CoChannel Interference
Text Sections: 3.53.7
Interference
Cochannel interference
· “cochannels” are nearby channels with the same frequency
· Cochannel interference causes
o Voice Channels: Loss of quality
o Control Channels: Dropped calls
· Increasing SNR does NOT solve cochannel interference (in fact, it can make it worse)
· Reduce cochannel interference by increasing distance between cochannels
o R = radius of each hexagonal cell
o D = distance between centers of cells
o Q = cochannel reuse ratio = D/R = sqrt(3N) for hexagonal cells
§ Small Q increases system capacity (N is small)
§ Small Q increases cochannel interference (less distance between cells)
Adjacent channel interference:
Channels that are adjacent in frequency are supposed to be unable to interfere with each other. In practice, electronics are imperfect, and adjacent channels may have sidebands that interfere. This is why FCC regulates the “out of band” noise that communication transmitters can have. It is also why engineers design “tight” input filters so that their systems do not pick up out of band noise.
If a nearby transmitter has just a little bit of outofband noise, it might swamp out the desired signal transmitted by a transmitter far away. This nearfar problem is reduced by controlling the power level that is transmitted by the mobiles to keep everyone on as close to the same power level as possible at the receiver base station. This means that antennas far away must transmit larger power than those nearby. This saves on battery life, as well as reducing adjacent channel interference.
Interesting note: When you are using your cell phone inside your car, it is partially blocked by the metal car structure. It must send much higher power levels in order to get the power to the base station. This is the “worstcase” scenario for power deposition in the head and for interference and lack of adequate power to the base station.
S = Signal Strength (power)
I = cochannel interference strength (power)
I_{i} = power of cochannel interference from i^{th} cell
To find total interference, sum up interference power from all cells:
_{}
Typically S/I must be 1518 dB for good reception.
Propagation measurements show:
P_{r} = Power received
d_{o} = near distance in the far field of the transmitter
d = far away distance (also in the far field of the transmitter)
_{}
n = path loss exponent, depends on environment
Table 3.2 

Environment 
n 
Free
Space 
2 
Urban
area cellular radio 
2.7 to
3.5 
Shadowed
urban area 
3 to 5 
In
building line of sight 
1.6 to
1.8 
obstructed
in building 
4 to 6 
obstructed
in factories 
2 to 3 
Converting this to the form above:
S =
d_{o } = distance to where S is measured = R
I_{i} = P_{r }=Power of interference from the i^{th} cell (received power from cochannel cells is not desired, and is therefore interference)
d = distance to the i^{th} cell D_{i}
Substituting into the S/I equation:
_{}
The cells that are the farthest away have much less interference. For nearestneighbors only (i_{o} = 1)
_{}
Examples for Problem 2.3
TDMA can tolerate S/I = 15 dB
What is the optimal value of N for omnidirectional antennas? Path loss = 4. We will not discuss trunking efficiency
Cochannel
Interference 













Equation

Variable







cluster
size 

N 
7 
(choices
4,7,12) 


path loss
exponent (meas) 

n 
4 


3.4 
cochannel
reuse ratio 
Q 
sqrt(3N) 
4.582576 



distance
between cochannels 

D 

meter 


radius of
cells 

R 

meter 

3.4 
Ratio of
distance to radius 
Q 
D/R 
4.582576 



number of
neighboring cells 
io 

6 
# of
sides of hexagon 

3.9 
signal to
interference ratio 
S/I 
(D/R)^n /
io 
73.5 



convert
to dB 
S/I 
10log(S/I) 
18.66287 
dB 









If S/I is
greater than required, it will work:
YES! 



Equation

Variable






cluster
size 

N 
4 
(choices
4,7,12) 

path loss
exponent (meas) 

n 
4 

3.4 
cochannel
reuse ratio 
Q 
sqrt(3N) 
3.464102 


distance
between cochannels 

D 

meter 

radius of
cells 

R 

meter 
3.4 
Ratio of
distance to radius 
Q 
D/R 
3.464102 


number of
neighboring cells 
io 

6 
# of
sides of hexagon 
3.9 
signal to
interference ratio 
S/I 
(D/R)^n /
io 
24 


convert
to dB 
S/I 
10log(S/I) 
13.80211 
dB 







If S/I is
greater than required, it will work:
NO! 


Equation

Variable






cluster
size 

N 
7 
(choices
4,7,12) 

path loss
exponent (meas) 

n 
3 

3.4 
cochannel
reuse ratio 
Q 
sqrt(3N) 
4.582576 


distance
between cochannels 

D 

meter 

radius of
cells 

R 

meter 
3.4 
Ratio of
distance to radius 
Q 
D/R 
4.582576 


number of
neighboring cells 
io 

6 
# of
sides of hexagon 
3.9 
signal to
interference ratio 
S/I 
(D/R)^n /
io 
16.03901 


convert
to dB 
S/I 
10log(S/I) 
12.05178 
dB 







If S/I is
greater than required, it will work:
NO! 


Effect of sectoring:
FIGURE 3.10,11
120° sectoring:
· Using a directional antenna with a 120 degree beamwidth.
_{· }Forward interference (primary interferers) are now 2 instead of 6 .. see figure 3.11_{}
60° sectoring:
_{· }Using a directional antenna with a 60 degree beamwidth. _{}
_{· }Forward interference (primary interferers) are now 1 instead of 6 _{}
Equation

Variable







cluster
size 

N 
7 
(choices
4,7,12) 


path loss
exponent (meas) 

n 
3 


3.4 
cochannel
reuse ratio 
Q 
sqrt(3N) 
4.582576 



distance
between cochannels 

D 

meter 


radius of
cells 

R 

meter 

3.4 
Ratio of
distance to radius 
Q 
D/R 
4.582576 



number of
neighboring cells 
io 

1 
# of
sides of hexagon 

3.9 
signal to
interference ratio 
S/I 
(D/R)^n /
io 
96.23409 



convert
to dB 
S/I 
10log(S/I) 
19.83329 
dB 









If S/I is
greater than required, it will work:
YES! 



_{ }