A processor unit used to determine a quality indicator, QI, of a communication channel. The processor unit receives received complex symbols at an input, executes a predetermined sequence of transformations on the received complex symbols and computes the error vector magnitude, EVM. The quality indicator, QI, of the communication channel is determined based on the determined error vector magnitude, EVM. Data representing the quality indicator, QI, is outputted at an output of the processor unit. The predetermined sequence of transformations transfers all the received complex symbols to a single predetermined region containing a single target location. The error vector magnitude, EVM, is then calculated as average distance of all the processed received complex symbols in the predetermined single region to the single target location.