For the next-generation wireless back-haul network, free space optical (FSO) communications are considered in non-terrestrial networks. The fading issues for the atmospheric turbulence and the misalignment become important to achieve high received power for seamless and high data rate communications. The spatial diversity technique could be the solution to mitigate these fading issues in FSO systems with a few meters of distance between transmitters larger than the coherence length. However, the distant arrangement of the transmitters causes additional alignment errors in the misalignment detection process in the pointing, acquisition, and tracking (PAT) systems, which increases the pointing loss. Therefore, the increased pointing errors should be considered to obtain desired diversity gain. In this work, we develop a statistical misalignment model due to multiple beam transmissions and analyze the transmission performance for the spatial diversity based vertical FSO links. The proposed misalignment model is investigated by the log-normal atmospheric fading channels, the distant arrangement of transmitters, and the centroid algorithm in the PAT systems. The increased alignment error in the misalignment process for multiple beam transmissions is experimentally demonstrated. The spatial diversity based FSO systems require larger beam width to compensate for the increased pointing error. The simulation results show that the system optimization with the misalignment model can increase achievable diversity gain as the number of channels increases. The proposed scheme provides an enhanced link budget to design seamless FSO based mobile back-haul networks.
We propose an asymmetric 3×2 multi-input multi-output (MIMO) system for polarization division multiplexing (PDM) transmission in visible light communication (VLC). In PDM transmission, independent channels are constructed by polarization orthogonality, which is vulnerable to misalignment of polarization between the transmitter and the receiver. Although PDM-based VLC system may not maintain polarization orthogonality, the proposed asymmetric MIMO system can achieve the polarization diversity required for a multichannel system. We experimentally demonstrate the performance enhancement of PDM VLC transmission using the proposed asymmetric 3×2 MIMO technique.
A modulation format of differential pulse amplitude modulation in visible light communication using a single transmitter module composed of multiple semiconductor light‐emitting diode (LED) chips is proposed and experimentally demonstrated. Precoded different nonreturn‐zero on–off‐keying signals were assigned to each LED chip, and linearly overlapped signals were detected by a single photodiode. In this demonstration, pre‐equalization and optical filtering were applied to compensate the modulation bandwidth limitation and nonlinearity of the light source. Data rate up to 200 Mbit/s with less than 7.8×10−5 bit error rate was verified by experiment using the proposed modulation method.
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