Adjacent Channel Interference: Interference resulting from signals which are close in frequency to the desired signal is called adjacent channel interference.
Adjacent channel interference results from imperfect receiver filters which allow nearby frequencies to leak in to pass band.
It is serious problem can be a particularly serious if an adjacent channel user is transmitting very close range to a subscribers receiver, while receiver attempts to receive a BS on the desired channel this is referred to as near far effect
Next channel interference:
Interference resulting from signal frequency which is immediately next to the desired signal frequency is called next channel interference.
Suppose the desired frequency of receiver is 90.3 MHz. If it captures the frequency 91.3 MHz transmitter then it results in next channel interference.
To reduce the interference:
careful channel assignment
There should be adequate frequency separation between the spectrum of the adjacent channels in a cell
if the frequency reuse factor is large or cluster size is small the adjacent channel at the base station will be too close to each other in the frequency domain and this will increase the interference.