StarGuide Digital Networks StarGuide III, Руководство пользователя

Страница: 56 / 58

46 StarGuide Digital Networks
7.3.1 E
b
/N
0
versus Signal to Noise (S/N)
Excerpt from: "Digital Communications, Fundamentals and Applications", Bernard Sklar,
p.158.
E
b
/N
0
is a ratio of two power readings so it can be expressed in dB this way: 10log
10
(E
b
\N
0
)
E
b
= Bit energy (only bits transmitting information are considered;
framing and error correction bits are not.)
N
0
= noise infinitesimal
For the mathematically minded:
S = avg modulating signal power
R = bit rate
T = 1/R = bit time duration
W = bandwidth
N = N
0
W = avg noise power
From the above definitions:
N
0
= N/W (For W approaching infinity)
E
b
= ST
and :
E
b
/N
0
= S/N (W/R)
because :
E
b
/N
0
= ST/N
0
= S/RN
0
= SW/RN
0
W = S/N (W/R)
E
b
/N
0
is a standard measure of digital system modulation and detection quality. When two
methods are being compared, the one with the smaller E
b
/N
0
is the more efficient . The
StarGuide’s E
b
/N
0
is 4.5dB.
E
b
\N
0
may also be used as a measure of signal quality in a particular downlink installation, in
which case the larger the E
b
/N
0
the better . Remember the threshold is 4.5dB - the receiver will
mute if the signal drops any lower. Establish a 7.5dB reading or better.
Following receiver installation recommendations: dish size, LNB noise temperature, cable type
and length, will keep system noise to a minimum, and received signal level to a maximum.
These quantities will be reflected in the system's E
b
/N
0
reading.