Heli-Coder 4 Transmitter Operation Manual | Doc. No. 6051452100 Rev. C
Broadcast Microwave Services, Inc.
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16
COFDM Characteristics
The main COFDM modulation parameters are:
Number of sub-carriers (about 2000 for DVB-T)
Guard interval (GI) duration between COFDM symbols
Constellation scheme used for individual sub-carrier modulation
Data redundancy code rate used for error correction
The transmission robustness depends on constellation and code rate.
The following table gives the useful transmission bit rate for each COFDM mode. It also specifies the Carrier-to-Noise operation limit in the case of a perfect
line-of-sight (Gaussian) channel and in the case of a typical multipath terrestrial (Rayleigh) channel.
Constellation
Code
Rate
Useful Bitrate (Mb/s)
C/N for perfect
channel (dB)
C/N for typical
channel (dB)
GI=1/4
GI=1/8
GI=1/16
GI=1/32
QPSK
1/2
4,98
5,53
5,85
6,03
3.1
5.4
2/3
6,64
7,37
7,81
8,04
4.9
8.4
3/4
7,46
8,29
8,78
9,05
5.9
10.7
5/6
8,29
9,22
9,76
10,05
6.9
13.1
7/8
8,71
9,68
10,25
10,56
7.7
16.3
16-QAM
1/2
9,95
11,06
11,71
12,06
8.8
11.2
2/3
13,27
14,75
15,61
16,09
11.1
14.2
3/4
14,93
16,59
17,56
17,10
12.5
16.7
5/6
16,59
18,43
19,52
20,11
13.5
19.3
7/8
17,42
19,35
20,49
21,11
13.9
22.8
64-QAM
1/2
14,93
16,59
17,56
18,10
14.4
16.0
2/3
19,91
22,12
23,42
24,13
16.5
19.3
3/4
22,39
24,88
26,35
27,14
18.0
21.7
5/6
24,88
27,65
29,27
30,16
19.3
25.3
7/8
26,13
29,03
30,74
31,67
20.1
27.9
Notice that a low code rate (high data redundancy) is necessary to insure a good efficiency in a multipath
environment.
The guard interval determines the maximum echoes length dispersion that the system can tolerate. From this figure, we can estimate the maximum
transmission range that the system might offer for a typical terrestrial channel (with adequate RF power).
