CSI reporting and codebook structure for Doppler-delay codebook-based precoding in a wireless communications systems

A communication device for providing a channel state information, CSI, feedback in a wireless communication system is described. The communication device includes a transceiver configured to receive, from a transmitter a radio signal via a time-variant, frequency-selective MIMO channel, the radio signal including downlink reference signals according to a reference signal configuration comprising a number of antenna ports, and downlink signals comprising the reference signal configuration; and a processor. The processor is configured to - estimate an explicit CSI in the frequency domain using measurements on the downlink reference signals on the radio channel, the downlink reference signals provided over a certain observation time, - select a Doppler-delay precoder matrix (W) for a composite Doppler-delay-beam three- stage precoder, the Doppler-delay-beam three-stage precoder being based on one or more codebooks, the one or more codebooks including o one or more transmit-side spatial beam components of the composite Doppler- delay-beam three-stage precoder, o one or more delay components of the composite Doppler-delay-beam three- stage precoder, and o one or more Doppler-frequency components of the composite Doppler-delay- beam three-stage precoder, - calculate either one or more of a channel quality indicator, CQI, and/or a precoder matrix indicator, PMI, and/or a rank indicator, Rl, using the explicit CSI and the composite Doppler-delay-beam three-stage precoder with the selected Doppler-delay- beam precoder matrix (W), and - report to the transmitter the CSI feedback including either one or more of the CQI, and/or the PMI and/or the Rl, wherein the PMI and Rl are used to indicate the Doppler-delay-beam three-stage composite precoder matrix for the configured antenna ports. The communication device incudes one or more parameters (D, F), the parameter indicating a number of delay components, like delay DFT vectors (I), and/or a number of Doppler-frequency components, like Doppler DFT vectors (II), to be used by the communication device when calculating the Doppler-delay precoder matrix (W). The communication device is configured to select from the one or more codebooks a subset of D delay components and/or a subset of F Doppler-frequency components and to use the selected subset of delay components for each polarization (p) and each spatial beam (u) and/or the selected subset of Doppler-frequency components for each polarization (p), each spatial beam (u) and each delay (d), when calculating the Doppler-delay precoder matrix (W).