Analysis of spatial correlation of precoding indoor MIMO visible light communication system

The construction of the fifth-generation mobile communication infrastructure has been accelerated, but the seamless coverage based on the three-dimensional virtual network needs to be solved urgently. Visible light communication utilizes the high-frequency flicker characteristics of light-emitting diodes (LEDs) to transmit information, which can not only guarantee the security of information transmission in the electromagnetic harsh area but also provide high-speed flexible access, accurate and convenient positioning services indoor with massive bandwidth requirements.

In recent years, visible light communication technology has developed rapidly. However, the modulation bandwidth of ordinary LEDs limits the information transmission rate. Multiple-input multiple-output (MIMO), which transmits multiple signals in space in parallel, can effectively solve the capacity problem of visible light communication system. In a multi-user system, users compete for limited time-frequency resources simultaneously, so there is an inevitable interference of co-channel. The precoding technology represented by block diagonalization designs the coding matrix at the transmitter to decompose the signal space into independent subspaces, which can eliminate the influence of this interference on the desired signal significantly. Because the matrix decomposition makes the subchannel transmit signals with different capabilities, and there is a certain correlation between different subchannels formed by LED and photodetectors, and the system capacity is affected. Therefore, on the premise of ensuring reasonable allocation of user resources, it is of great significance to study the spatial correlation of block diagonalization precoding MIMO channel to improve the bit error rate performance of MIMO indoor visible light communication system.

Professor Ke Xizheng and Dr. Zhang Ying's team at Xi'an University of Technology is dedicated to the research of multi-user indoor visible light communication systems. Based on the research of multi-user detection in visible light communication, they established the system model of multi-user MIMO indoor visible light communication (as shown in Fig.1) to further improve the system's bit error rate performance. By multi-dimensionalizing the transmitted signal to form a substream, they studied the performance of the sub-stream selected block diagonalization precoding algorithm. Using the control variable method, they analyzed the impact of channel correlation on system capacity and bit error rate performance when different terminal positions and different MIMO modes are adopted between LEDs and user terminals. It is found that under the condition of the same number of sub-channels, the distance between leds and the distance between detectors will affect the channel correlation and then the channel capacity. However, the increase of the number of sub-channels has no obvious effect on the channel capacity and only plays a key role in improving the bit error rate performance of the system. At the same time, the spatial correlation is inversely proportional to the bit error rate performance of the system, and the subflow selection block diagonalization algorithm can provide more than 4dB gain for the system. This is because the proposed algorithm optimizes the number of substreams and allocates the substream data according to the singular value of each substream channel, and combines the precoding algorithm to overcome the multi-user interference, so as to improve the bit error rate performance of the system. Therefore, the subflow selection block diagonalization precoding algorithm achieves a compromise between system capacity and bit error rate performance. The further expansion of this research can also be used to solve the interference suppression problem of indoor visible light communication multi-user data transmission.

The studies are supported by the fund project: Key Industry Innovation Chain Project of Shaanxi Province (2017ZDCXL-GY-06-01); Natural Science Foundation of Shaanxi Provincial Department of Education(No.17JK0569、 NO.18JK0341), and Xi'an Science and Technology Innovation Guidance Project(NO:201805030YD8CG14(12)).

Fig.1 Substream selected block diagonalization precoding MU-MIMO indoor visible light communication system model

About The Group

Professor Ke Xizheng and Dr. Zhang Ying's team of xi 'an university of technology is an innovative and active research team, including 11 associate professors, lecturers and students. The research group mainly carried out research on signal processing, visible light communication system design, optoelectronic heterogeneous network and photoelectric material design in visible light communication. They have won 1 first prize, 5 second prize and 5 third prize of science and technology progress of provincial and ministerial level. And they have also won the second prize of China industry-university-research cooperation innovation achievement award. They have taken 3 national projects, 1 provincial key project, 3 general projects and 4 entrusted projects. They published 34 papers, 16 of which were indexed by SCI, and they were also granted 5 invention patents and 3 software copyrights. Their major works include “Wireless Optical Communication”, “Space-Time coding theory for wireless optical MIMO systems” and “Principle and application of orthogonal frequency division multiplexing”.

Article

Zhang Ying, Gao Yue, Ke Xizheng. Analysis of spatial correlation of precoding indoor MIMO visible light communication system[J]. Opto-Electronic Engineering, 2020, 47(3): 190666-1-190666-7.

 DOI:10.12086/oee.2020.190666