Scalable LO Distribution Schemes For Building Large-scale Phased Arrays

Abstract

Facing the increasing demands of mmWave communication systems nowadays and the inherent adversity of higher signal attenuation in mm-wave wireless transmission, there is a continuous effort towards building larger-scale phased arrays to achieve better effective isotropically radiated power (EIRP) and directivity. One of the critical challenges in the array design is how to achieve frequency and phase synchronization in a scalable manner, which requires not only cell-level circuit optimizations but more importantly innovations at the architectural level. In this thesis, we present three scalable schemes for building large-scale active arrays with multiple local oscillators (LOs). First, a coupled oscillator array (COA) scheme based on unidirectional coupling is proposed. We derive the equilibrium states for the COA and demonstrate that our scaling scheme will preserve the steady state modes during scaling resulting in a predictable phase profile. We further evaluate the stability condition using the perturbation method. Based on the simulation results, we find that as long as the mismatches in the free-running frequencies are below a theoretical threshold, the scaled array could tolerate the presented element-to-element variations and could achieve frequency synchronization in a scalable manner. Next, we present a two-dimensional coupled phase-locked loop array (CPLLA) scheme using a type-II phase-locked loop (PLL) as a unit cell. Besides the concept of phase locking between multiple oscillators, we implement a chip prototype of the PLL array at 28GHz. The chip performance is characterized by on-wafer probing. The measurement indicates that the out-of-band phase noise of the distributed array does not depend on the number of elements. However, we find several undesired issues including in-band phase noise degradation and stability issue with the taped-out chip. In order to identify the reason behind the flawed performance, a theory of phase control conflicting between the PLL loop and injection locking is proposed. We later verify our theory with the post-taped-out simulations that achieve consistent results with the measured data. Finally, We adjust the method of mm-wave signal generation and distribution method, and propose a new phase self-aligning array architecture. A prototype is made to demonstrate the phase self-aligning between a 1 × 2 array. The measured results verify our theoretical analysis and show accurate phase control and a fast switching time withhigh spectral purity. As a result, we believe that the presented element-to-element self-alignment method is applicable for distributed tuning and control of scalable phased arrays without the need for extensive baseband calibration.