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SATEL OY
// SATELLAR MANUAL // RADIO UNIT // USER GUIDE // V. 1.8
6. Data transmission
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6.8 Automatic QAM modulation with packet routing
Link specific modulation can also be set to “Auto” mode which let the modem decide the selected
modulation level for each link based on radio path characteristics. These radio path characteris-
tics are deduced from the Signal To Noise ratio (SNR) that the device is detecting. Automatic QAM
modulation is dynamically selecting the best modulation level for each individual data transmis-
sion based on information it receives from the modem that is on the opposite side of the radio link.
In order for automatic modulation to function properly the radio traffic needs to have continuous
nature and also bidirectional as the information about the radio link quality is transferred via normal
radio traffic. If the radio traffic is otherwise unidirectional Handshaking should be set to ON.
As with link specific modulation in general, the network will have a basic modulation level set by
the “modulation” selection. It should have a value that is safe for all possible cases for the whole
network as the automatic modulation will use it as lowest possible modulation level.
The automatic protocol to set the modulation is as follows for Modem A <--> Modem B case (start of
the transimission):
1. Modem A transmit to modem B using basic modulation
2. Modem B answers back to modem A using basic modulation
3. Modem A transmit to modem B with highest modulation level suitable
4. Modem B answers back to modem A with highest modulation level suitable
Thereafter every transmission is adjusted based on previous response between modems A and
B and highest suitable modulation level is used. If there are changes in radio path characteristics
every data package may have different modulation level.
In typical cases automatic QAM modulation selection works without further adjustment, but if
needed it is possible to set “Auto QAM SNR Level Adjust” value from -5 to +20 dB and thus control
how the modulation level is set. By setting a positive value it possible to move the selection level for
more pessimistic value (while making the connection slower) so that each +3..+4 dB change equals
approximately one modulation level (64-QAM 32-QAM or 16-QAM 8-QAM etc.). By selecting nega-
tive value automatic QAM modulation is set higher than in normally would and thus the connection
will be that much faster. It is worth noting that negative value may cause connection to be totally
lost if the radio path can’t work properly with this higher modulation level. This setting can also be
used in office-table type testing if the SNR value stays high even when the radio signal strength goes
very low e.g. by using attenuators in noise free environment.
Automatic QAM modulation can’t be used together with manual link specific modulation i.e. auto-
matic mode should be selected for the whole network.