cochannel interference reduction in gsm

8/7/2019 COCHANNEL INTERFERENCE REDUCTION IN GSM

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SUBASH CHANDRA SAHOOROLL NO-08ET124REGD.NO-0821223022



` Solves the problem of spectral congestion and user capacity.` Offer very high capacity in a limited spectrum without major

technological changes.` Reuse of radio channel in different cells.

` Enable a fix number of channels to serve an arbitrarily largenumber of users by reusing the channel throughout the coverageregion.



` Each cellular base station is allocated a group of radio channelswithin a small geographic area called a cell.

` Neighbouring cells are assigned different channel groups.` By limiting the coverage area to within the boundary of the cell, the

channel groups may be reused to cover different cells.` Keep interference levels within

tolerable limits.` Frequency reuse or frequency

planning seven groups of

channel from A to G



Co-channel Interference and System

Capacity To reduce co-channel interference, co-channel cell must beseparated by a minimum distance.

When the size of the cell is approximately the sameco-channel interference is independent of the transmitted power

co-channel interference is a function of R: Radius of the cell

D: distance to the centr e of the near est c o-c hannel cell



For a hexagonal geometry

Q = D/R = ¥3N A small value of Q provides large capacity A large value of Q improves the transmission quality -smaller level of co-channel interference A trade-off must be made between these two objectives



` Methods for improving capacity in cellular systems

± Cell Splitting: subdividing a congested cell intosmaller cells.

± Sectoring: directional antennas to control theinterference and frequency reuse.



` Increasing the separation between twocochannel cells

` Using directional antennas at the base station

` lowering the antenna heights at the base station.` The use of directional antennas in each cell can serve

two purposes:- further reduction of cochannel interference

- increasing the channel capacity when thetraffic increases.



` Decrease the co-channelinterference and keepthe cell radius R unchanged

± Replacing single omni-directionalantenna by several directionalantennas

± Radiating within a specifiedsector



` The interfering site could cause interference atthose cells within a 19 sector in front of the cell.

` In an ideal situation the antenna pattern of theserving cell must be rotated clockwise by 10 suchthat the notch can be aimed properly at the

interfering cell



` Purely mechanical tilt` Fixed electrical tilt

` variable electrical tilt (VET)` Remote electrical tilt (RET)` Continuously adjustable electrical downtilt

(CAEDT).



Elements playing a major role in justifying antennatilting-

` The antenna vertical pattern` The antenna height



` For controlling the energy in a confined area, theumbrella-pattern antenna can be developed byusing a monopole with a top disk

` The size of the disk determines the tilting angle of the pattern. The smaller the disk, the larger thetilting angle of the umbrella pattern.



` Energy is confined to the immediate area of theantenna

` Reduces cochannel and long-distance

interference.` We can increase antenna height still reduce

cochannel interference` The frequency-reuse distance can be shortened



� Antennas are placed at the outer edges of the cell� Any channel may be assigned to any zone by the base

station� Mobile is served by the zone with the strongest signal.� Handoff within a cell

± No channel reassignment ± Switch the channel to a different zone site

` Reduce interference

± Low power transmitters are employed



` Ensure each mobile transmits the smallest power necessary to maintain a good quality link on the

reverse channel` Can be controlled only by mobile station

± long battery life ± increase SIR



` Creating a desired pattern in a certain direction.` currents appearing in several parasitic antennas

are caused by radiation from a nearby drive

antenna.` Ways to combine a driven antenna & a single

parasite :- Normal spacing

- Close spacing



a)One-quarter wavelengthspacing

b) one-half wavelength spacingc ) combination of a and b.



1. Length of both elements same- Forms a null at the centre- Directive gain of 3 dB relative to a single element

2. Length of parasite greater than active element- Parasite acts as reflector - Directive gain of 6 dB

3. Length of parasite less than active element

- Parasite acts as director - Directive gain of 8 dB



Dominant interfererence likely exists in GSM

Number of nearby co channel interference is rather smallCancellation of it can improve the receiver performanceIt depends on the dominant to rest of interference ratioDIR.



` Receiver algorithm identifies dominant interferingsignal

` It removes it by joint phase demodulation

` System performance improves by 1.3 dB

cochannel interference reduction in gsm

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