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SATEL OY
// SATELLAR MANUAL // RADIO UNIT // USER GUIDE // V. 1.8
5. Interfaces
3
5.2 Radio
The RU has a TNC female RF connector with impedance of 50 ohms. The frequency range of the RU is
coded in the type designation, which can be seen on the label back of the RU. The details of this are
explained in chapter 8.
The RF frequency can be set in 6.25 kHz steps. The RU supports three different channel spacing settings
that can be selected by software. For FSK-radio these are 12.5, 25 and 150 kHz, and for QAM-radio 6.25,
12.5 and 25 kHz. Also three different modulation methods are supported. For FSK-radio these are 4-, 8-
and 16-FSK and for QAM-radio these are 2-, 4-, 8-, 16-, 32- and 64-QAM.
The output power can be adjusted with steps 0.1, 0.2, 0.5, 1, 2 and 5 W. With for FSK radio these are con-
stant envelope values but with QAM-radio these power levels are peak values due to crest factor (the ratio
of peak values to the effective value) in QAM modulation. Crest factor varies between QAM modulation
levels and the difference between peak power value and average power value can be in range of 6-9 dBm.
E.g. for 5 W (37dBm) peak power value, the average power level is ~1W (30dBm). It should be noted that
average values should be used when defining radio link budgets of a network.
Channel spacing together with the modulation method determines the air speed as clarified in the techni-
cal specification in chapter 2. Air speed can be set independently of the data rate of the serial port.
The modulation method also affects the receiver sensitivity. The best sensitivity can be obtained by the
lowest level modulation, i.e. 4-FSK/2-QAM in SATELLAR XT 5R case. For typical sensitivities in different
conditions see the technical specification in chapter 2.
Another method to improve the sensitivity of the receiver is to use Forward Error Correction (FEC), this can
be used for SATELLAR XT 5R with 4- and 8-FSK modulations. This improvement effects the user data rate:
the air speed remains the same but the fraction of bits available for the user is as indicated by the code
rate of the FEC. The RU offers two different code rates, 0.67 and 0.5. For example, if 4-FSK is used with 25
kHz and the FEC is switched on with the code rate of 0.5 the user bit rate goes down to 9600 bits/s. The
effect of the FEC on the sensitivity depends on the code rate and the level of BER (Bit Error Rate) at which
the radio link is operating.
Changing of the modulation method or using FEC helps to improve the receiver sensitivity in noisy con-
nections, i.e. the bit errors are mostly evenly distributed over the entire transmission period. If the errors
happen in bursts these methods are not very efficient. For this reason the FEC is used together with the
interleaving method. This means that before transmitting the data from the DTE, the RU collects a certain
amount of data to the buffer and rearranges it according to a certain rule. The receiver knows the rule and
recovers the original order of data bits. The receiver then sees the errors scattered and the FEC can cor-
rect the errors. It should, however, be noted that FEC and interleaving increase the latency and should be
avoided in transparent mode in cases where a low latency is a primary requirement.
