- [Narrator] Use of the internet on the go
or when making mobile phone calls is made possible
thanks to the invisible electromagnetic waves
that mobile phones emit or receive.
However, did you know
that this electromagnetic frequency range
or spectrum is a highly precious resource?
In 2008 the U.S. government
made a whopping 19 billion dollars
through a spectrum auction.
In this video we will look at the scientific reasons
why this invisible electromagnetic frequency range
is such a valuable resource.
Whenever you use your phone,
either for internet use or making a voice call,
you send or receive information
using invisible electromagnetic waves.
These electromagnetic waves are picked up
by your cell tower,
transmitted towards the destination tower,
and then finally to the end user.
In cellular communication, geographical areas
are divided into different cells
but the difficult thing in cellular communication
is that if there are five users in a cell area,
then five different frequencies should be used
to transmit their signals.
To understand why, let's consider an example.
Assume balls of three different colors
are ejected from a toy like a water fountain.
The red ball is supposed to reach the red cup
and the blue ball the blue cup, etc.
In this arrangement there is no guarantee
that this condition will be met.
However, if you assign a particular size
to a particular color,
this condition can then be easily met.
This is exactly what we do in cellular communication.
The cell tower takes the place of the fountain
and users take the place of the cups.
In a similar way to having balls of different sizes,
we make sure that each user's frequency is unique.
This way your mobile phone will only accept the signal
which has your frequency assigned to it.
All other frequency signals will be rejected
by your device's filter circuit.
This also means each active user in the world
has to have a unique frequency,
otherwise you will overhear the signal of someone else.
Throughout this discussion we have kept on saying
that each user has a unique frequency.
However, to be precise, we should've used the words
frequency range instead of frequency.
Antennas are capable to transmit signals
into frequency range or band.
Your phone's filtering circuit also allows a frequency band.
Moreover, we need a modulation technique
to transmit any signal.
If we are using frequency modulation,
we require two frequencies to transmit signals.
The upper frequency can represent one
and the lower frequency can represent zero.
This could lead to some questions about privacy.
What happens if a hacker sets our frequency in his device
and tries to get access?
However, it's not that simple.
The signals sent over the frequency are protected
using encryption which means only your device
will be able to decrypt it.
In total, there are approximately five billion
cell phone users in the world
and because the entire frequency range
of the electromagnetic spectrum is not useful
for cellular communications for various reasons,
assigning a unique frequency range to each user
is an impossible task.
Let's look at the reasons why the frequency band is limited
and how we are overcoming this issue
using modern technology.
The electromagnetic spectrum
ranges from radio waves to gamma rays.
The higher frequencies of electromagnetic radiation
have the worst penetration capabilities
so we don't use those frequencies
for cellular communication.
The lower frequencies of electromagnetic spectrum
have lower energy that would require a larger antenna
for transmission
and this is practically impossible to design.
In the next portion
of the electromagnetic spectrum as shown,
the propagation is dependent
on the properties of the ionosphere layer.
So due to all of these restrictions,
only a small portion
of the electromagnetic spectrum is left.
These frequency slots
are also used for several other purposes
which further reduces cellular technology's share of them.
The spectrum is one of the resources
which is normally managed by a government body
in most countries.
The government earns
by distributing various portions of the spectrum
between companies using an auction or a lottery system.
In the United States two agencies,
the FCC and NTIA govern the radio spectrum
whereas in Europe the CEPT does the job.
At the global level, the ITU,
International Telecommunication Union,
manages all technical things related to the spectrum.
The frequency modulation we explained at the beginning
is already obsolete.
It was used in 1G and analog form.
In the fourth generation, a technique called QAM is used
in which we can send more bits at a time.
Let's see how it works.
In QAM, variations are made to the amplitude and phasing
to generate different digital signals.
One such example is shown here.
Hence we see that there are limited frequency ranges
available for cellular communication
so it becomes necessary to use these ranges efficiently.
To achieve this,
the cellular system divides the available range
into a number of frequency slots
and these frequency slots
are then distributed to several towers.
This distribution has the advantage
of using the same frequency slot for different cell towers
but there is a catch here.
Neighboring cell towers
cannot be allocated the same frequency slot.
This is done in order to restrict your phone
from receiving signals meant for neighboring cell towers.
Assigning a different frequency slot
for different cell areas
is known as frequency slot distribution.
So during a call if you are moving into a neighboring cell,
you are allocated a different frequency
from your neighboring cell tower
without the call dropping.
The next challenge
is to manage all the active users
within a cell area at the same time.
Here the cell tower frequency
has to be shared amongst several users.
This challenge is solved
by using one of the multiple access techniques.
These multiple access techniques
can be understood by an analogy.
Think about how 100 musicians can record a song.
They can record in different rooms or at different times
or in different languages.
Similarly in cellular communication,
the cell tower and multiple users
can share information over a different frequency slot
or in a different time slot
or using different encoding techniques.
In cellular communications each generation
has introduced different multiple access techniques.
The main focus of these techniques
has been to utilize the available spectrum efficiently.
For instance, 4G came up with OFDNA
and OFDNA available range
is split into a large number of smaller ranges
known as sub-carriers.
These sub-carriers
are mathematically orthogonal to each other
and each of them are modulated individually.
We hope this video gives you a clear understanding
about what is spectrum
and how your government is able to garner a huge revenue
just by acting as a regulatory body.
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Thank you.