A remote radio unit, RRU, comprising, a band-pass filter, BPF, a binary phase shift keying, BPSK, modulator, a variable gain amplifier and an optical signal generator, wherein: the BPF is configured to derive a local oscillator, LO, signal from a downlink optical signal received via a downlink radio over fiber, DL-ROF, from a central unit, CU the BPSK modulator is configured to modulate a BPSK waveform on to the LO signal to generate a stimulus signal and provide the stimulus signal to the variable gain amplifier, the variable gain amplifier is configured to apply a gain to the stimulus signal and provide the stimulus signal after amplification to the optical signal generator; the optical signal generator is configured to generate a to-be-transmitted uplink optical signal based on the stimulus signal for transmission via uplink radio over fiber, UL-ROF, to the CU and transmit the to-be-transmitted uplink optical signal to the CU; wherein: a received uplink optical signal corresponding to the to-be-transmitted optical signal is defined by the following equation: u BB = h UL, CU ∗ Φ UL, ROF g r BB + n BB, where u BB denotes a digital baseband representation of the received uplink optical signal, h UL,CU denotes linear distortion introduced at the CU, Φ UL,ROF denotes a non-linear distortion introduced by the UL-ROF, g denotes a gain applied to the stimulus signal by the variable gain amplifier, r BB denotes a digital baseband representation of the BPSK waveform and n BB denotes a distortion signal, wherein the variable gain amplifier is further configured to receive via a control channel a varying gain control signal sent by the CU and vary a gain applied to the stimulus signal in response to a variation in the gain control signal, whereby the CU is enabled to identify an amplitude-to-amplitude modulation, AM-AM, response of the UL-ROF and further to identify a non-linear distortion introduced by the UL-ROF. the BPF is configured to derive a local oscillator, LO, signal from a downlink optical signal received via a downlink radio over fiber, DL-ROF, from a central unit, CU the BPSK modulator is configured to modulate a BPSK waveform on to the LO signal to generate a stimulus signal and provide the stimulus signal to the variable gain amplifier, the variable gain amplifier is configured to apply a gain to the stimulus signal and provide the stimulus signal after amplification to the optical signal generator; the optical signal generator is configured to generate a to-be-transmitted uplink optical signal based on the stimulus signal for transmission via uplink radio over fiber, UL-ROF, to the CU and transmit the to-be-transmitted uplink optical signal to the CU; wherein: a received uplink optical signal corresponding to the to-be-transmitted optical signal is defined by the following equation: u BB = h UL, CU ∗ Φ UL, ROF g r BB + n BB, where u BB denotes a digital baseband representation of the received uplink optical signal, h UL,CU denotes linear distortion introduced at the CU, Φ UL,ROF denotes a non-linear distortion introduced by the UL-ROF, g denotes a gain applied to the stimulus signal by the variable gain amplifier, r BB denotes a digital baseband representation of the BPSK waveform and n BB denotes a distortion signal, wherein the variable gain amplifier is further configured to receive via a control channel a varying gain control signal sent by the CU and vary a gain applied to the stimulus signal in response to a variation in the gain control signal, whereby the CU is enabled to identify an amplitude-to-amplitude modulation, AM-AM, response of the UL-ROF and further to identify a non-linear distortion introduced by the UL-ROF.