Stephen Weinreich

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MS, Electrical Engineering, Stanford University, 2016

BS, Electrical Engineering, Brown University, 2014

Email: weinreich AT stanford DOT edu

An adaptive NB-IoT antenna interface using frequency-translated baseband impedances

Wide-area narrowband internet-of-things cellular standards, recently adopted in Release 13 of the 3GPP LTE standard, address wide coverage for high-value concise internet connectivity in a constrained energy budget. One of these new standards is NB-IoT (category NB1), enabling 10 year battery life at 10 kilometer distances from the base-station and into buildings and basements. Despite this, the antenna-receiver interface is a bottleneck due to the desire to exploit propagation benefits of longer wavelength (sub-GHz) spectrum and the electromagnetic realities of small antennas. This project looks at innovative solutions to handle this problem using frequency-translated baseband impedances.

The NB-IoT standard

The NB-IoT standard targets low-end IoT devices. Peak data rates are limited to 250 kbps due to the 180 kHz signal bandwidth but as a result ultra-low power consumption is achievable. Industrial and environmental sensors are among the IoT applications which require long battery life with low requirements on data rate. Additionally, NB-IoT has multiple deployment options due to its small signal bandwidth. One of these options is within currently unused LTE guard-bands, enabling network access at a lower cost. Another option is deployment within current LTE or GSM spectrum.[1]

Frequency-translated impedances

Recent advances in RF interfaces have seen the use of the passive mixer to perform both the filtering and downconversion operations on silicon[2]. Such frequency-translational circuits have been around for over 50 years[3], but it is only with recent technology scaling that they have become practical at radio frequencies. In addition to providing an easily tunable, low area, and high Q filter, passive mixers translate the baseband impedances up to RF as seen in Figure 1. In this project we will use a mixer-first receiver in order to exploit this property and achieve enhanced impedance matching for small, high-Q antennas through the use of flexible baseband impedances.

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Figure 1: Concept of frequency-translated baseband impedance

[1] 3GPP TS36.101, “3rd Generation Partnership Project; Technical Specification Group Radio Access Network; Evolved Universal Terrestrial Radio Access (E-UTRA); User Equipment (UE) radio transmission and reception,” v13.5.0, Sept. 2016;
[2] Andrews, C., & Molnar, A. (2010). A passive mixer-first receiver with digitally controlled and widely tunable RF interface. IEEE Journal of Solid-State Circuits, 45(12), 2696–2708.
[3] Franks, L. E., & Sandberg, I. W. (1960). An Alternative Approach to the Realization of Network Transfer Functions: The N -Path Filter. Bell System Technical Journal, 39(5), 1321–1350.

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