1. Technical Field
Embodiments of the present invention relate to methods for transmission of a pilot channel in a cellular communication network as well as to a corresponding cellular communication networks.
2. Description of Related Art
Existing third generation (3G) cellular networks suffer from a high number of dropped calls in case of high interference in the uplink of a cell. Such high uplink interference may be due to a high number of mobile terminals that are simultaneously communicating or due to high traffic generated by some terminals in the uplink or due to power generated in the 3G frequency band by external devices. For example, devices to inhibit bomb explosions transmit radio signals in the 3G cellular frequency band and therefore add to the uplink interference in 3G networks. The effect on the uplink interference of an inhibitor device being located near a base station, for example when the car in which it is installed passes by or parks near a base station, is considerable.
When uplink interference in a cell increases, the mobile terminals in the coverage edge of the cell cannot reach the base station any more, because the signal/noise ratio of the uplink signals of these mobile terminals becomes too low for correct signal decoding by the base station. A solution provided by some manufacturers to this problem is to trigger a handover of mobile terminals not only based on the power of the downlink pilot channel received by the mobile terminal but additionally based on the uplink power of the mobile terminal received by the base station. However, even in this case call drops or failed call set-ups may occur. For example, at call set-up in a cell with high uplink interference at the base station there is no time for handing over the mobile terminals at the cell coverage edge, because the handover process takes four seconds on average. Since the radio link situation is bad from the beginning of the call set-up, the call set-up is already abandoned before the handover takes place. Furthermore, in case of Soft Handover one of the legs may be in a cell from which the mobile terminal receives the pilot channel with relatively high signal strength but the cell has high uplink interference. The other leg may be in a cell from which the mobile terminal receives the pilot channel with lower signal strength and the cell has lower uplink interference. In this situation, there will probably be a radio link failure in the cell with the high uplink interference. As a consequence, the Radio network Controller (RNC) of the 3G network removes this leg but then it will add it again because the received signal strength of the downlink pilot channel of this cell at the mobile terminal is higher than the received signal strength of the pilot channels of all other cells. The result may be a ping-pong effect with the chance of removing the cell with the lower uplink interference, which would lead to a dropped call.