The power control regulates the transmit power of the terminal and base station, which results in less interference and allows more users on the same carrier. Transmit power regulation thus provides more capacity in the network. With a frequency re-use of 1, it is very important to have efficient power control in order to keep the interference at a minimum. For each subscriber service the aim is that the base station shall receive the same power level from all handsets in the cell regardless of distance from the base station. If the power level from one handset is higher than needed, the quality will be excessive, taking a disproportionate share of the resources and generating necessary interference with the other subscribers in the network. On the other hand, if power levels are too low this will result in poor
quality. In order to keep the received power at a suitable level, WCDMA has a fast power control that updates power levels 1500 times every second. By doing that the rapid change in the radio channel is handled. To ensure good performance, power control is implemented in both the up-link and the down-link,which means that both the output powers of the handset and the base station are frequently updated.Power control also gives rise to a phenomenon called “cell breathing”. This is the trade-off between coverage and capacity, which means that the size of the cell varies depending on the traffic load. When the number of subscribers in the cell is low (low load),good quality can be achieved even at a long distance from the base station. On the other hand, when the number of users in the cell is high, the large number of subscribers generates a high interference level and subscribers have to get closer to the base station to achieve good quality.
Figure 3.4 figure 3.5