Interferences in cellular mobile communication AJAL.A.J Assistant Professor –Dept of ECE,Federal Institute of Science And Technology (FISAT) TM MAIL: email@example.com
How to provide better frequency management ?• Inorder to provide better frequency management , frequency reuse is adopted in cellular mobile communication• The reuse of frequency bands results in the existence of co – channels (these channels are having the same set of frequency allocated but located at different space points.)
Under normal conditions :--• Co channels will not create any problem as they are located at a distance from where they cannot send interfering signals.• This is because the radiation of signal may not travel beyond the boundary of the cell• The power of radiation is selected in such a way that for the crossing attenuation , the signal strength will go below the threshold of the receiver.• Hence co channel signal will not have any effect under normal conditions in the required signal reception.
Er effect • However, co channel interference will be there through troposcattering effect• Note - The troposcatter is due to the back lobe and side lobe radiation of the base station antenna.
Spill over• Even a small amount of co channel from each cell will try to sum up to produce a heavy interfering signal .• Whatever maybe the precautions taken in the design, under abnormal conditions certain amount of spill over may occur due to co channels
Co channel interference - cause1. The duct kind of propagation without attenuation may travel unexpectedly a longer distance resulting in co channel interference2. Improper selection of transmission power may result in co channel interference
Nature of co channel interference !• The co – channel signals arrive from the direction at which frequency reuse is adopted.• Hence co channels are many , and the final interfering signal is due to the vectorial addition of the n co channel signals.
Cross talk• Even though the frequency reuse provides an incremental coverage area, it is the main cause for the co channel interference.• This results in chaos in information reception.• The co channel interference can produce un necessary cross – talk , which may at times mask the desired information
Measurement of co channel interference• The co channel interference power can be measured from both The mobile station reception unit & The base station reception unit• Co channel interference may occur in all channels equally in a given area.
Test conditions – 3 steps• The test is performed by creating test conditions in the surrounding cells
Step -1 (P1)• Co channels of a particular mobile unit are kept in off condition and the mobile unit is allowed to receive the local base station signal.• Let the power received be P1 in dBm .• This power is due to noise and signal
Step 2 -- (P2)• Local base station is kept in off condition and the mobile unit is turned to the co channel signal only.• The power received in dBm is measured .• As the local base station is in off condition , the power measured is due to the noise and co channel interference (P2)
Step 3 -- (P3)• Both the local and the distant co channels are kept in “off” condition.• The available mobile unit receiver power is measured in dBm.• This power received is due to the noise alone which can be taken as (P3).
assumption• Assuming that noise power is small compared to signal and interference power
The carrier to interference ratio:• The carrier to interference ratio: P1 / P2 = P1 ( dBm) - P2 ( dBm)
The carrier to Noise ratio:• The carrier to noise ratio: P1 / P3 = P1 ( dBm) - P3 ( dBm)
Frequency re use• The frequency reuse scheme drastically increases the spectrum efficiency.• But this advantage occurs at the cost of co – channel interference.• The co channels are present in all directions• Eventhough the interfering stations are situated at a distance , they are many in number.
Note:• Much small level interference can create appreciable power level with it.• Hence its reduction is important.
Co- channel interference reduction factor• The co channel spacing = D• Cell radius = R Co- channel reduction ratio = q= D /R• Mwhen q value is high , the co channel interference reduction is good
Q value is high @ 2 situations1. Where the distance d between the co channel is large2. When the radius of the cell is small
Practical cell arrangement• In a praactical cell arrangement , smaller radius may result in small D , provided the numberr of cell in a set of frequency band is large.
The minimum distance for frequencyreuse depends on many factors VIZ :1.number of co channel cells in the vicinity2. the type of Geographic contour3. the antenna height4. the transmitted power at each cell site
Co channel interference in omni -directional radiation• Usually the interference will dominate at the boundary area of any cell.• At the boundary , invariably the signal received will be will be quite low in strength.• The co channel signals willdrive from a n umber of directions
Worst case condition !• The weeak signal received at the boundary may not be sufficient enough to override the vectorial addition of a number of interfering signals.• Hence this particular case is called as worst case condition in cellular mobile communication systems interference.
Directional Antennas to reduce co channel interference ?• We call this method to reduce interference by using directional antennas as cell sectoring
4 other methods of co channel reduction1)• The base station antenna height must be decided just to cover the horizon distance allotted for the cell .• In certain elevated locations like hills , mountains, .. Etc minimum height will sense the purpose.• Hence in valley region covering the height of antenna will be more effective .
2)• Co channel interference can be reduced by means of a tilted antenna pattern .• The pattern is called umbrella radiation pattern in elevation.• In this pattern energy is confined to the near about area of the antenna.• Also proper design technique can minimize the spill over towards atmosphere.
3)• Directional antennas can be used at the base station to reduce co channel interference; highly directive antennas can concentrate radiation in the desired direction and avoid in unwanted direction.
4)• The chance of interfering with the other co channel sites can be reduced by lowering the power level• Reduction inpower radiated may also reduce the near end– far end interference ratio
How to reduce interference in CDMA system ?• Diversity reception Will also reduce co channel interference significantly.
Precautions to be taken in thedesign for co channel interference !• Before evolving a system for service propagation , prediction model should be developed• Co- channel interference phenomena must be well analyzed in the propagation model.• Necessary techniques to avoid the interference should be adopted