenna
G AR
P as =kT AR B
r(t)=s(t)+n(t)
G a
C
C
–– ––
( N Rx
( N
n 2 (t)
Noise-free
linear stage
Excess receiver
noise
P x =kT e B
(
Det
Detector
∼ m (t)
Model for receiver system
consisting of the cascaded
stages: LNA, transmission line,
down converter, and IF amplifier
Figure 8–24
Receiver
Communication system model for link budget evaluation.