Cellular telecommunications systems are widely used today and need to offer very efficient use of the available frequency spectrum. With billions of mobile phones in use around the globe today, it is necessary to re-use the available frequencies many times over without mutual interference of one cell phone to another.
Early schemes for radio telephones schemes used a single central transmitter to cover a wide area. These radio telephone systems suffered from the limited number of channels that were available. Often the waiting lists for connection were many times greater than the number of people that were actually connected.
The need for a spectrum efficient system
To illustrate the need for efficient spectrum usage for a radio telecommunications system, take the example where each user is allocated a channel. While more effective systems are now in use, the example will take the case of an analogue system. Each channel needs to have a bandwidth of around 25 kHz to enable sufficient audio quality to be carried as well as enabling there to be a guard band between adjacent signals to ensure there are no undue levels of interference. Using this concept it is only possible to accommodate 40 users in a frequency band 1 MHz wide. Even of 100 MHz were allocated to the system this would only enable 4000 users to have access to the system. Today cellular systems have millions of subscribers and therefore a far more efficient method of using the available spectrum is needed.
Cell system for frequency re-use
The method that is employed is to enable the frequencies to be re-used. Any transmitter will only have a certain coverage area. Beyond this the signal level will fall to a limited below which it cannot be used and will not cause significant interference to users associated with a different transmitter. This means that it is possible to re-use a channel once outside the range of the transmitter. The same is also true in the reverse direction for the receiver, where it will only be able to receive signals over a given range. In this way it is possible to arrange split up an area into several smaller regions, each covered by a different transmitter / receiver station.
These regions are conveniently known as cells, and give rise to the name of a cellular telecommunications system. Diagrammatically these cells are often shown as hexagonal shapes that conveniently fit together. In reality this is not the case. They have an irregular boundary because of the terrain over which they travel. Hills, buildings and other objects all cause the signal to be attenuated and diminish differently in each direction.
It is also very difficult to define the exact edge of a cell. The signal strength gradually reduces and towards the edge of the cell performance will fall. As the mobiles themselves will have different levels of sensitivity, this adds a further greying of the edge of the cell. Therefore it is never possible to have a sharp cut-off between cells. In some areas they may overlap, whereas in others there will be a "hole" in coverage.
Cell clusters
To overcome this problem, in a basic cellular system, adjacent cells are allocated different frequency bands so that they can overlap without causing interference. In this way cells can be grouped together in what is termed a cluster.
Often these clusters contain seven cells, but other configurations are also possible. Seven is a convenient number, but there are a number of conflicting requirements that need to be balanced when choosing the number of cells in a cluster:
* Limiting interference levels
* Number of channels that can be allocated to each cell site
It is necessary to limit the interference between cells having the same frequency. The topology of the cell configuration has a large impact on this. The larger the number of cells in the cluster, the greater the distance between cells sharing the same frequencies.
In the ideal world it might be good to choose a large number of cells to be in each cluster. Unfortunately there is only a limited number of channels available. This means that the larger the number of cells in a cluster, the smaller the number available to each cell, and this reduces the capacity.
This means that there is a balance that needs to be made between the number of cells in a cluster, and the interference levels, and the capacity that is required.
Cell size
Even though the number of cells in a cluster can help govern the number of users that can be accommodated, by making all the cells smaller it is possible to increase the overall capacity of the network. However a greater number of transmitter receiver or base stations are required if cells are made smaller and this increases the cost to the operator. Accordingly in areas where there are more users, small low power base stations are installed.
The different types of cells are given different names according to their size and function:
* Macro cells
* Micro cells
* Pico cells
* Selective cells
* Umbrella cells
Macro cells are large cells that are usually used for remote or sparsely populated areas. These may be 10 km or possibly more in diameter. Micro cells are those that are normally found in densely populated areas which may have a diameter of around 1 km. Picocells may also be used for covering very small areas such as particular areas of buildings, or possibly tunnels where coverage from a larger cell is not possible. Obviously for the small cells, the power levels used by the base stations are much lower and the antennas are not position to cover wide areas. In this way the coverage is minimised and the interference to adjacent cells is reduced.
Other types of cell may be used for some specialist applications. Sometimes cells termed selective cells may be used where full 360 degree coverage is not required. They may be used to fill in a hole in the coverage, or to address a problem such as the entrance to a tunnel etc. Another type of cells known as an umbrella cell is sometimes used in instances such as those where a heavily used road crosses an area where there are microcells. Under normal circumstances this would result in a large number of handovers as people driving along the road would quickly cross the microcells. An umbrella cell would take in the coverage of the microcells (but use different channels to those allocated to the microcells). However it would enable those people moving along the road to be handled by the umbrella cell and experience fewer handovers than if they had to pass from one microcell to the next.
Early schemes for radio telephones schemes used a single central transmitter to cover a wide area. These radio telephone systems suffered from the limited number of channels that were available. Often the waiting lists for connection were many times greater than the number of people that were actually connected.
The need for a spectrum efficient system
To illustrate the need for efficient spectrum usage for a radio telecommunications system, take the example where each user is allocated a channel. While more effective systems are now in use, the example will take the case of an analogue system. Each channel needs to have a bandwidth of around 25 kHz to enable sufficient audio quality to be carried as well as enabling there to be a guard band between adjacent signals to ensure there are no undue levels of interference. Using this concept it is only possible to accommodate 40 users in a frequency band 1 MHz wide. Even of 100 MHz were allocated to the system this would only enable 4000 users to have access to the system. Today cellular systems have millions of subscribers and therefore a far more efficient method of using the available spectrum is needed.
Cell system for frequency re-use
The method that is employed is to enable the frequencies to be re-used. Any transmitter will only have a certain coverage area. Beyond this the signal level will fall to a limited below which it cannot be used and will not cause significant interference to users associated with a different transmitter. This means that it is possible to re-use a channel once outside the range of the transmitter. The same is also true in the reverse direction for the receiver, where it will only be able to receive signals over a given range. In this way it is possible to arrange split up an area into several smaller regions, each covered by a different transmitter / receiver station.
These regions are conveniently known as cells, and give rise to the name of a cellular telecommunications system. Diagrammatically these cells are often shown as hexagonal shapes that conveniently fit together. In reality this is not the case. They have an irregular boundary because of the terrain over which they travel. Hills, buildings and other objects all cause the signal to be attenuated and diminish differently in each direction.
It is also very difficult to define the exact edge of a cell. The signal strength gradually reduces and towards the edge of the cell performance will fall. As the mobiles themselves will have different levels of sensitivity, this adds a further greying of the edge of the cell. Therefore it is never possible to have a sharp cut-off between cells. In some areas they may overlap, whereas in others there will be a "hole" in coverage.
Cell clusters
To overcome this problem, in a basic cellular system, adjacent cells are allocated different frequency bands so that they can overlap without causing interference. In this way cells can be grouped together in what is termed a cluster.
Often these clusters contain seven cells, but other configurations are also possible. Seven is a convenient number, but there are a number of conflicting requirements that need to be balanced when choosing the number of cells in a cluster:
* Limiting interference levels
* Number of channels that can be allocated to each cell site
It is necessary to limit the interference between cells having the same frequency. The topology of the cell configuration has a large impact on this. The larger the number of cells in the cluster, the greater the distance between cells sharing the same frequencies.
In the ideal world it might be good to choose a large number of cells to be in each cluster. Unfortunately there is only a limited number of channels available. This means that the larger the number of cells in a cluster, the smaller the number available to each cell, and this reduces the capacity.
This means that there is a balance that needs to be made between the number of cells in a cluster, and the interference levels, and the capacity that is required.
Cell size
Even though the number of cells in a cluster can help govern the number of users that can be accommodated, by making all the cells smaller it is possible to increase the overall capacity of the network. However a greater number of transmitter receiver or base stations are required if cells are made smaller and this increases the cost to the operator. Accordingly in areas where there are more users, small low power base stations are installed.
The different types of cells are given different names according to their size and function:
* Macro cells
* Micro cells
* Pico cells
* Selective cells
* Umbrella cells
Macro cells are large cells that are usually used for remote or sparsely populated areas. These may be 10 km or possibly more in diameter. Micro cells are those that are normally found in densely populated areas which may have a diameter of around 1 km. Picocells may also be used for covering very small areas such as particular areas of buildings, or possibly tunnels where coverage from a larger cell is not possible. Obviously for the small cells, the power levels used by the base stations are much lower and the antennas are not position to cover wide areas. In this way the coverage is minimised and the interference to adjacent cells is reduced.
Other types of cell may be used for some specialist applications. Sometimes cells termed selective cells may be used where full 360 degree coverage is not required. They may be used to fill in a hole in the coverage, or to address a problem such as the entrance to a tunnel etc. Another type of cells known as an umbrella cell is sometimes used in instances such as those where a heavily used road crosses an area where there are microcells. Under normal circumstances this would result in a large number of handovers as people driving along the road would quickly cross the microcells. An umbrella cell would take in the coverage of the microcells (but use different channels to those allocated to the microcells). However it would enable those people moving along the road to be handled by the umbrella cell and experience fewer handovers than if they had to pass from one microcell to the next.
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