Capacity and Fairness in Multihop Wireless Backhaul Networks

1
Capacity and Fairness in Multihop Wireless
Backhaul Networks

• Ashu Sabharwal
• ECE, Rice University

2
Wireless UtopiaMobile 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

3
Transit Access PointsMulti-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
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

5
Characteristics of TAP Networks

• No mobility in backbone
• TAPs dont 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

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

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Multi-hop Networks
0
2

• 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

3
1
8
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

9
Cooperative PHY
0
2
3
1

• Send packet(s) to other TAPs

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Cooperative PHY
0
2
3
1

• 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

11
Throughput Gains
70
60
Throughput (Mbits/s)
Maximum Available Routes

• Rule Choose best k-out-of-m routes leading to
  minimum total delay
• Substantial gains for moderate network size

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 ?

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

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Throughput Performance
2-route OCR
3-route OCR
d
4-route OCR
2
Throughput (Mbits/s)
0
1
200 m
2-hop 802.11
3
Distance from source (d)

• Throughput gains (20-30) outweigh spatial reuse
  loss
• 2-4 routes give max gain due to handshake overhead

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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

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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

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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

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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
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Removing Spatial Bias

• Spatial Bias Removal (SBR)
• Find the bottleneck link of each flow
• Share of all flows traversing bottleneck equal
• SBRTemporal Fair Equal temporal share in
  bottleneck links
• SBR Throughput Fair Equal throughput for all
  flows regardless of their paths

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Throughput Comparisons
Example
20Mbps
10Mbps

5Mbps



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

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

23
TAP Testbed Goals

• Prototype network on and around Rice campus
• Measurement studies from channel conditions to
  traffic patterns

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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