Channel Estimation for MIMO-OCDM Systems as an Emerging 6G Radio Access Technology

In multiple-input multiple-output (MIMO) systems, channel estimation is of crucial importance to ensure the reliable recovery of the high-speed MIMO signals through severe multi-path propagation. In this work, we propose a novel and efficient channel estimation algorithm for the emerging MIMO orthogonal chirp-division multiplexing (OCDM) systems by taking advantage of the unique features of OCDM signals. In the proposed algorithm, a set of pilot signals is deliberately designed based on the Fresnel basis, which are essentially linearly frequency-modulated (or chirped) waveforms and mutually orthogonal. The pilot signals are assigned to different transmit antennas and transmitted simultaneously, occupying the same time interval and frequency band for channel acquisition. Based on the convolution-preservation theory of the Fresnel transform, the transfer matrices of the MIMO channel can be readily estimated at the receiver without any inter-antenna interference even if the pilots overlap in both time and frequency. The proposed algorithm avoids bandwidth waste in the conventional channel estimators, in which silent pilots are required in time and/or frequency domain to ensure that the received MIMO pilots are still distinguishable. Moreover, by virtue of the unique features of the chirped pilots, the proposed algorithm is unbiased and thus has better accuracy. Numerical results are provided to validate the advantages of the proposed estimator with improved performance over other algorithms. As a result, the proposed channel estimation algorithm holds great promise for future MIMO-OCDM as an emerging radio access technology for 6G wireless systems.