Wireless networks - Lecture 3: Introduction to Wireless communication. The main topics covered in this chapter include: multiplexing; multiplexing - carrying multiple signals; transmission mediums; propagation modes; multi-path propagation; fading;...
Wireless Networks Lecture Introduction to Wireless Communication Dr Ghalib A Shah Outlines Review of previous lecture #2 Multiplexing Transmission Mediums Propagation modes Multi-path propagation Fading Summary of today’s lecture Last Lecture Review Wireless Transmission ► ► ► ► Digital data analog signal Baseband/bandpass signal Encoding techniques/Modulation Receiver synchronization / Demodulation Noises ► ► ► ► ► Thermal noise Intermodulation noise Crosstalk Impulse Noise Manmade noise / Natural noise Losses / Gain Multiplexing Capacity of transmission medium usually exceeds capacity required for transmission of a single signal Multiplexing carrying multiple signals on a single medium ► More efficient use of transmission medium Multiplexing Reasons for Widespread Use of Multiplexing Cost per kbps of transmission facility declines with an increase in the data rate Cost of transmission and receiving equipment declines with increased data rate Most individual data communicating devices require relatively modest data rate support Multiplexing Techniques Frequencydivision multiplexing (FDM) ► Takes advantage of the fact that the useful bandwidth of the medium exceeds the required bandwidth of a given signal Timedivision multiplexing (TDM) ► Takes advantage of the fact that the achievable bit rate of the medium exceeds the required data rate of a digital signal Frequencydivision Multiplexing Timedivision Multiplexing 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 10 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 19 Sky Wave Propagation 20 Sky Wave Propagation Signal reflected from ionized layer of atmosphere back down to earth Signal can travel a number of hops, back and forth between ionosphere and earth’s surface Reflection effect caused by refraction Examples ► Amateur radio ► CB radio 21 LineofSight Propagation 22 LineofSight 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 23 LineofSight Equations Optical line of sight d 3.57 h Effective, or radio, line of sight d • • • 3.57 h d = distance between antenna and horizon (km) h = antenna height (m) K = adjustment factor to account for refraction, rule of thumb K = 4/3 24 LineofSight Equations Maximum distance between two antennas for LOS propagation: 3.57 h1 • h1 = height of antenna one • h2 = height of antenna two h2 25 Example Let h1 =100 m, h2 =0 or the second antenna is at ground level D =3.57 ( 4/3x100)^1/2 +0 =41 km Now suppose that h2 =10m To achieve same distance, what must be h1? 41 =3.57(Kh1)^1/2+(13.3)^1/2 h1 =46.2m 26 Propagation Factors 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 27 27 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 28 Multipath Propagation 29 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 30 Types of Fading Fast fading ► Rapid variation in signal strength occurs over distance about onehalf of wavelength ► At 900 Mhz cellular badn, lambda is 0.33 m Slow fading ► Users cover distance well in excess of a wavelength as it passes buildings of different heights, vacant lots and so on ► A slow variation in signal strength Flat fading Selective fading 31 Fading channel Additive white Gaussian noise (AWGN) channel ► Signal is degraded only by thermal noise ► Accurate for space communication and some wire communication such as coaxial cable Rayleigh fading ► Fading occurs when there are multiple indirect paths but no direct LOS path ► Suitable for Outdoor environment Rician fading ► When there exist a direct LOS path in addition to multiple paths ► Suitable for smaller cells and indoor environment 32 Summary of today’s lecture Multiplexing ► FDM, TDM Transmission Mediums ► ► Guided media Unguided media • • • Propagation modes ► ► ► Microwave Radio waves Infra red Ground wave propagation Sky-wave propagation LOS propagation Multi-path propagation Fading Next lecture ► Error detecting and correcting techniques 33 ... Review of previous lecture #2 Multiplexing Transmission Mediums Propagation modes Multi-path propagation Fading Summary of today’s lecture Last Lecture Review Wireless Transmission ► ►... Long haul telecommunications service • • • 4 – 6 GHz band is common But due? ?to? ?increased congestion 11 GHz is coming into use now Microwave links provide TV signals? ?to? ?local CATV and then distributed to? ?subscribers via coaxial cable... attenuation due to atmospheric absorption ► Mostly use 5.92 5-6 .425 for uplink and 3. 7-4 .2 GHz for downlink referred as 4/6 GHz band ► Due to saturation, 12/14 GHz band has been developed Uplink: 1 4-1 4.5,