<s>
Pulse-position	B-Device
modulation	I-Device
(	O
PPM	O
)	O
is	O
a	O
form	O
of	O
signal	O
modulation	B-Application
in	O
which	O
M	O
message	O
bits	O
are	O
encoded	O
by	O
transmitting	O
a	O
single	O
pulse	O
in	O
one	O
of	O
possible	O
required	O
time	O
shifts	O
.	O
</s>
<s>
An	O
ancient	O
use	O
of	O
pulse-position	B-Device
modulation	I-Device
was	O
the	O
Greek	O
hydraulic	O
semaphore	O
system	O
invented	O
by	O
Aeneas	O
Stymphalus	O
around	O
350	O
B.C.	O
</s>
<s>
In	O
modern	O
times	O
,	O
pulse-position	B-Device
modulation	I-Device
has	O
origins	O
in	O
telegraph	O
time-division	B-Protocol
multiplexing	I-Protocol
,	O
which	O
dates	O
back	O
to	O
1853	O
,	O
and	O
evolved	O
alongside	O
pulse-code	B-Algorithm
modulation	I-Algorithm
and	O
pulse-width	B-Algorithm
modulation	I-Algorithm
.	O
</s>
<s>
In	O
the	O
early	O
1960s	O
,	O
Don	O
Mathers	O
and	O
Doug	O
Spreng	O
of	O
NASA	O
invented	O
pulse-position	B-Device
modulation	I-Device
used	O
in	O
radio-control	O
(	O
R/C	O
)	O
systems	O
.	O
</s>
<s>
PPM	O
is	O
currently	O
being	O
used	O
in	O
fiber-optic	B-Application
communications	I-Application
,	O
deep-space	O
communications	O
,	O
and	O
continues	O
to	O
be	O
used	O
in	O
R/C	O
systems	O
.	O
</s>
<s>
Therefore	O
,	O
it	O
is	O
often	O
implemented	O
differentially	O
as	O
differential	O
pulse-position	B-Device
modulation	I-Device
,	O
whereby	O
each	O
pulse	O
position	O
is	O
encoded	O
relative	O
to	O
the	O
previous	O
,	O
such	O
that	O
the	O
receiver	O
must	O
only	O
measure	O
the	O
difference	O
in	O
the	O
arrival	O
time	O
of	O
successive	O
pulses	O
.	O
</s>
<s>
Multipath	O
in	O
Pulse	B-Device
Position	I-Device
Modulation	I-Device
systems	O
can	O
be	O
easily	O
mitigated	O
by	O
using	O
the	O
same	O
techniques	O
that	O
are	O
used	O
in	O
Radar	O
systems	O
that	O
rely	O
totally	O
on	O
synchronization	O
and	O
time	O
of	O
arrival	O
of	O
the	O
received	O
pulse	O
to	O
obtain	O
their	O
range	O
position	O
in	O
the	O
presence	O
of	O
echoes	O
.	O
</s>
<s>
One	O
of	O
the	O
principal	O
advantages	O
of	O
PPM	O
is	O
that	O
it	O
is	O
an	O
M-ary	O
modulation	B-Application
technique	O
that	O
can	O
be	O
implemented	O
non-coherently	O
,	O
such	O
that	O
the	O
receiver	O
does	O
not	O
need	O
to	O
use	O
a	O
phase-locked	O
loop	O
(	O
PLL	O
)	O
to	O
track	O
the	O
phase	O
of	O
the	O
carrier	O
.	O
</s>
<s>
This	O
makes	O
it	O
a	O
suitable	O
candidate	O
for	O
optical	O
communications	O
systems	O
,	O
where	O
coherent	O
phase	O
modulation	B-Application
and	O
detection	O
are	O
difficult	O
and	O
extremely	O
expensive	O
.	O
</s>
<s>
The	O
only	O
other	O
common	O
M-ary	O
non-coherent	O
modulation	B-Application
technique	O
is	O
M-ary	O
frequency-shift	O
keying	O
(	O
M-FSK	O
)	O
,	O
which	O
is	O
the	O
frequency-domain	O
dual	O
to	O
PPM	O
.	O
</s>
<s>
Optical	O
communications	O
systems	O
tend	O
to	O
have	O
weak	O
multipath	O
distortions	O
,	O
and	O
PPM	O
is	O
a	O
viable	O
modulation	B-Application
scheme	O
in	O
many	O
such	O
applications	O
.	O
</s>
<s>
One	O
common	O
application	O
with	O
these	O
channel	O
characteristics	O
,	O
first	O
used	O
in	O
the	O
early	O
1960s	O
with	O
top-end	O
HF	B-Algorithm
(	O
as	O
low	O
as	O
27MHz	O
)	O
frequencies	O
into	O
the	O
low-end	O
VHF	B-Algorithm
band	I-Algorithm
frequencies	O
(	O
30MHz	O
to	O
75MHz	O
for	O
RC	O
use	O
depending	O
on	O
location	O
)	O
,	O
is	O
the	O
radio	O
control	O
of	O
model	O
aircraft	O
,	O
boats	O
and	O
cars	O
,	O
originally	O
known	O
as	O
"	O
digital	O
proportional	O
"	O
radio	O
control	O
.	O
</s>
<s>
Servos	O
made	O
for	O
model	O
radio	O
control	O
include	O
some	O
of	O
the	O
electronics	O
required	O
to	O
convert	O
the	O
pulse	O
to	O
the	O
motor	O
position	O
–	O
the	O
receiver	O
is	O
required	O
to	O
first	O
extract	O
the	O
information	O
from	O
the	O
received	O
radio	O
signal	O
through	O
its	O
intermediate	O
frequency	O
section	O
,	O
then	O
demultiplex	B-Architecture
the	O
separate	O
channels	O
from	O
the	O
serial	O
stream	O
,	O
and	O
feed	O
the	O
control	O
pulses	O
to	O
each	O
servo	O
.	O
</s>
<s>
Each	O
channel	O
(	O
up	O
to	O
8	O
)	O
is	O
encoded	O
by	O
the	O
time	O
of	O
the	O
high	O
state	O
(	O
PPM	O
high	O
state	O
+	O
(	O
0.3	O
×	O
PPM	O
low	O
state	O
)	O
=	O
servo	O
PWM	B-Algorithm
pulse	O
width	O
)	O
.	O
</s>
<s>
More	O
sophisticated	O
radio	O
control	O
systems	O
are	O
now	O
often	O
based	O
on	O
pulse-code	B-Algorithm
modulation	I-Algorithm
,	O
which	O
is	O
more	O
complex	O
but	O
offers	O
greater	O
flexibility	O
and	O
reliability	O
.	O
</s>
<s>
The	O
advent	O
of	O
2.4GHz	O
band	O
FHSS	B-Protocol
radio-control	O
systems	O
in	O
the	O
early	O
21st	O
century	O
changed	O
this	O
further	O
.	O
</s>
<s>
Pulse-position	B-Device
modulation	I-Device
is	O
also	O
used	O
for	O
communication	O
with	O
the	O
ISO/IEC	O
15693	O
contactless	O
smart	O
card	O
,	O
as	O
well	O
as	O
in	O
the	O
HF	B-Algorithm
implementation	O
of	O
the	O
Electronic	O
Product	O
Code	O
(	O
EPC	O
)	O
Class	O
1	O
protocol	O
for	O
RFID	B-Application
tags	I-Application
.	O
</s>
