Adap%ng to the Wireless Channel II: SampleWidth

Transcription

1 Adap%ng to the Wireless Channel II: SampleWidth M038/GZ06 Mobile and Adap-ve Systems Kyle Jamieson Lecture 8 Department of Computer Science University College London 1

2 Outline A Case for Adap%ng Channel Width in Wireless Networks. Chandra et al., Proceedings of the ACM SIGCOMM Conference, August, Reducing power and increasing range 2. Improving flow throughput 3. Improving fairness 4. Improving capacity 5. The SampleWidth algorithm 2

3 Narrower bandwidth, slower in %me SNR: 3 db Amplitude vs. frequency ( Spectrum ) SNR: +3 db Signal 2W 1/ (W/2) 1/ (2W) 1/ W W W/2 Noise Frequency (Hz) è Amplitude vs. %me (2W) W (W/2) T/2 T 2T Time (sec) è 3

4 Narrower bandwidth: higher SNR Amplitude vs. frequency ( Spectrum ) Signal SNR=3 db SNR=6 db SNR=9 db Noise Frequency (Hz) è 4

5 Reducing bandwith and transmit power Amplitude vs. frequency ( Spectrum ) Signal Noise Frequency (Hz) è Amplitude vs. %me Time (sec) è 5

6 Is it just noise, or a signal in noise? Received signal Transmi7ed signal + or 0 +? 6

7 Lower bandwidth filter reduces noise Narrower signal è narrower matched filter Noise Signal Noise Signal High SNR Lower SNR 7

8 Adap%ng channel width: OFDM RF front end clock determines channel width Baseband/MAC processor clock determines sampling period T Same PLL drives both clocks! OFDM Symbol OFDM Symbol OFDM Symbol... Time Guard interval OFDM guard interval prevents the echoes of one OFDM symbol from corrup%ng the next (intersymbol interference) 5MHz 10 MHz 20 MHz 40 MHz Symbol Duration 16 µs 8 µs 4 µs 2 µs SIFS 40 µs 20 µs 10 µs 5 µs Slot Duration 20 µs 20 µs 20 µs 20 µs Guard Interval 3.2 µs 1.6 µs 0.8 µs 0.4 µs 8

9 Adap%ng channel width: MAC Modify backoff slot dura%on to be the same across channel widths: backoff fairness between different nodes What about SIFS? 5MHz 10 MHz 20 MHz 40 MHz Symbol Duration 16 µs 8 µs 4 µs 2 µs SIFS 40 µs 20 µs 10 µs 5 µs Slot Duration 20 µs 20 µs 20 µs 20 µs Guard Interval 3.2 µs 1.6 µs 0.8 µs 0.4 µs 9

10 Outline A Case for Adap%ng Channel Width in Wireless Networks. Chandra et al., Proceedings of the ACM SIGCOMM Conference, August, Reducing power and increasing range 2. Improving flow throughput 3. Improving fairness 4. Improving capacity 5. The SampleWidth algorithm 10

11 Experimental Setup Clean experiment using alenuator and channel emulator Tested channel widths of 5, 10, 20, 40 MHz Only use OFDM bit rates, for consistency Indoor experiments at MSR & UCSB Outdoor experiments in large park (results not in paper) 11

12 Is peak throughput propor%onal to channel width? Shannon capacity tells us: # C = B " log% 1+ S $ N & ( ' Higher relacve overhead at high B for inter- frame spacings 12

13 Modeling throughput Extending model by Gast, Wireless Networks: The Defini%ve Guide, O Reilly, MHz 10 MHz 20 MHz 40 MHz Symbol Duration 16 µs 8 µs 4 µs 2 µs SIFS 40 µs 20 µs 10 µs 5 µs Slot Duration 20 µs 20 µs 20 µs 20 µs Guard Interval 3.2 µs 1.6 µs 0.8 µs 0.4 µs B = 20 MHz B Consider total transaccon Cme t, for a single packet: t = t CW + t DIFS + t data + t SIFS + t ACK ( ) + B (!) + Bt SIFS + B (!) ( ) = 8t slot + 2t slot + Bt SIFS = 10t slot + B! Analy%cal model throughput = 1/t Mbits/s Constant term plus term propor%onal to 1/B 13

14 Throughput model matches experiments Measurements from emulator experiments Reference point: analy%cal throughput model Alribute slowdown at high B to beacons, background noise 14

15 Lower channel width, lower loss rate Emulator experiment at 6 Mbit/s modula%on Range threshold: alenua%on at which loss rate < 10%

16 Empirical vs theore%cal SNR gain Amplitude vs. frequency ( Spectrum ) Signal Noise SNR Symbol %me T/2 SNR +3 db Symbol %me T Frequency (Hz) à SNR= +6 db Symbol %me 2T Empirical 7 db gain short of 9 db theore%cal gain from 5 MHz à 40 MHz 16

17 More choices for each range Range threshold (db) +6 db 17

18 Transla%ng SNR gain to real distance Assume signal power decays as 1/d α " A =10log P % send Consider alenua%on A: $ ' =10( logd ) d =10 A /10( # & d 2 d 1 = 10A2 /10" 10 A 1 /10" =10#A /10" P recv ΔA = 6 db, α=4 d 2 /d 1 = % 18

19 Narrower channels reduce power Reduce power because of slower clock speed. 19

20 Channel widths: room for improvement (a) Emulator Similar mo%va%on to SampleRate: can adapt to stay on best channel width 20

21 Outline A Case for Adap%ng Channel Width in Wireless Networks. Chandra et al., Proceedings of the ACM SIGCOMM Conference, August, Reducing power and increasing range 2. Improving flow throughput 3. Improving fairness 4. Improving capacity 5. The SampleWidth algorithm 21

22 Improving fairness and load balancing Case 1 Case 2 40 MHz 10 MHz Client A 40 MHz 0 MHz Client A AP 1 AP 2 AP 1 AP 2 20 MHz 10 MHz 20 MHz 20 MHz AP 3 AP 4 AP 3 AP 4 Scenario AP 1 AP 2 AP 3 AP 4 T FI Case 1: (fixed) 1/6 1 1/ Case 1: (adaptive) 2/6 1/2 1/3 1/ Case 2: (fixed) 1/6 X 1/3 1/ Case 2: (adaptive) 2/6 X 1/3 1/

23 Capacity improvement Two laptop sender- receiver pairs, receivers in adjacent offices Move the two senders to 24 different loca%ons (x- axis) Maximum gain in the far- near case because of rate anomaly +10% (CSMA sharing) (Separated) +65% Average 23

24 SampleWidth algorithm overview How to realize these capacity gains in prac-ce? Find best channel width over one link Bit rate adapta%on works in the background Challenge: large two- dimensional search space (channel width bit rate) Start at narrowest channel width, adapt when send data Probe different channel widths Probe only adjacent channel widths Probe only if disconnec%on unlikely 24

25 SampleWidth: algorithm details Measure each second: Average throughput T cur Average bit rate chosen R cur Rule 1: R cur < α Mbps è probe narrower R cur > β Mbps è probe wider Choosing α, β: Measure efficiency: frac%on of an op%mal algorithm s throughput β =12 β =18 β =24 β =36 α = α = α = α = Rule 2: Else, choose B i : max i {T i } Probing table: B i T i R i 40 MHz 20 MHz T cur R cur 10 MHz 5 MHz 25

26 Probing versus switching Probing widths based on bit rates (R cur < α, R cur > β) Switching widths is based on throughput SampleWidth dis%nguishes between: Poor link quality è probe and move narrower High conten%on è probe but stay wide 26

27 We try all fixed widths and SampleWidth. Figure 13(a) shows the power consumption behavior in detail for all configurations at the sender. The fixed width systems start out at their idle mode power consumption, move to their send mode consumption level, and then come back to their idle mode levels. SampleWidth starts out at the idle mode level for 5 MHz, because that is least costly. When the transfer starts, it moves to the power consumption No local minimum level of 40condiCon MHz, because (λ > that 1): yields the least powerper-byte ratio. When the transfer finishes, Throughput it comes back to the Throughput 5MHzlevel.Figure13(b)showsthatthroughthisadaptation,SampleWidth T(Bis i ) able to consume the least total amount T(B i ) of energy. T(B i )/λ T(B i )/λ Does SW get stuck in local minima? 6.4 Efficiency of Autorate & Smoothness SampleWidth uses autorate to probe channel widths and find an efficient data rate. We justify this design choice by showing that i- 1 i i+1 i- 1 i i+1 modern autorate algorithms are indeed capable of achieving close to optimal throughput. Channel Figurewidth 14 shows the suboptimality inchannel terms width of reduction Smoothness in throughput at B i : of using Atheros s Smoothness proprietary criterion: autorate implementation on # S( B i ) = max T Windows ( B XP in i) T( B i"1 ), T( B i ) & comparison to using the best possible modulation $ in a stationary' indoor S = setting. min i S( BThe i ) "1 important for SW convergence observation is that % at all measurement T( B i+1 )( points, autorate performs within at most 16% of the optimal data rate. Empirical measurement using bit rate adaptacon: Rate S In order to see whether autorate is sufficiently close to the optimum in order for SampleWidth to converge, recall the definition of 27

28 [Slide: Ranveer Chandra] SampleWidth adapts to best channel width Emulator experiment: vary alenua%on in 1 db steps Run UDP throughput measurement Two nodes (sender, receiver); sta%c configura%on 28

29 Close match between emulator, indoors Emulator experiment Adjust alenua%on (db) in wireless emulator Indoor experiment Separate sender and receiver by varying number of offices between them (a) Emulator. The labels depict transition points where that width becomes better that the adjacent wider channel. (a) Emulator. The labels depict transition points where that (b) Indoor 29 Figure 12: Comparison of throughput achieved using SampleWidth with that of static width schemes in emulator and indoor settings. Figure 1 ferent c involves

30 [Slide: Ranveer Chandra] SampleWidth saves energy One sender, one receiver One minute experiment; transfer one 25 MB file with TCP Try sta%c fixed widths, and SampleWidth Total Energy (Joules) MHz 10MHz 20MHz 40MHz SW 30

31 [Slide: Ranveer Chandra] How SampleWidth saves energy 31

32 [Slide: Ranveer Chandra] Applica%on Scenarios 1. Throughput/energy- aware song sharing 2. Load aware spectrum alloca%on in WLANs 3. Improved capacity in Cogni%ve (DSA- based) networking 32

33 Channel width interoperability issues RTS/CTS approaches will not work (need to decode) Can flows on different widths share the medium with just CSMA? Far scenario: senders can only par%ally hear each other (50% FLR) One flow always at 20 MHz; vary other flow s channel width 33

34 In wireless, SNR is unpredictable! Bit error rate é Fast fades QAM second BPSK Time è 34