Overcoming the Antennas-Per-AP Throughput Limit in MIMO Shyamnath Gollakota Samuel David Perli and Dina Katabi

MIMO LANs Today, MIMO delivers as many concurrent packets as the antennas on the AP Talk presents a practical technique to double the concurrent packets in MIMO LANs

MIMO Primer AP Bob Antenna 1 Antenna 2 h ij is the channel from antenna i to antenna j

AP AP receives the sum of these vectors MIMO Primer Bob

AP AP projects on a direction orthogonal to interference p2 p2 p1 p1 How does the AP decode each packet? Current MIMO decodes as many concurrent packets as there are antennas per AP Current MIMO decodes as many concurrent packets as there are antennas per AP MIMO Primer Bob

Can We Get More Concurrent Packets? Bob AP p3 p3 p3 p3 No direction is orthogonal to all interference  AP can’t decode All current MIMO LANs are limited by number of antennas-per-AP Alice

Let the APs Coordinate Over the Ethernet Naive solution: Emulate 4-antenna AP by sending every signal sample over Ethernet

Let the APs Coordinate Over the Ethernet Impractical Overhead, Naive solution: Emulate 4-antenna AP by sending every signal sample over Ethernet p3 p3 Raw samples E.g., a 3 or 4-antenna system needs 10’s of Gb/s Can we leverage the Ethernet with minimal overhead?

p1 p1 Bob AP1 p3 p3 p3 p3 Align P3 with P2 at AP1 AP1 broadcasts P1 on Ethernet AP2 subtracts/cancels P1  decodes P2, P3 p1p1 p2p2 p3p3 AP2 Alice  AP1 decodes P1 to its bits Interference Alignment and Cancellation (IAC) IAC overcomes the antennas-per-AP throughput limit In IAC, a packet is decoded, then broadcasted once on the Ethernet  minimal overhead IAC overcomes the antennas-per-AP throughput limit In IAC, a packet is decoded, then broadcasted once on the Ethernet  minimal overhead

Contributions First MIMO LAN to overcome the antennas-per-AP limit IAC synthesizes interference alignment and cancellation Proved that IAC almost doubles MIMO throughput Implemented IAC in software radios showing practical throughput gains

How to Change Packet Direction?

Client AP

How to Change Packet Direction? Client AP Sender controls packet direction by multiplying with a vector

How Do We Align? Bob AP1 Alice AP2

How Does Alignment Work in Presence of Modulation? Real Imaginary Modulated samples are complex numbers with different phases Real Imaginary Sample in P3 Sample in P2 Alignment is in the antenna domain not the modulation domain Antenna 1 Antenna 2 Alignment works independent of modulation phases

How Does AP2 Subtract Interference from P1? Can’t subtract the bits in packet Need to subtract interference signal as received by AP2 Solution AP2 Re-modulate P1’s bits AP2 estimate and apply the channel P1 traversed to itself on modulated signal – Channel estimation in the presence of interference as in [ZigZag, SIGCOMM’08] Subtract!

How Does IAC Generalize to M-Antenna MIMO?

Theorem In a M- antenna MIMO system, IAC delivers 2M concurrent packets on uplink max{2M-2, 3M/2} concurrent packets on downlink How Does IAC Generalize to M-Antenna MIMO? E.g., M=2 antennas 4 packets on uplink 3 packets on downlink

Theorem In a M- antenna MIMO system, IAC delivers 2M concurrent packets on uplink max{2M-2, 3M/2} concurrent packets on downlink How Does IAC Generalize to M-Antenna MIMO? E.g., M=10 antennas 20 packets on uplink 18 packets on downlink For a large M, IAC doubles MIMO throughput For a large M, IAC doubles MIMO throughput

What if There is a Single Client? Client AP1 Current MIMO exploits diversity and pick best of two APs Can’t have more than 2 concurrent packets, but … IAC can pick the best antenna pair across APs AP2 IAC provides higher diversity than Current MIMO Diversity gain applies to one or more clients IAC provides higher diversity than Current MIMO Diversity gain applies to one or more clients

IAC MAC Requirements Allow concurrent packets Clients are oblivious to each other Works even when channel changes (i.e., the matrix H changes)

IAC MAC Leverages PCF mode Clients are simple: APs compute v vectors and send them to clients in the Grant message IAC adapts to changing channels because APs get a new channel estimate from each ACK packet CF- End Contention-free Contention Downlink Uplink..... ACKs P4 P5 P6 P1 P2 P3 Time

Performance

Implementation GNURadio software 2-antenna MIMO USRP nodes Carrier Freq: 2.4GHz

Testbed 20-node testbed All nodes within radio range of each other Each run randomly picks APs and clients

Gain = Client throughput in IAC Client throughput in current MIMO Metric

Uplink Gain CDF of Runs Per-Client Throughput Gain

Uplink Gain CDF of Runs Per-Client Throughput Gain On uplink, IAC’s median gain is 2.1x Gain is partially due to diversity but more to concurrency On uplink, IAC’s median gain is 2.1x Gain is partially due to diversity but more to concurrency

Downlink Gain CDF of Runs Per-Client Throughput Gain On downlink, IAC’s median gain is 1.5x

Gains as a Function of SNR

SNR in dB Uplink Throughput Gain IAC is beneficial across the operational range of SNRs

Related Work Interference Alignment [AMK’08,JS’08] Interference Cancellation [GC’80,HWA’08] MU-MIMO [NJ’06] IAC provably provides more throughput, and doubles the number of concurrent packets

Conclusion First MIMO LAN to overcome the antennas-per-AP limit IAC synthesizes interference alignment and cancellation Proved that IAC almost doubles MIMO throughput Implemented IAC in software radios showing that it works in practice

IAC MAC Leverages PCF mode APs compute and send v vectors in Grant  Clients are oblivious to each other APs can track channels, i.e., H, from using ACKs CF- End Contention-free Contention Downlink Uplink..... ACKs P4 P5 P6 P1 P2 P3 Time

Uplink: for M=2 antennas, IAC delivers 2M=4 packets Clients APs p1p1 p2p2 p4 p3

APs Clients p1p1 p2p2 p3p3 Downlink: - Clients can’t coordinate over Ethernet - For M=2 antennas, IAC delivers 3M/2 = 3 packets

IAC’s concurrency increases capacity bound C = d log(SNR) + o(log(SNR)) IAC increases degrees of freedom d is degrees of freedom Interference cancellation does not increase degrees of freedom but provides a better use of them