Reference Manual

05-4446A01, Rev. E Mercury Reference Manual 75
Modulation
Protection and
Hysteresis Margins
Table 3-3 on Page 75 shows the relationship between the radio’s Protec-
tion Margin, Hysteresis Margin, and the SNR range allowed for each
form of modulation.
Column A lists the available modulation types for the radio, while col-
umns B and C show the minimum SNR range required to operate in each
modulation. For example, an SNR of 5.8 dB in Column B is required for
QPSK modulation with an FEC rate of 1/2. An SNR of 8.4 dB is
required for QPSK modulation with an FEC rate of 3/4.
Columns B and C have a Hysteresis Margin of 0 dB. This means there
is no overlap between the maximum SNR for BPSK (5.8 dB) and the
minimum SNR for QPSK-1/2 (5.8 dB).
Columns D and E show the SNR ranges with a Protection Margin and
Hysteresis Margin of 3 dB. The Protection Margin is added to each
value in Columns B and C to get the corresponding value in Columns D
and E. The Hysteresis Margin is then added to the Max SNR value.
For example, the third SNR value in Column D is 11.4 dB (8.4 + 3 =
11.4 dB), and the third SNR value in Column E is 17.1 (11.1 + 3 + 3 =
17.1 dB). Note that with a Hysteresis Margin of 3 dB, there is an overlap
of 3 between the Max SNR of one modulation and the Min SNR of the
next higher modulation.
In this case, if a link is operating with an SNR of 15 dB, then QPSK-3/4
modulation is used. The SNR must go above 17.1 dB before the link
shifts up to 16QAM-1/2 modulation. Conversely, the SNR will need to
drop below 11.4 dB before the link shifts down to QPSK-1/2.
The blank entries (--) in the table indicate infinite SNR (i.e., the top of
the range). For example, in columns B and C, 64QAM-3/4 modulation
is used for all SNR values from 20 dB and up.
Table 3-3. Adaptive Modulation Protection and Hysteresis Margins
A B C D E F G
Protection 0 Protection 3 Protection 5
Hysteresis 0 Hysteresis 3 Hysteresis 3
Min SNR Max SNR Min SNR Max SNR Min SNR Max SNR
BPSK 3.3 5.8 3.3 11.8 3.3 13.8
QPSK-1/2 5.8 8.4 8.8 14.4 10.8 16.4
QPSK-3/4 8.4 11.1 11.4 17.1 13.4 19.1
16QAM-1/2 11.1 14.4 14.1 20.4 16.1 22.4
16QAM-3/4 14.4 18.4 17.4 24.4 19.4 26.4
64QAM-2/3 18.4 20 21.4 26 23.4 28
64QAM-3/4 20 -- 23 -- 25 --