Introduction

1. Introduction Lecture # 1

2. Overview • Fundamentals of wireless communication technology • The electromagnetic spectrum • Radio propagation mechanisms • Characteristics of the wireless channel • Modulation techniques • Multiple access techniques • Error control • Computer networks • Computer network software • IEEE 802 networking standard • Wireless networks

3. Reference Material • Ad hoc Wireless Networks - Ch #1 Architectures and Protocols By C. Siva Ram Murthy • Wireless Communication - Ch #5 and Networks By William Stallings

4. Fundamentals of Wireless Communication Technology • Wireless communication is based on the principle of broadcast where electromagnetic waves are used for communication • Waves are characterized by frequency (f) and wavelength (λ), and measured in Hertz (Hz) • f – cycle/second • λ – distance between two consecutive maxima or minima • Speed of propagation of these waves (c) varies from medium to medium, except in a vacuum where all electromagnetic waves travel at the same speed, the speed of light c = λ x f where c is the speed of light (3 x 108 m/s)

5. Electromagnetic Spectrum • Various frequency bands in electromagnetic spectrum are defined by International Telecommunication Union (ITU) • Amount of the information that can be carried by an electromagnetic wave is determined by the • Band width • Encoding technique (number of bits encoded/Hz)

6. Frequency Bands & their Common Uses

9. The Electromagnetic Spectrum • Low-frequency bands • Comprised of the radio, microwave, infrared, and visible light portion of the spectrum • Can be used for information transmission by modulating amplitude, frequency and phase of the waves • High-frequency bands • X-rays and Gamma rays • Theoretically better for information propagation, but are not used because of the difficulty in generation and harm full to living things • Also high frequency waves do not propagate well through the buildings

10. Radio Waves • Widely used for both indoor and out door communication • Easy to generate • Pass through buildings • Travel long distances • Transmission is Omni directional – no need to align transmitter and receiver • At low frequencies waves can pass through obstacles easily • Power falls with an inverse-squared relation with respect to distance • At higher frequencies waves are more prone to absorption by rain drops, and they get reflected by obstacles • Interference between wireless transmissions is a problem

11. VLF, LF, and MF Bands • Propagation of waves, also called as ground waves, follows the curvature of earth • Maximum transmission ranges of the order of a few hundred kilometers • Used for low bandwidth transmissions such as AM radio broadcasting

12. HF and VHF Bands • Absorbed by the atmosphere near the earth’s surface • However, the portion of the radiation, called the sky waves, radiated outward and upward to the ionosphere in the upper atmosphere • Ionosphere contains ionized particles formed due to sun’s radiation • These ionized particles reflects the sky waves back to the earth • A powerful sky wave may get reflected several times between earth and the ionosphere • Sky waves are used by amateur ham radio operators and for military communications

13. Microwave Transmission (SHF) • Tend to travel in straight lines and hence can be narrowly focused • Widely used for long distance telephony before being replaced by fiber optics • Also used for mobile phones and television transmission • High signal-to-noise ratio (SNR) • Due to high frequency of operation can not pass through buildings • Proper alignment between transmitter and receiver is required • May require repeater, as microwaves get attenuated by objects found in their path

14. Infrared Waves & Waves in the EHF Band (millimeter waves) • Used for short-range communication • Widely used in television, VCR, and stereo remote controls • Relatively directional inexpensive to build • Can not travel through obstacles

15. Visible Light • Unguided optical signaling using visible light provides very high bandwidth at a very low cost • Use of lasers to connect LANs on two buildings through roof-top antennas • But the main disadvantage here is that it is very difficult to focus a very narrow uni-directional laser beam, which limits the maximum distance between the transmitter and receiver • Also can not penetrate through rain or thick fog

16. Spectrum Allocation • International Telecommunications Union Radio communication (ITU-R) coordinates the spectrum allocation • ITU has designed some frequency bands, called the ISM (industrial, scientific, medical) bands, for unlimited usage • These bands commonly used by wireless LANs and PANs are around 2.4 GHz bands (12.5 cm wavelength) • Parts of the 900 MHz (33.3 cm wavelength) and the 5 GHz bands (5.2 cm wavelength) are also available in USA and Canada

17. Wireless Spectrum • Broadcast TV • VHF: 54 to 88 MHz, 174 to 216 MHz • UHF: 470 to 806 MHz 30 MHz 300 MHz 3 GHz 30 GHz • FM Radio • 88 to 108 MHz • Digital TV • 54 to 88 MHz, 174 to 216 MHz, 470 to 806 MHz

18. Wireless Spectrum • 3G Broadband Wireless • 746-794 MHz, 1.7-1.85 GHz, 2.5-2.7 GHz 30 MHz 300 MHz 3 GHz 30 GHz • Cellular Phone • 800-900 MHz • Personal Communication Service (PCS) • 1.85-1.99 GHz

19. Wireless Spectrum • Wireless LAN (IEEE 802.11b/g) • 2.4 GHz • Wireless LAN (IEEE 802.11a) • 5 GHz 30 MHz 300 MHz 3 GHz 30 GHz • Bluetooth • 2.45 GHz • Local Multipoint Distribution Services (LMDS) • 27.5-31.3 GHz

20. Introduction • An antenna is an electrical conductor or system of conductors • Transmission - radiates electromagnetic energy into space • Reception - collects electromagnetic energy from space • In two-way communication, the same antenna can be used for transmission and reception

21. Radiation Patterns • Radiation pattern • Graphical representation of radiation properties of an antenna • Depicted as two-dimensional cross section • Beam width • Measure of directivity of antenna • Reception pattern • Receiving antenna’s equivalent to radiation pattern

22. Types of Antennas • Isotropic antenna (idealized) • Radiates power equally in all directions z y z ideal isotropic radiator y x x

23. Types of Antennas • Dipole antennas • Half-wave dipole antenna (or Hertz antenna) • Consists of two straight collinear conductors of equal lengths , separated by a small feeding gap • Length of the antenna is one-half the wavelength of the signal that can be transmitted most effectively • Quarter-wave vertical antenna (or Marconi antenna) • Commonly used for automobile radios and portable radios

24. Types of Antenna /4 /2 simple dipole y y z x z x side view (xy-plane) side view (yz-plane) top view (xz-plane)

25. Antennas: Directed and Sectorized • Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley) y y z directed antenna x z x side view (xy-plane) side view (yz-plane) top view (xz-plane) z z sectorized antenna x x top view, 3 sector top view, 6 sector

26. Parabolic Reflective Antenna • Used in terrestrial microwave and satellite applications • If a source of electromagnetic energy is places at the focus of the paraboloid, and if the paraboloid is a reflecting surface, then the wave will bounce back in lines parallel to the axis of paraboloid • If incoming waves are parallel to the axis of the reflecting paraboloid, the resulting signal will be concentrated at the focus

27. Antenna Gain • Antenna gain • Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna) • Effective area • Related to physical size and shape of antenna

28. Antenna Gain • Relationship between antenna gain and effective area • G = antenna gain • Ae= effective area • f = carrier frequency • c = speed of light (» 3 ´ 108 m/s) •  = carrier wavelength

29. Radio Propagation Mechanisms • Reflection • When the propagating radio wave hits an object which is very large compared to its wavelength (surface of earth or tall buildings), the wave gets reflected by the objects • Reflection causes a phase shift of 180 degrees between the incident and the reflected waves

30. Radio Propagation Mechanisms • Diffraction • Occurs when a wave hits an impenetrable object • Waves bends at the edges of the object and, thereby propagating in different directions • Dimension of the object is comparable to the wavelength of the wave • Bending causes the wave to reach places even behind the objects which generally can not be reached by the line-of-sight transmission • Amount of diffraction is frequency-dependent, with the lower frequency waves diffracting more

31. Radio Propagation Mechanisms • Refraction • It occurs because the velocity of an electromagnetic wave is a function of the density of the medium through which it travels • When an electromagnetic wave moves from a medium of one density to a medium of another density, its speed changes • Refraction depends upon the wavelength • Cause a one-time bending of the direction of the wave at the boundary between the two media • Under normal propagation conditions, the refractive index of the atmosphere decreases with height so that radio waves travels more slowly near the ground than at higher altitudes • Result is the slight bending of the radio waves toward the earth

32. Radio Propagation Mechanisms • Scattering • When the wave travels through a medium, which contains many objects with dimensions small compared to its wavelength, scattering occurs • Wave gets scattered into several weaker outgoing signals • In practice, objects such as street signs, lamp posts, causes scattering

33. Propagation Modes • Ground Wave Propagation • Follows contour of the earth • Found in frequencies up to 2 MHz • Several factors account for • Electromagnetic waves induces a current in the earth’s surface, the result is to slow the wavefront near the earth, causing the wavefront to tilt downward and hence follows the earth’s curvature • Diffraction • Examples: AM radio

34. Propagation Modes • Sky Wave Propagation • A signal is reflected from the ionized layer of the atmosphere back down the earth • Reflection effect caused by refraction • Signal can travel through number of hops, bouncing back and forth between the ionosphere and the earth’s surface • A signal can be picked up from thousands of kilometers from the transmitter • Examples: Amateur radios, international broadcasts such as BBC

35. Propagation Modes • Line-of-Sight Propagation • Transmitting and receiving antennas must be within line of sight • Satellite communication – signal above 30 MHz not reflected by ionosphere • Ground communication – antennas within effective line of site due to refraction • Microwaves are bend or refracted by the atmosphere • Velocity of electromagnetic wave is a function of the density of the medium • When wave changes medium, speed changes • Wave bends at the boundary between mediums • Therefore, propagate farther than the optical line of sight

37. Characteristics of the Wireless Channels (Impairments) • Susceptible to a variety of transmission impediments • Attenuation and attenuation distortion • Path Loss or Free space loss • Noise • Atmospheric absorption • Multipath • Refraction • Thermal noise • These factors restricts the range, data rate, and reliability of the wireless channel • Effects depends upon the environmental conditions and the mobility of the transmission and receiver

38. Attenuation • Strength of signal falls off with distance over transmission medium • Guided media • Attenuation is generally logarithmic and typically expresses as a constant number of decibels per unit distance • Unguided media • Attenuation is a more complex function of distance makeup of atmosphere • Attenuation factors for unguided media: • Received signal must have sufficient strength so that circuitry in the receiver can interpret the signal • Signal must maintain a level sufficiently higher than noise to be received without error • Attenuation is greater at higher frequencies, causing distortion

39. Path Loss Or Free Space Loss • For any type of wireless communication the signal disperses with distance • This form of attenuation is also expressed as Free space loss • Expressed as the ratio of the power of the transmitted signal to the power of the same signal received by the receiver, on a given path • Important for designing and deploying the wireless communication networks • Dependent of • Radio frequency used • Nature of the terrain • Distance • Different estimation (model) for different environment

40. Free Space Propagation Model • There is a direct-path signal between the transmitter and receiver, with no atmospheric attenuation or multipath components Pr = PtGtGr (λ/4πd)2 Pr – Power receiver Pt – Power transmitted Gr – Receiver antenna gain Gt – Transmitter antenna gain d – distance between the transmitter and receiver λ – c/f, wavelength of the signal

41. Two-Ray Model OR Two-Path Model • Assumes the multiple paths (two paths) from transmitter to receiver • One a line-of-sight path • Other reflected, refracted, or scattered Pr = PtGtGr (hthr / d2)2 hthr - heights of transmitter and receiver respectively

42. Isotropic Antenna • In general isotropic antennas, power of the transmitted signal is the same in all direction Pr = PtGtGr (λ/4πd)2 1/dγ γ – Propagation coefficient varies between 2 (free-space coefficient) and 5 (strong attenuation)

43. Noise • For any data transmission event, the received signal will consists of • Transmitted signal, modified by the various distortions imposed by the transmission systems • Additional unwanted signals that are inserted somewhere between transmission and reception • These unwanted signals are referred as noise • Thermal Noise • Intermodulation noise • Crosstalk • Impulse Noise

44. Thermal Noise • Thermal noise due to agitation of electrons • Present in all electronic devices and transmission media • Cannot be eliminated • Function of temperature • Noise is assumed to be independent of frequency • Particularly significant for satellite communication • Received signal strength is quite low and it has significant thermal noise

45. Noise • Intermodulation noise – occurs if signals with different frequencies share the same medium • Interference caused by a signal produced at a frequency that is the sum or difference of two original frequencies or multiple of those frequencies • Crosstalk – unwanted coupling between signal paths, can also occur when unwanted signals are picked up by microwave antennas • Often dominates in ISM bands • Impulse noise – irregular pulses or noise spikes • Short duration and of relatively high amplitude • Caused by external electromagnetic disturbances, or faults and flaws in the communications system

46. Other Impairments • Atmospheric absorption – water vapor and oxygen contribute to attenuation • Multipath propagation – obstacles reflect signals so that multiple copies with varying delays are received