UMTS power control

As with any CDMA system it is essential that the base station receives all the UEs at approximately the same power level. If not, the UEs that are further away will be lower in strength than those closer to the node B and they will not be heard. This effect is often referred to as the near-far effect. To overcome this the node B instructs those stations closer in, to reduce their transmitted power, and those further away to increase theirs. In this way all stations will be received at approximately the same strength.

It is also important for node Bs to control their power levels effectively. As the signals transmitted by the different node Bs are not orthogonal to one another it is possible that signals from different ones will interfere. Accordingly their power is also kept to the minimum required by the UEs being served.

To achieve the power control there are two techniques that are employed: open loop; and closed loop.

Open loop techniques are used during the initial access before communication between the UE and node B has been fully established. It simply operates by making a measurement of the received signal strength and thereby estimating the transmitter power required. As the transmit and receive frequencies are different, the path losses in either direction will be different and therefore this method cannot be any more than a good estimate.

Once the UE has accessed the system and is in communication with the node B, closed loop techniques are used. A measurement of the signal strength is taken in each time slot. As a result of this a power control bit is sent requesting the power to be stepped up or down. This process is undertaken on both the up and downlinks. The fact that only one bit is assigned to power control means that the power will be continually changing. Once it has reached approximately the right level then it would step up and then down by one level. In practice the position of the mobile would change, or the path would change as a result of other movements and this would cause the signal level to move, so the continual change is not a problem.