In low power wide area networks (LPWAN), a packet transmitted by a device can be received and decoded by all surrounding base stations (BS). If at least one of the base stations decodes the packet, the latter is delivered to the system. This scheme is called selection combining (SC) macro diversity and has been studied in the literature. However, the existing research only cares about the average packet-loss probability over the entire networks. In this paper, we consider as the main performance indicator the outage probability, which better takes into account the real availability of the service in any part of the area: the outage probability. Given an outage probability constrain, we study the minimum required BS spatial density of SC macro diversity based LPWAN. The numerical results show that if both target packet loss rate and out-age probability are 10%, the minimum BS spatial density required by SC macro diversity is at most half of that required by traditional BS attach method. The performance gain is more significant when the network load increases.