Novel OCDM Transceiver Design for Doubly-Dispersive Channels

Orthogonal chirp division multiplexing (OCDM) is a new multi-carrier scheme that has been emerging as a new candidate for 6G waveform taking advantage of the unique features of the chirp spread spectrum that makes it immune to intersymbol interference raised due to delay spread. However, a thorough analysis of OCDM under doubly-dispersive channels has not been conducted yet to verify its robustness against Doppler spread as well. In this paper, we investigate the input-output relationship of an OCDM system under doubly-selective channel, where we demonstrate that the circular convolution property of OCDM partially holds even under Doppler spread. Building on previous results, we show the difficulties and problems associated with estimating and equalizing the channel at the receiver side in conventional OCDM systems, especially in case of having fractional delay and Doppler shifts. Then, we propose a new OCDM transceiver by adding fast Fourier transform (FFT) and windowing blocks to ensure channel tap separability and reduce the effect of fractional Doppler shift, respectively. Accordingly, a new channel estimation scheme is developed for the proposed OCDM system. The numerical and simulation results validate the advantages of the proposed OCDM system performance under doubly-dispersive channels over the conventional where the proposed leverages a bit-error-rate (BER) gain in perfect channel state information (CSI) of 2 dB and 3 dB for minimum mean squared error (MMSE) and message passing (MP) equalizers, respectively, and show that it holds great promise as an emerging radio access technology for 6G wireless systems.