Mobile Network: How the world gets connected?

When we make a call to any person, we just dial his/her Mobile no. and press call and the call connects to the person and we can talk to him/her regardless of being very close or very far away. This is made possible by the invention of the mobile network or the cellular network which helps to communicate to any mobile devices wirelessly.

Hmm, now what exactly is Mobile network. A cellular network or mobile network is a communications network where the last link is wireless. The network is distributed over land areas called cells, each served by at least one fixed-location transceiver, known as a cell site or base station. This base station provides the cell with the network coverage which can be used for transmission of voice, data and others. In a cellular network, each cell uses a different set of frequencies from neighboring cells, to avoid interference and provide guaranteed bandwidth within each cell.

When joined together these cells provide radio coverage over a wide geographic area. This enables a large number of portable transceivers (e.g., mobile phones, pagers, etc.) to communicate with each other and with fixed transceivers and telephones anywhere in the network, via base stations, even if some of the transceivers are moving through more than one cell during transmission.

Radiocommunication: Basic Technology on which Mobile phones work

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.

RA produces a Radio Frequency Allocation Information Sheet, which gives details of the types of services operating in any particular band. This can be obtained from our website.

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 Radio Frequency carrier wave is made proportional to the size of the electrical modulating signal. Figure below demonstrates this concept.

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 Stations 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.

'Cellular' Radio

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  below shows the cell structure of a mobile phone network1. 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 a 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.

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. 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.

Cell signal encoding

To distinguish signals from several different transmitters, frequency division multiple access (FDMA) and code division multiple access (CDMA) were developed.

With FDMA, the transmitting and receiving frequencies used in each cell are different from the frequencies used in each neighbouring cell. In a simple taxi system, the taxi driver manually tuned to a frequency of a chosen cell to obtain a strong signal and to avoid interference from signals from other cells.

The principle of CDMA is more complex, but achieves the same result; the distributed transceivers can select one cell and listen to it.

Other available methods of multiplexing such as polarization division multiple access (PDMA) and time division multiple access (TDMA) cannot be used to separate signals from one cell to the next since the effects of both vary with position and this would make signal separation practically impossible. Time division multiple access, however, is used in combination with either FDMA or CDMA in a number of systems to give multiple channels within the coverage area of a single cell.

Movement from cell to cell and handover

In a primitive taxi system, when the taxi moved away from a first tower and closer to a second tower, the taxi driver manually switched from one frequency to another as needed. If a communication was interrupted due to a loss of a signal, the taxi driver asked the base station operator to repeat the message on a different frequency.

In a cellular system, as the distributed mobile transceivers move from cell to cell during an ongoing continuous communication, switching from one cell frequency to a different cell frequency is done electronically without interruption and without a base station operator or manual switching. This is called the handover or handoff. Typically, a new channel is automatically selected for the mobile unit on the new base station which will serve it. The mobile unit then automatically switches from the current channel to the new channel and communication continues.

The exact details of the mobile system's move from one base station to the other varies considerably from system to system (see the example below for how a mobile phone network manages handover).

Cellular frequency choice in mobile phone networks

The effect of frequency on cell coverage means that different frequencies serve better for different uses. Low frequencies, such as 450 MHz NMT, serve very well for countryside coverage. GSM 900 (900 MHz) is a suitable solution for light urban coverage. GSM 1800 (1.8 GHz) starts to be limited by structural walls. UMTS, at 2.1 GHz is quite similar in coverage to GSM 1800.

Higher frequencies are a disadvantage when it comes to coverage, but it is a decided advantage when it comes to capacity. Pico cells, covering e.g. one floor of a building, become possible, and the same frequency can be used for cells which are practically neighbours.

Cell service area may also vary due to interference from transmitting systems, both within and around that cell. This is true especially in CDMA based systems. The receiver requires a certain signal-to-noise ratio, and the transmitter should not send with too high transmission power in view to not cause interference with other transmitters. As the receiver moves away from the transmitter, the power received decreases, so the power control algorithm of the transmitter increases the power it transmits to restore the level of received power. As the interference (noise) rises above the received power from the transmitter, and the power of the transmitter cannot be increased any more, the signal becomes corrupted and eventually unusable. In CDMA-based systems, the effect of interference from other mobile transmitters in the same cell on coverage area is very marked and has a special name, cell breathing.

One can see examples of cell coverage by studying some of the coverage maps provided by real operators on their web sites or by looking at independently crowdsourced maps such as OpenSignal. In certain cases they may mark the site of the transmitter, in others it can be calculated by working out the point of strongest coverage.