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AirShare
Distributed Coherent
Transmission Made Seamless
Omid Abari
Hariharan Rahul, Dina Katabi and Mondira Pant
Wireless networks are getting
denser and denser
Wi-Fi connections
Factories
Smart Homes
But, wireless spectrum is limited
Distributed
MIMO
Distributed
Modulation
Distributed cooperative protocols
Distributed
lattice coding
Distributed
compressed
sensing
Noisy network
coding
Protocols assume:
Wireless nodes transmit at exactly the same frequency
higher throughput & higher efficiency
Reality:
Nodes have small offsets in their frequencies
Carrier Frequency Offset (CFO)
Multiple wireless nodes transmit
concurrently
Q
Ideal
Independent wireless nodes have
slightly different carrier frequencies
Reality
Q
Transmissions from different nodes
rotate relative to each other
I
I
Root Cause of CFO is Clock
Each node uses its own clock as a reference
Wireless Node
Radio
Clock
Reference
Signal
10 MHz
Carrier Signal
2.4 GHz
Root Cause of CFO is Clock
Each node uses its own clock as a reference
Wireless Node
Clock
10 MHz +1Hz
Radio
Carrier Signal
Wireless Node
2.4 GHz+240Hz
Carrier Signal
2.4 GHz-720Hz
Clock
Radio
10 MHz -3Hz
Crystals have slightly different frequencies
Different nodes have offset in their carrier frequency (CFO)
which varies over time
How can we eliminate CFO?
Naïve Solution
Connect all nodes to a shared reference clock
Wireless Node
Wireless Node
Clock
Radio Node
Wireless
Radio
Wireless Node
Radio
Wireless Node
Radio
Wireless Node
Radio
Carrier Signal
Radio
Defeats the notion of a wireless network
Our Idea
Transmit a reference over-the-air
Wireless Node
Wireless Node
Radio
Radio
Clock
Wireless Node
Wireless Node
Radio
Radio
AirShare
transmits the reference clock over the air
&
eliminates CFO
Protocol independent
Supports mobility
Cheap and Low-Power
AirShare Architecture
Emitter
Clock
Emitter
Recipient
Challenges
Emitter
Clock
Emitter
Recipient
Challenges
Emitter
Clock
Emitter
How can emitter transmit a clock?
Recipient
How can we build a cheap and
low-power recipient?
Emitter
Clock
Emitter
How can emitter transmit a clock?
Problem: Reference clocks are typically 10-40 MHz
- FCC forbids transmitting such a low-frequency signal
- Requires large antennas
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
Clock
Emitter
Emitter
Recipient
fref
f1
sin 2 
fref = 10 MHz
f1
f2
f2
sin 21  + sin 22 
f2 , f1 = any frequency
fref
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
Clock
Emitter
Emitter
fref
f1
sin 2 
f2
sin 21  + sin 22 
fref = 10 MHz
f2 , f1 = any frequency
?
Recipient
f1
f2
fref
?
Recipient
f1
fref
f2
receives the signal and multiplies the signal by itself
f1
f1
f2
sin 21  + sin 22 
f2
f1
f2
× sin 21  + sin 22 
Using trigonometric identities:
1
1
(α) ×   = (α + ) +  α − 
2
2
AirShare transmits the reference clock
without
DC violating
f2-f1 = 10 MHzFCC2fregulations
1 f1+f2 2f2
Challenges
Emitter
Clock
Emitter
How can emitter transmit a clock?
Transmit a Differential-reference
Recipient
How can we build a cheap and
low-power recipient?
Use Passive Architecture
f1
f1
f1
f2
f2
f2
2f1 f1+f2 2f2
DC f2-f1
Passive
Passive
BandPass
Filter
LNA
Mixer
Reference
Signal
Wireless
Node
Simple, passive, off-the-shelf components
 Cheap and Low-power
Our AirShare Prototype
We built a prototype of recipient in
a custom designed PCB
• Low power consumption:
• < 10% for wireless sensors
• < 0.1% for Wi-Fi APs
• Low cost:
• Off-the-shelf components
• Costs only a few dollars
Antenna
Evaluation
• Implemented AirShare using off-the-shelf components
• Evaluated AirShare in an indoor testbed using USRPs
• Evaluated two applications:
– Distributed Rate Adaptation
– Distributed MIMO
Synchronization Accuracy
Measured CFO between nodes at 2.4 GHz
– 500 Experiments
– Different nodes and locations
1
AirClock
Today Nodes
0.8
0.6
CDF
2-3 orders of magnitude
0.4
0.2
0
0.01
Ideal Zone
[sigcomm’12]
0.1
1
10
CFO(Hz)
100
1000
10000
AirShare reduces the CFO by multiple orders of
magnitude
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
1
2
3
N
I
Throughput Gain
• Data throughput for 6 sensors
TDMA
This Work
Throughput (Kbps)
120
80
40
0
5-12
12-18
16-24
SNR (dB)
20-25
Throughput gains of 1.6-3× over today sensors
for 6 sensors
Application 2: Distributed MIMO
1
2
N
Ethernet
1
2
N
Multiple APs transmit to multiple clients concurrently
Network throughput scales with the number of APs
Throughput Gain
Distributed MIMO network including 5 clients and 5 APs
Throughput Gain
5
4
3
2
1
2
3
44
Receivers
of Receivers
Number
Number of
55
Throughput gain of 4.4× over traditional 802.11
for 5 transmitters
Related Work
• 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
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.
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