Interference basics | Interference types
This page describes Interference basics and Interference types. The common types of interference include adjacent channel Interference, co-channel Interference, Electromagnetic Interference, ICI (Inter Carrier Interference), ISI (Inter Symbol Interference), light Interference, Sound Interference etc.
Interference term can be applied to many domains including communication, entertainment, science, sports, politics, personal life and so on. We will discuss types of interference applied to communication domain.
What is Interference?
There are many meanings or definitions of interference.
• In communication, interference results into poor pictures or sounds being received on TV, radio or mobile phones. These communication systems use EM (Electromagnetic) waves as transport medium.
• When someone tries to interfere in daily life or in politics, it is referred as an act of interference. The same is applied to sports and other domains.
• In science, the process in which light, sound or electromagnetic waves interfere with desired communication path is also known as interference.
• Interference term can also be defined in other contexts based on situations or scenarios.
➤Based on the spacing of interfering signal from the desired signal there are two types viz. co-channel and adjacent channel.
These interferences are observed in single carrier transmission systems such as satellite, GSM, microwave etc.
➤Based on type of signal which is interfering there are various types such as Electromagnetic, Sound, light etc.
➤Multi-carrier systems use OFDM and OFDMA techniques. ICI and ISI type of interferences are observed in these baseband techniques.
Co-Channel Interference (CCI)
The interference caused by transmitting at the same frequency by two or more wireless systems is known as co-channel interference. In order to handle huge number of calls with limited number of channels frequency reuse concept is applied to the cellular system. In frequency reuse same frequency is reused in multiple cells within their own boundaries without causing any interference. These cells are known as co-channel cells.
To reduce co-channel interference, co-channel cell must be separated by minimum distance.
When the size of the cell is approx. same, following can be applied to it.
• Co-channel interference is independent of the transmit power.
• Co-channel interference is function of radius (R) of cell and distance (D) to the center of the nearest co-channel cell.
• By increasing ratio Q (= D/R), interference is reduced.
• Q is known as co-channel reuse ratio.
• For hexagonal geometry of cell, Q = D/R = (3*N)0.5
• Larger value of Q improves transmission quality as it will have smaller level of co-channel interference.
Adjacent channel Interference (ACI)
The interference caused to the desired signal (or channel) from adjacent frequency signals is known as adjacent channel interference. This interference is caused by leakage of frequencies from imperfect filters into passband of desired channel. Moreover it is result of near-far effect.
Adjacent channel interference can be reduced by careful filtering and channel assignments by RF planners. In order to achieve this, frequency separation between channels is kept large. Ensure that each mobile transmits smallest power necessary to maintain good quality link.
Electromagnetic Interference (EMI)
Interference caused by EM signal at one frequency with the EM signal at the same frequency or at the other frequency is known as EM interference. Electromagnetic wave consists of electric field and magnetic field which are perpendicular to each other. The EM waves travel in vacuum at the speed of 3 x 108 m/sec.
EM interference can be between systems operating at the same frequencies or at different frequencies.
These interference types are known as co-channel and adjacent/alternate channel interference.
Refer EM radiation >> and EMI vs EMC >> for more information.
As we know sound waves are produced and detected by ultrasonic transducers. The speed of sound through the air is 344 m/sec at 20oC. Two sound waves can cause constructive interference or destructive interference based on whether they are in phase or out of phase to each other respectively.
Example: Sound produced by speakers can interfere in normal communication of people. Similarly other sound producing devices can interfere.
When Light signals at different wavelengths or at the same interfere with each other, it causes light interference. Light signal can also cause interference to other communication system operating using other transmission media. The speed of light in vacuum is about 186282 miles/second (i.e. 299792 km/sec).
Normal functioning of light communication can be interfered by oil floating on the water or by thin film of soap bubbles. Light waves can cause interference to other waves. This light interference is caused due to addition and substraction of different waves when they overlap either in phase or out of phase.
Inter Carrier Interference (ICI)
As we know, in OFDM carriers are densely packed, where in peak of one sub-carrier is at null of other sub-carriers. This is referred as orthogonality. In other words, for OFDM to be efficient modulation, subcarriers should be orthogonal to each other. ICI (Inter Carrier Interference) is caused when sub-carriers lose orthogonality.
ICI is result of following two reasons.
• Delay spread of radio channel exceeds the CP interval(i.e. guard interval)
• Frequency offset at the receiver.
ICI is reduced or can be mitigated by estimation of frequency offset and correction of sub-carrier spacing accordingly.
Inter Symbol Interference (ISI)
In OFDM based systems, the transmission takes place symbol by symbol. Before the symbol transmission, symbols are packed with complex modulated data symbols. After the symbol is formed, CP (Cyclic Prefix) is appended to each of the OFDM symbols individually. As the symbols travel one by one to the other end, the path from transmit to receive end will introduce delay spread in time domain. This results into OFDM symbol getting spread out and hence will interfere with consecutive OFDM symbols. This is referred as ISI (Inter Symbol Interference).
ISI interference can be mitigated or reduced using CP (Cyclic prefix) concepts explained above. Here CP length is choosen as more than channel delay spread.
Refer difference between ICI vs ISI >> for more information.
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