1. 1 Wireless Sensor Networks Physical Layer Mario Čagalj mario.cagalj@fesb.hr FESB University of Split 9/04/2010. Based on “Protocols and Architectures for Wireless Sensor Networks”, Holger Karl, 2005.

2. 2 Goal of this lecture oGet an understanding of the peculiarities of wireless communication >“Wireless channel” as abstraction of these properties – e.g., bit error patterns >Focus is on radio communication oImpact of different factors on communication performance >Frequency band, transmission power, modulation scheme, etc. >Transciever design oUnderstanding of energy consumption for radio communication

3. 3 oWhich part of the electromagnetic spectrum is used for communication >Not all frequencies are equally suitable for all tasks – e.g., wall penetration, different atmospheric attenuation oVLF = Very Low FrequencyUHF = Ultra High Frequency oLF = Low Frequency SHF = Super High Frequency oMF = Medium Frequency EHF = Extra High Frequency oHF = High Frequency UV = Ultraviolet Light oVHF = Very High Frequency 1 Mm 300 Hz 10 km 30 kHz 100 m 3 MHz 1 m 300 MHz 10 mm 30 GHz 100  m 3 THz 1  m 300 THz visible light VLFLFMF HF VHFUHFSHFEHFinfraredUV optical transmission coax cable twisted pair Radio spectrum for communication

4. 4 oSome frequencies are allocated to specific uses >Cellular phones, analog television/radio broadcasting, DVB-T, radar, emergency services, radio astronomy, … oParticularly interesting: ISM bands (“Industrial, scientific, medicine”) – license-free operation Some typical ISM bands FrequencyComment 13,553-13,567 MHz 26,957 – 27,283 MHz 40,66 – 40,70 MHz 433 – 464 MHzEurope 900 – 928 MHzAmericas 2,4 – 2,5 GHzWLAN/WPAN 5,725 – 5,875 GHzWLAN 24 – 24,25 GHz Frequency allocation

5. 5 http://www.ntia.doc.gov/osmhome/allochrt.pdf Electromagnetic Spectrum

6. 6 Transmitting data using radio waves oProduced by a resonating circuit (e.g., LC) oTransmitted through and antenna oBasics: Transmiter can send a radio wave, receiver can detect whether such a wave is present and also its parameters oParameters of a wave (e.g, a sine function) s(t)=A(t) sin( 2πf(t)t +  (t) ) >Parameters: amplitude A(t), frequency f(t), phase  (t) oManipulating these three parameters allows the sender to express data; receiver reconstructs data from signal oSimplification: Receiver “sees” the same signal that the sender generated – not true, see later!

7. 7 Time and Frequency Domains Different representations of the same signal. Spectral representation obtained using FFT (Fast Fourier Transform) frequency amplitude A f A/3 3f frequency amplitude A f A/3 3f

8. 8 Signal Modulation (I) oHow to manipulate a given signal parameter? >Set the parameter to an arbitrary value: analog modulation >Choose parameter values from a finite set of legal values: digital keying oModulation? >Data to be transmitted is used to select transmission parameters as a function of time >These parameters modify a basic sine wave, which serves as a starting point for modulating the signal onto it >This basic sine wave has a center frequency f c >The resulting signal requires a certain bandwidth to be transmitted (centered around center frequency)

9. 9 Signal Modulation (II) oUse data to modify the amplitude of a carrier frequency - Amplitude Shift Keying (ASK) oUse data to modify the frequency of a carrier frequency - Frequency Shift Keying (FSK) oUse data to modify the phase of a carrier frequency - Phase Shift Keying (PSK) © Tanenbaum, Computer Networks

10. 10 Signal Modulation (III) oQuadrature PSK (QPSK): Two bits >00 ≈ A sin(2πft + 7π/4) >01 ≈ A sin(2πft + 5π/4) >10 ≈ A sin(2πft + 3π/4) >11 ≈ A sin(2πft + π/4) oQuadrature Amplitude and Phase Modulation (QAM) QAM-4, QAM-16, QAM-64, QAM-256 >s(t) = I(t) cos( 2πf c t) - Q(t) sin( 2πf c t) I(t) and Q(t) are the modulating signals (analog modulation) >I(t) > “in-phase” componenet, Q(t) > “quadrature” component >s(t) is a linear combination of two orthogonal signal waveforms >Received signal (ideal case) I(t) component is demodulated as r(t) = s(t) cos( 2πf c t) = ½ I(t) + ½ [I(t) cos( 4πf c t) + Q(t) sin( 4πf c t)] >By filtering a low-pass filter we can recover the I(t) and the Q(t) terms

11. 11 Signal Modulation (IV) oQuadrature PSK (QPSK): Two bits >00 ≈ A sin(2πft + 7π/4) >01 ≈ A sin(2πft + 5π/4) >10 ≈ A sin(2πft + 3π/4) >11 ≈ A sin(2πft + π/4) oQuadrature Amplitude and Phase Modulation (QAM) QAM-4, QAM-16, QAM-64, QAM-256 Q I 01 11 00 10 Q I QAM-4 QAM-16 Q I 0 1 Binary

12. 12 Signal Modulation (examples) Carrier Modulating data Modulating data 11100100 101110011000 Resulting signal QPSK Resulting signal QAM-8

13. 13 Bit rate vs. Baud rate oBit rate = bits/second oBaud (Symbol) rate = Symbols/second oBinary PSK, 1 symbol encodes 1 bit oQAM-4, 1 symbol encodes 2 bits oQAM-16, 1 symbol encodes 4 bits Q I 01 11 00 10 Q QAM-4 QAM-16 Q I 0 1 Binary

14. 14 Receiver: Demodulation oThe receiver looks at the received wave form and matches it with the data bit that caused the transmitter to generate this wave form >Necessary: one-to-one mapping between data and wave form >Because of channel imperfections, this is at best possible for digital signals, but not for analog signals oProblems caused by >Carrier synchronization: frequency can vary between sender and receiver (drift, temperature changes, aging, …) >Bit synchronization (actually: symbol synchronization): When does symbol representing a certain bit start/end? >Frame synchronization: When does a packet start/end? >Biggest problem: Received signal is not the transmitted signal!

15. 15 Antenna (I) oA resonating circuit (e.g., LC) connected to an antenna causes an antenna to emit EM (electromagnetic) waves oA receiving antenna converts the EM waves into electrical current oMany types of antennas with different gains (G) Gain: 10-55dB Isotropic Directional Omnidirectional Gain: 2dB

16. 16 dB, dBm, dBi,... dBm = dB value of Power / 1 mWatt Used to describe signal strength. dBW = dB value of Power / 1 WattUsed to describe signal strength. dBi = dB value of antenna gain relative to0dBi is by default the gain of an the gain of an isotropic antennaisotropic antenna A linear number is converted into dB, using the following formula: X(dB) = 10log 10 (X) X(dBm) = 10log 10 (X/1mW) E.g. 1W = 0dBW = +30dBm

17. 17 Antenna (II): Gain vs. Beamwidth ©Constantine A. Balanis, Antenna Theory: Analysis and Design, 3rd Edition oAntenna radiation pattern >Beamwidth of a pattern is the angular separation between two identical points on opposite side of the pattern maximum oFNBW > First Null BeamWidth oHPBW > Half-Power BeamWidth >The power reduced by half or 3dB of its maximum -> 3dB beamwidth

18. 18 Antenna (III): Gain vs. Beamwidth oGain of an antenna: >The ratio of the intensity, in a given direction, to the radiation intensity that would be obtained if the power accepted by antenna were radiated isotropically. ©D. Adamy, A First Course on Electronic Warfare

19. 19 Transmitted signal <> received signal! oWireless transmission distorts any transmitted signal >Received <> transmitted signal; results in uncertainty at receiver about which bit sequence originally caused the transmitted signal >Abstraction: Wireless channel describes these distortion effects oSources of distortion >Attenuation – energy is distributed to larger areas with increasing distance >Reflection/refraction – bounce of a surface; enter material >Diffraction – start “new wave” from a sharp edge >Scattering – multiple reflections at rough surfaces >Doppler fading – shift in frequencies (loss of center)

20. 20 Signal Propagation: Diffraction, Reflection, Scattering oReflection: When the surface is large relative to the wavelength of signal (λ = c/f), c = speed of light >May cause phase shift from original / cancel out original or increase it oDiffraction: When the signal hits the edge of an impenetrable body that is large relative to the wavelength λ >Enables the reception of the signal even if Non-Line-of-Sight (NLOS) oScattering: obstacle size is in the order of λ. (e.g., a lamp post) oIn LOS (Line-of-Sight) diffracted and scattered signals not significant compared to the direct signal, but reflected signals can be (multipath effects) oIn NLOS, diffraction and scattering are primary means of reception Reflection Scattering Diffraction

21. 21 Doppler shift oIf the transmitter and/or receiver are mobile, the frequency of the received signal changes >When they are moving closer, the frequency increases >When they are moving away, the frequency decreases Frequency difference = velocity/wavelength oExample: λ ( 2.4 GHz) = 3x10 8 /2.4x10 9 = 0.125m o120km/hr = 33.3 m/s oFreq. diff = 33.3/.125 = 267 Hz

22. 22 Signal Propagation (attenuation and path loss) oEffect of attenuation: received signal strength is a function of the distance R between sender and receiver oCaptured by Friis equation (a simplified form) >Gr and Gt are antenna gains for the receiver and transmiter > λ is the wavelength and α is a path-loss exponent (2 - 5) >Attenuation depends on frequencies, for free-space α=2 oPath loss (PL)

23. 23 Suitability of different frequencies – Attenuation oAttenuation depends on the used frequency oCan result in a frequency-selective channel >If bandwidth spans frequency ranges with different attenuation properties © http://141.84.50.121/iggf/Multimedia/Klimatologie/physik_arbeit.htm

24. 24 Signal Propagation (Strength) ©D. Adamy, A First Course on Electronic Warfare XMTR RCVR Path through link Signal Strength (dBm) Transmitted Power Antenna Gain Received Power LINK LOSSES Spreading and Atmospheric Loss To calculate the received signal level (in dBm), add the transmitting antenna gain (in dB), subtract the link losses (in dB), and add the receiving antenna gain (dB) to the transmitter power (in dBm).

25. 25 Receiver sensitivity oThe smallest signal (the lowest signal strength) that a receiver can receive and still provide the proper specified output. oExample: >Transmitter Power (1W) = +30dBm >Transmitting Antenna Gain = +10dB >Spreading Loss = 100dB >Atmospheric Loss = 2dB >Receiving Antenna Gain = +3dB Receiver Power (dBm) = +30dBm + 10dB – 100dB – 2dB + 3dB = -59dBm Receiver 1 sensitivity is -62dBm and the receiver 2 is -65dBm > receiver 1 and 2 will receive the signal as if there is still 3dBm and 6dBm of margin on the link, respectively. Recv 2 is 3dB (a factor of two) better than recv 1; recv 2 can hear signals that are half the strength of those heard by recv1.

26. 26 Distortion effects: Non-line-of-sight paths oBecause of reflection, scattering, …, radio communication is not limited to direct line of sight communication >Effects depend strongly on frequency, thus different behavior at higher frequencies oDifferent paths have different lengths = propagation time >Results in delay spread of the wireless channel >Closely related to frequency-selective fading properties of the channel >With movement: fast fading Line-of- sight path Non-line-of-sight path Signal at receiver LOS pulses Multipath pulses © Jochen Schiller, FU Berlin

27. 27 Wireless signal strength in a multi-path environment oBrighter color = stronger signal oObviously, simple (quadratic) free space attenuation formula is not sufficient to capture these effects © Jochen Schiller, FU Berlin

28. 28 Generalizing the attenuation formula oTo take into account stronger attenuation than only caused by distance (e.g., walls, …), use a larger path-loss exponent α > 2 >Rewrite in logarithmic form (in dB): oTake obstacles into account by a random variation >Add a Gaussian random variable with 0 mean, variance  2 to dB representation >Equivalent to multiplying with a lognormal distributed random variable in metric units > lognormal fading (R 0 is a referent distance)

29. 29 From waves to bits: symbols and bit errors oExtracting symbols out of a distorted/corrupted wave form is filled with errors >Depends essentially on strength of the received signal compared to the corruption >Captured by signal to noise and interference ratio (SINR) oSINR allows to compute bit error rate (BER) for a given modulation >Also depends on data rate R (# bits/symbol) of modulation >E.g., for simple DPSK

30. 30 Examples for SINR to BER mappings BER SINR

31. 31 WSN-specific channel models oTypical WSN properties >Small transmission range >Implies small delay spread (nanoseconds, compared to micro/milliseconds for symbol duration) >Frequency-non-selective fading, low to negligible inter-symbol interference oSome example measurements > α - path loss exponent >Shadowing variance  2 >Reference path loss at 1 m Average α

32. 32 Transceiver design oStrive for good power efficiency at low transmission power >Some amplifiers are optimized for efficiency at high output power >To radiate 1 mW, typical designs need 30-100 mW to operate the transmitter WSN nodes: 20 mW (mica motes) >Receiver can use as much or more power as transmitter at these power levels Sleep state is important oStartup energy/time penalty can be high >Examples take 0.5 ms and ¼ 60 mW to wake up oExploit communication/computation tradeoffs >Might payoff to invest in rather complicated coding/compression schemes

33. 33 Transceiver design oOne exemplary design point: which modulation to use? >Consider: required data rate, available symbol rate, implementation complexity, required BER, channel characteristics, … >Tradeoffs: the faster one sends, the longer one can sleep Power consumption can depend on modulation scheme >Tradeoffs: symbol rate (high?) versus data rate (low) Use m-ary transmission to get a transmission over with ASAP But: startup costs can easily void any time saving effects oAdapt modulation choice to operation conditions >Similar to dynamic voltage scaling (DVS) introduced in the last lecture, introduce Dynamic Modulation Scaling >When there are no packets present, a small value for m (bits per symbol) can be used, having low energy consumption. As backlog increases, m is increased as well to reduce the backlog quickly and switch back to lower values of m.

34. 34 Summary oWireless radio communication introduces many uncertainties into a communication system oHandling the unavoidable errors will be a major challenge for the communication protocols oDealing with limited bandwidth in an energy-efficient manner is the main challenge