Due to the low sampling rate of the camera based receiver, the movement of communication devices becomes a factor that cannot be ignored in the demodulation of optical camera communications (OCC). In this paper, we propose the block matching demodulation (BMD) algorithm, which is a movement-robust and optical-interference-resistant demodulation method for OCC. The BMD algorithm can resist the movement of communication devices by locating communication light source per image and immunize optical interference by utilizing the fast time-varying characteristic of communication light source. Moreover, in order to enhance the reliability of communication systems, BMD algorithm and Manchester decoding are combined to realize error detection and correction without increasing the overhead of encoding. Finally, we build a practical mobile OCC system to test the performance of BMD algorithm. Experimental results show that the BMD algorithm can resist the movement of communication devices effectively and achieve reliable demodulation.

Although optical camera communication (OCC) is gaining increasing attention in research, developing a practical OCC system to increase data rate and transmission distance is still an issue. The rolling shutter can increase data rate, but it also limits the transmission distance at the same time. When the transmission distance is long, the thresholding method will become difficult and the block sequence number (BSN) is easy to be wrong due to bit errors, which will make it impossible to reorder the frame sequences. In this paper, in order to increase the transmission distance, we propose the efficient thresholding method which transforms partial mean gray values of per column nonlinearly to increase the contrast between light and dark bands. In addition, this method takes the frame image of the camera as the basic unit to adjust the threshold for the environment in which each frame of the image is located. Experimental results show that this method is better than the polynomial regression. Moreover, we perform the error correction of BSN by using its continuity to specify the legal BSNs and the illegal BSNs.

Due to the directionality of light, the hidden device problem and the obstruction cannot be ignored for carrier sense multiple access with collision avoidance (CSMA/CA)-based uplink visible light communication (VLC). In this paper, we introduce multipacket reception (MPR) to handle the hidden device problem in VLC system. We model the traffic of the device with on/off Markov source. With the unsaturated traffic, we formulate a two dimensional (2D) Markov chain to model the CSMA/CA-based slotted random access procedure to evaluate the effects of hidden devices and obstructions on the performance of MPR-aided VLC system, which are mapped into the transition probabilities of the Markov chain. Then, we analyze the throughput and the reception power efficiency (RE) of MPR-aided VLC system with the obstructed optical channel. Numerical results show that the effect is negative when hidden devices or obstructions appear solely. But when they appear simultaneously, they will interact with each other to mitigate the negative effects.

The medium access control (MAC) protocol for indoor visible light communication (VLC) with energy harvesting is explored in this paper. The unfairness of throughput exists among devices due to the significant difference of their energy harvesting rates which changes with distance, acceptance angle and the obstruction probability. We propose an energy harvesting model, a new obstruction probability model and an energy adaptive contention algorithm to overcome the unfairness problem. This device can adjust its contention window according to the energy harvesting rate. As a result, the device with lower energy harvesting rate can get shorter contention window to improve its transmission opportunity. Simulation results show that our MAC protocol can achieve a higher degree of fairness.

A location-adaptive transmission scheme for indoor visible light communication (VLC) system is proposed in this paper. In this scheme, the symbol error rate (SER) of less than 10^-3should be guaranteed. And the scheme is realized by the variable multilevel pulse-position modulation (MPPM), where the transmitters adaptively adjust the number of time slots n in the MPPM symbol according to the position of the receiver. The purpose of our scheme is to achieve the best data rate in the indoor different locations. The results show that the location-adaptive transmission scheme based on the variable MPPM is superior in the indoor VLC system.

This paper exploits an optical large multiple input multiple output (MIMO) system. We first establish the non-reciprocity compensation correction factor to solve the channel non-reciprocity problem. Then we propose an antenna selection algorithm with the goal of realizing maximum energy efficiency (EE) when satisfying the outage EE. The simulation results prove that this non-reciprocity compensation correction factor can compensate beam energy attenuation gap and spatial correlation gap between uplink and downlink effectively, and this antenna selection algorithm can economize the number of transmit antennas and achieve high EE performance. Finally, we apply direct current- biased optical orthogonal frequency division multiplexing (DCO-OFDM) modulation in our system and prove that it can improve the bit error rate (BER) compared with on-off keying (OOK) modulation, so the DCO-OFDM modulation can resist atmospheric turbulence effectively.

To facilitate the efficient support of quality-of-service (QoS) for promising free-space optical (FSO) communication systems, it is essential to model and analyze FSO channels in terms of delay QoS. However, most existing works focus on the average capacity and outage capacity for FSO, which are not enough to characterize the effective transmission data rate when delay-sensitive service is applied. In this paper, the effective capacity of FSO communication systems under statistical QoS provisioning constraints is investigated to meet heterogeneous traffic demands. A novel closed-form expression for effective capacity is derived under the combined effects of atmospheric turbulence conditions, pointing errors, beam widths, detector sizes and QoS exponents. The obtained results reveal the effects of some significant parameters on effective capacity, which can be used for the design of FSO systems carrying a wide range of services with diverse QoS requirements.

Considering that the collision caused by hidden terminal is particularly serious due to the narrow beams of optical devices, the multi-packet reception (MPR) is introduced to mitigate the collisions for IEEE 802.15.7 visible light communication (VLC) system. To explore the impact of MPR on system performance and investigate the interaction between physical (PHY) layer and media access control (MAC) layer, a three dimensional (3D) integrated PHY-MAC analytical model of carrier sense multiple access/collision avoidance (CSMA/CA) is established based on Markov chain theory for VLC system, in which MPR is implemented through the use of orthogonal code sequence. Throughput is derived to evaluate the performance of VLC system with MPR capability under imperfect optical channel. The results can be used for the performance optimization of a VLC system with MPR capability.

采用一种简单有效的原位水热合成方法, 使用石墨烯氧化物(GO)作为反应物和晶体生长基底成功制备出了还原氧化石墨烯/硒化锌(r-GO/ZnSe)纳米复合材料。采用X射线粉末衍射(XRD)、透射电子显微镜(TEM)、高分辨透射电镜(HRTEM)以及红外-可见光谱(FT-IR)等方法对r-GO/ZnSe纳米复合材料进行了检测。结果表明, 平均粒径在30 nm的立方闪锌矿晶体结构的ZnSe粒子均匀分散在氧化石墨烯片层上, 构成纳米复合结构。 UV-Vis光谱显示, 纳米复合材料的光学吸收的起始波长在445 nm附近。PL光谱显示, 纳米复合材料在470 nm附近存在一个很强的发射峰。这种石墨烯基纳米复合材料在白光二极管领域中有重要的应用价值。

为了提高IEEE 802.16d系统的吞吐量和无线信道的视频质量，提出了估计物理层正交频分复用(OFDM)符号错误程度的方法，设计了视频跨层容错传输机制。在物理层用幸存路径的平均量度实时估计OFDM符号错误程度，得到了数据错误级因子(Ef)；MAC层根据容错门限和Ef值决定是否上传CRC校验出错的数据包。同时，应用层采用优化的分组级FEC算法恢复丢弃和出错的视频帧。实验结果表明，容错门限取值在0.02~0.06时可以达到用户可接受的视频峰值信噪比(PSNR)。与无容错传输功能的802.16d系统相比，本文设计的视频容错传输系统可在各种信道条件下获得更好的视频质量和更高的吞吐量。