"Antennas, Propagation & Signal Encoding Techniques"
Antennas and Propagation Lecture Learning Outcomes Understand the radiation pattern of an antenna and calculate parameters for different antenna types. Understand the basis of signal propagation. Lecture Learning Outcomes Understand the concepts associated with LoS transmissions. Been able to calculate noise parameters, antenna gain and transmission losses for different types of antennas in LoS transmissions. Class Contents Antennas • Radiation Patterns • Antenna Types & Gains Propagation Modes • Ground Wave • Sky Wave • Line of Sight Line of Sight Transmission • Attenuation • Free Space Loss • Noise • Atmospheric Absorption • Multipath • Refraction Fading in the Mobile Environment • Multipath Propagation Antennas An antenna is an electrical conductor or system of conductors used either for radiating electromagnetic energy into space or for collecting electromagnetic energy from space. Radiation Patterns is a graphical representation of the radiation properties of an antenna as a function of space coordinates. Radiation patterns are almost always depicted as 2-dimensional cross section of the three-dimensional pattern The Isotropic Antenna An Isotropic Antenna radiates power in all directions equally. (Omnidirectional Antenna) Beam Width (Half-Power Width) Is the angle within which the power radiated by the antenna is at least half of what is in the most preferred radiation position. Directional Antenna: Power radiated in the direction of B is greater than that radiated in the direction of A Antenna Types & Gains Dipoles • Half-Wave Dipole (Hertz Antenna) • Quarter Wave Dipole (Marconi Antenna) Half-Wave Marconi Dipole Radiation Pattern Dipole Antenna Parabolic Reflective Antenna (a) Parabola Properties (b) Parabolic Antenna: principle of operation (c) Radiation Pattern Typical beam width for parabolic antennas at 12 GHz Antenna Diameter (m) Beam Width (degrees) 0.5 3.5 0.75 2.33 1.0 1.75 1.5 1.166 2.0 0.875 2.5 0.7 5.0 0.35 Antenna Gain Is a measure of directionality of an antenna It is defined the power output in a particular direction compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna). G antenna gain 4 Ae 4 Ae f 2 G A e effectivearea 2 c2 f carrier frequency c speed of ligth (3x108 m/s) carrier wa velength Effective Area of typical antennas Type of Antenna Effective Area Ae Power Gain (m2) (Relative to Isotropic) Isotropic 1 / 4 2 1.5 2 / 4 Infinitesimal 1.5 Dipole or loop Half-Wave Dipole 1.64 2 / 4 1.64 Parabolic (face area A) 0.56 A 7 A / 2 Propagation Modes Ground Wave Propagation Sky Wave Propagation Line of Sight Ground Wave • Frequency Below 2 MHz • Slowed down wave front due to EM current induced into the earth. (downwards tilt) • Suffer from difraction and scattering from the atmosphere • Classical Example: AM radio Sky Wave • Frequency between 2 and 30 MHz • Transmitted signal is refracted by the ionosphere and reflected By the earth. • Bouncing allows signal to be picked up thousands of kilometres from the transmitter. • Classical Example: Amateur radio, CB radio and international broadcast (BBC & Voice of America) Line of Sight • Above 30 MHz, ground wave and sky wave do not operate • There is no reflection from the ionosphere (allowing satellite communications not beyond the horizon and back). • For Ground Based communications, the antennas need to be in LOS with each other. Optical and radio LOS Optical LOS with no intervening obstacles d 3.57 h Radio LOS K is and adjustment factor d 3.57 K h used to compensate for the refraction Optical and radio LOS Maximum distance between two antennas (radio LOS) with K=4/3 d 3.57 K h1 K h2 4.12 h1 h2 • h is measured in metres • d is measured in kilometres •K depends on weather conditions Perfect Ideal Average Hard Bad Standard Atmosphere Without mist Sub-standard Surface Ducts, Wet Mist Light Mist ground over mist water Typical Mild Climate (Non Dry, Mountainous Plains, some Tropical Coast Coast tropical), air mix without mist mist day and night K 1,33 1,33 1 1 0,66 0,66 0,5 0,5 0,4 Line of Sight Transmission Sources of Impairment Attenuation & Attenuation Distortion Noise Atmospheric Absorption Multipath Refraction Attenuation & Attenuation Distortion Attenuation Defined as the loss of strength of the signal over the communications channel. It is a complex function of the distance and the make of the atmosphere. Attenuation Distortion Occurs when the frequency components of the received signal have different relative strengths than the frequency components of the transmitted signal. Factors encountered when dealing with attenuation Strength on the received signal (solved using amplifiers or repeaters in the communications path). SNR considerations (must be high enough to avoid errors in the transmission) – solved using amplifiers of repeaters. Attenuation increase with frequency (known as attenuation distortion) – solved using equalizing techniques across a band of frequencies. Free Space Loss Is the ratio of power radiated by the transmitter antenna to the power received by the receiver antenna. PT 4 d PT=transmitted power (W) 2 Isotropic : L PR=received power (W) Antenna PR 2 d = distance = wavelength (same units as distance It is usually expressed in dB PT(dB) PR(dB) L dB f is expressed in Hz L dB 20 log(d ) 20 log( f ) 147.56 dB d is expressed in m PT(dB) PR(dB) 20 log( d ) 20 log( f ) 147 .56 dB Free Space Loss – Other Antennas For non-isotropic antennas, the gain of the antenna, with respect to isotropic, should be taken into consideration: L PT 4 d 2 2 PR GT GR Expressed in dB: PT(dB) PR(dB) 20 log( d ) 20 log( f ) 10 log(G T G R ) 147 .56 dB PT(dB) and PR(dB) must be expressed in the same dB unit: dBW or dBm The gains inside the logarithm should be expressed in adimensional Quantities. If expressed in dB, they should be in dBi PT(dB) PR(dB) 20 log( d ) 20 log( f ) G T(dBi ) G R(dBi) 147 .56 dB Free Space Loss – Other Antennas Free space loss can also be expressed in terms of effective area: L dB 20 log( d ) 20 log( f ) 20 log( A eT A eR ) 169 .54 dB Noise Noise are unwanted signals that combine and distort the signal intended for transmission and reception in a communications system. Thermal Noise Intermodulation Noise Crosstalk Impulsive Noise Thermal Noise Due to thermal agitation of electrons It is present in all electronic devices and transmission media. It is a function of the temperature The amount of thermal noise is defined as noise power density in watts per 1 Hz of bandwidth. N 0 k T ( W/Hz) K is the Boltzmann’s constant: 1.3803x10-23 J/K T is the absolute temperature in Kelvins Thermal Noise At room temperature (250 C), the noise power density is: N 0 dB 10 log(1.38 10 23 (J/K ) (298 .15 K)) 203 dBW/Hz For any given bandwidth B, the noise present in the band is: N0 k T B in dBW N 0 228 .6 dBW 10 log(T ) 10 log( B) Intermodulation Noise Produced when there is nonlinearities in the transmitter, receiver or transmission system, when 2 or more different frequencies share the medium. The effect is the production of new signals at frequencies that are the sum or difference of the original frequency and multiples of those frequencies. Cross Talk Defined as unwanted coupling between signal paths. Can occur when unwanted signals are picked up by microwave antennas or by electrical coupling between twisted pair (in guided media transmissions) Can be identified when in the telephone line, another conversation can be heard. Typically is in the same order of magnitude or less than the Thermal Noise Impulsive Noise Non-continuous noise consisting of irregular pulse or noise spikes of short duration and relatively high amplitude. Causes include external electromagnetic disturbances (lightning) and faults and flaws in the communication system. It is a minor concern in analogue signals, but is a major concern when dealing with digital data transmissions Impulsive Noise Example In a voice communication, impulsive noise will generate clicks and crackles of short duration, however, the conversation will still be intelligible. In a digital transmission, a small spark of energy (10 ms in duration) would wash out 560 bits of data being transmitted at 56 kbps. Ratio of Signal Energy per bit to Noise Power Density The short name for this equivalent is the Eb/N0 expression The advantage of Eb/N0 over SNR is that the latter depends on the bandwidth Ratio of Signal Energy per bit to Noise Power Density A signal containing a binary data transmitted at a data rate of R, is subjected to thermal noise N0 The Energy per bit in such a signal is: S = signal power E b S Tb Tb = time needed to transmitt 1 bit: The expression Eb/No can be written: Tb = 1/R S S k = Boltzman Constant Eb N0 (1.3803x1023 J/K) R N0 R k T T = Temp in Kelvin Eb N SdBW 10 log( R ) 228.6 dBW - 10 log(T) 0 dB Ratio of Signal Energy per bit to Noise Power Density Example: Suppose a signal encoding technique requires that Eb/N0 = 8.4 dB for a bit error rate of 10-4. If the effective noise temperature is 290K (room temperature) and the data rate is 2.4 Kbps, what received signal level is required to overcome thermal noise Solution: Eb N SdBW 10 log( R ) 228.6 dBW - 10 log(T) 0 dB 8.4 dB SdBW 10 (3.38) 228.6 - 10 (2.46) SdBW 161.8 dBW Achievable Spectral Density The parameter N0 is the noise power density in watts/hertz. The noise in a signal with a bandwidth B is: N N0 B Substituting in the Eb/N0 expression S S B Eb N0 R N0 N R Considering that the Shannon’s capacity formula (in bps) C B log 2 (1 S N ) S C 2 B 1 N Achievable Spectral Density Equating the channel capacity C with the data rate R, and using the Eb/N0 expression: B CB Eb N 0 2 1. C This expression is a formula that relates the achievable spectral efficiency C/B to Eb/No Atmospheric Absorption Additional loss between the transmitting and receiving antenna. The main contributors are the water vapour and oxygen present in the atmosphere. Water Vapour generates attenuation peaks at frequencies close to 22 GHz Absorption due to oxygen has a peak in the vicinity of 60 GHz Rain and Fog cause scattering of radio waves that results in attenuation Multipath Occurs in environments where there is no direct LOS between the transmitting and receiving antenna due to the presence of intervening obstacles. Obstacles can reflect the signal creating multiple copies that arrive at delayed times to the receiver. This copies acts as noise to the received signal. Refraction Is the bend that suffer radio waves when propagating through the atmosphere It is caused by changes of speed of the signal with altitude or by other spatial changes in atmospheric conditions. Normally the speed of the signal increases with altitude, causing the radio waves to bend downwards. Fading in the Mobile Environment Fading refers to the time variation of the received signal power caused by changes in the transmission medium or path(s). The most important fading mechanism is multipath propagation. Multipath propagation Reflection (surface > wavelength) Diffraction (edge of body > wavelength) Scattering (obstacle = wavelength) Effects of multipath propagation Copies of the signal arriving at different phases. If copies add destructively, SNR declines Signal interpretation then becomes difficult. Intersymbol interference (ISI)