How Cell Phones work
D. Indumathi, The Institute of Mathematical Sciences, Chennai

Practically every adult person you know owns a cell phone, right? Perhaps you have one of your own too. Cell phones have become even more popular and handy than TVs, and are perhaps considered even more necessary than fridges and washing machines which are so useful.
What is a cell phone and how does it work? Unlike its first and early version, cell phones can do a lot more than just make and receive phone calls. In fact, they are handy computers and almost just as powerful. With a cell phone you can also send or receive e-mail, access the internet for information or news, send or receive text messages and take photos or videos and share them as well. You don't expect a landline telephone to take photos or send e-mail. In fact, at its most basic, a cell phone is a radio -- an extremely sophisticated radio! So what makes the cell phone so special and versatile?

The "cell" in the cell phone
Earlier, in developed countries, doctors and policemen had radio telephones in their cars. There was one central antenna tower per city, and perhaps 25 frequencies (channels) available on that tower. Each call was made on a different frequency. (If two people used the same frequency, there would be cross-talk between the two calls and total confusion would ensue). So only a few people could use radio telephones because there just were not enough channels.
The break-through occurred when the cellular system was installed. Here, the city is divided into small cells. So the same frequency can be re-used across a city, so that millions of people can use cell phones simultaneously.

Duplex systems
A walkie-talkie is a half-duplex device. Only one person can talk at a time. That's why you may have heard people in movies using walkie-talkies say "Over and out". This is because both the people on the call use the same frequency, so only one person can talk at a time. A cell phone is a full-duplex device. That means that you use one frequency for talking and a second, separate frequency for listening. Both people on the call can talk at once. So a walkie-talkie typically has one channel, a typical cell phone can communicate on 1,664 channels or more. That's why you can put one call on hold and talk to another person. Or even have group calls.

How does the cell help? If you are travelling in a moving vehicle, the cell phone switches cells as they move around. That is, the phone latches on to the signal from a different cell phone tower. These cells give mobile phones incredible range. Someone using a cell phone can drive hundreds of kilometers and maintain a conversation the entire time because of the cellular approach. In fact, you may have heard an irritating passenger talking loudly on his phone during an entire bus or train ride! But the bottom line is that each phone only accesses a nearby tower and so does not need a very high power transmitter. Because of this, they have compact batteries.

BOX on Base Stations and Cell Phone Towers
Each cell has a base station that consists of a tower and a small building containing the radio equipment. The tower make a wireless connection to the mobile phone. Each carrier in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of the phone connections to the normal land-based phone system and controls all of the base stations in the region.
It is very common to see cell phone towers rising into the air on a lattice steel structure. Mostly they are on the top of buildings. The antennae on the tower are clearly visible. If you look at the base of a tower, you can see provider equipment.
The tower typically supports antennae and one or more sets of transmitter/receivers transceivers, digital signal processors, control electronics, a GPS receiver for timing, primary and backup electrical power sources, and sheltering. The box houses the radio transmitters and receivers that let the tower communicate with the phones. The radios connect with the antennae on the tower through a set of thick cables. If you look closely, you will see that the tower and all of the cables and equipment at the base of the tower are heavily grounded (electrically earthed).
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BOX on Cell phone Codes
. Electronic Serial Number (ESN): a unique 32-bit number programmed into the phone when it is manufactured.
. Mobile Identification Number (MIN): a 10-digit number derived from your
phone's number.
. System Identification Code (SID): a unique 15-bit number that is
assigned to each carrier by the Federal Communications Commission (FCC).
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How a call is made
A typical large city can have hundreds of towers. Each carrier (such as BSNL, Airtel, etc) in each city also runs one central office called the Mobile Telephone Switching Office (MTSO). This office handles all of the phone connections to the normal land-based phone system and controls all of the base stations in the region. It does this by storing all mobile sim data. All calls are directed to other cities and countries through MTSO. Every city has 1 MTSO that is connected to many mobile towers through fibre optics.
All cell phones have special codes associated with them. These codes are used to identify the phone, the phone's owner and the service provider. (See the Box). What happens during a call?
When you first power up the phone, it listens for an SID (see box) on the control channel, which is a special frequency that the phone and base station use to talk to one another. When it receives the SID, the phone compares it to the SID programmed into the phone. If the SIDs match, the phone knows that the cell it is communicating with is part of its home system.
Along with the SID, the phone also transmits a registration request, and the MTSO keeps track of the phone's location in a database -- this way, the MTSO knows which cell you are in when it wants to ring your phone.
Now, when your friend calls you, the MTSO gets the call, and tries to find you. It looks in its database to see which cell you are in. Then it communicates with your phone over the control channel to tell it which frequencies to use, and then the call is connected. Now, you are talking by two-way radio to your friend!
When you are in a moving vehicle, the base station realises that your signal strength is decreasing. But there is another base station that you are moving towards, which sees that your phone's signal strength is increasing. The two base stations coordinate with each other through the MTSO, and at some point, your phone gets a signal on a control channel telling it to change frequencies. This handoff switches your phone to the new cell. As you travel, the signal is passed from cell to cell. Amazing, isn't it?

Phone roaming
Let's say you're on the phone and you move from one cell to another -- but the cell you move into is covered by another service provider, not yours. Instead of dropping the call, it'll actually be handed off to the other service provider. If the SID on the control channel does not match the SID programmed into your phone, then the phone knows it is roaming. The MTSO of the cell that you are roaming in contacts the MTSO of your home system, which then checks its database to confirm that the SID of the phone you are using is valid. Your home system verifies your phone to the local MTSO, which then tracks your phone as you move through its cells. And the amazing thing is that all of this happens within seconds. Of course, you will be charged more for "roaming calls" but the system usually warns you about this.

The SIM card
Of course, the phone will not work without a SIM card from the service provider. SIM or Subscriber identity module is an integrated circuit which securely stores the international mobile subscriber identity (IMSI) number and its related key, which are used to identify the subscriber. This smart card is usually made of PVC with embedded contacts and semiconductors, with the SIM as its primary component.
The SIM contains a unique serial number (ICCID), international mobile subscriber identity (IMSI) number, security authentication and ciphering information, temporary information related to the local network, a list of the services the user has access to, and two passwords: a personal identification number (PIN) for ordinary use, and a personal unblocking key (PUK) for PIN unlocking.

BOX on The First Cell Phone Call
The very first cell phone call was placed in 1973 from a Manhattan, New York, street corner by Martin Cooper, the manager of Motorola's cellular phone program, to his counterpart at AT&T, which was a rival phone company. Cooper doesn't recall his exact words, but essentially he told his rival that he had beaten AT&T! Another decade passed before the first mobile phone made it into consumer hands. It weighed more than 1 kg (small laptops weigh this much!) and cost nearly $4,000 in 1983. That's about Rs 2 lakh 40 years ago.
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Digital versus Analog
Modern cell phone technology is digital-signal based, while older 1G technology was analog based. What does this mean? It depends on the kind of signal being transmitted.
An analog signal is any signal that varies continuously with time. For instance, if someone is singing, then the amplitude (loudness) as well as the pitch (frequency) of the signal changes with time in a continuous way. This can be transmitted on a microphone/speaker system, for instance, as waves of different amplitude and frequency. The waves may be electrical waves instead of the original sound waves. That is, the sound wave is converted into a matching electrical wave which is transmitted by wires from the microphone to the loud-speaker which then converts it back to audio for us to hear. 
In a digital signal, there is no continuous wave. The signal is transmitted as a signal with discrete values, which can only take on the values 0 and 1 (called binary system). The same singer's voice can be transformed into a digital signal by sampling. The analog signal is sampled at short intervals, say, every 50 microseconds. Each reading is then converted to digital 0s and 1s so each reading is represented with a fixed number of bits. The resulting stream of numbers is stored as digital data and forms the digital signal.
Analog systems can't be compressed and manipulated as easily as true digital signals. This is why cable companies switched to digital -- to fit more channels within a given bandwidth. Digital phones convert your voice into binary information (1s and 0s) and then compress it. This compression allows between three and 10 digital cell phone calls to occupy the space of a single analog call.

BOX on Cell Phone Network Technologies
You must have heard that 5G phones are now the craze. What does this mean? 2G and 3G were the earlier technologies which are called network protocols. Here G stands for generation. While 2G was the first digital technology (1G is analog, not digital), 3G came along because of the need for accessing data rather than just making phone calls or using SMS messaging services.
3G technology was intended for the true multimedia cell phone -- typically called smartphones -- which could access web-based applications such as watching youtube videos which need more bandwidth. It actually contains many technologies within it. While 3G networks take about 15 seconds to download a 3-minute MP3 song, the fastest 2G phones took about 8 minutes! You would never listen to songs let alone watch movies if the technology had remained 2G (I wonder whether that is a good thing or a bad thing!) So 3G phones are like mini-laptops.
The original 4G network was simply due to an increase in speed. But what made all the difference was 4G-LTE. LTE stands for Long Term Evolution and signifies not just higher speeds but new hardware that is optimised for data and voice calls at the same time. In older 3G networks, a different system was required for voice calls and for network/data. LTE supports everything -- data, voice (VoLTE), instant messaging and video -- over a single interface. It is also nearly 10 times faster than 3G networks. Obviously 5G will be even faster, about 100 times faster than 4G! This is because of improvements both in the phone and base station's hardware as well as software optimisation. The graph show the comparison between the different technologies.
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What's inside your digital phone
Cell phones are some of the most intricate devices people use on a daily basis. Modern digital cell phones can process millions of calculations per second in order to compress and decompress the digital voice stream that is being transmitted by them. But more amazing still is the software that drives the hardware. The Android Smartphones have advanced software platforms. Android Operating System (OS) has been the best-selling OS worldwide on smartphones since 2011. Many phones such as Nokia advertise themselves as pure Android phones. Others such as Samsung also use Android software but adapt it somewhat for their optimum use. Apple phones such as i-phone use different software.
All modern cell phones contain only a few individual parts:
. A circuit board containing the brains of the phone
. An antenna
. A liquid crystal display (LCD) or LED display
. A microphone
. A speaker
. A battery
apart from cameras and perhaps even a GPS.
The circuit board is the heart of the system. Its microprocessor handles all of the housekeeping chores for the display, including the keyboard, and deals with command and control signaling with the base station and also coordinates the rest of the functions on the board.
Cell phones have such tiny speakers and microphones that it is incredible how well most of them reproduce sound. What is amazing is that all of that functionality -- which only 30 years ago would have filled an entire floor of an office building -- now fits into a package that sits comfortably in the palm of your hand!

Sources: https://electronics.howstuffworks.com/smartphone5.htm, https://en.wikipedia.org/wiki/SIM_card