Interferences
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1) Frequency reuse in cellular networks results in co-channel interference from signals using the same frequency band but located in different cells. 2) Under normal conditions, co-channel signals do not interfere due to being located outside the cell boundary. However, troposcattering and transmission power issues can cause co-channel interference. 3) Measuring the carrier-to-interference ratio and carrier-to-noise ratio can help quantify co-channel interference levels. Frequency reuse increases spectrum efficiency but also co-channel interference, so reduction techniques are important.
Interferences
- 1. Interferences in cellular mobile communication AJAL.A.J Assistant Professor –Dept of ECE, Federal Institute of Science And Technology (FISAT) TM MAIL: ec2reach@gmail.com
- 2. 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.)
- 3. 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.
- 4. 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.
- 5. 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
- 6. Co channel interference - cause 1. The duct kind of propagation without attenuation may travel unexpectedly a longer distance resulting in co channel interference 2. Improper selection of transmission power may result in co channel interference
- 7. 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.
- 8. 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
- 9. 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.
- 10. Test conditions – 3 steps • The test is performed by creating test conditions in the surrounding cells
- 11. 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
- 12. 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)
- 13. 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).
- 14. calculation • Noise + signal = P1 • Noise + interference = P2 • Noise alone = p3
- 15. assumption • Assuming that noise power is small compared to signal and interference power
- 16. The carrier to interference ratio: • The carrier to interference ratio: P1 / P2 = P1 ( dBm) - P2 ( dBm)
- 17. The carrier to Noise ratio: • The carrier to noise ratio: P1 / P3 = P1 ( dBm) - P3 ( dBm)
- 18. 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.
- 19. Note: • Much small level interference can create appreciable power level with it. • Hence its reduction is important.
- 20. 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
- 21. Q value is high @ 2 situations 1. Where the distance d between the co channel is large 2. When the radius of the cell is small
- 22. 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.
- 23. The minimum distance for frequency reuse depends on many factors VIZ : 1.number of co channel cells in the vicinity 2. the type of Geographic contour 3. the antenna height 4. the transmitted power at each cell site
- 24. 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
- 25. 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.
- 26. Directional Antennas to reduce co channel interference ? • We call this method to reduce interference by using directional antennas as cell sectoring
- 27. 4 other methods of co channel reduction 1) • 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 .
- 28. 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.
- 29. 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.
- 30. 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
- 31. How to reduce interference in CDMA system ? • Diversity reception Will also reduce co channel interference significantly.
- 32. Precautions to be taken in the design 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