1. Antennas and Propagation
Chapter 5
Signal propagation

2. Introduction
An antenna is an electrical conductor or system of conductors
Transmission - radiates electromagnetic energy into space
Reception - collects electromagnetic energy from space
In two-way communication, the same antenna can be used for transmission and reception
Technique background on networking. Fundamental background for wireless transmission.
For transmission of signal, radio-frequency electrical energy from the transmitter, is converted into electromagnetic energy by the antenna, and radiated into the surrounding environment (atmosphere, space, water)
For reception of a signal, electromagnetic energy impinging on the antenna is converted into radio-frequency electrical energy and fed into the receiver.
Same frequency is used in both direction.

3. Radiation Patterns Radiation pattern
Graphical representation of radiation properties of an antenna
Depicted as two-dimensional cross section
The relative distance from the antenna position in each direction determines the relative power.
How to characterize the performance of an antenna, as a function of space coordinates. Idealized isotropic antenna. A point in space that radiates equally in all directions. Sphere. Of its three dimensional pattern.
Idealized antenna, directional antenna in which the preferred direction of radiation is along one axis. To determine the relative power in a given direction, a line is drawn from the antenna position at the appropriate angle, and the point of intercept with the radiation pattern is determine.
B vector is longer than A vector, indicating that more power is radiated in B direction than in A direction. The relative lengths of the two vectors are proportional to the amount of power radiated in the two directions.

4. Radiation Patterns Beam width (or half-power beam width)
Measure of directivity of antenna
The angle within which the power radiated by the antenna is at least half of what it is in the most preferred direction.
Reception pattern
Receiving antenna’s equivalent to radiation pattern
The longest section of the pattern indicates the best direction for reception.

5. Types of Antennas Isotropic antenna (idealized) Dipole antennas
Radiates power equally in all directions
Dipole antennas
Half-wave dipole antenna (or Hertz antenna)
Quarter-wave vertical antenna (or Marconi antenna)
The half-wave dipole consists of two straight collinear conductors of equal length, separated by a small gap. The length of the antenna is one-half the wavelength of the signal that can be transmitted most efficiently.
Automobile radios and portable radios

6. Types of Antennas Parabolic Reflective Antenna
Terrestrial microwave and satellite application
A parabola is locus of all points equidistant from a fixed line and a fixed point not on the line. Focus, directrix
If a parabola is revolved about its axis, the surface generated is called a paraboloid. A cross section through the paraboloid parallel to its axis forms a parabola a cross section perpendicular to the axis
Automobile headlights, telescopes. Reflecting. Wave bounce back

7. Antenna Gain Antenna gain Effective area
A measure of the directionality of an antenna.
Power output, in a particular direction, compared to that produced in any direction by a perfect omnidirectional antenna (isotropic antenna)
Effective area
Related to physical size and shape of antenna
If an antenna has a gain of 2 db. That the antenna improves based on isotropic antenna in that direction by 2 db. The increased power radiated in a given direction is at the expense of other directions. Increased power is radiated in one direction by reducing the power radiated in other directions.
Does not refer to obtaining more output power than input power but rather to directionality.

8. Antenna Gain Relationship between antenna gain and effective area
G = antenna gain
Ae = effective area
f = carrier frequency
c = speed of light (» 3 ´ 108 m/s)
 = carrier wavelength

9. Antenna gain and effective areas
Type of antenna
Effective area
Power gain
Isotropic
ג2/4 1
Infinitesimal dipole or loop
1.52/4
1.5
Half-wave dipole
1.642/4
1.64
Horn, mouth area A
0.81A
10A/ 2
Parabolic, face area A
0.56A
7A/ 2
turnstile
1.152/4
1.15

10. Propagation Modes Ground-wave propagation Sky-wave propagation
Line-of-sight propagation
A signal radiated from an antenna travels along one of three routes

11. Ground Wave Propagation
Ground wave propagation follows the contour of the earch. Can propagate considerable distances, well over the visual horizon.
The electromagnetic wave induces a current in the earth’s surface, the result of which is to slow the wavefront near the earth, causing the wavefront to tilt downward and hence follow the earth curvature. diffraction

12. Ground Wave Propagation
Follows contour of the earth
Can Propagate considerable distances
Frequencies up to 2 MHz
Example
AM radio

13. Sky Wave Propagation
Hard reflecting surface. Amateur radio, CB

14. 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

15. Line-of-Sight 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

16. Line-of-Sight 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 sight of each other 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
A brief discussion about refraction is warranted. Such as light or radio wave travels at approximately . Vacuum
If moving from a less dense to a more dense medium, the wave will bend toward the more dense medium. Shorter and bent
Continuous, gradual bending

17. Line-of-Sight Equations
Optical line of sight
Effective, or radio, line of sight
d = distance between antenna and horizon (km)
h = antenna height (m)
K = adjustment factor to account for refraction, rule of thumb K = 4/3
With no intervening obstacles, the optical line of sight can be expressed as kilometers

18. Line-of-Sight Equations
Maximum distance between two antennas for LOS propagation:
h1 = height of antenna one
h2 = height of antenna two
The maximum distance between two antenna for LOS transmission if one antenna is 100 m high and the other is at ground level is :
Now suppose that the receiving antenna is 10 m high. To achieve the same distance, how high must the transmitting antenna be?

19. Exercise
The maximum distance between two antenna for LOS transmission if one antenna is 100 m high and the other is at ground level is :
Now suppose that the receiving antenna is 10 m high. To achieve the same distance, how high must the transmitting antenna be?
This is a saving of over 50 m in the height of the transmitting antenna. Illustrate the benefit of raising receiving antennas above ground level to reduce the necessary height of the transmitter

20. LOS Wireless Transmission Impairments
Attenuation and attenuation distortion
Free space loss
Noise
Atmospheric absorption
Multipath
Refraction
With any communications system, the signal that is received will differ from the signal that is transmitted, due to different transmission impairments. For digital signal, bit errors are introduced. Vice verse

21. 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
Attenuation is greater at higher frequencies, causing distortion

22. Free Space Loss Free space loss, ideal isotropic antenna
Pt = signal power at transmitting antenna
Pr = signal power at receiving antenna
 = 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)

23. Free Space Loss
Free space loss equation can be recast:

24. Free Space Loss Free space loss accounting for gain of other antennas
Gt = gain of transmitting antenna
Gr = gain of receiving antenna
At = effective area of transmitting antenna
Ar = effective area of receiving antenna

25. Free Space Loss
Free space loss accounting for gain of other antennas can be recast as

26. Categories of Noise Thermal Noise Intermodulation noise Crosstalk
Impulse Noise

27. Thermal Noise Thermal noise due to agitation of electrons
Present in all electronic devices and transmission media
Cannot be eliminated
Function of temperature
Particularly significant for satellite communication

28. Thermal Noise
Amount of thermal noise to be found in a bandwidth of 1Hz in any device or conductor is:
N0 = noise power density in watts per 1 Hz of bandwidth
k = Boltzmann's constant = ´ J/K
T = temperature, in kelvins (absolute temperature)

29. Thermal Noise Noise is assumed to be independent of frequency
Thermal noise present in a bandwidth of B Hertz (in watts):
or, in decibel-watts

30. Noise Terminology
Intermodulation noise – occurs if signals with different frequencies share the same medium
Interference caused by a signal produced at a frequency that is the sum or difference of original frequencies
Crosstalk – unwanted coupling between signal paths
Impulse noise – irregular pulses or noise spikes
Short duration and of relatively high amplitude
Caused by external electromagnetic disturbances, or faults and flaws in the communications system

31. Expression Eb/N0
Ratio of signal energy per bit to noise power density per Hertz
The bit error rate for digital data is a function of Eb/N0
Given a value for Eb/N0 to achieve a desired error rate, parameters of this formula can be selected
As bit rate R increases, transmitted signal power must increase to maintain required Eb/N0
A achievable spectral efficiency C/B

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

33. Fading in a mobile environment
The term fading refers to the time variation of received signal power caused by changes in the transmission medium or paths.
Atmospheric condition, such as rainfall
The relative location of various obstacles changes over time
One of the two antennas is moving relative to the others. The relative location of various obstacles changes over time.

34. Multipath Propagation
Urban environment. Lamp post or traffic signs

35. 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
Is scattered into several weaker outgoing signals. Influence the system performance in various ways depending on the local conditions

36. 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
A second phenomenon, of particular importance for digital transmission,

37. Types of Fading Fast fading Slow fading Flat fading Selective fading
Rayleigh fading
Rician fading
Rapid variation in signal strength occurs over distances of about one-half a wavelength. 900Mhz mobile cellular application a wavelength is 0.33m. Spatial variation of amplitude.
Flat fading, all frequency components of the received signal fluctuate in the same proportions simultaneously. Affects unequally the different spectral components. Occurs a portion of the whole bandwidth, selective.

38. The fading channel
Additive White Gaussian Noise (AWGN) channel  thermal noise as well as electronics at the transmitter and receiver
Rayleigh fading  there are multiple indirect paths between transmitter and receiver and no distinct dominant path, such as an LOS path
Rician fading  there is a direct LOS path in additional to a number of indirect multipath signals
Estimate the effects of multipath fading and noise on mobile channel. Simplest channel model, from the point of view of analysis. Thermal noise and physical

39. The fading channel
K=0 Rayleign
K=∞ AWGN
When K=0 rayleign

40. The fading channel
Bit error rate as a function of ratio AWGN fairly good performance. Large K.

41. Error Compensation Mechanisms
Forward error correction
Adaptive equalization
Diversity techniques
The effects to compensate for the errors and distortions introduced by multipath fading fall into three general categories

42. Forward Error Correction
Transmitter adds error-correcting code to data block
Code is a function of the data bits
Receiver calculates error-correcting code from incoming data bits
If calculated code matches incoming code, no error occurred
If error-correcting codes don’t match, receiver attempts to determine bits in error and correct
Digital transmission. A receiver, using only information contained in the incoming digital transmission, correct bit errors in the data. The receiver may detect the presence of errors and then sends a request back the transmitter to retransmit the data in error. Practical wireless applications. Satellite communication, the amount of delays involved. Mobile communications. The bit errors are so high.
A coding algorithm. The threshold . The length of code and the nature of algorithm.
2-3

43. Adaptive Equalization
Can be applied to transmissions that carry analog or digital information
Analog voice or video
Digital data, digitized voice or video
Used to combat intersymbol interference
Involves gathering dispersed symbol energy back into its original time interval
Techniques
Lumped analog circuits
Sophisticated digital signal processing algorithms
Linear equalizer circuit. For each output symbol, the input signal is sampled at five uniformly spaced intervals of time, separated by a delay T. the samples are individually weighted by the coefficients Ci, and sumed to the output symbol. Adaptive adjusted training sequence.

44. Diversity Techniques
Diversity is based on the fact that individual channels experience independent fading events
Space diversity – techniques involving physical transmission path
Frequency diversity – techniques where the signal is spread out over a larger frequency bandwidth or carried on multiple frequency carriers
Time diversity – techniques aimed at spreading the data out over time
We provide multiple logical channels in some sense between transmitter and receiver and sending part of the signal over each channel. The technique does not eliminate errors but it does reduce the error rate, because we have spread the transmission out to avoid being subjected to the highest error rate that might occur.
Space: multiple nearby antennas may be used to receive the message, then reconstruct the most likely transmitted signal. Multiple directional antennas, each oriented to a different reception angle with the incoming signals, then combined to reconstruct the transmitted signal.
So that a noise burst affects fewer bits. If a mobile host is moving slowly, it may remain in a region of a high level of fading for a relatively long interval. Even powerful error correction codes may be unable to cope with this error. If the data is transmitted in a TDM, in multiple users share the same physical channel by the use of time slots. interleaving