Thorough Analysis of Downlink Capacity in a WCDMA Cell

In WCDMA networks, the Call Blocking Probability (CBP) assessment is necessary for proper cell capacity determination, in respect of traffic load in erlangs, and network dimensioning. This paper focuses on the downlink capacity estimation, through CBP calculation in a WCDMA cell. To this end, we study an analytical model for the WCDMA cell by taking into consideration the effects of the following: the multi-service environment, the soft blocking, the imperfect power control and multipath propagation. In this model, the maximum transmission power of a base station in the downlink is considered as the shared system resource. To analyze the system, we follow the methodology proposed by Mäder & Staehle, and describe the WCDMA cell by a Markov chain, where each system state represents a certain number of resources occupied by mobile users. We solve the Markov chain and provide an efficient recurrent formula for the system occupancy distribution, as well as the so-called local blocking probabilities. Based on them, we calculate the CBP of different service-classes accommodated in the cell, versus the total offered traffic load. We evaluate the analytical model through simulation. The results show that the accuracy of the model is very satisfactory. The main contribution of this paper is the improved determination of several parameters involved in the downlink capacity estimation, in comparison to the calculations that appear in the work of Mäder & Staehle. In addition, we show the effect of the intra-cell interference (due to orthogonality factor) on the erlang capacity of the cell.