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Information Technology
Mobile Computing
Module: Signal Propagation Effects
Learning Objectives
- Signal definition
- Modes of signal propagation
- Ground wave propagation
- Sky wave propagation
- LOS propagation
- Phenomena effecting signal
- Reflection, Refraction, Diffraction, Scattering
- Effects on signal due to phenomena
- Multipath propagation, Delay spread, inter symbol interference
Introduction
In the previous modules we learnt what is wireless communication, different modes of wireless communication, how data and voice travel in the form of signals, which of electromagnetic spectrum are used for wireless communication, frequencies used for different wireless technologies. When E.M. waves carry data in the form of voice, images etc. it is known as signal. Signal on its way from sender to receiver is effected by several factors like distance as well as the environmental objects it interacts with. According to the properties of the object, earth’s atmosphere and wavelength of signal, the signal undergoes different phenomena like reflection, refraction, diffraction and scattering. Due to these phenomena, various effects occur which destructively or constructively effect the communication. In this module, we will learn about these phenomena and the effects due to them.
Signal
The physical representation of data by the transmission of which communication takes place is known as signal. The representation can be electrical, electronic or optical. Voice, images, letters or numbers can be represented using signals. Signal can be analog or digital.
Figure 1 Analog Signal v/s Digital signal
Factors affecting strength and path of EM waves
Distance between transmitter and receiver Frequency/wavelength of wave Earth’s atmosphere Environmental objects
Signal Propagation modes
A signal on its journey from transmitter to receiver propagates in three modes. Type of Propagation is determined by the carrier frequency.
Ground waves or surface waves propagation Sky waves or Ionospheric propagation Space waves or Line-of-sight propagation
Ground Waves Propagation also known as Surface Wave (<2MHz)
Waves with low frequencies follow the earth’s surface. Due to very less frequency range, also known as low frequency/medium waves. Antennas are bigger in size and located near the ground. They follow curvature of the Earth. These waves are used in Submarine Communication, A.M radio
Figure 2 Ground Waves Propagation
If there are obstructs like buildings, hills, a distance travelled will be less due to attenuation
Sky Wave Propagation or Ionospheric Propagation (2-30 MHz)
Atmosphere consists of different layers. When cosmic rays from sun fall on atmospheric layers, atoms get ionised. At higher layers, the density is less; less ions are produced. Solar radiation penetrates that layer and reaches the middle layer. Middle layer is denser; more ions are produced. Radiation penetrates further; but less radiation reaches the lower layers; though denser less ions are produced. Middle layer maximum ionisation; known as Ionosphere. High frequency waves (3-32 MHz) when reaches this layer and gets reflected to the ground station hence covering large distance. Phenomena are similar to total internal reflection in optics. A signal from antenna at a station on earth strikes the ionosphere; It is reflected back down the earth. Sky wave signal bounces back and forth between the ionosphere and earth’s surface and can travel along a long distance across the globe. Used in Amateur radio, CB radio, International Broadcast, BBC, voice of America.
Figure 4 Sky Waves Propagation Distance traveled by sky waves depends on
Angle of Radiation: If the angle of radiation from the transmitting antenna towards sky is travel short distance and if angle of radiation is more, sky waves will travel longer distance
Strength of Ionosphere: If the strength of Ionosphere is more, attenuation of waves will be less, (this is how sky wave broadcast travel all over the world) they will travel longer distance and if strength is less, it will offer more attenuation to the sky waves hence they will be received at a shorter distance
Power of transmitter: More the power, more the distance from transmitter to receiver
Level of ionization: It depends on Level of radiation which decreases with altitude, time of day of year and season. As per the sun condition, the ionization increases and decreases. More the ionization, more the reflection. Therefore, the propagation also changes as per day and night. Frequencies below 10MHz propagation effectively by sky wave at night whereas frequencies above 10 MHz propagate effectively during the day.
Line-of-sight or Space wave
The communication follows straight line-of-sight at frequency higher than 40 MHz At this frequency, the signal is not reflected by ionosphere and it penetrates the ionosphere and escape. In satellite communication, this signal straight reaches the satellite overhead and back to the ground station at Earth penetrating layers of atmosphere. For ground based communications, this propagation follows line-of-sight i.e. the transmitter and receiver should be in line-of-sight. Therefore the space wave propagation is also called as line of sight propagation.
Example satellite communication, FM broadcast, two-way radio, AM aircraft communication, Aircraft navigation aids, cellular phone, radar, microwaves links, cordless phones, all Infrared and optical communications use Line-of-sight Propagation or space wave propagation.
Figure 5 Space wave Communication
Factors effecting distance travelled by space waves
Line of sight distance : D istance between transmitting antenna and receiving antenna at which they can see each other, which is also called range of communication
metallic surface offer reflects. The strength of reflected signal will be decreased if the medium or the object offers absorption of signal. Though reflection degrades the signal quality, it also helps the signal to reach the points where LOS transmitter cannot reach
Diffraction
Diffraction of radio waves is caused by obstructs or surfaces that has sharp irregularities (edge). Corners of buildings, furniture, back of trees all offer diffraction. The phenomena causes bending of radio wave as it passes the edge of the object. It is due to redistribution of energy within a wave front when it passes near the edge of an opaque object and allows radio wave to propagate around corners. Due to this effect, radio signals propagate around the surface of the earth, beyond the horizon and behind the obstructions. Diffraction results in a change of direction of part of the wave energy from the normal line-of-sight path. This change makes it possible to receive energy around the edges of an obstacle. The lower the frequency or the longer the wavelength, the greater the bending of the wave. Radio waves are more readily diffracted than light waves.
Figure 6 Reflection from sea
Figure 7 Reflection from different object
Scattering
When the dimensions of the objects in the path of radio wave are very small as compared to the wavelength of the waves takes place. Example: forage, street signs, lamp posts offer scattering. The incoming signal is scattered into several weaker outgoing signals.
Figure 9 Scattering
For optical light of wavelength of order nanometer, water molecules offer scattering. For example: when sunlight fall a water droplet in the cloudlets it gets scattered into
Figure 8 Diffraction
Figure 10 Scattering
Or
For other antennas, gain of antenna should be considered
Therefore received power depends on
i. Increase frequency of propagation ii. Distance depends between transmission and receiver iii. Depends on gain of transmitting and receiving power in case antenna is not isotropic
Free Space loss
Free space propagation the transmitting and receiving antenna are located in an empty environment. The obstacles are absent in such an environment. Radio waves propagate as light does i.e. follow line-of-sight. But even if no matter exists between sender and receiver, the signal experience free space loss. This is the main mode of loss, the strength of the signal falls with increase in distance. This is because for isotropies radiation, there is omni directional propagation i.e. it disperses with distance in the form of a sphere with transmitter as the center. If no obstacles are there, this sphere grows and the sending energy is equally distributed on the surface of the sphere area with radius as the distance from the transmitter equal to 4πd^2. Therefore, signal strength decreases with square of distance from the transmitter. Example: If the transmitter sends signal of strength at distance of 100m. Then signal strength at 400m (4 times increase distance)
Figure 11 Attenuation in free space
We have discussed that signal strength in free space decreases only as the distance. It behaves very much like light but in real life, the propagation likes in mobile phone, WLAN etc. do not follow line-of-sight. Particularly in urban area there are many objects like buildings, skyscrapers, trees etc. are there. They also affect quality of signal. Besides these atmosphere components likes rain, fog, air, smog influences the propagation. They absorb much of radiation of antenna.
Blocking or shadowing
Blocking or shadowing is extreme form of attenuation in which the radio signals are blocked by obstacles. The effect is frequency dependent greater the frequency more it behaves likes light gets obstructed even by small obstacle. Lower the frequency, penetration is more. Long waves can even penetrate through the ocean while small waves can blocked even by a tree. Water vapor and oxygen in the atmosphere attenuates signals in the range of 22GHz. Oxygen absorbs signals of 30GHz – 60GHz frequency. Rain and fog attenuates by scattering. Therefore in the areas of precipitates lower frequency bands should be used. Flat surfaces with dimensions larger than a wavelength often reflection whereas rough surface contribute to scattering.
Multipath Propagation
When the signal travels from sender to receiver through the radio channel, it is reflected, refracted, diffracted or scattered by the objects in the environment like buildings, atmosphere etc in mobile telephony and furniture in WLAN. As a result the signal does not follow line-of-sight and follow different propagation paths in the form of multipath components. Each multipath component travel with different amplitude, delay and phase shifts among each other and reach the receiver. In the figure below it can be seen that signal is taking many paths. One is direct line of sight and others are followed after reflection, diffraction and scattering.
Inter Symbol Interference
Each multipath component arrive the receiver as a sequence of pulses with different time, amplitude and phase called Inter symbol interference. This effect occurs when data is being transmitted. In the Fig. symbols s1, s2, s3, s4 are transmitted. At first path, there is no delay. In second path, there is delay of one symbol period. In there it is of two symbol periods.
Therefore, at receiver, the symbols interfere as shown in figure.
Interference of multipath components
When multipath components with different power arrive at the receiver in phase with each other, they add constructively and phenomena is known as constructive interference. Resultant power is sum of powers of individual components.
Figure 14 Inter symbol Interference
Figure 15 Constructive interference
When multipath components with different power arrive at the receiver out of phase with each other, they add destructively and phenomena is known as destructive interference. Resultant power is difference of powers of individual components known as fading.
Figure 16 Destructive Interference
Solutions for Multipath propagation
MIMO and OFDM are the solution multipath problem. OFDM signals can be made orthogonal to each other so that they do not interfere. It is used by 802.11n, Wi-Fi, LTE-advanced, Wimax and many more technologies. MIMO is a technology which uses multiple antennas to setup multiple data streams on the same channel to increase its capacity. It takes advantage of multipath propagation in providing additional capacity to the channel
Figure 17 MIMO
