Cellular systems for Mobile Communications implement SDM. Each transmitter, typically called a base station, comprises a particular area, a cell. Cell radii can differ from tens of meters in buildings, and hundreds of meters in cities, tens of kilometers in the countryside. The pattern of cells are never perfect circles or hexagons but depend on the circumstances, weather conditions, and sometimes even on system load. Typical systems using the method are mobile telecommunication systems, where a mobile station within the cell around a base station interacts with this base station and vice versa.
Advantages of the cellular system with small cells are the following:
To avoid interference, different transmitters between each other's interference range use FDM. If FDM is combined with TDM, the hopping pattern has to be coordinated. The general goal is never to use the same frequency at the same time within the interference range.
Cells are combined in clusters. The hexagonal and time are taken as an easy way to demonstrate the model. This model also explains the repetition of the same frequency sets. The transmission power of a sender therefore has to be limited to avoid interference with the next cell using the same frequencies.
To decrease interference even further sectorized antennas can be used. The fixed assignment of frequencies to cell clusters and cells respectively is not very effective if traffic load differs. In the case of a heavy load in one cell and a light load in a neighboring cell, for instance, it could make sense to borrow frequencies. Cells with higher traffic are dynamically allocated more frequencies. This system is known as borrowing channel allocation (BCA), while the first fixed scheme is called fixed channel allocation (FCA).
A dynamic channel allocation (DCA) scheme has been implemented in DECT. In this scheme, frequencies can only be borrowed, but it is also possible to assign frequencies to cells. With the dynamic assignment of frequencies to cells, the danger of interference with cells using the same frequency exists. Thus, the borrowed frequency can be blocked in the surrounding cells.
A cellular system using CDM instead of FDM does not need such elaborate channel allocation schemes. Here users are separated through the code they use, and not through the frequency. But here cell planning faces another problem - the cell size depends on the current load. Accordingly, CDM cells are commonly said to breathe. While a cell can cover a larger area under a light load, it shrinks if the load increases. The reason for this is the growing noise level if more users are in a Cell.
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