Cellular systems for Mobile Communications

Dec, 30 2020, 01:10 pm [IST]

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:

• Higher Capacity: implementing SDM allows frequency reuse. If one transmitter is very distant from another, i.e., outside the resistance range, it can reuse the same frequencies. As most mobile phone systems assign to specific users, this frequency is prevented for other users. But frequencies are a limited resource and, thus, the number of concurrent users per cell is very limited to lesser possible users per km2.
• Less transmission power: while power aspects are not a big problem for base stations, they are problematic for mobile stations. A receiver very distant from a base station would require much more transmit power than the current few watts. But energy is a severe problem for mobile handheld devices.
• Local interference only: having long distances between sender and receiver results in even more interference problems. With small cells, mobile stations only have to deal with local interference.
• Robustness: cellular systems are decentralized and, thus, more robust against failures of single components. If one antenna fails, this defect only influences communication within a small area.

Disadvantages:

• The infrastructure needed: cellular systems need a complex infrastructure to connect all based stations. This Infrastructure includes many antennas, switches for call forwarding, location registers to find a mobile station, etc. This makes the whole system quite expensive.
• Handover required: the mobile station has to work a handover when switching from one cell to another. Depending on the cell size and the speed of movement, This can occur quite often.
• Frequency planning: to avoid interference between transmitters using the same frequencies, frequencies have to be distributed carefully. On 1The one hand interference should be avoided, on the other hand, only a limited number of frequencies is available.

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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