R&S FSL Cable TV Measurements (Option K20)
1300.2519.12 2.65 E-11
The lower pane shows the final CTB ratio and whether it is passed or failed. It also shows the most
i
mportant correction factors used to calculate the CTB ratio.
Digital TV Basics
Cable TV networks use single carrier QAM signals. These signals are continuously modulated. The
Cable TV Measurements option does not support burst signals as used in cable modems (e.g.
DOCSIS) which rely on TDMA techniques that share the same channel with several subscribers. To get
a better understanding, we now want to have a closer look at an ideal QAM transmitter.
Binary
Source
Symbol
Mapping
bits
I
dirac
(t)
Q
dirac
(t)
RRC
Filter
RRC
Filter
I
RRC
(t)
Q
RRC
(t)
cos(2
f
C
F
t)
/
2
IQ
R
F
(t)
Fig. 2-44 Ideal QAM transmitter
To keep things simple we start with a binary source providing a never ending bit stream. Please keep in
mind that in reality these bits originate from a video stream which will be source encoded e.g. by a
MPEG encoder. To allow errors during the transmission via the cable channel coding (e.g. convolutional
coding) will be applied. Finally we get something like "…010010111101010101110110101111010…''
The symbol mapping block transforms the digital information (bits) into the continuous signals I
dirac
(t)
and Q
dirac
(t). I
dirac
(t) and Q
dirac
(t) (see Fig. 2-45) consist of dirac pulses that appear at times t=n*T
symbol
and that can be distinguished by their in–phase "I'' and quadrature "Q'' levels. For example a 16QAM
constellation has 16 different I and Q combinations and 4 different I and Q levels (4*4=16). Typically
this is visualized in a constellation diagram (see Fig. 2-46). With 16=2
4
we are able to transmit 4 bits per
symbol. Therefore we can calculate:
bit_rate = symbol_rate * 4 = 4/ T
symbol
[bits/second]
or more general:
bit_rate = symbol_rate * log2(M) [bits/second] for MQAM.
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