Digital Signal Processing Reference
In-Depth Information
Thus, the demands for a minimum C/N fluctuate within a wide range
from about 3 dB for QPSK with a code rate of 1/2 in a Gaussian channel
up to about 28 dB for 64QAM with a code rate of 7/8 in a Rayleigh chan-
nel. Practical values are about 18 to 20 dB (64QAM, code rate 2/3 or 3/4)
for stationary reception and about 11 to 17 dB (16QAM, code rate 2/3 or
3/4) for mobile reception.
Table 20.15.
Theoretical minimum C/N with hierarchical modulation (QPSK,
64QAM, Α = 2); low priority path (LP)
Modulation
Code rate
Gaussian channel
Rice channel
Rayleigh channel
[dB]
[dB]
[dB]
QPSK
1/2
6.5
7.1
8.7
2/3
9.0
9.9
11.7
3/4
10.8
11.5
14.5
64-QAM
1/2
16.3
16.7
18.2
2/3
18.9
19.5
21.7
3/4
21.0
21.6
24.5
5/6
21.9
22.7
27.3
7/8
22.9
23.8
29.6
20.8 DVBT SingleFrequency Networks (SFN)
COFDM is well suited to single frequency operation. As the name indi-
cates, in single frequency operation, all transmitter operate at the same fre-
quency which makes for great economy with regard to frequency re-
sources. All transmitters radiate the identical signal and have to operate in
complete synchronism with each other. Signals from adjacent signals are
seen by a transmitter as if they were simply echoes. Frequency synchroni-
zation is the easiest condition because frequency accuracy and stability had
to meet high demands even in analog television. In DVB-T, the transmitter
RF is locked to the best reference available: the signal from the GPS
(Global Positioning System) which is available throughout the world and
is now also used for synchronizing the transmitting frequencies of a
DVB-T single-frequency network. The GPS satellites radiate a 1 pps
(pulse per second) signal to which a 10 MHz oscillator in professional
GPS receivers is locked which, in turn, acts as reference signal for the
DVB-T transmitters.
However, there is also a strict requirement with respect to the maximum
distance between transmitters (Fig. 20.23. and Tables 20.16, 20.17. and
20.18.). This distance is related to the length of the guard interval and the
velocity of light, i.e. the associated signal delay. Intersymbol interference
can only be avoided if in the case of multipath reception, the delay on any
path is no longer than the length of the guard interval. The question about
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