Capacity and Fairness in Multihop Wireless Backhaul Networks

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

2. 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 Rice University

3. 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 Rice University

4. 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 Rice University

5. 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 accesscould be topology aware • Qos routingcan be reliably done Opportunity for optimization based on topology via feedback at multiple time-scales Rice University

6. 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 Rice University

7. Multi-hop Networks 0 • 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 2 3 1 Rice University

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

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

10. 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 0 2 3 1 Rice University

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

12. 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 ? Rice University

13. 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 Rice University

14. Throughput Performance • Throughput gains (20-30%) outweigh spatial reuse loss • 2-4 routes give max gain due to handshake overhead 2-route OCR 3-routeOCR d 4-route OCR 2 Throughput (Mbits/s) 0 200 m 1 2-hop 802.11 3 Distance from source (d) Rice University

15. 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 Rice University

16. Unfairness in Current Protocol • IEEE 802.11, 5 MUs/TAP • TAP1 completely starved • Same for TCP • Caused mainly by information assymetry • In general, closest to the wire TAP wins Rice University

17. 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 Rice University

18. TA (1) TA (2) TA (3) Internet TAP4 TAP1 TAP2 TAP3 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 20Mbps 10Mbps 5Mbps Rice University

19. 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 Rice University

20. Throughput Comparisons Example 20Mbps 10Mbps 5Mbps Rice University

21. 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 Rice University

22. 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 Rice University

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

24. 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 Rice University