Hitchhike: Riding Control on Preambles Xiaoyu Ji Xiaoyu Ji, Jiliang Wang, Mingyan Liu, Yubo Yan, Panlong Yang and Yunhao Liu INFOCOM, 2014, Toronto Hong Kong University of Science and Technology Tsinghua University University of Michigan PLA University of Science & Technology

Background 2 Control overhead – Dedicated control channel – Data plane acting as control plane E.g., RTS/CTS The drawback – Extra air time – Collision Hi, Bob Hi, Alice AliceBob I wanna send you sth. Sure DATA Got it!

The State-of-the-art Carrying control on data packets payload – Utilize the PHY SNR margin of payload – Achieve a side-channel 1 – Build a decoupled control plane 2 3 Ref 1: K. Wu, etc. Side channel: bits over interference, MobiCom’10 Ref 2: A. Cidon, etc. Flashback: decoupled lightweight wireless control, Sigcomm’12

The Problems High SNR of the payload – Redundancy with DSSS/OFDM – Hurting payload bits Control may be submerged Never supporting multiple users 4 ? Is there any better choice?

Motivation Preamble – What is it? – Repeated pattern (e.g., 32 ‘0’ in ) – Auto-correlation The SNR gap: detection vs. decoding 5 Preamble can be a good carrier for control messages.

The Basic Idea 6 Put controls onto preambles Encode control messages with waveforms Decode control messages by correlation

Design of Control Messages Control message design – Control Preamble – Controls have little cross-correlation x The length of controls x is a critical point! 7 Large control space Less impact on preamble

Cont’d PN sequences in x = 64 can be a good candidate 8

Detecting Control Messages Correlation detection multiple signals The normalized correlation 9 Detect multiple controls!

The Subtraction-Detection Algorithm Imperfection – Controls cannot be totally orthogonal to preamble – Cross-correlation among controls Sub and then detect 10

Some Other Concerns Frequency offset – Small in homogeneous protocols, e.g., 260 Hz and thus can be omitted – Large in heterogeneous protocols, can reach 200k Hz, profound affect 11

Implementation 12 Sender maps the controls on preamble of the packet on the air Receiver exploits the sub-detect algorithm

Evaluation Signal trace analysis Single user scenario – Detection accuracy of preamble – Detection accuracy of controls Multiple-user scenario WiFi interference scenario Comparison with payload based mechanism 13

Settings Modulation scheme: IEEE , center frequency GHz Control message: 64 chips Power  SNR range: [10-26] dBm Detection threshold:

Signal Traces 15 One control: power enhancement Three controls: larger power peak WiFi interference has significant influence (frequency offset)

Detection of Preamble 16 Control messages affect slightly (<1%) Wi-Fi signals can be more detrimental Frequency offset estimation is distorted!

Detection of Controls 17 Correlation increases with larger SNR difference Sub-detection algorithm increases the correlation by more than 0.1

Multi-user Scenario 18 Sender 1,2 and 3 transmit control messages while sender 4 not Three spikes for the three users, with clear difference in correlation

Hitchhike vs. Side-Channel 19 Side-channel: interfere single chip/symbol (k=1), interfere 2 consecutive chips (k=2) Side-channel suffers from false “error” bits Side-channel has larger loss rate, especially in 8~16 dBm region

Summary Carrying controls with data packets is a promising direction Data payload and control messages interfere with each other We propose Hitchhike, which put controls on preamble Design and implement Hitchhike in Comprehensive evaluation is conductd 20

Thank you ！ 21