33.4: A Wideband Bidirectional Calibration-Free Frequency/Switching-Staggering $360^{\circ}$ D-Band Phase Shifter with Frequency-Invariant Codes Achieving lt; {2.38}^{\circ}/{0.63}\text{dB}$ RMS-Errors Over 24% Bandwidth

With the increasing need for high data-rate and channel throughput, the D-band (110 to 170GHz) has been actively explored for beyond-5G and 6G wireless communication, sensing, and radar applications [1], [2]. To overcome the severe free-space path loss at D-band, large-scale phased arrays are essential to focus and steer the radiation beams [3]–[5]. As summarized in Fig. 33.4.1 (top), D-band phased arrays come with a list of system challenges, which affects the design requirements of the front-end building blocks, in particular the phase shifters (PSs). First, the array element should fit into a $\sim\lambda/2\times\lambda/2$ grid dictated by the antenna spacing (1mm×1mm at 150GHz). This requires compact PSs with a desired bidirectional ability to enable PS sharing between a transmitter and receiver of each element. Second, due to the narrow beamwidth of large-scale phased arrays, accurate beamforming is essential, requiring PSs with $360^{\circ}$ phase range, accurate phase control (low rms phase error), and sufficiently fine resolution (~4 to 6b). Third, to simplify the phase/gain calibration of the array elements for fast beam forming/steering, PSs with minimum gain variations across phase states (low rms gain error) and phase codes independent of frequency are highly desirable. Fourth, with the compact element area of D-band arrays, low element-level power consumption is necessary to lower thermal density. This either requires active PSs with low DC power or zero-power passive PSs with low insertion loss (IL). Finally, supporting large data-rates demands wideband front-ends, requiring PSs that can achieve all the aforementioned desirable features across wide bandwidths.