The system reverse paths make use of the 5 MHz to 65 MHz frequency band and subscribers’ signals are transmitted by using QPSK or 16-QAM methods. Signals transmission over the cable network of a CATV system worsens the quality of service (QoS) due to noise and distortions inherent to the active devices in the HFC system such as laser transmitters, optical receivers, optical and RF amplifiers

The C/N degradation as a signal passes through a single RF amplifier is given by the formula In the downstream direction, coaxial distribution networks with multiple, identical amplifiers in cascade are usually designed and operated so that the gain, measured from amplifier output to amplifier output, is unity. For a cascade of i identical RF amplifiers, each followed by loss equal to the gain of selected amplifier type, end-of-line C/N (in dB) can be easily calculated as follows: