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This communication presents a millimeter-wave (mmW) compact wideband high-gain $8\times8$ circularly polarized (CP) filtenna array with densified 16-way substrate integrated waveguide (SIW) feeding network. First, a parallel-series $2\times2$ CP patch subarray without a power distribution network is designed by exciting four and two quasi- modes, simultaneously, in a single SIW cavity. By properly setting perturbed via-slot groups, hexagonal radiating apertures, and square patches etched with a 45°-inclined center slot, three axial ratio (AR) minima are created to realize wide AR bandwidth. Additional bandpass filtering response can also be observed, which helps to realize high-selectivity CP filtenna array design. The densified 16-way feeding network mainly consists of four sequentially rotated SIW four-way power dividers, and two filtering SIW TE10-to-TE20 mode two-way power dividers. The former is constructed based on the anti-phase field distribution of TE20 SIW mode, and the latter features an additional filtering response and simple structure. An $8\times8$ CP filtenna array centered at 28 GHz was fabricated and tested. The measured results show a-10dB impedancebandwidth of 14.4%, a 3 dB AR bandwidth of 15.3%, a peak gain of up to 23.9 dBic, and high out-of-band rejection levels better than 39 dB.