Interference Alignment By Motion Swarun Kumar Fadel Adib, Omid Aryan, Shyamnath Gollakota and Dina Katabi

Major Advances in MIMO E.g. Interference Alignment Significant gains in throughput

Single-Antenna Devices Single Antennas, due to limits on power and size Largely left out of these MIMO benefits Single Antennas, due to limits on power and size Largely left out of these MIMO benefits

Bring MIMO Benefits to Single Antenna Devices “Interference Alignment” Goal

Interference Alignment antenna 1 C1 C2 C3 antenna 2 AP 1 C1 interfere 2-antenna node can decode only 2 signals 1 2 C2 C3

Interference Alignment antenna 1 C1 C2 C3 antenna 2 AP 1 C1 interfere 1 2 C2 C3 2-antenna node can decode only 2 signals

Interference Alignment antenna 1 C1 C2 C3 antenna 2 AP 1 C1 interfere 1 2 C2 C3 “align” 2-antenna node can decode only 2 signals

Interference Alignment antenna 1 C1 C2 C3 antenna 2 AP 1 C1 interfere 1 2 C2 C3 “align” 2-antenna node can decode only 2 signals

Interference Alignment antenna 1 C1 antenna 2 C1 interfere 1 C2 C3 one unwanted interferer 2 AP 1 “align” 2-antenna node can decode only 2 signals

Single-Antenna Devices C1 interfere 1 C2 C3 antenna 1 C1 C2 C3 antenna 2 Can we still perform interference alignment? Signals from all clients will change 2 AP 1

Single-Antenna Devices C1 interfere 1 C2 C3 antenna 1 C1 C2 C3 antenna 2 Signals from all clients will change 2 AP 1 Perform Interference Alignment purely at the AP Eliminates feedback/cooperation with clients Brings benefits of alignment to new devices Perform Interference Alignment purely at the AP Eliminates feedback/cooperation with clients Brings benefits of alignment to new devices Can we still perform interference alignment?

MoMIMO Moves the AP’s antenna to positions that achieve interference alignment Needs to only displace antenna by up to 2 inches Achieves 1.98x gain in throughput over n

1. How do we “find” positions of alignment? 2. How does it impact general wireless networks?

Feasibility of “Alignment by Motion” Record antenna displacement for interference to drop below noise AP 1 2 inch radius 21 interfere desired C1C2 C3

Feasibility of “Alignment by Motion”

Why is the required displacement small? Median: 0.3 inch 90 th Percentile: 1 inch

A Simple Example antenna 1 C1 antenna 2 align Reference C1 1 2 AP 1

A Simple Example antenna 1 0 align Reference C1 1 2 AP 1 Goal: Minimize signal from C1 to antenna 2 C1 antenna 2

Paths combine constructively or destructively based on phase Indoor Environments Rich in Multipath High (poor alignment) C1 1 2 AP 1

Indoor Environments Rich in Multipath Paths differ by extra 2” Paths combine constructively or destructively based on phase For Wi-Fi, 2” ≈ λ/2 λ 0°360° C1 1 2 AP 1

Indoor Environments Rich in Multipath Paths combine constructively or destructively based on phase For Wi-Fi, 2” ≈ λ/2 In-phase paths now out-of-phase! Low (good alignment) 0° 180° λ 2 C1 1 2 AP 1 Paths differ by extra 2” Small displacement suffices for alignment Generalizes to many reflectors, any alignment Small displacement suffices for alignment Generalizes to many reflectors, any alignment

{ { How Can We Find Good Alignment? We must quantify goodness of alignment antenna 1 C1 C2 antenna 2 interference C1 C2 interference Poor Good C1 C2 interference ≈ 0 Goal: Find antenna location that minimizes interference

Naïve solution: Random walk Simulated the spatial profile of interference Ten reflectors placed in randomly chosen locations Applied standard multipath models Does not work!

Interference (dB) Naïve solution: Random walk High interference x (in) y (in)

Interference (dB) Naïve solution: Random walk Low interference Goal: Find blue spots y (in) x (in)

Naïve solution: Random walk x (in) y (in) Blue spots of low interference are small  Hard to stumble upon in a random walk Goal: Find blue spots Interference (dB)

Key Observation: Interference is smooth Wireless channels are continuous and smooth functions over space x (in) y (in) Interference (dB)

Solution: A Hill Climbing Algorithm M ove in random direction and track interference x (in) y (in) Interference (dB)

Solution: A Hill Climbing Algorithm M ove in random direction and track interference – If interference  : continue in that direction x (in) y (in) Interference (dB)

Solution: A Hill Climbing Algorithm M ove in random direction and track interference – If interference  : continue in that direction x (in) y (in) Interference (dB)

Solution: A Hill Climbing Algorithm M ove in random direction and track interference – If interference  : continue in that direction – If interference  : continue in opposite direction x (in) y (in) Interference (dB) Algorithm converges to spot of minimum interference Guides antenna to find positions of alignment Algorithm converges to spot of minimum interference Guides antenna to find positions of alignment

1. How do we “find” positions of alignment? 2. How does it impact general wireless networks?

Interference Alignment AP 1 C1C2 C3 AP 2AP 3 Align C2 and C3

Interference Alignment AP 1 C1C2 C3 AP 3 Align C1 and C3 AP 2

Interference Alignment AP 1 C1C2 C3 Align C1 and C2 AP 2AP 3

Interference Alignment AP 1 C1C2 C3 AP 2AP 3 3 concurrent streams  Gain in throughput! N antenna APs enable N+1 concurrent uplink streams 3 concurrent streams  Gain in throughput! N antenna APs enable N+1 concurrent uplink streams

What about downlink traffic? AP 1 C1C2 C3 AP 2AP 3

What about downlink traffic? AP 1 C2 C3

AP 1 has 2 antennas null ?? 2 antenna node can null interference at up to 1 antenna Nothing! C2 & C3 aligned at AP 1 AP 1 C2 C3

AP 1 has 2 antennas null 2 antenna node can null interference at up to 1 antenna C2 & C3 aligned at AP 1 AP 1 C2 C3 null for free!

Uplink Wireless Channels AP 1 C2 C3 h1h1 h2h2 h3h3 h4h4 antenna 1 antenna 2 (h 1, h 2 ) (h 3, h 4 ) h1h2h1h2 h3h4h3h4 =

Downlink Wireless Channels AP 1 C2 C3 h1h1 h2h2 h3h3 h4h4 Channel Reciprocity x h 1 x + h 2 αx null αxαx

Downlink Wireless Channels AP 1 C2 C3 h1h1 h2h2 h3h3 h4h4 Channel Reciprocity xαxαx h 1 x + h 2 αx = 0 null

Downlink Wireless Channels AP 1 C2 C3 h1h1 h2h2 h3h3 h4h4 Channel Reciprocity xαxαx α =α = null -h 1 h 2 α =α = -h 3 h 4 h1h2h1h2 h3h4h3h4 = null Alignment on the uplink enables nulling on the downlink, with no extra movement

Downlink Traffic AP 1 C1C2 C3 AP 2AP 3

Downlink Traffic AP 1 C1C2 C3 AP 2AP 3

Downlink Traffic AP 1 C1C2 C3 AP 2AP 3

Downlink Traffic AP 1 C1C2 C3 AP 2AP 3 3 concurrent streams on the downlink MoMIMO provides gains to uplink & downlink traffic 3 concurrent streams on the downlink MoMIMO provides gains to uplink & downlink traffic

Experimental Results

MoMIMO Implementation Implemented on USRP N210 Mounted antenna on Roomba to emulate sliding antennas Compare MoMIMO with n, n+

Testbed Randomly assign nodes to red locations Class Room Office Space

Can Alignment Reduce Interference? CDF Interference (dB)

Can Alignment Reduce Interference? CDF n MoMIMO Downlink Median: -2.5dB Interference (dB)

Throughput Heterogeneous mix of 1 & 2-antenna nodes CDF Network Throughput (Mbps)

Throughput Heterogeneous mix of 1 & 2-antenna nodes 1.98x CDF Network Throughput (Mbps) n MoMIMO

Throughput Heterogeneous mix of 1 & 2-antenna nodes 1.31x CDF Network Throughput (Mbps) n n+ MoMIMO

Conclusion Performs Interference Alignment purely by moving an antenna of the AP Displaces antenna by up to 2 inches New applications at intersection of networking and robotics