Omid Abari Hariharan Rahul, Dina Katabi and Mondira Pant 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 Smart Homes Factories

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

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

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

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

How can we eliminate CFO?

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

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

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

AirShare Architecture Emitter Recipient

Challenges Emitter Recipient

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

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 sin 2𝜋 𝑓 𝑟𝑒𝑓 𝑡 sin 2𝜋 𝑓 1 𝑡 + sin 2𝜋 𝑓 2 𝑡 Emitter fref = 10 MHz f2 , f1 = any frequency Recipient

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 ?

? 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 = 1 2 𝑐𝑜𝑠 (α+𝛽) + 1 2 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

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

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

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

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 2-3 orders of magnitude Ideal Zone [sigcomm’12] AirShare reduces the CFO by multiple orders of magnitude

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

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

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

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 802.11 for 5 transmitters

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

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.