<s>
Frequency-hopping	B-Protocol
spread	I-Protocol
spectrum	I-Protocol
(	O
FHSS	B-Protocol
)	O
is	O
a	O
method	O
of	O
transmitting	O
radio	O
signals	O
by	O
rapidly	O
changing	O
the	O
carrier	O
frequency	O
among	O
many	O
frequencies	O
occupying	O
a	O
large	O
spectral	O
band	O
.	O
</s>
<s>
FHSS	B-Protocol
is	O
used	O
to	O
avoid	O
interference	O
,	O
to	O
prevent	O
eavesdropping	O
,	O
and	O
to	O
enable	O
code-division	B-Protocol
multiple	I-Protocol
access	I-Protocol
(	O
CDMA	B-Protocol
)	O
communications	O
.	O
</s>
<s>
FHSS	B-Protocol
offers	O
four	O
main	O
advantages	O
over	O
a	O
fixed-frequency	O
transmission	O
:	O
</s>
<s>
FHSS	B-Protocol
signals	O
are	O
highly	O
resistant	O
to	O
narrowband	O
interference	O
because	O
the	O
signal	O
hops	O
to	O
a	O
different	O
frequency	O
band	O
.	O
</s>
<s>
Signals	O
are	O
difficult	O
to	O
intercept	O
if	O
the	O
frequency-hopping	B-Protocol
pattern	O
is	O
not	O
known	O
.	O
</s>
<s>
FHSS	B-Protocol
transmissions	O
can	O
share	O
a	O
frequency	O
band	O
with	O
many	O
types	O
of	O
conventional	O
transmissions	O
with	O
minimal	O
mutual	O
interference	O
.	O
</s>
<s>
FHSS	B-Protocol
signals	O
add	O
minimal	O
interference	O
to	O
narrowband	O
communications	O
,	O
and	O
vice	O
versa	O
.	O
</s>
<s>
Spread-spectrum	O
signals	O
are	O
highly	O
resistant	O
to	O
deliberate	O
jamming	O
unless	O
the	O
adversary	O
has	O
knowledge	O
of	O
the	O
frequency-hopping	B-Protocol
pattern	O
.	O
</s>
<s>
Military	O
radios	O
generate	O
the	O
frequency-hopping	B-Protocol
pattern	O
under	O
the	O
control	O
of	O
a	O
secret	O
Transmission	O
Security	O
Key	O
(	O
TRANSEC	O
)	O
that	O
the	O
sender	O
and	O
receiver	O
share	O
in	O
advance	O
.	O
</s>
<s>
United	O
States	O
military	O
radios	O
that	O
use	O
frequency	B-Protocol
hopping	I-Protocol
include	O
the	O
JTIDS/MIDS	O
family	O
,	O
the	O
HAVE	O
QUICK	O
Aeronautical	O
Mobile	O
communications	O
system	O
,	O
and	O
the	O
SINCGARS	O
Combat	O
Net	O
Radio	O
,	O
Link-16	O
.	O
</s>
<s>
In	O
the	O
US	O
,	O
since	O
the	O
Federal	O
Communications	O
Commission	O
(	O
FCC	O
)	O
amended	O
rules	O
to	O
allow	O
FHSS	B-Protocol
systems	O
in	O
the	O
unregulated	O
2.4GHz	O
band	O
,	O
many	O
consumer	O
devices	O
in	O
that	O
band	O
have	O
employed	O
various	O
FHSS	B-Protocol
modes	O
.	O
</s>
<s>
eFCC	O
CFR	O
47	O
part	O
15.247	O
covers	O
the	O
regulations	O
in	O
the	O
US	O
for	O
902	O
–	O
928MHz	O
,	O
2400	O
–	O
2483.5MHz	O
,	O
and	O
5725	O
–	O
5850MHz	O
bands	O
,	O
and	O
the	O
requirements	O
for	O
frequency	B-Protocol
hopping	I-Protocol
.	O
</s>
<s>
Some	O
walkie-talkies	B-Application
that	O
employ	O
FHSS	B-Protocol
technology	O
have	O
been	O
developed	O
for	O
unlicensed	O
use	O
on	O
the	O
900MHz	O
band	O
.	O
</s>
<s>
FHSS	B-Protocol
technology	O
is	O
also	O
used	O
in	O
many	O
hobby	O
transmitters	O
and	O
receivers	O
used	O
for	O
radio-controlled	O
model	O
cars	O
,	O
airplanes	O
,	O
and	O
drones	O
.	O
</s>
<s>
The	O
overall	O
bandwidth	O
required	O
for	O
frequency	B-Protocol
hopping	I-Protocol
is	O
much	O
wider	O
than	O
that	O
required	O
to	O
transmit	O
the	O
same	O
information	O
using	O
only	O
one	O
carrier	O
frequency	O
.	O
</s>
<s>
While	O
providing	O
no	O
extra	O
protection	O
against	O
wideband	O
thermal	O
noise	O
,	O
the	O
frequency-hopping	B-Protocol
approach	O
reduces	O
the	O
degradation	O
caused	O
by	O
narrowband	O
interference	O
sources	O
.	O
</s>
<s>
One	O
of	O
the	O
challenges	O
of	O
frequency-hopping	B-Protocol
systems	O
is	O
to	O
synchronize	O
the	O
transmitter	O
and	O
receiver	O
.	O
</s>
<s>
The	O
transmitter	O
's	O
data	O
is	O
identified	O
by	O
a	O
special	O
sequence	O
of	O
data	O
that	O
is	O
unlikely	O
to	O
occur	O
over	O
the	O
segment	O
of	O
data	O
for	O
this	O
channel	O
,	O
and	O
the	O
segment	O
can	O
also	O
have	O
a	O
checksum	B-Algorithm
for	O
integrity	O
checking	O
and	O
further	O
identification	O
.	O
</s>
<s>
The	O
transmitter	O
and	O
receiver	O
can	O
use	O
fixed	O
tables	O
of	O
frequency-hopping	B-Protocol
patterns	O
,	O
so	O
that	O
once	O
synchronized	O
they	O
can	O
maintain	O
communication	O
by	O
following	O
the	O
table	O
.	O
</s>
<s>
Both	O
FHSS	B-Protocol
and	O
direct-sequence	B-Device
spread-spectrum	O
(	O
DSSS	B-Device
)	O
systems	O
can	O
transmit	O
at	O
1	O
watt	O
,	O
a	O
thousandfold	O
increase	O
from	O
the	O
1	O
milliwatt	O
limit	O
on	O
non-spread-spectrum	O
systems	O
.	O
</s>
<s>
The	O
earliest	O
mentions	O
of	O
frequency	B-Protocol
hopping	I-Protocol
in	O
open	O
literature	O
are	O
in	O
,	O
awarded	O
to	O
Nikola	O
Tesla	O
on	O
March	O
17	O
,	O
1903	O
,	O
and	O
in	O
radio	O
pioneer	O
Jonathan	O
Zenneck	O
's	O
book	O
Wireless	O
Telegraphy	O
(	O
German	O
,	O
1908	O
,	O
English	O
translation	O
McGraw	O
Hill	O
,	O
1915	O
)	O
,	O
although	O
Zenneck	O
writes	O
that	O
Telefunken	O
had	O
already	O
tried	O
it	O
.	O
</s>
<s>
Nikola	O
Tesla	O
does	O
n't	O
mention	O
the	O
phrase	O
"	O
frequency	B-Protocol
hopping	I-Protocol
"	O
directly	O
,	O
but	O
certainly	O
alludes	O
to	O
it	O
.	O
</s>
<s>
The	O
German	O
military	O
made	O
limited	O
use	O
of	O
frequency	B-Protocol
hopping	I-Protocol
for	O
communication	O
between	O
fixed	O
command	O
points	O
in	O
World	O
War	O
I	O
to	O
prevent	O
eavesdropping	O
by	O
British	O
forces	O
,	O
who	O
did	O
not	O
have	O
the	O
technology	O
to	O
follow	O
the	O
sequence	O
.	O
</s>
<s>
Jonathan	O
Zenneck	O
's	O
book	O
Wireless	O
Telegraphy	O
was	O
originally	O
published	O
in	O
German	O
in	O
1908	O
,	O
but	O
was	O
translated	O
into	O
English	O
in	O
1915	O
as	O
the	O
enemy	O
started	O
using	O
frequency	B-Protocol
hopping	I-Protocol
on	O
the	O
front	O
line	O
.	O
</s>
<s>
Wireless	O
Telegraphy	O
includes	O
a	O
section	O
on	O
frequency	B-Protocol
hopping	I-Protocol
,	O
and	O
,	O
as	O
it	O
became	O
a	O
standard	O
text	O
for	O
many	O
years	O
,	O
it	O
probably	O
introduced	O
the	O
technology	O
to	O
a	O
generation	O
of	O
engineers	O
.	O
</s>
<s>
In	O
1942	O
,	O
actress	O
Hedy	O
Lamarr	O
and	O
composer	O
George	O
Antheil	O
received	O
for	O
their	O
"	O
Secret	O
Communications	O
System	O
"	O
,	O
an	O
early	O
version	O
of	O
frequency	B-Protocol
hopping	I-Protocol
using	O
a	O
piano-roll	O
to	O
switch	O
among	O
88	O
frequencies	O
to	O
make	O
radio-guided	O
torpedoes	O
harder	O
for	O
enemies	O
to	O
detect	O
or	O
jam	O
.	O
</s>
<s>
Lamarr	O
's	O
and	O
Antheil	O
's	O
idea	O
was	O
rediscovered	O
in	O
the	O
1950s	O
during	O
patent	O
searches	O
when	O
private	O
companies	O
were	O
independently	O
developing	O
direct-sequence	B-Device
Code	B-Protocol
Division	I-Protocol
Multiple	I-Protocol
Access	I-Protocol
,	O
a	O
non-frequency-hopping	O
form	O
of	O
spread-spectrum	O
,	O
and	O
has	O
been	O
cited	O
numerous	O
times	O
since	O
.	O
</s>
<s>
A	O
practical	O
application	O
of	O
frequency	B-Protocol
hopping	I-Protocol
was	O
developed	O
by	O
Ray	O
Zinn	O
,	O
co-founder	O
of	O
Micrel	O
Corporation	O
.	O
</s>
<s>
Using	O
frequency	B-Protocol
hopping	I-Protocol
and	O
sweep	O
modes	O
,	O
Zinn	O
's	O
method	O
is	O
primarily	O
applied	O
in	O
low	O
data	O
rate	O
wireless	O
applications	O
such	O
as	O
utility	O
metering	O
,	O
machine	O
and	O
equipment	O
monitoring	O
and	O
metering	O
,	O
and	O
remote	O
control	O
.	O
</s>
<s>
In	O
2006	O
Zinn	O
received	O
for	O
his	O
"	O
Wireless	O
device	O
and	O
method	O
using	O
frequency	B-Protocol
hopping	I-Protocol
and	O
sweep	O
modes.	O
"	O
</s>
<s>
Adaptive	O
frequency-hopping	B-Protocol
spread	I-Protocol
spectrum	I-Protocol
(	O
AFH	O
)	O
as	O
used	O
in	O
Bluetooth	B-Protocol
improves	O
resistance	O
to	O
radio	O
frequency	O
interference	O
by	O
avoiding	O
crowded	O
frequencies	O
in	O
the	O
hopping	O
sequence	O
.	O
</s>
<s>
This	O
sort	O
of	O
adaptive	O
transmission	O
is	O
easier	O
to	O
implement	O
with	O
FHSS	B-Protocol
than	O
with	O
DSSS	B-Device
.	O
</s>
<s>
For	O
example	O
,	O
if	O
there	O
are	O
several	O
colocated	O
frequency-hopping	B-Protocol
networks	O
(	O
as	O
Bluetooth	B-General_Concept
Piconet	I-General_Concept
)	O
,	O
they	O
are	O
mutually	O
interfering	O
and	O
AFH	O
's	O
strategy	O
fails	O
to	O
avoid	O
this	O
interference	O
.	O
</s>
<s>
The	O
problem	O
of	O
dynamic	O
interference	O
,	O
gradual	O
reduction	O
of	O
available	O
hopping	O
channels	O
and	O
backward	O
compatibility	O
with	O
legacy	O
Bluetooth	B-Protocol
devices	O
was	O
resolved	O
in	O
version	O
1.2	O
of	O
the	O
Bluetooth	B-Protocol
Standard	O
(	O
2003	O
)	O
.	O
</s>
<s>
Such	O
a	O
situation	O
can	O
often	O
happen	O
in	O
the	O
scenarios	O
that	O
use	O
unlicensed	B-Algorithm
spectrum	I-Algorithm
.	O
</s>
<s>
In	O
addition	O
,	O
dynamic	O
radio	O
frequency	O
interference	O
is	O
expected	O
to	O
occur	O
in	O
the	O
scenarios	O
related	O
to	O
cognitive	B-Protocol
radio	I-Protocol
,	O
where	O
the	O
networks	O
and	O
the	O
devices	O
should	O
exhibit	O
frequency-agile	B-Algorithm
operation	O
.	O
</s>
<s>
Chirp	O
modulation	O
can	O
be	O
seen	O
as	O
a	O
form	O
of	O
frequency-hopping	B-Protocol
that	O
simply	O
scans	O
through	O
the	O
available	O
frequencies	O
in	O
consecutive	O
order	O
to	O
communicate	O
.	O
</s>
<s>
Frequency	B-Protocol
hopping	I-Protocol
can	O
be	O
superimposed	O
on	O
other	O
modulations	O
or	O
waveforms	O
to	O
enhance	O
the	O
system	O
performance	O
.	O
</s>
