FFT-Domain Signal Processing for Spectrally-Enhanced CP-OFDM Waveforms in 5G New Radio

Fast Fourier transform (FFT)-domain signal processing has been considered recently as an effective tool for spectrum enhancement of orthogonal frequency-division multiplexing (OFDM)-based waveforms, which is a central element in the fifth generation new radio (5G-NR) developments. Fast-convolution (FC) filtering approximates linear convolution by effective FFT-based block-wise circular convolutions using partly overlapping processing blocks. In earlier work, we have shown that FC processing is a very flexible and efficient tool for filtered OFDM signal generation and receiver-side subband filtering, e.g., for the mixed numerology scenarios of the 5G-NR. However, with the continuous overlap-save and overlap-add processing models with fixed block-size and overlap, the FC-processing blocks are not synchronized to all OFDM symbols of a transmission frame. Furthermore, 5G-NR numerology does not allow to use transform lengths smaller than 128 because this would lead to non-integer cyclic prefix (CP) lengths. Here we present a modified FC processing scheme which avoids these limitations. The scheme is based on extrapolating the CP samples which, as an example, makes it possible to use 16-point transforms in case of a 12-subcarrier-wide subband allocation, greatly reducing the implementation complexity. Also the FC-processing blocks can be aligned with each OFDM symbol, reducing complexity and latency, e.g., in mini-slot transmissions.