Wi-Fi BackscatterWi-Fi Backscatter

By Shivangi Gupta

Outline

Introduction

Background

Components

Working Working

Conclusion

References

Introduction

Wi-Fi Backscatter is used for connecting “battery free or RF powered device” with the Internet.

For ex-

1. Sensors could be embedded ,to help monitor and 1. Sensors could be embedded ,to help monitor and track everything for the safety of bridges .

2. Health monitoring and communication updates on our smart watches.

Background

Ambient-Backscatttering

- Enables two low powered devices to communicate using “ambient(surrounding) RF signals” as the only source of power. source of power.

- As it only uses existing signals, it does not require any new infrastructure.

How does it works?

Suppose there are 2 battery less devices with interaction capabilities(antenna). They want to communicate

Alice wants to send some packet to Bob.

Alice has a antenna ,which creates electrical energy from radio signals(RF signals) .

It uses this energy to backscatter towards Bob . While backscattering, it uses Switch that sets the state of backscattering, it uses Switch that sets the state of device .

Reflective-1

Absorbed -0

• Bob will receive the signal and decodes it.

Example

• Suppose a person left his keys on the couch

Couch is being attached with small ambient backscatter tag.

Such tags are embedded with sensing capabilities.

As soon as we left the couch , it will send a message to our mobile that “ You lost your keys” using ambient RF signals.

Example

Ambient backscattering

Components

Wi-Fi Backscatter has three main components:

1. Wi-Fi device .

2. Wi-Fi helper (Router or other access point)

3. RF-powered Wi-Fi Backscatter tag.

The device and helper can be any Wi-Fi device, including routers and mobile devices.

Wi-Fi backscatter tag is embedded in the low powered de vice.

Components

Wi-Fi AP/Wi-

Wi-Fi Device/Wi-Fi Reader

Wi-Fi AP/Wi-Fi Helper

Wi-Fi Backscatter

Tag

Wi-Fi Backscatter tag

Wi-Fi Backscatter follows a request-response model.

Wi-Fi Backscatter’s communication has two main components:

-An uplink from the Wi-Fi Backscatter tag to the -An uplink from the Wi-Fi Backscatter tag to the

Wi-Fi device. - A downlink from the Wi-Fi device to the Wi-Fi

Backscatter tag.

Uplink

• Wi-Fi Backscatter tag communicates with a Wi-Fi device by modulating its Wi-Fi channel.

• Specifically, it conveys a ‘1’ and a ‘0’ bit by either reflecting or absorbing the Wi-Fi packets received by the reflecting or absorbing the Wi-Fi packets received by the mobile device.

• The reflected signals changes the per-packet CSI andRSSI measurements, which the mobile device uses todecode messages from the Wi-Fi Backscatter tag.

UPLINK

1.Modulating the Wi-Fi signals

The Wi-Fi Backscatter tag conveys information by modulating the Wi-Fi channel.

To do this, it uses a “low power switch” that allows the antenna impedance to be modulated by a transmitted bitimpedance to be modulated by a transmitted bitstream.

When we do modulation then CSI(Channel state information) and RSSI(Received signal strength indicator) changes.

“The antenna’s impedance affects the amount of signal that is reflected by the tag.”

By modulating this impedance, the tag can convey ‘1’ and ‘0’ bits.

Antenna

Wi-Fi backscatter uses a special antenna which has both impedance modulation as well as Wi-Fi energy harvesting capability.

CSI/RSSI Modulation

Backscatter tag

2. Decoding at Wi-Fi reader

Decoding is done in 3 steps

Signal Conditioning. Signal Conditioning.

Exploiting Wi-Fi Frequency/Spatial Diversity

Decoding bits from the Wi-Fi Backscatter tag.

Signal conditioning-

The goal of this step is two-fold:

1) Remove the natural temporal variations in the channel measurements due to noise & mobility in the

environment.

2) normalize the channel measurements by mapping to -1 and +1 values.

o -1

1 +1

Exploiting Wi-Fi Frequency Diversity

Wi-Fi transmission span a bandwidth of 20MHz.At such a wide bandwidth, signal experience “selective frequency fading”.

Now in order to get best reflected signal following steps are Now in order to get best reflected signal following steps are followed

1. Identify the good Wi-Fi subchannel and antennas-To do this Wi-Fi reader uses correlation method. Everytime, the backscatter tag adds one preamble at the message beginning. When a transmission arrives ,the Wi-Fi reader sorts sub-channel based on correlation value, then it will select TOP TEN channels.

2. Combining information across the good Wi-Fi sub channels Sub-channel with low noise variance are given higher

weight. Sub-channel with higher noise are given lower weights.

CSIweighted = ∑ CSIi / (Ω i) 2CSIweighted = ∑ CSIi / (Ω i)

CSIi- It is the normalized CSI computed on the ith good Wi-Fi sub-channel.

(Ωi )2 = the noise variance in the ith sub-channel.CSI - is the Channel state information which tells about scattering , fading , power decay effect over signal.

3.Decoding bits from Wi-Fi backscatter

If the CSI weighted computed is greater then 1,then the bit is interpreted as 1.

Otherwise interpreted as 0. Otherwise interpreted as 0.

Downlink

It is from Wi-Fi device to Backscatter tag.

The challenges are

-Wi-Fi device can only send Wi-Fi packets.

-Wi-Fi backscatter cannot decode Wi-Fi transmissions.

• The mobile device sends a pattern of short Wi-Fi packets — the presence (absence) of the short Wi-Fi packet encodes the ‘1’ (‘0’) bit.

• The Wi-Fi Backscatter tag decodes this information by using our low-power energy detector to differentiate between the presence and absence of Wi-Fi packets.

Downlink

• For solving these challenges we have to follow 2 steps

1. Encoding at Wi-Fi reader.

2. Designing backscatter tag which can decode Wi-fitransmissions.

1.Encoding at Wi-Fi device

Encoding is done by sending or not sending the packet.

Let's say the device want to transmit 101000011, and so on.

The Wi-Fi device will transmit The Wi-Fi device will transmit

packet 1

no packet/silence 0

packet 1

Problem

Since we are encoding a “0” as silence, other Wi-Fi devices around us will detect the medium as unoccupied and can transmit in the silence spaces, completely messing up our transmission.

To prevent this we leverage a feature of the 802.11 protocol called CTS (clear to send), is used to reserve the channel.

CTS-to-Self 0 0 0001 1 11

Wi-Fi Wi-Fi

Device

Wi-Fi Backscatter

Tag

•So before transmitting data, the Wi-Fi devices sends a CTS-to-self packet to reserve the channel. •Other Wi-Fi Devices will respect the 802.11 protocol and abstain from transmitting in the silence periods.

2. Designing Backscatter tag

Taking benefit from one of the feature of Wi-Fi signal.

Wi-Fi uses OFDM.

OFDM uses multiple carrier wave to modulate one signal.signal.

OFDM has high Peak to Average power ratio, which means the average energy in the Wi-Fi signal is small, with occasional peaks spread out during the transmission.

Example

You can see that the OFDM signal spends lots of its time with very low amplitudes and occasionally has one of these vary large peaks. This means that OFDM has a very high peak-to-average power ratio, and we can use this to our advantage.

Circuit of Backscatter tag

EnvelopeDetector

Antenna

Peak finder Set threshold Comparator

Envelope Detector- It remove the carrier frequency of the Wi-Fi transmissions. Peak Finder Circuit- It finds the peaks in OFDM signals.

Set threshold- The capacitor computes a threshold using this peak value.

Comparator- It takes 2 input , the threshold value and the received signal

It gives output as “1” bit, whenever the received signal is greater than the threshold value and “0” bit otherwise.

This whole packet detection circuit requires only few

µW of power.