A Radio Multiplexing Architecture for High Throughput Point to Multipoint Wireless Networks

1. 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

2. 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? Networked 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 Networked Systems for Developing Regions (NSDR)

4. Typical rural scenario Networked Systems for Developing Regions (NSDR)

5. Typical rural scenario Networked Systems for Developing Regions (NSDR)

6. Point-Multipoint (PMP) networks Networked 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 Networked Systems for Developing Regions (NSDR)

8. 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 Networked Systems for Developing Regions (NSDR)

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

10. Towers are the hidden cost Requirements Status quo Large initial costs Interference lowers throughput Expensive and tedious to realign or troubleshoot Adding capacity and links impossible once “maxed out” • Low cost per user • Good performance (throughput) • Grassroots deployment and management • Scalable expansion Networked Systems for Developing Regions (NSDR)

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

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

13. Talk outline • Why multiplexing architecture? • Architectural principles and implications • Evaluation Networked Systems for Developing Regions (NSDR)

14. Single sector scenario • Clients: c1 ,c2 …cn • Single base-station 5 7 4 9 6 3 8 11 12 1 10 2 Networked Systems for Developing Regions (NSDR)

15. 5 7 4 9 6 3 8 11 12 1 10 2 Multiple-sector scenario (today) • Clients: c1 ,c2 …cn • Radios: R1 ,R2 …Rm • Sector antennas Ch: 1 Ch: 1 Ch: 1 Networked Systems for Developing Regions (NSDR)

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

17. 5 7 4 9 6 3 8 11 12 1 10 2 Principle 1: Separate channels for more degrees of freedom • Clients: c1 ,c2 …cn • Radios: R1 ,R2 …Rm • Sector antennas • Each sector on different channel • Both directional and frequency separation gains Ch: 2 Ch: 3 Ch: 1 Networked Systems for Developing Regions (NSDR)

18. 5 7 4 9 6 3 8 11 12 1 10 2 Principle 2: Exploit spatial reuse • Multiple channels per sector antenna • Channels as widely separated as possible • Spatial diversity and multiplexing gains Networked Systems for Developing Regions (NSDR)

19. Principle 3: Use cheap h/w to increase capacity • Wireless cards cheap • Commodity splitters and combiners cheap • Linear capacity increase possible • But ensure sufficient RF isolation! Networked Systems for Developing Regions (NSDR)

20. RF isolation Tx • Isolation from commodity splitters may not be enough • TDMA MAC solves this problem nicely Rx Networked Systems for Developing Regions (NSDR)

21. 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 Networked Systems for Developing Regions (NSDR)

22. Key architectural benefits 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 Before • Number of sectors: S • Number of orthogonal channels: C • Total #antennas: S*C • Peak #clients per sector: C Networked Systems for Developing Regions (NSDR)

23. 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 Networked Systems for Developing Regions (NSDR)

24. Talk outline • Why multiplexing architecture? • Architectural principles and implications • Evaluation Networked Systems for Developing Regions (NSDR)

25. 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 Networked Systems for Developing Regions (NSDR)

26. Aggregate UDP throughput Muxing works as expected for 3 radios, even for Rx/Tx Networked Systems for Developing Regions (NSDR)

27. Throughput vs. isolation At sufficient channel separation and isolation, aggregate CSMA throughput unaffected. Need for TDMA otherwise. Networked Systems for Developing Regions (NSDR)

28. 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 Networked Systems for Developing Regions (NSDR)

29. Thank You! Networked Systems for Developing Regions (NSDR)