<s>
Amplitude	B-Device
and	I-Device
phase-shift	I-Device
keying	I-Device
(	O
APSK	B-Device
)	O
is	O
a	O
digital	B-General_Concept
modulation	B-Application
scheme	O
that	O
conveys	O
data	O
by	O
modulating	O
both	O
the	O
amplitude	B-Application
and	O
the	O
phase	O
of	O
a	O
carrier	O
wave	O
.	O
</s>
<s>
In	O
other	O
words	O
,	O
it	O
combines	O
both	O
amplitude-shift	O
keying	O
(	O
ASK	O
)	O
and	O
phase-shift	B-General_Concept
keying	I-General_Concept
(	O
PSK	O
)	O
.	O
</s>
<s>
This	O
allows	O
for	O
a	O
lower	O
bit	O
error	O
rate	O
for	O
a	O
given	O
modulation	B-Application
order	O
and	O
signal-to-noise	O
ratio	O
,	O
at	O
the	O
cost	O
of	O
increased	O
complexity	O
,	O
compared	O
to	O
ASK	O
or	O
PSK	O
alone	O
.	O
</s>
<s>
Quadrature	B-Device
amplitude	I-Device
modulation	I-Device
(	O
QAM	B-Device
)	O
can	O
be	O
considered	O
a	O
subset	O
of	O
APSK	B-Device
because	O
all	O
QAM	B-Device
schemes	O
modulate	O
both	O
the	O
amplitude	B-Application
and	O
phase	O
of	O
the	O
carrier	O
.	O
</s>
<s>
Conventionally	O
,	O
QAM	B-Device
constellations	O
are	O
rectangular	O
and	O
APSK	B-Device
constellations	O
are	O
circular	O
,	O
however	O
this	O
is	O
not	O
always	O
the	O
case	O
.	O
</s>
<s>
The	O
distinction	O
between	O
the	O
two	O
is	O
in	O
their	O
production	O
;	O
QAM	B-Device
is	O
produced	O
from	O
two	O
orthogonal	O
signals	O
.	O
</s>
<s>
The	O
advantage	O
of	O
APSK	B-Device
over	O
conventional	O
QAM	B-Device
is	O
a	O
lower	O
number	O
of	O
possible	O
amplitude	B-Application
levels	O
and	O
therefore	O
a	O
lower	O
peak-to-average	O
power	O
ratio	O
(	O
PAPR	O
)	O
.	O
</s>
<s>
The	O
resilience	O
of	O
APSK	B-Device
to	O
amplifier	O
and	O
channel	O
non-linearities	O
afforded	O
by	O
its	O
low	O
PAPR	O
have	O
made	O
it	O
especially	O
attractive	O
for	O
satellite	O
communications	O
,	O
including	O
DVB-S2	O
.	O
</s>
<s>
There	O
are	O
many	O
APSK	B-Device
constellations	O
.	O
</s>
<s>
There	O
may	O
be	O
multiple	O
circular	O
constellations	O
of	O
the	O
same	O
order	O
,	O
for	O
example	O
16-APSK	O
could	O
be	O
implemented	O
using	O
a	O
(	O
1	O
,	O
5	O
,	O
10	O
)	O
constellation	O
or	O
a	O
(	O
5	O
,	O
11	O
)	O
constellation	O
.	O
</s>
<s>
Other	O
APSK	B-Device
constellations	O
include	O
triangular	O
,	O
rectangular	O
and	O
hexagonal	O
constellations	O
.	O
</s>
<s>
For	O
the	O
regular	O
QAM	B-Device
constellations	O
,	O
a	O
gap	O
of	O
1.56dB	O
is	O
observed	O
.	O
</s>
<s>
The	O
previous	O
solution	O
,	O
where	O
the	O
constellation	O
has	O
a	O
Gaussian	O
shape	O
,	O
is	O
called	O
constellation	B-Device
shaping	I-Device
.	O
</s>
