Effect of Power Control and DTX in GSM
Both the power control and the DTX are standard GSM features, which are designed to minimize the interfering transmission when possible. They are both mandatory features in the mobile terminals, but it is up to the network operator to decide whether to use them or not.
DTX prevents unnecessary transmissions when there is no need to transfer information. Power control is used to optimise the transmitted signal strength so that the signal strength at the receiver is still adequate. The both features can be individually activated for uplink and downlink. Operators have been widely using both features in UL direction mainly in order to maximise the battery life in mobiles.
In a non-hopping network these features provide some quality gain for some users, but this gain cannot be transferred effectively to increased capacity, since the maximum interference experienced by each user is likely to remain the same. Also the power control mechanism doesn’t function optimally because the interference sources are stable causing chain effects where the increase of transmission power of one transmitter causes worse quality in the interfered receiver, which in turn causes the power increase in another transmitter and so on. This means that, for example, one mobile located in a coverage limited area may severely limit the possibility of several other transmitters to reduce their power.
In a random hopping network the quality gain provided by both features can be efficiently
exploited to capacity gain because the gain is more equally distributed among the users. Since the typical speech activity factor (also called DTX factor) is less than 0.5, DTX effectively cuts the network load in half when it is used. In a soft blocking limited network this means that the DTX can theoretically provide up to 100% capacity increase. Also, the power control works more efficiently because each user has many interference sources. Thus, if one interferer increases its power, the effect on the quality of the connection is not seriously affected. In fact, it is probable that some other interferers are decreasing their powers at the same time. Thus, the system is more stable and chaining effects mentioned earlier do not occur frequently.
The simulated gain for power control and DTX with different mobile speeds can be seen in the following Figure.