Introduction to Wireless Technologies Dr. Farid Farahmand

Wired Vs. Wireless Communication CS 625, IITK CSE Jan3, 2002 Wired Vs. Wireless Communication Wired Wireless Each cable is a different channel One media (cable) shared by all Signal attenuation is low High signal attenuation High interference noise; co-channel interference; adjacent channel interference No interference Introduction to wireless communication

Why go wireless ? Limitations Bandwidth Fidelity Power (In)security CS 625, IITK CSE Jan3, 2002 Why go wireless ? Limitations Bandwidth Fidelity Power (In)security Advantages Sometimes it is impractical to lay cables User mobility Cost Introduction to wireless communication

Wireless Systems: Examples CS 698T Wireless Systems: Examples Broadcast (analog) AM, FM Radio TV Broadcast Satellite Broadcast 2-way Radios Cordless Phones Satellite Links Mobile Telephony Systems Wireless Local Loop (WLL) Microwave Links Wireless LANs Infrared LANs 2-way communication (analog) 2-way communication (digital) Q1. Can you think of any wireless system not already listed here? Q2. These systems are not randomly listed. Can you spot any trend here? OB1. It is interesting that operating frequency of these systems increases as we go down the list. Q3. Discuss difference between analog and digital systems. P. Bhagwat

Wireless Systems: Range Comparison CS 698T Wireless Systems: Range Comparison 1 m 10 m 100 m 1 Km 10 Km 100 Km 1,000 Km Satellite Links SW Radio MW Radio FM Radio Mobile Telephony, WLL WLANs Q1. Can you think of a system whose range of communication is more than satellite links? Q2. Give an example of a system whose range of communication is shorter than IR links. Point to ponder: Why does range of communication increase on logarithm scale? Blueooth IR P. Bhagwat

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

Frequency Band Allocations RADIO IR VISIBLE UV X-RAYS GAMMA RAYS RADIO VLF LF MF HF VHF UHF SHF EHF 3k 30k 300k 3M 30M 300M 3G 30G 300GHz VLF: Very Low Frequency LF: Low Frequency MF: Medium Frequency HF: High Frequency VHF: Very High Frequency UHF: Ultra High Frequency SHF: Super High Frequency EHF: Extremely High Frequency

Wavelengths of Frequency Bands VLF, LF  long waves MF  medium waves HF, VHF  short waves UHF, SHF  microwaves  EHF  millimeter waves Above microwave region, only certain windows of frequencies propagate freely through air, rain, etc. Infrared and visible light will not penetrate walls X-rays and gamma rays interact with matter Propagate well beyond line of sight The distance the signal travels decreases as the frequency increases

Electromagnetic Signals Electromagnetic Signals are emitted and received in wireless systems Requires a transmitting and receiving antenna The EM signal goes through the unguided medium Free space (vacuum) Earth’s atmosphere EM propagation is also referred to radio frequency propagation Wireless communications examples Terrestrial radio Microwave radio Broadband radio Mobile radio Cellular phone

What is EM? EM involves both a varying electric field (E) and a varying magnetic field (H) E and H appear at right angles to each other and to the direction of travel of the wave (Z-axis) The power passing a given signal is called the power density (P) P (Watt/m2)= H.E http://info.ee.surrey.ac.uk/Teaching/Courses/EFT/transmission/html/TEMWave.html

EM Propagation Electromagnetic waves are invisible We use the concept of rays to describe them When radiating uniformly over a spherically we refer to it as isotropic radiation Power Density (W/m2) = P_radiated / Area of sphere As we get further from the source the radiation (received power) becomes smaller

Example of Power Density Assume the isotropic radiated power from an antenna is 100 W. Assuming the receiving antenna is 100 m away, calculate the received power density (assume vacuum). P(density) = 100W / 4p(100)2 =0.796 mW/m2 TX RX R=100 m

Free Space Loss The signal disperses with distance Free space loss, ideal isotropic antenna Pt = signal power at transmitting antenna (watt) Pr = signal power at receiving antenna (Watt)  = carrier wavelength d = propagation distance between antennas c = speed of light (» 3 ´ 10 8 m/s) where d and  are in the same units (e.g., meters)

Example of Power Radiation Assume the isotropic radiated power from an antenna is 100 W. Assuming the receiving antenna is 100 m away, calculate the received power (assume vacuum and frequency of radiation is 100 MHz). P(received) =Pr =100W / (100x106x4p(100)/3x108)2 =0.057 mW  very little power received! TX RX R=100 m

Attenuation Strength of signal falls off with distance over transmission medium 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 Note: Attenuation in dB can be calculated by

Signal Loss and Attenuation Pulse spreading in free space Attenuation in non-vacuum Attenuation due to particles absorbing the EM energy Called “wave absorption” Remember: Attenuation = 10 log (Pout/Pin)

Other Impairments Multipath – obstacles reflect signals so that multiple copies with varying delays are received Refraction – bending of radio waves as they propagate through the atmosphere Atmospheric absorption – water vapor and oxygen contribute to attenuation

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

Multipath Propagation & Fading Reflection – occurs when signal encounters a surface that is large relative to the wavelength of the signal Diffraction - occurs at the knife-edge of an impenetrable body that is almost the same 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

Multipath Propagation & Fading Three basic propagation mechanisms (D is the size of the material) Reflection λ << D Diffraction λ  D Scattering λ >> D

Refraction Refraction – bending of microwaves by the atmosphere Thin Air Dense 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

References Narayan Mandayam, Tomasi