Novel Filterbank Multicarrier Waveform for L-band Digital Aeronautical Communications: Initial Field Test Results

Growth in aviation has resulted in crowding of the traditional aviation VHF spectrum in many parts of the world. In response to this, several organizations have investigated use of the L-band (~1-2 GHz) for future aviation communications. The International Civil Aviation Organization (ICAO) Digital Communications Infrastructure Working Group (DCI-WG) is currently working on standardizing an L-band Digital Aeronautical Communication System (LDACS, formerly LDACS-1). This system uses orthogonal frequency division multiplexing (OFDM) multicarrier modulation, with multiple modulation types (QPSK, QAM) in a 500 kHz signal bandwidth, designed to operate in spectral "gaps" in the L-band between currently assigned Distance Measurement Equipment (DME) transmission channels. Under a NASA research project, we have recently investigated an alternative modulation scheme, filterbank multicarrier (FBMC). The FBMC scheme is by design more spectrally contained than OFDM, and this can yield several benefits: better robustness to adjacent channel DME signals, less interference to those adjacent channel DME signals, and larger spectral efficiency than OFDM. The FBMC approach gains these benefits at a cost of increased computational complexity—for modern-day signal processing however, this additional complexity is not prohibitive. Our NASA work has gone from analyses, to computer simulations, to laboratory testing of our FBMC designs, and in this paper we report on the designs, and provide some results of initial terrestrial field tests. Our current plan is to flight test these radios in Spring-Summer 2021, but prior to flight testing, to gain confidence in our designs, we undertook some short-range, low-power "drive tests," in which the aircraft radios are conveyed in an automobile. Our primary application is Controller-Pilot Data Link Communications (CPDLC), but our designs are not inherently restricted to this low-rate application. In this paper, we provide some background to our NASA project, describe the FBMC signal processing and radio frequency design, the key new features that we include in our FBMC radio tests, and example drive test results that illustrate our success. Some of the new features include, (i) adaptive modulation up to 1024 QAM, (ii) the use of a smaller number of subcarriers than LDACS (16 instead of 64) for greater Doppler resilience, (iii) simultaneous L-band and C-band transmission, (iv) a 5 MHz C-band FBMC link, and (v) the use of diversity ground station antennas for increased range/reliability.