A cellular network or mobile network is a radio network is a radio network distributed over land areas called cells, each served by at least one fixed-location transceiver, known as a cell site or base station. 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.
Cellular networks offer a number of advantages over alternative solutions:
• flexible enough to use the features and functions of almost all public and private networks
• increased capacity
• reduced power use
• larger coverage area
• reduced interference from other signals
Concept
In a cellular radio system, a land area to be supplied with radio service is divided into regular shaped cells, which can be hexagonal, square, circular or some other regular shapes, although hexagonal cells are conventional. Each of these cells is assigned multiple frequencies (f1 – f6) which have corresponding radio base stations. The group of frequencies can be reused in other cells, provided that the same frequencies are not reused in adjacent neighboring cells as that would cause co-channel interference.
The increased capacity in a cellular network, compared with a network with a single transmitter, comes from the fact that the same radio frequency can be reused in a different area for a completely different transmission. If there is a single plain transmitter, only one transmission can be used on any given frequency. Unfortunately, there is inevitably some level of interference from the signal from the other cells which use the same frequency. This means that, in a standard FDMA system, there must be at least a one cell gap between cells which reuse the same frequency. In the simple case of the taxi company, each radio had a manually operated channel selector knob to tune to different frequencies. As the drivers moved around, they would change from channel to channel. The drivers knew which frequency covered approximately what area. When they did not receive a signal from the transmitter, they would try other channels until they found one that worked. The taxi drivers would only speak one at a time, when invited by the base station operator (this is, in a sense, time division multiple access (TDMA)).
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.
Cellular Data Usage
Push Notifications
Some applications from the App Store use the Apple Push Notification service to alert you of new data. Applications that rely extensively on push notifications (such as instant messaging applications) will consume data. To disable push notifications, go to Settings > Notifications and set Notifications to Off. Note that this does not prevent new data from being received when the application is opened. Also, the Notifications setting will not be visible if you do not have any applications installed that support push notifications.
Push email and over-the-air syncing
Applications such as Mail can be set to fetch data wirelessly at specific intervals. The more frequently email or other data is fetched, the faster data is consumed. To fetch new data manually, from the Home screen choose Settings > Mail, Contacts, Calendars > Fetch New Data and tap Manually. To increase the fetch interval, go to Settings > Mail, Contacts, Calendars > Fetch New Data and tap Hourly. Note that this is a global setting and applies to all applications that do not support push services.
Operation Of The Cellular Phone
When the mobile unit is active (i.e. when a mobile phone is switched on), it registers with the appropriate BS , depending on its location, and its cell position is stored at the responsible MSC. When a call is set-up (when a user makes a call), the base station monitors the quality of the signal for the duration of the call, and reports that to the controlling MSC, which in turn makes decisions concerning the routing of the call. When a cellular phone moves from one cell to the other, the BS will detect this from the signal power and inform the MSC of that. The MSC will then switch the control of the call to the BS of the new cell, where the phone is located. This is called handover . It normally takes up to 400ms, which is not noticeable for voice transmission.
A cellular phone user can only use his/her mobile within the covered area of the network. Roaming is the capacity of a cellular phone, registered on one system, to be able to enter and use other systems. Those other systems must be compatible to enable roaming (i.e. they must have the same type of networks). In Europe, the standard cellular network is called GSM (Global System for Mobile Communication). Incoming calls to GSM[8] users are routed to them, irrespective of where they are, as long as they are within Europe.
The benefits of Microcellular systems
• Interference reduced (compared to decreasing the cell size)
• Handovers reduced (also compared to decreasing the cell size) since the microcells within the cell operate at the same frequency; no handover occurs when the mobile unit moves between the microcells
• Size of the zone apparatus. The zone site equipment are small, so the can be mounted on the side of a building of on poles.
• Increased system capacity. The microcell is an intelligent cell. The new microcell knows where to locate the mobile unit in a particular zone of the cell and deliver the power to that zone. Since the signal power is reduced, the microcells can be closer and therefore increase capacity. However, in microcellular system, the transmitted power to a mobile phone within a microcell has to be precise; too much power results in interference between microcells, while too little power and the signal might not reach the mobile phone.
This is a drawback of microcellular systems, since a change in the surrounding (a new building, say, within a microcell) will require a change of the transmission power.
Multiple Access Systems
In digital cellular networks, Multiple access systems are used to allow simultaneous users to share the same channel within a cell. The common methods are time division multiple access (TDMA) and frequency division multiple access (FDMA).
In TDMA, the bandwidth allocated for the channel is divided into time slots - the number of slots depends on the system. Each user is then allocated a slot, and hence multiple users share the same frequency but at different times.
In FDMA, the channel is divided into frequency bands, and each user is allocated a frequency band.
A New Technology
Due to the rapid growth in the cellular communication industry, there is an increased need for greater system capacity. Code division multiple access is the new technology, and it does not need a cellular structure.
CDMA is a 'spread spectrum' technology; it spreads the information contained in a particular signal over the entire bandwidth allocated for the mobile communication. With CDMA, unique digital codes, rather than radio frequencies or channels, are used to differentiate the different signals. These codes are shared by both the transmitter and receiver; and hence the receiver (the mobile phone) receives all the signals but can only recognise the one with the same codes.
CDMA has many advantages over the existing cellular systems
• Increases capacity and improves quality of the signal.
• Simplified system planning through the use of the same frequency all over the covered area.
• Enhanced privacy
However, CDMA can only be used for digital transmission, unlike the cellular system. This makes the move towards CDMA in some countries, which do not use the digital system, a distant future.
The Future
Currently, there are different standards of cellular systems in different parts of the world; the major ones are the GSM in Europe, and the PCS in North America. Roaming is not possible between these two systems. The next step would be to have duel-mode phones which could operate in the two different systems at the touch of a button. Having one global cellular communication system is ideal, but will take a while, since it will require altering one system's hardward (which costs a lot of money!).
Since the "information age" and due to the rapid growth of the cellular system, one could predict that in the very near future, everyone will have a portable communication terminal, which is small in size, fast in accessing the internet and transmit/receive data, cheaper, and could virtually be used from anywhere in the system.
Finally, celluar networks will soon be replaced by the spread specturm technology; this move has already began in some parts of the world and the rest will sure to follow.
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