h channels. For non-uniform panel array, the antenna elements in the horizontal/vertical direction cannot be viewed as a uniform array. Thus array response vector is not in DFT form, and additional phase difference exists between the panels. In other words, the DFT vector cannot capture the actual channel response but can cause beam distortion and beam gain reduction. Moreover, panels at one transmit/ receive point are not easily calibrated in a practical implementation, where a fixed or ran- dom phase may exist among different panels. A good codebook design should use phase/ amplitude factors among panels to combine the beamforming vectors of the panels to match the array response [59]. In multi-panel MIMO codebook design, 2D-DFT vectors may be used as the per-panel beamforming vectors representing the spatial propagation properties for each panel. Furthermore, an inter-panel co-phasing factor is added across the DFT-based codeword to reduce performance loss. Let us use the two-panel case as an example, one can design the multi-panel codebook W for dual-polarized antenna array as W = [W|W1 W2W24] where W1 and W1 consist of DFT vectors reflecting long-term channel characteristics of antenna panel 1 and panel 2, respectively; W2 and W2 consist of phase factors reflecting the short-term and frequency-selective channel elements of panels 1 and 2, respectively, and P is the inter-panel co-phasing factor, which is designed for feedback in a wideband manner or subband manner according to the use cases. The precoding vectors for both panels are restricted to be the same in order to reduce the feedback overhead [59]. For a uniform or non-uniform planer array with ideal synchronization or small phase offset, coherent transmission can be used to achieve spatial multiplexing/diversity gain. However, the performance of coherent transmission may be degraded in practice due to different reflection and refraction propagation paths which cause different average channel delay and different average channel gain for a given directi