UNB is dedicated to long range and low power transmission in IoT networks. The channel access is Random-FTMA, where nodes select their time and frequency in a random and continuous way. This randomness leads to a new behavior of the interference which has not been theoretically analyzed yet, when considering the pathloss of nodes located randomly in an area. In this paper, in order to quantify the system performance, we derive and exploit a theoretical expression of the packet error rate in a UNB based IoT network, when taking into account both interference due to the spectral randomness and path loss due to the propagation.