SMIQ
Digital Modulation
1125.5555.03
E-9
2.91
Differential coding according to VDL is shown in the following table:
Modulation symbol d
n
(binary, MSB, LSB)
000
001
010
011
100
101
110
111
Phase difference
∆ϕ
0°
45°
135°
90°
315°
270°
180°
225°
Phase differential coding INMARSAT and PHASE DIFF correspond to system standards Inmarsat-M
and DVB according to ETS 300 429. The INMARSAT coding can generally be used for modulation types
with 2 bit/symbol, such as QPSK. It uses the following algorithm.
dc0
n
dc1
n
(MSB)
INMARSAT
CODING
d0n
d1
n
(MSB)
dc1
n
= [NOT (d1
n
EXOR d0/
n
)
AND (d1
n
EXOR dc1
n-1
)] OR
[(d1
n
EXOR d0/
n
)
AND (d1
n
EXOR dc/0/
n-1
)]
dc0/
n
= [NOT (d1
n
EXOR d0/
n
)
AND (d0/
n
EXOR dc0/
n-1
)] OR
[(d1
n
EXOR d0/
n-1
) AND
(d0/
n
EXOR dc1
n-1
)]
2.10.1.5 Setting
Conflicts
As already mentioned in the previous sections, the combination of the above modulation methods with
modulation parameters such as symbol rate, filtering and coding is limited. This limitation inevitably
causes setting conflicts if a parameter is changed which would lead to an impermissible combination.
Table 2-10 Examples of settings conflicts
Original status
Change by selection
Conflict
Solution
1
MOD TYPE
π
/4DQPSK
SYMBOL RATE 10 Msymb/s
MOD TYPE
→
GMSK
GMSK is only possible
for symbol rates up to
7.5 Msymb/s.
Reduction of symbol rate to
a value of <7.5 MHz.
2
MOD TYPE
2FSK
FILTER
GAUSS
FILTER PARAMETER 1.0
FILTER
→
COS
Filter COS 1.0 not
possible, the maximum
roll-off factor is 0.7.
Setting of FILTER
PARAMETER to a value
between 0.15 and 0.7.
3
SYMBOL RATE 270 ksymb/s
POWER RAMP INT
SYMBOL RATE
→
5 Msymb/s
Envelope control is only
possible for symbol rates
up to 2.5 Msymb/s.
Switch-off of envelope
control.
If SMIQ cannot resolve a setting conflict, error message SETTINGS CONFLICT is indicated in the
status line of the display. In this case, SMIQ uses the user-defined setting in the display. However, the
generated modulation signal does not correspond to this indication. The setting conflict can be
eliminated by a change of parameters. Error message SETTINGS CONFLICT disappears as soon as a
conflict-free setting is reinstated. For a list of possible setting conflicts and error messages for digital
modulation see Annex B, thumbnail divider 7.
