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1] REFLECTION ->Reflection occurs when the signal encounters a large solid surface, whose size is much larger than the wavelength of the signal, e.g., a solid wall.
2] DIFFRACTION-> Diffraction occurs when the signal encounters an edge or a corner, whose size is larger than the wavelength of the signal, e.g., an edge of a wall.
3] REFRACTION->Refraction occurs when an RF signal passes between objects or material of different densities.
4] Scattering -> occurs when the medium through which the wave travels consists of objects with dimensions that are small compared to the wavelength.
5] Large scale signal propagation ->Large scale effects typically occur in the order of hundreds to thousands of meters in distance. Small scale effects are localized and occur temporally (in the order of a few seconds) or spatially (in the order of a few meters).
6] Lognormal shadowing -> The log-normal distribution describes the random shadowing effects which occur over a large number of measurement locations which have the same T-R separation, but have different levels of clutter on the propagation path. This phenomenon is referred to as log -normal shadowing.
7] Doppler shift 🌟 ->The relative motion between the transmitter and the receiver causes Doppler shifts. Local scattering typically comes from many angles around the mobile. This scenario causes a range of Doppler shifts, known as the Doppler spectrum. The maximum Doppler shift corresponds to the local scattering components whose direction exactly opposes the mobile trajectory.
Due to Doppler spread, fading effects can also be classified as fast fading and slow fading.
There is always a relative motion between the cell-site transmitter and the mobile receiver. As a result Doppler effect occurs in the shift of the received carrier frequency. Doppler spectrum is the spectrum of the fluctuations of the received signal strength.
Doppler effect results in the inaccurate operation of the system. Proper compensation technique needs to be implemented to minimise this effect.
8] Narrowband and wideband fading models -> Narrowband refers to radio communications whose signal bandwidth is within the coherence band of a frequency channel. This means the bandwidth of the signal does not significantly exceed the coherent bandwidth of the frequency channel.
Wideband refers to broadband communications that uses a relatively wide range of frequencies. It refers to radio channels whose operational bandwidth may significantly exceed the coherence bandwidth of the channel.
9] Power delay profile -> The power delay profile of a channel represents the average power of the received signal in terms of the delay with respect to the first arrival path in multi-path transmission.
10] Rms delay spread -> RMS Delay Spread is a measure of the channel's average delay spread in seconds
11] Coherence bandwidth ->In communications systems,a communication channel may change with time. Coherence time is the time duration over which the channel impulse response is considered to be not varying.
12] Coherence time -> The coherence time is the characteristic time over which the channel gain does not change appreciably.
13] flat and frequency selective fading -> Flat fading is countered by error correction coding and diversity reception. Frequency selective fading occurs when the symbol length is shorter than the delay spread, or equivalently when signal bandwidth is larger than the channel bandwidth.
14] Fast Fading ->It varies quickly with the frequency. Fast fading originates due to effects of constructive and destructive interference patterns which is caused due to multipath.
Doppler spread leads to frequency dispersion and time selective fading.
Fast Fading results due to following:
High Doppler Spread
Coherence Time < Symbol Period
15] Slow Fading ->It does not vary quickly with the frequency. It originates due to effect of mobility. ..
Slow Fading results due to following:
Low Doppler Spread
Coherence Time >> Symbol Period
16] Average Fade Duration (AFD) -> It is the measurement of average time over which the signal below a certain level
17] Level crossing rate ->The LCR is a measure for the average number of times a radio signal drops below a given threshold level,
18 . Explain Channel Scattering Function.
The autocorrelation function of the WSSUS channel model can be represented in the frequency domain by taking Fourier to transform with respect to one or both variables - the difference in time and the propagation delay. Of the two forms, the Fourier transform on the variable gives specific insight to channel properties in terms of propagation delay and the Doppler Frequency simultaneously.
The Fourier transform of the above two-dimensional autocorrelation function on the variable is called the scattering function. Multipath fading and user mobility lead to a time and frequency-dependent channel. The Transfer function of a particular sample channel does not necessarily provide enough details about the stochastic behavior of the radio channel. Such stochastic properties are captured in the scatter function. The scatter function combines information about
1. Doppler spread (which relate to angles of arrival)
2. path delays.
The scatter function provides a statistical model for the channel.
19. What do you mean by Doppler spread?
This is a measure of spectral broadening caused by the rate of change of the mobile radio channel. It is caused by either relative motion between the mobile and base station or by the movement of objects in the channel.
When the velocity of the mobile is high, the Doppler spread is high, and the resulting channel variations are faster than that of the baseband signal, this is referred to as fast fading. When channel variations are slower than the baseband signal variations, then the resulting fading is referred to as slow fading.
20. What are the types of signal propagation?
Space wave propagation:
The radio waves from the transmitting antenna propagate through space, around the ground and reach the receiver either directly or by reflection from the ground, this kind of wave propagation I called space wave propagation. It is also called the line of sight propagation.
The radio waves which travel directly from the transmitting antenna to the receiving antenna are also called space waves. Waves in the MHz frequency range are used for space wave propagation. We know that our earth is spherical therefore to make the long-distance we need the height of the antenna as long as possible.
Skywave propagation:
The radio waves from the transmitting antenna propagate through the sky towards the receiver, either directly or by reflection from the ground, this kind of wave propagation is called skywave propagation. It is also called ionospheric wave propagation.
The refractive index of the ionosphere is less than its free space value and it decreases with increases in height. Therefore radio waves go under total internal reflection. Radio waves of frequency less than 3 MHz are observed in the ionosphere and of frequency greater than 30 MHz can pass through the ionosphere after suffering a small deviation.
Ground wave propagation:
When the radio waves from the transmitting antenna propagate along the surface of the earth, to reach the receiving antenna, the wave propagation is called ground waves or surface wave propagation. Ground waves are radio waves that travel or propagate along the surface of the earth. These radio waves induce a current in the ground from which they pass. While propagating through the medium waves will have losses as well as absorption. Therefore there will be a loss of signal or power. The more the frequency of waves the more will be losses. That is why it is favourable only for low-frequency waves only.
Generally, this kind of technique is used for broadcasting purposes.
21. Explain different types of fading and the sub- categories.🌟
Large Scale Fading: This refers to the attenuation of signal power due to obstacles between the transmitter and receiver. It also covers the attenuation and fluctuations of signal when the signal is transmitted over a long distance (usually in kilometres).
Small Scale Fading: This refers to the fluctuations in signal strength and phase over short distance and small duration of time. It is also called Rayleigh Fading. Small Scale Fading affects almost all forms of wireless communication and overcoming them is a necessity to increase efficiency and decrease error.
Multipath Fading: It occurs when a signal reaches the receiver from various path i.e. when multipath propagation takes place. Multipath fading can affect all ranges of frequencies starting from low frequency to microwave and beyond. It affects both the amplitude and the phase of the signal causing phase distortions and ISI. Multipath fading can affect signal transmission in two ways:
Path Loss: It refers to the attenuation when a signal is transmitted over large distances. Wireless signals spread as they propagate through the medium and as the distance increases, the energy per unit area starts decreasing. This is a fundamental loss that is independent of the type of transmitter and medium. Although, we can minimize its effects by increasing the capture area/dimension of the receiver.
Shadowing: This refers to the loss in signal power due to the obstructions in the path of propagation. There are a few ways in which shadowing effects can minimize signal loss. One that is most effective, is to have a Line-Of-Sight propagation.
Reflection: This effect occurs if the object is large compared to the wavelength of the transmitted signal, e.g. huge buildings, mountains or surface of the earth etc. After striking with such object, the signal gets reflected in different directions. In this effect, some power of the signal gets absorbed by the object therefore the reflected signal is not as strong as original. The more often the signal is reflected, the weaker it becomes.
Refraction: This effect occurs because the velocity of EM waves depends on the density of the medium through which it travels. Those waves which travel into a denser medium are bent towards the medium. This is the reason for line of sight radio waves being bent towards the earth because the density of the atmosphere is higher closer to the ground. Scattering: While Shadowing and Reflection are caused by the objects much larger than the wavelength of the signal, the scattering occurs due to obstacles i.e. in order of the wavelength or less. We can say that if the obstacle size is much less than the wavelength of the transmitted signal then waves can be scattered. An incoming signal is scattered into several weaker outgoing signals.
Diffraction: This effect shows the wave character of radio signals. This effect means that the radio waves will be deflected at an edge and propagated in different directions. The results of diffraction are patterns with varying signal strengths depending on the location of the receiver.
22. What are various issues in signal propagation in mobile computing?🌟
In free space, signals propagate quite similar to how light does i.e. they follow a straight line-of-sight path. But in real life, we rarely have a line-of-sight between the sender and receiver of radio signals. Before reaching upto receiver, signals have to face a lot of disturbances like, mountains, valleys, big buildings etc. Here several effects occur in addition to the attenuation caused by the distance between sender and receiver. These effects are
1. Shadowing
2. Reflection
3. Refraction
4. Scattering
5. Diffraction
23. Explain Doppler Power Spectrum, Doppler Spread, and Coherence Time.
1. Doppler power spectrum is defined with respect to Ac(Af; At) = F7[Ac(7, ^t)]
2. Specifically, the Doppler power spectrum is Sc(p) FAt[Ac(^Æ’ = 0, ^t)^ = Ac( which measures channel intensity as a function of Doppler frequency.
3. The maximum value of p for which Sc(p) > 0 is called the channel Doppler spread, which is denoted by Bd.
4. By the Fourier transform relationship, Ac(At) 0 for At> 1/Bd. Thus, the channel becomes uncorrelated over a time of 1/Bd seconds.
5. We define the channel coherence time as Tc = 1/Bd. A deep fade lasts approximately Tc seconds. Hence, if the coherence time greatly exceeds a bit time, the signal experiences error bursts lasting Tc seconds.
Wideband models characterized by scattering function, which measures average channel gain relative to delay and Doppler.
Scattering function used to obtain key channel characteristics of rms delay spread and Doppler spread, which are important for system design.
Multipath delay spread defines the maximum delay of significant multipath components. Its inverse is the channel coherence bandwidth. Signals separated in
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