A Zero-Math Basics of Cellular Networks and Mobile Communications

If you enrolled in an Electronic or Telecommunication Engineering class in a university like the ones in my country, to learn the usual engineering stuff that always held your interest, you would be disappointed.

^{Free Pixabay Image}

Of course they would teach you some technical engineering stuff but you would spend most of your time doing math and studying ancient technology like the one I shared here in my last post. But that is not a problem because it turns out that if you understand the math and the ancient technology, you would do just fine with the new stuff. For instance, when it comes to telecommunications or telecommunication engineering, there are three things that never change:

• Information Source
• Transmitter
• Channel (Medium)
• Receiver

The system could be as simple as:

^{Wikimedia Commons: A Basic Communication System}

Or a little bit more complex like:

^{Wikimedia Creative Commons: Signal Processing Communication System}

Whether we are discussing Radio Broadcasting or Mobile Telecommunications, the basic principle remains that information from a source is fed to a transmitter where it is encoded and sent to the receiver through a medium which varies from cables to optic fibres to radio waves and microwaves. Whatever the medium or channel, the encoded information must be received at the other end and decoded by the receiver. In some cases, like in cellular communications, both the transmitter and the receiver are housed in a single device called a transceiver. Today, I would like to discuss Cellular Communications

Cellular Communication

When the first mobile communication carrier came to Nigeria around 2001, most people did not know what was going on especially the older generation. They noticed that many young people had formed the habit of looking at their palms while walking on the road. Something very interesting also seemed to be happening to the palm of their hands because they would often stop on the road, look at their palms and smile. It was really strange but what was stranger was their behaviour when they were constrained to locations with weak cellular network coverage. You would see people climb trees and walk up a mountain in search of cellular network. But all that are in the past now. There is widespread cellular network coverage as the number of users have grown to over 142 million active subscriptions in Nigeria and an estimated 7.4 billion subscriptions worldwide. That is more than the human population of the earth! Now we shall examine how cellular phones work.

Wireless Technology

All mobile phones work through a wireless technology. One of those wireless technology is the Cellular Network or Mobile Network. For simplicity, we define a cellular network as a communication network where the last link is wireless. This definition suggests that there could be some physically connected components of a cellular network but that last link which is the link that you and I are aware of, is wireless. A cellular network is made possible by sending and receiving signals through electromagnetic waves. At the risk of repeating it too many times, electromagnetic waves are undulating electric and magnetic waves that vibrate at the speed of light (3x10₈), capable of travelling thousands of miles from the point of propagation.

In cellular terminology, a cell phone (or mobile phone) or simply mobile is called a Mobile Station(MS).

When a person speaks into a mobile phone, the tiny microphone built into it receives the voice information in the form of a fluctuating audio waves and converts it into a fluctuating electrical signal. The audio signal comes out on the other end of the microphone as discrete measurements shown in red on the Digital Signal Processing image. A microchip inside the cellular phone takes this electrical signal and converts it into a string of binary (ones and zeros) numbers.

As was mentioned in the post about FM radio, audio signals must be embedded in a carrier signal before it is transmitted to the receiver. In an analogue system, the problems with transmitting audio (baseband) signal without embedding it in a carrier wave (modulation) could range from requiring huge antennas to low penetration of the transmitted signal. In digital communication such as cellular communication, modulation offers extra advantage in the form of squeezing as much data in as little frequency band as possible. This is called spectral efficiency.

Therefore, the digital signal from the microchip is embedded in a carrier wave through a variety of technologies ranging from amplitude shift keying (similar to AM), frequency shift keying (similar to FM) and several other combinations of modulation that improve spectral efficiency in cellular communication. Anyway, when the signal is embedded in a carrier wave, it is beamed out through the antenna of the mobile phone (mobile station) to the nearest cellphone mast which is called a Base Transceiver Station(BTS) which is part of the Base Station Subsystem(BSS). When the BTS receives the call, it routes it to the BSS which takes care of all calls that takes place within a local part of the cellular network called cell.

Cell

The cellular network is distributed over land areas called cells. Each cell is served by at least one BTS or optimally, up to three BTSs. These BTS provide the cells with the network coverage with which voice and data are transmitted from mobile phones to other cells and other transceivers and ultimately to the receiver. A cell typically uses a different set of frequencies from neighboring cells, to avoid interference but the same frequencies may be used for non-neighbouring cells.

Attached to the BSS is a register called Home Location Register and another called Visitor Location Register both of which are updated with information concerning cell phones that are located in the cell and cell phones which entered the cell perhaps through roaming. These would help the network make call routing calculations. When calls are made from a cellphone to another cellphone on the same network, the call is routed to the base station nearest to the receiver phone based on the information on the network then finally, from the base station, the call is routed to the destination phone.

Moving on to BTS

The Base Transceiver Stations contain the mast which house the high power antenna. These antenna's do the heavy lifting of transmitting the signal from the cell phone so that the cell phones won't have to. It ensures that the the cell phone can transit signals with as little power as possible. Without those masts, the battery requirement and the antenna requirements would be so high that they would be too cumbersome to lift. So the job of the cell phone is limited to receiving audio signal, digitizing it, embedding in a carrier wave, finding the nearest mast that would require the least power to beam the signal to it. Because the cell phone is always searching for the cell with the strongest signal, this would explain why the cell phone battery runs down faster in an area with low cellular network coverage. Once the signal is beamed, the BTS kicks in.

So, if A calls B, the call signal is sent to the mast nearest to A. This mast takes the signal to its base station from where it is routed to the mast nearest to B. The second mast would take the signal to its own station and it is routed to B's cell phone to be received by B. But cell phones are a mobile technology, so what happens when a caller drives through one cell to another cell?

The Handoff Procedure

Cell phone users are often on the move. Therefore it is to be expected that a user could move from one cell (a region covered by one BTS) to another cell (a region covered by another BTS). This is called roaming. As explained earlier, the cell phone is constantly searching for the strongest signal. As a passenger rides in a car and moves around a region, the strongest signal the cell phone would be receiving is the signal from its cell. However, the passenger passes from a region where the signal from that cell is strongest to a region where the signal of another cell is stronger, there would be a point where the first cell would "hand-off" the call to the cell with the stronger signal. This often happens without the notice of the passenger.

References

1. Information Week | IoT Will Surpass Mobile as Most Connected Device

2. DAEnotes | The Need for Modulation

3. The Electronic Design | Understanding Modern Digital Modulation Techniques

4. Wikipedia | Cellular Network

5. Explain That Stuff | Cell Phones

6. How Stuff Work: Electronics | Cell Phone
   
   

Thank you.