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Spectrum and encrypted signal- Beginning

Use of the internet on google 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 article, 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 colures 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 colour, 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 to take the place of the cups. In a similar way to having balls of different sizes, we make sure that each users 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 devices 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 word frequency range instead of frequency.

How signals are transmitted?

Antennas are capable to transmit signals into frequency range or band. Your phones 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 to 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 communications.

The lower frequencies of the 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, 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. This frequency slots are also used for several other purposes which further reduces cellular technology share of them. The spectrum is one of the resources which is normally managed by a government body in most countries.

Role of Government

The government earns 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 analogue 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.

QAM and Challenges

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 this 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. Neighbouring cell towers cannot be allocated the same frequency slot.

This is done in order to restrict your phone from receiving signals meant for neighbouring 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 neighbouring cell, you are allocated a different frequency from your neighbouring 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 is modulated individually.

We hope this article gives you a clear understanding of what is a spectrum and how your government is able to garner a huge revenue just by acting as a regulatory body.

Thank you.

About Admin MC3

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