Thursday, August 25, 2016

SIGCOMM 2016 - Session 8 - Wireless - Paper 2: "Enabling Practical Backscatter Communication for On-body Sensors".

Presenter: Pengyu Zhang
Authors: Pengyu Zhang, Mohammad Rostami, Pan Hu, Deepak Ganesan

Summary
The driving question of this paper is the same as the last one, can these implanted devices communicate directly with mobile devices such as smartphones, watches and tablets? Popular wireless radios, e.g., WiFi, Bluetooth, etc., consumes too much energy for lower power sensors. Therefore, this paper also focuses on applying backscatter as the go-to technique, but with a different approach to tackle its practicality.

In particular, the authors focuses on the fact that in order to extract the weak backscattered signal, the system needs to deal with self interference from the wireless carrier (WiFi or Bluetooth) without
relying on built-in capability to cancel or reject the carrier interference. The proposed solution is called FS-Backscatter, which was motivated by a simple but effective intuition: if a backscatter tag can shift an incident WiFi or Bluetooth carrier to a clean WiFi or Bluetooth band, then the receiver can see a clean, carrier-interference free backscattered signal in the shifted band.

In order to achieve a high data rate, the characteristics of packet-level decoder and bit-level decoder are studied, and a low-power ring oscillator-based clock generator is designed for the FSBackscatter tag which operates at tens of micro-watts but also has temperature-induced frequency variations. The authors implement a prototype of FS-Backscatter and demonstrate that a 20MHz frequency shift is enough for enabling an FS-Backscatter tag to communicate with commercial WiFi and Bluetooth radios. It is also shown from evaluation that an FS-Backscatter tag operates at a power budget of 45 microwatt through the use of a ring oscillator based clock design, and is robust to frequency variations induced by environmental changes.


Q&A:
Q1. What are the differences between this work and the Interscatter work presented just now?
A: We share the same vision in that backscatter is a promising technique in supporting on-body/inbody sensors, and we take different approaches to provide practical backscatter solutions. It is expected that certain designs in Interscatter can be applied to improve the performance of FS-Backscatter.

Q2. What is the reason to shift the frequency by 20MHz?
A: We shift the singal by 20MHz because it is the bandwidth of a channel in WiFi. As long as the carreir is shifted to a clean channel, the receiver can see a clean, carrier-interference free backscattered signal in the shifted band.