24.3 A 236-to-266GHz 4-Element Amplifier-Last Phased-Array Transmitter in 65nm CMOS

Abstract

Because the vast majority of the frequencies are unallocated and atmospheric absorption is low above 250GHz, 300GHz-band transceivers are appealing for the sixth-generation (6G) wireless communication technologies to support over 100Gb/s data rate. The main challenge of the 300GHz-band transmitter (TX) is achieving a large equivalent isotropic radiated power (EIRP) to compensate for the high free-space loss. One solution is to adopt a two-dimensional (2D) phased-array to boost the antenna gain and enable beam steering, which is much more practical than a high-gain antenna or lens. Another way is increasing the output power of each TX element. There are compound semiconductor processes with transistors exhibiting high unity-power-gain frequency (f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf> ) that can easily achieve this target; however, they tend to be incompatible with digital circuits and are expensive, which make them less practical. CMOS processes enable RF front-end circuits to integrate with baseband circuits at a much lower cost, but their low f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf> degrades power-amplifier (PA) performance. To address the f <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">max</inf> limitation of CMOS, Fig. 24.3.1 shows some recent 300GHz-band TX architectures. The multiplier-last architecture can generate high TX output power, however, the constellation maps of higher-order modulation schemes are degraded and thus fail to support higher data rates [1]. To satisfy its mixer linearity requirement, the square mixer-last architecture needs to operate at power back-off (PBO), and it consumes a lot of area due to power-combining [2]. The sub-harmonic mixer-last architecture suffers from low output power and needs PBO in the TX [3]. While the outphasing topology can operate at around output power at 1dB compression point (OP <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1dB</inf> ), the generation of the outphasing angles is problematic in digital baseband because their operation mechanism is intrinsically nonlinear [4]. Because of the absence of an RF PA, all the above works suffer from small EIRP, low power-added efficiency, and large chip area.