Impact and modeling of phase noise in mmW beamforming systems

Abstract

Due to the exponential growth of wireless communication, mobile communication applications require more bandwidth available in higher operating frequencies. High centre frequency makes the systems sensitive for phase variations caused by the phase noise (PN) of the imperfect local oscillators (LOs) used in wireless transceivers. Moreover, wide bandwidth also makes the faster phase variations of the phase noise spectra have an impact on the overall system performance by reducing effective signal-to-noise-ratio. These fast variations seen in the high offset frequencies in the phase noise spectra are typically ignored in the communication systems because the traditional system bandwidths are in order of megahertz, or in maximum few gigahertz. In mmW frequencies, i.e., at 30–300 GHz, the transceivers are typically using multiple antenna elements to achieve the required link range by highly directional beams. Often so-called phased arrays are used to implement the multi-antenna transceiver, where the beamforming is mostly performed in the analog domain by digitally controllable mmW phase shifters. For generating multiple beams from the same transceivers, more than one phased array is typically used in the same platform. The phased arrays often share a single LO, for multiple antenna elements. A typical LO generation architecture is containing a base clock, phased-locked loop (PLL), and some frequency multipliers to achieve the target mmW operating frequency. In multi-array systems, the LO signal can be divided into phased arrays in multiple domains, i.e., the arrays can have an independent clock, and a shared clock, but independent PLLs, shared PLL, or even the final mmW LO can be shared. In different architectures, the phase noise has different behavior, and it can have an impact for example on the beamforming accuracy. This thesis focuses on the effects of phase noise on milimeter-wave (mmW) beamforming systems to study different LO routing architectures. We mainly focus on LO architecture with multiple phased arrays that intend to make a common beamformer and their impact on overall system-level phase noise performance. The specific focus is given to the behavior of the wideband phase noise. The phase noise is modeled by using baseband equivalent models where a gaussian phase noise source is filtered by a filter modeling the equivalent phase noise spectra. The parameterization of the model is based on commercial LO phase noise spectra. The behavior is studied in different LO schemes in single-beam and multi-beam scenarios by using simple examples. The simulations are mostly carried out by using continuous-wave signals, but also the single-carrier modulated QAM waveform is demonstrated. The simulations are performed in MATLAB.