Hi. I’m Mohamad Katanbaf, from University of Washington.
Today I present “Relacks"
Reliable Backscatter Communication in Indoor Environments.
My Co-authors in this work are
Dr.Vivek Jain from Bosch,
and Prof. Joshua Smith from University of Washington.
We are surrounded by devices capable of wireless communication.
For example, most appliances and many light bulbs are
equipped with embedded radios,
that connect them to a smart home networks.
These radios can cooperate to form backscatter networks and
delivere low-cost Wireless connectivity too many everyday objects.
This would enable many new applications such as monitoring
the temperature inside a pot or tracking human interactions
with different objects in living Spaces.
While previous works
have proved the "possibility" of backscatter communication
with BLE, WiFi, LoRa, or other wireless standards,
the question of reliability of these links is not investigated
thoroughly. In this work,
We try to answer these questions:
"How reliable are backscatter links?"
and "How can we improve their reliability?"
To better understand the reliability challenge of backscatter systems,
I have simulated the received signal strength for a BLE link
and a backscatter link using free-space path loss model.
The number on each contour shows the received signal
strength at the origin,
if the transmitter is placed on that contour.
We can see that for a BLE transmitter with 0 dBm output
power at 100 meters,
the received signal strength is -80 dbm.
A typical BLE receiver has a sensitivity better than -95dBm.
Yet, we know ble as a short range of standard with practical ranges
of 10 to 30 meters.
This margin contributes to the reliability of BLE radios, in
non-line-of-sight applications, and against multipath losses.
We draw the same contours for a bistatic backscatter link
with 20dBm carrier power, where the carrier source
and receiver are 10 meters away from each other.
Even with 100 times more power,
The received signal strength at most points is much closer to
the receiver sensitivity level.
We conclude that
the backscatter links
must operate on much narrower link budget margins.
To investigate the question of bistatic backscatter reliability,
we design a BLE backscatter prototype.
Here is the block diagram of the backscatter tag.
We use an FPGA to implement the BLE bit stream processing for
the backscatter tag.
We also add a low-power wake up
receiver to the tag.
For transceivers,
we use a commercial BLE chip and add a power amplifier to
amplify the output for backscatter communications.
Also, each transceiver supports two antennas.
Please check the paper for implementation details.
To better understand the effect of communication parameters
on the backscatter link, We design an experiment. The transceivers
are placed on the opposite side of a 60 square meters room.
And the tag is placed at 14 different points inside the room.
At each point, we sweep the communication parameters,
including the frequency transceivers antenna, and the
transceiver to wake-up the tag and initiate the communication.
We try each set of parameters 50 times.
Each Square on these heat maps shows the result
for one parameter set.
At each row,
we see the effect of frequency diversity.
If you look at points 5 or 10, you can clearly see that some
frequencies have a superior performance.
At point 4, we get good results from antenna pair (0,1), which
highlights the benefits of antenna diversity at the
transceiver units.
At Point 12, we get good results if we use the
transceiver1 to wake up the tag.
So, we can expect high reliability
if a select the communication parameters wisely.
To this end,
We must move from an open-loop system where the
transceivers follow predefined patterns, the a closed loop one.
A closed-loop system filters
high loss channels and focuses on the good channels.
Now, the question is how should we implement this?
Besides the three devices, we use a controller in our system
to analyze the collected information.
The controller initiates the operation.
We use an active packet between transceivers, to share the
frequency, antenna and wake up selection.
Then we start the backscatter communication phase.
Once all the results are collected, the controller
selects the parameters for next round.
We implement a scoring system to determine the best channels
for communication
and follow this flow chart to ensure FCC compatibility, and
explore other possible Good channels
for more details on the implementation,
please check the paper.
We repeat the success rate test at the same 14 points using three
different selection algorithms, random, proposed algorithm with
antenna diversity, and without antenna diversity, for both BLE
coded and uncoded PHYs.
Our evaluation shows that following
the proposed algorithm with antenna diversity achieves 81%
average success rate over all Points, which is 2.7
times improvement compared to random selection.
For more evaluations, including antenna orientation effects,
environment variation effects, or long-time evaluation,
as well as a sample application, please refer to the paper.
So in summary, we design and implement a bistatic
backscatter system and use it to investigate the effect of
frequency antenna and wake up diversity on the reliability of
backscatter links.
We propose a closed-loop configuration selection
algorithm and show that it significantly improves the
backscatter systems reliability.
And finally, we evaluate our system in many different static
and dynamic environments and show one potential application.
Thank you.