User's Manual

6051412900 Rev -
26
ANNEX A: COFDM Modes Characteristics
The main COFDM modulation parameters are:
• Number of sub-carriers (1705)
• 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 mainly depends on constellation scheme 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 (Raylegh) channel.
Selection may be compromised due to the distance between the transmitter and receiver sites.
We can notice that a low code rate (= high data redundancy) is necessary to insure a good efficiency in
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).
The following table summarizes the results that come out from the 4 possible guard interval values:
Guard Interval
Ratio
Guard Interval
Duration (us)
Maximum echoes
dispersion (km)
Maximum transmission
distance (km)
1/32 7 2.1 2-6
1/16 14 4.2 4-12
1/8 28 8.4 8-24
1/4 56 16.8 16-48
Maximum transmission distances can be increased when using directive antennas, but signal break-ups
can nevertheless occur when long echoes occasionally enter the receiving antenna.
Useful Bitrate (Mb/s) Constellation
Scheme
Code
Rate
GI=1/4 GI=1/8 GI=1/16 GI=1/32
C/N for perfect
channel (dB)
C/N for typical
channel (dB)
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
QPSK
7/8
8,71 9,68 10,25 10,56 7.7 16.3
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
16-QAM
7/8
17,42 19,35 20,49 21,11 13.9 22.8
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
64-QAM
7/8
26,13 29,03 30,74 31,67 20.1 27.9