Mobile Computing CS 4830

1. Mobile Computing CS 4830 Basics of Wireless Communication Mr. Abdul Haseeb Khan

2. Instructor Pre-requisite Text books Mr. Abdul Haseeb Khan Data Communication and Networks Wireless Communication and Networks, 2nd Ed., W. Stalling. Wireless Communications: Principles and Practices, 2nd Ed., T. S. Rappaport. The Mobile Communications Handbook, J. D. Gibson Chapter Basics

3. Introduction to Wireless Communication • The Wireless vision • Radio Waves • Channel Capacity • Motivation for Specialized MAC • Hidden and Exposed Terminals • Far and Near Terminals • Multiplexing • Antennas • Signal Propagation • Access Methods(SDMA, FDMA, TDMA, CDMA

4. The Wireless vision • What is wireless communication? • Unguided media • The basis for all wireless transmission is the electromagnetic spectrum, in which lie the different frequency bands that are used for wireless communication. • What are the driving factors? • increase in demand of tether less connectivity • Dramatic progress in VLSI technology • Implementation of efficient signal processing algorithms. • New Coding techniques • Success of 2G wireless standards (GSM), 3 G & 4G

5. Wired Vs. Wireless Communication Each cable is a different channel One media (cable) shared by all Highsignal attenuation Signal attenuation is low High interference noise; co-channel interference; adjacent channel interference No interference

6. Why go wireless ? • Advantages • Sometimes it is impractical to lay cables • User mobility • Cost • Limitations • Bandwidth • Fidelity (Reliability) • Power • (In) security

7. Electromagnetic Signal • EM signal is used as a means to transmit information. • Function of time • Can also be expressed as a function of frequency • Signal consists of components of different frequencies

8. ISM band 902 – 928 Mhz 2.4 – 2.4835 Ghz FM radio S/W radio TV TV AM radio cellular 5.725 – 5.785 Ghz VHF UHF SHF EHF LF MF HF  300MHz 30MHz 30GHz 300GHz 3GHz 3MHz 30kHz 300kHz  100mm 10cm 10m 1cm 1m 100m 10km 1km X rays Gamma rays visible UV infrared  1 MHz 1 GHz 1 kHz 1 THz 1 EHz 1 PHz Propagation characteristics are different in each frequency band EM Spectrum

9. Electromagnetic Signals • Radio waves (EM spectrum longer than infrared light) • Microwaves (wavelengths ranging from as long as one meter to as short as one millimeter, or equivalently, with frequencies between 300 MHz (0.3 GHz) and 300 GHz) • Infrared light (lies between the visible and microwave portions of the electromagnetic spectrum). • Light Waves (electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight).

10. Time-Domain Concepts • Analog signal - signal intensity varies in a smooth fashion over time • No breaks or discontinuities in the signal • Digital signal - signal intensity maintains a constant level for some period of time and then changes to another constant level • Periodic signal - analog or digital signal pattern that repeats over time • Aperiodic signal - analog or digital signal pattern that doesn't repeat over time

11. Time-Domain Concepts • Peak amplitude (A) - maximum value or strength of the signal over time; typically measured in volts • Frequency (f ) • Rate, in cycles per second, or Hertz (Hz) at which the signal repeats • Period (T ) - amount of time it takes for one repetition of the signal • T = 1/f • Phase () - measure of the relative position in time within a single period of a signal

12. Time-Domain Concepts • Wavelength () - distance occupied by a single cycle of the signal • Or, the distance between two points of corresponding phase of two consecutive cycles  = vT Square wave Sine wave

13. Sine Wave Parameters

14. Frequency-Domain Concepts • Fundamental frequency - when all frequency components of a signal are integer multiples of one frequency, it’s referred to as the fundamental frequency • Spectrum - range of frequencies that a signal contains • Absolute bandwidth - width of the spectrum of a signal • Effective bandwidth (or just bandwidth) - narrow band of frequencies that most of the signal’s energy is contained in

15. Spectrum and bandwidth • Bandwidth is the difference between the upper and lower frequencies in a continuous set of frequencies. It is measured in hertz, and may sometimes refer to pass band. • Electromagnetic signals are made up of many frequencies • The 2nd frequency is an integer multiple of the first frequency • When all of the frequency components of a signal are integer multiples of one frequency, the latter frequency is called fundamental frequency (f) • period of the resultant signal is equal to the period of the fundamental frequency • Period of s(t) is T=1/f

16. Relationship between Data Rate and Bandwidth • The greater the bandwidth, the higher the information-carrying capacity • Conclusions • Any digital waveform will have infinite bandwidth • BUT the transmission system will limit the bandwidth that can be transmitted • AND, for any given medium, the greater the bandwidth transmitted, the greater the cost • HOWEVER, limiting the bandwidth creates distortions

17. Bandwidth Allocation • Necessary to avoid interference between different radio devices • Microwave woven should not interfere with TV transmission • Generally a radio transmitter is limited to a certain bandwidth • 802.11channel has 30MHz bandwidth • Power and placement of transmitter are regulated by authority

18. About Channel Capacity • Impairments(losses), such as noise, limit data rate that can be achieved • For digital data, to what extent do impairments limit data rate? • Channel Capacity – the maximum rate at which data can be transmitted over a given communication path, or channel, under given conditions

19. Concepts Related to Channel Capacity • Data rate - rate at which data can be communicated (bps) • Noise - average level of noise over the communications path • Error rate - rate at which errors occur • Error = transmit 1 and receive 0; transmit 0 and receive 1

20. Design Challenges • Two fundamental aspects of wireless communication • Channel fading (disappearing) • Multipath fading • Path loss via distance attenuation • Shadowing by obstacles • Interference • Multiple transmitters to a common receiver • Multiple transmitters to multiple receivers

21. Design Challenges goals • The primary concern in wireless systems is to increase the reliability of air interface. • This is achieved by controlling the channel fading and interference. • Recently the focus has shifted to spectral efficiency.

22. Motivation for Specialized MAC

25. Multiplexing • A fundamental mechanism in communication system and networks • Multiplexing - carrying multiple signals on a single medium, Enables multiple users to share a medium • More efficient use of transmission medium • Multiplexing Techniques • Frequency-division multiplexing (FDM) • Time-division multiplexing (TDM) • For wireless communication, multiplexing can be carried out in four dimensions: space, time, frequency and code

26. Multiplexing

27. Classifications of Transmission Media • Transmission Medium • Physical path between transmitter and receiver • Guided Media • Waves are guided along a solid medium • E.g., copper twisted pair, copper coaxial cable, optical fiber • Unguided Media • Provides means of transmission but does not guide electromagnetic signals • Usually referred to as wireless transmission • E.g., atmosphere, outer space

28. Unguided Media • Transmission and reception are achieved by means of an antenna • Configurations for wireless transmission • Directional • Omnidirectional

29. Antenna • An electrical conductor or system of conductors used for radiating electromagnetic energy into space or for collecting electromagnetic energy from the space • An integral part of a wireless system

30. Radiation Patterns • Antenna radiates power in all directions • but typically does not radiate equally in all directions • Ideal antenna is one that radiates equal power in all direction • called an isotropic antenna • all points with equal power are located on a sphere with the antenna as its center

31. Antenna location Omnidirectional Antenna Omnidirectional Antenna • Produces omnidirectional radiation pattern of equal strength in all directions • Vector A and B are of equal length

32. A B Directional Antenna • Radiates most power in one axis (direction) • radiates less in other direction • vector B is longer than vector A : more power radiated along B than A • directional along X

33. Dipole Antenna • Half-wave dipole or Hertz antenna consists of two straight collinear conductor of equal length • Length of the antenna is half the wavelength of the signal. Half-wave dipole λ/2

34. Quarter-wave antenna • Quarter-wave or marconi antenna has a veritcal conductor of length quarter of the wavelength of the signal λ/4

35. 3 sector antenna Sectorized Antenna • Several directional antenna combined on a single pole to provide sectorized antenna • each sector serves receivers listening it its direction

36. Multipath Propagation • Reflection - occurs when signal encounters a surface that is large relative to the wavelength of the signal • Diffraction - occurs at the edge of an impenetrable body that is large compared to wavelength of radio wave • Scattering – occurs when incoming signal hits an object whose size in the order of the wavelength of the signal or less

37. Multipath Propagation

38. The Effects of Multipath Propagation • Multiple copies of a signal may arrive at different phases • If phases add destructively, the signal level relative to noise declines, making detection more difficult • Intersymbol interference (ISI) • One or more delayed copies of a pulse may arrive at the same time as the primary pulse for a subsequent bit

39. Propagation Modes • Ground-wave propagation • Sky-wave propagation • Line-of-sight propagation

40. Ground Wave Propagation

41. Ground Wave Propagation • Follows contour of the earth • Can Propagate considerable distances • Frequencies up to 2 MHz, which are low frequencies and have tendency to tilt downwards • EM waves of low frequency are scattered by the atmosphere such that they do not penetrate the upper atmosphere. • Example • AM radio

42. Sky Wave Propagation

43. Line-of-Sight Propagation

44. 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 • Refraction – bending of microwaves 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

45. Propagation Factors 45 The transmitter’s power output The frequency being transmitted The effect of the Earth’s shape in between the points The conductivity of the Earth along the transmission path The microclimate through which the signal passes

46. Multiple Access Techniques • Frequency Division Multiple Access (FDMA) • Time Division Multiple Access (TDMA) • Random Access • ALOHA • Slotted ALOHA • Reservation-based ALOHA • Code Division Multiple Access (CDMA)

47. Code User 4 User 3 User 2 User 1 Time Frequency FDMA • FDMA was the initial multiple-access technique for cellular systems • Separates large band into smaller channels. • Each channel has the ability to support user. • Guard bands are used to separate channel preventing co-channel interference • Narrow bandwidth (30 khz). f1

48. FDMA • Advantages • Simple to implement in terms of hardware. • Fairly efficient with a small base population and with constant traffic. • Disadvantages • Network and spectrum planning are intensive and time consuming. • Channels are dedicated for a single user, idle channels add spectrum inefficiency.

49. Code User 4 User 3 User 2 User 1 Time Frequency TDMA • Entire bandwidth is available to the user for finite period of time. • Users are allotted time slots for a channel allowing sharing of a single channel. • Requires time synchronization. • Each of the user takes turn in transmitting and receiving data in a round robin fashion.

50. How it works? • User presses Push-to-Talk (PTT) button • A control channel registers the radio to the closest base station. • The BS assigns an available pair of channels. • Unlike FDMA, TDMA system also assigns an available time slot within the channel. • Data transmission is not continuous rather sent and received in bursts. • The bursts are reassembled and appear like continuous transmission.