Interference Centric Wireless Networks

Sachin KattiAssistant ProfessorEE&CS, Stanford University

Interference is Everywhere

WiFiZigbee

BluetoothHow to maximize throughput

in the presence of interference?

Current Approach to Interference• Fears & avoids interference at all costs• Impacts all aspects of wireless design– Radios are half duplex–MAC protocols try to schedule one link at a

time– Coexisting networks use different channels

if possible–…….

Current Approach Cannot ScaleDense and chaotic wireless deployments Interference is unavoidable– Hidden terminals cause collisions– Coexisting networks interfere with each other– Legacy interferers (e.g. microwave) …..

Moreover,Limited spectrum + interference avoidance design Achievable capacity is fundamentally limited

This TalkFundamental rethink:Exploit interference instead of avoiding it

High-Level Approach• Infer interference structure• Exploit structure to better decode

interfered packets and increase throughput

Exploiting Interference in All Contexts• Exploiting In-Link Interference– Full Duplex Radios (Mobicom 10,11)

• Exploiting In-Network Interference– Rateless & Collision Resilient PHY

(Sigcomm11)• Exploiting Cross-Network

Interference– Detecting Degrees of Freedom

(Sigcomm11)

Exploiting In-Link Interference:Full Duplex Radios

Jain et al, “Practical Real Time Full Duplex Wireless”Mobicom 2010, 2011

“It is generally not possible for radios to receive and transmit on the same frequency band because of the interference that results. Thus, bidirectional systems must separate the uplink and downlink channels into orthogonal signaling dimensions, typically using time or frequency dimensions.” - Andrea Goldsmith, “Wireless Communications,” Cambridge Press, 2005.

In-Link Interference Half Duplex Radios

TX RX TX RX

Self-interference is millions to billions (60-90dB) stronger than received signal

Analog Self Interference Analog

Received Signal

Digital Self Interference Digital

Received Signal

Tx Rx

max

- max

ADC

Self-interference drowns out received signal

In-Link Interference Half Duplex Radios

Our Approach1. Infer interference structure– Easy, we know what we are

transmitting!

2. Exploit knowledge of interference structure to subtract and decode

First Attempt: Antenna Cancellation

d d + λ/2

TX1 TX2RX

• Signal null at RX antenna• ~30dB self-interference cancellation

Bringing It Together

QHX220

ADC

HardwareCancellation

TX Signal

AntennaCancellation

RX

DigitalCancellation ∑TX

Samples

+-Clean RX samples

RF

Baseband

Our Prototype

AntennaCancellation

HardwareCancellation

Digital InterferenceCancellation

TX1 TX2

Only TX1 Active

Antenna Cancellation: Performance

TX1 TX2

Only TX2 Active

Antenna Cancellation: Performance

Only TX1 Active

TX1 TX2

Only TX1 ActiveOnly TX2 Active

Both TX1 & TX2 Active

Antenna Cancellation: Performance

NullPosition

TX1 TX2

Only TX1 ActiveOnly TX2 Active

Both TX1 & TX2 Active

Antenna Cancellation: Performance

~25-30dBNull

Position

Bandwidth ConstraintA λ/2 offset is precise for one frequency

fc

d d + λ/2 TX1 TX2RX

Bandwidth ConstraintA λ/2 offset is precise for one frequencynot for the whole bandwidth

fc fc+Bfc -Bd d + λ/2

TX1 TX2RX

Bandwidth ConstraintA λ/2 offset is precise for one frequencynot for the whole bandwidth

fc fc+Bfc -Bd d + λ/2

TX1 TX2RX

d2 d2 + λ+B/2 TX1 TX2RX

d1 d1 + λ-B/2 TX1 TX2RX

Bandwidth Constraint

fc fc+Bfc -Bd d + λ/2

TX1 TX2RX

d2 d2 + λ+B/2 TX1 TX2RX

d1 d1 + λ-B/2 TX1 TX2RX

WiFi (2.4G, 20MHz) => ~0.26mm precision error

A λ/2 offset is precise for one frequencynot for the whole bandwidth

Bandwidth Constraint

2.4 GHz

5.1 GHz

300 MHz

Bandwidth Constraint

2.4 GHz

5.1 GHz

300 MHz

• WiFi (2.4GHz, 20MHz): Max 47dB reduction

• Bandwidth⬆ => Cancellation⬇• Carrier Frequency⬆ => Cancellation⬆

First prototype gives 1.84x throughput gain with two radios compared to half-duplex with a single radio.

Limitation 1: Need 3 antennasLimitation 2: Bandwidth constrained (802.15.4 works)Limitation 3: Doesn’t adapt to environment

Our Approach1. Infer interference structure– Easy, we know what we are

transmitting!

2. Exploit knowledge of interference structure to subtract and decode

Poor Man’s Subtraction

2.4 GHz

5.1 GHz

300 MHz

Cancellation using Phase Offset Self-

Interference

Cancellation Signal

∑

Cancellation using Phase Offset Self-

Interference

Cancellation Signal

∑

Self-Interference

Cancellation Signal

∑

Frequency dependent, narrowband

Self-Interference

Cancellation Signal

∑

Self-Interference

Cancellation Signal

∑

Frequency and bandwidth independent

Cancellation using Signal Inversion

Time

Xt +Xt/2

-Xt/2

BALUN

Second Design: Balanced to Unbalanced Conversion

Traditional Design

R

TX Frontend

RX Frontend

T R+aT

aT

1. Invert the Signal

aT R

TX Frontend

RX Frontend

2T

+T -T R+aT

balun

2. Subtract Signal

R

TX Frontend

RX Frontend

ΣR+aT-T

R+aT

balun

aT

2T

-T+T

3. Match Signals

R

TX Frontend

RX Frontend

Σ v-vT R+aT-vT

R+aT

balun

attenuator anddelay line

aT

2T

-T+T

Can Receive If v = a!

R

TX Frontend

RX Frontend

Σv-vT

R+aTattenuator anddelay line

balun

aT

2T

-T+T

R+aT-aT

+Xt/2

•Measure wideband cancellation•Wired experiments•240MHz chirp at 2.4GHz to measure response

TimeSignal Inversion Cancellation: Wideband Evaluation

TX

RX

Signal Inversion Cancellation Setup

∑ TX RX

Phase Offset Cancellation Setup

∑

RF Signal

Splitte

r

Xt +Xt/2

-Xt/2

Xt

+Xt/2

λ/2 Delay

Time

Lower isbetter

Higher isbetter

Time

~50dB cancellation at 20MHz bandwidth with balun vs ~38dB with phase offset cancellation.Significant improvement in wideband cancellation

Lower isbetter

Higher isbetter

Time

•From 3 antennas per node to 2 antennas•Parameters adjustable with changing

conditions

Attenuator and

Delay Line

TX RX

TX Frontend

Xt

+Xt/2 -Xt/2

∑

RX Frontend

Other advantages

• Need to match self-interference power and delay

• Can’t use digital samples: saturated ADC

Adaptive RF CancellationTX RX

Wireless Receiver

Wireless Transmitter

RF Cancellation

TX Signal Path

RX Signal Path

RF Reference Σ

Balun

Attenuation & Delay

• Need to match self-interference power and delay

• Can’t use digital samples: saturated ADC

Adaptive RF Cancellation

RSSI : Received Signal Strength Indicator

TX RX

Attenuation & Delay

Wireless Receiver

Wireless Transmitter

RF Cancellation

TX Signal Path

RX Signal Path

RF ReferenceΣ

Balun

RSSI

• Need to match self-interference power and delay

• Can’t use digital samples: saturated ADC

Adaptive RF Cancellation

Use RSSI as an indicator of self-interference

TX RX

Attenuation & Delay

Wireless Receiver

Wireless Transmitter

RF Cancellation

TX Signal Path

RX Signal Path

RF ReferenceΣ

Balun

RSSI

Control Feedback

Objective: Minimize received powerControl variables: Delay and Attenuation

TX RX

Attenuation & Delay

Wireless Receiver

Wireless Transmitter

RF Cancellation

TX Signal Path

RX Signal Path

RF ReferenceΣ

Balun

RSSI

Control Feedback

Adaptive RF Cancellation

➔ Simple gradient descent approach to optimize

Objective: Minimize received powerControl variables: Delay and Attenuation

Adaptive RF Cancellation

Digital Interference Cancellation

TX RX

Attenuation & Delay

RF ➔ Baseband

ADC

Baseband ➔ RFDAC

Encoder Decoder

Digital Interference Reference

RF Cancellation

TX Signal Path

RX Signal Path

RF ReferenceΣ

FIR filter ∆

RSSI

Control Feedback

Channel Estimate

Balun

Bringing It All Together

Performance

• WiFi full-duplex: with reasonable antenna separation• Not enough for cellular full-duplex: need 20dB more

Full Duplex Implications• Breaks a fundamental assumption in

wireless• Could eliminate the need for paired

spectrum• Impacts higher layer design– Reduce control overhead (Radunovic et al)

• Other applications– Security & Privacy (Gollakota et al)

• Many more …..

Exploiting In-Network InterferenceRateless & Collision-Resilient Codes

Gudipati, Katti “Strider: Automatic Rate Adaptation”SIGCOMM 2011

In-Network Interference Collisions

Carrier sense failure Packet collisions and lossCurrent Approach: Conservative backoff, RTS/CTS

Our Approach: Infer Interference Structure• Current approach: – Measure channel SINR and pick modulation,

coding rate– If channel SINR < decoding threshold,

decoding fails– Collision SINR < decoding threshold

• Key insight: Novel rateless codes for wireless

• Rateless code no need to know SINR in advance, automatically achieves optimal throughput

Our Approach: Infer Interference Structure• Key technique: Novel rateless codes• P1 acts as interference to P2 and vice versa

1. Use rateless code to decode P1 Infer interference

P1 P2

Decode P1P1

P1 P2

Our Approach: Exploit Interference Structure• Key insight: Exploit rateless code to

decode one packet, subtract it and decode next packet Decode P1

Subtractinterference

1. Use rateless code to decode P1 Infer interference

2. Subtract P1 from received signal and decode P2

_ __

P1

Exploiting Cross-Network InterferenceDetecting Degrees of Freedom

Hong, Katti “DOF: A Local Wireless Information Plane”SIGCOMM 2011

WiFiZigbee

Bluetooth

Cross-Network Interference Coexistence

How to maximize throughput in the presence of cross-network interference?

Microwave

Smart Transmitter

Smart Receiver

1. The protocol types operating in the local vicinity 2. The spectrum occupancy of each type

3. The spatial directions of each type

DOF infers coexisting interference structure

WiFi AP

Heart Monitor

AoA

Freq2.3 GHz

2.5 GHz

0 °18

0°

AoA

Freq2.3 GHz

2.5 GHz

0 °18

0°

Freq2.3 GHz

2.5 GHzFreq

2.3 GHz

2.5 GHz

Our Approach: Infer Interference Structure

“Man-made” signals hidden repeating patterns that are unique and necessary for operation

Key Insight

CP CP CPData Data Data …………………….

Repeating Patterns in WiFi OFDM signals

Repeating Patterns in Zigbee signalsTime

Leverage unique patterns to infer 1) type, 2) spectral occupancy, and 3)

spatial directions

Exploit interference structure knowledge• Policy 0 – Only use unoccupied spectrum

WiFi

Microwave

Smart Tx

AoA

Freq2.3 GHz

2.5 GHz

Frequency

2.5 GHz

Smart Rx

AoA

Freq2.3 GHz

PSD

• Policy 1 – Use unoccupied spectrum + mw oven spectrum• Policy 2 – Use unoccupied spectrum + mw oven spectrum + compete for WiFi spectrum

Heart Monitor(ZigBee Based)

Our Approach: Exploit Interference Structure

To ConcludeFuture: dense, chaotic and limited spectrum

Interference is the dominant determinant of future wireless network capacity• Point to point link speeds are close to Shannon

Our approach: Fundamental rethink of wireless to manage and exploit interference• Increase concurrency Increase network capacity