Synchronous Two-Phase Rate and Power Control in WLANs

Transcription

1 Synchronous Two-Phase Rate and Power Control in WLANs Kishore Ramachandran, Ravi Kokku, Honghai Zhang, and Marco Gruteser WINLAB, Rutgers University and NEC Laboratories America

2 Towards All-Wireless Enterprises Last-mile network access predominantly High bandwidth, low/no cost Voice and data converging onto mobile devices 100 million WiFi handsets over the next 5 years. Source: IDC, Frost & Sullivan, Infonetics success => changing requirements Greater density Higher mobility Increased battery life

3 Why Power and Rate Control? Integral Components in Overall Solution Adaptive transmit power control (TPC) can: Improve spatial reuse and network capacity Reduce energy consumption for mobile devices Compensate for link changes due to mobility Hard problem in WLANs due to distributed operation and unlicensed spectrum

4 Symphony: A new approach to transmit power control (TPC) in WLANs ~50 ft. ~20 ft. Dense Enterprise WLANs Goals: increase network capacity and improve battery life

5 Related Work Domain Solution Granularity Realization Deployability Energy Capacity Rate Channel Access Asymm. Hidden Nodes WLANs [Sheth02] [Qiao03] [Akella05] [Chevillat05] Per-link Per-link Per-link Per-link [Mhatre07] Per-cell Ad-hoc Nets [Monks01] [Jung02] [Muqattash03] [Muqattash05] [Sheth05] [Kim06] [Shah07] [Narayanaswa my02] Per-link Per-link Per-link Per-link Per-link Per-link Per-link Per-network No solution is both comprehensive and realizable

6 Outline Introduction Challenges to TPC in WLANs Symphony Design Experimental Results Summary

7 Challenge #1: Receiver-side Interference and Asymmetric Channel Access AP1 C AP2 AP1 AP2 B B A Receiver-side interference A Asymmetric channel access Receiver-side interference: Incorrect power reduction results in increased packet error rate Asymmetric channel access: Incorrect power reduction results in increased channel access time

8 Challenge #2: Interaction with Rate Adaptation maxrate Rate +ve -ve (minrate, minpower) Power maxpower Measuring SINR at fine time scales --- infeasible with mobility Use delivery ratio over a window of packets [Wong06] Selecting rate and power non-trivial Incorrect power reduction results in reduced rate

9 Challenge #3: Mobility With mobility, challenges #1 and #2 can occur intermittently Observations: Performance at maximum power --- key reference point Mobility => Periodic reference measurement needed Some form of co-ordination between transmitters --- needed

10 Outline Introduction Challenges to TPC in WLANs Symphony Design Experimental Results Summary

11 Symphony Design: Two-phase Execution 1 ms have passed REFERENCE Track performance at max. power OPERATIONAL Reduce power if possible 2 ms have passed Goal: Link s performance should be at least as good as in the baseline maximum power network

12 Symphony Design: Synchronous Operation Phase REF At T1 OPT Time Phase REF At T2 OPT Time All transmitters cycle through the two phases in synchrony

13 Outline Introduction Challenges to TPC in WLANs Symphony Design Experimental Results Summary

14 Symphony Evaluation Does it deal well with the challenges? Does Symphony increase spatial reuse? Does it reduce energy consumption? Is it incrementally deployable (with non-compliant nodes)?

15 Platform: Indoor Office Testbed and ORBIT Static Nodes Problem scenarios Mobile clients Hardware: Dell laptops or ORBIT nodes (with Atheros NICs) Software: Linux with MadWifi driver (v0.9.3.x) Synchronization: NTP between APs, broadcast msg. for clients Traffic: Mix of emulated VoIP, 200pps, and TCP

16 Results Synopsis Channel Access Asymmetry Challenges Hidden Terminals Goals Battery life Spatial Reuse Improvement 300% throughput improvement Detect and correct in 1 sec. Up to 50% improvement Up to 46% improvement From a deployment perspective: Symphony opportunistically reduces the transmit power by 6dB even in the presence of non-compliant nodes

17 Summary Symphony is Effective in addressing TPC challenges Easy to realize in a WLAN Readily deployable (even with non-compliant nodes)

18 Thanks! For more information:

19 Extra Slides

20 Traditional approach in WLANs: Static or coarsely dynamic per-cell TPC AP1 AP2 A B D C AP1 A interferes with AP2 B Strategy 1: Fixed TPC For all clients, use maximum power (18-20dBm) Strategy 2: Per-cell TPC [Mhatre07] For all clients in a cell, use power level for weakest client Adjust power at coarse time intervals (10s of seconds)

21 Symphony: Dynamic per-link TPC AP1 AP2 A B D C AP1 A and AP2 B can talk simultaneously Choose power on a per-client (per-link) basis Adjust power at fine-grained time intervals Goal: Use as much power as needed and adapt to mobility

22 Symphony: Implementation Network Layer Rcv. pkts Snd. pkts Device driver Symphony Receive path Transmit path SET Power Rate RTS Rate Power adaptation R O R O R O R-side Asymm. access tx_pkt() tx_pkt_complete() Interface card Implemented in MadWifi v

23 Symphony: Enables up to 18dB power reduction Blah blah

24 Symphony: Negligible R-score degradation Average R-score difference: 2 Worst case R-score difference: 3.4

25 Symphony: Up to 46% more battery life Active Mode Battery life Improvement Setup ORBIT Indoor Office Symphony Up to 46% (relative to max. power) Up to 33% (relative to max. power)

26 Symphony: 30-50% throughput improvement Two links Three links

27 Results Summary Improves throughput of asymmetry-affected links by 300% Is responsive to receiver side interference even at short timescales of 1 second. For mobile VOIP clients, opportunistically reduces transmit power by up to 97%, with negligible voice quality degradation Increases network throughput by up to 50% Increases battery life by up to 46% (relative to max. power)

28 Evaluation Details Hardware: Dell laptops or ORBIT nodes (with Atheros NICs) Software: Linux with MadWifi driver (v0.9.3.x) Synchronization: NTP between APs, broadcast msg. for clients Traffic: Mix of emulated VoIP, 200pps, and TCP Traditional Power: 18dBm fixed Rate: Variable (RRAA+) Symphony Power: 3dB step size Rate: Variable (RRAA+)

29 Future Work