Millions of people in the India and around the world use cellular phones. They are such great gadgets — with a cell phone, you can talk to anyone on the planet from just about anywhere!

But have you ever wondered how a cell phone works? What makes it different from a regular phone? What do all those confusing terms like PCS, GSM, CDMA and TDMA mean?

Here in this  page we will discuss the technology behind cell phones so that you can see how amazing they really are.

The Cell Approach:

One of the most interesting things about a cell phone is that it is really a radio  — an extremely sophisticated radio, but a radio nonetheless. The telephone was invented by Alexander Graham Bell in 1876, and wireless communication can trace its roots to the invention of the radio in 1894 by a young Italian named Guglielmo Marconi. It was only natural that these two great technologies would eventually be combined!

In the dark ages before cell phones, people who really needed mobile communications ability installed radio telephones in their cars. In the radio telephone system, there was one central antenna tower per city, and perhaps 25 channels available on that tower. This central antenna meant that the phone in your car needed a powerful transmitter — big enough to transmit 40 or 50 miles. It also meant that not many people could use radio telephones — there just were not enough channels.

The genius of the cellular system is the division of a city into small cells. This allows extensive frequency reuse across a city, so that millions of people can use cell phones simultaneously. In a typical analog cell phone system in the United States, the cell phone carrier receives about 800 frequencies to use across the city. The carrier chops up the city into cells. Each cell is typically sized at about 10 square miles (26 square kilometers).

From Cell to Cell:

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.

Let’s say you have a cell phone, you turned it on, and someone tries to call you. Here is what happens to the call:

  • When you first power up the phone, it listens for an SID (see sidebar) on the control channel. The control channel is a special frequency that the phone and base station use to talk to one another about things like call set-up and channel-changing. If the phone cannot find any control channels to listen to, it knows it is out of range, and displays a “no service” message.
  • 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 your phone’s location in a database — this way, the MTSO knows which cell you are in when it wants to ring your phone.
  • The MTSO gets the call, and it tries to find you. It looks in its database to see which cell you are in.
  • The MTSO picks a frequency pair that your phone will use in that cell to take the call.
  • The MTSO communicates with your phone over the control channel to tell it what frequencies to use, and once your phone and the tower switch on those frequencies, the call is connected. You are talking by two-way radio to a friend!
  • As you move toward the edge of your cell, your cell’s base station will note that your signal strength is diminishing. Meanwhile, the base station in the cell you are moving toward (which is listening and measuring signal strength on all frequencies, not just its own one-seventh) will be able to see your phone’s signal strength increasing. The two base stations coordinate themselves through the MTSO, and at some point, your phone gets a signal on a control channel telling it to change frequencies. This hand off switches your phone to the new cell.

Radio communication:

Mobile phones may be a relatively new technology, but radio has been used as a means of communication for over a hundred years. Marconi made the very first radio transmission in 1895. Within thirty years radio was being used on a daily basis for broadcasting and for two-way radio communication by the military and the police. Today, a little over a hundred years since Marconi’s first transmission, 60% of the UK population – around 40 million people – enjoy the benefits of mobile phone use.

What is a radio wave?

Mobile phones and their base stations transmit and receive signals using electromagnetic waves (also referred to as electromagnetic fields, or radio waves). Electromagnetic waves are emitted by many natural and man-made sources and play a very important part in our lives. We are warmed by the electromagnetic emissions of the sun and we see using the part of the electromagnetic spectrum that our eyes detect as visible light. All electromagnetic radiation consists of oscillating electric and magnetic fields and the frequency, which is the number of times per second at which the wave oscillates, determines their properties and the use that can be made of them. Frequencies are measured in hertz or Hz, where 1 Hz is one oscillation per second, 1 kHz a thousand, 1 MHz is a million, and 1 GHz, is a thousand million. Frequencies between 30 kHz and 300 GHz are widely used for telecommunication, including broadcast radio and television, and comprise the radio frequency band.

In the UK, AM radio uses frequencies between about 180 kHz and 1.6 MHz, FM radio ranges from 88 to 108 MHz, and TV ranges from 470 to 854 MHz. Cellular mobile services operate within the frequency ranges 872-960 MHz, 1710-1875 MHz and 1920 – 2170 MHz. Waves at higher frequencies but within the RF region, up to 60 GHz, are referred to as microwaves and have a wide variety of uses. These include radar, telecommunication links, satellite communications, weather observation and medical diathermy.

Ofcom produces a set of Radio Frequency Allocation Information Sheets, which give details of the types of services operating in any particular band.

How radio communication works?

A radio frequency wave used for radio communication is referred to as a carrier wave. The radio frequency carrier wave of any system is produced by the transmitter as a sine wave. A sine wave conveys very little information since it simply repeats over and over. However, it can be switched on and off and this was the technique used in the earliest radio transmissions which used Morse code.

If the radio wave is to convey more information, such as speech or computer data etc., this information has to be added to the carrier wave in some way, a process known as modulation. The modulation process involves some feature of the carrier wave being varied in accordance with the information transmitted. For example, for AM (amplitude modulation) transmission, the electrical signal from a microphone produced by speech or music is used to vary the amplitude of the carrier wave, so that at any instant the size or amplitude of the RF carrier wave is made proportional to the size of the electrical modulating signal. Figure 1 below demonstrates this concept.

Amplitude Modulation

There are many different types of modulation technique, each with different characteristics, and each suitable for different applications. You might be familiar with the frequency modulation (FM) used for radio broadcasting, or the digital techniques used by mobile phones. All work by varying some property of the carrier wave in a way by which the information to be communicated can be conveyed or carried by the radio frequency carrier wave.

Mobile Phone Networks

Base Station and handsets

A mobile phone sends and receives information (voice messages, fax, computer data, etc) by radio communication. Radio frequency signals are transmitted from the phone to the nearest base station and incoming signals (carrying the speech from the person to whom the phone user is listening) are sent from the base station to the phone at a slightly different frequency. Base stations link mobile phones to the rest of the mobile and fixed phone network.

Once the signal reaches a base station it can be transmitted to the main telephone network, either by telephone cables or by higher frequency radio links between an antenna (e.g. dish) at the base station and another at a terminal connected to the main telephone network.

Each base station provides radio coverage to a geographical area known as a cell. Base stations are connected to one another by central switching centres, which track calls and transfer them as the caller moves from one cell to the next. Diagram 2 below shows the cell structure of a mobile phone network . An ideal network may be envisaged as consisting of a mesh of hexagonal cells, each with a base station at its centre. The cells overlap at the edges to ensure the mobile phone users always remain within range of the base station. Without sufficient base stations in the right locations, mobile phones will not work.

The size of each cell depends on three factors. First, the local terrain; radio signals are blocked by trees, hills and buildings. Second, the frequency band in which the network operates (in general, the higher the radio frequency, the smaller the cell). Third, the capacity (i.e. number of calls) needed in any given area. Base stations are typically spaced about 0.2-0.5 km in towns and 2-5 km apart in the countryside.

If a person with a mobile phone starts to moves out of one cell and into another, the controlling network hands over communications to the adjacent base station.

‘Cellular’ Radio

Why are so many base stations required?

Transmitted signal strength falls off rapidly with distance from base stations, and mobile phones require a certain minimum signal strength to ensure adequate reception. The current generation of GSM base stations cannot communicate over distances greater than 35 km because the delay in receiving radio signals becomes too great. However, the decline of signal strength with distance places a practical limit on coverage of around 10 km. For these reasons an extensive network of base stations is needed to ensure coverage throughout the UK.

Why can’t one base station serve my town?

Radio spectrum is a precious natural resource with many different demands upon it (for example, radio and TV broadcasting, emergency communication, navigation aids etc). Consequently the amount made available to each mobile phone operator is limited and this means base stations can only carry a limited number of calls at any one time.

To accommodate the steadily increasing volume of users, network operators have to use the limited number of radio frequencies licensed to them to support the maximum number of mobile phone users. This is achieved by re-using any given radio frequency many times in a network and carefully controlling base station power so that signals arising in different parts of the network do not interfere with each other. This concept of frequency re-use is illustrated in figure 3. The cells are grouped into clusters, with the frequencies allocated to a particular cell within a cluster not being re-used until the corresponding cell in adjacent clusters. This gives a repeating pattern of cells and clusters which can be expanded to provide national coverage.

To increase the capacity of their networks, operators have to build additional base stations and thus reduce cell size. It is for this reason that one large base station cannot serve a whole town.

Frequency Re-use

Posted By

Shiva Chaitanya(ECE 2/4) MGIT

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