Radio over free space optical (RoFSO) communication has demonstrated its utility in ultra-high speed and spectral-efficient data transmission in an unguided medium, i.e., free space [1]. It has been used to transport large volumes of interference-free secure data in less time with less power consumption over license-free bandwidth. In RoFSO, the radio frequency (RF) signal is used to transmit using an optical carrier through free space as the channel. As a result, RoFSO has emerged as an alternative solution for various applications such as wireless local area networks (WLAN), 5G mobile data transmission, and internet of things (IoT) services [2]. However, the RoFSO link has to face various types of atmospheric attenuations due to different weather conditions such as haze, dust, fog, rain, and snow during optical signal transmission [3]. The channel models for different weather conditions implemented in this research work have been reported and discussed in Section 3 in detail. To increase the transmission range while maintaining high-speed data transmission under such weather conditions, some major modifications such as modulation techniques, appropriate light source, estimation of the transmitting power levels, selection of transmission wavelength, and appropriate optical detector have to be implemented at the transmitter and receiver side [4].
In adverse weather conditions, FSO systems generally transmit data through adaptive threshold transmission schemes such as On-Off keying modulation. These schemes are not only difficult to implement, but they significantly increase the likelihood of fading errors in received data in bad weather.
In this paper, we implemented a QAM-16 modulation scheme to fix this problem. In adverse weather conditions where the transmission link degrades rapidly, QAM-16 facilitates high data rate transmission while maintaining excellent phase margin. As the QAM order increases, the distance between the constellation points reduces, and the probability of data errors increases. Furthermore, transmission of high-order QAM schemes requires a very high signal to noise ratio (SNR) to maintain a threshold SNR level to avoid poor link performance. QAM-16 provides a balance between data throughput and the requisite signal-to-noise ratio below 20 km of transmission range [5].
According to recent research, various multiplexing techniques, such as time division (TDM), wavelength division (WDM) and mode division multiplexing (MDM), has been found highly efficient in increasing transmission range while improving transmission rate and spectral efficiency. Among these approaches, mode division multiplexing (MDM) has been identified as a novel and promising candidate that optimize spectrum efficiency and data rates and extends the transmission range in FSO transmission systems at acceptable SNR. These spatial laser modes are produced by using a various technique, such as spatial light modulators, photonic crystal fibers [6]. Recently, different studies proposed the implementation of Laguerre–Gaussian (LG) and Hermite Gaussian (HG) modes in FSO systems employing multiple channels under high attenuation environments such as dense dust, fog, haze and strong rain weather conditions.
The transmission of an OOK modulated single channel at a data rate of 2.5 Gb/s over various dusty weather conditions was examined. Under dense dust, a link range of less than 0.2 km was achieved for 22 dB transmitted power. It is improved to 1 km in
light dust conditions. BER, SNR, and channel capacity were used to evaluate performance. The results demonstrate a minor improvement in FSO system performance under dusty conditions, hence it is regarded as an ultimate impairment. No multiplexing techniques have been reported to increase transmission range [9].
The 8 x 5 Gb/s Channel MDM-FSO system with LG modes transmission was developed for clear sky to rain conditions to achieve the greatest possible link range at a 1550 nm wavelength. The FSO system transported data at a rate of 40 Gb/s for a distance of 3 km in clear weather and 0.650 km in severe rain conditions. Quality data reception has shown that BER performance has improved. [10]. 10 Gb/s information was sent at 10 dBm power of employing a modified Duo-binary return to zero modulation (MDRZ) technique for optimal reception at a single channel receiver with an equal gain combining technique. An analysis of optical signal scintillation owing to rain-induced attenuation losses has been examined. for a link range of less than 1 km Suriza, Carbonnea, Japan, and Samir models were used to estimate rainfall-induced attenuation for a given rainfall rate. An increase in BER, Q factor, and received optical power implies that the MDRZ-based FSO link has improved significantly [11].
A high-speed passive network with a 10 Gb/s FSO hybrid link has also been reported to provide a solution to last-mile bottleneck problems. A passive fibre network of 10 to 20 km in length was combined with the FSO system. Under strong atmospheric turbulence conditions, the FSO system was capable of transmitting high-speed data over a link range of 0.1 to 1 km. An MDM has also been integrated into this system to improve the efficiency of the FSO link, employing four LG00, LG01, LG10, and LG11 laser modes. Among these 4 modes, LG00 mode has found highly suitable in increasing the link tinge up to 1 km for acceptable SNR [12].
The incorporation of the decision feedback equalization (DFE) technique into the MDM-FSO system improved the link range even more. The system's performance was evaluated in fog, rain, and haze conditions. To compensate for atmospheric attenuation, the MDM-FSO receiver used the minimum mean square error (MMSE) technique in tandem with DFE. The proposed system included three channels, each of which carried 2.5 Gb/s data rates using HG modes. Under medium fog, rain, haze, and clear weather conditions, a transmission rate of 7.5 Gb/s was achieved over 0.40 km, 0.8 km, 1.4 km, and 2 km, respectively. The designed system's simulation results confirmed excellent throughput with the desired BER. [13].
The performance of the MZM-MDM-FSO system is compared to that of the MZM-MDM-FSO system based on QAM-16 modulation in this proposed research work. To produce a 20 Gb/s-10 GHz optical signal, two independent channels, each carrying 10 Gb/s of data, are modulated with MZM/QAM-16 modulation schemes and transported via two HG (HG00 and HG01) laser modes at 10 GHz RF signal employing mode division multiplexing. Section 2 discusses in detail into the mathematical expression and modelling of various HG modes [15]. This MDM signal is transmitted at a wavelength of 1550 nm through free space link to assess the effects of different clear sky, dense haze, dense fog, and severe rain conditions. Section 3 also discusses and presents the mathematical modelling of atmospheric attenuation produced by these climatic conditions. For strong turbulence, the refractive index structural parameter of the free space link is chosen as\(C_{n}^{2}={10^{ - 13}}{m^{2/3}}\). We use some performance metrics such as BER, eye diagram, Quality factor with respect to change in transmission range and the findings identified finally are compared with the precious research work in section 5. The findings confirm the significant improvement in transmission link by using mainly HG00 laser mode transmission.
Section 5 finally conclude the entire proposed work and its research findings. It also identifies the possible improvements in the existing proposed work. The content of this research paper is finally summarized as illustrated in Fig. 1 in the form of work flow.