Enabling Low-power Communication, Sensing, and Computation on Internet-of-Things Pengyu Zhang Stanford University 1
Massive number of IoT devices are emerging Internet of Things Smartphone, Tablet, PC 9 Number of Devices (Billions) IoT surpasses 6.75 4.5 2.25 0 2004 2005 2006 2007 2008 2009 2010 2011 2012 2013 2014 2015 2016 2017 2018 Year Vision — ubiquitous IoT deployment in the next five years! Source: strategy analytics October 2014 2
This vision is limited by problems arise in three domains Data Analytics Wireless Energy-efficient Connectivity computing 3
How to provide low power wireless connectivity? Data Analytics Backscatter with Commodity Radios [SIGCOMM 16a, SenSys 16 * , CoNext 17a] Wireless Energy-efficient Connectivity computing * best paper awards or nomination 4
Wireless communication consumes lots of power 1,000,000 wireless radios 100,000 Power (uW) 10,000 5x 10 2 x 1,000 10 4 x 100 10 1 Accel MCU SRAM BLE Bluetooth ZigBee WiFi ideal Wireless communication consumes orders of magnitude higher power compared to computation, storage, and sensing Source: www.ti.com 5
Why a wireless radio consumes lots of power? baseband AMP baseband analog circuits processing Can we TX data with minimum baseband processing and RF circuits? 6
Lessons from Radio Frequency Identification (RFID) RFID reader Backscatter tag Carrier Wave TX baseband AMP logic RF harvester LNA RX baseband Reflected Signal An RFID tag communicates its data at µWatts and does not need battery 7
Lessons from Radio Frequency Identification (RFID) RFID reader Backscatter tag Carrier Wave TX baseband AMP logic RF harvester LNA RX baseband Reflected Signal The RFID reader infrastructure is NOT widely deployed 8
Can we leverage commodity radios (WiFi and Bluetooth) to provide low-power wireless connectivity? 9
Contribution— backscatter with WiFi, Bluetooth and ZigBee radios [SIGCOMM 2016, SenSys 2016*, CoNEXT 2017] packet packet packet packet packet packet packet packet WiFi/Bluetooth/ZigBee WiFi/Bluetooth/ZigBee transmitter receiver We enable backscatter communication with commodity radios, such as WiFi, Bluetooth and ZigBee * best paper awards nomination 10
Demo: backscattering ECG sensing data with WiFi radios Backscatter tag WiFi transmitter WiFi receiver 11
How to inject and decode tag data on top of unknown WiFi traffic? 12
Limited number of codewords in a codebook The codewords used by 802.11b WiFi codeword signal +1+1+1-1-1-1+1-1-1+1-1 0 packets 0101100… 1 -1-1-1+1+1+1-1+1+1-1+1 WiFi transmitter WiFi receiver codebook WiFi uses a finite set of codewords to encode data 0 and data 1 13
Key observation — we can transform codewords used by WiFi We can transform a codeword i to another codeword j by performing simple operations codeword -1-1-1+1+1+1-1+1+1-1+1 X -1 +1+1+1-1-1-1+1-1-1+1-1 +1+1+1-1-1-1+1-1-1+1-1 = X -1 -1-1-1+1+1+1-1+1+1-1+1 -1-1-1+1+1+1-1+1+1-1+1 = +1+1+1-1-1-1+1-1-1+1-1 codebook The codewords shown are used by1Mbps 802.11b WiFi 14
How does a tag backscatter its own information? codeword i codeword i packet packet tag data 0 WiFi transmitter WiFi receiver 15
How does a tag backscatter its own information? codeword j codeword i packet packet tag data 1 WiFi transmitter WiFi receiver 16
Codeword translation — embed tag information on WiFi packets backscatter codeword tag data = = tag data XOR WiFi codeword backscatter codeword XOR WiFi codeword codeword j codeword i packet packet Input signal tag data 0 tag data 1 WiFi transmitter WiFi receiver Codeword i Codeword i Codeword j Codeword j Codeword j Codeword i 17
How to do codeword translation at low power? codeword j codeword i packet packet codeword i = codeword j X -1 WiFi transmitter WiFi receiver codeword j = codeword i X -1 How does tag introduce the X -1 at low power? 18
How to do codeword translation at low power? codeword j codeword i packet packet codeword i = codeword j X -1 WiFi transmitter WiFi receiver codeword j = codeword i X -1 X -1 means a 180 o phase change on the reflected signal 19
How to change the phase at low power? codeword j codeword i packet packet WiFi transmitter WiFi receiver phase shifter — 400 µWatts [1] 20 [1] Peregrine Semiconductor PE44820
How to change the phase at low power? codeword j codeword i packet packet WiFi transmitter WiFi receiver delay 180 o phase difference delay — 1 µWatts for a 10ns delay phase shifter — 400 µWatts [1] 21 [1] Peregrine Semiconductor PE44820
Codeword translation can be done in three dimensions S ( t ) = A e 2 π f t + θ A wireless signal can be presented by: Frequency modification Phase modification S ( t ) = Ae 2 π ft + θ S ( t ) = Ae 2 π ft + θ Amplitude modification We can embed tag data on 802.11 b/g/n WiFi, Bluetooth and ZigBee 22
What is the impact of scrambler and interleaving? 23
What is the impact of scrambler on backscattering 802.11b? x[n] y[n] z[n] packets scrambler descrambler packets 802.11b transmitter 802.11b receiver y[n] y[n] Z -1 Z -1 Z -1 Z -1 Z -1 Z -1 Z -1 x[n] Z -1 Z -1 Z -1 Z -1 Z -1 Z -1 Z -1 z[n] z[n] = y[n] + y[n-4] + y[n-7] y[n] = x[n] + y[n-4] + y[n-7] = x[n] 24
What is the impact of scrambler on backscattering 802.11b? tag[n] x[n] y[n] y*[n] z*[n] tag packets scrambler descrambler packets 802.11b transmitter 802.11b receiver y*[n] = y[n] + tag[n] y[n] y*[n] Z -1 Z -1 Z -1 Z -1 Z -1 Z -1 Z -1 x[n] Z -1 Z -1 Z -1 Z -1 Z -1 Z -1 Z -1 z*[n] z*[n] = y*[n] + y*[n-4] + y*[n-7] y[n] = x[n] + y[n-4] + y[n-7] = x[n] + tag[n] + tag[n-4] + tag[n-7] 25
What is the impact of scrambler on backscattering 802.11b? tag[n] x[n] y[n] y*[n] z*[n] tag packets scrambler descrambler packets 802.11b transmitter 802.11b receiver y*[n] = y[n] + tag[n] y[n] y*[n] Z -1 Z -1 Z -1 Z -1 Z -1 Z -1 Z -1 x[n] Z -1 Z -1 Z -1 Z -1 Z -1 Z -1 Z -1 z*[n] z*[n] = y*[n] + y*[n-4] + y*[n-7] y[n] = x[n] + y[n-4] + y[n-7] = x[n] + tag[n] + tag[n-4] + tag[n-7] Feed received data to a scrambler to decode the tag data 26
What is the impact of interleaving on backscattering 802.11g/n? 802.11g/n block diagram z[n] x[n] y[n] deinterleaving data data interleaving 802.11g/n transmitter 802.11g/n receiver x[n] = b 0 b 1 b 2 … b n y[n] = b n b 10 b 1 b 5 … b 2 b 36 z[n] = x[n] = b 0 b 1 b 2 … b n-1 b n 27
What is the impact of interleaving on backscattering 802.11g/n? 802.11g/n block diagram z*[n] x[n] y[n] y*[n] deinterleaving data data interleaving tag tag[n] = 101010….10 802.11g/n transmitter 802.11g/n receiver x[n] = b 0 b 1 b 2 … b n y[n] = b n b 10 b 1 b 5 … b 2 b 36 z*[n] = b 0 b 1 b 2 … b n-1 b n != x[n] xor tag[n] y*[n] = y[n] + tag[n] = b 0 b 1 b 2 … b n-1 b n = b n b 10 b 1 b 5 … b 2 b 36 Interleaving is done on data bits of one OFDM symbol 28
What is the impact of interleaving on backscattering 802.11g/n? 802.11g/n block diagram z*[n] x[n] y[n] y*[n] deinterleaving data data interleaving tag data 0 802.11g/n transmitter 802.11g/n receiver x[n] = b 0 b 1 b 2 … b n y[n] = b n b 10 b 1 b 5 … b 2 b 36 z*[n] = b 0 b 1 b 2 … b n One OFDM y*[n] = b n b 10 b 1 b 5 … b 2 b 36 29
What is the impact of interleaving on backscattering 802.11g/n? 802.11g/n block diagram z*[n] x[n] y[n] y*[n] deinterleaving data data interleaving tag data 1 802.11g/n transmitter 802.11g/n receiver x[n] = b 0 b 1 b 2 … b n y[n] = b n b 10 b 1 b 5 … b 2 b 36 z*[n] = b 0 b 1 b 2 … b n One OFDM y*[n] = b n b 10 b 1 b 5 … b 2 b 36 Decode the tag data by checking whether the bits in an OFDM symbol is modified 30
How to do spectrum efficient data injection? 31
Interference from the WiFi transmitter codeword j codeword i packet packet WiFi transmitter WiFi receiver Signals from the WiFi transmitter is much louder than the reflected signal 32
Frequency-shifting the backscattered signal B(t) = S(t) x Tag(t) S(t) packet packet Tag(t) WiFi transmitter WiFi receiver WiFi signal Power backscatter Frequency Enabling Practical Backscatter Communication for On-body Sensors Pengyu Zhang, Mohammad Rostami, Pan Hu, Deepak Ganesan 33 SIGCOMM 2016
Inefficient spectrum usage — double-side band backscatter B(t) = S(t) x Tag(t) S(t) packet packet Tag(t) WiFi transmitter WiFi receiver WiFi signal Power backscatter Interference for other WiFi Frequency 34
Single-side band backscatter WiFi signal backscatter + Signal 1 Signal 2 = 35
Software and hardware prototype • Open sourced software and hardware backscatter radio board https://github.com/pengyuzhang/HitchHike • Used for teaching Stanford EE107 Networked Embedded Systems image sensing board 36
Experiment Setup WiFi RX Intel 5300 WiFi transmitter Backscatter tag Apple Macbook Pro laptop 37
Throughput across distances in line-of-sight deployment 400 15dBm 30dBm Throughput (kbps) 300 200 100 0 0 5 9 14 19 23 28 33 37 42 47 51 56 Tag-to-laptop distance (m) 38
Throughput across distances in non-line-of-sight deployment 400 Throughput (kbps) 300 200 100 0 0 3 6 9 11 14 17 20 23 26 28 31 34 Tag-to-laptop distance (m) 39
Recommend
More recommend
