TETRA Antenna Combiner
Radio Systems Information Ltd.
Page 6
2
Combiner Design
2.1
Overview
When operating TETRA radio terminals in close proximity there are several mechanisms by
which one terminal can affect the performance of another terminal. Although the TETRA system
utilises Time Division Multiple Access (TDMA) there is a possibility that one of the radio units
may transmit at the same time as another co-located unit is attempting to receive. The 4 main
mechanisms that can cause problems are:
2.1.1 Blocking
This occurs when a transmitter is operating close to a receiver although they may be several
MHz apart in frequency, the transmitter saturates the receiver front-end and causes de-
sensitisation.
The ETSI required specification is for a signal more than 500kHz from the received signal to be
no greater than –25 dBm when measured at the receiver port. (en 300 392-2 para. 6.5.1.2)
The output from the transmitter is 3 W i.e +34 dBm, therefore at least 59 dB isolation is required
between the ports at the transmit frequency is required to avoid desensitisation
The combiner typically achieves a figure of around 64 dB or better attenuation.
2.1.2 Wideband noise
TETRA transmitter will normally have wideband noise extending for several MHz into the receive
band. Therefore a co-located receiver will receive in-band noise.
The ETSI specified level greater than 5 MHz away from the carrier is less than 100 dBc which
equates to –66 dBm for a 3W transmitter. (en 300 392-2 para. 6.4.2.3)
The maximum usable sensitivity of the receiver is about –110 dBm for 10
-2
BER which with 18dB
S/N ratio for TETRA requires a maximum interfering signal of –128 dBm. Therefore at least
62dB isolation is required between the ports at the receive frequency.
The combiner easily achieves 70dB attenuation of wideband noise with figures often being much
greater.
2.1.3 Intermodulation products
Two or more TETRA transmitters can combine and produce intermodulation products that can
be in the transmit band and cause interference to a base station, or in the receive band and
cause interference to a third receiving TETRA terminal.
There should be at least 60dB isolation between one terminal and another in the transmit band
to ensure any intermodulation products are negligible.
Similarly, there should be at least 60dB isolation from the antenna port back to the terminals to
prevent signals from other nearby radio terminals creating intermodulation products.
The combiner typically manages 80dB isolation for the Model A and 60dB for the Model B.
Overall, this ensures that any intermodulation products are at least -60dBc.