1. Omid Abari Hariharan Rahul, Dina Katabi and Mondira Pant
AirShare
Distributed Coherent Transmission Made Seamless
Omid Abari
Hariharan Rahul, Dina Katabi and Mondira Pant

2. Wireless networks are getting denser and denser
Wi-Fi connections
Smart Homes
Factories

3. But, wireless spectrum is limited
Distributed MIMO
Distributed Modulation
Distributed cooperative protocols
Distributed
lattice coding
Noisy network coding
Distributed compressed sensing
Protocols assume:
Wireless nodes transmit at exactly the same frequency
higher throughput & higher efficiency
Reality:
Nodes have small offsets in their frequencies

4. Carrier Frequency Offset (CFO)
Ideal
Multiple wireless nodes transmit concurrently
Independent wireless nodes have slightly different carrier frequencies
Reality
Transmissions from different nodes rotate relative to each other

5. Root Cause of CFO is Clock
Each node uses its own clock as a reference
Carrier Signal
2.4 GHz
10 MHz

6. Root Cause of CFO is Clock
Each node uses its own clock as a reference
Carrier Signal
Carrier Signal
2.4 GHz+240Hz
2.4 GHz-720Hz
10 MHz +1Hz
10 MHz -3Hz
Crystals have slightly different frequencies
Different nodes have offset in their carrier frequency (CFO)
which varies over time

7. How can we eliminate CFO?

8. Naïve Solution Connect all nodes to a shared reference clock
Carrier Signal
Defeats the notion of a wireless network

9. Transmit a reference over-the-air
Our Idea
Transmit a reference over-the-air

10. transmits the reference clock over the air
AirShare
transmits the reference clock over the air
&
eliminates CFO
Protocol independent
Supports mobility
Cheap and Low-Power

11. AirShare Architecture
Emitter
Recipient

12. Challenges
Emitter
Recipient

13. Challenges How can emitter transmit a clock?
Recipient
How can we build a cheap and
low-power recipient?

14. How can emitter transmit a clock?
Problem: Reference clocks are typically MHz
- FCC forbids transmitting such a low-frequency signal
- Requires large antennas

15. Transmit a Differential-reference
Instead of transmitting a signal at the clock frequency (10 MHz)
Transmits two signals separated by the clock frequency
10 MHz
sin 2𝜋 𝑓 𝑟𝑒𝑓 𝑡
sin 2𝜋 𝑓 1 𝑡 + sin 2𝜋 𝑓 2 𝑡
Emitter
fref = 10 MHz
f2 , f1 = any frequency
Recipient

16. Transmit a Differential-reference
Instead of transmitting a signal at the clock frequency (10 MHz)
Transmits two signals separated by the clock frequency
10 MHz
sin 2𝜋 𝑓 𝑟𝑒𝑓 𝑡
sin 2𝜋 𝑓 1 𝑡 + sin 2𝜋 𝑓 2 𝑡
Emitter
fref = 10 MHz
f2 , f1 = any frequency
Recipient ?

17. ? AirShare transmits the reference clock
Recipient ?
receives the signal and multiplies the signal by itself
sin 2𝜋 𝑓 1 𝑡 + sin 2𝜋 𝑓 2 𝑡 × sin 2𝜋 𝑓 1 𝑡 + sin 2𝜋 𝑓 2 𝑡
Using trigonometric identities:
𝑠𝑖𝑛 (α) × 𝑠𝑖𝑛 𝛽 = 𝑐𝑜𝑠 (α+𝛽) 𝑐𝑜𝑠 α−𝛽
AirShare transmits the reference clock
without violating FCC regulations
= 10 MHz
Extract the 10 MHz reference-clock without
transmitting a 10MHz signal

18. Challenges How can emitter transmit a clock?
Transmit a Differential-reference
Recipient
How can we build a cheap and
low-power recipient?

19. Use Passive Architecture
Simple, passive, off-the-shelf components
 Cheap and Low-power

20. Our AirShare Prototype
We built a prototype of recipient in
a custom designed PCB
Antenna
Low power consumption:
< 10% for wireless sensors
< 0.1% for Wi-Fi APs
Low cost:
Off-the-shelf components
Costs only a few dollars

21. Evaluation Implemented AirShare using off-the-shelf components
Evaluated AirShare in an indoor testbed using USRPs
Evaluated two applications:
Distributed Rate Adaptation
Distributed MIMO

22. Synchronization Accuracy
Measured CFO between nodes at 2.4 GHz
500 Experiments
Different nodes and locations
2-3 orders of magnitude
Ideal Zone
[sigcomm’12]
AirShare reduces the CFO by multiple orders of magnitude

23. Distributed Rate Adaptation
Application 1:
Distributed Rate Adaptation
Ideally: Better channel quality  Higher throughput
Problem: Sensors support only single low data rate
Solution: Distributed Rate Adaptation
multiple sensors transmit together  higher throughput Q I

24. Throughput gains of 1.6-3× over today sensors
Data throughput for 6 sensors
Throughput gains of 1.6-3× over today sensors
for 6 sensors

25. Application 2: Distributed MIMO
Multiple APs transmit to multiple clients concurrently
Network throughput scales with the number of APs

26. Throughput gain of 4.4× over traditional 802.11
Distributed MIMO network including 5 clients and 5 APs
Throughput gain of 4.4× over traditional
for 5 transmitters

27. AirShare Related Work Supports mobility Protocol independent
Using wires or power-lines to distribute a shared clock [SenSys’09, SIGCOMM’14]
Designing algorithms to estimate and correct for CFO [SIGCOMM’12, ToN’2013]
Equip each node with a GPS disciplined oscillator [Trimble, Jackson Labs]
AirShare
Supports mobility
Protocol independent
Cheap, Low-Power

28. Conclusion
Described AirShare, a simple method to eliminate CFO in wireless nodes
Provides large throughput gains
Enables many new applications such as distributed MIMO, distributed modulation, etc.