User guide

SATELLINE-3AS NMS / NMS Epic / NMS 869 / VHF
User Guide, Version 1.0
39
6.2
Receiver
The sensitivity of the receiver depends on the channel spacing of the radio modem (=data
speed of the radio interface) and on the mode of the FEC (Forward Error Correction) according
to the table below:
SATELLINE-3AS(d) NMS Epic radio modem contains two separate receivers, which are used
simultaneously. The selection between received signals is made in order that the stronger/better
signal is used. By using this method, the signal fading caused by multipath propagation will be
smaller than in case, where just one antenna and receiver is used. The recommended minimum
distance between two receiving antennas is  wavelengths, app. 50 cm at 450 MHz frequency
range.
6.2.1
RSSI and RSSI threshold level
The radio modem measures constantly the received signal strength of the receiver. The RSSI
Threshold Level of the receiver is the important parameter determining the level, above which
the search for the messages from the radio receiver transfer signal is active. It is recommended
that values given in the table 6.2 are used as a basis. If the Signal Threshold Level setting is set
too low (the CD LED is ON constantly), it is possible that the receiver is trying to synchronise
itself with noise. In such case the actual data transmission might remain unnoticed. If the RSSI
threshold is set too high, the weak data transmissions will be rejected although they could be
otherwise acceptable.
The RSSI threshold should only be changed for a reason. For example in the following cases:
• Continuous interference is present and the desired signal is strong. In this case the RSSI
threshold can be increased to prevent the modem from synchronising to the interfering
signal(s) and /or possible noise.
• Maximum sensitivity should be achieved and the desired signal is very weak. In this case the
sensitivity could increase by decreasing the RSSI threshold. This type of situation is usually a
sign of a poorly constructed radio network / contact. Bit errors and momentary loss of
signals can be expected in this kind of a situation. Some data might be successfully
transferred.
The radio modems equipped with an LCD display show the RSSI of the last received message in
dBm units. The RSSI can be requested also locally by using a special SL command (SL@R?). The
RSSI value is available 7s after the receiving the message. After that the value is returned to
zero. SATELLINE-3AS NMS Epic responds by the stronger RSSI value of the two receivers.
SATELLINE-3AS NMS / NMS Epic / NMS 869 / VHF radio modems do not include an
analogue RSSI signal - it is a special hardware assembly option that must be specified
in the order.
Table 6.2. Effect of settings on the receiver sensitivity
Channel spacing Sensitivity FEC OFF Sensitivity FEC ON
25 kHz -110 dBm -113 dBm
12,5 kHz -112 dBm -115 dBm
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