COMMUNITY ANTENNA TELEVISION (CABLE TELEVISION)
A model showing the concept of hybrid fiber-coaxial cable CATV system. TX, fiber-optic
transmitter; RC, fiber-optic receiver.
HFC system layout for optimal performance (one-way).
on a coaxial cable and uses that as the modulating signal. The second method also uses
analog amplitude modulation, but the modulating signal is a grouping of subcarriers that
are each frequency modulated. One off-the-shelf system multiplexes in a broad FDM
configuration, eight television channels, each on a separate subcarrier. Thus a 48-channel
CATV system would require six fibers, each with eight subcarriers (plus 8 or 16 audio
Link Budget for an AM System
. We will assume a model using a distributed
feedback laser (DFB) with an output of
+5 dBm coupled to a pigtail. The receiver is a
PINFET with a threshold of
-5 dBm. This threshold will derive approximately 52-dB
S/N in a video channel. The C/N required is about 49.3 dB [see formulas (17.4) and
(17.5)]. This is a very large C/N value and leaves only 10 dB to be allocated to fiber loss,
splices, and link margin. If we assign 2 dB for the link margin, only 8 dB is left for the
fiber/splices loss. At 1550-nm operation, assuming a conservative 0.4-dB/km fiber/splice
loss, the maximum distance from the headend to the coax hub or first fiber-optic repeater
is only 8/0.4 or 20 km. Of course, if we employ an EDFA (erbium-doped fiber amplifier)
with, say, 20 dB gain, the distance can be extended by 20/0.4 or 50 km.
442 COMMUNITY ANTENNA TELEVISION (CABLE TELEVISION) Figure 17.7 A model showing the concept of hybrid fiber-coaxial cable CATV system. If we assign 2 dB for the link margin, only 8 dB is left for the fiber/splices loss.