In this letter, we analyze statistics of signal-to-interference ratio in wireless networks operating over arbitrary fading channels in interference-limited scenarios with heterogeneous Poisson fields of transmitters and interferers. First, we derive expressions for the moment generating functions of aggregate interference and its negative fractional power. Then, we obtain a formula for the outage probability. Depending on an operational scenario, this formula presents either an exact expression or an upper bound. Some obtained results are given in terms of Mittag-Leffler's function. This special function is implemented in standard software, which makes the presented results convenient for analysis and design of wireless networks.