Hitchhike: Riding Control on Preambles

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

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Background• 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

Alice Bob

I wanna send you sth.

Sure

DATA

Got it!

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The State-of-the-art• Carrying control on data packets– Utilize the PHY SNR margin of payload– Achieve a side-channel1

– Build a decoupled control plane2

Ref 1: K. Wu, etc. Side channel: bits over interference, MobiCom’10Ref 2: A. Cidon, etc. Flashback: decoupled lightweight wireless control, Sigcomm’12

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The Problems• High SNR of the payload– Redundancy with DSSS/OFDM– Hurting payload bits

• Control may be submerged• Never supporting multiple users

?Is there any better choice?

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Motivation• Preamble – What is it?– Repeated pattern (e.g., 32 ‘0’ in 802.15.4)– Auto-correlation

• The SNR gap: detection vs. decoding

Preamble can be a good carrier for

control messages.

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The Basic Idea

• Put controls onto preambles• Encode control messages with waveforms• Decode control messages by correlation

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Design of Control Messages• Control message design– Control Preamble– Controls have little cross-correlation

• The length of controls x is a critical point!

Large control space

Less impact on preamble

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Cont’d• PN sequences in 802.15.4• x = 64 can be a good candidate

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Detecting Control Messages• Correlation detection multiple signals

• The normalized correlation

1

*

1

2

1

[ ]

, ,

, ,

N

i ii

L

k

L

i i ik

y n H x n n

C s y q s k y k q

C s y q H s k

1

, ,, , L

k

C s y qN s y q

s k

Detect multiple controls!

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The Subtraction-Detection Algorithm

• Imperfection– Controls cannot be totally orthogonal to preamble – Cross-correlation among controls

• Sub and then detect

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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

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Implementation

• Sender maps the controls on preamble of the packet on the air

• Receiver exploits the sub-detect algorithm

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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

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Settings• Modulation scheme: IEEE 802.15.4, center

frequency 2.432 GHz• Control message: 64 chips• Power SNR range: [10-26] dBm• Detection threshold: 0.3

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Signal Traces

• One control: power enhancement• Three controls: larger power peak• WiFi interference has significant influence

(frequency offset)

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Detection of Preamble

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

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Detection of Controls

• Correlation increases with larger SNR difference• Sub-detection algorithm increases the correlation

by more than 0.1

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Multi-user Scenario

• Sender 1,2 and 3 transmit control messages while sender 4 not

• Three spikes for the three users, with clear difference in correlation

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Hitchhike vs. Side-Channel

• 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

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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 802.15.4• Comprehensive evaluation is conductd

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Thank you！