1-Bit Polarization Rotating and Beam-Switching Transmitarray for Millimeter Wave MIMO Applications

This paper presents a low profile, 1-bit phase quantized, polarization rotating (PR), and switchable beam transmitarray for millimeter-wave (29.7-30.7GHz) MIMO applications. The transmitarray is based on polarization rotating elements, designed on two substrate layers and have a volume of 0.21 lambda(0) x 0.21 lambda(0) x 0.30 lambda(0) (lambda(0) is corresponding to frequency at 30GHz). The polarization rotating elements have low insertion loss of 2.5dB in the frequency band of 29.5 GHz - 39.5GHz with a phase difference of pi between two phase quantized bits and thus exhibit a wideband nature. The transmit panel is designed using 29 x 29 phase quantized unit cells and a 3-port patch MIMO antenna operating around 30GHz is used as a feed element to achieve one dimensional beam-switching (BS) of +/- 13 degrees with respect to the broadside direction.The focal distance between the center feed element and the transmitarray panel is 30 mm with the maximum dimension of the panel as 60.9 mm giving an f/d ratio of 0.49. The MIMO antenna system has a port to port isolation greater than 30dB in the operating frequency band which is obtained by keeping the inter-element spacing as lambda(0). The transmitarray prototype is fabricated and measured. The simulated and measured results are found to be in close agreement. A peak gain of 19.3dBi with a switching loss of 1dB, 3dB beamwidth of 10.1 degrees and aperture efficiency of 21% are obtained in measurement. The wideband nature of the transmitarray is validated by feeding the surface with a wideband (26.5GHz- 40 GHz) horn antenna. The switchable beam transmitarray can be used for beam-forming in base stations for future 5G millimeter wave communications.