CELLULAR COMMUNICATIONS 2. Radio Wave Propagation

Radio Waves  Radio waves is a form of radiation known as electromagnetic waves  Nature of radio waves and the way they travel are a key elements in cellular system  Properties of radio waves affect  Modulation (encoding of a speech or data using radio wave)  Spectrum allocation (Transmission frequency)  Network planning (location and characteristics of antennas)

Electric fields  Any electrically charged object (static or current) has an electrical field associated with it  Like charges repel one another and opposite charges attract  The electric field radiates out from any item with an electric potential.  Strength of the field falls away as the inverse square of the distance

Electric Potential around a charged sphere

Variation of potential with distance

Magnetic Field  Field around the magnets and electric current  Have two types of poles (north and south) analogous two 2 types of electric charge (positive and negative)  Strength of the field falls away as the inverse square of the distance

Magnetic Forces Around The Current

Radio Signals  Form of the electromagnetic radiation (same as light, ultraviolet or infrared)  Characterized by specific range of frequencies (wavelength)  Inseparable magnetic and electric components

Radio Wave

Wave Frequency and Wavelength

Electromagnetic Wave Spectrum

Radio Wave Spectrum UHF (450 MHz, 800/900 MHz, 1800/1900MHz and 2100 MHz)are typically used in communications system:

Radio Waves Cont’d  Ground wave (<2 MHz): Waves with low frequencies  follow the earth’s surface and can propagate long distances  AM(LF,MF) radio.  Sky wave (2–30 MHz): Short waves that are reflected at the ionosphere.  bounce back and forth between the ionosphere and the earth’s surface, travelling around the world.  International Radio Broadcasts  Line-of-sight (>30 MHz): Mobile phone systems, satellite systems, cordless

Radio signals in “free space”  Can travel over enormous distance  Become weaker as have to cover greater area  Area of surface (sphere) is proportional to the distance square  Strength of the signal is inversely proportional to the square of the distance

Radio waves in typical terrestrial environment  Tree, buildings and other objects  Signal strength dies away at much faster rate  Around 1/d^4  Typical formula used by network designer  1/d^3.8

Reflection and Refraction Sea and most areas are good reflector Sand absorb large portion of the energy Air with different temperatures/pressure might result in refraction Snell’s law (refraction index)

Refraction Demo

Some Consequences of refraction  Slowly changing refraction index  Steady bending of the wave  Coverage of the antenna can be extended below line of sight by 30%

Diffraction

Fading  Signals to/from mobile handset vary greatly in strength as the users moves from place to place  Variations is larger over the small distances, could not be explained by distance from the antenna alone  Usually categorized into two types: slow fading and fast fading  Slow fading: phone moves behind the building

Fast fading: Multipath

How fast is the fast fading?  Assume only 2 paths  Frequency is 2Ghz  Wavelength c/f=3*10^8/2*10^9=0.15m.  If 2 paths of the same length, the received signal is in-phase  Move handset to increase the distance of one path by 7.5cm, the signals will become out of in phase  The two signals will almost completely cancel out

Combined effect of fading

Inter-Symbol Interference(ISI)  In digital transmission bits of information are transmitted using radio wave  Radio wave is modulated to carry binary information  Simple example :on-off modulation

ISI cont’d  Assume large variation between path distances  The receiver may be receiving a signal via the direct path that may be one bit of data  The reflected signal may be delayed to a sufficient extent that it is carrying the previous bit of data

ISI example  Assume data transmission rate is 50 kbps  1 bit of data should be sent during 1/50*1024 about 20 micro second (10^-6)  During this time the radio signal will travel about 6km  Reflected signal from the building 3km behind the handset will cause ISI

Fixing ISI and fading  Equalization: Model the channel distortion by sending known signal. Use model to fix real signal  Diversity: Send different data portion over multiple carriers, multiple times, separated in time

Cells around the transmitter

Experimental Data