Impact of aggregate interference on meteorological radar from secondary users

In this paper, we investigate the impact of aggregate interference in a secondary spectrum access system. Particularly, meteorological radar operating in 5.6 GHz band is considered to be a primary user. Secondary users are WLAN devices spreading in a large area which induce aggregate interference to the radar. We develop a mathematical model to derive the probability distribution function (PDF) of the aggregate interference. The derivation considers dynamic frequency selection (DFS) mechanism for the protection of the radar such that the transmission of each WLAN is regulated by an interference threshold. Numerical experiments are performed with various propagation environments and densities of WLAN devices. It is observed that the effect of aggregate interference is severe in a rural environment. The interference threshold for individual WLAN should be much lower than the maximum tolerable interference at the radar. Thus, only a limited number of WLANs can transmit at the same time. On the other hand, adverse effect of the aggregate interference is not shown in an urban environment, where up to 10 WLANs per square kilometer can use the radar spectrum without considering the aggregate interference. The framework discussed in this paper can readily be adapted to assess the aggregate interference for other types of radars.