Enhanced MIMO-FSO fronthaul design based hybrid VCSEL with wideband travelling-wave semiconductor optical amplifier for 5G C-RAN applications

Abstract
This work proposed two promising architectures over multi input multi output free-space optics\r\n(MIMO-FSO) channel, namely hybrid and non-hybrid optical modulators, aiming to improve\r\nthe capacity of fronthaul connectivity for cloud radio access network networks in case of severe\r\nweather conditions utilizing end-to-end Gamma–Gamma turbulence channel. A proposed hybrid\r\ntransmitter design combined a vertical-cavity surface-emitting laser (VCSEL) pumping an external\r\nMach–Zehnder modulator (MZM) whereas the non-hybrid system utilized only MZM. To com-\r\npensate atmospheric losses, both designs employ a wideband travelling-wave semiconductor\r\noptical amplifier (WTWSOA) to improve the received signal intensity. The hybrid scheme utilized\r\nto leverages the VCSEL’s low-threshold efficiency and MZM’s chirp control. Based on the simu-\r\nlation results obtained by Optisystem 21 simulator, the hybrid optical modulator demonstrated\r\nsignificant performance improvement compared with the counterpart non-hybrid modulator.\r\nThus in severe rain condition with 19.275 dB km−1 attenuation, the hybrid MIMO-FSO tech-\r\nnique works quite well, with Q-factor values of 29.8 at the link length of 1.9 km, while the non-\r\nhybrid architecture (MIMO-MZM FSO), achieves Q-factor of 10 at the same range. The simula-\r\ntion results report that the hybrid VCSEL–MZM modulator has a 2.1–2.8 average Q-factor gain\r\nover a single MZM transmitter about 35% drop in bit error rate at 50 dB km−1 attenuation. Under\r\nmild turbulence, MIMO diversity increases the possible link distance by 1.5–2 km. Considering\r\nstrong turbulence Cn2 = 10−13 m−2/3, the hybrid modulator achieves a peak Q-factor of approx-\r\nimately 27 at strong attenuation (around 40 dB km−1), maintaining Q-factor >6 even when atten-\r\nuation increases to 55 dB km−1. Meanwhile at weak turbulence condition the reported Q-factor\r\nis 43 at attenuation of 40 dB km−1 and remains acceptable Q-factor ⩾6 up 60 dB km−1. These\r\nresults show that the suggested hybrid modulator along with WTWSOA are crucial techniques in\r\nimproving the transmitted signal over FSO fronthaul especially in cities where there is a lot of fog\r\nand turbulence.

Author
raghad zuhair yousif

DOI
https://doi.org/10.1088/2040-8986/ae3df5

ISSN
2040-8986

Publish Date: 10-Feb-2026