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An All-Digital 1 Mbps, 57 pJ/bit Bluetooth Low Energy (BLE) Backscatter ASIC in 65 nm CMOS

James Daniel Rosenthal

We present an all-digital application specific integrated circuit (ASIC) that implements Bluetooth Low Energy (BLE)-compatible backscatter communication. The ASIC was fabricated in a 65 nm CMOS process and occupies an active area of 0.12 mm(2) while consuming a total of 205 mu W DC power from 0.48 V and 1 V supplies. Of the total power consumption, 56% (115 mu W) is consumed by the digital logic, 16% (33 mu W) by the on-chip clock oscillator, and 28% (57 mu W) by the RF switch used as a backscatter modulator. The ASIC broadcasts up to 1000 BLE advertising packets per second at a data rate of 1 Mbps, yielding a backscatter modulator efficiency of 57 pJ/bit. The device was validated in both cabled and wireless (over-the-air) measurement setups, demonstrating compatibility with unmodified smartphones as well as commercially available BLE chips, such as the Nordic Semiconductor nRF51822. With the wireless test setup used in this work, and assuming a +10 dBm carrier source, the ASIC has a theoretical maximum read range of 4.9 m using a smartphone as the receiver. Building from previous work in BLE backscatter communication using FPGA-based prototypes, this work provides an important quantitative demonstration of the size and power savings that can be achieved in a BLE-compatible backscatter ASIC.

IEEE2022

An All-Digital 1.25 Mbps 5-Subcarrier OFDM Backscatter Uplink with Delta-Sigma Modulation for Improved Spurious-Free Dynamic Range

James Daniel Rosenthal

This paper describes an all-digital backscatter modulation approach leveraging delta-sigma modulation (DSM) to improve the in-channel spectral characteristics of orthogonal frequency division multiplexed (OFDM) backscatter communication. We demonstrate through numerical simulations and experimental validation that DSM can improve the spurious-free dynamic range (SFDR) of OFDM subcarriers generated by a low-resolution impedance digital-to-analog converter (DAC), such as an RF switch having two or four different impedance states. We present the design and validation of a prototype OFDM backscatter uplink with DSM implemented with all-digital logic in an FPGA. A single-pole-four-throw CMOS RF switch (i.e. 2-bits of impedance DAC resolution) serves as the backscatter modulator. We experimentally validated the DSM approach with a 2.4 GHz, five-subcarrier OFDM backscatter uplink and a four-times oversampling DSM at up to 1.25 Mbps. In this scenario, the DSM improved the SFDR by 43 dB within the subcarrier band while reducing the overall noise floor in the same band by 11.3 dB. These results confirm that a DSM approach can be used to control quantization noise and improve the spectral characteristics of low-resolution digital impedance modulators for backscatter communication in scenarios where in-channel SFDR is more important than wideband noise performance.

IEEE2022

Adaptive Wireless Power Transfer and Backscatter Communication for Perpetual Operation of Wireless Brain-Computer Interfaces

James Daniel Rosenthal

Brain-computer interfaces (BCIs) are neural prosthetics that enable closed-loop electrophysiology procedures. These devices are currently used in fundamental neurophysiology research, and they are moving toward clinical viability for neural rehabilitation. State-of-the-art BCI experiments have often been performed using tethered (wired) setups in controlled laboratory settings. Wired tethers simplify power and data interfaces but restrict the duration and types of experiments that are possible, particularly for the study of sensorimotor pathways in freely behaving animals. To eliminate tethers, there is significant ongoing research to develop fully wireless BCIs having wireless uplink of broadband neural recordings and wireless recharging for long-duration deployment, but significant challenges persist. BCIs must deliver complex functionality while complying with tightly coupled constraints in size, weight, power, noise, and biocompatibility. In this article, we provide an overview of recent progress in wireless BCIs and a detailed presentation of two emerging technologies that are advancing the state of the art: ultralow-power wireless backscatter communication and adaptive inductive resonant (AIR) wireless power transfer (WPT).

IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC2021