For most of us, a mobile phone is a part of our lives, but I am sure your curious minds have always been struck by such questions as to how a mobile phone makes a call, and why there are different generations of mobile communications. Let’s explore the technology behind mobile communications.
To understand the sophistication of the cell phone let's compare it with walkie talkie. Walkie talkies are half duplex devices that mean when 2 people communicating; use the same frequency. So the only one person can talk at a time. Whereas a cellphone is a full duplex device that means you can use one frequency for talking and a second frequency for listening. So both people can talk on the call at once.
When you speak on your phone, your voice is picked up by your phone’s microphone. The microphone turns your voice into a digital signal with the help of an MEMS sensor and IC (Fig:1A). The digital signal contains your voice in the form of 0s and 1s. An antenna inside the phone, receives these 0s and 1s and transmits them in the form of electromagnetic waves (Fig:1B). Electromagnetic waves transmit the 0s and 1s by altering the wave characteristics such as the amplitude, frequency, phase or combinations of these. For example, in the case of frequency, 0 and 1 are transmitted by using low and high frequencies respectively.
So if you could find a way to transmit these electromagnetic waves to your friend’s phone, you would be able to establish a call, however electromagnetic waves are incapable of travelling long distances. They lose their strength due to the presence of physical objects, electrical equipment and some environmental factors as shown in Fig:2.
In fact if there were no such issues, even then electromagnetic waves would not carry on forever due to the earth’s curved structure (Fig:3). To overcome these issues, cell towers were introduced using the concept of cellular technology.
In cellular technology, a geographic area is divided into hexagonal cells (Fig:4). Hexagonal cell shape is perfect over square or triangular cell shapes in cellular architecture because it covers an entire area without overlapping, which means they can cover the entire geographical region without any gaps. Each cell is having having its own tower and frequency slot. This is the tower whose signal strength is shows in phone screen. Generally, these cell towers are connected through wires, or more specifically, optical fiber cables. These optical fiber cables are laid under the ground, or the ocean, to provide national or international connectivity.
The electromagnetic waves produced by your phone are picked up by the tower in your cell (Fig:5A), and converted them into high frequency light pulses (Fig:5B). These light pulses are carried to the Base Transceiver Box, located at the base of the tower, for further signal processing. After processing, your voice signal is routed towards the destination tower (Fig:5C). Upon receiving the pulses, the destination tower radiates it outwards, in the form of electromagnetic waves, and your friend’s phone then receives the signal. This signal undergoes a reverse process, and your friend hears your voice (Fig:5D). So it’s true that mobile communications are not entirely wireless, they do use a wired medium too.
This is how mobile communications are carried out, however there is a big issue that we intentionally left unanswered. Mobile communication is only successful when your tower transfers the signal to your friend’s tower, but how does your tower know in which cell tower area your friend is located? Well for this process the cell tower gets help from something called a Mobile Switching Center. The MSC is the central control for a group of cell towers and database (Fig:6).
Before moving further, let's explain more information about the MSC. When you purchase a SIM card, all the subscription information is registered in a specified MSC. This MSC will be your Home MSC. The Home MSC stores information such as service plans, your current location and your activity status. If you move outside the range of your home MSC, the new MSC, which serves you instead, is known as a Foreign MSC. As you enter a foreign MSC region, it communicates with your home MSC (Fig:7). In short, your home MSC always knows which MSC area you are in. In foreign MSC, your phone is registered temporarily so that you can receive and place calls from outside the home MSC area.
To understand in which cell location a subscriber is, within the MSC area, the MSC uses a few techniques.
1. One way is to update the subscriber location after a certain period.
2. When the phone crosses a predefined number of towers, the location update is again done.
3. The last one of these is when the phone is turned on.
By location update procedure it become easy for MSC to locate a phone at the time of call request.
Let’s try to understand all of these procedures with an example. Suppose Emma wants to call John. When Emma dials John's number, the call request arrives at Emma's home MSC as shown in Fig:8A. Upon receiving John's number, the request will be forwarded to John's home MSC, now, John's MSC checks for his current MSC. If John is in his home MSC, the call request will be immediately sent to his current cell location. The cell tower of the current cell broadcast the John’s number to check whether John is engaged on another call, or if his mobile is switched off. If everything is positive, John’s phone receives this message and responds to its cell tower by identifying itself. Upon identification, John’s phone rings, and the call will be connected. Upon connection, both MSC instructs the respective cell towers to move the call on their unused frequency (Fig:8B). This whole process takes less than 3 sec.
However, if John is not in his home MSC, John’s home MSC simply forwards the call request to the foreign MSC. The foreign MSC will follow the previously explained procedure to locate John’s phone, and will then establish the call (Fig: 9).
Now, let’s discuss why the frequency spectrum is quite important in mobile phone communications. To transfer 0s and 1s in digital communication, each subscriber is allocated a frequency range. However, the frequency spectrum available for cellular communications is quite limited and there are billions of subscribers. This issue is solved with the help of 2 technologies.
In the Frequency slot distribution, different frequency slots are carefully allocated to different cell towers (Fig: 10A). This distribution has the advantage of using the same frequency slot for different cell towers. But there is a catch here, neighbouring cell towers are not allocated the same frequency slot. This is done in order to restrict your phone from receiving signals for neighbouring cell towers. So during the call if you are moving into neighbouring cell, you are allocated different frequency from your neighbouring cell tower without call drop. This process is known as handoff or handover. The decision of frequency allocation was made by MSC in 1G whereas from 2G to till now, this decision is taken by mobile itself.
Another challenge is managing several users at the same time within a cell. Here the cell tower frequency has to be shared amongst several users. This problem is solved by using one of the multiple access technique. In the multiple access technique, this frequency slot is efficiently distributed amongst all the active users in the cell area (Fig: 10B).
Now, the big question, why are there different generations of mobile phone technologies?
From 1G to 4G, the world has observed a drastic increase in the number of cell phone users. The main aiding factor to that increase has been significant changes in terms of data speed that were observed with the introduction of every new generation.
1G originally allowed users, for the first time, to carry a phone without a cable attached to it (Fig:11), but 1G suffered from two major problems. The first problem was that the wireless transmission was in an analog format. Analog signals are easily altered by external sources. So it provided poor voice quality and poor security. The second problem was that it used the Frequency Division Multiple Access (FDMA) technique, which used the available spectrum in an inefficient way. In FDMA, total available spectrum is chop up into frequency slots. Each user is allocated a unique frequency slot. During the period of the call, no other user could share the same frequency slots.
These factors paved the way for the second generation of mobile communications (Fig:12). 2G used digital multiple access technologies, namely Time Division Multiple Access (TDMA) or Code Division Multiple Access (CDMA) technology. TDMA system allows user to share common frequency spectrum but in different time slots. In CDMA each users information coded with unique code and transmit those codes over the common frequency slot. The second generation also introduced a revolutionary data service, SMS and Internet browsing.
3G technology was focused on giving a higher data transfer speed (Fig:13). It used a Wide Code Division multiple access (WCDMA) technique, along with an increase in bandwidth, to achieve this. The 3G speed of 2mbps, allowed the transfer of data for uses such as GPS, videos, voice calls etc. 3G was a huge step in the transformation of the basic phone to a smartphone.
Next came 4G, which achieved speeds of 20 to 100 mbps, this was suitable for high-resolution movies and television (Fig:14). This higher speed was made possible due to Orthogonal Frequency Division Multiple Access (OFDMA) Technology and Multiple Input Multiple Output (MIMO) technology. In OFDMA, available range is split into a large number of smaller range known as subcarriers. These subcarriers are mathematically orthogonal to each other and each of them are modulated individually. MIMO uses multiple transmitter-receiver-antennas inside both the mobile phones and the towers.
The Next Generation of Mobile communication (5G), to be rolled out soon, will use enhanced MIMO technology and millimeter waves. It will provide seamless connectivity to support the Internet of Things, such as driverless cars and ‘Smart’ homes (Fig:15).
This article is written by Prerna Gupta, a post graduate in Control and Instrumentation. Currently she is working at Imajey consulting engineers pvt. ltd. as a Visual Educator. Her areas of interest are Telecommunication, Semiconductor Material and devices, Embedded systems and design. To know more about the author check this link