Power control
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In cellular telecommunication systems Cellular phones adaptively adjust their power level so as not to swamp all the other users in the system. Cell phones close to a cell tower transmit using very little power. As they get further away, the cell tower tells them to transmit using more power. This adaptive transmission has 2 benefits:
- A fixed transmit power level would either (close to the tower) use more energy than necessary, reducing the battery life of the cell phone, or (far from the tower) transmit too quietly to be heard, reducing the range of the cell phone.
- Co-channel interference (cross-talk) to nearby cells using the same channel frequency can be avoided if each cellphone sends with minimum power that is required for undistrubed reception in the base station. This makes it possible to reduce the distance between cells that reuse the same channel frequence, and thus achive higher capacity due to increased spectral efficiency.
- In DS-CDMA spread spectrum, for example in the IS-95 and 3G cellular systems, rapid power control is required in the uplink to avoid co-channel interference within the same cell, since all users in a cell simultaneously send at the same frequency band. The cell tower adjusts the power level of each cell phone belonging to that cell so that -- at the tower -- they all have close to equal power levels. This makes it much easier to separate all the signals from each other -- if one phone were much "louder" than the others, it would be more likely to bleed through into the other signals. Otherwize the base station receiver would not be able to dicern signals from a cell phone that for the moment is attenuated due to long distance or fading.
- A prioritized user that require high quality-of-service may be offered a slightly higher power than other users. Thus a less robust modulation, channel code or spreading code can be used, resulting in higher data rate.
