Wireless@VT Student Seminars: Kickoff

Wireless@VT Seminars

Walid SaadWireless@VT, Durham 447

walids@vt.edu

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Wireless@VT Seminars Fall Logistics

Weekly meetings in SEB 135 SEB 125 used 10/24, 11/07, and 12/05 Contacts: SaiDhiraj Amuru, Harpreet Dhillon, Walid Saad

For the students, by the students So what am I doing here? Due to short notice, no student has stepped up

Who will participate? The goal is to have most speakers be students Faculty will be attending sporadically

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Wireless@VT Seminars What are the goals of these seminars?

Sleeping hour for grad students? A form of torture? “Debate group”: engaging

discussions between groups “Integrating” new students

by giving them insights on how to identify and formulate a research problem, how to choose a technique, etc.

Practice prelims, defense, etc. Free pizza (outside)!

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Additional Goals Have talks that discuss new directions in wireless or other

areas we’re engaged in… Talks/tutorials that discuss how to use certain tools For example: how to program a USRP? How to use NS3

for wireless simulations? We will solicit such questions from the students Distinguished Speakers from inside and outside VT

We will strive to get at least one distinguished speaker per year from outside VT

Build a culture of research in our students community => Sustainable only via active students’ participation

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Best Practices The purpose of these meetings is not to overly defend

your research …but rather get feedback from fellow students and explain to

them what you’re doing

Strongly suggest to explain a basic problem you’re working on, while clarifying methodology, assumptions made, etc.

You know this but…. use more figures and illustrations to clarify your ideas However, we need sufficient technical details in these

seminars so that new students can learn from the rest

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Best Practices

Avoid overly “texty” slides Concise bullets preferred Avoid weird fonts And weird colors Make eye contact with your audience! Prepare for questions, if no questions, you

can start the debate

Be enthusiastic, not bored!

On the Physical Layer Security of Backscatter Systems

Today’s Talk

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Outline Introduction to RFID systems and the backscatter

Motivation and key features Backscatter communication System model

Basic communication model What happens in the presence of eavesdroppers?

Physical layer security of backscatter systems Introduction and motivation Basic model Main analysis

Simulation results Conclusions and future outlook

RFID Systems

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A Radio Frequency Identification (RFID) system is composed of two key components RFID tag: electronic device that can store and transmit

data to a reader in a contactless manner RFID reader: Transceiver that can read and

communicate with RFID tags

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RFID Systems Three types of RFID tags

Passive: cheapest – does not include any power source, harvests energy from the reader

Semi-Passive: can include some basic sotrage elements as well as a possible small battery to power the circuitry (but not for TX)

Active: has transmission capabilities – basically a small sensor

Two main technologies Near-field communication (NFC): operate at very short distances,

e.g., few centimeters, over HF frequencies (i.e., 13.56 MHz). Ultra high frequency (UHF) RFIDs: operate at UHF frequencies

(typically 433 MHz), can be used at longer ranges (10 meters is common but recent research is looking at larger distances) Of interest in this talk

RFID Applications

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Basic RFID Communication

Reader transmits a continuous wave (CW) carrier signal Induces an RF voltage across the tag antenna that is converted to DC by a

power harvesting circuit to power the tag’s circuitry and demodulate the received command

Tag transmits back its stored information by controlling the amount of backscatter of the impinging downlink signal by varying the impedance (mis)-match of the antenna front-end Role of the reader in energizing the tag

Interrogate and Power Tag

Responds with data, by modulating the CW

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RFID Backscatter: Challenges Tags must remain cheap and passive tags must harvest energy

Limitations on circuit design. Design of innovative RFIDs is a key challenge.

The uplink tag-to-reader communication has a strictly limited bandwidth due to limited backscatter power and tag collisions How to improve the uplink communication? Crucial to allow higher distances, better rate, and new applications!

Security is probably the most critical issue Fact: Classical cryptography hard to implement on RFID tags Current solutions (lightweight cryptography) are non-scalable Security solutions with little complexity => Physical layer security!

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System Model Basic UHF RFID system with one reader and one tag The signal backscattered by a tag k toward the receive

antenna of a reader i is given by (Griffin et al., IEEE Trans. Antenna and Propagation, Feb 2008):

Tag-reader

channel Reader-tag

channel

Tag’s reflection coefficient

(useful signal)

Reader’sTX signal

Noise

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Presence of Eavesdroppers

Interrogateand power

Tag

TagResponds

with data/ID

Passive tagrequires

the reader’scontinuous

wave to power up!

A nearby eavesdropper can tap into the communication

Two main critics of PHY security But cryptography works fine in

cellular systems! But not in RFID

You assume eaves. location known RFIDs work on small

distances! Does the backscatter constitute the

ideal PHY security application? Let’s see

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Key Backscatter Features Eavesdropping in a backscatter system is mostly

useful at close distances Due to low-power, low-cost nature of the system as

well as the reliance on passive tags Eavesdroppers are mainly interested in the uplink

signal/tag data So they must focus their effort on the tag, not the

reader The downlink reader-tag signal is also received by

the eavesdropper during backscatter A feature to exploit for PHY security

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Signal at the Eavesdropper The signal backscattered by a tag toward the receive

antenna of a reader as received at the eavesdropper

In a classical system, the eavesdropper knows the CW x and, thus, the second term can be cancelled and does not hinder eavesdropping

Tag-eave channel

Reader-tag

channel

Tag’s reflection coefficient

(useful signal)

Reader continuous CW transmission,

Reader-eave channel

Noise

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Using Artificial Noise Observation: the backscatter channel provides a

mean for “reader-induced” interference, due to the nature of the communication

A random noise signal added to the CW transmitted by the readers will be detrimental to the eavesdroppers Can improve the physical layer security of the system While this has been used in PHY security for

conventional systems, it often requires MIMO, in RFID, no need for MIMO!

Easily implementable, with little changes at the tag

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Using Artificial Noise Each reader transmits x + z with z being a random

narrowband artificial noise known to the reader With noise injection, the reader’s signal becomes:

And the eavesdropper’s received signal:

z and z’ are the same signal, but received by the eavesdropper over different paths and at possibly different time instants

Power of the received noise from z’ will naturally be much larger than that of the backscattered noise

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PHY Security: Basics

Notion of secrecy capacity Maximum rate sent from a wireless node to its destination in

the presence of eavesdroppers Here, secrecy capacity of user 1 : C1 = (Cd

1 - Ce1)+

For RFID, we use the secrecy capacity as a theoretical performance metric to assess how artificial noise can improve secrecy Practical RFID-tailored notions is an interesting direction

Cd1

Ce1

I can hear User 1!

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RFID with Artificial Noise Approximation of secrecy capacity of the backscatter

communication between reader and tag:

The received powers follow the Friis model We assume worst-case scenario at eavesdropper, i.e.,

noise is 0, hereinafter.

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Positive Secrecy Key question: For this basic model, what is the condition

for positive secrecy? Let’s look at two baseline cases

Case 1: all terminals have omnidirectional antennas

Positive secrecy is challenging with eaves. close to the tag. Case 2: any antennas at the eavesdropper

Let’s look at antenna impact numerically

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Case 2

Even for highly directional

antenna, positive secrecy

is possible with reasonable noise

power

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Back to Case 1

Even for an eavesdropper

placed at 10 cm ofthe tag, positive

secrecyis possible with reasonable noise

power

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Optimal Power Allocation Reader has a finite transmit power budget that

must be split between noise and main CW

How to allocate the reader’s finite power between noise and real signal?

Analytical closed-form solution can be derived easily (simple optimization problem)

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Simulation results (1)

Positive secrecy Achievable

even for smalldistances

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Simulation results (2)

Positive secrecy Achievable

even for highly directive antennas

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Simulation results (3)

Only a smallnoise

injectionis sufficientfor positive

secrecy

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The Multi-Reader Case What if we have multiple readers? Key tradeoff: more power for noise, worse SINR at

eavesdroppers, but Less power for the “useful” signal Coupling in actions: It can lead the other readers to change

their power allocation, due to the spreading interference

New dimension to the problem Adding noise can protect against eavesdroppers but… …the power of each others’ noise will impact our

performance We now have a problem where decisions between

readers are interdependent

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The Multi-Reader Case What are the techniques we can now use? We can still use an optimization problem for the

entire system Advantage: ensure system-wide optimality Disadvantage: could require additional overhead and explicit

control of all readers (may not be possible in practice)

Game theory Why? Interdependence in decisions between the readers Advantage: allows individual optimization per reader (kind of

like we did before) Disadvantage: could yield an inefficient point and may still

require some form of “information” at each reader.

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RFID Game Formulation What kind of game?

Noncooperative, because we assume no explicit communication between readers

We formulate the problem as a strategic, noncooperative game with The readers being the players The action of each reader being the fraction of power

allotted to the noise and the useful signal The utility being the sum of secrecy rates at all tags,

i.e.,

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Game Solution Definition: A Nash equilibrium is a strategy profile s*

with the property that no player i can do better by choosing a strategy different from s*, given that every other player j ≠ i .

For each player i with payoff function ui , we have:

Does the Nash equilibrium exist? Not always, multiplicity, efficiency, algorithmic aspects

Algorithmic solution via best response Not necessarily the best solution, but can give you insights on a

problem

Stage II – Actual data acquisition

Stage I – Best Response Dynamics, until finding a Nash equilibrium

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Best-Response SolutionInitial State:

-No artificial noise being used-- Discovery of neighboring readers

Sequentially,Each reader decides on its optimal power distribution,

under the currently observed network state

- Readers send out the interrogatecommands using the derived powers/secrecy

- The tags respond using backscatter

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Simulation results (1)

Case with 6Eavesdroppers:Performance

advantageparticularly

when the numberOf eavesroppers >Number of readers

Classical approach=

Regular channelWith no noise

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Simulation results (2)

ReasonableNumber

of iterations

Summary The physical layer security of backscatter RFID systems is

still a largely unchartered territory Most natural application for PHY security, due to infeasibility of

cryptography and the need for cost-effective, practical solutions Our preliminary results show that a simple solution can lead to

significant gains, at least theoretically

Future work Advanced signal processing and information theoretic analysis More system-level analysis, multi-reader/tag/eavesdropper Theoretical analysis on performance of RFID PHY security Innovative optimization techniques, beyond basic artificial noise

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Finally….

Thank YouQuestions ?