Mobile and Wireless Compu2ng CITS4419 Week 2: Wireless Communica2on

Transcription

1 Mobile and Wireless Compu2ng CITS4419 Week 2: Wireless Communica2on Rachel Cardell- Oliver School of Computer Science & So8ware Engineering semester

2 MoBvaBon (for CS students to study radio propagabon) Understanding the wireless channel is essenbal for design, deployment and management of wireless networks The complexity of the radio channel make wireless networks far more complicated than wired ones

3 Topics Wireless propagabon basics Wireless spectrum ProperBes of wireless channels End user metrics

4 Wireless PropagaBon Basics

5 Important characterisbcs of a wireless channel Achievable signal coverage Measured as received signal strength as a funcbon of distance Modelled using path loss models Maximum data rate Limited by mulbpath structure, fading, signaling scheme, and receiver design Rate of channel fluctuabons Caused by movement of the transmiser, receiver, or objects in between

6 Signal propagabon (in theory)

7 Signal PropagaBon (in prabce) Power Level 1 Source: UWA Honours Thesis by Niraj Vitvani

8 Link Budget A simple link budget equabon: Received Power (dbm) = Transmit Power (dbm) + Gains (db) Losses (db) Note that decibels are logarithmic measurements, so adding decibels is equivalent to mulbplying the actual numeric rabos.

9 Receive Signal Strength (dbm) Decibel (db) is a logarithmic unit for expressing the rabo of 2 physical values Usually with one standard reference value dbm indicates a reference power of 1 milliwas

10 Path loss Path Loss is the reducbon in power density of an electromagnebc wave as it propagates through space Path loss is used in the analysis and design of communicabons link budgets It determines how far apart 2 sensors can be and sbll have reliable communicabon

11 Free Space Path Loss Model Free space propagabon: ideal model of loss that would occur in a region free of all objects that might observe or reflect radio energy. For tx power P t and rx power P r distance d and L 0 a constant depending on tx frequency P r =L 0 P t /d 2

12 Reality In reality the drop in signal strength is d n rather than d 2 (n ranges from 2 to 6 in pracbce) so P r =L 0 P t /d n In decibels (dbm is rabo relabve to 1 mw) P r (dbm) = P t (dbm) + L 0 (db) - 10 n log 10 (d) And path loss is given by L p = 10 n log 10 (d) - K(dBm)

13 Plane Earth Path Loss Model n = 4 standard, n>4 foliage or buildings d = distance in meters h = height of transmitter and receiver antennas LPE is independent of frequency

14 Bushland LPE Model (n=4.45)

15 Urban LPE Model (n=5.07)

16 Log distance model Constants B and n and reference distance d0 = 1 meter

17 LoRa tx path loss observed

18 Signal to Noise RaBo Not actually a rabo but the difference in decibels between the received signal and background noise (noise floor). Example: a radio receives signal of - 60 dbm, noise floor is - 90 dbm, SNR is 30 db SNR of db usually means unreliable communicabon SNR of 20 or more is good, 25+ for voice

19 Signal to Noise RaBo Source: hsps://documentabon.meraki.com/mr/wifi_basics_and_best_pracbces

20 Measured SNR SNR quanbfies how much of a signal (meaningful info) has been corrupted by noise (unwanted signal) SNR = rabo of signal power to noise power

21 1 km UWA 40MHz campus range test

22 Kings Park 40 MHz range test m 1.8 k

23 Wireless Spectrum

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27 Industrial, ScienBfic and Medical (ISM) InternaBonal agreement radio bands Does not require permission from the radio licensing authority (ACMA) Does not require extensive tests to ensure that transmissions do not interfere with other licensed bands Unlicensed spectra of choice for sensor networks are the 433 MHz all, 868MHz band in Europe, the 916MHz band in the USA and Aus, and the 2.4GHz band that is available almost everywhere in the world. The 2.4GHz band has the widest available bandwidth and is becoming a popular choice for (indoor) sensor networks. Downside: Microwave ovens 2.45 GHz, IEEE WLANs, cordless phones, Bluetooth, and many other wireless devices also operate in these bands; significant interference in areas of dense deployment. AddiBonally: frequency allocabons for short range devices (< mw power) eg power meters (169.4 MHz), RFID (13.5 MHz), radio- controlled models, garage door openers, car keys

28 Radio spectrum for sensor networks

29 Radio Frequency Trade- Offs Opera2ng Frequency Band 40 MHz 868 MHz 2.4 GHz Wavelength 7.5 m 35cm 12cm Product example Mannheim 40MHz node Xbee- PRO 868 (Europe) Xbee- PRO ZB Data rate 20 bits ps 24 kbits ps 250 kbits ps Range (indoor/urban) Range (outdoor line of sight) Tx power output Tx current 1800 m tested 250 m tested 550 m dsheet 90 m dsheet 12 km tested Up to 40km dsheet 40 mw(16dbm) 30 ma ma 1.5 km dsheet 63 mw (+18 dbm) 205 ma Rx sensibvity dbm - 112dBm dbm

30 ProperBes of Wireless Channels

31 CommunicaBon Link CharacterisBcs 1. Path Loss 2. Noise: probabilisbc packet recepbon 3. Time varying link quality 4. Asymmetric links 5. Packet Collisions 6. Limited communicabon range (hidden terminal problem) 22 Sep 2010 Mannheim Summer School 31

32 Noisy Radio RecepBon Footprints 22 Sep 2010 Mannheim Summer School 32

33 Grey Zone Packet recepbon has a transi2onal (grey) zone where PRR is hard to predict

34 Time varying link quality Jingbo Sun PhD 2009 Wireless links are dynamic and lossy Time Space Energy is limited for communicabon

35 Packets received per 6 hours from 1 to 31 October Number of Packets Received node-10 node-11 node-13 node-14 node-15 node-16 node-18 node Oct 08-Oct 13-Oct 18-Oct 23-Oct 28-Oct

36 Link quality variabon: LoRa Strawberry Day of Year RSSI Tomato 1 Day of Year RSSI

37 Asymmetric links are common Broadcast packet delivery probability % 30-70% 1-30% 1 kilometer Measurements from an b Mesh Network Daniel Aguayo et al, SIGCOMM 2004

38 Interference

39 Packet Collisions

40 Hidden Terminal

41 Channel Metrics for End- Users

42 Channel Quality Metrics Bit error rate (BER): The number of bit errors divided by the total number of transferred bits during a studied Bme interval Tx BER = 2/8 = 0.25 Rx Packet Delivery Rate: The rabo of correctly received packets (including re- transmissions) to transmised packets Data Delivery Rate: The rabo of unique received packets to all transmised packets (bits per second, or packets/sec)

43 Homework Reading Read SecBons 13.4 and 13.5 (pp 570 to 580) of the following overview. Pahlavan, K. and Krishnamurthy, P Networking Fundamentals: Wide, Local and Personal Area Communica2ons Wiley. Chapter 13: Wireless Sensor Networks, SecBons 13.4 and 13.5 hsp://media.johnwiley.com.au/product_data/excerpt/ 91/ / pdf