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different areas in order to provide radio coverage over a wider area than the area of one cell. Cellular networks are inherently asymmetric with a set of fixed main
transceivers each serving a cell and a set of distributed(generally, but not always, mobile) transceivers which provide services to the network's users.
Cellular networks offer a number of advantages over alternative solutions,
A good (and simple) example of a cellular system is an old taxi driver's radio system where a city will have several transmitters based around a city. We'll use that as an example and assume that each transmitter is handled separately by a different operator.
The primary requirement for a cellular network is a way for the distributed stations to distinguish the signal from its own transmitter from the signal from other transmitters. There are two common solutions to this, frequency division multiple access (FDMA) and code division multiple access (CDMA). FDMA works by using a different frequency for each neighbouring cell. By tuning to the frequency of a chosen cell the distributed stations can avoid the signal from other neighbours. 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 Polarisation division multiple access (PDMA) and
time division multiple access (TDMA) cannot be used toseparate 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.
In the case of our taxi company, each radio has a knob. The knob acts as a channel selector and allows the radio to tune to different frequencies. As the drivers move around, they change from channel to channel. The drivers know which frequency covers approximately what area, when they don't get a signal from the transmitter, they also try other channels until they find one which works. The taxi drivers only speak one at a time, as invited by the operator (in a sense TDMA).
Practically every cellular system has some kind of broadcast mechanism. This can be used directly for distributing information to multiple mobiles, commonly, for example in mobile telephony systems, the most important use of broadcast information is to set up channels for one to one communication between the mobile transceiver and the base station. This is called paging.
The details of the process of paging vary somewhat from network to network, but normally we know a limited number of cells where the phone is located (this group of cells is called a location area in the GSM system). Paging takes place by sending the broadcast message on all of those cells. In a few cases paging messages can be used for information transfer. This happens in
pagers and also in the UMTS system where it allows for low downlink latency in packet based connections.Our taxi network is a very good example here. The broadcast capability is often used to tell about road conditions and also to tell about work which is available to anybody. On the other hand, typically there is a list of taxis waiting for work. When a particular taxi comes up for work, the operator will call their number over the air. The taxi driver acknowledge s that they are listening, then the operator reads out the address where the taxi driver has to go.
The increased capacity in a cellular network, as compared to 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.
The frequency reuse factor is the rate at which the same frequency can be used in the network. It is 1/n where n is the number of cells which cannot use a frequency for transmission.
Code division multiple access based systems use a wider frequency band to achieve the same rate oftransmission as FDMA, but this is compensated for by the ability to use a frequency reuse factor of 1. In other words, every cell uses the same frequency and the different systems are separated by codes rather than frequencies.
Depending on the size of the city, a taxi system may not have any frequency reuse in its own city, but certainly in other nearby cities, the same frequency can be used. In a big city, on the other hand, frequency reuse could certainly be in use.
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