In this study a mathematical model, named ‘Single Interaction ScatTEring Reflecting (SISTER)’, to characterise wireless communication channel for outdoor and indoor environments is introduced which falls into semi-deterministic channel models. The aim of this study was to bridge the gap between the complicated theory of radio signal propagation and the idealised models using statistical channel properties for analysing multiple input multiple output (MIMO) systems. Therefore proposed model is developed based on the electromagnetic scattering and reflection and fundamental physics; however, it has been kept simple through appropriate assumptions. This model is validated by a three-dimensional ray-tracing tool. The numerical results obtained for different scenarios are presented and discussed in Part 2 of this paper.