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A Radio Multiplexing Architecture for High
Throughput Point to Multipoint Wireless Networks
Ramakrishna Gummadi
Rabin Patra,Sergiu Nedevschi, Sonesh Surana, Eric BrewerUC Berkeley
WiNS-DR 2008
MIT CSAIL
A radio multiplexing architecture
Architecture (noun): the manner in which the components of a computer or computer system are organized and integrated
- For what particular wireless configuration?- Why do we care about a radio architecture?
19 September 2008 Networked Systems for Developing Regions (NSDR)
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18 August 2008 Networked Systems for Developing Regions (NSDR)
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Rural network connectivity What is the need?
Divide: Rural vs urban, Intranet vs Internet
Applications: Health, Education, Information access
Requirements for rural networks Low cost per user Good performance (throughput) Grassroots deployment and management Scalable expansion
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Typical rural scenario
Com3Com3Com3
Village
City
Optical Fibre uplink
City
Village
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Typical rural scenario
Com3Com3Com3
Village
City
Optical Fibre uplink
City
Village
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Point-Multipoint (PMP) networks
Com3Com3Com3
Village
City
Optical Fibre uplink
City
Village
18 August 2008 Networked Systems for Developing Regions (NSDR)
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PMP features Base station:
Multiple sector or steerable antennas Multiple radios
Client: Single radio Directional antenna
Distances: Up to 20km
Traffic: Demands are time-varying and bursty
Why a radio architecture for PMP? Point-point (P-P) links needs high
throughput Know how to do this well for P-P (e.g., 2P,
WiLDNet) But cannot extend to PMP directly
System as a whole susceptible to interference Maintaining links tedious and error-prone Incremental scalability hard Inflexible to bursty traffic Most importantly, high total cost of ownership
18 August 2008 Networked Systems for Developing Regions (NSDR)
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High cost?
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Cost: $70,000
Cost: $3,000
In relative GDP terms, costs can be comparable!
Towers are the hidden costRequirements Low cost per user Good performance
(throughput) Grassroots
deployment and management
Scalable expansion
Status quo Large initial costs Interference lowers
throughput Expensive and tedious to
realign or troubleshoot Adding capacity and links
impossible once “maxed out”
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Goal
Design and evaluate high-throughput yet low-cost radio multiplexing architectures for PMP n/w
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Where is the architecture?
18 August 2008 Networked Systems for Developing Regions (NSDR)
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.
.
.
Architecturegoes here
Cheap$$,
lower interference with larger sector separation
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Talk outline Why multiplexing architecture?
Architectural principles and implications
Evaluation
18 August 2008 Networked Systems for Developing Regions (NSDR)
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5
6
21
4
12
3
11
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9
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Single sector scenario Clients: c1 ,c2 …cn
Single base-station
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Multiple-sector scenario (today)
Clients: c1 ,c2 …cn
Radios: R1 ,R2 …Rm
Sector antennas5
6
21
4
12
3
11
10
8
9
7
Ch: 1
Ch: 1
Ch: 1
But interference can kill
18 August 2008 Networked Systems for Developing Regions (NSDR)
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A
B
C
1
2
Simultaneousreceive
α A
B
C
1
2
Simultaneoussend
α
α should be large enough!
Principle 1: Separate channels for more degrees of freedom
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Clients: c1 ,c2 …cn
Radios: R1 ,R2 …Rm
Sector antennas Each sector on
different channel Both
directional and frequency separation gains
5
6
21
4
12
3
11
10
8
9
7
Ch: 1
Ch: 2
Ch: 3
Principle 2: Exploit spatial reuse
Multiple channels per sector antenna
Channels as widely separated as possible
Spatial diversity and multiplexing gains
18 August 2008 Networked Systems for Developing Regions (NSDR)
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5
6
21
4
12
3
11
10
8
9
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Principle 3: Use cheap h/w to increase capacity
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Wireless cards cheap
Commodity splitters and combiners cheap
Linear capacity increase possible
But ensure sufficient RF isolation!
RF isolation Isolation from
commodity splitters may not be enough
TDMA MAC solves this problem nicely
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Tx
Rx
Principle 4: Allocate radios dynamically for bursty traffic
Client traffic is bursty Static radio assignment
sub-optimal A multiplexing
controller after splitter switches radios to clients dynamically
2/4-port muxers affordable; higher port counts lossy and costly
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Key architectural benefits
Number of sectors: S Number of orthogonal
channels: C
Total #antennas: S*C
Peak #clients per sector: C
After Still S*C cards, but:
Total #antennas: S Towers can be smaller
Peak #clients per sector: S*C Greater spectral efficiency So, more throughput per
client, or more clients
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Before
Additional benefits Low cost per user
Fewer antennas with more channels and radios Grassroots deployment and management
Shorter towers means easier alignment If radio or link fails, switch to under-used or
spare Scalable expansion
New clients added by allocating radios permanently
18 August 2008 Networked Systems for Developing Regions (NSDR)
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18 August 2008 Networked Systems for Developing Regions (NSDR)
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Talk outline Why multiplexing architecture?
Architectural principles and implications
Evaluation
Evaluation 3 clients, 3 PMP links Radios: 25 dBm
max. 3-way muxer, 20 dB
isolation 20 dB attenuators Metrics:
Simultaneous Tx/Rx, Tx+Rx throughput
Effect of channel separation, isolation
18 August 2008 Networked Systems for Developing Regions (NSDR)
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Aggregate UDP throughput
18 August 2008 Networked Systems for Developing Regions (NSDR)
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#Radios
Channels
All radios
Tx
All radios
Rx
MixTx/Rx
2 1,6 14.51 14.89 14.77
2 6,11 14.74 13.98 14.16
2 1,11 13.90 13.79 13.70
3 1,6,11 21.14 20.34
Muxing works as expected for 3 radios, even for Rx/Tx
Throughput vs. isolation
18 August 2008 Networked Systems for Developing Regions (NSDR)
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At sufficient channel separation and isolation, aggregate CSMA throughput unaffected.
Need for TDMA otherwise.
Conclusions Radio multiplexing can reduce large-towers
Maintains throughput and simplifies management Commodity splitter and combiners can be
used Need to think about RF isolation carefully Cost<->complexity trade-off can be hard
At PHY layer, complementary to WiLDNet Future work
Look into actual deployments TDMA MAC must synchronize Tx/Rx across radios
18 August 2008 Networked Systems for Developing Regions (NSDR)
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18 August 2008 Networked Systems for Developing Regions (NSDR)
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Thank You!