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maintain the Eb/No within the parameters you have set. Logging the AUPC delta power allow
you to, if questioned, be able to specify to the satellite operator the exact power you were
transmitting throughout the day (or at a specific time in question).
8.12
TUTORIAL ON CARRIER/NOISE & Eb/No MEASUREMENTS
8.12.1 Introduction
This is the
source of much confusion and `Specmanship`. This brief tutorial explains what the terms
mean and how to measure and interpret the figures you see. The diagram on the following page
attempts to summarise this as clearly as possible visually with the text below adding detail for which
there is no room on the diagram. Finally a table provides a quick look up reference to convert values
measured on a spectrum analyser to a real Eb/No.
8.12.2 Derivation of Eb/No from (C+N)/N
It is possible to describe the Carrier / Noise ratio within the Rx system at an arbitrary reference point,
eg at the input to the demodulator, at the input to the Forward Error Correction decoder (FEC), at the
input outer Reed-Solomon FEC decoder, or even at the terrestrial data interface. Many of the terms
you come across represent this Rx Carrier / Noise ratio at these different reference points.
It is common to measure
Ideally we are after the Carrier / Noise ratio, but as we cannot turn off the atmospheric noise (!) we
have to make the measurement with the noise on, and then convert back to simply Carrier / Noise. In
the equation below the term (C+N)/N is the (Carrier+Noise) / Noise ratio read from the spectrum
analyser.
Carrier
Noise
We can express this same Carrier / Noise ratio at the input of the demod in terms of the Energy per
Symbol / Noise power density, which is written as E
powers measured in the same bandwidth (the resolution bandwidth of the analyser), this is the same
as E
/ N
.
s
o
Within the demodulator, each Symbol is converted back into the Transmitted Bits. For BPSK each
Symbol represents only 1 Transmitted Bit, for QPSK or OQPSK each Symbol represents 2
Transmitted Bits, and for 8PSK each Symbol represents 3 Transmitted Bits. The expression E
(or E
/ N
) represents the Carrier to Noise ratio referenced to this Transmitted Bit rate. For QPSK
bt
o
and 8PSK the Tran
smitted Bit rate is higher than the Symbol rate, and so E
Noise referenced to Transmitted Bit rate) is lower than the E
the Symbol rate), as this same power is referenced to a higher bit rate. Allowing for the change in bit
E
N
rate in the demod gives:
Next in the demod chain comes the FEC Decoder. The receive Carrier / Noise referenced to this point
is referred to as E
/ N
Dec
provide corrected data at a lower rate. Because the data rate reduces through the FEC Decoder E
/ N
is higher than E
/ N
o
t
o
in
bit rate in the FEC decoder gives:
P300H
P300 Series Modem Installation and Operating Handbook
(Carrier + Noise) / Noise at the demodulator input on a spectrum analyser.
C
=
=
10
(
log
10
10
N
E
t
s
=
-
10
log
(No
of
10
N
0
0
. In the FEC Decoder the bit rate is reduced, as the data is decoded to
o
as the same power is referenced to a lower bit
E
E
Dec
t
=
-
10
log
(FEC
10
N
N
0
0
E
(C
+
N)
/N
s
(
)
-
1)
=
10
N
0
. Because `Carrier / Noise` is a ratio of two
/ N
s
o
/ N
(the Carrier / Noise referenced to
s
o
Bits
/
Symbol)
rate. Allowing for the change
Code
Rate)
/ N
t
o
/ N
(ie the Carrier /
t
o
Dec
Page 161
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