An analytic model for deriving the node-B transmit power in heterogeneous UMTS networks

UMTS operates with WCDMA over the air interface. Most studies dealing with the capacity of CDMA networks consider the uplink (reverse link) and evaluate the coverage or capacity of a cell or network. This focus on the uplink originates in the fact that the IS-95 network was a single voice network, such that the network performance was limited by the uplink. Furthermore, fast power control was then implemented on the uplink only. The introduction of 3G networks leaves the pure voice networks behind and instead provides a variety of different services which produce asymmetric traffic, with the bulk on the downlink (forward link). This traffic asymmetry shifts the capacity limit from the uplink to the downlink. The 3gpp standard prescribes the use of fast power control for the downlink as well. We propose a model to calculate the first and second moment of the node-B transmit power which allows us to approximate its distribution and to determine the probability that the system becomes unstable when a certain transmit power is exceeded.