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