Wireless Networked Systems. Lec #1b: PHY Basics

Transcription

1 Wireless Networked Systems CS 795/895 - Spring 2013 Lec #1b: PHY Basics Tamer Nadeem Dept. of Computer Science

2 Wireless Communication Page 2 Spring 2013 CS 795/895 - Wireless Networked Systems

3 Radio Signal Basics Page 3 Spring 2013 CS 795/895 - Wireless Networked Systems

4 Radio Signal When the electrons run through a wire (transmitting antenna), they create electromagnetic sinusoidal waves that can propagate through the space electric field magnetic field propagation direction By attaching an antenna of the appropriate size to an electrical circuit, the electromagnetic waves can be broadcast efficiently and received by a receiver distance away Page 4 Spring 2013 CS 795/895 - Wireless Networked Systems

5 Basics of Signal The number of oscillations per second of an electromagnetic wave is called its frequency, f, measured in Hertz The distance between two consecutive maxima is called the wavelength, designated by λ By knowing the frequency of a wave and its wavelength, we can find its velocity (v): v = λf Velocity of a wave is only affected by the properties of the medium. In vacuum, waves travel at the speed of light c = 3x10 8 m/sec In copper or fiber, the speed slows down to about 2/3 of this value Page 5 Spring 2013 CS 795/895 - Wireless Networked Systems

6 Fourier Transform Sinusoidal waves, also called harmonics, are represented as: with frequency f, amplitude A, phase shift φ Every Signal Can be Decomposed as a Collection of Harmonics Page 6 Spring 2013 CS 795/895 - Wireless Networked Systems

7 Time Domain v.s. Frequency Domain Page 7 Spring 2013 CS 795/895 - Wireless Networked Systems

8 Signal Interference Page 8 Spring 2013 CS 795/895 - Wireless Networked Systems

9 Signal Bandwidth A typical signal will include many frequencies (Fourier theorem) Spectrum: The range of frequencies in a signal In this case: [f1 f2] Bandwidth: The width of the Spectrum In this case: (f2 f1) Page 9 Spring 2013 CS 795/895 - Wireless Networked Systems

10 Signal Bandwidth The amount of information a wireless channel can carry is related to its bandwidth Most wireless transmissions use a narrow frequency band ( f << f) where Δf: frequency band f: middle frequency where transmission occurs New technologies use spread spectrum techniques a wider frequency band is used for transmission Page 10 Spring 2013 CS 795/895 - Wireless Networked Systems

11 Spectrum Allocation Radio Frequencies: 3KHz < RF < 300GHz * FCC = Federal Communications Commission Page 11 Spring 2013 CS 795/895 - Wireless Networked Systems

12 Page 12 Spring 2013 CS 795/895 - Wireless Networked Systems

13 Frequencies and Regulations ITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) * ITU = International Communication Union Page 13 Spring 2013 CS 795/895 - Wireless Networked Systems

14 Signal Propagation Basics Page 14 Spring 2013 CS 795/895 - Wireless Networked Systems

15 Wireless Communication Page 15 Spring 2013 CS 795/895 - Wireless Networked Systems

16 Radio Propagation As it turns out, radio waves are easy to generate can travel long distances can penetrate buildings are omni-directional: can travel in all directions can be narrowly focused at high frequencies (greater than 100MHz) using parabolic antennas (like satellite dishes) Properties of radio waves are frequency dependent at low frequencies, they pass through obstacles well, but the power falls off sharply with distance from source at high frequencies, they tend to travel in straight lines and bounce of obstacles (they can also be absorbed by rain) are subject to interference from other radio wave sources Page 16 Spring 2013 CS 795/895 - Wireless Networked Systems

17 Basics of Radio Propagation At VLF, LF, and MF bands, radio waves follow the ground. AM radio broadcasting uses MF band At HF bands, the ground waves tend to be absorbed by the Earth The waves that reach the ionosphere ( km above Earth surface), are reflected back to the Earth Ionosphere reflection absorption Page 17 Spring 2013 CS 795/895 - Wireless Networked Systems

18 Basics of Radio Propagation VHF transmission LoS path Reflected wave n Directional antennas are used n Waves follow more direct paths n LoS: Line-of-Sight communication n Reflected waves may interfere with the original signal Page 18 Spring 2013 CS 795/895 - Wireless Networked Systems

19 Basics of Radio Propagation Receiving power additionally influenced by shadowing (e.g. through a wall or a door) refraction depending on the density of a medium reflection at large obstacles scattering at small obstacles diffraction at edges Page 19 Spring 2013 CS 795/895 - Wireless Networked Systems

20 Basics of Radio Propagation We are interested in propagation characteristics and models for waves with frequencies in range: few MHz to a few GHz Modeling the radio channel is important to determine the coverage area of a transmitter the transmitter power requirement the battery lifetime adopt modulation and coding schemes to improve the channel quality determine the maximum channel capacity Page 20 Spring 2013 CS 795/895 - Wireless Networked Systems

21 Basics of Radio Propagation As the receiver unit moves, propagated signals have an impact on the received signal strength either constructively or destructively As the receiver unit moves over small distances, the instantaneous received signal will fluctuate rapidly giving rise to small-scale fading (Rayleigh fading) in small scale fading, the received signal power may change as much as 3 or 4 orders of magnitude (30dB or 40dB), when the receiver moves a fraction of the wavelength As the receiver unit moves away from the transmitter over larger distances, the mean received signal gradually decreases. This is called large-scale fading (path loss) The models that predict the mean signal strength for an arbitraryreceiver transmitter (T-R) separation distance are called large-scale propagation models useful for estimating the coverage area of transmitters Page 21 Spring 2013 CS 795/895 - Wireless Networked Systems

22 Small- and Large-scale fading Received Power (dbm) T-R Separation (meters) Page 22 Spring 2013 CS 795/895 - Wireless Networked Systems

23 What is a decibel (db)? By convention, a decibel (db) decibel is a logarithmic unit used to describe a ratio For example, given two values P1 and P2, the difference between log P1 and log P2 is expressed in db by 10 log (P1/P2) db for example if the transmit power P1 = 100W, and the received power is P2 = 1 W, the path loss (P1/P2) in db is 10 log (100/1) = 20dB Page 23 Spring 2013 CS 795/895 - Wireless Networked Systems

24 What is a decibel (db)? The use of db is convenient as large numbers (rather, their difference) is expressed by numbers of small size examples: Tx power = 100W, received power = 1W Tx power is 100 times the received power difference is 20dB Tx power = 100W, received power = 1mW Tx power is 100,000 times the received power difference is 50dB Tx power = 1000W, received power = 1mW Tx power is 1,000,000 times the received power difference is 60dB Page 24 Spring 2013 CS 795/895 - Wireless Networked Systems

25 What is a decibel (db)? Page 25 Spring 2013 CS 795/895 - Wireless Networked Systems

26 mw and dbm Page 26 Spring 2013 CS 795/895 - Wireless Networked Systems

27 Free-Space Propagation Model Used to predict the received signal strength when transmitter and receiver have clear, unobstructed LOS path between them. The received power decays as a function of T-R separation distance raised to some power. Path Loss: Signal attenuation as a positive quantity measured in db defined as the difference (in db) between the effective transmitter power and the received power Page 27 Spring 2013 CS 795/895 - Wireless Networked Systems

28 Free-Space Propagation Model In free space, receiving power proportional to 1/d² (d = distance between transmitter and receiver) Writing path loss in log scale: PL = 10 log(pt) 10log(Pr) Page 28 Spring 2013 CS 795/895 - Wireless Networked Systems

29 Long Distance Path Loss Model The average large-scale path loss for an arbitrary T-R separation is expressed as a function of distance by using a path loss exponent n The value of n depends on the propagation environment: for free space it is 2; when obstructions are present it has a larger value d PL ( d) ( ) d PL(d) denotes the average large-scale path loss at a distance d (denoted in db) 0 n Reference distance (d 0 ): In large coverage cellular systems, 1km reference distances are commonly used In microcellular systems, much smaller distances are used: such as 100m or 1m Page 29 Spring 2013 CS 795/895 - Wireless Networked Systems

30 Path Loss Exponent for Different Environments Environment Path Loss Exponent, n Free space 2 Urban area cellular radio 2.7 to 3.5 Shadowed urban cellular radio 3 to 5 In building line-of-sight 1.6 to 1.8 Obstructed in building 4 to 6 Obstructed in factories 2 to 3 Page 30 Spring 2013 CS 795/895 - Wireless Networked Systems

31 Multipath Fading Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction Multipath Affect Signal Strength Multi-path components are delayed depending on path length (delay spread) Phase shift causes frequency dependent constructive / destructive interference Page 31 Spring 2013 CS 795/895 - Wireless Networked Systems

32 Multipath Fading Example: reflection from the ground: received power decreases proportional to 1/d 4 instead of 1/d² due to the destructive interference between the direct signal and the signal reflected from the ground Due to constructive and destructive interference of multiple transmitted waves, signal strength may vary widely as a function of receiver position Page 32 Spring 2013 CS 795/895 - Wireless Networked Systems

33 Multipath Fading Channel characteristics change over location & time sin sin sin Page 33 Spring 2013 CS 795/895 - Wireless Networked Systems

34 Multipath Fading Page 34 Spring 2013 CS 795/895 - Wireless Networked Systems

35 Multipath Can Spread Delay Page 35 Spring 2013 CS 795/895 - Wireless Networked Systems

36 Multipath Can Cause ISI µs Page 36 Spring 2013 CS 795/895 - Wireless Networked Systems

37 Path loss and Received Power In log normal propagation environment: PL(d) (path loss) and Pr(d) (received power at a distance d) are random variables with a normal distribution in db about a distance dependent mean. Sometime we are interested in answering following kind of questions: What is mean received P r (d) power (mean_p r (d))at a distance d from a transmitter What is the probability that the receiver power P r (d) (expressed in db power units) at distance d is above (or below) some fixed value γ (again expressed in db power units such as dbm or dbw). Page 37 Spring 2013 CS 795/895 - Wireless Networked Systems

38 Received Power and Normal Distribution In answering these kind of question, we have to use the properties of normal (gaussian distribution). P r (d) is normally distributed that is characterized by: a mean (µ) a standard deviation (σ) We are interested in Probability that P r (d) >= γ or Pr(d) <= γ Page 38 Spring 2013 CS 795/895 - Wireless Networked Systems

39 Received Power and Normal Distribution PDF Figure shows the PDF of a normal distribution for the received power P r at some fixed distance d ( m = 10, s = 5) Note: x-axis is received power, y-axis probability EXAMPLE: Probability that P r is smaller than 3.3 (i.e. Prob(P r <= 3.3)) is given by measure of hashed area Page 39 Spring 2013 CS 795/895 - Wireless Networked Systems

40 Signal Modulation Basics Page 40 Spring 2013 CS 795/895 - Wireless Networked Systems

41 Wireless Communication Page 41 Spring 2013 CS 795/895 - Wireless Networked Systems

42 Why Modulation? Page 42 Spring 2013 CS 795/895 - Wireless Networked Systems

43 ON/OFF Keying (OOK) Simplest, oldest form of modulation Morse code (1837) developed for telegraphy Page 43 Spring 2013 CS 795/895 - Wireless Networked Systems

44 Carrier Modulation Continuous radio wave carrier has zero bandwidth (single frequency) but carries no information Want to change (modulate) the wave over time to convey a message Will increase bandwidth: More information -> More bandwidth Receiver must demodulate the carrier to get back the original signal Page 44 Spring 2013 CS 795/895 - Wireless Networked Systems

45 Carrier Modulation Objective encode data into analog signals at the right frequency range with limited usage of spectrum Basic schemes Analogue Modulation: Amplitude Modulation (AM) Frequency Modulation (FM) Digital Modulation Phase Shift Keying (PSK) Quadrature Amplitude Modulation (QAM) Page 45 Spring 2013 CS 795/895 - Wireless Networked Systems

46 Amplitude Modulation Page 46 Spring 2013 CS 795/895 - Wireless Networked Systems

47 Amplitude Modulation Page 47 Spring 2013 CS 795/895 - Wireless Networked Systems

48 AM Spectrum Page 48 Spring 2013 CS 795/895 - Wireless Networked Systems

49 Varying Modulation Index Page 49 Spring 2013 CS 795/895 - Wireless Networked Systems

50 Frequency Modulation Page 50 Spring 2013 CS 795/895 - Wireless Networked Systems

51 Frequency Modulation Page 51 Spring 2013 CS 795/895 - Wireless Networked Systems

52 FM Spectrum Sideband structure is more complicated. Theoretically an infinite number of sidebands is generated. But most power is in the first m+1 sidebands. Bandwidth: 2(m+1)f m Page 52 Spring 2013 CS 795/895 - Wireless Networked Systems

53 Digital Modulation Page 53 Spring 2013 CS 795/895 - Wireless Networked Systems

54 Phase Shift Keying: BPSK BPSK (Binary Phase Shift Keying): bit value 0: sine wave bit value 1: inverted sine wave very simple PSK Properties robust, used e.g. in satellite systems Page 54 Spring 2013 CS 795/895 - Wireless Networked Systems

55 Phase Shift Keying: QPSK QPSK (Quadrature Phase Shift Keying): 2 bits coded as one symbol symbol determines shift of sine wave often also transmission of relative, not absolute phase shift: DQPSK - Differential QPSK Page 55 Spring 2013 CS 795/895 - Wireless Networked Systems

56 Quadrature Amplitude Modulation Quadrature Amplitude Modulation (QAM): combines amplitude and phase modulation It is possible to code n bits using one symbol 2 n discrete levels Example: 16-QAM (4 bits = 1 symbol) Symbols 0011 and 0001 have the same phase φ, but different amplitude a and 1000 have same amplitude but different phase Page 56 Spring 2013 CS 795/895 - Wireless Networked Systems

57 Signal-to-Noise (S/N) SNR: signal-to-noise ratio Larger SNR easier to extract signal from noise (a good thing ) SNR versus BER tradeoffs Given physical layer: increase power à increase SNR à decrease BER Given SNR: choose physical layer that meets BER requirement, giving highest throughput SNR may change with mobility: dynamically adapt physical layer (modulation technique, rate) Page 57 Spring 2013 CS 795/895 - Wireless Networked Systems

58 The Nyquist Limit A noiseless channel of bandwidth B can at most transmit a binary signal at a capacity 2B» E.g. a 3000 Hz channel can transmit data at a rate of at most 6000 bits/second» Assumes binary amplitude encoding For M levels: C = 2B log 2 M» M discrete signal levels More aggressive encoding can increase the actual channel bandwidth Factors such as noise can reduce the capacity Page 58 Spring 2013 CS 795/895 - Wireless Networked Systems

59 Shannon Capacity Formula Equation: C = B log 2 (1+SNR) Represents error free capacity Result is based on many assumptions Bandwidth B and noise N are not independent» N is the noise in the signal band, so it increases with the bandwidth Shannon does not provide the coding that will meet the limit, but the formula is still useful Page 59 Spring 2013 CS 795/895 - Wireless Networked Systems

60 Example of Nyquist and Shannon Formulations Spectrum Spectrum of a channel between 3 MHz of a channel between 3 MHz and 4 MHz ; SNR db = 24 db B = 4 MHz 3 MHz = 1 MHz SNR db = 24dB = 10 log 10 (SNR) à SNR = 251 Using Shannon s formula C = 10 6 x log 2 (1+251) 10 6 x 8 = 8 Mbps How How many signaling levels are many signaling levels are required? C = 2B log 2 M 8 x 10 6 = 2 x 10 6 x log 2 M à M = 16 Page 60 Spring 2013 CS 795/895 - Wireless Networked Systems

61 Why Spread Spectrum? C = B*log2(1+S/N)... [Shannon] To achieve the same channel capacity C Large S/N, small B Small S/N, large B Increase S/N is inefficient due to the logarithmic relationship power power signal noise, interferences signal frequency B e.g. B = 30 KHz B e.g. B = 1.25 MHz Page 61 Spring 2013 CS 795/895 - Wireless Networked Systems

62 Spread Spectrum Methods for spreading the bandwidth of the transmitted signal over a frequency band (spectrum) which is wider than the minimum bandwidth required to transmit the signal. Reduce effect of jamming Military scenarios Reduce effect of other interferences More secure Signal merged in noise and interference Page 62 Spring 2013 CS 795/895 - Wireless Networked Systems

63 Direct Sequence SS Direct sequence (DS): most prevalent Signal is spread by a wide bandwidth pseudorandom sequence (code sequence) Signals appear as wideband noise to unintended receivers Not for intra-cell multiple access Nodes in the same cell use same code sequence Page 63 Spring 2013 CS 795/895 - Wireless Networked Systems

64 Frequency Hopping SS (FHSS) 2.4GHz band divided into 75 1MHz subchannels Sender and receive agree on a hopping pattern (pseudo random series). 22 hopping patterns defined One possible pattern f f f f f f f f f f f Different hopping sequences enable co-existence of multiple BSSs Robust against narrow-band interferences Page 64 Spring 2013 CS 795/895 - Wireless Networked Systems

65 Questions Page 65 Spring 2013 CS 795/895 - Wireless Networked Systems

66 Warming UP Read posted materials about: How to read, write, and present papers: Pick 5 conferences: ACM Mobicom, MobiHoc, MobiSys, Sigcomm, NSDI, Hotnets, HotMobile IEEE ICNP, ICDCS, SECON, WoWMoM Get the Program lists for last three years (2010, 2011, 2012) of each of your conference. Page 66 Spring 2013 CS 795/895 - Wireless Networked Systems

67 Warming UP For each paper in a program list (max 35 papers), compile a list of the keywords related to wireless topics. DO NOT read the whole paper In most cases, title w/o paper abstract is enough. Utilize your instincts. Using a Tag Cloud tool, draw the corresponding tag cloud (word cloud, or weighted list in visual design) of the compiled keywords. me one pdf page with your cloud tag. Send me an ordered list of only 5 papers you mostly liked! Again use instincts/guts Your presentation paper(s) could be among this list Deadline: Jan 16 th, 5:45pm. Page 67 Spring 2013 CS 795/895 - Wireless Networked Systems