Coverage extended MMF-based indoor OWC using overfilled launch and diversity reception.

Speckle patterns generated as coherent optical beams are reflected by scattering elements. Multimode fibers (MMFs) can modify the transverse intensity distribution of speckle patterns with macro perturbations, i.e., pressures, providing a simple and low-cost way to achieve equivalent beam-steering for indoor optical wireless communications (OWCs) with divergent optical beams. However, the received optical power (ROP) variance severely limits the mobility of user terminals. In this paper, the issue is alleviated by using the overfilled launch of MMFs and the diversity gain of multi-receivers. By adjusting the axial spatial coupling distance between the MMF and the single mode fiber (SMF) emitting coherent laser, the number of excited modes of MMF can be significantly increased at 1550 nm with negligible coupling and bending losses. In addition, the signal-to-noise ratio (SNR) enhancement obtained by applying two receivers is theoretically analyzed for the case when either thermal noise or shot noise is dominant. The experimental results demonstrate that the proposed scheme can efficiently compensate for the ROP inhomogeneity, and at the same time it can extend the achievable full steering angle up to 12° at a 1.5-m free-space distance for bit error rate (BER) values of less than 3.8 × 10-3.

Sub-column pixel neural network scheme for modulation format shifting based optical camera communications.

In this Letter, a sub-column pixel neural network (SCPNN) scheme is proposed to label the pixel patterns in modulation format shifting based optical camera communication (MFS-OCC) systems. The MFS scheme, by identifying on-off keying (OOK) and Manchester signal formats, can additionally harvest one-bit information without sacrificing the pixel width per bit, resulting in a higher transmission rate for OCC without multi-level modulation. Compared with the conventional multi-threshold decision (MTD) scheme, the proposed SCPNN can effectively mitigate the impact of inter-symbol interference (ISI) and noise on signal decision by making full use of all pixels in one-bit period. The experimental results show that, in the case of the illuminance as low as 300 lux, the SCPNN scheme can attain a data rate of 8.16 Kbps with an average bit-error-rate (BER) of 2.2 × 10-3 below the HD-FEC limitation of 3.8 × 10-3. Meanwhile, the SCPNN scheme achieves remarkable BER performance improvement compared to both MFS-OCC with MTD scheme and conventional OOK-OCC system.

Monolithic integrated two-stage cascaded SOA-PIN receiver for high-speed optical wireless communication.

Monolithic integrated receivers are highly desired due to the potential of mass production and the reduction of device size and cost. In this Letter, a monolithic integrated optical wireless communication (OWC) receiver with optical preamplifiers is designed, fabricated, and investigated to achieve high sensitivity based on photonic integration technology. The proposed receiver consists of one waveguide PIN photodetector integrated with two semiconductor optical amplifiers (SOAs). Compared with using a one-stage optical amplifier, using two independent SOAs as a two-stage amplifier offers the advantage of optimizing the noise figure of each amplifier independently by tuning their injection currents, which leads to the reduction of the total noise and an improvement of the receiver sensitivity. The achieved sensitivity for a 10-Gb/s OOK signal with 10-dBm launch power at 1550-nm wavelength by using the designed receiver is up to -27.5 dBm at a bit-error-ratio (BER) level of 3.1×10-3 over a 0.9-m indoor free-space link. The experimental results show the potential to achieve a high-speed OWC link with high sensitivity by using a cascaded SOA/PIN monolithic integrated receiver.

User-specific power level allocation optimization for OFDM-NOMA in a multi-user RoF system.

In this Letter, we propose a digital signal processing (DSP) -aided technique to optimize the power ratio among users for orthogonal frequency division multiplexing (OFDM) -based non-orthogonal multiple access (NOMA) in an integrated optical fiber and millimeter wave (mmWave) wireless communication system. In this way, a central or distributed unit can leverage the proposed techniques to maintain the uniformity of the signal-to-noise ratios (SNRs) among subcarriers without requiring any channel information feedback. The proposed mechanism can facilitate the power allocation management by treating all subcarriers equally as an independent channel. As an illustration, multiple NOMA scenarios, in which a near user with 10 km fiber transmission and far user with either longer fiber distance or additional wireless propagation, are experimentally investigated. Experimental results demonstrate that when the conventional OFDM-NOMA without the proposed DSP-aided technique is used, the optimal power ratios vary rapidly when the subcarrier quality index changes due to high-frequency fading in a mmWave radio over fiber (RoF) system, whereas, by using the proposed techniques, including both orthogonal circulant matrix transform and discrete Fourier transform, the optimal power ratios on all effective subcarriers are optimized at the same level and the users' performance is significantly improved.