Restrictions of the voice activity gain and consequences for CDMA based mobile communication systems

The system capacity of Code Division Multiple Access (CDMA) based mobile communication systems can be increased by applying the sectorization of the radio cells and by the exploitation of quiet periods of the voice. But, in the literature the resulting capacity gains are assessed too optimistic. In this article the capacity gain by applying voice activity detection is analyzed more exactly. Paul Brady has already investigated this fact in the late sixties, but the voice activity factor (VAF) was assumed to be constant for each subscriber. Practically, the VAF is different for each subscriber. Thus, by having only a small number of active subscribers we found out that these fluctuations of the voice activity factor effectuate a loss of system capacity. This effect was analyzed by means of simulations. Furthermore, an analytical estimation was derived.