Embodiments are provided for implementing a CSMA-CA half window scheme in 802.11 networks or other suitable wireless networks that could benefit from such scheme. The half window scheme improves a back-off time calculation by adding a probability prediction factor. The back-off time is part of a delay time in accessing a wireless transmission medium by a station (STA). The probability prediction factor is used to adjust a contention widow (CW) used to calculate the back-off time, based on the STA's medium access probability. The STA splits the CW into two at least half windows and then chooses one of the windows according to the window's information gain for the probability prediction. The selected window is used to select a random number for the back-off time. The improved back-off time calculation reduces contention between STAs in accessing the medium.