Specifying a TETRA bi-directional amplifier for a tunnel or car park

Underground car parks, road and rail tunnels and the cores of large buildings are where professional radio drops out first. For TETRA, the usual answer is a bi-directional amplifier (BDA): a repeater that picks up the network outside and radiates it inside, in both directions. This note lists what an integrator has to decide before ordering one.

Repeater or indoor base station?

A BDA needs a usable signal from an outdoor base station at a pick-up point: a rooftop or the tunnel portal, where a directional donor antenna can see the network. Where no such signal exists, or where the traffic inside is heavy, the network operator may provide an indoor base station instead; the distribution system behind it can be the same. Which of the two applies is the operator's decision, so it is settled first.

Band and channels

TETRA for public safety uses the 380–400 MHz range, typically as an uplink and a downlink block, for example 380–385 / 390–395 MHz. Write down the exact blocks or channels the system must carry. A band-selective BDA with adjustable bandwidth passes only that block, so it does not amplify neighbouring services or the noise around them. Other bands, such as DMR or P25 on VHF and UHF, or 700 MHz broadband, are specified the same way.

Gain, isolation and the uplink

A repeater oscillates when the signal it radiates inside leaks back into its own donor antenna with more strength than the gain takes away. The isolation between the donor and the service antennas therefore has to be comfortably above the gain: a margin of about 15 dB is the usual rule of thumb. Isolation is measured on site before the gain is set, not assumed from drawings.

The uplink deserves the same care as the downlink. Every BDA adds noise at the input of the base station it talks to. Uplink gain is set no higher than the link budget needs, so the indoor system is heard without raising the noise floor for the rest of the cell. The operator will want to see these settings.

Tunnels: portal, fibre and radiating cable

In a tunnel, the signals are usually combined at the portal: TETRA and, where required, the cellular operators. From there, fibre carries them to remote units placed along the tunnel, each feeding a section of radiating cable. Fibre does not lose signal over the distance the way coaxial cable does, so the length of the tunnel sets the number of remote units rather than the power at the portal.

Power, alarms and monitoring

Ask early how the system is powered and whether backup power is required, and how alarms must be reported: dry contacts, SNMP or a management server. Per-band level and VSWR monitoring shows a failing antenna or a cut cable before users notice. Decide who watches the alarms, the integrator or the operator.

What to send for a proposal

• The network and band, for example TETRA 380–385 / 390–395 MHz (uplink / downlink)
• The channels or the bandwidth to carry
• How the signal is fed: a pick-up antenna towards an outdoor base station, or an indoor base station
• The building or tunnel: floors, area or length, and floor plans
• The available power supply, any backup-power requirement, and how alarms must be reported

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