There are provided methods and devices for determining a quality parameter characterizing an optical communication signal, the methods being performed by signal detection devices. At the transmitting end, there are obtained a signal power P1 of a first optical signal, a signal power P2 of a second optical signal, a signal power P3 of a third optical signal, optionally a signal power P4 of a fourth optical signal, and a total signal power Ps of a channel where the first, second, third and optional fourth optical signals are located. At a detection point, there are further obtained a signal power P1′ of the first optical signal, a signal power P2′ of the second optical signal, a signal power P3′ of the third optical signal and optionally a signal power P4′ of the fourth optical signal. There are then determined a signal deformation factor SDF and/or an optical signal to ASE noise ratio OSNR from the obtained signal powers.