Optimization of AMI-MDM-RoFSO under atmospheric turbulence

Radio over Free Space (Ro-FSO) is promising candidate for providing ubiquitous digital services especially in rural areas. This work investigates the performance of MDM of two 5Gbps-10GHz data channels over FSO link using LP 01 and LP 02 modes under the effect of atmospheric turbulences. The signal to noise ratio (SNR), total received power, modal decomposition at receiver at the receiver is also reported. The reported result shows the successful transmission of two channels with acceptable SNR over FSO link under atmospheric turbulences.


Introduction
The plethora of wireless network platforms includes numerous operators with independent radio base stations and networks. The diversity of wireless services leads to excessive investment on infrastructure and redundancy of equipment, which may affect cellular architecture efficiency. According to reports by the International Telecommunication Union (ITU), cellular services witnessed a remarkable growth of 7.5 billion subscribers in 2013 [1]. Despite this, radio frequency is spectrum is still a scarce commodity.
The limitation of radio frequency amidst the growth in mobile subscribers has motivated the development of Ro-FSO, a technology that transmits multiple RF signals through a high-speed optical carrier without investing excessively on optical fiber cabling [2, 3]. As Ro-FSO utilizes optical carriers to exploit various electromagnetic spectrum for mobile networks, it efficiently eliminates congestion issues and licensing of RF signals prevalent in the current wireless networks. Ro-FSO is a multi-faceted technology that combines radio-over-fiber (RoF) and free-space optical (FSO) technologies. RoF is used for centralizing processes including RF handoff, up-down conversion, coding, switching, and multiplexing among the base stations [4]. FSO is used for transporting data signals at a higher bandwidth through atmosphere than radio signals. Thus, Ro-FSO is efficient for integrating radio and optical networks [5].
However, the performance of Ro-FSO can be affected by atmospheric turbulences arising from fog, rain and snow [6]. In order to achieve high capacity in Ro-FSO while optimizing network resources, researchers have highlighted the use of advanced modulation formats [7]. Alternate mark inversion (AMI) has been used for improving the capacity of fiber transmission. AMI is a bipolar encoding system which has binary 0 as the neutral voltage and binary 1 as the alternating positive and negative voltages. However, AMI has not been exploited in free-space optical network systems. Another bandwidth-enhancing strategy, known as mode division multiplexing (MDM), is used for enhancing the capacity of free-space optical networks [8][9][10][11][12] by leveraging the abundance of fiber eigenmodes for transmitting various channels by generating specific modes by means of optical signal processing [13][14][15] , spatial light encoding [16][17][18], few mode fibers [19].
The current study aims to utilize both aforementioned strategies, specifically a new combination of AMI and MDM in Ro-FSO systems for increasing the capacity and transmission distance, which thus far has not been covered in any previous studies. In order to achieve this objective, a simulation of an MDM system on Linearly Polarized (LP) 01 and 02 modes using AMI-encoded signals will be performed over a 3 km FSO link to optimize the parameters of the system under various atmospheric conditions.

System Description
MDM-RoFSO modelled in OptiSystem TM . AMI is used for 5 Gbps data encoding while its line codes are EPJ Web of Conferences 162, 01020 (2017) DOI: 10.1051/epjconf/201716201020 InCAPE2017 generated using delay-and-add and delay-and-subtract operations [20,21]. Data is transmitted through pulse amplitudes of on-off-keying (OOK) while impressing its phase modulation using mathematical operations of binary input signal x(n) = [22] and output signal yAMI (n) as: The two channels are launched from LP 01 mode and LP 02 mode, where yAMI is the AMI sequence, modulated using a Lithium Niobate dual-port optical modulator and 10GHz radio signal through a 850 nm spatial laser respectively after propagation through a multimode fiber.
The two modes are transmitted over a 3 km FSO along with the pre and post compensation using optical amplifiers. The FSO link equation is given as [24]: 2 /10 Re where dR is the receiver aperture diameter, dT is the transmitter aperture diameter, is the beam divergence, R the range, and the atmospheric attenuation.
The performance of the proposed Ro-FSO system under specified atmospheric conditions is investigated by adjusting the atmospheric attenuation value for different atmospheric conditions, i.e, 0.14 dB for clear weather conditions, 4 dB for hazy weather, 9 dB/km for thin fog, 13 dB/km for light fog, 16 dB/km for thick fog, and 22 dB/km for heavy fog [25,26].
An Avalanche photodetector with low pass filter is used for recovering the original baseband transmitted signal. The mean squared error minimization of the estimate of sum of electric fields and the output field is performed [27] to retrieve the modal composition at the receiver [27].

Results and Discussion
As shown in Fig. 2  From Fig. 2 and Fig. 3, it is evident that the FSO link reach is extended under various atmospheric conditions: 3900 m in hazy weather, 2700 m in light fog, 2100 m in moderate fog, and 1600 m in heavy fog with acceptable SNR values for channel 1. The largest amount of power for channel 1 is coupled into LP 01 mode, as shown in Fig. 4.

Conclusion
Numerical simulations demonstrate that MDM system on LP 01 mode and LP 02 mode using AMI-encoded signals are viable for extending the range of the FSO link with acceptable SNR. The results show that the for channel 1, FSO links prolong to 3.9 km in hazy weather whereas the links prolong to 2.7 km in case of light fog. In case of moderate fog, the FSO links prolong to 2.1 km and to 1.6 km in heavy fog. For channel 2, FSO link prolongs to 2200 m in hazy weather whereas 1375 m in light fog. In case of moderate fog, FSO link prolongs to 1050 m and to 900 m in heavy fog.