Multi-level modulation scheme based on PAK algorithm for optical camera communications

In an optical camera communication (OCC) system, multi-level modulation is essential for data rate enhancement with the finite frame rate of the receiving camera constraint, where the K-means algorithm is widely used as a thresholding scheme. The result of K-means clustering is sensitive to initial cluster centers. In this paper, we propose a multi-level modulation scheme utilizing the pilot-aided K-means (PAK) algorithm. PAK algorithm innovates in both obtaining the state of the stripes propagated through the optical channel under different environments and overcoming the susceptibility of K-means. Our scheme could prompt data rate and improve the performance of OCC. Finally, non-orthogonal multiple access (NOMA) pattern is designed based on the proposed scheme to achieve multiplex communications.     

Dimming control technique for compressed sensing-based optical camera communications

Compressed sensing (CS)-based optical camera communications (CS-OCC) technology was previously studied to support accurate optical data detection with down-sampled measurements. This paper extends the CS-based OCC scheme to discuss the methods supporting dimming control functionality. Based on measurement matrices consisting of a random distribution of pseudo-noise (PN) code for sparse reconstruction, various solutions for a dimmable CS-OCC method are provided. The simulation results show that the CS-OCC scheme of Gaussian PN code with a varying mean signal shows the best data retrieval performance, while providing robust dimming control.     

Rolling shutter aided optical camera communications with increasing communication distance

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.     

Decoding algorithm with multiple features based on optical camera communication system

The performance of decoding algorithm is one of the important influential factors to determine the communication quality of optical camera communication (OCC) system. In this paper, we first propose a decoding algorithm with adaptive thresholding based on the captured pixel values under an ideal environment, and then we further propose a decoding algorithm with multiple features, which is more suitable under the existence of the interference of light sources. The algorithm firstly determines the light-emitting diode (LED) array profile information by removing the interfering light sources through geometric features, and then identifies the LED state by calculating two grayscale features, the average gray ratio (AGR) and the gradient radial inwardness (GRI) of the LEDs, and finally obtains the LED state matrix. The experimental results show that the bit error ratio (BER) of the decoding algorithm with multiple features decreases from 1×10-2 to 5×10-4 at 80 m.     

Mismatch robustness and security of chaotic optical communications based on injection-locking chaos synchronization

Mismatch robustness and its effects on security of chaotic optical communication system based on injection-locking chaos synchronization are studied numerically by establishing the corresponding SIMULINK model. Unlike previous studies, we focus on the communication relating issues when parameter mismatches are considered. The mismatch robustness of generalized synchronization is discussed firstly in terms of cross-correlation coefficient and synchronization error. Decoding performances as well as the effects of message strength are examined for both with and without mismatch cases. Effects of injection strength on system decoding performances are also investigated by examining the chaos-pass filtering effect. Finally, a modified decoding scheme is brought forward to improve the system decoding capability. Results show that the system under consideration exhibits unconspicuous difference in both synchronization and decoding characteristics when large parameter mismatches are considered. The system based on injection-locking chaos synchronization is inappropriate for the applications where high transmission security is necessary.     

Modulators for optical communications

This paper reviews the field of high-speed small-aperture modulators for applications in optical communications, with emphasis on electrooptic modulation. The capabilities and limitations of electrooptic modulators are discussed based on a review of the physical origin of the electrooptic effect. Thermal and photoconduction phenomena, which may severely limit the operation of practical devices, are emphasized. The modulation power and bandwidth limitations using various schemes of electrooptic interaction are derived and compared. It is shown that lumped modulators are capable of efficient modulation for bandwidths up to about 1 GHz for visible wavelengths and are also attractive for their simplicity. For broader bandwidth capability the traveling wave or zigzag types of interaction become more efficient but with added complexity. Finally, acoustooptic and magnatooptic modulators are briefly discussed and compared with electrooptic modulators.     

Coherent optical sources for communications

The development of coherent optical sources, producing usable amounts of power, has provided a stimulus for communications research. Coherent sources in the form of lasers and parametric oscillators are available at wavelengths which span the entire optical spectrum. This paper reviews the state of the art of coherent optical sources with major emphasis on the most highly developed sources.     

Glass fibers for optical communications

Glass optical waveguides with attenuations below 20 dB/km have made possible a new approach to optical communications. These glass fibers satisfy requirements for transmission over kilometer lengths with experimental systems utilizing existing devices for sources and detectors. The realization of material and fabrication advances necessary for this accomplishment are the topic of this paper. Basic theoretical principles are introduced in a review fashion. The application of these principles in choice of materials and fabrication is described. Results in fiber performance following this framework are given in a section on evaluation, which includes information capacity, attenuation, and some environmental requirements. Preliminary experiments in bundling and cabling are discussed, followed by concluding remarks.     

采用光流估计的数字相机自动对焦算法

自动对焦技术对于数字相机至关重要,它是获取清晰图像的重要手段。针对复杂环境下多目标场景图像,提出了一种基于光流场估计的自动对焦算法。通过计算输入图像序列的光流场,对场景中的运动目标进行检测,根据目标运动属性准确判断出感兴趣目标。改进了Brenner清晰度评价方法,利用目标的二维边缘梯度信息建立评价函数,并且通过非线性增益提高评价函数的灵敏度,减小了噪声对评价值的影响。实验证明,该算法能够在主辅目标景深比达50倍的情况下分辨出感兴趣主目标,并在方差为0.02的随机噪声干扰下能有效地评价图像的清晰度;此算法将Brenner等评价函数的峰值稳定余量提高了1至4倍,对于不同图像具有良好的鲁棒性,易于硬件实现。     

Robustness analysis of O-FDE algorithm for dispersion compensation in coherent fiber optical communications

The overlapping frequency domain equalization(O-FDE) in digital signal processing(DSP) is frequently employed to provide dispersion compensation in long-distance coherent fiber optical communications. However, the change in overlapping symbol length that occurs during the processing of the O-FDE algorithm will typically be influenced by the decision and zero filling of the last subblock, which is harmful to the robustness of the O-FDE algorithm. In this study, with a thorough robustness analysis...     

Demodulation of low-level broad-band optical signals with semiconductors

A simple approach to the problem of detection of optical signals with semiconductors leads to a general understanding of the performance. It shows that the low impedance level of broad-band circuits prevents high efficiency conversion of the incident power. Because of this, the envelope detector has a signal-to-noise ratio (SNR) which is determined by the noise of the following amplifier. As in radio communication, addition of a coherent optical signal from a local oscillator can increase SNR; however, for optical signals the limiting value of the noise figure is determined by the shot noise of the conversion current. Comparison with the limiting SNR expected from a photo-emission detector reveals no important distinction between them for frequency conversion. The unit which uses material with the higher quantum conversion efficiency will have the possibility of better SNR. For envelope detection, the photo emitter can be far superior to the photoconductive detector. The two cases of a photovoltaic and a photoelectromagnetic solid-state device are analyzed and general expressions as well as numerical examples given for the design parameters. The devices are comparable both in dimensions and performance.     

System design for optical PPM communications with diode combining

The most efficient way to encode laser diodes for maximum data rate is studied. Three different system architectures are considered, with combining achieved by dichroic mirrors operating in conjunction with a pulse-position-modulated (PPM) format. The basic criterion is to maximize the data rate with increasing number of diodes, while maintaining diode power constraints and decoding bit error probability. The three systems are: power combining into a single pulse, followed by PPM encoding; parallel channels, in which each diode is separately PPM encoded; and color coding, in which diodes are encoded over a common wavelength-time slot alphabet. Data rate equations are presented as a function of the number of diodes, mirror combining losses, PPM alphabet size, and the operating optical signal-to-noise ratio     

一种改进的稳健自适应波束形成算法

当快拍数较小时,自适应波束形成算法的性能将会降低,而对角加载算法是提高这类自适应波束形成算法稳健性的简单而有效的方法,但是至今没有一种比较有效的方法来确定对角加载值。本文提出了一种确定加载值的方法,这种方法在加载值和协方差矩阵的估计误差之间建立联系,它能够根据阵列的输出数据动态的调整加载值。计算机仿真证实了该算法的有效性。     

Gigabit receiver ICs for optical communications

The authors discuss gigabit receiver ICs for optical communications, focusing on their circuit and package design, the performance of receivers that were fabricated, and their application to a 1.6 Gb/s optical receiver. The key technologies for the receivers are discussed, and a design based on these key technologies is proposed. The proposed design is used to fabricate six receiver ICs (eight chips) using an ultra-high-speed bipolar process with transistors having a unity gain bandwidth of 6-8 GHz. The receivers are suitable for long-haul optical transmission at bit rates up to 1.6 Gb/s. Experimental results show that the 1.6 Gb/s receiver has an optical dynamic range of more than 23 dB without any adjustment, and the received average optical power required to maintain a 10^-11 error rate is less the -31 dBm     

Multicomponent glass fibers for optical communications

The preparation of low-loss multicomponent glasses, and the methods by which both step- and graded-index fibers can be drawn from them are described. The optical performance of fiber prepared by each technique is discussed.     

光通信用垂直腔面发射激光器的进展

垂直腔面发射激光器(VCSEL)以其优异的性能有望成为信息时代的新光源.文章介绍了用于光通信的VCSEL的开发现状.     

High performance receivers for optical communications

The principles underlying optical detection are reviewed and the practical realisation of devices and receivers for digital optical communications is discussed. Some emphasis is placed on the advanced receiver signal processing techniques-both electronic and optoelectronic-that are now coming forward to provide a performance approaching the fundamental limits imposed by quantum mechanical considerations     

Deep-space optical communications

A major problem in deep-space communication systems is that of obtaining high data rates (of the order of 107 bits per second). This article proposes some design concepts that indicate the probable feasibility of achieving wide-band communications by means of the laser. The example selected here is a hypothetical mission to Venus, chosen because of its great brightness and, hence, high background-noise level. Since no earth satellite relay is assumed, the communication channel includes the atmosphere. The down-link is the one considered because of its high-information-rate requirement.     

Miniature acoustooptic modulators for optical communications

Broad-band acoustooptic modulators have been designed and developed for fiber optical-communication applications. The specific function of these modulators is to impress signal information, up to 50 Mb/s, on the CW emission of Nd:YAG lasers.