Advanced Physical-Layer Technologies in VHF Data Link Communications

The VHF band has been widely used for aviation voice communication using analog communication systems for decades. However, due to fast increasing transportation and high usage of VHF channels, the band is becoming much more crowded, and use of analog waveforms will likely not maintain the required performance and quality of service in the future. Therefore, digital communication systems have been considered and studied due to their larger spectral efficiency. Notably, digital systems proposed for VHF are broadband VHF (B-VHF) using multi carrier-code division multiple access (MC-CDMA), and VHF data link modes 2 and 3 (VDL2/3) using differential 8-state phase shift keying (D8PSK) modulation. Compared to B-VHF, VDL2/3 has received more attention due to its simplicity and more constant amplitude waveform, yielding lower peak-to-average power ratio (PAPR) and hence better energy efficiency. Recently, advanced VHF digital link (A-VDL) was proposed for VHF [1]. This scheme enables use same platform as VDL except for the physical layer processing, including modulation. The proposed A-VDL, following digital video broadcasting satellite second-generation (DVB-S2) standard, uses amplitude and phase-shift keying (APSK) modulation with higher modulation order than VDL, hence providing higher spectral efficiency than VDL. Compared to the widely used quadrature amplitude modulation (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 techniques used in DVB-S2, low-density parity-check (LDPC) codes, more efficient standardized voice encoders, as well as better pulse shaping filters than the classical square-root raised-cosine (SRRC) 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.