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Database-Assisted Multi-AP Network on TV White Spaces: Architecture, Spectrum Allocation and AP Discovery. Xiaojun Feng , Jin Zhang, and Qian Zhang Hong Kong University of Science and Technology DySPAN 2011. How far away are we from mass deployment of cognitive radio networks?.

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slide1

Database-Assisted Multi-AP Network on TV White Spaces: Architecture, Spectrum Allocation and AP Discovery

Xiaojun Feng, Jin Zhang, and Qian Zhang

Hong Kong University of Science and Technology

DySPAN 2011

how far away are we from mass deployment of cognitive radio networks
How far away are we from mass deployment of cognitive radio networks?

Real and Mass deployment of cognitive radio network.

How can a multi-AP system really works on TV white spaces?

2008, 2010 FCC releases TV white space for secondary access

10 years of study of Cognitive Radio

1999, Cognitive Radio was proposed by Joseph Mitola and Gerald Q. Maguire, JR

some backgrounds
Some backgrounds
  • TV White Spaces
    • Free TV channels after Analog to Digital Transition
  • FCC rules
    • Sep. 2010
    • Available TV channels: channel 2-51 (except 37), 54MHz~698MHz
    • Geo-location Database architecture
existing systems
Existing Systems
  • WhiteFi, SIGCOMM 09
    • Single AP, multiple users
    • Adaptive channel width
    • Sensing based architecture
  • Jello, NSDI 10
    • Several link pairs
    • Non-continuous spectrum for each link
    • Coordination through common backup channel
  • SenseLess, DySPAN 11
    • Database architecture
    • Mainly focus on the database design and implementation
our aim
Our Aim
  • Our system should be compatible with the database architecture
  • Multiple APs, each AP supports multiple users
  • Non-continuous spectrum allocation for each AP in a distributed manner
  • Challenges
    • Detailed database specification
    • How to allocate spectrum for each AP distributedly while ensuring fairness and performance?
    • How can users discover APs operating on non-continuous band?
our system whitenet
Our System: WhiteNet

AP Discovery and Connection: a novel beacon scheme

Spectrum Sharing among APs: rule regulated spectrum allocation

Incumbent Protection: propose a local database compatible with FCC’s final rule

outline
Outline
  • WhiteNet Design
    • WhiteNet Local Database
    • Distributed Spectrum Allocation
    • AP Discovery
  • System Implementation
  • Evaluation
  • Conclusion
whitenet the local database
WhiteNet: the Local Database
  • Why a local database?
    • Act as an proxy between global database and local APs, provide information of available channels
    • Facilitate spectrum among APs
  • What’s in the local database?
    • Available TV channel list
    • Information about APs in the system
      • Location
      • Power
      • Channel occupation
    • Anything be more?
whitenet local database
WhiteNet Local Database
  • Proposed Local Database Architecture:
whitenet local database1
WhiteNet Local Database
  • Vacant TV Database (TDB)
    • Store information about available TV channels obtained from the central database
  • Local AP Database (ADB)
    • Store information about APs in the local area network, such as location, power, occupied channel
  • Contention Database (CDB)
    • Define rules to resolve contention between APs during spectrum allocation
    • One simple example:
      • CDB generate random primes periodically
      • Two contending APs generate their own numbers and compared the residuals through CDB
whitenet distributed spectrum allocation
WhiteNet: Distributed Spectrum Allocation
  • Why distributed?
    • There may not be any central controller, there are only central database.
    • The controller may become a bottleneck in the centralized scheme
  • Our assumptions
    • All APs are good people
    • They can follow some rules
  • The problem is: How to define such “rules”?
existing rule based spectrum allocation schemes
Existing Rule-based Spectrum Allocation Schemes
  • Assign each AP with spectrum around its “fair” share, so everyone will be happy
  • But, what’s a “fair” share?
    • Poverty Line: a lower bound on spectrum allocation to one AP, when the system converges
unique challenges in tvws
Unique challenges in TVWS
  • Frequency Heterogeneity of TV band
    • Higher frequency  higher path loss
    • Non-uniform path loss non-uniform interference graphs in different bands
  • Fragmented Spectrum in a very wide range (54MHz-698MHz), but limited capability for each AP

A limited number of contiguous spectrum width, especially in urban areas

Figure from WhiteFi, SIGCOMM09

whitenet s way b safe distri b uted s pectrum a llocation f or whitespac e
WhiteNet’s Way: B-SAFE: DistriButed Spectrum Allocation For whitespacE
  • For Frequency Heterogeneity:
    • Use different interference graphs in different bands
    • Define an “aggregated poverty line” to jointly value the fair share of each AP in all available bands
  • For Frequency Fragmentation:
    • Enable NC-OFDM to utilize fragmented spectrum
    • Consider the spectrum span constraint of each AP
b safe key ideas
B-SAFE: Key Ideas
  • Aggregated Poverty Line

: Number of spectrum chunks in white space i

: Number of neighbors of AP j in white space i

: Number of white spaces

Now the problem becomes: How to assigned each AP with spectrum of width APL

b safe key ideas1
B-SAFE: Key Ideas
  • Aggregated Poverty Line
b safe key ideas2
B-SAFE: Key Ideas
  • Consideration of spectrum span constraint
    • One radio can access limited spectrum span
    • Partition all white spaces according to the maximum spectrum span
    • Calculate APL for each partition
    • AP chooses one partition to do spectrum allocation
b safe algorithm outline
B-SAFE: Algorithm Outline
  • APs contend through database to assign spectrum one by one
  • AP calculates its APL in each partition
  • AP select one partition and use Best Fit to do spectrum allocation
  • Other APs update available spectrum

Until all APs are assigned spectrum

whitenet ap discovery
WhiteNet: AP Discovery
  • In WiFi
    • AP is operating on one of the pre-defined WiFi channels
    • AP periodically broadcast beacon messages
    • A Client scans APs of each WiFi channels and records their corresponding signal strengths
  • Challenges in WhiteNet
    • AP may be assigned with non-continuous spectrum
    • Both the bandwidth and center frequency of each AP can be variable
ap discovery
AP Discovery
  • Our Key Ideas
    • Each AP uses its left most frequency for beacon message

Beacon for AP1

Beacon for AP1

ap discovery1
AP Discovery
  • Our Key Ideas
    • Each AP uses its left most frequency for beacon message
    • All APs can be discovered one by one

Cancel out the spectrum of AP1, AP2’s beacon can be found

outline1
Outline
  • WhiteNet System Architecture
    • WhiteNet Database Design
    • Spectrum Allocation Algorithm
    • AP Discovery
  • System Implementation
  • Evaluation
  • Conclusion
system implementation
System Implementation
  • Platform
    • 7 USRP nodes
  • Hardware limitations
    • USRP cannot support a very wide bandwidth
    • No proper antenna for TV band
  • WhiteNet’s solution
    • Define 40 channels, each of width 125KHz, in 2.4GHz band
    • Set different power on different bands to obtain different interference relationships
system implementation1
System Implementation
  • Proof of concept implementation of the WhiteNet Local Database
    • Run on one linux machine as an user level process, data stored in memory.
    • APs use socket to connect this machine through wired backhaul
    • Define communication protocol between AP and database process
system implementation2
System Implementation
  • NC-OFDMA
    • Support multiple users
    • Currently, only downlink is supported, since it is hard to synchronize multiple users
    • Uplink implementation will be left as our future work
  • AP discovery
    • MAC protocols for both AP and the user to set up connection
outline2
Outline
  • WhiteNet System Architecture
    • WhiteNet Database Design
    • Spectrum Allocation Algorithm
    • AP Discovery
  • System Implementation
  • Evaluation
  • Conclusion
system evaluation
System Evaluation
  • Three topologies
  • Two metrics
    • System goodput
    • Proportional fairness of the spectrum allocation
  • Four spectrum algorithms
    • B-SAFE
    • Uni-Conservative: use the interference graph on the lowest frequency for all white spaces
    • Uni-Aggressive: use the interference graph on the highest frequency for all white spaces
    • Continuous allocation
efficiency of b safe vs uniform interference graph
Efficiency of B-SAFE(vs. uniform interference graph)

System goodput in topology 1, similar results in other topologies

Proportionally fairness of the system in topology 1

efficiency of b safe vs continuous allocation
Efficiency of B-SAFE(vs. continuous allocation)

Results of system goodput in topology 1

Results of proportionally fairness of the system in topology 1

system evaluation1
System Evaluation
  • Efficiency of the AP discovery scheme
  • Topology
    • Two APs: 1 & 3
    • Clients: 2, 4, 5, 7
    • Clients try to connect one AP one by one
  • The baseline scheme
    • Check all the combinations of different center frequencies and different spectrum width
  • Metric
    • Time to discover all neighboring APs
time in ap discovery
Time in AP Discovery

Results are an average of 8 round tests

simulation evaluation
Simulation Evaluation
  • To evaluate B-SAFE in a larger scale network
  • Scenario
    • 20-120 APs randomly distributed in a 1km × 1km area.
    • Randomly generated 6−30 vacant TV channels
    • Interference graph generated according to the path loss model
  • Metrics
    • Average goodput of each AP
    • Average utility of each AP
simulation results
Simulation Results

vs. uniform interference graph

vs. continuous allocation

conclusion
Conclusion
  • WhiteNet architecture coherent with the FCC’s rule
  • Distributed spectrum allocation algorithm –

B-SAFE

  • AP discovery and connection based on a novel beacon scheme
  • Both system and simulation evaluation show the efficiency of the proposed algorithm
thanks questions
Thanks!Questions?

Database-Assisted Multi-AP Network on TV White Spaces: Architecture, Spectrum Allocation and AP Discovery

Xiaojun Feng

xfeng@cse.ust.hk