2 Lecture Learning Outcomes Understand the radiation pattern of an antenna and calculate parameters for different antenna types.Understand the basis of signal propagation.
3 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.
4 Class Contents Antennas Radiation Patterns Antenna Types & Gains Propagation ModesGround WaveSky WaveLine of SightLine of Sight TransmissionAttenuationFree Space LossNoiseAtmospheric AbsorptionMultipathRefractionFading in the Mobile EnvironmentMultipath Propagation
5 AntennasAn 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 Patternsis 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
6 The Isotropic AntennaAn Isotropic Antenna radiates power in all directionsequally. (Omnidirectional Antenna)
7 Beam Width (Half-Power Width) Is the angle within which the power radiated by the antennais at least half of what is in the most preferred radiationposition.Directional Antenna: Power radiated in the direction of B is greater than that radiated in the direction of A
10 Typical beam width for parabolic antennas at 12 GHz Antenna Diameter (m)Beam Width (degrees)0.53.50.752.331.01.751.51.1662.00.8752.50.75.00.35
11 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).
12 Effective Area of typical antennas Type of AntennaEffective Area Ae (m2)Power Gain(Relative to Isotropic)Isotropic1Infinitesimal Dipole or loop1.5Half-Wave Dipole1.64Parabolic (face area A)
13 Propagation Modes Ground Wave Propagation Sky Wave Propagation Line of Sight
14 Ground Wave Frequency Below 2 MHz Slowed down wave front due to EM current induced intothe earth. (downwards tilt)Suffer from difraction and scattering from the atmosphereClassical Example: AM radio
15 Sky Wave Frequency between 2 and 30 MHz Transmitted signal is refracted by the ionosphere and reflectedBy the earth.Bouncing allows signal to be picked up thousands of kilometresfrom the transmitter.Classical Example: Amateur radio, CB radio and internationalbroadcast (BBC & Voice of America)
16 Line of Sight Above 30 MHz, ground wave and sky wave do not operate There is no reflection from the ionosphere (allowing satellitecommunications not beyond the horizon and back).For Ground Based communications, the antennas need to bein LOS with each other.
17 Optical and radio LOS Optical LOS with no intervening obstacles K is and adjustment factorused to compensate for therefraction
18 Optical and radio LOS h is measured in metres Maximum distance between two antennas (radio LOS) with K=4/3h is measured in metresd is measured in kilometresK depends on weather conditionsPerfectStandard AtmosphereIdealWithout mistAverageSub-standard Light MistHardSurface Ducts, ground mistBadWet Mist over waterTypicalMild Climate (Non tropical), air mix day and nightDry, Mountainous without mistPlains, some mistTropical CoastCoastK1,331,33 11 0,660,66 0,50,5 0,4
19 Line of Sight Transmission Sources of ImpairmentAttenuation & Attenuation DistortionNoiseAtmospheric AbsorptionMultipathRefraction
20 Attenuation & Attenuation Distortion Defined as the loss of strength of the signal over the communicationschannel. It is a complex function of the distance and the make of theatmosphere.Attenuation DistortionOccurs when the frequency components of the received signalhave different relative strengths than the frequency componentsof the transmitted signal.
21 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.
22 Free Space LossIs the ratio of power radiated by the transmitter antennato the power received by the receiver antenna.PT=transmitted power (W)PR=received power (W)d = distance= wavelength (sameunits as distanceIsotropicAntenna:It is usually expressed in dBf is expressed in Hzd is expressed in m
23 Free Space Loss – Other Antennas For non-isotropic antennas, the gain of the antenna, with respectto isotropic, should be taken into consideration:Expressed in dB:PT(dB) and PR(dB) must be expressed in the same dB unit: dBW or dBmThe gains inside the logarithm should be expressed in adimensionalQuantities. If expressed in dB, they should be in dBi
24 Free Space Loss – Other Antennas Free space loss can also be expressed in terms ofeffective area:
25 Noise Noise are unwanted signals that combine and distort the signal intended for transmission and reception ina communications system.Thermal NoiseIntermodulation NoiseCrosstalkImpulsive Noise
26 Thermal Noise Due to thermal agitation of electrons It is present in all electronic devices and transmissionmedia.It is a function of the temperatureThe amount of thermal noise is defined as noise power densityin watts per 1 Hz of bandwidth.K is the Boltzmann’s constant: x10-23 J/KT is the absolute temperature in Kelvins
27 Thermal Noise At room temperature (250 C), the noise power density is: For any given bandwidth B, the noise present in the band is:in dBW
28 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.
29 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
30 Impulsive NoiseNon-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
31 Impulsive NoiseExampleIn 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 databeing transmitted at 56 kbps.
32 Ratio of Signal Energy per bit to Noise Power Density The short name for this equivalent is the Eb/N0 expressionThe advantage of Eb/N0 over SNR is that the latter depends on the bandwidth
33 Ratio of Signal Energy per bit to Noise Power Density A signal containing a binary data transmitted at a data rate of R, issubjected to thermal noise N0The Energy per bit in such a signal is:S = signal powerTb = time needed totransmitt 1 bit:Tb = 1/Rk = Boltzman Constant(1.3803x1023 J/K)T = Temp in KelvinThe expression Eb/No can be written:
34 Ratio of Signal Energy per bit to Noise Power Density Example:Suppose a signal encoding technique requires that Eb/N0 = 8.4 dBfor a bit error rate of If the effective noise temperature is 290K(room temperature) and the data rate is 2.4 Kbps, what receivedsignal level is required to overcome thermal noiseSolution:
35 Achievable Spectral Density The parameter N0 is the noise power density in watts/hertz.The noise in a signal with a bandwidth B is:Substituting in the Eb/N0 expressionConsidering that the Shannon’s capacity formula (in bps)
36 Achievable Spectral Density Equating the channel capacity C with the data rate R, and using the Eb/N0 expression:This expression is a formula that relates the achievablespectral efficiency C/B to Eb/No
37 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 GHzAbsorption due to oxygen has a peak in the vicinity of 60 GHzRain and Fog cause scattering of radio waves that results in attenuation
38 MultipathOccurs 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.
39 RefractionIs the bend that suffer radio waves when propagating through the atmosphereIt 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.
40 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.
41 Multipath propagation Reflection(surface > wavelength)Diffraction(edge of body > wavelength)Scattering(obstacle = wavelength)
42 Effects of multipath propagation Copies of the signal arriving at different phases.If copies add destructively, SNR declinesSignal interpretation then becomes difficult.Intersymbol interference (ISI)