Research of a frequency band¶

When designing an RF link, one of the first tasks to realise is to define the frequency band that is adapted to the application foreseen. In fact, the frequency spectrum is shared among users in order to regulate RF emissions, and avoid interferences among various equipments.

There are international and national regulation agencies that allocate frequency band to users, with a limited bandwidth and a maximum emission power authorized for the equipment. In France, the ANFR (Agence Nationale des Fréquences) is the agency in charge of the frequency allocation and regulation, and the TNRBF (Tableau National de Répartition des Bandes de Fréquences), describes how the frequency spectrum is shared among services and users.

There are two approaches when researching a suitable frequency band for an application:

• Approach n°1 is the fastest, and consists in searching among “free bands” one that could fit our application, in term of bandwidth and emission power available.
• Approach n°2 requires more time, and consists in searching in the TNRBF an available bandwidth and sending a “request for allocation” to the ANFR.

1. Issue of free frequency band

For the design of radio systems with no license dedicated to data transmission, special authorized frequency bands exists and lead to simple et efficient solution. These bands are précised in the ANNEX 7 of the TNRBF, which specifies the free bands dedicated to low range and power equipment. In particular, the section for non-specific equipment is particularly adapted for TM/TC activities, data and video transmission (c.f. ANNEX I ).

Considering the high range required for the link (minimum 50km), there are two suitable bands with different characteristics:

• 868,7-869,2 MHz offers a 500 kHz bandwidth and ERP=25 mW leading to EIRP=41 mW
• 869,4-869,65 MHz offers a 250 kHz bandwidth and ERP=500 mW leading to EIRP=820 mW

With

ERP: effective radiated power (ERP), often used to describe low frequencies systems.
EIRP: effective isotropic radiated power (EIRP), often used to describe high frequencies systems.

And

EIRP=ERP ×1,64

Considering our application, the 500 mW ERP band seems to be better adapted, but there is a trade off to make with the bandwidth available, especially for the video transmission that requires large bandwidth.

Although it appears to be the fastest and easiest way to define the frequency range of a system, the use of a free band is risky for the user. Indeed, there are no guaranties of non-jamming for users of license-free radio systems. In our case, a permanent link should be maintained between the aircraft and the ground station, considering that the aircraft will fly around the city of Toulouse and transmit uniformly to a region of more than 100km all around its antenna. We cannot allow that telecommand data don’t reach the aircraft, or that ground station loose the telemetry information of the plane: the solution is to send a frequency allocation request to the ANFR.

1. Request for frequency band allocation

Regulatory aspects are parameters that define the system but yet don’t depend on the user. The process of frequency allocation is long and administratively complex, that’s why we asked for information about frequency allocation process at an early stage of the project. In the mean time, as the answers were sending us back to the TNRBF, we decided to consider free bands, but we soon realised it was not a reliable solution for our design.

Our first idea was to search for frequency bands dedicated to the service named MBA (Mobile for Aeronautical services) and affected to the user DEF (Ministry of Defence), because ISAE is under the tuition of Defence Ministry. There, we find an available band 5091-5150 MHz (c.f. ANNEX II )