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Symbol boundary alignment, with respect to waveform selection, has an important impact on the numerology design for fifth-generation mobile applications. The current symbol boundary alignment, along with orthogonal frequency division multiplexing (OFDM) waveform, strongly suffers from intercarrier interference (ICI) especially in unmanned aerial vehicles (UAV) communications. This happens when the mobility causes Doppler effect which results in loss of orthogonality in OFDM. The available solutions for overcoming the ICI problem suffer from high complexity, low spectral efficiency, and incompatibility with the current radio access technologies. This paper presents a novel symbol boundary alignment, called Low ICI Symbol (LICIS) boundary alignment numerology, to avoid the disadvantages of the available solutions. LICIS utilizes large subcarrier-spacing to reduce the ICI power (e.g., around 5-dB ICI power reduction with subcarrier spacing of 30 kHz in high-speed UAV communications). Moreover, LICIS is based on the same reference clock as local thermal equilibrium (LTE) which guarantees its compatibility with the current LTE numerology. In addition, this approach places only one guard-interval at the end of a sequence of OFDM symbols and creates a subslot. This leads to less overhead and preserves the spectral efficiency. Furthermore, a pre-fast Fourier transform (FFT) multipath channel equalizer is considered for removing the intersymbol interference between the OFDM symbols occurring within the subslot. Only one additional FFT and IFFT operations are required for the equalizer which creates an acceptable complexity increment compared to the complexity of other available solutions. Numerical and analytical evaluations show the superior performance of the proposed technique in terms of reliability and spectral efficiency |
