Channel Estimation in an Over-water Air-Ground Channel Using Low Complexity OFDM-OQAM Modulations Hosseinali Jamal, David W. Matolak Department of Electrical Engineering University of South Carolina Columbia, SC, USA {hjamal@email, matolak@}sc.edu 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. Keywords— L-DACS1; OFDM-OQAM filter bank modulation; Preamble-based CE; Air-to-ground communication. I. INTRODUCTION Today many L-band communication systems are deployed for aeronautical or air-ground (AG) links. In 2010, EUROCONTROL [1] released final investigation results for a technology similar to broadband VHF (B-VHF), which was funded by the European Commission starting in January 2004 [2]. These new technologies operate in the aeronautical L-band (960-1164 MHz), which has recently been made potentially available for the Aeronautical Mobile (Route) Service. In [1] they released the final report on the L-DACS1 system, whose physical layer is based on cyclic-prefix orthogonal frequency division multiplexing (CP-OFDM) multicarrier modulation. These waveforms may also be used for unmanned aircraft systems (UAS) which are seeing explosive growth in both commercial and consumer use. These UAS have a large variety of applications [3], and integration of them into the worldwide airspace will require reliable and spectrally-efficient waveforms. This motivates our study of channel estimation. Multicarrier modulations are promising candidates to get high data rate transmission in frequency selective and time varying channels. Among these modulations, CP-OFDM has been studied extensively, and is a candidate for L-band AG communication systems. CP-OFDM is an efficient multicarrier modulation (MCM) to combat multi-path fading channels. Its robustness comes from adding guard intervals in time and frequency domains to combat ISI due to multi-path fading, but these guard intervals decrease spectral efficiency. In L-DACS1 a physical layer based on CP-OFDM modulation has been proposed. In this modulation system the time duration of symbols is extended up to 0.15 percent of the useful symbol time for a cyclic prefix, and 14 out of 64 subcarriers are nulled for adjacent-channel guard bands. The rectangular OFDM symbols lead to a sin(x)/x frequency spectrum. By using windowing techniques, one can decrease the side-lobes. This windowing also helps to reduce the interference from adjacent symbols. L-DACS1 uses raised- cosine windowing with a small roll-off factor. Several alternatives have been investigated to find better MCM schemes relative to the frequency and/or time-frequency localization criteria. As suggested in many publications, e.g., [4]-[6], OFDM-Offset Quadrature Amplitude Modulation (OQAM) is a very good scheme, which may be an attractive alternative. In this modulation each subcarrier is modulated by staggered Offset Quadrature Amplitude Modulation (OQAM) symbols which are real-valued symbols, in comparison to OFDM which transmits complex-valued symbols. The OQAM symbol rate is twice that of OFDM, and the two schemes therefore have similar spectral efficiency. In [5] and [6] OFDM-OQAM has been suggested as a very reliable alternative to OFDM, which can also be implemented with low complexity algorithms like the fast Fourier transform and polyphase networks. The largest difference between CP- OFDM and OFDM-OQAM is that the latter uses waveforms that are better localized in time and frequency domains [7]. In addition, OFDM-OQAM can attain a higher spectral efficiency by removing the cyclic prefix.