In a cellular system, a BS (Base Station) is located in the center of each cell1. The Base Station includes an antenna, a control system and a series of transceivers to communicate on the channels assigned to this cell.
If the system is used by an excessive number of users, the frequencies assigned to a cell may become insufficient to handle all calls. To respond to this criticality, various approaches are adopted, the main ones of which are listed.
- Addition of new cells: in general, when configuring a system on the territory, not all channels are used and therefore it is possible to predict growth and expansion in an orderly way, simply by adding new channels.
- Subtraction of frequencies: in the simplest case, the frequencies are subtracted from the congested cells to be assigned to the adjacent cells. Frequencies can also be assigned to cells dynamically.
- Subdivision of the cells: the distribution of traffic and the morphological characteristics of the territory are not uniform and this presents opportunities to increase capacity. Cells in areas of high usage can be divided into smaller cells. In general, the cells have a radius that varies from a few kilometers to 10 kilometers. Even the smallest cells can be further divided; however, in practical use, there must be a minimum cell radius of 1.5 kilometers less than the use of microcells. To use a smaller cell, the power level used must be reduced in order to keep the signal only inside the cell. Therefore, while the mobile terminal is moving, it will move from one cell to another and this will require the transfer of the call from one basic transceiver to another (handoff2 process). As the size of the cells decreases, the handoff operation becomes more and more frequent.
- Cell sectors: A cell is divided into a number of sectors, each with its own set of channels, generally providing for three to six sectors per cell. Each sector is assigned a separate subset of the cell channels and directional antennas are used to precisely cover each sector.
- Microcells: as shown in Figure 1, where as cells are reduced in size, the antennas must move from the top of tall buildings or hills to the top of small buildings or the facade of buildings and finally on the lighting poles , until creating microcells. Any reduction in the size of a cell is accompanied by a reduction in the level of power radiated by the base station and mobile units. Microcells are very useful in congested areas of cities, along fast roads and inside large public buildings. Using small cells reduces the emitted power and offers better propagation conditions.
Figure 1 – Subdivision of the territory into cells of different sizes.
To take into account the different distribution of the traffic required in a given area, the territory is therefore divided into cells of different sizes. In this way, in areas with high user density it will be possible to decrease the reuse distance of the channels thus increasing the number of users that can be served.
It is evident that technologies such as 5G which, according to the ITU3 guidelines, will have a peak speed of 20 Gb / s for the downlink and 10 Gb / s for the uplink, will require the use of these optimization techniques in a path that it will lead from territorial coverage operated with macro cells to the adoption of femtocells.
1 The concept of cellular radio electric coverage is based on the distribution of several low-power Base Stations (BS) which cover small areas called cells (hence the name). Each cell is assigned a group of radio channels that are reused to cope with the large number of users and the limited frequency spectrum available for mobile radio networks.
2 By handoff we mean the procedure that consists in the passage of a mobile unit from one base station to another due to its passage from one cell to another.
3 The International Telecommunication Union (ITU) is the international organization that sets standards for telecommunications and the use of radio waves.