In a communication system, a method comprising: calculating, by a Fast Fourier Transform, FFT, calculator, a FFT output for one or more frequency null subcarriers in a first frequency channel of the communication system adjacent a second frequency channel of another communication system, comprising: calculating a FFT output for one or more upper frequency side null subcarriers in an upper side of the first frequency channel; and calculating a FFT output for one or more lower frequency side null subcarriers in a lower side of the first frequency channel; calculating a FFT output for one or more center data subcarriers in a center frequency part of the first frequency channel; calculating power values associated with the one or more frequency null subcarriers using the FFT output; calculating a power value associated with the center data subcarriers; calculating a first ratio (γ 1) between the power value for the one or more upper frequency side null subcarriers to the power value for the one or more center data subcarriers; calculating a second ratio (γ u) between the power value for the one or more lower frequency side null subcarriers to the power value for the one or more center data subcarriers; and determining, by an adjacent channel interference, ACI, evaluator, a presence of ACI from an adjacent frequency channel of the other communication system in the first frequency channel of the communication system comprising comparing (810, 816) the first ratio (γ 1) and the second ratio (γ u) to a threshold to determine the presence of ACI. calculating, by a Fast Fourier Transform, FFT, calculator, a FFT output for one or more frequency null subcarriers in a first frequency channel of the communication system adjacent a second frequency channel of another communication system, comprising: calculating a FFT output for one or more upper frequency side null subcarriers in an upper side of the first frequency channel; and calculating a FFT output for one or more lower frequency side null subcarriers in a lower side of the first frequency channel; calculating a FFT output for one or more upper frequency side null subcarriers in an upper side of the first frequency channel; and calculating a FFT output for one or more lower frequency side null subcarriers in a lower side of the first frequency channel; calculating a FFT output for one or more center data subcarriers in a center frequency part of the first frequency channel; calculating power values associated with the one or more frequency null subcarriers using the FFT output; calculating a power value associated with the center data subcarriers; calculating a first ratio (γ 1) between the power value for the one or more upper frequency side null subcarriers to the power value for the one or more center data subcarriers; calculating a second ratio (γ u) between the power value for the one or more lower frequency side null subcarriers to the power value for the one or more center data subcarriers; and determining, by an adjacent channel interference, ACI, evaluator, a presence of ACI from an adjacent frequency channel of the other communication system in the first frequency channel of the communication system comprising comparing (810, 816) the first ratio (γ 1) and the second ratio (γ u) to a threshold to determine the presence of ACI.