Plane Wave Expansion-based Codebook Design for 6G Near-field MIMO

To meet the stringent spectrum efficiency requirements of International Mobile Telecommunications (IMT)-2030 (6 G), it is essential to explore the application potential of higher frequency bands and extremely large antenna arrays (ELAA). This shift introduces near-field effects, where electromagnetic waves deviate from the far-field plane wavefront assumption and must be modeled as spherical waves under certain distance conditions. Consequently, introducing beam focusing codebooks for the near-field is crucial. The typical fifth-generation New Radio (5G NR) high-precision Type II codebook, constructed from a two-dimensional discrete Fourier transform (DFT)-based grid of beams, enables the channel state information (CSI) feedback of a set of orthogonal beam selections along with corresponding amplitude and phase information. However, it is not optimized for near-field characteristics. To satisfy the 3rd Generation Partnership Project (3GPP) Releases’ requirement for backward compatibility, we introduce the plane wave expansion (PWE) method of focused beams. This method allows for the generation of focused beams through the weighted superposition of multiple DFT beams. Based on this approach, we define a set of additional feedback contents and formats within the NR Type II codebook framework, enabling a more accurate representation of the near-field line-of-sight (LoS) CSI for feedback. Numerical simulation results substantiate the performance of the proposed NR Type II codebook enhancement scheme, achieving a performance within 2dB of the optimal beam focusing scheme with appropriate configuration.