Channel estimation in an over-water air-ground channel using low complexity OFDM-OQAM modulations

Abstract

Demands for very reliable and robust communications systems are becoming commonplace, and this is true for air-to-ground (AG) systems, which are seeing increased usage, particularly for unmanned aircraft. Available spectrum exists for AG communications systems in the L-band (960–1164 MHz). Two robust L-Band Digital Aeronautical Communication Systems (L-DACS) were recently defined, LDACS1 and LDACS2. The L-DACS1 scheme employs a fairly broadband (0.5 MHz) transmission using Cyclic-Prefix Orthogonal Frequency-Division Multiplexing (CP-OFDM) together with adaptive coding and modulation. In this paper we propose a new AG communication system based on OFDM-OQAM filter bank modulation, using features similar to those of the L-DACS1 physical layer, and show how better subcarrier filtering can improve spectrum efficiency in AG channels. Channel estimation (CE) in OFDM-OQAM systems must employ a pilot method completely different from classical pilot-based CP-OFDM systems, since in OFDM-OQAM the pilots should remove pilot subcarrier interference from neighboring subcarriers. We employ a low complexity pilot-based CE on the OFDM-OQAM signal, and show BER results for L-DACS1 and for comparable OFDM-OQAM based systems in an example AG channel. The AG channel we employ is one based upon a recent extensive measurement campaign. We also estimate channel throughput and mean square error (MSE) for these two systems, and show that for similar BER performance, both channel estimation and throughput of the new system are better than that for L-DACS1.