<s>
Multiple	B-Device
frequency-shift	I-Device
keying	I-Device
(	O
MFSK	B-Device
)	O
is	O
a	O
variation	O
of	O
frequency-shift	O
keying	O
(	O
FSK	O
)	O
that	O
uses	O
more	O
than	O
two	O
frequencies	O
.	O
</s>
<s>
MFSK	B-Device
is	O
a	O
form	O
of	O
M-ary	B-Device
orthogonal	I-Device
modulation	I-Device
,	O
where	O
each	O
symbol	O
consists	O
of	O
one	O
element	O
from	O
an	O
alphabet	O
of	O
orthogonal	O
waveforms	O
.	O
</s>
<s>
In	O
a	O
M-ary	O
signaling	O
system	O
like	O
MFSK	B-Device
,	O
an	O
"	O
alphabet	O
"	O
of	O
M	O
tones	O
is	O
established	O
and	O
the	O
transmitter	O
selects	O
one	O
tone	O
at	O
a	O
time	O
from	O
the	O
alphabet	O
for	O
transmission	O
.	O
</s>
<s>
MFSK	B-Device
is	O
classed	O
as	O
an	O
M-ary	O
orthogonal	O
signaling	O
scheme	O
because	O
each	O
of	O
the	O
M	O
tone	O
detection	O
filters	O
at	O
the	O
receiver	O
responds	O
only	O
to	O
its	O
tone	O
and	O
not	O
at	O
all	O
to	O
the	O
others	O
;	O
this	O
independence	O
provides	O
the	O
orthogonality	O
.	O
</s>
<s>
Typical	O
values	O
in	O
practice	O
range	O
from	O
4	O
to	O
64	O
,	O
and	O
MFSK	B-Device
is	O
combined	O
with	O
another	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
scheme	O
to	O
provide	O
additional	O
(	O
systematic	O
)	O
coding	O
gain	O
.	O
</s>
<s>
Spectral	B-Protocol
efficiency	I-Protocol
of	O
MFSK	B-Device
modulation	O
schemes	O
decreases	O
with	O
increasing	O
of	O
modulation	O
order	O
M	O
:	O
</s>
<s>
Like	O
any	O
other	O
form	O
of	O
angle	B-Device
modulation	I-Device
that	O
transmits	O
a	O
single	O
RF	O
tone	O
that	O
varies	O
only	O
in	O
phase	O
or	O
frequency	O
,	O
MFSK	B-Device
produces	O
a	O
constant	O
envelope	O
.	O
</s>
<s>
It	O
is	O
possible	O
to	O
combine	O
two	O
MFSK	B-Device
systems	O
to	O
increase	O
the	O
throughput	O
of	O
the	O
link	O
.	O
</s>
<s>
Perhaps	O
the	O
most	O
widely	O
used	O
2-tone	O
MFSK	B-Device
system	O
is	O
dual-tone	O
multi-frequency	O
(	O
DTMF	O
)	O
,	O
better	O
known	O
by	O
its	O
AT&T	O
trademark	O
of	O
"	O
Touch	O
Tone	O
"	O
.	O
</s>
<s>
However	O
,	O
two	O
audio	O
tones	O
can	O
be	O
sent	O
simultaneously	O
on	O
a	O
conventional	O
,	O
constant-envelope	O
FM	B-Application
RF	O
carrier	O
,	O
but	O
the	O
noncoherent	O
detection	O
of	O
the	O
FM	B-Application
signal	O
at	O
the	O
receiver	O
would	O
destroy	O
any	O
signal-to-noise	O
ratio	O
advantage	O
the	O
multitone	O
scheme	O
might	O
have	O
.	O
</s>
<s>
Skywave	O
propagation	O
on	O
the	O
high	B-Algorithm
frequency	I-Algorithm
bands	O
introduces	O
random	O
distortions	O
that	O
generally	O
vary	O
with	O
both	O
time	O
and	O
frequency	O
.	O
</s>
<s>
Understanding	O
these	O
impairments	O
helps	O
one	O
understand	O
why	O
MFSK	B-Device
is	O
such	O
an	O
effective	O
and	O
popular	O
technique	O
on	O
HF	O
.	O
</s>
<s>
This	O
is	O
because	O
summing	O
two	O
or	O
more	O
paths	O
with	O
different	O
delays	O
creates	O
a	O
comb	B-Algorithm
filter	I-Algorithm
even	O
when	O
the	O
individual	O
paths	O
have	O
a	O
flat	O
frequency	O
response	O
.	O
</s>
<s>
A	O
fading	O
channel	O
effectively	O
imposes	O
an	O
unwanted	O
random	O
amplitude	B-Device
modulation	I-Device
on	O
the	O
signal	O
.	O
</s>
<s>
With	O
appropriate	O
parameter	O
selection	O
,	O
MFSK	B-Device
can	O
tolerate	O
significant	O
Doppler	O
or	O
delay	O
spreads	O
,	O
especially	O
when	O
augmented	O
with	O
forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
.	O
</s>
<s>
A	O
long	O
delay	O
spread	O
with	O
little	O
Doppler	O
spreading	O
can	O
be	O
mitigated	O
with	O
a	O
relatively	O
long	O
MFSK	B-Device
symbol	O
period	O
to	O
allow	O
the	O
channel	O
to	O
"	O
settle	O
down	O
"	O
quickly	O
at	O
the	O
start	O
of	O
each	O
new	O
symbol	O
.	O
</s>
<s>
Forward	B-Error_Name
error	I-Error_Name
correction	I-Error_Name
is	O
especially	O
helpful	O
in	O
this	O
case	O
.	O
</s>
<s>
Because	O
of	O
the	O
wide	O
variety	O
of	O
conditions	O
found	O
on	O
HF	O
,	O
a	O
wide	O
variety	O
of	O
MFSK	B-Device
schemes	O
,	O
some	O
of	O
them	O
experimental	O
,	O
have	O
been	O
developed	O
for	O
HF	O
.	O
</s>
<s>
Piccolo	O
was	O
the	O
original	O
MFSK	B-Device
mode	O
,	O
developed	O
for	O
British	O
government	O
communications	O
by	O
Harold	O
Robin	O
,	O
Donald	O
Bailey	O
and	O
Denis	O
Ralphs	O
of	O
the	O
Diplomatic	O
Wireless	O
Service	O
(	O
DWS	O
)	O
,	O
a	O
branch	O
of	O
the	O
Foreign	O
and	O
Commonwealth	O
Office	O
.	O
</s>
<s>
MFSK8	B-Device
and	O
MFSK16	B-Device
were	O
developed	O
by	O
Murray	O
Greenman	O
,	O
ZL1BPU	O
for	O
amateur	O
radio	O
communications	O
on	O
HF	O
.	O
</s>
<s>
Olivia	B-Device
MFSK	I-Device
is	O
also	O
an	O
amateur	O
radio	O
mode	O
.	O
</s>
<s>
Automatic	B-Device
link	I-Device
establishment	I-Device
(	O
ALE	B-Device
)	O
is	O
a	O
protocol	O
developed	O
by	O
the	O
United	O
States	O
military	O
and	O
used	O
mainly	O
as	O
an	O
automatic	O
signalling	O
system	O
between	O
radios	O
.	O
</s>
<s>
"	O
CIS-36	O
MFSK	B-Device
"	O
or	O
"	O
CROWD-36	O
"	O
(	O
)	O
is	O
the	O
western	O
designation	O
of	O
a	O
system	O
similar	O
to	O
Piccolo	O
developed	O
in	O
the	O
former	O
Soviet	O
Union	O
for	O
military	O
communications	O
.	O
</s>
<s>
Recently	O
the	O
system	O
was	O
also	O
described	O
as	O
"	O
MFSK-20	O
"	O
.	O
</s>
<s>
MFSK	B-Device
modes	O
used	O
for	O
VHF	B-Algorithm
,	O
UHF	O
communications	O
:	O
</s>
<s>
FSK441	B-Protocol
,	O
JT6M	B-Protocol
and	O
JT65	B-Protocol
are	O
parts	O
of	O
the	O
WSJT	B-Protocol
family	O
or	O
radio	O
modulation	O
systems	O
,	O
developed	O
by	O
Joe	O
Taylor	O
,	O
K1JT	O
,	O
for	O
long	O
distance	O
amateur	O
radio	O
VHF	B-Algorithm
communications	O
under	O
marginal	O
propagation	O
conditions	O
.	O
</s>
<s>
These	O
specialized	O
MFSK	B-Device
modulation	O
systems	O
are	O
used	O
over	O
troposcattering	O
,	O
EME	O
(	O
earth-moon-earth	O
)	O
and	O
meteoscattering	O
radio	O
paths	O
.	O
</s>
<s>
It	O
is	O
frequently	O
used	O
for	O
telecommand	O
(	O
remote	O
control	O
)	O
applications	O
over	O
VHF	B-Algorithm
and	O
UHF	O
voice	O
channels	O
.	O
</s>
