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Advanced Physical-Layer Technologies in VHF Data Link Communications

Due to the rapidly increasing number of flights and high usage of VHF channels, the VHF spectrum is becoming much more crowded, and use of analog waveforms will likely not maintain the required performance. Therefore, digital communications have been considered due to larger spectral efficiency. Notably, VHF data link modes 2 and 3 (VDL2 and 3) using D8PSK modulation has been proposed. VDL2 and 3 received attention due to its simplicity and more constant amplitude waveform, yielding lower PAPR. Recently, advanced VHF digital link (A-VDL) was proposed for VHF. This scheme enables use of essentially the same platform as VDL except for the physical layer processing, including modulation. The proposed A-VDL, following DVB-S2 standard, uses APSK modulation with higher modulation order than VDL, hence providing higher spectral efficiency than VDL. Compared to the widely used QAM, APSK is more resistant to amplifier amplitude and phase distortions. Thus, APSK has become of interest for satellite communications, as well as VHF communications in A-VDL. In this paper, we investigate other advanced technologies such as channel encoding technique low-density parity-check (LDPC), more efficient standardized voice encoders, as well as better pulse shaping filters than the classical square-root raised-cosine filter used in VDL and A-VDL. Via simulations and analysis, we compare the proposed scheme's link margin, PAPR, and spectral efficiency compared to VDL and A-VDL, which both use Reed Solomon (RS) encoding. In addition, as another way of generating the same VDL waveforms (or possibly other single-carrier aeronautical band waveforms), we investigate the single-carrier type waveform used in cellular LTE and 5G uplink communication links, the discrete Fourier transform-spread OFDM (DFT-s-OFDM), and discuss how we can take advantage of using the same LTE and 5G hardware resources.

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