Atmospheric Effects on Microwave

Fresnel Zone:

Fresnel Zone is the volume of space enclosed by an ellipsoid which has two antennas at the ends of a radio link at its foci.

AB = Direct Path

ACB = Indirect Path

ACB - AB = nλ/2

 n = 1, for 1st Fresnel clearance

It is sufficient to have 60% of the first Fresnel clearance

 

For 1st Fresnel zone clearance:

h= 17.3 √{d1x d2/ f(d1+d2)}

F= Frequency in GHz 
d1 and d2 = Distance in Km
h = Highest point in meters

Example:
a. 10 Km long LOS path
b. Frequency of operation= 915MhZ 
c. Data from path profile:
    i.  Highest point on path is 3Km from one end
    ii. Direct path clears the obstacle by 18 m
Do we have adequate Fresnel clearance?
 

Solution:

Here, d1 = 3Km, d2 = 7 Km, f = 0.915Ghz, h=26.2m
A clearance of 18m is about 70% of this.
Hence Fresnel zone is cleared

Polarization:
Polarization is the physical orientation of radiated wave in space.

Polarization is important in wireless communications systems. The physical orientation of a wireless antenna corresponds to the polarization of the radio waves received or transmitted by that antenna. Thus, a vertical antenna receives and emits vertically polarized waves, and a horizontal antenna receives or emits horizontally polarized waves. The best short-range communications is obtained when the transmitting and receiving (source and destination) antennas have the same polarization. The least efficient short-range communications usually takes place when the two antennas are at right angles (for example, one horizontal and one vertical). Over long distances, the atmosphere can cause the polarization of a radio wave to fluctuate, so the distinction between horizontal and vertical becomes less significant. 

Some wireless antennas transmit and receive EM waves whose polarization rotates 360 degrees with each complete wave cycle. This type of polarization, called elliptical or circular polarization, can be either clockwise or counterclockwise. The best communications results are obtained when the transmitting and receiving antennas have the same sense of polarization (both clockwise or both counterclockwise). The worst communications usually takes place when the two antennas radiate and receive in the opposite sense (one clockwise and the other counterclockwise).  

Polarization affects the propagation of EM fields at infrared (IR), visible, ultraviolet (UV), and even X-ray wavelengths. In ordinary visible light, there are numerous wave components at random polarization angles. When such light is passed through a special filter, the filter blocks all light except that having a certain polarization. When two polarizing filters are placed so a ray of light passes through them both, the amount of light transmitted depends on the angle of the polarizing filters with respect to each other. The most light is transmitted when the two filters are oriented so they polarize light in the same direction. The least light is transmitted when the filters are oriented at right angles to each other.







 

Types:
1. Linearly Polarized: 

    Waves have same alignment in space.Waves from most antennas are linearly polarized. 

       a. Vertical Polarization: 

          All the electrical intensity vectors are vertical.

       b. Horizontal Polarization: 

          All the electrical intensity vectors are horizontal.
       c. Circular Polarization:
          Circular polarization of an electromagnetic wave is a polarization in which the electric field of the

          passing wave does not change strength but only changes direction in a rotary manner.

       d. Elliptical Polarization: 

          Elliptical polarization is the polarization of electromagnetic radiation such that the tip of the electric

          field vector describes an ellipse in any fixed plane intersecting, and normal to the direction of

          propagation.

2. Randomly Polarized: 
    Waves do not have same alignment in space. Light from incoherent sources like sun, light bulb.


Fading:

Fading is a phenomenon in which the MW does not follow the desired path in the atmosphere. It occurs due to the repeated refraction of the wave as it passes through a medium having variable refractive index.

In wireless systems, fading may either be due to multipath propagation, referred to as multipath induced fading, or due to shadowing from obstacles affecting the wave propagation, sometimes referred to as shadow fading.

a. Flat fading: 

    The coherence bandwidth of the channel is larger than the bandwidth of the signal. Therefore, all 

    frequency components of the signal will experience the same magnitude of fading. It is independent of 

    frequency of operation.

            -Ducting
            -Rain Attenuation

b. Frequency-selective fading: 

   The coherence bandwidth of the channel is smaller than the bandwidth of the signal. Different frequency

   components of the signal therefore experience uncorrelated fading.

             -Atmospheric multipath fading
             -Ground reflection multipath fading 

*Coherence bandwidth Wc in rad/s is given approximately by the equation:

Wc = 2∏/D

*Also coherence bandwidth Bc in Hz is given approximately by the equation:

Bc = 1/D

Avoid Fading by:
i. Do link budget analysis again
               -Change in Antenna
               -Change in Distance between Links
               -Change in operating frequency
ii. Adopt Diversity
               -Diversity is the transmission of information on separate paths. Paths may be separated in
                 space-time or frequency.

Diversity:

• Time diversity: Multiple versions of the same signal are transmitted at different time instants. Alternatively, a redundant forward error correction code is added and the message is spread in time by means of bit-interleaving before it is transmitted. Thus, error bursts are avoided, which simplifies the error correction.

• Frequency diversity: The signal is transmitted using several frequency channels or spread over a wide spectrum that is affected by frequency-selective fading. Middle-late 20th century microwave radio relay lines often used several regular wideband radio channels, and one protection channel for automatic use by any faded channel. Later examples include:
  • OFDM modulation in combination with subcarrier interleaving and forward error correction
  • Spread spectrum, for example frequency hopping or DS-CDMA.

• Space diversity: The signal is transmitted over several different propagation paths. In the case of wired transmission, this can be achieved by transmitting via multiple wires. In the case of wireless transmission, it can be achieved by antenna diversity using multiple transmitter antennas (transmit diversity) and/or multiple receiving antennas (reception diversity). In the latter case, a diversity combining technique is applied before further signal processing takes place. If the antennas are far apart, for example at different cellular base station sites or WLAN access points, this is called macro-diversity or site diversity. If the antennas are at a distance in the order of one wavelength, this is called micro-diversity. A special case is phased antenna arrays, which also can be used for beam-forming, MIMO channels and space–time coding (STC).

• Polarization diversity: Multiple versions of a signal are transmitted and received via antennas with different polarization. A diversity combining technique is applied on the receiver side.

• Multiuser diversity: Multiuser diversity is obtained by opportunistic user scheduling at either the transmitter or the receiver. Opportunistic user scheduling is as follows: at any given time, the transmitter selects the best user among candidate receivers according to the qualities of each channel between the transmitter and each receiver. A receiver must feed back the channel quality information to the transmitter using limited levels of resolution, in order for the transmitter to implement Multiuser diversity.

• Cooperative diversity: Achieves antenna diversity gain by using the cooperation of distributed antennas belonging to each node.

Space Diversity:

Spatial diversity employs multiple antennas, usually with the same characteristics, that are physically separated from one another. Depending upon the expected incidence of the incoming signal, sometimes a space on the order of a wavelength is sufficient. Other times much larger distances are needed. Cellularization or sectorization, for example, is a spatial diversity scheme that can have antennas or base stations miles apart. This is especially beneficial for the mobile communication industry since it allows multiple users to share a limited communication spectrum and avoid co-channel interference.
 

Pattern diversity consists of two or more co-located antennas with different radiation patterns. This type of diversity makes use of directive antennas that are usually physically separated by some (often short) distance. Collectively they are capable of discriminating a large portion of angle space and can provide a higher gain versus a single omnidirectional radiator.
 

Polarization diversity combines pairs of antennas with orthogonal polarizations (i.e. horizontal/vertical, ± slant 45°, Left-hand/Right-hand CP etc.). Reflected signals can undergo polarization changes depending on the medium through which they are traveling. A polarisation difference of 90° will result in an attenuation factor of up to 34dB in signal strength. By pairing two complementary polarizations, this scheme can immunize a system from polarization mismatches that would otherwise cause signal fade. Additionally, such diversity has proven valuable at radio and mobile communication base stations since it is less susceptible to the near random orientations of transmitting antennas.
Transmit/Receive diversity uses two separate, collocated antennas for transmit and receive functions. Such a configuration eliminates the need for a duplexer and can protect sensitive receiver components from the high power used in transmit.
 

Adaptive arrays can be a single antenna with active elements or an array of similar antennas with ability to change their combined radiation pattern as different conditions persist. Active electronically scanned arrays (AESAs) manipulate phase shifters and attenuators at the face of each radiating site to provide a near instantaneous scan ability as well as pattern and polarization control. This is especially beneficial for radar applications since it affords a signal antenna the ability to switch among several different modes such as searching, tracking, mapping and jamming countermeasures.