Radio coverage and link budget

A weak point at the tunnel portal

Talbahn suspects insufficient usable received power at the tunnel portal. Before changing sites, it compares gains and losses along an assumed radio path.

A link budget is this power balance between transmitter and receiver. It shows the received power expected under the assumed conditions. It models neither all temporal radio variations nor automatically interference from other transmitters.

dBm is a power level referenced to one milliwatt. A negative dBm value describes power below one milliwatt. dB, by contrast, describes a ratio, such as a loss or gain. It indicates how power changes relative to a reference power. A level in dBm can therefore be combined with gains and losses in dB. dBi describes antenna gain relative to an idealised reference radiating equally in all directions. In an appropriate link budget, that gain enters as a dB contribution.

A fully specified teaching calculation

We consider only the stated direction. All numbers are fictional planning values.

Expected received power is: 30 + 8 − 2 − 130 + 3 − 2 = −93 dBm.

For this teaching example, a required received level of −100 dBm is assumed. The margin is the difference between the calculated received level and that required level. Here, −93 − (−100) = 7 dB.

An additional loss of 4 dB reduces received power to −97 dBm. The margin falls to 3 dB. Both results apply only if all other assumptions remain the same.

Why a positive margin is insufficient

The required level depends on factors including transmission method, data rate, bandwidth and interference conditions. It is not a universal FRMCS limit. A strong unwanted transmitter may impair the connection even if the desired received power exceeds the assumed level.

Uplink means the direction from vehicle to network. The vehicle transmits in this direction. Downlink means the reverse. Here the vehicle receives from the network. Different transmit powers and installation conditions can create different margins. A good downlink calculation therefore does not replace an uplink assessment.

The frequency band also matters. The harmonised n100 and n101 ranges from 2.3 provide a framework; they do not alone determine site spacing or coverage. Tunnel geometry, vehicle structure, antennas and real interference must be considered. [S08, S12]

Coexistence in radio planning

During migration, Talbahn still needs GSM-R coverage. New FRMCS transmitters, frequency use and antennas must therefore be coordinated with the existing system. A band plan alone does not demonstrate interference-free coexistence. Check mutual interference, available resources and coverage of both systems in each migration state, among other things. A good FRMCS margin must not come at the cost of an undetected degradation of existing GSM-R.

From calculation to case file

Talbahn documents the origin of every value, the direction considered and the required margin. It then compares assumptions with measurements at critical locations. Only afterwards does it connect the radio findings to service measurements such as the group call.

Check your understanding

An additional installation loss is 6 dB rather than the 4 dB examined. Calculate received power and margin. Can a successful group call be inferred?

Model answer

−93 − 6 = −99 dBm; a 1 dB margin remains against −100 dBm. This is only the result of the power balance. Interference, temporal changes, the reverse direction and the complete service chain remain to be checked. A successful group call is not demonstrated.

What to remember

A link budget explains an assumption about the radio path; measurements test its validity.

Your Talbahn case file

Record the link budget, direction and origin of assumptions. Add the tunnel-portal measurement and checks needed for GSM-R coexistence.