Goal. A tutorial overview of wireless communication. Antennas, propagation and (de)modulation

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1 Goal A tutorial overview of wireless communication Antennas, propagation and (de)modulation Focus on a single wireless link Operating on a small slice of spectrum called a channel, characterized by centre frequency and bandwidth 1

2 Internet Protocol Stack application: supporting network applications FTP, SMTP, HTTP transport: process-process data transfer TCP, UDP network: routing of datagrams from source to destination IP, routing protocols link: data transfer between neighboring network elements PPP, Ethernet physical: bit pipe application transport network link physical

3 source message segment datagram frame H l H n H n H t H t H t M M M M application transport network link physical H l H n H t M link physical switch H l H n H n H t H t H t M M M M destination application transport network link physical H l H n H n H t H t M M network link physical router

4 Source Digital Communication System Destination Bit stream for transmission Source Encoder Source Decoder Received bit stream Channel Encoder Channel Decoder Transmitted waveform Channel Received waveform Physical Layer 4

5 Channel Encoder/Decoder Layers 1. Error Correction Coder/Decoder 2. Modulator/Demodulator (Baseband) 3. Frequency Conversion (Passband) 5

6 Source Wireless Communication System Destination Bit stream for transmission Source Encoder Channel Encoder Wireless channel Source Decoder Received bit stream Channel Decoder Transmitted Antenna Antenna Received waveform waveform Physical Layer 6

7 Antennas

8 Antenna Antenna design goal: ensure the conversion process is efficient, i.e., direct as much power as possible in useful directions 8

9 Field Regions L (antenna diameter) and λ (wavelength) 9

10 Antenna Radiation Pattern Plot of far-field radiation from the antenna Radiation intensity, U: power radiated from an antenna per unit solid angle Isotropic antenna with spherical pattern vs. omnidirectional (e.g., hertzian dipole) vs. directional Azimuth plane (x-y plane), Elevation plane (x-z plane) 10

11 Isotropic Antenna Antenna Radiation Pattern (contd.) z x y 11 Hertzian Dipole Antenna

12 Radiation Pattern of a Generic Directional Antenna Half-power beamwidth (HPBW): angle subtended by the half-power points of the main lobe Front-back ratio: ratio between peak amplitudes of main and back lobes Side lobe level: amplitude of the biggest side lobe 12

13 Antenna Directivity, Efficiency and Gain Directivity, D: ratio of max radiation intensity of antenna to radiation intensity of isotropic antenna radiating the same total power D ~ 41,000/Θ o HPφ o HP ; Θ o HP ( φ o HP ) are vertical (horizontal) plane half-power beamwidths in degrees Radiation Efficiency, e: ratio of radiated power to power accepted by antenna Sometimes specified via Voltage Standing Wave Ratio (VSWR) (Power) Gain, G = e * D 13

14 Antenna Polarization Orientation of the electric field of an electromagnetic wave relative to the earth In general described by an ellipse Two special cases of elliptical polarization: linear and circular polarizations 14

15 Electromagnetic Wave Propagation 15

16 Wireless Propagation 16

17 Decibels Power ratio in decibels = 10log 10 (P/P ref ) Power ratios dB, dB, dB, Similarly, power ratios dB, dB, dB, 3dB (power ratio = 2), -3dB (power ratio = ½) Voltage ratio in decibels = 20log 10 (V/V ref ), since P = V 2 /R Absolute power with respect to standard reference power in decibels: dbw (P ref = 1W) and dbm (P ref = 1mW) 1W = 0 dbw = +30 dbm; 1mW = 0 dbm = -30 dbw Antenna gains: dbi (P ref is power radiated by an isotropic reference antenna) and dbd (P ref is power radiated by a half-wave dipole) 0 dbd = 2.15 dbi db for gains and losses (e.g., path loss, SNR) Why in decibels? Signal strength often falls off exponentially, so loss easily expressed in terms of decibel (a logarithmic unit) Net gain or loss via simple addition and subtraction 17

18 Noise Types in a Wireless Channel Multiplicative Antenna directionality Attenuation or Absorption (walls, trees, atmosphere) Reflection (smooth surfaces) Scattering (rough surfaces and small objects) Refraction (atmospheric layers, layered/graded materials) Diffraction (edges of buildings and hills) Additive 18 Internal sources within the receiver: thermal and shot noise in passive and active components External sources: Interference from other transmitters and appliances Atmospheric effects Cosmic radiation

19 Three Scales of Multiplicative Noise Large-scale propagation effects Path loss Shadowing (or slow fading) leads to variations over distances in the order of metres Could be over 10s or 100s of metres in outdoor environments 19 Fast fading (or multipath fading), a small-scale propagation effect: causes variations of over very short distances in the order of the signal wavelength

20 Fading Processes Illustrated 20

21 Another Illustration of Path Loss, Shadowing and Multipath Fading 21

22 Path Loss Received power, P R = P T G T G R /L T LL R Effective Isotropic Radiated Power (EIRP) At transmitter, P TI = P T G T /L T At receiver, P R = P RI G R /L R Path loss, L = P TI /P RI = P T G T G R /P R L T L R 22

23 Free-Space (or Spreading) Loss Illustration on a Point-to-Point Wireless Link Assume antennas T and R Arranged such that their directions of maximum gain are aligned With matching polarizations Separated by distance d, large enough that antennas are in each other s far-field regions 23

24 Free-Space Loss For simplicity, assume no feeder losses, i.e., L T = L R = 1 P T : transmit power S: power density incident on receiver antenna = P T G T /4Πd 2 Receiver antenna effective area (aperture), A er = G R λ 2 /4Π Receiver input power, P R = P T G T A er /4Πd 2 Friss transmission formula: P R /P T = G T G R (λ/4πd) 2 Propagation loss in free space, L F = P T G T G R /P R = (4Πd/λ) 2 = (4Πdf/c) 2 c, speed of light (3 x 10 5 Km per second) L F (db) = log(d) + 20log(f), d in Km and f in MHz 24

25 Path Loss Exponent (α) Free-space loss is the minimum path loss for a given distance Path loss in practice much higher (includes average shadowing) because of attenuation due to signal encounters with the environment Path loss exponent, α: a term used to indicate how fast signal power degrades with distance α = 2 in free space; typically, 2 α 5 25

26 Two-Ray (Plane Earth Loss) Model P R = P T G T G R (h t h r ) 2 /r 4 26

27 Two-Slope Model [Schwartz 05] Two-ray model only holds for long distances Oscillation at short distances due to the constructive and destructive combination of the two rays Instead, two-slope model used (for microcells) City n 1 n 2 d b (m) London Melbourne Orlando

28 Receiver Sensitivity Received power level, P R min, at which just acceptable communication quality Assuming only thermal noise in the receiver electronic circuitry For a given transmission bit-rate (i.e., physical layer data rate) Determines maximum range Path loss corresponding to P R min is called maximum acceptable path loss 28

29 Link Budget Analysis Link budget(ing): a calculation of signal powers, noise powers and/or signal-to-noise ratios for a complete communication link Simple, but useful calculation of system performance at design stage Max acceptable propagation loss [db] = Predicted loss + Fade margin Predicted loss given by distance-dependent path loss model (e.g., free space, plane earth models) Fade margin for resilience against signal fading effects (e.g., 20dB) greater fade margin greater reliability and smaller max range 29

30 Shadowing Represents medium-scale fluctuations of the received signal power occurring over distances from few metres to tens or hundreds of meters Due to signal encounters with terrain obstructions such as hills or man-made obstructions (e.g., buildings, trees) Measured signal power may differ substantially at different locations even though at the same radial distance from transmitter 30

31 Multipath Fading Effects Rapid changes in signal strength over a small physical distance or time interval Time dispersion (echoes) caused by multipath propagation delays Random frequency modulation due to Doppler shifts on different multipath signals 31 Influencing Factors Multipath propagation The transmission bandwidth of the signal Speed of the mobile Speed of surrounding objects

32 Multipath Propagation (a) constructive phase interference (b) destructive phase interference 32

33 Delay Spread Depends on the environment Typically around 40-70ns in indoor office environments, can go up to 200ns in some cases Can cause inter-symbol interference (ISI) 33

34 Doppler Shift Vehicle motion with respect to the incoming ray introduces a doppler frequency shift, f k = vcosβ k /λ Hz Frequency of received signal with doppler shift = f c + f k, where f c is carrier frequency 34

35 Multipath Channel Parameters Doppler spread (B D ) and coherence time (T C ) describe the time-varying (frequency dispersive) nature of the channel due to relative motion of transmitter and receiver or movement of surrounding objects T C α 1/B D Delay spread (τ t ) and coherence bandwidth (B c ) describe the frequency-selective (time dispersive) nature of the channel due to delays between different propagation paths Τ t α 1/B c 35

36 Mitigating Multipath Fading Coding techniques for error detection and correction Interleaving for combating fast fading Diversity techniques (space, frequency, time and polarization dimensions) Equalization also to mitigate frequency-selective fading Orthogonal frequency division multiplexing (OFDM) to mitigate frequency-selective fading 36

37 Signal-to-Noise Ratio (SNR) Crucial factor determining wireless transmission quality Shannon s Channel Capacity Theorem for band-limited additive white Gaussian noise (AWGN) channel: C = W log 2 (1+SNR) C, channel capacity in bits per second W, channel bandwidth in Hz SNR, signal-to-noise ratio 37 So long as data rate below C, error probability can made arbitrarily lower with the use of more sphisticated coding schemes

38 SNR versus Distance 38

39 Modulation Schemes and Constellations Bits to symbols 39

40 Wireless Link Throughput Modulation scheme used determines the transmission bit-rate Use of a modulation scheme also implies a relationship between SNR and bit-error rate (BER) Frame error rate (FER) = 1 (1- BER) L L, frame length Throughput = bit-rate * (1-FER) = bit-rate * (1- BER) L 40

41 BER versus SNR Assume a symbol rate of 1M symbols per second and AWGN channel 41

42 Bit-Level Throughput versus SNR 42

43 Frame-Level Throughput versus SNR 43

44 Error Correction Coding and Coding Rate (R) Determines the number of redundant bits added Ratio of number of data bits transmitted to the number of coded bits If K redundant bits are added for every N data bits transmitted, then R = N / (N+K) 44

45 Orthogonal Frequency Division Multiplexing (OFDM) A wide channel is divided into several component orthogonal subcarriers Use multiple subcarriers in parallel for a single transmission by multiplexing data over all of them Physical layer in a/g is based on OFDM Similar to the discrete multi-tone (DMT) in DSL systems 45

46 The a/g Case Total 52 subcarriers for a 20MHz channel 48 subcarriers used for data and the remaining 4 are pilot subcarriers 46

47 802.11a/g Bit-Rates Modulation and Coded bits per Coded bits per Data bits per Bit rate (Mbps) coding rate (R) sub-carrier a symbol symbol b 6 BPSK, R=1/ BPSK, R=3/ QPSK, R=1/ QPSK, R=3/ QAM, R=1/ QAM, R=3/ QAM, R=2/ QAM, R=3/ a Coded bits per sub-carrier is a function of the modulation (BPSK, QPSK, 16-QAM, or 64-QAM). b The data bits per symbol is a function of the rate of the convolutional code. 250,000 symbols per second across 48 subcarriers 47

48 Multiple Access Techniques Frequency Division Multiple Access (FDMA) Time Division Multiple Access (TDMA) Code Division Multiple Access (CDMA) 48

49 Frequency Division Multiple Access (FDMA) frequency time Early cellular systems were based on (analog) FDMA OFDM (and OFDMA) similar to FDMA, except frequency division done digitally Closer frequency spacing Dynamic allocation of subcarriers (in WiMAX and LTE) 49

50 Time Division Multiple Access (TDMA) frequency More difficult to implement than FDMA since the users must be time-synchronized But easier to accommodate multiple data rates with TDMA since multiple timeslots can be assigned to a given user Random access: asynchronous version of TDMA ALOHA, Slotted ALOHA, CSMA, CSMA/CD (Ethernet), CSMA/CA (WiFi) 50 time

51 Wireless Random Multiple Access Issues Half-duplex operation collision detection at transmitter very difficult Location-dependent carrier sensing Hidden terminals Exposed terminals Capture 51

52 Code Division Multiple Access (CDMA) Used in several wireless broadcast channel standards (e.g., 2G/3G cellular, satellite) Unique code assigned to each user; i.e., code set partitioning All users share same frequency, but each user has own chipping sequence (i.e., code) to encode data Encoded signal = (original data) X (chipping sequence) Decoding: inner-product of encoded signal and chipping sequence Allows multiple users to coexist and transmit simultaneously with minimal interference (if codes are orthogonal ) Issues: code selection, near-far problem

53 (a) FDMA, (b) TDMA, (c) CDMA. 53

54 CDMA Encode/Decode channel output Zi,m Zi,m= di. cm data bits d1 = -1 d0 = sender code slot 1 channel output slot 0 channel output slot 1 slot 0 M Di = Σ Zi,m. cm m=1 received input M d1 = -1 d0 = 1 code slot 1 channel output slot 0 channel output receiver slot 1 slot 0

55 CDMA: two-sender interference 6: Wireless and 6-55 Mobile Networks

56 References R. G. Gallager, Principles of Digital Communication, Cambridge University Press, S. R. Saunders and A. Aragon-Zavala, Antennas and Propagation for Wireless Communication Systems, Second Edition, John Wiley, M. Schwartz, Mobile Wireless Communications, Cambridge University Press, C. Haslett, Essentials of Radio Wave Propagation, Cambridge University Press, E. McCune, Practical Digital Wireless Signals, Cambridge University Press,

57 References (Contd.) M. S. Gast, Wireless Networks, O Reilly, J. C. Bicket, Bit-Rate Selection in Wireless Networks, Master s thesis, MIT, Feb J. F. Kurose and K. W. Ross, Computer Networking: A Top-Down Approach, 5th edition, Pearson Education, A. C. V. Gummalla and J. O. Limb, Wireless Medium Access Control Protocols, IEEE Communications Surveys & Tutorials, C. Cox, Essentials of UMTS, Cambridge University Press,