QPSK Demodulator Application Notes
01-0868-401B 12/02
61
Output Spectrum Width
To prevent intersymbol interference, the outgoing symbols are filtered by a DVB-
compliant transmit filter. The filter type is a square-root raised cosine filter with
roll-off factor alpha = 0.35. The result is that the output spectrum has a width of
(1.35 * Symbol Rate).
The following table provides examples of information bit rates versus transponder
bandwidth at each code rate.
Estimating E
b
/N
0
The TRD60 provides an estimate of the E
b
/N
0
. To access this estimate, select the
Input>Status
menus.
E
b
/N
0
Definition
The E
b
/N
0
expression is defined as follows:
■
E
b
= Energy per information bit.
■
N
0
= Noise energy per Hz.
The formula for E
b
/N
0
, in dB is:
E
b
/N
0
= C(dBm) - N
0
(dBm/Hz) - 10log(IR)
where:
■
C = Carrier Power
■
IR = Information Rate
E
b
/N
0
Estimation Algorithm
Within the FEC decoder IC, the Viterbi decoder output is re-encoded and
compared to a delayed version of the original encoder input. Incoming bit errors
that are corrected will result in a difference between the original input and the
encoded output. These differences are accumulated for a fixed amount of time.
This accumulation register can be read by the TDR60 microprocessor and gives
an approximation of the signal to noise level of the input signal. Once these
reading have been calibrated using an input with a known E
b
/N
0
, they are used by
the TDR60 to accurately estimate the E
b
/N
0
of the incoming signal.
Note that the estimated E
b
/N
0
reading is accurate only for relatively low E
b
/N
0
readings (less than 10). This is because there are not enough bit errors at higher
E
b
/N
0
to generate an accurate estimation.
Transponder
BW
(MHz)
QPSK
1/2
Mbps
QPSK
2/3
Mbps
QPSK
3/4
Mbps
QPSK
5/6
Mbps
QPSK
7/8
Mbps
54
36.8627
49.1503
55.2941
61.4379
64.5098
46
31.4016
41.8668
47.1024
52.3360
54.9528
41
27.9884
37.3178
41.9826
46.6473
48.9797
36
24.5752
32.7669
36.8627
40.9586
43.0065
33
22.5272
30.0363
33.7908
37.5454
39.4227
30
20.4793
27.3057
30.7190
34.1322
35.8388
27
18.4314
24.5752
27.6471
30.7190
32.2549
18
12.2876
16.3834
18.4314
20.4793
21.5033
