A Radio Multiplexing Architecture for High Throughput Point to Multipoint Wireless Networks Ramakrishna Gummadi Rabin Patra, Sergiu Nedevschi, Sonesh Surana, Eric Brewer UC 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 2008Networked Systems for Developing Regions (NSDR) 2

18 August 2008Networked Systems for Developing Regions (NSDR) 3 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 2008Networked Systems for Developing Regions (NSDR) 4 Typical rural scenario

18 August 2008Networked Systems for Developing Regions (NSDR) 5 Typical rural scenario

18 August 2008Networked Systems for Developing Regions (NSDR) 6 Point-Multipoint (PMP) networks

18 August 2008Networked Systems for Developing Regions (NSDR) 7 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 2008Networked Systems for Developing Regions (NSDR) 8

High cost? 18 August 2008Networked Systems for Developing Regions (NSDR) 9 Cost: $70,000 Cost: $3,000 In relative GDP terms, costs can be comparable !

Towers are the hidden cost Requirements  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 2008Networked Systems for Developing Regions (NSDR) 10

Goal Design and evaluate high-throughput yet low-cost radio multiplexing architectures for PMP n/w 18 August 2008Networked Systems for Developing Regions (NSDR) 11

Where is the architecture? 18 August 2008Networked Systems for Developing Regions (NSDR) Architecture goes here Cheap $$, lower interference with larger sector separation

18 August 2008Networked Systems for Developing Regions (NSDR) 13 Talk outline  Why multiplexing architecture?  Architectural principles and implications  Evaluation

18 August 2008Networked Systems for Developing Regions (NSDR) Single sector scenario  Clients: c 1,c 2 …c n  Single base-station

18 August 2008Networked Systems for Developing Regions (NSDR) 15 Multiple-sector scenario (today)  Clients: c 1,c 2 …c n  Radios: R 1,R 2 …R m  Sector antennas Ch: 1

But interference can kill 18 August 2008Networked Systems for Developing Regions (NSDR) 16 A B C 1 2 Simultaneous receive α A B C 1 2 Simultaneous send α α should be large enough!

Principle 1: Separate channels for more degrees of freedom 18 August 2008Networked Systems for Developing Regions (NSDR) 17  Clients: c 1,c 2 …c n  Radios: R 1,R 2 …R m  Sector antennas  Each sector on different channel  Both directional and frequency separation gains 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 2008Networked Systems for Developing Regions (NSDR)

Principle 3: Use cheap h/w to increase capacity 18 August 2008Networked Systems for Developing Regions (NSDR) 19  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 2008Networked Systems for Developing Regions (NSDR) 20 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 2008Networked Systems for Developing Regions (NSDR) 21

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 2008Networked Systems for Developing Regions (NSDR) 22 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 2008Networked Systems for Developing Regions (NSDR) 23

18 August 2008Networked Systems for Developing Regions (NSDR) 24 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 2008Networked Systems for Developing Regions (NSDR) 25

Aggregate UDP throughput 18 August 2008Networked Systems for Developing Regions (NSDR) 26 #RadiosChannelsAll radios Tx All radios Rx Mix Tx/Rx 21, , , ,6, Muxing works as expected for 3 radios, even for Rx/Tx

Throughput vs. isolation 18 August 2008Networked Systems for Developing Regions (NSDR) 27 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 2008Networked Systems for Developing Regions (NSDR) 28

18 August 2008Networked Systems for Developing Regions (NSDR) 29 Thank You!