<s>
Microwave	B-Application
transmission	I-Application
is	O
the	O
transmission	O
of	O
information	O
by	O
electromagnetic	O
waves	O
with	O
wavelengths	O
in	O
the	O
microwave	O
frequency	O
range	O
of	O
300MHz	O
to	O
300GHz(1 m - 1 mm wavelength )	O
of	O
the	O
electromagnetic	O
spectrum	O
.	O
</s>
<s>
Microwave	O
signals	O
are	O
normally	O
limited	O
to	O
the	O
line	O
of	O
sight	O
,	O
so	O
long-distance	O
transmission	O
using	O
these	O
signals	O
requires	O
a	O
series	O
of	O
repeaters	O
forming	O
a	O
microwave	B-Application
relay	I-Application
network	O
.	O
</s>
<s>
Although	O
an	O
experimental	O
microwave	O
telecommunication	O
link	O
across	O
the	O
English	O
Channel	O
was	O
demonstrated	O
in	O
1931	O
,	O
the	O
development	O
of	O
radar	B-Application
in	O
World	O
War	O
II	O
provided	O
the	O
technology	O
for	O
practical	O
exploitation	O
of	O
microwave	B-Application
communication	I-Application
.	O
</s>
<s>
10	O
,	O
which	O
used	O
microwave	B-Application
relays	I-Application
to	O
multiplex	B-Architecture
eight	O
telephone	O
channels	O
over	O
long	O
distances	O
.	O
</s>
<s>
In	O
the	O
post-war	O
era	O
,	O
the	O
development	O
of	O
microwave	O
technology	O
was	O
rapid	O
,	O
which	O
led	O
to	O
the	O
construction	O
of	O
several	O
transcontinental	O
microwave	B-Application
relay	I-Application
systems	O
in	O
North	O
America	O
and	O
Europe	O
.	O
</s>
<s>
Communication	O
satellites	B-Application
took	O
over	O
the	O
television	O
broadcast	O
market	O
during	O
the	O
1970s	O
and	O
80s	O
,	O
and	O
the	O
introduction	O
of	O
long-distance	O
fibre	B-Architecture
optic	I-Architecture
systems	O
in	O
the	O
1980s	O
and	O
especially	O
90s	O
led	O
to	O
the	O
rapid	O
rundown	O
of	O
the	O
relay	O
networks	O
,	O
most	O
of	O
which	O
are	O
abandoned	O
.	O
</s>
<s>
In	O
recent	O
years	O
,	O
there	O
has	O
been	O
an	O
explosive	O
increase	O
in	O
use	O
of	O
the	O
microwave	O
spectrum	O
by	O
new	O
telecommunication	O
technologies	O
such	O
as	O
wireless	B-General_Concept
networks	I-General_Concept
,	O
and	O
direct-broadcast	O
satellites	B-Application
which	O
broadcast	O
television	O
and	O
radio	O
directly	O
into	O
consumers	O
 '	O
homes	O
.	O
</s>
<s>
Microwaves	O
are	O
widely	O
used	O
for	O
point-to-point	B-Architecture
communications	O
because	O
their	O
small	O
wavelength	O
allows	O
conveniently-sized	O
antennas	O
to	O
direct	O
them	O
in	O
narrow	O
beams	O
,	O
which	O
can	O
be	O
pointed	O
directly	O
at	O
the	O
receiving	O
antenna	O
.	O
</s>
<s>
This	O
frequency	O
reuse	O
conserves	O
scarce	O
radio	B-Algorithm
spectrum	I-Algorithm
bandwidth	B-Algorithm
.	O
</s>
<s>
Another	O
advantage	O
is	O
that	O
the	O
high	O
frequency	O
of	O
microwaves	O
gives	O
the	O
microwave	O
band	O
a	O
very	O
large	O
information-carrying	O
capacity	O
;	O
the	O
microwave	O
band	O
has	O
a	O
bandwidth	B-Algorithm
30	O
times	O
that	O
of	O
all	O
the	O
rest	O
of	O
the	O
radio	B-Algorithm
spectrum	I-Algorithm
below	O
it	O
.	O
</s>
<s>
Microwave	B-Application
radio	I-Application
transmission	O
is	O
commonly	O
used	O
in	O
point-to-point	B-Architecture
communication	I-Architecture
systems	O
on	O
the	O
surface	O
of	O
the	O
Earth	O
,	O
in	O
satellite	B-Application
communications	O
,	O
and	O
in	O
deep	O
space	O
radio	O
communications	O
.	O
</s>
<s>
Other	O
parts	O
of	O
the	O
microwave	B-Application
radio	I-Application
band	O
are	O
used	O
for	O
radars	B-Application
,	O
radio	O
navigation	O
systems	O
,	O
sensor	O
systems	O
,	O
and	O
radio	O
astronomy	O
.	O
</s>
<s>
The	O
next	O
higher	O
frequency	O
band	O
of	O
the	O
radio	B-Algorithm
spectrum	I-Algorithm
,	O
between	O
30GHz	O
and	O
300GHz	O
,	O
are	O
called	O
"	O
millimeter	O
waves	O
"	O
because	O
their	O
wavelengths	O
range	O
from	O
10mm	O
to	O
1mm	O
.	O
</s>
<s>
More	O
recently	O
,	O
microwaves	O
have	O
been	O
used	O
for	O
wireless	B-Application
power	I-Application
transmission	I-Application
.	O
</s>
<s>
Microwave	B-Application
radio	I-Application
relay	O
is	O
a	O
technology	O
widely	O
used	O
in	O
the	O
1950s	O
and	O
1960s	O
for	O
transmitting	O
information	O
,	O
such	O
as	O
long-distance	O
telephone	O
calls	O
and	O
television	O
programs	O
between	O
two	O
terrestrial	O
points	O
on	O
a	O
narrow	O
beam	O
of	O
microwaves	O
.	O
</s>
<s>
In	O
microwave	B-Application
radio	I-Application
relay	O
,	O
a	O
microwave	B-Application
transmitter	I-Application
and	O
directional	O
antenna	O
transmits	O
a	O
narrow	O
beam	O
of	O
microwaves	O
carrying	O
many	O
channels	O
of	O
information	O
on	O
a	O
line	O
of	O
sight	O
path	O
to	O
another	O
relay	O
station	O
where	O
it	O
is	O
received	O
by	O
a	O
directional	O
antenna	O
and	O
receiver	O
,	O
forming	O
a	O
fixed	O
radio	O
connection	O
between	O
the	O
two	O
points	O
.	O
</s>
<s>
Chains	O
of	O
microwave	B-Application
relay	I-Application
stations	O
were	O
used	O
to	O
transmit	O
telecommunication	O
signals	O
over	O
transcontinental	O
distances	O
.	O
</s>
<s>
Microwave	B-Application
relay	I-Application
stations	O
were	O
often	O
located	O
on	O
tall	O
buildings	O
and	O
mountaintops	O
,	O
with	O
their	O
antennas	O
on	O
towers	O
to	O
get	O
maximum	O
range	O
.	O
</s>
<s>
Beginning	O
in	O
the	O
1950s	O
,	O
networks	O
of	O
microwave	B-Application
relay	I-Application
links	O
,	O
such	O
as	O
the	O
AT&T	O
Long	O
Lines	O
system	O
in	O
the	O
U.S.	O
,	O
carried	O
long-distance	O
telephone	O
calls	O
and	O
television	O
programs	O
between	O
cities	O
.	O
</s>
<s>
Through	O
the	O
1950s	O
,	O
they	O
deployed	O
a	O
network	O
of	O
a	O
slightly	O
improved	O
version	O
across	O
the	O
U.S.	O
,	O
known	O
as	O
TD2	B-Algorithm
.	O
</s>
<s>
The	O
launch	O
of	O
communication	O
satellites	B-Application
in	O
the	O
1970s	O
provided	O
a	O
cheaper	O
alternative	O
.	O
</s>
<s>
Much	O
of	O
the	O
transcontinental	O
traffic	O
is	O
now	O
carried	O
by	O
satellites	B-Application
and	O
optical	B-Architecture
fibers	I-Architecture
,	O
but	O
microwave	B-Application
relay	I-Application
remains	O
important	O
for	O
shorter	O
distances	O
.	O
</s>
<s>
Because	O
the	O
radio	O
waves	O
travel	O
in	O
narrow	O
beams	O
confined	O
to	O
a	O
line-of-sight	O
path	O
from	O
one	O
antenna	O
to	O
the	O
other	O
,	O
they	O
do	O
not	O
interfere	O
with	O
other	O
microwave	O
equipment	O
,	O
so	O
nearby	O
microwave	B-Application
links	I-Application
can	O
use	O
the	O
same	O
frequencies	O
.	O
</s>
<s>
Typical	O
types	O
of	O
antenna	O
used	O
in	O
radio	B-Application
relay	I-Application
link	I-Application
installations	O
are	O
parabolic	O
antennas	O
,	O
dielectric	O
lens	O
,	O
and	O
horn-reflector	O
antennas	O
,	O
which	O
have	O
a	O
diameter	O
of	O
up	O
to	O
4	O
meters	O
.	O
</s>
<s>
Multipath	O
fades	O
are	O
usually	O
deep	O
only	O
in	O
a	O
small	O
spot	O
and	O
a	O
narrow	O
frequency	O
band	O
,	O
so	O
space	O
and/or	O
frequency	O
diversity	B-Protocol
schemes	I-Protocol
can	O
be	O
applied	O
to	O
mitigate	O
these	O
effects	O
.	O
</s>
<s>
The	O
longest	O
microwave	B-Application
radio	I-Application
relay	O
known	O
up	O
to	O
date	O
crosses	O
the	O
Red	O
Sea	O
with	O
a	O
360km	O
(	O
200mi	O
)	O
hop	O
between	O
Jebel	O
Erba	O
(	O
2170m	O
a.s.l.	O
,	O
,	O
Sudan	O
)	O
and	O
Jebel	O
Dakka	O
(	O
2572m	O
a.s.l.	O
,	O
,	O
Saudi	O
Arabia	O
)	O
.	O
</s>
<s>
The	O
target	O
was	O
to	O
supply	O
the	O
requested	O
availability	O
for	O
the	O
whole	O
block	O
(	O
Plesiochronous	B-Protocol
digital	I-Protocol
hierarchy	I-Protocol
,	O
PDH	O
,	O
or	O
synchronous	B-Protocol
digital	I-Protocol
hierarchy	I-Protocol
,	O
SDH	O
)	O
.	O
</s>
<s>
During	O
1990s	O
microwave	B-Application
radio	I-Application
links	O
begun	O
widely	O
to	O
be	O
used	O
for	O
urban	O
links	O
in	O
cellular	O
network	O
.	O
</s>
<s>
Furthermore	O
,	O
link	O
planning	O
deals	O
more	O
with	O
intense	O
rainfall	O
and	O
less	O
with	O
multipath	O
,	O
so	O
diversity	B-Protocol
schemes	I-Protocol
became	O
less	O
used	O
.	O
</s>
<s>
Another	O
big	O
change	O
that	O
occurred	O
during	O
the	O
last	O
decade	O
was	O
an	O
evolution	O
toward	O
packet	B-Application
radio	I-Application
transmission	O
.	O
</s>
<s>
Therefore	O
,	O
new	O
countermeasures	O
,	O
such	O
as	O
adaptive	B-Protocol
modulation	I-Protocol
,	O
have	O
been	O
adopted	O
.	O
</s>
<s>
These	O
microwave	B-Application
transmissions	I-Application
use	O
emitted	O
power	O
typically	O
from	O
0.03	O
to	O
0.30	O
W	O
,	O
radiated	O
by	O
a	O
parabolic	O
antenna	O
on	O
a	O
narrow	O
beam	O
diverging	O
by	O
a	O
few	O
degrees	O
(	O
1	O
to	O
3-4	O
)	O
.	O
</s>
<s>
In	O
the	O
last	O
decade	O
the	O
dedicated	O
spectrum	O
for	O
each	O
microwave	O
band	O
has	O
become	O
extremely	O
crowded	O
,	O
motivating	O
the	O
use	O
of	O
techniques	O
to	O
increase	O
transmission	O
capacity	O
such	O
as	O
frequency	O
reuse	O
,	O
polarization-division	O
multiplexing	B-Architecture
,	O
XPIC	O
,	O
MIMO	O
.	O
</s>
<s>
However	O
the	O
low	B-Algorithm
frequency	I-Algorithm
and	O
medium	B-Algorithm
frequency	I-Algorithm
radio	O
waves	O
used	O
during	O
the	O
first	O
40	O
years	O
of	O
radio	O
proved	O
to	O
be	O
able	O
to	O
travel	O
long	O
distances	O
by	O
ground	O
wave	O
and	O
skywave	O
propagation	O
.	O
</s>
<s>
In	O
1931	O
an	O
Anglo-French	O
consortium	O
headed	O
by	O
Andre	O
C	O
.	O
Clavier	O
demonstrated	O
an	O
experimental	O
microwave	B-Application
relay	I-Application
link	O
across	O
the	O
English	O
Channel	O
using	O
dishes	O
.	O
</s>
<s>
Telephony	O
,	O
telegraph	O
,	O
and	O
facsimile	O
data	O
was	O
transmitted	O
over	O
the	O
bidirectional	O
1.7GHz	O
beams	O
between	O
Dover	O
,	O
UK	O
,	O
and	O
Calais	B-General_Concept
,	O
France	O
.	O
</s>
<s>
A	O
1933	O
military	O
microwave	B-Application
link	I-Application
between	O
airports	O
at	O
St.	O
Inglevert	O
,	O
France	O
,	O
and	O
Lympne	O
,	O
UK	O
,	O
a	O
distance	O
of	O
,	O
was	O
followed	O
in	O
1935	O
by	O
a	O
300MHz	O
telecommunication	O
link	O
,	O
the	O
first	O
commercial	O
microwave	B-Application
relay	I-Application
system	O
.	O
</s>
<s>
The	O
development	O
of	O
radar	B-Application
during	O
World	O
War	O
II	O
provided	O
much	O
of	O
the	O
microwave	O
technology	O
which	O
made	O
practical	O
microwave	B-Application
communication	I-Application
links	O
possible	O
,	O
particularly	O
the	O
klystron	O
oscillator	O
and	O
techniques	O
of	O
designing	O
parabolic	O
antennas	O
.	O
</s>
<s>
After	O
the	O
war	O
,	O
telephone	O
companies	O
used	O
this	O
technology	O
to	O
build	O
large	O
microwave	B-Application
radio	I-Application
relay	O
networks	O
to	O
carry	O
long-distance	O
telephone	O
calls	O
.	O
</s>
<s>
During	O
the	O
1950s	O
a	O
unit	O
of	O
the	O
US	O
telephone	O
carrier	O
,	O
AT&T	O
Long	O
Lines	O
,	O
built	O
a	O
transcontinental	O
system	O
of	O
microwave	B-Application
relay	I-Application
links	O
across	O
the	O
US	O
that	O
grew	O
to	O
carry	O
the	O
majority	O
of	O
US	O
long	O
distance	O
telephone	O
traffic	O
,	O
as	O
well	O
as	O
television	O
network	O
signals	O
.	O
</s>
<s>
The	O
main	O
motivation	O
in	O
1946	O
to	O
use	O
microwave	B-Application
radio	I-Application
instead	O
of	O
cable	O
was	O
that	O
a	O
large	O
capacity	O
could	O
be	O
installed	O
quickly	O
and	O
at	O
less	O
cost	O
.	O
</s>
<s>
It	O
was	O
expected	O
at	O
that	O
time	O
that	O
the	O
annual	O
operating	O
costs	O
for	O
microwave	B-Application
radio	I-Application
would	O
be	O
greater	O
than	O
for	O
cable	O
.	O
</s>
<s>
There	O
were	O
two	O
main	O
reasons	O
that	O
a	O
large	O
capacity	O
had	O
to	O
be	O
introduced	O
suddenly	O
:	O
Pent	O
up	O
demand	O
for	O
long-distance	O
telephone	O
service	O
,	O
because	O
of	O
the	O
hiatus	O
during	O
the	O
war	O
years	O
,	O
and	O
the	O
new	O
medium	O
of	O
television	O
,	O
which	O
needed	O
more	O
bandwidth	B-Algorithm
than	O
radio	O
.	O
</s>
<s>
The	O
TDX	O
was	O
upgraded	O
to	O
the	O
TD2	B-Algorithm
system	O
,	O
which	O
used	O
[	O
the	O
Morton	O
tube	O
,	O
416B	O
and	O
later	O
416C	O
,	O
manufactured	O
by	O
Western	O
Electric ]	O
in	O
the	O
transmitters	O
,	O
and	O
then	O
later	O
to	O
TD3	O
that	O
used	O
solid-state	O
electronics	O
.	O
</s>
<s>
Remarkable	O
were	O
the	O
microwave	B-Application
relay	I-Application
links	O
to	O
West	O
Berlin	O
during	O
the	O
Cold	B-Application
War	I-Application
,	O
which	O
had	O
to	O
be	O
built	O
and	O
operated	O
due	O
to	O
the	O
large	O
distance	O
between	O
West	O
Germany	O
and	O
Berlin	O
at	O
the	O
edge	O
of	O
the	O
technical	O
feasibility	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
telephone	O
network	O
,	O
also	O
microwave	B-Application
relay	I-Application
links	O
for	O
the	O
distribution	O
of	O
TV	O
and	O
radio	O
broadcasts	O
.	O
</s>
<s>
Military	O
microwave	B-Application
relay	I-Application
systems	O
continued	O
to	O
be	O
used	O
into	O
the	O
1960s	O
,	O
when	O
many	O
of	O
these	O
systems	O
were	O
supplanted	O
with	O
tropospheric	O
scatter	O
or	O
communication	O
satellite	B-Application
systems	O
.	O
</s>
<s>
One	O
example	O
from	O
the	O
USA	O
is	O
the	O
RCA	O
CW-20A	O
1	O
–	O
2GHz	O
microwave	B-Application
relay	I-Application
system	O
which	O
utilized	O
flexible	O
UHF	O
cable	O
rather	O
than	O
the	O
rigid	O
waveguide	O
required	O
by	O
higher	O
frequency	O
systems	O
,	O
making	O
it	O
ideal	O
for	O
tactical	O
applications	O
.	O
</s>
<s>
The	O
typical	O
microwave	B-Application
relay	I-Application
installation	O
or	O
portable	O
van	O
had	O
two	O
radio	O
systems	O
(	O
plus	O
backup	O
)	O
connecting	O
two	O
line	O
of	O
sight	O
sites	O
.	O
</s>
<s>
These	O
radios	O
would	O
often	O
carry	O
24	O
telephone	O
channels	O
frequency-division	B-Protocol
multiplexed	I-Protocol
on	O
the	O
microwave	O
carrier	O
(	O
i.e.	O
</s>
<s>
Long-distance	O
microwave	B-Application
relay	I-Application
networks	O
were	O
built	O
in	O
many	O
countries	O
until	O
the	O
1980s	O
,	O
when	O
the	O
technology	O
lost	O
its	O
share	O
of	O
fixed	O
operation	O
to	O
newer	O
technologies	O
such	O
as	O
fiber-optic	B-Architecture
cable	I-Architecture
and	O
communication	O
satellites	B-Application
,	O
which	O
offer	O
a	O
lower	O
cost	O
per	O
bit	O
.	O
</s>
<s>
During	O
the	O
Cold	B-Application
War	I-Application
,	O
the	O
US	O
intelligence	O
agencies	O
,	O
such	O
as	O
the	O
National	O
Security	O
Agency	O
(	O
NSA	O
)	O
,	O
were	O
reportedly	O
able	O
to	O
intercept	O
Soviet	O
microwave	O
traffic	O
using	O
satellites	B-Application
such	O
as	O
Rhyolite	O
.	O
</s>
<s>
Much	O
of	O
the	O
beam	O
of	O
a	O
microwave	B-Application
link	I-Application
passes	O
the	O
receiving	O
antenna	O
and	O
radiates	O
toward	O
the	O
horizon	O
,	O
into	O
space	O
.	O
</s>
<s>
By	O
positioning	O
a	O
geosynchronous	O
satellite	B-Application
in	O
the	O
path	O
of	O
the	O
beam	O
,	O
the	O
microwave	O
beam	O
can	O
be	O
received	O
.	O
</s>
<s>
At	O
the	O
turn	O
of	O
the	O
century	O
,	O
microwave	B-Application
radio	I-Application
relay	O
systems	O
are	O
being	O
used	O
increasingly	O
in	O
portable	O
radio	O
applications	O
.	O
</s>
<s>
A	O
microwave	B-Application
link	I-Application
is	O
a	O
communications	O
system	O
that	O
uses	O
a	O
beam	O
of	O
radio	O
waves	O
in	O
the	O
microwave	O
frequency	O
range	O
to	O
transmit	O
video	O
,	O
audio	O
,	O
or	O
data	O
between	O
two	O
locations	O
,	O
which	O
can	O
be	O
from	O
just	O
a	O
few	O
feet	O
or	O
meters	O
to	O
several	O
miles	O
or	O
kilometers	O
apart	O
.	O
</s>
<s>
Microwave	B-Application
links	I-Application
are	O
commonly	O
used	O
by	O
television	O
broadcasters	O
to	O
transmit	O
programmes	O
across	O
a	O
country	O
,	O
for	O
instance	O
,	O
or	O
from	O
an	O
outside	O
broadcast	O
back	O
to	O
a	O
studio	O
.	O
</s>
<s>
Terrestrial	O
microwave	B-Application
relay	I-Application
links	O
are	O
limited	O
in	O
distance	O
to	O
the	O
visual	O
horizon	O
,	O
a	O
few	O
tens	O
of	O
miles	O
or	O
kilometers	O
depending	O
on	O
tower	O
height	O
.	O
</s>
<s>
Tropospheric	O
scatter	O
(	O
"	O
troposcatter	O
"	O
or	O
"	O
scatter	O
"	O
)	O
was	O
a	O
technology	O
developed	O
in	O
the	O
1950s	O
to	O
allow	O
microwave	B-Application
communication	I-Application
links	O
beyond	O
the	O
horizon	O
,	O
to	O
a	O
range	O
of	O
several	O
hundred	O
kilometers	O
.	O
</s>
<s>
Signal	O
clarity	O
obtained	O
by	O
this	O
method	O
depends	O
on	O
the	O
weather	O
and	O
other	O
factors	O
,	O
and	O
as	O
a	O
result	O
,	O
a	O
high	O
level	O
of	O
technical	O
difficulty	O
is	O
involved	O
in	O
the	O
creation	O
of	O
a	O
reliable	O
over	B-Application
horizon	I-Application
radio	I-Application
relay	I-Application
link	I-Application
.	O
</s>
<s>
Troposcatter	O
links	O
are	O
therefore	O
only	O
used	O
in	O
special	O
circumstances	O
where	O
satellites	B-Application
and	O
other	O
long-distance	O
communication	O
channels	O
cannot	O
be	O
relied	O
on	O
,	O
such	O
as	O
in	O
military	O
communications	O
.	O
</s>
