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 2𝜋𝑓𝑟𝑒𝑓 𝑡 fref = 10 MHz f1 f2 f2 sin 2𝜋𝑓1 𝑡 + sin 2𝜋𝑓2 𝑡 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 2𝜋𝑓𝑟𝑒𝑓 𝑡 f2 sin 2𝜋𝑓1 𝑡 + sin 2𝜋𝑓2 𝑡 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 2𝜋𝑓1 𝑡 + sin 2𝜋𝑓2 𝑡 f2 f1 f2 × sin 2𝜋𝑓1 𝑡 + sin 2𝜋𝑓2 𝑡 Using trigonometric identities: 1 1 𝑠𝑖𝑛(α) × 𝑠𝑖𝑛 𝛽 = 𝑐𝑜𝑠(α + 𝛽) + 𝑐𝑜𝑠 α − 𝛽 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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