Adjacent channel interference is the results of closely spaced radio channel in frequency, so the spectral content of the adjacent channels is spilling into the bandwidth of the desired channel. Factors determining the level of the interference are signal power and spectral distribution, power and spectral distribution of the interfering signal intercepted by the receiver and distance dependence of the transmission losses between interfering transmitter and the desired signal receiver. ACI can be somewhat controlled by tight filtering, however limiting factors are spectral utilization of the system and, on the other hand, finite slope that can be achieved for filtering functions. Another way to improve adjacent channel performance is the design of spectrally efficient modulation techniques that have largely concentrated power in a limited bandwidth with good roll-off properties, which in turn requires more elaborated demodulation schemes. Also adjacent channel interference in cellular systems can be avoided by careful frequency planning, so adjacent channels are not used within the same cell or in the neighbouring cells.