CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 U.S. National Phase of International Application No. PCT/JP2019/033475, filed on Aug. 27, 2019, which claims priority to Japanese Application No. 2018-168639 filed on Sep. 10, 2018. The entire disclosures of the above applications are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to an optical/RF wireless hybrid communication system for transmitting data by channels of an RF wireless link and an optical wireless link, and a control method therefor.

BACKGROUND ART

High-speed Internet services using fiber-to-the-home (FTTH) and long term evolution (LTE) are essential tools in daily lives. Particularly in recent years, a wide variety of applications and services such as IP data communications, Internet of Things (IoT)/machine to machine (M2M), 4K/8K high-definition video distribution services, online video distribution services, and video uploading through SNS have rapidly become widespread due to the popularization of cloud use and the spread use of mobile terminals. In addition, new workstyles such as teleworking with ICT have been proposed, and the demand for network-based services is expected to grow in the future. At present, stable optical broadband services are provided to homes owing to the popularization of FTTH, but wireless communication systems for achieving high throughput have been sought after in home environments from the viewpoint of utilization of multiple devices other than PCs and routing of cables and against the background of diversity of services described above.

Wireless communication systems can be roughly divided into two types across 3 THz from the viewpoint of frequency regions. The frequency band of 3 THz or lower is called “radio region”, and includes cellular systems such as LTE and 5G and wireless LANs that use unlicensed bands. Research and development in this region are active, and high throughput can be achieved by extending a frequency band used or using a high frequency band. Regarding the extension of the frequency band, there is a report that high throughput is achieved by, for example, using frequency bands of 2.4 GHz and 5.2 GHz and taking advantage of advanced technologies such as multi-level modulation, control of sub-carrier spacing, and MIMO technology, which transmits and receives data with a plurality of antennas (NPL 1). Regarding the use of high frequency bands, the Ministry of Internal Affairs and Communications has permitted the use of the 60 GHz band, and in recent years, wireless LAN standards utilizing the millimeter wave band, such as IEEE 802.11ad, which are not bound by the Radio Act, have been standardized. However, concerns over power consumption and high cost of devices in the millimeter wave band have raised the bar for the rapid spread of millimeter wave wireless LAN standards in the future.

The frequency band of 3 THz or higher (30 PHz or lower) is, on the other hand, called “optical wave region”, and includes methods using infrared communication and visible light and Li-Fi, which transmits and receives by means of high-speed modulation of LEDs recently used in lighting. The frequency band in this region is high, and hence it is relatively easy to achieve high throughput with a simple transmission reception configuration using the intensity modulation-direct detection (IM-DD). At the research level, transmission experiments of 1 m/4 Gbps using wavelength division multiplexing (WDM) by LEDs, which are incoherent light, and 8PAM and of 50 m/320 Gbps by 8-wavelength dense WDM (DWDM) at 50 GHz intervals using coherent light have been reported (NPL 2 and NPL 3).

CITATION LIST

Patent Literature

[PTL 1] Japanese Translation of PCT Application No. 2005-506718

Non Patent Literature

[NPL 1] Akira Yamada, Daisuke Nojima, Takahiro Asai, “Musen LAN Kanren Sisutemu no Kokusai Hyoujunka Doukou(Trend of International Standardization of Wireless LAN-based Systems)”, IEICE Communication Society Magazine, No. 38, Autumn, pp. 74-79, 2016.[NPL 2] Mengjie Zhang, Meng Shi, Fumin Wang, Jiaqi Zhao, Yingjun Zhou, Zhixin Wang, Nan Chi, “4.05-Gb/s RGB LED-based VLC system utilizing PS-Manchester coded Nyquist PAM-8 modulation and hybrid time-frequency domain equalization”, Proc OFC2017, W2A.42.[NPL 3] Wen-Shing Tsai, Hai-Han Lu, Chung-Yi Li, Ting-Chieh Lu, Hung-Hsien Lin, Bo-Rui Chen, Chang-Jen Wu, “A 50 m/320 Gbps DWDM FSO communication”, IEEE Photonics Conference (IPC), pp. 43-44, 2016.

SUMMARY OF THE INVENTION

Technical Problem

In the wireless communication systems in the radio wave region and the optical wave region, the throughput can be expected to increase by the above-mentioned method, but the respective wireless communication systems are considered to have the following problems from the viewpoint of stability of communication conditions of wireless links.

(Problems Due to Use of Frequency Band in Radio Wave Region)

Deterioration in communication quality due to multipath fadingReduction in throughput due to radio interference
(Problem Due to Use of Frequency Band in Optical Wave Region)Propagation through obstacles is impossible (communication is disconnected when an obstacle is present in a path)

These problems can be overcome by a hybrid wireless communication system in which wireless communication systems using the frequency bands in the radio wave region and the optical wave region cooperate with each other. The specific problems and solutions are as follows.

(Problems Due to Use of Frequency Band in Radio Wave Region and Solutions)

Deterioration in communication quality due to multipath fading

#(Optical wave region) Using characteristics that directivity is extremely high

Reduction in throughput due to radio interference

#(Optical wave region) Using characteristics that crosstalk due to optical wave interference is low

(Problem Due to Use of Frequency Band in Optical Wave Region and Solution)

Propagation through obstacles is impossible (communication is disconnected when an obstacle is present in a path)

#(Radio wave region) Using diffraction effect and transmission of radio waves although depending on a frequency band used

As described above, when only a wireless communication system in the radio wave region or the optical wave region is used, the communication conditions become unstable due to the above-mentioned problems, but the disadvantages (above-mentioned problems) of one system are complemented by the advantages of the other system so that stable wireless links independently from the radio conditions and the communication bandwidth can be constructed. The above-mentioned hybrid wireless communication system is effective from the viewpoint of securing frequency resources in the radio wave region by using a frequency band in the optical wave region, which is not affected by limitation of the Japanese Radio Law.

Such hybrid communication systems using the frequency bands in the radio wave region and the optical wave region have already been proposed. PTL 1 discloses RF and optical wireless hybrid communication links provided with functions for monitoring weather forecast, a weather device in a controller, or transmission/reception power to determine the quality of the respective link conditions. In PTL 1, however, the state of each wireless link is determined from transmission/reception power, but, for example, the above-mentioned multipath fading may result in reduction in throughput because signals interfere with each other due to the propagation of different paths, which can significantly fluctuate reception intensity. Signal quality deterioration due to waveform distortion (jitter) caused by radio/optical wave arrival time different (transmission delay variation) may also be considered. Thus, the quality of the link conditions cannot be determined simply by transmission/reception power of each link. Further, PTL 1 indicates that after the conditions of the links are determined by the above-mentioned method, whether the link is an RF wireless link or an optical wireless link is uniquely determined and the determined link is used. This indicates that only one of the RF wireless link and the optical wireless link is used for data transmission, and does not solve the instability of the link conditions caused by the above-mentioned problems.

It is therefore an object of the present invention to provide an optical/RF wireless hybrid communication system and a control method capable of solving the instability of link conditions of an RF wireless link and an optical wireless link.

Means for Solving the Problem

In order to achieve the above-mentioned object, an optical/RF wireless hybrid communication system according to the present invention determines quality of link conditions from signal quality received through channels of both an RF wireless link and an optical wireless link, and decides distribution of data to be transmitted through the respective links on the basis of the determination result.

Specifically, an optical/RF wireless hybrid communication system according to the present invention for transmitting frames by using channels of an RF wireless link and an optical wireless link includes:

a block formation unit for forming blocks of a predetermined number of frames as one piece of data;

a signal quality measurement unit for measuring signal quality of frames transmitted through the channel of the RF wireless link and signal quality of frames transmitted through the channel of the optical wireless link;

a distribution determination unit for determining, on the basis of the signal quality measured by the signal quality measurement unit, a distribution ratio of the frames transmitted through the channel of the RF wireless link and the frames transmitted through the channel of the optical wireless link in the data; and

a distribution unit for dividing the data into two pieces on the basis of the distribution ratio determined by the distribution determination unit, and distributing one frame group to the channel of the RF wireless link and the other frame group to the channel of the optical wireless link or distributing all frame groups in the data to the channel of the RF wireless link and the channel of the optical wireless link.

A control method according to the present invention for an optical/RF wireless hybrid communication system for transmitting data by using channels of an RF wireless link and an optical wireless link includes:

a block formation procedure for forming blocks of a predetermined number of frames as one piece of data;

a signal quality measurement procedure for measuring signal quality of data transmitted through the channel of the RF wireless link and signal quality of data transmitted through the channel of the optical wireless link;

a distribution determination procedure for determining, on the basis of the signal quality measured in the signal quality measurement procedure, a distribution ratio of data transmitted through the channel of the RF wireless link and data transmitted through the channel of the optical wireless link in the data; and

a distribution procedure for dividing the data into two pieces on the basis of the distribution ratio determined by the distribution determination procedure, and distributing one frame group to the channel of the RF wireless link and the other frame group to the channel of the optical wireless link or distributing all frame groups in the data to the channel of the RF wireless link or the channel of the optical wireless link.

The optical/RF wireless hybrid communication system and the control method therefor according to the present invention form blocks of frames input during data transmission for each predetermined number k of frames, and measure, for each blocked frame group, received signal quality in channels of both the RF wireless link and the optical wireless link, and distribute the frame group to the RF wireless link or the optical wireless link on the basis of the measurement result. Thus, the links can be flexibly switched depending on the transmission conditions such as disturbance. Consequently, the present invention can provide an optical/RF wireless hybrid communication system and a control method capable of solving the instability of link conditions of an RF wireless link and an optical wireless link.

The optical/RF wireless hybrid communication system and the control method therefor according to the present invention add, to a head of the frame group, control information used for the distribution unit to distribute the frame group to the channel of the RF wireless link and the channel of the optical wireless link. The signal quality measurement unit detects the control information, and changes a frequency to measure signal quality of a frame on the basis of the distribution ratio. The frequency to measure the signal quality can be reduced by the distribution ratio.

Further, in the optical/RF wireless hybrid communication system and the control method therefor according to the present invention, it is preferred that, in bidirectional communication, one direction be capable of transmitting the data while distributing the data to the channels of both the RF wireless link and the optical wireless link, and the other direction transmit the data through only the RF wireless link.

By using optical/RF wireless hybrid communication for only a downlink direction in bidirectional communication between a master station and a terminal, the need of disposing an optical transmitter for uploading on the terminal can be eliminated to reduce the cost.

Effects of the Invention

According to the present invention, in the RF/optical wireless hybrid communication type in which the RF wireless communication type and the optical wireless communication type are blended to secure frequency resources in a radio wave region in a wireless transmission section and increase the throughput, the quality of link conditions is determined from signal quality received through channels of both the RF wireless link and the optical wireless link, and the distribution of data to be transmitted through the respective links is determined on the basis of the determination result.

Consequently, the present invention can provide an optical/RF wireless hybrid communication system and a control method capable of solving the instability of link conditions of the RF wireless link and the optical wireless link.

