Red
MAX
™
Base Station
user manual
Doc. #70-00058-01-01-DRAFT
Proprietary Redline Communications © 2006
November
29,
2006
Page 80 of 106
Chapter
6
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6.1 Co-Channel
Operation
As the complexity of the modulation technique increases, a greater carrier-to-noise (C/N)
ratio is required to maintain adequate throughput. In the presence of an interferer, the
useful signal is degraded; the ratio of the useful signal to interfering signal is called
carrier-to-interference ratio. For a general discussion we assume that interferer signal
power is additional noise power, and we can estimate the overall C/I+N ratio as:
)
10
10
log(
10
/
/
10
/
10
/
N
C
I
C
N
C
I
C
CINR
+
−
+
=
, where all variables are in decibels.
This formula corresponds with our expectation that if the signal is very strong (high C/N)
it is able to withstand higher levels of interference (low C/N). If the signal is weak, even
low interference levels will significantly degrade the useful signal. Consider the
following examples:
Case 1: The threshold CINR for 64 QAM 3/4 is around 23.25 dB. If the C/N ratio is 24
dB, the system will operate at 64 QAM 3/4 (signal is close to the 64 QAM 3/4 threshold,
but still above it). If we introduce an interferer with a signal strength 30 dB weaker than
the signal (i.e., C/I = 30 dB), the overall CINR will drop to around 23 dB, forcing the
system to change modulation rate to 64 QAM 2/3.
Case 2: If the signal is very strong and the C/N ratio is around 28 dB, it will require a
much stronger interferer to force a drop in modulation. According to the formula above,
for a C/N of 28 dB, the C/I should be 25 dB in order to force a drop in modulation. In this
case the interferer was 5 dB stronger than the first case.
The following table lists experimentally obtained minimal C/I ratios required for the
system to not change modulation:
Table 32
:
Op. Notes - Co-channel C/I dB Measured Results
Modulation/Coding
CINR threshold dB
C/I: Case 1 dB
C/I: Case 2 dB
64 QAM 3/4
23.25
34
24.4
64 QAM 2/3
21.75
32
22.7
16 QAM 3/4
18
28
18.2
16 QAM 1/2
15
26
16.4
QPSK 3/4
11.6
21
12.1
QPSK 1/2
6.75
19
9.4
In case 1 the signal is already very close the threshold rate (C/N is less than 1 dB above
the CINR threshold) and even weak interference can force the system to change to a