Visible light communication (VLC), which has the advantages of being high speed, license-free, anti-electromagnetic interference, and high security, is receiving more and more global attention [1
]. In the era of 4th/5th generation wireless communication, VLC is even more considered to be a mutual enhancement to radio frequency (RF) communication, especially for indoor applications. In recent years, achieving high speed data transmission in a VLC system has been a research focus. Introducing multiple input multiple output (MIMO) into a VLC system can multiply channel capacity and spectrum utilization without increasing system bandwidth. However, in a VLC system, channel correlation is even larger than that in RF communication systems. The reliability of system will be greatly reduced when MIMO is directly applied to VLC. Cooperative transmission can effectively improve reliability of wireless communication systems by resisting channel fading. It is also called a virtual MIMO. The distributed layout of luminaires in indoor VLC systems is very suitable for employing cooperative transmission. It provides an effective way in practice to implement MIMO in indoor VLC systems.
Many studies have focused on the use of spatial modulation (SM) to improve spectrum utilization and reliability in VLC systems. In [3
], SM was first applied to VLC systems. As a result, the data rate of this modulation was higher than that of traditional optical modulations, such as on-off keying modulation or pulse position modulation, and spectrum utilization was significantly improved. In [4
], a generalized SM scheme which could activate multiple light emitting diodes (LEDs) to simultaneously transmit information was proposed in VLC systems. Eventually, this scheme could not only improve spectrum utilization, but also reduce symbol error rate. In VLC, multipath effects can destroy the orthogonality of subcarriers of orthogonal frequency division multiplexing optical symbols during transmission and influence demodulation at the receiving device. This is called inter channel interference (ICI). To eliminate ICI and improve spectrum efficiency of VLC systems, an index-based optical spatial modulation (OSM) scheme was proposed in [5
]. In [6
], a channel-adaptation spatial modulation (CASM) that channel-adapted spatial constellations with maximum size of the best combinations of active LEDs when ICI was below a given threshold was proposed. It established a general criterion of balancing spectral efficiency and error performance by analyzing special features of the system with the channel state information known for luminaires, and was very suitable for large-scale MIMO VLC systems. As a result, performance of bit error rate (BER) was better than conventional OSMs, and the greater the channel correlation, the more obvious advantages of CASM. To eliminate the adverse effects of random channel assignment in conventional OSM systems, a novel SM scheme based on index modulation was proposed in [7
]. Both the active LED index and the modulated information were transmitted on a designed symbol through the best VLC channel. As a result, the complexity of the receiving device could be greatly reduced and significant performance improvement could be achieved compared to conventional OSM with random channel assignment. In conventional OSM or optical spatial multiplexing (OSMP), symbols sent by LEDs are independently generated from constellations, and this is not optimal when considering energy utilization. Therefore, a novel constellation design where two symbols from two spaces in the MIMO VLC system collaborate so closely that the average optical power is minimized under the constraint that the minimum Euclidean distance is fixed was proposed in [8
]. Though spatial diversity was adopted in traditional approaches and the novel design, the latter has a better BER performance than conventional OSM or OSMP. In [9
], the design and implementation methodology of a spatially modulated VLC MIMO system using eight power LEDs as transmitters and eight photodiodes as receivers were described, and furthermore, spatial modulation techniques and special lens arrangements were introduced to mitigate co-channel interference (CCI) and increase the robustness of system. The literature above demonstrated that the use of SM enables MIMO techniques to play a role in VLC systems. However, in order to further improve performance of VLC systems, several approaches tried to make use of cooperative transmission. In [10
], an indoor cooperative incremental hybrid decode–amplify–forward (IHDAF) protocol for a SM VLC system was proposed, which has a significant performance improvement in resisting channel propagation compared with conventional IHDAF. In [11
], a space frequency block code with frequency-switched transmit diversity in indoor VLC systems was proposed. It can mitigate CCI through cooperative transmission, which occurs when the coverage areas of LEDs are overlapped and source luminaires send different information, and thereby improve the reliability of the system.
The main aim of this paper was to put forward a cooperative strategy based on space–time labeling diversity for indoor VLC systems to achieve high reliability. This scheme was implemented through a Monte Carlo simulation. Finally, we presented BER performance comparisons with other schemes in indoor VLC systems.
The main contributions of this paper were as follows:
(1) Searching and investigating various SM schemes which have been proposed in VLC systems in recent years.
(2) Employing the idea of cooperative transmission in indoor VLC systems to realize MIMO in practice.
(3) Putting forward a space–time labeling diversity structure for cooperative indoor VLC systems.
The key achievement of this paper was to employ the idea of cooperative transmission to construct virtual MIMO for indoor VLC systems to eliminate channel correlation in MIMO VLC systems. Moreover, a new diversity structure of space–time labeling was designed to further improve the reliability of the system.
This paper is organized as follows. Section 2
presents related knowledge and theory with regard to cooperative communication and space–time labeling diversity. Section 3
first describes a model of cooperative indoor VLC systems, and then introduces space–time labeling diversity into the constructed model, finally deriving the BER of the proposed cooperative indoor VLC system. Monte Carlo simulations and analyses of results are found in Section 4
. Finally, the conclusions of this paper are summarized in Section 5