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

Ashu SabharwalRice University Wireless Utopia: Mobile Broadband WiFi Hot-spots –Reasonable speeds –Expensive + poor coverage  low subscriber rates, failing companies,… 3G –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 SabharwalRice University Transit Access Points: Multi-hop Backbone Few wires –Most TAPs multi-hop to wired gateways –Add wires to TAPs as demand grows Use both licensed and unlicensed spectrum –Licensed spectrum: protected, allows guarantees –Unlicensed spectrum: free, more, less interference outdoors Multiple radios & MIMO

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

Ashu SabharwalRice University Cooperative PHY Why use only one route every time ? –Carrier sense will shut off many TAPs –Use their power and antenna resources

Ashu SabharwalRice University Cooperative PHY Send packet(s) to other TAPs

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

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

Ashu SabharwalRice 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 SabharwalRice 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 SabharwalRice University Throughput Performance Throughput gains (20-30%) outweigh spatial reuse loss 2-4 routes give max gain due to handshake overhead Distance from source (d) Throughput (Mbits/s) m d 2-hop route OCR 3-route OCR 4-route OCR

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

Ashu SabharwalRice 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 SabharwalRice 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% Increase in number of hops  decrease in throughput TAP1TAP2TAP3 TA(1) TAP4 TA(2) TA(3) Internet 20Mbps 5Mbps 10Mbps

Ashu SabharwalRice 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 SabharwalRice University Throughput Comparisons 20Mbps 5Mbps 10Mbps Example

Ashu SabharwalRice 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 SabharwalRice 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 –4x4 MIMO system Open-source design –Both hardware and software

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

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