Capacity and Fairness in Multihop Wireless Backhaul Networks

Capacity and Fairness in Multihop Wireless Backhaul Networks Ashu Sabharwal ECE, Rice University Ashu Sabharwal Rice University

Wireless Utopia: Mobile Broadband • Wi. Fi Hot-spots – Reasonable speeds – Expensive + poor coverage low subscriber rates, failing companies, … • 3 G – Ubiquitous, allows mobility but low data rates – Expensive to deploy slow deployments • Major costs – Wired connection to backbone – Spectral fees – Uneasy “on-demand” growth Ashu Sabharwal Rice University

Transit Access Points: Multi-hop Backbone • Few wires – Most TAPs multi-hop to wired gateways – Add wires to TAPs as demand grows Multiple radios & MIMO • Use both licensed and unlicensed spectrum – Licensed spectrum: protected, allows guarantees – Unlicensed spectrum: free, more, less interference outdoors Ashu Sabharwal Rice University

Major Challenges • High information density around wires – Capacity per gateway log(n) • Service quality transparent to user location – Users close to wire can win big – TCP on RTT time-scale, too slow Ashu Sabharwal Rice University

Characteristics of TAP Networks • No mobility in backbone – TAPs don’t move static topology • Slow variability can be used at all time-scales – Physical layer can use fast feedback – Medium access could be topology aware – Qos routing can be reliably done Opportunity for optimization based on topology via feedback at multiple time-scales Ashu Sabharwal Rice University

Outline • Opportunistic Cooperative Relaying [Sadeghi, Chawathe, Khoshnevis, Sabharwal] – Route diversity – Cooperative PHY – OCR • TAP Fairness [Gambiroza, Sadeghi, Knightly] – Performance of current protocols – Inter-TAP fairness model • Rice TAP Testbed Ashu Sabharwal Rice University

Multi-hop Networks 0 2 • Multiple routes to destination – Many routes exist to destination – Route quality function of time 3 1 • Coherence time – Time for which channel SNR remains constant – For low mobility channels, several packets long Route diversity Ashu Sabharwal Rice University

Cooperative PHY 0 2 3 1 • Why use only one route every time ? – Carrier sense will shut off many TAPs – Use their power and antenna resources Ashu Sabharwal Rice University

Cooperative PHY 0 2 3 1 • Send packet(s) to other TAPs Ashu Sabharwal Rice University

Cooperative PHY 0 2 3 1 • Send packet(s) to other TAPs • All TAPs together “forward” the packet – Acts like a 3 M x M antenna system (in above picture) – Simplest form of network coding Ashu Sabharwal Rice University

Throughput (Mbits/s) Throughput Gains ~70% ~60% Maximum Available Routes • Rule: Choose best “k-out-of-m” routes leading to minimum total delay • Substantial gains for moderate network size Ashu Sabharwal Rice University

Challenges in Realizing Route Diversity • Quality of routes unknown – Use of a route depends on its current condition – Thus, routes have to measured before every use • Multiple TAP coordination – Medium access has to coordinate multiple TAPs • Knowledge of routes – Many routes exist – Which subset to actively monitor ? Ashu Sabharwal Rice University

Opportunistic Cooperative Relaying • 4 -way multi-node handshake – Allows source (TAP 0) to know all channel qualities – AND coordinate participating TAPs – TAP 0 chooses the smallest delay route • Multi-hop MAC – Forwarded packets do not contend again – Slot reservation ensures safe passage to destination Ashu Sabharwal Rice University

d 2 0 200 m 3 1 Throughput (Mbits/s) Throughput Performance 2 -route OCR 3 -route OCR 4 -route OCR 2 -hop 802. 11 Distance from source (d) • Throughput gains (20 -30%) outweigh spatial reuse loss • 2 -4 routes give max gain due to handshake overhead Ashu Sabharwal Rice University

Outline • Opportunistic Cooperative Relaying [Sadeghi, Chawathe, Khoshnevis, Sabharwal] – Route diversity – Cooperative PHY – OCR • TAP Fairness [Gambiroza, Sadeghi, Knightly] – Performance of current protocols – Inter-TAP fairness model • Rice TAP Testbed Ashu Sabharwal Rice University

Unfairness in Current Protocol • IEEE 802. 11, 5 MUs/TAP • TAP 1 completely starved – Same for TCP – Caused mainly by information assymetry • In general, closest to the wire TAP wins Ashu Sabharwal Rice University

Inter-TAP Fairness • Ingress Aggregation – Flows originating from a TAP treated as one – TAPs implement inter-flow fairness • Temporal fairness – Different links have different throughputs – Throughput fairness hurts good links • Removal of Spatial Bias – Equal temporal share not sufficient – More hop flows get lesser bandwidth Ashu Sabharwal Rice University

Throughput with Temporal Fairness • Temporal Fairness – Equal time shares to all flows – Flow receives 1/F of the throughput of the case it was the only flow • Shares: 18%, 21%, 61% TA(1) TA(2) TA(3) Internet TAP 1 20 Mbps TAP 2 TAP 3 5 Mbps TAP 4 10 Mbps • Increase in number of hops decrease in throughput Ashu Sabharwal Rice University

Removing Spatial Bias • Spatial Bias Removal (SBR) – Find the bottleneck link of each flow – Share of all flows traversing bottleneck equal • SBR+Temporal Fair = Equal temporal share in bottleneck links • SBR + Throughput Fair = Equal throughput for all flows regardless of their paths Ashu Sabharwal Rice University

Throughput Comparisons Example 20 Mbps 5 Mbps Ashu Sabharwal 10 Mbps Rice University

Outline • Opportunistic Cooperative Relaying [Sadeghi, Chawathe, Khoshnevis, Sabharwal] – Route diversity – Cooperative PHY – OCR • TAP Fairness [Gambiroza, Sadeghi, Knightly] – Performance of current protocols – Inter-TAP fairness model • Rice TAP Testbed Ashu Sabharwal Rice University

TAP Hardware Design • Platform for new PHY + Protocol Design • Generous compute resources – High-end FPGAs with fast interconnects – Simulink GUI environment for development • 2. 4 GHz ISM band radios – 4 x 4 MIMO system • Open-source design – Both hardware and software Ashu Sabharwal Rice University

TAP Testbed Goals • Prototype network on and around Rice campus • Measurement studies from channel conditions to traffic patterns Ashu Sabharwal Rice University

Summary • Transit Access Points – Wi. Fi “footprint” is dismal – 3 G too slow and too expensive – Removing wires is the key for economic viability • Challenges – Enabling high capacity backbone – Multi-hop fairness Ashu Sabharwal Rice University