The quantum efficiency of GaN-based micro-light-emitting diodes (micro-LEDs) is of great significance for their luminescence and detection applications. Optimized passivation process can alleviate the trapping of carriers by sidewall defects, such as dangling bonds, and is regarded as an effective way to improve the quantum efficiency of micro-LEDs. In this work, an AlN passivation layer was prepared by atomic layer deposition to improve the electro-optical and photoelectric conversion efficiency in GaN-based micro-LEDs. Compared to conventional Al₂O₃ passivation, the AlN passivation process has a stronger ability to eliminate the sidewall defects of micro-LEDs due to the homogeneous passivation interface. Our experiments show that the AlN-passivated device exhibits two orders of magnitude lower forward leakage and a smaller ideality factor, which leads to significantly enhanced external quantum efficiency (EQE). For 25*25 μm² micro-LEDs, the EQE of the AlN-passivated device was 18.3% and 57.7% higher than that of the Al₂O₃-passivated device in luminescence application and detection application, respectively.

Nowadays, how to further enhance the spectrum efficiency is one of the key technologies for band limited scenarios in underwater visible light communication (UVLC) systems. In this paper, a novel subcarrier component extraction (SCE) with complex independent component analysis (ICA) algorithm is proposed in the non-orthogonal multiband CAP UVLC systems. With the proposed SCE-ICA algorithm, the non-orthogonal multiband CAP system can support a higher overlapping level in contrast to the conventional CAP scheme and therefore hold a higher spectral efficiency. In the simulation, varying degrees of performance enhancement is achieved in comparison with the conventional CAP scheme with different roll-off, compression factor (CF) and signal-noise ratio (SNR). Then, we first discover that in the simulated frequency-selective fading channels, moderate overlapping among subcarriers can achieve the best performance in system-level in contrast to the non-overlapping case. Moreover, the UVLC system experimental investigation of overlapping 2-CAP and 5-CAP scheme is carried out. The experiment demonstration shows that the utmost spectral efficiency enhancement of 40% and 18% are achieved compared with the non-compression CAP case and the conventional demodulation of the overlapping CAP case, respectively. Finally, with the low computational complexity of O(N), the proposed algorithm can be a potential candidate to handle the equalization of non-orthogonal multiband CAP schemes.

In multiuser access visible light communication (VLC) systems, many multiplex techniques can be utilized to fully exploit the limited bandwidth. In frequency-division multiplexing systems, one possible solution is to dynamically allocate limited bandwidth resources for users to support multiple-service access. To enable high-speed transmissions, the super-Nyquist multiband concept has been proposed in recent years. Specifically, the center frequency spacing is narrowed, and the subcarriers are nonorthogonally overlapped to enable more services and users in multiband systems. Studies have shown that this strategy can support higher spectral efficiency (SE) while simultaneously, introducing intersymbol interference (ISI) and intercarrier interference (ICI). Therefore, a well-designed ICI mitigation scheme is required. In multiband carrierless amplitude phase ( m -CAP) modulation systems, it is technically challenging to achieve the SE beyond one baud/s/Hz under the pre-forward error correction (pre-FEC) threshold without increasing the modulation order. Inspired by the distinctive pulse shaping filter (PSF) structure in a non-orthogonal m -CAP scheme, we propose the iterative decision feedback kurtosis minimum (DFKM) scheme and achieve the super-Nyquist SE of approximately 1.06 baud/s/Hz. In contrast to existing ICI mitigation schemes, the theoretical framework reveals that symbol-level crosstalk filters are indispensable to reap the benefit of super-Nyquist transmission, not limited to VLC.

In this paper, we propose a novel sparse data-to-symbol neural network (SDSNN) receiver for bandwidth-limited underwater visible light communication (UVLC) based on non-orthogonal multi-band carrierless amplitude and phase modulation (NM-CAP). Bandwidth limited NM-CAP signals usually carry severe inter-symbol interference (ISI) and inter-band interference (IBI). The SDSNN receiver directly converts the received NM-CAP data with ISI and IBI into quadrature amplitude modulation symbols without distortion for each sub-band. In contrast, the conventional receiver requires the least mean square (LMS) equalizer to cancel ISI, and the subcarrier component extraction with complex independent component analysis (SCE-ICA) to cancel IBI, respectively. SDSNN provides a novel receiving structure to replace post-equalization, matched filtering, and SCE-ICA. A blue-LED based UVLC system has been demonstrated utilizing NM-CAP16 with 3 sub-bands. The experimental results show that NM-CAP with the SDSNN receiver case reaches the highest spectral efficiency, where an enhancement of 43%, 20%, 6% has been measured over the orthogonal multi-band CAP case, NM-CAP with LMS equalizer case, and NM-CAP with joint LMS equalizer and SCE-ICA case, respectively. Compared with joint LMS equalizer and SCE-ICA case, the proposed SDSNN receiver can achieve 98% reduction of computational complexity.

Next generation fronthaul interface (NGFI) has been adopted as the mobile fronthaul (FH) transportation solution of the next generation (NG) mobile network, which requires high bandwidth and architecture flexibility. Spectral efficiency (SE) is a key performance indicator that worth discussion for NG mobile FH. In this paper, a novel inter-band interference (IBI) cancellation method based on complex independent component analysis (ICA) is proposed in non-orthogonal multiband CAP (NO-m-CAP) system. The interfering distortion caused by spectral overlapping between adjacent sub-bands in non-orthogonal multi-band CAP is mitigated by the proposed method, and the overall SE has been improved. As a proof of concept, an experimental demonstration of 100Gb/s/λ NGFI-II data transmission over 25km SSMF is carried out. Simple intensity modulator at central unit (CU) and a coherent detection at distributed unit (DU) are applied to achieve high-performance, low-cost and flexible mobile FH data transmission. A maximum SE improvement of 15% is obtained with the proposed method for both 4-band and 8-band assuming a 7% HD-FEC limit. Flexible data rate allocating, from 12.5Gb/s to 100Gbps with a bandwidth granularity of 12.5Gb/s, is also demonstrated.

In this paper, we demonstrate a Toeplitz concatenated matrix aided independent component analysis (TCM-ICA) equalizer that can skillfully compensate for the inter-channel interference (ICI) of super-Nyquist multiband carrierless amplitude and phase modulation (m-CAP) system. In the point-to-point super-Nyquist m-CAP visible light communication (VLC) scenario, we experimentally demonstrated a system-level average spectral efficiency (SE) enhancement of 0.50 b/s/Hz in comparison to the conventional scheme. In the multiple-input single-output (MISO) super-Nyquist 5-CAP scenario, the subcarrier-level Q factor enhancements of 4.4 dB, 5.2 dB, and 6.5 dB are achieved in comparison with the least-mean-square (LMS) post-equalizer for the 2^nd, 3^rd and 4^th subcarrier, respectively. As far as we know, the TCM-ICA equalizer is the most effective ICI compensation scheme to enhance SE in the super-Nyquist m-CAP systems.