There is provided a method for defining at least one channel reservation window the method comprising: setting length for each of the at least one channel reservation window such that each channel reservation window comprises a plurality of sub-frames; dividing the length of each channel reservation window into a transmission part and an idle part, wherein the idle part consists of one or more symbols in one sub-frame; maximizing, at a symbol level accuracy, the length of the transmission part such that a ratio between the length of the idle part and the length of the transmission part fulfills a predetermined criterion; and concatenating the transmission part and the idle part so as to form the channel reservation window.