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Welcome to the lecture series of millimetre
wave technology so roughly by millimetre wave we understand a section of electromagnetic
spectrum of (())(0:37) of dimension ten millimetre to approximately one millimetre so it translate
to a frequency scale three hundred gigahertz to thirty gigahertz But we also have some practical applications
which are starting at twenty six point five gigahertz We have a band and also we have
some other applications at twenty four gigahertz so for practical applications by millimetre
wave we understand from twenty four gigahertz to three hundred gigahertz approximately So
now why do we need millimetre wave technology So for wireless communication already we have
some popular applications like Bluetooth wifi wireless LAN So for Bluetooth how much data rate it can
support So maximum in Kbps range in wifi or wireless LAN it can be increased to mega bit
per second Or (())(1:42) of mega bit per second but if we want gigabits per second Let’s
say through output so in that case we have only option that we have to increased the
carrier frequency to millimetre wave So not only that high resolution radar so high resolutions
imaging these are also possible by using millimetre wave frequency range So there are so many advantages at millimetre
wave frequency range then why don’t we use millimetre wave frequencies Why it’s not
so popular So obviously there are many problems and we have to solve this challenges some
of them are already solved So in this class we are going to learn all this challenges
How to solve them and umm slowly slowly we will see starting from the source to umm receive
here rece receiver so what are the different component we use at millimetre wave frequencies So let’s start with the band designation
So at If you look at the different band designation mark by this yellow color So the Ka band its
starts from twenty six point five gigahertz to forty gigahertz So the whole millimetre
wave spectrum it is divided into several bands So among these some popular bands are Ka band
then the V band which is from fifty to seventy five gigahertz then the W band seventy five
to one hundred and ten gigahertz frequency range And they are already in use for satellite
application for Radar applications and now people are trying to if use this band for
wireless communications So if I Let us just look at a circuit millimetre
wave circuit So this is a millimetre wave circuit which is fabricated on a printed circuit
board You can see the black color dielectric material and the shiny is the copper So if
I look at the left hand side it has actually two parts One high frequency component parts
which is millimetre wave frequency component parts and in the right hand side this is the
low frequency component parts So it is a its frequency range in micro f
frequency or in RF frequency range Now for any wireless system we need antennas so this
is an example of FMCW RADAR circuit so you can see these rectangular patches These are
actually the radiating antennas and at millimetre wave frequency the antenna dimension is very
small So a typically the resonating antennas its length is lambda g by two That means half
wave length at the operating frequency So at sixty gigahertz for example The wave length is already five millimetre
in free space and then half of it so its two point five millimetre and not only that when
we fabricate this antenna on some printed circuit board so its dimension further reduces
by a factor of root f side on arc Where f side on arc it is called the dielectric constant
of the substrate So at millimetre wave frequencies antenna dimension it becomes very small just
a few millimetre So as you can see in the circuit So this is
actually an array of antennas three by twelve elements So these antennas then it is followed
by low noise amplifier then by mixer So mixer it actually can down convert or UP convert
the carrier frequency We can use non-linearity of any active element like a PN junction diode
or a transistor to UP convert and down convert frequency So once millimetre wave frequency down convert
it to micro f frequency or RF frequencies then the conventional RF components or micro
f components can be used So this right hand side it consist of those micro f components So this is a typical RF umm millimetre wave
transmitter and receiver circuit there is conceptual diagram So in the transmitter circuit
you can see that we have a sixty gigahertz millimetre wave continuous source So which
is being modulated by the base band its coming from CMOS digital circuitry So the cost of
any millimetre wave system it depends on fabrication procedure So standard technologies in use are germanium
silicon and gallium arsenide material so in a group three group five like gallium arsenide
material the circuit performance is better but it is very expensive procedure So unless
we can minimize the cost it will not be applicable to consumer market So in CMOS or BICMOS procedure this cost is
quiet low but unfortunately CMOS or BICMOS process it cannot be umm used for millimetre
wave circuit design So in this particular example then we can use that gallium arsenide
or silicon germanium for six hundred and twenty gigahertz or millimetre wave generation and
for the basement part we can use the CMAS CMOS digital circuitry so then after modulation
umm an amplification it can be transmitted by one antenna At the receiver side we have one receiving
antenna then the receive signal it can be sixty gigahertz for example this one sixty
gigahertz pulse receiver then it is demodulated by CMOS digital circuitry So once it is demodulated
and down converted then we can use conventional CMOS digital circuitry So what is the advantage
of this system The advantage is that these systems can support
multi giga width communication So that means we can we use we can umm these millimetre
wave system for multi giga width communication without using any wiring So fiber optics it
can also support multi giga width communication but in that case we have to use wiring for
fiber optics So but millimetre wave communication it has
to face many challenges so one of them is atmospheric attenuation So this figure typically
it shows the attenuation in db per kilometre versus the wavelength So if you look at the
diagram we have a solid line and one dotted line so this dotted line it corresponds to
the measured attenuation in db per kilometre at a four kilometre height from the sea surface
sea level And the solid line it corresponds to attenuation
at sea level so if we measure attenuation at sea level it higher than that at higher
atmosphere now if I follow this curve so from left hand side to right hand side actual frequency
increases so at twenty two gigahertz we have some attenuation band This is due to the resonance of water molecule
H two O again we have a sixty gigahertz peak here where attenuation is very high this is
due to the absorption of oxygen umm so this is due to the resonance of oxygen molecule
so again if I further increase the frequency we have one more absorption band at one hundred
and eighteen gigahertz this is again due to the oxygen molecule and at one hundred and
eighty three gigahertz We have one more attenuation band which is
again due to water molecule so that means we have some propagation windows if we want
to communicate over long distance we have to use these windows So otherwise for example
if we used sixty gigahertz band for long distance communication so signal will be highly attenuated
So this attenuation it depends on frequency as well as height from the sea level So as
we see from this graph that at higher atmosphere this attenuation is much smaller compare to
sea level So not only that fog rain and sand storm dust
storm so all of these weather condition also affect millimetre wave propagation So this
is a typical attenuation plot umm with rain rate with difference frequency range so if
you follow any one curve so lower rain rate for example point two five millimetre per
hour to one point two five millimetre per hour So for this range the attenuation is typically
at millimetre wave frequencies one to two db or kilometre but for moderate rain for
example five to twenty five millimetre per hour it increases to ten db per kilometre
and for a cloud burst typically one hundred and fifty millimetre to two hundred and fifty
millimetre per hour this attenuation can increases to as high as hundred db per kilometre So why this attenuation happened because the
droplet rain droplet it scattered millimetre wave frequencies and because it’s the size
of the rain droplet is comparable to millimetre wave wavelength so that’s why this effect
is more prominent at millimetre wave frequency compared to the lower frequencies For example
umm if I follow any curve lets say for moderate rain rate twenty five millimetre per hour So below X band the attenuation its showing
two db per kilometre but in millimetre wave frequency range typically its increasing to
ten db per kilometre so this is a problem for long distance communication For example
for a satellite link So for satellite it has to communicate between
ground based system to satellite which is umm which can be umm as high as thirty six
thousand kilometre above the earth surface So we have to keep in mind when we are going
to design any millimetre wave system rain rate it can affect the channel performance So now long distance communication really
possible at millimetre wave frequencies So we have actually one formula we call it Friis
law by which we can calculate the free space path loss at different frequency range So
free space path loss it can be given by twenty log bas ten four pie r by lambda in decibel
So where r is the free space distance lambda is the wavelength So in this expression we
are not considering any atmospheric effect this is just due to the increasing distance
between transmitter and receiver So why if umm this loss increases with distance
you can consider one transmitter lets say it is placed at the origin of a polar co-ordinate
system and it is transmitting in all the direction now I have an receiving antenna so the received
power by receiving antenna depends on the effective aperture of this receiving antenna
So we can consider is fea is spherical surface and the receiving antenna is placed on that
surface Now the intensity on that spherical surface
it obviously depends on a distance arc So if I increase the distance from origin then
the power it decreases with r square So this law we call the path loss and its given by
this expression so it so its function of r as well as lambda the wavelength So here are
some calculated values of free space path loss for example this first table showing
the path loss value at r equal to ten meter so at two point four gigahertz this typical
path loss value is sixty db now if I increase the frequency to sixty gigahertz where free
space wavelength is five millimetre so this path loss increase to eighty eight db So not only that if I keep on increasing the
frequency lets say three hundred gigahertz (())(15:39) millimetre wave frequency range
so it further increases to hundred and two db so now if I recalculate this values at
r equal to one Km so at two point four gigahertz we can see the previous path loss was sixty
db now it increased to hundred db and at sixty gigahertz from eighty eight db it now increases
to one hundred and twenty eight db So if you send one watt of power at sixty
gigahertz it will be attenuated by one hundred and twenty eight db at one kilometre so this
calculation does not consider the attenuation due to atmosphere This is just the free space
path loss So this picture is showing a typical millimetre wave antenna a parabolic reflector
and this is the feeder horn So now what are the advantages and disadvantages
of millimetre wave communication So already we have seen that millimetre wave have high
atmospheric attenuation so if we increase frequency so attenuation will increase And
not only that at millimetre wave frequency we have some transmitting window so for any
wireless link so we have to use those windows to get minimum attenuation Rain feed that is another problem and we have
seen that attenuation it increases with rain rate so similarly it is also attenuated by
fog it is attenuated by sand storm dust storm So weather condition it affects millimetre
wave propagation Humidity that also has an degrading effect on millimetre wave propagation So at sixty gigahertz we have seen that attenuation
due to oxygen its quiet high it can be ten to fifty db per kilometre but if I go to that
window lets say seventy to eighty gigahertz frequency band so in that band the attenuation
due to oxygen is very small lets say point two to point three db per kilometre but in
humid condition lets say humidity is almost hundred percent this attenuation can increase
to three to four db per kilometre just due to humidity Then next is surface appear rougher so diffused
reflection increases So multipath propagation that cause another problem so particularly
from reflection from indoor walls and surfaces causes serious fading So this fading problem
you might have experienced even at lower frequencies for example FM radio So it might have notice
that the radio when I place at one corner of the room its working but if I place it
at other corner of the room it’s not working so this is due to fading So we have actually reflection of the signal
from walls from doors from furniture and this reflective signal from the various sources
the umm they provide they produces in constructive and distracting intervance so which is will
be a function of space so at some points then you will get some signal at some points of
the in the same room you may not get any signal so this is called fading effect And it’s very prominent at millimetre wave
frequency so even just umm if you displace your comp receiver by a few centimetre you
your signal level it will degrade So Doppler shift another problem So doppler
shift as we know that it depends on the frequency as well as it increases with the velocity
So but at the millimetre wave frequency the frequency is so high that even at pedestrian
speed this shift can be significant So whenever we are going to use any portable device we
have to be very careful about this Doppler shift So it should be consider in our design
and another problem is shadowing problem So at lower frequencies for example one point
eight gigahertz to two point four gigahertz wireless LAN application so electromagnetic
signal actually it can bend around our body this is due to the effect of distraction but
at millimetre wave frequencies the wave length is so small that it can bend around our body
so what we see what we observed at optical wavelength shadow effect so similar effect
we experience at millimetre wave frequency so if I have a transmitter sitting just at
back of me so in front of me you can’t get any signal so it’s called the shadowing
effect Millimetre wave it travels solely by line
of sight and are blocked by building walls and attenuated by foliage So long distance
communication its in question so only line of sight communication may be possible so
but it also has one advantage that we can design impact communication networks which
is called WPAN system through frequency reuse Os for example lets say we are designing a
wireless LAN like system in one building which is using lets say seventy seven gigahertz
frequency range and we know that it will be highly attenuated outside the building so
just in the next building we may not get any signal from that building so in that building
then we can in the second building we can use the same frequency spectrum to design
another wireless wireless LAN like system So this is called frequency reuse and highly
dense network is feasible due to it So millimetre wave it shows optical propagation
characteristics so that’s means it can be easily reflected or focused by small medium
surfaces So even a few square feet of antenna is sufficient to generate (())(22:27) like
beam and it is diffracted by building edges So the wavelength at millimetre wave as we
have discussed that its ten millimetre to one millimetre and for resonating antennas
which length is typically half wavelength so its then five millimetre to point five
millimetre So that means the antenna size it decreases
So we can design arrays of antenna at millimetre wave frequency easily because antenna size
is small so a an array of antenna we can easily fit over lets say a square feet area So potential application very high resolution
radar communication links typically more than ten gigabit per second So before starting the next part let me discuss
about a Sir J C Bose’s work So you will be surprised to know that the first millimetre
wave system were build by professor J C Bose in Kolkata in west bangal So in one thousand
eight hundred and ninety five actually umm he demonstrated a millimetre wave system and
its typically working at sixty gigahertz frequency band and he communicated over a distance of
twenty three meters So at that time umm these different sources
of millimetre wave frequencies how to transmit it How to receive it How to design different
components It so all of these were not known at that time so he has to design all of these
components starting from the transmitter How to receive it And then he had shown the refraction property
refract refraction property and the polarization of millimetre wave and also he has shown that
millimetre wave its nothing but another form of electromagnetic wave which follows more
or less optical properties So in his experiment typical wave length he used starting from
two point five centimetre to five millimetre so roughly it correspond to twelve gigahertz
to as high as sixty gigahertz So this an replica of the original system
design by Professor JC Bose in Kolkata So you can see this a box so inside this box
we have millimetre source which is nothing but a spark gap and then he used a tube metallic
hollow tube as the transmitter now we know that it behaves as an antenna and we have
a (())(25:33) table here So where he can use a umm prism and other
structure to show the refrect refraction and reflection property and on the right side
we have a receiver so this receiver receives millimetre wave signal then how to sense this
millimetre wave signal so he designed one detector which is a metal semi conductor junction
I am going to show the picture in next slide so when millimetre wave signal falls on this
detector it provides DC current simply DC voltage so we can then detect this DC voltage
by using a galvanometer So in this slide you can see the different
component used by Professor JC Bose this bottom right corner it shows this spark gap so we
have two gaps between this two pin High voltage is applied between these two pins which generate
sparks so sparks it content a white electromagnetic spectrum starting from optical wave to RF He used the millimetre wave frequency range
so he placed this spark gap inside this box And then you can see the transmitting part
here so inside we have the spark gap so from which electromagnetic wave is being transmitted
by this pipe now we call it horn antenna So this is a prism made of dielectric material
so to show the refraction property of electromagnetic signal and this is the reviver side So at
the receiver you can see top right figure so we have a point contact detector so this
is it It is made up a metal needle and bottom side
it’s a semi conductor material its galena lead sulphite so it has non linear characteristics
so almost similar to a P N junction and when electromagnetic signal falls on this it provides
DC voltage so then by measuring this DC voltage we can detect electromagnetic signal so he
invented this type of detector so it’s a long time back in one thousand eight hundred
and ninety five these are some polarisers So this umm this middle part you can see some
parallel metallic wire so when electromagnetic signal falls on it so if the electric field
is a perpendicular to this metallic wire it can pass through this wire mesh but if the
electric field is parallel to this wire then dif diffraction of the component incident
electromagnetic wave it will pass through this wire mesh so similarly so he designed
some other polarisers so its nothing but your book He used several metallic plates inside the
books so this plates are parallel to each other then any electromagnetic wave which
whose electric field is parallel to this metal it can pass through this book He also designed
another polarizer this is made of jute and inside he used different wires So we see that even at in the one thousand
eight hundred and ninety five long time back J C Bose in Kolkata He designed the world’s
first millimetre wave systems so after professor J C Bose’s work almost fifty to sixty year
there was no work at millimetre wave frequency So again its started at during Second World
War one thousand nine hundred and forty so now we will take a short break then again
we will start

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