Cable TV Measurements (Option K20) R&S FSL
1300.2519.12 2.66 E-11
Fig. 2-45 Pulse signals I
dirac
(t) and Q
dirac
(t) for 16QAM
I
Q
Fig. 2-46 16QAM constellation diagram
Unfortunately the required bandwidth for transmitting dirac pulses is infinite. Let's reduce the bandwidth
by applying a so–called pulse shaping filter (as referred to as TX filters). In most QAM systems root
raised cosine filters are used. Root raised cosine filters are exclusive supported by the Cable TV
Measurements option. Via the filter's roll–off factor the occupied bandwidth can be controlled.
OccupiedBandwidth = SymbolRate (1 + RollOff)
In a cable TV receiver, a filter of the same shape is used as RX filter. The combination of two root
raised cosine filters, one in the transmitter (TX) and another one in the receiver (RX), has a very strong
property: There will be no inter–symbol interference at the RX filter's output. Due to this property it is
very easy for the receiver to retrieve the transmitted symbols. Please note, that in a real–world scenario
the channel (causing echoes / multipath propagation) causes inter–symbol interference. In that case the
use of an equalizer is recommendable. Fig. 2-47 shows the signals I
RRC
(t) and Q
RRC
(t) that result from
filtering I
dirac
(t) and Q
dirac
(t) with the root raised cosine filter RRC.
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