Aeronautical Communications

Air traffic is significantly increasing for the last few years. About 33,000 flights a day makes the European airspace to one of the busiest of the world. Until the year 2035, EUROCONTROL expects the increase of number of flights by 50% compared to the year 2016.

This brings today’s communication systems to their limit. Therefore, it is necessary to develop new strategies to handle the increase of civil air traffic. For this purpose, the SESAR (Single European Sky Air traffic management Research program) was launched in year 2004.

The long-term aims of SESAR are:

Harmonization of the European airspace
Reduction of CO₂ emissions by 10% per flight
Reduction of flight times by 6% and departure delays up to 30%
Additional 10% flights landing at congested airports
Air traffic management system shall be capable to handle 100% more traffic

To achieve those aims new communication systems with higher throughput are necessary. SESAR consists of several research topics.

The Microwave Engineering Group focuses on communication between ground stations and aircraft and the improvement of the robustness of these systems.

L-Band Digital Aeronautic Communication Systems (LDACS)

LDACS is a terrestrial communication system that will be deployed in the aeronautic L-Band (962 MHz – 1213 MHz). It supports ground to aircraft communication and vice versa.

The L-Band is primarily used for navigation, surveillance and military systems. One can find the Distance Measuring Equipment (DME), Secondary Surveillance Radar (SSR), Joint Tactical Information Distribution System (JTIDS) and many other systems in the L-Band (see figure below). Thus, LDACS is an overlay system and, therefore, needs to cooperate with legacy systems. The most crucial thing for the deployment of new systems is that they must not interfere with already existing systems.

Diagram of L-band frequency users showing no free frequency for LDACS. Possible frequency bands with minimal interference with other systems are shown.

Frequency band proposals for LDACS

Ultra Linear LDACS Power Transmitter

LDACS is an OFDM based communication system with an extremely stringent spectral mask. The channel bandwidth is 625 kHz with 64 sub-carriers. The average transmit power is approx. 15 W with peak power levels around 230 W.

The figure below shows a block diagram of the LDACS transmitter prototype. It consists of an up-conversion stage, amplifier stage and a reference receiver for output monitoring.

Block diagram of the structure of an LDACS transmitter. Directional coupler, reference receiver, image rejection filters, controlled via microcontroller.

Block diagram of an LDACS transmitter

A baseband unit generates the input signal for the up-conversion in real-time. Due to high instantaneous output power the power amplifier (PA) is driven into its nonlinear range. Because of the nonlinear behavior intermodulation between the subcarriers broadens the output spectrum and that would violate the spectral mask. Thus, adjacent LDACS channels or even other communication systems will be distorted by this effect. Hence, it is necessary to linearize the output spectrum. This is done via digital predistortion (DPD). The output signal of the PA is down converted to baseband and compared with the sent baseband signal. With this information it is possible to model the behavior of the PA or even more the complete up-conversion chain. By altering phase and amplitude of the baseband signal to counteract the nonlinearity of the PA, it is possible to linearize the output signal again. Here, an impressive linearization performance of 76 dB was achieved with a fully LDACS-compliant prototype.

Graph: This measured spectrum superimposes the output signal with a targeted spectral mask. The extremely high linearity of the transmitter can be seen.

Output spectrum at +42 dBm power

LDACS transmitter prototype

Figure 4 shows the LDACS transmitter prototype. The main purpose of the LDACS transmitter prototype was to demonstrate that it does not disturb other deployed aeronautic communications systems.

Robust LDACS Receiver with Interference Mitigation

In an LDACS communication system, the receiver has to handle input signal levels between -104 dBma (average) up to -10 dBm (peak power). Due to co-site interference by other aeronautic systems the receiver has to withstand pulses up to +25 dBm. These co-site interferers (e.g. DME) cannot only damage the receiver front-end, they also result in saturating the first receiver stages. Thus, signal reception and decoding becomes impossible until the amplifier regains normal operation.

The main aim of the Microwave Engineering Group’s research activities is to detect and mitigate any input interference. The purpose is to reduce the blanking period of the receiver hardware and furthermore to give the decoding unit valuable information about the reliability of the input signal.

A typical blanking scenario due to DME pulse pairs. The measurement depicted below shows that a typical input amplifier is about 120 µs blanked until it regains normal operation. With one OFDM symbol being 120 µs long, a single co-site DME pulse pair is expected to impair two consecutive OFDM symbols, resulting in an unacceptable high bit error rate. By introducing interference detection and mitigation circuitry, the blanking time reduces significantly.

2 Graphs: Impact of DME interference on the forward transmission of an LDACS receiver front-end. Without measures, the gain breaks down for more than 10ms.

S21 over time of the front-end after DME interference

Combining analog front-end interference mitigation strategies with digital decoding is part of ongoing research within the group. This involves extensive simulations of digital decoding algorithms using interference information from the front-end. To verify the different approaches, a prototype of an LDACS receiver front-end with interference mitigation circuitry was developed by the group.

LDACS receiver prototype

Additional Information

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