Wireless Communication Lecture 3
Co-Channel Interference
Text Sections: 3.5-3.7
Interference
Co-channel interference
· “co-channels” are nearby channels with the same frequency
· Co-channel interference causes
o Voice Channels: Loss of quality
o Control Channels: Dropped calls
· Increasing SNR does NOT solve co-channel interference (in fact, it can make it worse)
· Reduce co-channel interference by increasing distance between co-channels
o R = radius of each hexagonal cell
o D = distance between centers of cells
o Q = co-channel reuse ratio = D/R = sqrt(3N) for hexagonal cells
§ Small Q increases system capacity (N is small)
§ Small Q increases co-channel 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.
Near-Far Effect
If a nearby transmitter has just a little bit of out-of-band noise, it might swamp out the desired signal transmitted by a transmitter far away. This near-far 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 “worst-case” scenario for power deposition in the head and for interference and lack of adequate power to the base station.
Signal-to-interference ratio
S = Signal Strength (power)
I = co-channel interference strength (power)
Ii = power of co-channel interference from ith cell
To find total interference, sum up interference power from all cells:
Typically S/I must be 15-18 dB for good reception.
Propagation measurements show:
Pr = Power received
do = 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
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Table 3.2 |
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Environment |
n |
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Free
Space |
2 |
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Urban
area cellular radio |
2.7 to
3.5 |
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Shadowed
urban area |
3 to 5 |
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In
building line of sight |
1.6 to
1.8 |
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obstructed
in building |
4 to 6 |
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obstructed
in factories |
2 to 3 |
Converting this to the form above:
S =
do = distance to where S is measured = R
Ii = Pr =Power of interference from the ith cell (received power from co-channel cells is not desired, and is therefore interference)
d = distance to the ith cell Di
Substituting into the S/I equation:
The cells that are the farthest away have much less interference. For nearest-neighbors only (io = 1)
Examples for Problem 2.3
TDMA can tolerate S/I = 15 dB
What is the optimal value of N for omni-directional antennas? Path loss = 4. We will not discuss trunking efficiency
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Co-channel
Interference |
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Equation
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Variable
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cluster
size |
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N |
7 |
(choices
4,7,12) |
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path loss
exponent (meas) |
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n |
4 |
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3.4 |
co-channel
reuse ratio |
Q |
sqrt(3N) |
4.582576 |
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distance
between co-channels |
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D |
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meter |
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radius of
cells |
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R |
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meter |
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3.4 |
Ratio of
distance to radius |
Q |
D/R |
4.582576 |
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number of
neighboring cells |
io |
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6 |
# of
sides of hexagon |
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3.9 |
signal to
interference ratio |
S/I |
(D/R)^n /
io |
73.5 |
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convert
to dB |
S/I |
10log(S/I) |
18.66287 |
dB |
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If S/I is
greater than required, it will work:
YES! |
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Equation
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Variable
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cluster
size |
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N |
4 |
(choices
4,7,12) |
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path loss
exponent (meas) |
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n |
4 |
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3.4 |
co-channel
reuse ratio |
Q |
sqrt(3N) |
3.464102 |
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distance
between co-channels |
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D |
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meter |
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radius of
cells |
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R |
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meter |
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3.4 |
Ratio of
distance to radius |
Q |
D/R |
3.464102 |
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number of
neighboring cells |
io |
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6 |
# of
sides of hexagon |
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3.9 |
signal to
interference ratio |
S/I |
(D/R)^n /
io |
24 |
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convert
to dB |
S/I |
10log(S/I) |
13.80211 |
dB |
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If S/I is
greater than required, it will work:
NO! |
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Equation
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Variable
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cluster
size |
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N |
7 |
(choices
4,7,12) |
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path loss
exponent (meas) |
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n |
3 |
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3.4 |
co-channel
reuse ratio |
Q |
sqrt(3N) |
4.582576 |
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distance
between co-channels |
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D |
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meter |
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radius of
cells |
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R |
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meter |
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3.4 |
Ratio of
distance to radius |
Q |
D/R |
4.582576 |
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number of
neighboring cells |
io |
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6 |
# of
sides of hexagon |
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3.9 |
signal to
interference ratio |
S/I |
(D/R)^n /
io |
16.03901 |
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convert
to dB |
S/I |
10log(S/I) |
12.05178 |
dB |
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If S/I is
greater than required, it will work:
NO! |
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Effect of sectoring:
FIGURE 3.10,11
120° sectoring:
· Using a directional antenna with a 120 degree beamwidth.
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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
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Equation
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Variable
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cluster
size |
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N |
7 |
(choices
4,7,12) |
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path loss
exponent (meas) |
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n |
3 |
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3.4 |
co-channel
reuse ratio |
Q |
sqrt(3N) |
4.582576 |
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distance
between co-channels |
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D |
|
meter |
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radius of
cells |
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R |
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meter |
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3.4 |
Ratio of
distance to radius |
Q |
D/R |
4.582576 |
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number of
neighboring cells |
io |
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1 |
# of
sides of hexagon |
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3.9 |
signal to
interference ratio |
S/I |
(D/R)^n /
io |
96.23409 |
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convert
to dB |
S/I |
10log(S/I) |
19.83329 |
dB |
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If S/I is
greater than required, it will work:
YES! |
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