|Abstract: ||The mobile communication networks in 5G and beyond aim to provide better reliability, improve the coverage, and increase the communication speed. The number of users increases drastically. In 5G, the number of base stations, called Transmission Reception Point (TRP) in 5G, will increase dramatically, and smaller sells will be
used. The central station (CS) must be placed in an easily accessible and securely maintained location. Implementing this scenario will increase the cost rapidly and would be challenging to manage, troubleshoot, and monitor. As an alternative, the central station can be placed in a central location and can be accessed easily. Only the TRP will placed in the location. The TRP then should be connected to the central station using Radio over Fiber (RoF) links. The TRP unit should be as simple as possible since this will lower the cost and increase the reliability. Orthogonal Frequency Division Multiplexing (OFDM) has been chosen as the main
waveform to be implemented in 5G as a transmission technique. OFDM is compatible with Multiple Input Multiple Output (MIMO), which is the key factor in the new generation communication systems. The large bandwidth and spectrum allocation of OFDM are suitable for 5G requirements. However, the bandwidth of
the wireless systems is still limited compared with very high bandwidth that can an optical transmission link provide. Since the bandwidth of 5G antennas is very wide, it is possible to send multiple technologies in the same fiber link from the central station to the TRP unit. For instance, in the same link, WiMax, WiFi, 4G, and 5G signals can be transmitted simultaneously. In the TRP unit, all these signals can be optically received and
wirelessly retransmitted at the same time. This approach will reduce costs dramatically, increase the reliability, and reduce the need to monitor and maintain each technology separately. In the previous approach, a reliable light source that can provide high-speed rate is crucial. The best candidate in this situation would be the Vertical-Cavity Surface-Emitting-Laser (VCSEL). VCSEL has many advantages such as low fabrication and
packaging costs, high performance, high efficiency, easy fiber coupling, and large modulation bandwidth.
However, VCSELs have some disadvantageous such as polarization instability, which is sometimes known as polarization switching, low output power, relative noise intensity, and nonlinearity. Sending multiple signals into the same optical system (transmitter, fiber link, and photodetector) will create unwanted harmonics which will deteriorate the analogue performance drastically. The primary source of nonlinearity in the Rof link of is the VCSEL. The second-order harmonic distortion and third-order intermodulation distortion are considered to be the main drivers of nonlinearity inside the VCSEL. To suppress the nonlinearities in the VCSEL or come around it. Multiple workarounds have been proposed. Different VCSELs have different harmonic distortion minima at different frequencies and different temperatures. Planing each OFDM signal to fall into the distortion valley will relieve the effect of the distortion on the signal. Another workaround is inherited from the WDM technology where each OFDM signal can be sent in one VCSEL in the VCSEL array. Another solution is the selective optical feedback where a fraction of the output power injected back into the VCSEL after changing the polarization of the reinjected signal. This solution shows to be a good approach to suppress the nonlinearities related to polarization instability in VCSEL. It has been shown that harmonic distortions strongly depends on the type of the polarization that injected back into the VCSEL since the Parallel Optical Feedback (POF) will increase the distortion and create new harmonics at higher frequencies. However, Orthogonal Optical Feedback (OOF) will suppress the harmonic distortion strongly.|