Wideband Blocker-Tolerant Receivers for Sub-7 GHz 5G Applications

Abstract

To meet the growing demand for higher data rates, the fifth generation (5G) of mobile communication has been introduced for low-band, mid-band, and high-band frequencies. In 5G applications, higher bandwidth and complex modulation schemes are employed for this purpose.

To mitigate the high path loss associated with mm-wave frequencies, greater focus has been placed on low-band and mid-band radios. Even the operating frequency of sub-6GHz radios has been extended to sub-7GHz. However, the congested sub-7GHz spectrum has kept the offset frequency of close-in blockers constant compared to the previous standards, such as 4G. This imposes stringent requirements on receiver (RX) selectivity and linearity.

This thesis presents reconfigurable wideband low noise transconductance amplifier (LNTA)-based RXs for sub-7GHz radios. The proposed RXs have high bandwidth and decent noise figure (NF) performance to employ highorder modulation schemes and achieve a high data rate. This thesis introduces techniques to enhance the RX selectivity for suppressing the close-in blockers of 5G user equipment, microcell base station, and local area base station applications. Moreover, this thesis proposes RXs with decent far-out out-ofband linearity for base station co-location applications where strong blockers are present from other standards.

Chapter 1 outlines the evolution of wireless communication leading to 5G applications. It introduces the 5G standard and highlights its stringent requirements on RX operating frequency, bandwidth, noise figure, and linearity. Following a brief discussion on N-path filters and their role in enabling wideband RXs, Chapter 1 reviews state-of-the-art RX designs and identifies their limitations for 5G applications. Finally, it defines the objectives and scope of this thesis.

Chapter 2 targets 5G user equipment applications and introduces a wideband blocker tolerant receiver fabricated in 40-nm bulk CMOS technology. By incorporating programmable zeros and a second-order transimpedance amplifier (TIA), the RX achieves enhanced selectivity and fulfills the stringent linearity requirements of 5G for close-in blockers. An auxiliary path is employed to reduce the RX input impedance at far-out offset frequencies, creating a current-sinking path for far-out blockers. In this way, the proposed RX achieves decent out-ofband linearity performance. To determine the component values for both the RF front-end and the second-order TIA, two design guides are developed based on the 5G standard. The proposed RX successfully meets 5G requirements for reference sensitivity and out-of-band blocking tests.

Chapter 3 presents a wideband RX for 5G microcell base station applications. This Chapter targets microcell co-location scenarios. Hence, it adopts a parallel preselect filter to achieve decent far-out out-of-band B1dB. Third-order RF and baseband filters deliver sixth-order channel selectivity to handle close-in blockers of base station applications, where the ratio of blocker offset frequency to RX bandwidth is 1/10. Additionally, a translational feedback network provides input matching and minimizes in-band gain ripple to below 0.5 dB. The RX’s reconfigurable architecture supports a low-noise mode and linear mode. Leveraging its current-mode operation and sharp filtering, the implemented RX in 40-nm CMOS technology complies with all 3GPP requirements for reference sensitivity, in-band blocking, and out-of-band blocking.

Chapter 4 introduces a wideband LNTA-based RX for 5G local area base station applications. The proposed RX covers both low- and mid-band frequencies. Firstly, this Chapter determines the optimal TIA architecture for 5G applications. To do so, the first-order and Rauch TIAs were thoroughly analyzed and compared in terms of transfer function, input impedance, loop gain, and noise performance. The Rauch TIA was selected for its superior selectivity and higher loop gain for out-of-band signals, with additional selectivity enhancement by adopting a third-order high-pass filter integrated in parallel with the TIA feedback resistor. The RX incorporates the Rauch TIAs with passive mixers and an LNTA featuring an N-path notch filter in its feedback. To enhance the RX’s operating frequency range, two switch sets at the LNTA output (one for the N-path notch filter and another for the down-converting mixers) were merged. Furthermore, the band-pass characteristic of the TIA input impedance is leveraged to introduce positive feedback in the LNTA, increasing the 3-dB bandwidth without compromising out-of-band rejection. The resulting design is fabricated using 40 nm bulk CMOS technology, and the proposed RX satisfies 3GPP requirements for reference sensitivity, in-band blocking, close-in blocking, and out-of-band blocking, making it a strong candidate for 5G local area base station applications.

Chapter 5 provides a brief discussion of the proposed ideas presented in this thesis and offers suggestions for future research directions.