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Abstract
Free-space optical communication (FSO) is used to provide network configuration flexibility. A network-flying platform-based vertical FSO connection can be employed to enhance mobile network coverage and capacity. Solar background noise can be a potential risk that disrupts the seamless connection in the vertical FSO downlink channel. In this paper, we propose signal transmission using an orbital angular momentum (OAM) beam. The OAM demodulation process can filter sunlight out of the optical receiver except for the signal corresponding to the azimuthal state. We experimentally verified that most of the solar background noise could be reduced. To verify the feasibility of the proposed scheme in a vertical FSO channel, we modeled a FSO vertical downlink with an OAM modulation/demodulation process.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Free-space optical communication (FSO) systems are used for communication between two points at a distance of up to several kilometers. Similar to conventional wired optical communication, laser diodes (LDs) are used as transmitters, and photodiodes (PDs) are used as receivers. Therefore, because it provides a very wide bandwidth compared to RF, a much higher transmission rate can be implemented. This technology is one of the candidates for releasing rapidly increasing data traffic demand. FSO can not only use a vast optical spectrum but can also provide features such as high security, high energy efficiency, and freedom from licensing [1].
In traditional wired optical communication, a waveguide for signal transmission is used as a fiber, whereas in an FSO system, a signal is transmitted in free space without fiber. FSO systems can be easily deployed and are cost-effective compared to wired optical communication [1]. This technology also attracts interest to applying for a wide range of applications such as backhaul for cellular systems, redundant link disaster recovery, and enterprise/campus connectivity. To implement the FSO system with flexible link configuration, the FSO transceiver should be carried on a vehicle. The network flying platforms (NFPs) is one of the types to implement enhanced network coverage, capacity, and network configuration flexibility. Unmanned aerial vehicles, balloons, or drones are employed to configure layers [2,3]. And each layer must be linked either horizontally or vertically.
In an FSO system, overcoming the limitations that degrade the system performance, such as geometric losses, background noise, and weather attenuation, is challenging. In the downlink connection from high-altitude platforms (HAPs) to the gateway on the ground, the optical receiver often faces the sun. Solar background noise is a potential factor that deteriorates the transmission performance and causes link failure. Therefore, the mitigation of solar background noise needs to implement seamless communication to realize services beyond 5G. Strategies have been developed to mitigate the effects of background noise in the FSO channel. Spectrum filtering, advanced signal modulation, and spatial filtering are solutions to release the impact of solar background noise [4,5]. Nevertheless, it is still challenging for incident solar background noise to be so strong that the angle of viewing the sun is small.
In this paper, we propose a spatial filtering method based on the orbital angular momentum (OAM) mode to enhance the spatial filtering performance by using the proposed scheme together with existing spectral filtering and a narrow field of view. We transmitted the signal by the OAM beam and received the signal by OAM demodulation. Through the OAM demodulation process, most of the solar background noise that does not belong to the corresponding OAM mode was removed. We experimentally verified that the OAM demodulation process can spatially separate the OAM carrying signal from the solar background noise. We also verified that the proposed scheme was effective in the simulated vertical FSO downlink channel. The simulator modeled split-step beam propagation containing several steps of phase distortion and diffraction.
2. Vertical FSO downlink and solar background noise
The solar background noise causes unwanted beating noise during signal reception. It can divide direct and indirect noise according to the type of traveling path of sunlight. Indirect sunlight also causes communication performance degradation; but direct sunlight causes link outages, which are fatal to seamless communication systems. Direct sunlight is expressed as
There are three steps to transfer the data from the core network to the FSO gateway as shown in Fig. 1: 1. vertical FSO uplink, 2. HAP horizontal FSO link, and 3. vertical FSO downlink. For the vertical FSO downlink, in which the optical receiver faces the sky, the intense solar background noise is a potential risk for link disruption in the vertical FSO downlink of NFPs. Before the proposed solar background noise mitigation technique, we assumed a vertical FSO downlink, as shown in Fig. 2.
Atmospheric turbulence varies with altitude. The structure constant Cn2 represents the turbulence strength and follows the Hufnagel-Valley 5/7 model. We modeled beam propagation in simulation by using split-step beam propagation method (SSBPM) [6]. When the beam propagates L distance, the diffraction of L/2 distance is considered in spatial frequency domain. And then, phase distortion is considered in the space domain. Finally, the diffraction of remained L/2 distance is considered in spatial frequency domain again. If this process is configured to be multiple sections, various phase distortion can be implemented. We modeled the vertical FSO channel by 3 sections.
In Fig. 2, each phase screen (PS) represents the phase distortion in each section. Optical diffraction occurred between each PS. PS1 represents the weak turbulence, Cn2 = 1×10−17, from the HAP to 2 km in height; PS2 represents the moderate turbulence, Cn2 = 1×10−15, from 2.0-0.2 km in height; and PS3 represents the strong turbulence, Cn2 = 1×10−13, from 0.2-0 km in height.
3. OAM beam for spatial separation of optical signal and solar background noise
OAM-carrying beam has a donut-shaped intensity profile with a phase singularity at the beam center. And there is orthogonality between the different discrete twisting rates, determined by angular momentum [7]. Based on orthogonality between the OAM modes, the OAM multiplexing technique to improve data capacity has attracted interest to improve data capacity representatively [8,9].
In this work, we proposed the OAM mode to separate the solar background noise by using orthogonality of OAM beam. The sunlight arriving at the receiver can be approximate the plane wave [10]. Based on this assumption, we propose that the solar background noise can be separated using the OAM beam. The process of the proposed technique is illustrated in Fig. 3. The Gaussian beam with the data signal transformed the OAM beam using a spatial light modulator (SLM). This OAM beam can be expressed as
Fig. 3. Beam profile of signal and noise from transmitter to receiver. (I: optical intensity, P: optical phase)
4. Experiments
The proposed scheme was verified experimentally as shown in Fig. 4. Diffraction between each step of the SSBPM could not be realized in a small-sized laboratory experiment. We verified the feasibility of the proposed spatial filtering method in a fixed structure constant. The transmission performance in the vertical FSO channel is verified by simulation in the following section.
The 10-Gbps OOK signal generated from the arbitrary waveform generator (AWG) drove to Mach-Zehnder modulator (MZM). The signal was transmitted from single mode fiber to the free space by a collimator. We divided the SLM (meadowlark optics, E512-1550) into upper and lower sections for independent operation in the experiment. We defined the upper section as SLM1 and the lower section as SLM2. The SLM1 is the SLM of the transmitter, and the SLM2 is the SLM of the receiver. The SLM1 transformed the Gaussian beam which is carrying the signal into an OAM beam which is OAM mode L = 4. The amplified spontaneous emission (ASE) noise of the erbium-doped fiber amplifier (EDFA) was used as the solar emulator, and the OAM mode of emulated noise is L = 0. The SLM2 loaded the azimuthal OAM state L = −4 to the incident signal and noise combined beam for OAM demodulation for the OAM beam carrying the signal. And the SLM2 also loaded phase distortion of the FSO channel. After loaded the OAM mode L = −4 at SLM2, the signal and noise combined beam profile in the receiver plane is shown in Fig. 5(g). We separately represent the signal and solar background noise for the explanation in Figs. 5(a)-5(f). The noise profile is changed from Fig. 5(a) to Fig. 5(c) after OAM demodulation. And the signal profile is changed from Fig. 5(b) to Fig. 5(d) after OAM demodulation. The Rx collimator acts as a spatial filter to filter out background noise by only receiving beams within the diameter of the Rx collimator. Then, the signal is received to the PD (New focus, 1544B) and digital signal oscilloscopes (DSO).
Fig. 5. Beam profile in the receiver plane before OAM demodulation: (a) noise, (b) signal, and after OAM demodulation: (c) noise, (d) signal, (e) noise (simulation), (f) signal (simulation), (g) noise + signal.
It is difficult to experimentally emulate a vertical FSO channel in which the turbulence strength varies with altitude. Therefore, we verified the similarity between the experiment and simulation under a fixed turbulence strength. We then measured the transmission performance in the vertical FSO channel through a simulation. The Rytov variance, relative beamwidth, and Fresnel number determine the relationship or correlation between the emulated link and simulation link. The Rytov variance is ${\sigma _R} = {(1.64{W^{5/3}}{t^{5/6}})^{1/2}}$, where W is the relative beamwidth, and t is the normalized propagation distance [11]. Figure 6 shows the beam profile on the Rx collimator plane and the scintillation index of the experiment and simulation. There was some discrepancy under a scintillation index of less than 0.05. This discrepancy might be caused by errors in the parameters or optical structure. However, the power fluctuation was only a few decibels at a low scintillation index. Therefore, it is considered that the correlation between the experiment and the simulation is sufficient to expand the experiment of the small-scale and fixed-turbulence conditions in the simulation.
Fig. 6. Beam profile in the receiver plane after OAM demodulation in various atmospheric turbulence.
Figure 7 shows the eye diagram of the proposed OAM based signal transmission. This process was performed by changing the OAM modulation and demodulation phase screen of the SLM. And we set the received signal power before OAM demodulation as −5 dBm. In the case of transmitting the signal on OAM mode 0, which is not employing the proposed scheme, the sunlight power is out of the allowable power level of the PD, and it may cause permanent damage to the PD. So, we cannot measure the receiving performance in OAM mode 0, and we can expect that it is impossible to receive the signal. The eye diagram of OAM modes 1 to 2 shows that signal reception is impossible. The bit error rates (BER) is 1.63×10−3 and 1.51×10−5 in OAM mode 3 and 4, respectively.
We experimentally measured the solar background noise reduction performance and signal power loss in various modulated and demodulated OAM modes. As the modulated OAM mode increases, the OAM beam size increases and effectively reduces the solar background noise. The proposed scheme reduced the solar background noise by more than 33 dB in the OAM mode of 4, as shown in Fig. 8(a). The power loss of the solar background noise is different between the OAM modes, but the received signal power does not change significantly. This result shows that the proposed scheme is effective in reducing the solar background noise.
Fig. 8. Results of normalized received power in solar background noise and signal in OAM modes in (a) experiment and (b) simulation.
Noise reduction performance has some differences between the experiment and simulation. This difference might have occurred to experimental errors such as fine misalignment errors [12]. The misalignment errors cause the power loss or intermodal OAM crosstalk. The significant difference between the experimental and simulation results is the signal power loss. The experiment only verifies the feasibility of the proposed scheme, and the convex lens collects all of the beam power. To verify the transmission performance in 20-km vertical FSO downlink, the simulation considered the parameters for aperture size and beam divergence loss, which was not considered in the experiment. By considering these parameters, signal power loss occurs during signal reception because of the donut-shaped OAM beam as shown in Fig. 8(b) [7]. The signal power loss increases as the OAM mode increases, which degrades the system performance.
Considering the other noise factor such as shot or thermal noise from the optical receiver, performance degradation due to signal loss is expected to be more significant than the transmission performance improvement because of the noise attenuation performance of 40 dB or more in simulation. Thus, we conducted a 20-km vertical downlink simulation using OAM mode L = 4.
5. Simulation
We used the SSBPM with Fresnel-Kirchhoff diffraction, shown in Fig. 2, to verify the transmission performance of the proposed scheme in the vertical FSO channel [6]. The total transmitted distance is L = L1 + L2 + L3, and the propagation distance of the Fresnel-Kirchhoff diffraction can be separated by the following steps: L1/2, L1/2 + L2/2, L2/2 + L3/2, and L3/2. Each propagation distance corresponds to z as follows:
Considering the OAM beam divergence scales as L, we set the initial Gaussian beam waist before OAM modulation as 1/4 of the transmitter aperture size [14]. Figure 9 shows the average receiving power at various distances. The transmitted power is 10 dBm at the transmitter. The diameter of the transmitter aperture was 260–340 mm, and the diameter of the Rx aperture was 300 mm. This result shows that the transmission performance improves as the transmitter aperture increases because the beam divergence is inversely proportional to the beam waist.
The signal-to-noise ratio (SNR) performance can be determined by the power received at the optical receiver. We derived the simulation results based on PIN PD. The SNR is obtained as follows [15]:
Figure 10 shows the BER performance at the 20-km vertical FSO channel. The BER for vertical FSO channels with different turbulence strengths according to altitude was obtained. For comparison with 20-km FSO channel with fixed turbulence strength, BER performance was obtained in fixed turbulence strength: Cn2 = 1×10−13, 1×10−14, 1×10−15, and 1×10−17. This result shows that the BER performance of the 20-km vertical FSO channel and fixed turbulence strength with 1×10−15 are similar. This result shows that the turbulence effect of low altitude, during which the turbulence is strong, is dominant in determining the BER performance even though the turbulence strength of most propagation distances is 1×10−17. Considering the 10-dBm transmission power and Reed-Solomon forward error correction limit, the power margin is 6 dBm in the 20-km vertical FSO channel.
Figure 11 shows the average SNR of Gaussian and proposed OAM-based transmission. In Gaussian-based transmission, the solar background noise severely degrades the transmission performance when the angle with the sun is narrow. However, the proposed OAM-based transmission effectively reduces the solar background noise. The proposed scheme is effective under the condition that solar background noise occurs at >−17 dBm, but it is less effective in a condition with lower solar background power. As a result, the proposed scheme is an effective solution to release the transmission performance limitation in a vertical FSO downlink where the solar background noise is a potential factor that deteriorates the transmission performance. Power penalty occurs owing to the use of OAM; however, it can be advantageous as it provides reliability of communication. In addition, we demonstrated that the OAM-based transmission system has solar background noise robust characteristics through this study.
6. Conclusion
Considering the fact that solar background noise can potentially disrupt the reliable connection of a vertical FSO communication link, an OAM state to transmit a signal with spatial separation was proposed. The solar background noise mitigation performance of the proposed scheme was verified by experiment and simulation. These results show that the noise reduction performance becomes improved for higher OAM mode. Although higher OAM mode is effective in noise reduction, the higher OAM mode is vulnerable to the signal power loss from beam divergence in considering the size of receiver aperture in 20-km distance transmission. The solar background noise was effectively mitigated up to 40 dB at L = 4 determined as a proper OAM mode in the vertical FSO channel. The simulation result verified the feasibility of 10-Gbps OOK transmission in the 20-km vertical FSO downlink with direct sunlight conditions. This solar noise reduction technique using the OAM state would be useful for enhanced SNR achievement in vertical FSO communications.
Funding
Institute for Information and Communications Technology Promotion (2019- 0-00685).
Acknowledgements
This work was supported by Institute of Information & communications Technology Planning & Evaluation (IITP) grant funded by the Korea government (MSIT) (No.2019-0-00685, Free space optical communication based vertical mobile network).
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
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