WO2021203487A1 - 光纤使能光无线通信系统及方法 - Google Patents

光纤使能光无线通信系统及方法 Download PDF

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Publication number
WO2021203487A1
WO2021203487A1 PCT/CN2020/086078 CN2020086078W WO2021203487A1 WO 2021203487 A1 WO2021203487 A1 WO 2021203487A1 CN 2020086078 W CN2020086078 W CN 2020086078W WO 2021203487 A1 WO2021203487 A1 WO 2021203487A1
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optical
signal
user terminal
base station
fiber
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PCT/CN2020/086078
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English (en)
French (fr)
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高西奇
孙晨
王家恒
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东南大学
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Publication of WO2021203487A1 publication Critical patent/WO2021203487A1/zh
Priority to US17/962,558 priority Critical patent/US20230044988A1/en

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    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/11—Arrangements specific to free-space transmission, i.e. transmission through air or vacuum
    • H04B10/114—Indoor or close-range type systems
    • H04B10/116—Visible light communication
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/40—Transceivers
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/501—Structural aspects
    • H04B10/503—Laser transmitters
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04B—TRANSMISSION
    • H04B10/00—Transmission systems employing electromagnetic waves other than radio-waves, e.g. infrared, visible or ultraviolet light, or employing corpuscular radiation, e.g. quantum communication
    • H04B10/50—Transmitters
    • H04B10/516—Details of coding or modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04J—MULTIPLEX COMMUNICATION
    • H04J14/00—Optical multiplex systems
    • H04J14/02—Wavelength-division multiplex systems
    • H04J14/0201—Add-and-drop multiplexing
    • H04J14/0202—Arrangements therefor

Definitions

  • the invention relates to an optical wireless communication technology, in particular to an optical antenna, a fiber-enabled optical wireless communication (FE-OWC, fiber enabled optical wireless communication) device, an FE-OWC system and a method, and belongs to the field of mobile communication technology.
  • FE-OWC fiber-enabled optical wireless communication
  • optical wireless communication uses optical bands to provide very rich spectrum resources and supports high-speed data transmission. It is a highly potential wireless communication method. Compared with high-frequency radio frequency wireless communication, optical wireless communication also has the advantages of low complexity of receiving and sending signals, and mature communication devices and equipment.
  • Visible light communication is a research direction in optical wireless communication.
  • the signal is modulated to the amplitude of visible light, and LEDs are used to provide illumination while transmitting data signals to user terminals.
  • LEDs In order to meet the needs of lighting, LEDs generate wide beams to cover the entire communication area.
  • the channel coefficients of optical wireless communication have a high degree of correlation.
  • a single optical transmitting node transmits an omnidirectional signal, which is usually regarded as a single transmitting antenna, which can only transmit a single data signal, and the number of user terminals served by the system at the same time is limited.
  • the modulation bandwidth of the LED is about 20MHz
  • the abundant spectrum resources in optical wireless communication cannot be fully utilized, and the transmission rate of the system is low, which cannot meet the demand for ultra-high-speed data transmission.
  • the current visible light wireless communication system only considers the downlink transmission from the base station to the user terminal, and cannot support two-way communication.
  • infrared optical wireless communication By generating a directional narrow beam, the optical signal energy is concentrated to the user terminal, which greatly improves the receiving energy.
  • the laser is used to generate optical signals to support a transmission rate of tens of Gbps.
  • the infrared light beam generated by the laser has extremely strong directivity, the transceiver device needs to be accurately aligned, which greatly increases the complexity of the system in scenarios such as user movement, and most infrared optical wireless optical communication systems only Considering point-to-point single link transmission, it cannot support simultaneous communication of multiple or a large number of user terminals.
  • the purpose of the present invention is to provide an optical antenna, and based on this
  • the FE-OWC device, FE-OWC system and method of the optical antenna can make full use of the abundant spectrum resources of the optical band, realize the full coverage of the signal in the communication area and support the high-rate two-way communication of the terminal mobility, and meet the needs of future mobile communication applications .
  • the optical antenna of the present invention includes an array of optical fiber transceiver ports and a lens or reflector; in the process of signal transmission, the light emitted by a single optical fiber transceiver port is refracted by the lens or reflected by the reflector. A light beam with a certain angle range is generated in a certain direction, and the light from different optical fiber transceiver ports is refracted or reflected to different directions; in the process of receiving signals, the received light from different directions is coupled into after being refracted by a lens or reflected by a mirror.
  • Different optical fiber transceiver ports receive, and different optical fiber transceiver ports receive optical signals in different directions.
  • the optical fiber transceiver port in the optical antenna includes an optical fiber port and a micro lens; in the process of transmitting a signal, the optical signal from a single optical fiber port is refracted by the micro lens to generate a light beam with a certain angle range; In the signal process, the micro lens couples the optical signal within a certain angle range into the fiber port.
  • the optical antenna uses an array of optical fiber transceiving ports and lenses or mirrors to generate light beams in different directions. Different optical beams cover different areas. All optical beams generated by the optical fiber transceiving port array cover the entire communication area, realizing a full beam in the communication area. cover.
  • the FE-OWC device of the present invention includes the above-mentioned optical antenna and an optical transceiver link; the optical antenna is used to send and receive optical signals in different directions; the optical antenna and the optical transceiver link are directly connected through optical fibers, or through optical switching Unit connection; the optical transceiver link is used to realize the mutual conversion between optical signals and electrical signals; a single FE-OWC device performs wireless communication with a single or a group of FE-OWC devices.
  • the optical transceiver link is used to realize the mutual conversion between optical signals and electrical signals; in the process of sending signals, the electrical signals are added with a bias current to drive the laser to generate optical signals corresponding to the electrical signals, or use External modulator mode, the optical signal and electrical signal generated by the laser source are input to the external modulator to generate the corresponding optical signal.
  • the optical signal is amplified by the optical amplifier and transmitted to the optical antenna through the optical fiber; in the process of receiving the signal, the optical antenna receives
  • the signal is transmitted to the optical transceiver link through the optical fiber, and after being amplified by the optical amplifier, the optical signal is detected by the optical detector and converted into the corresponding electrical signal.
  • each optical transceiver link corresponds to a fiber transceiver port, and the number of optical transceiver links is the same as the number of fiber transceiver ports; when the optical antenna and the optical transceiver link pass through When the optical switch unit is connected, the number of optical transceiver links is less than or equal to the number of fiber transceiver ports.
  • the optical switch unit is used to switch the correspondence between the optical transceiver link and the fiber transceiver port, and generate the signal in the optical transceiver link with the optical antenna. Corresponding to the beams.
  • the FE-OWC device on the base station side further includes a baseband signal processing unit, the baseband signal processing unit includes A/D and D/A modules and a digital baseband processing and control module; in the downlink transmission process, the digital baseband processing on the base station side
  • the and control module is used to realize user scheduling and multi-user precoding transmission, and generate the transmission signal of each user terminal.
  • the D/A module is used to convert the transmission signal generated by the digital baseband processing and control module into an analog signal and input into the optical transceiver link ;
  • the A/D module is used to convert the electrical signal output by the optical transceiver link at the base station side into a digital signal, and the digital baseband processing and control module is used to detect the signals received by multiple users and restore each user terminal’s Send the signal.
  • the user terminal side FE-OWC device further includes a baseband signal processing unit, the baseband signal processing unit includes A/D and D/A modules and a digital baseband processing and control module; in the downlink transmission process, the A/D module is used In order to convert the electrical signal output by the optical transceiver link on the user terminal side into a digital signal, the digital baseband processing and control module is used to detect the received signal and restore the transmission signal of the base station; in the uplink transmission process, the digital baseband processing on the user terminal side The and control module is used to realize precoding transmission, and the D/A module is used to convert the generated transmission signal into an analog signal and input the optical transceiver link.
  • the FE-OWC device on the base station side further includes a baseband signal processing unit, the baseband signal processing unit includes a baseband modulation and baseband demodulation module and a digital baseband processing and control module; in the downlink transmission process, the baseband side digital baseband processing and The control module is used to allocate non-overlapping beam sets for different user terminals to generate digital baseband signals sent to each user terminal.
  • the baseband modulation module is used to generate analog baseband signals sent to each user terminal and transmit them to the corresponding optical transceiver chain.
  • the baseband demodulation module is used to demodulate the analog baseband received signal output by the optical transceiver link on the base station side to generate a digital baseband signal
  • the digital baseband processing and control module It is used to restore the transmitted signal of each user terminal according to the result of beam allocation and the digital baseband signal on the corresponding beam of each user terminal.
  • the user terminal side FE-OWC device further includes a baseband signal processing unit, the baseband signal processing unit includes a baseband modulation and baseband demodulation module and a digital baseband processing and control module; in the downlink transmission process, the user terminal side baseband demodulation The module is used to demodulate the analog signal output by the optical transceiver link to generate a digital baseband signal.
  • the baseband signal processing unit includes a baseband modulation and baseband demodulation module and a digital baseband processing and control module; in the downlink transmission process, the user terminal side baseband demodulation The module is used to demodulate the analog signal output by the optical transceiver link to generate a digital baseband signal.
  • the digital baseband processing and control module is used to select the corresponding fiber transceiver port of the base station, and restore the transmitted signal on the base station side according to the received digital baseband signal;
  • the digital baseband processing and control module on the user terminal side is used to generate the uplink digital baseband transmission signal
  • the baseband modulation module is used to generate the analog baseband transmission signal, which is transmitted to the optical transceiver link, using the fiber transceiver port corresponding to the base station Send it.
  • the base station and user terminal configurations of the system are configured with the above-mentioned FE-OWC device.
  • Multi-user MIMO or massive MIMO optical wireless communication or beam division multiple access BDMA optical wireless communication is realized between the base station and the user terminal.
  • the FE-OWC method of the present invention is based on the FE-OWC system, which calculates the link budget of single link transmission and establishes the transmission channel model of the electrical signal at the transceiver end;
  • the link budget includes the transmission end electrical-optical conversion, optical Wireless channel gain, receiving end photoelectric conversion, and receiving end electrical noise;
  • the transmitting end electro-optical conversion part establishes the corresponding relationship between the optical power output by the transmitting end and the input electrical signal according to the photoelectric characteristics of the electro-optical conversion device;
  • the optical wireless channel gain is the transmission The gain of the wireless channel between the optical fiber transceiver port at the receiving end and the optical transceiver port at the receiving end;
  • the photoelectric conversion part at the receiving end considers the two processes of receiving optical signals through optical amplifier amplification and optical detector detection to establish the relationship between the input optical signal and the output electrical signal at the receiving end
  • the electrical noise at the receiving end includes electrical noise introduced by optical amplifiers and photodetectors.
  • the relationship between signal power and noise power in the received electrical signal of a single link is established; on the basis of a single link budget, a complete A channel model for electrical signal transmission from the sender to the receiver; based on this channel model, a multi-user MIMO or massive MIMO or beam division multiple access optical wireless communication method is implemented between the base station and the user terminal.
  • the optical wireless channel gain describes the channel gain of optical wireless transmission from the transmitting end to the receiving end, including beam modeling at the transmitting end, free space transmission channel gain, the receiving power ratio of the optical fiber transceiver port at the receiving end, and the coupling efficiency of the optical fiber port.
  • beam modeling at the transmitting end describes the light intensity distribution of a single beam after the light emitted by the fiber transceiver port is refracted by a lens or reflected by a reflector.
  • a single fiber transceiver port is The light intensity of the user terminal is progressively orthogonal; the free space transmission channel gain considers the transmission process of the optical beam from the sender to the receiver through the free space, which is inversely proportional to the square of the transmission distance; the receive power ratio of the fiber transceiver port at the receiver describes a single fiber transceiver The ratio of the optical power received by the port to the total received power of the user is proportional to the common area of the received light projection on the receiving plane and the optical fiber transceiver port; the coupling efficiency of the optical fiber port is the ratio of the received light at the optical fiber port that can be coupled into the optical fiber, It is proportional to the common area of the angle range of the incident light at the fiber port and the acceptance angle of the fiber port.
  • the FE-OWC method of the present invention is a multi-user MIMO or massive MIMO optical wireless communication method based on the FE-OWC system.
  • the specific communication process includes the following steps:
  • the base station broadcasts a downlink synchronization signal, and the user terminal uses the received signal to establish and maintain synchronization with the base station;
  • Channel sounding the user terminal sends an uplink sounding signal, and the base station estimates the channel information of each user terminal according to the received sounding signal;
  • Downlink transmission The base station uses the channel information of each user terminal and the low-rank characteristics of the channel for precoding transmission, and simultaneously sends all user terminal signals, including pilot signals and data signals, and each user terminal estimates channel information based on the received pilot signals , And use the channel information to recover the data signal;
  • Uplink transmission The user terminal uses precoding transmission to simultaneously send signals to the base station, including pilot signals and data signals.
  • the base station receives the signal superimposition of all user terminals, estimates the channel information of each user terminal based on the pilot signal, and uses the low channel
  • the rank characteristic carries on the receiving processing, restores the data signal of each user terminal.
  • the base station estimates the uplink channel from each user terminal to the base station through the uplink detection process, uses the channel reciprocity to obtain the downlink channel, decomposes the channel matrix into the product of the column vector and the row vector, and calculates each The received signal-to-interference-to-noise ratio and reachability and rate of the user terminal are designed to maximize the system and rate under the power constraints of a single fiber transceiver port to design the optimal linear precoding; or the base station uses the maximum line vector decomposed according to the channel matrix Compared with the transmission MRT or the regularized zero-forcing RZF precoding method, the precoding vector is used to simultaneously send all user terminal signals, including pilot signals and data signals; during the uplink transmission process, the base station receives the signal superimposition of all user terminals.
  • the pilot signal estimates the uplink channel matrix of each user terminal and decomposes it into the product of the column vector and the row vector to design the optimal linear receiver with the goal of maximizing the system and rate; or the base station decomposes it according to the channel matrix of each user
  • the column vector adopts the maximum ratio to combine the MRC receiver, and the linear receiver is used to detect the received data signal and restore the transmitted signal of each user terminal.
  • each user terminal estimates the downlink transmission channel matrix according to the received pilot signal, and decomposes it into the product of the column vector and the row vector, designing the optimal linear receiver with the goal of maximizing the system and rate ,
  • the user terminal uses the optimal linear receiver to detect the received data signal; in the uplink transmission process, each user terminal uses the reciprocity of the channel to obtain the uplink channel information according to the downlink channel estimation, and decomposes the channel matrix into column vectors and The product of the row vector, under the condition of total power constraint, design the optimal precoding vector with the goal of maximizing the system and rate, and the user terminal uses the precoding vector to simultaneously transmit uplink signals, including pilot signals and data signals.
  • the FE-OWC method of the present invention is a beam division multiple access optical wireless communication method based on the FE-OWC system.
  • the channel information for different user terminals allocates beams that do not overlap each other. Each beam sends and receives signals from at most one user terminal, and uses optical beams in different directions to communicate with the user terminal at the same time; the user terminal selects the corresponding direction of the base station according to the channel information Use a single beam to communicate with a single base station; the specific communication process includes the following steps:
  • the base station broadcasts a downlink synchronization signal, and the user terminal uses the received signal to establish and maintain synchronization with the base station;
  • Channel sounding user terminals send uplink sounding signals, and the base station allocates beams to each user terminal according to the received sounding signals. For users communicating with the base station on the same time-frequency resource, the allocated beams do not overlap each other and each user is allocated only one beam ;
  • the base station sends an independent signal on the beam allocated by each user terminal according to the beam allocation result, and each user terminal selects the beam corresponding to the base station for reception detection according to the received signal;
  • Each user terminal sends a signal on the beam corresponding to the base station, and the base station receives and detects the transmitted signal of each user terminal on the beam allocated by each user terminal according to the result of the beam allocation.
  • the base station sends a signal to each user terminal on the beam allocated by the beam according to the result of the beam allocation.
  • Different beams send signals from different user terminals, and the multi-user downlink transmission link is decomposed into multiple parallel ones.
  • the base station uses baseband modulation to generate user analog baseband transmission signals; in the uplink transmission process, the base station receives and detects the transmitted signal of the user terminal on the beam allocated by each user terminal according to the result of beam allocation.
  • the base station uses different The beam receives and detects signals from different user terminals, the multi-user uplink transmission link is decomposed into multiple parallel single-user links, and the base station uses baseband demodulation to generate digital baseband signals for each user terminal.
  • each user terminal selects the beam corresponding to the base station according to the strength of the received signal, and uses the baseband demodulation method on the corresponding beam to generate a digital baseband signal; in the uplink transmission process, each user terminal corresponds to the base station On the beam, the baseband modulation method is used to generate the analog baseband signal.
  • the base station uses optical antennas composed of optical fiber transceiver port arrays and lenses or mirrors to generate multiple or a large number of beams in different directions, different beams cover different areas, and the base station uses different beams to communicate with multiple or a large number of user terminals at the same time , Which greatly increases the number of user terminals supported by optical wireless communications.
  • the present invention utilizes the ultra-high rate data transmission supported by optical fiber to significantly improve the transmission rate of each user terminal link and the system throughput.
  • Base stations and user terminals use optical fiber transceiver ports to send and receive signals, which can realize two-way communication and solve the limitation of one-way transmission in optical wireless communication.
  • the base station uses optical antennas to generate multiple or a large number of beams in different directions to achieve full beam coverage of the communication area.
  • the base station only needs to switch the beam corresponding to the user terminal, and no complicated tracking system is required.
  • the base station uses different beams to communicate with different user terminals, and decomposes the multi-user transmission link into multiple single-user transmission links.
  • Each single-user transmission link can adopt optical modulation and optical demodulation (such as OOK modulation).
  • OOK modulation optical demodulation
  • A/D and D/A equipment may not be needed, which significantly reduces the complexity of implementing ultra-high-speed optical wireless communication systems.
  • optical antenna and the optical transceiver link are connected by optical fiber. Due to the extremely low loss in the optical fiber transmission process, the optical antenna can be flexibly deployed, and the optical antenna is a passive system with low cost, which can greatly reduce the system construction cost.
  • the optical antennas of the base station and the user terminal input the received optical signal into the optical fiber, and then use the optical fiber amplifier to amplify the optical receiving process, which can greatly improve the receiving capacity, thereby reducing the transmitting power of the transmitting end or increasing the communication distance.
  • the proposed optical fiber-enabled optical wireless communication method can use mature optical fiber communication technology, devices and equipment to efficiently construct a high-speed optical wireless communication system to meet the needs of future mobile communication for transmission rates and system capacity orders of magnitude or more after 5G. Demand for a magnitude increase.
  • the proposed optical fiber-enabled optical wireless communication system can also be conveniently docked with an optical fiber communication network to realize the extension of the optical fiber communication network to wireless coverage, thereby realizing mobile optical communication and all-optical communication supporting terminal mobility.
  • Figure 1 is a schematic diagram of the architecture of a fiber-enabled optical wireless communication system
  • Figure 2 is a schematic diagram of an optical transceiver link
  • Figure 3 is a schematic diagram of the optical antenna structure
  • Figure 4 is a schematic diagram of a beam pattern, (a) a single fiber transceiver port, (b) an array of fiber transceiver ports;
  • Figure 5 is a schematic diagram of system throughput performance, (a) downlink transmission, (b) uplink transmission.
  • the present invention discloses a fiber-enabled optical wireless communication (FE-OWC, fiber enabled optical wireless communication) system.
  • the system architecture is shown in Figure 1.
  • a single base station (BS, base station) simultaneously serves K user terminals (UT, user). terminal), both the base station and the user terminal are equipped with FE-OWC devices.
  • the FE-OWC device includes an optical antenna, an optical transceiver link, and a baseband signal processing unit.
  • the optical antenna and the optical transceiver link are connected by an optical fiber. Due to the extremely low loss during the optical fiber transmission process, the optical antenna can be flexibly deployed.
  • Optical antennas are used to send and receive optical signals, including optical fiber transceiver port arrays and lenses or mirrors.
  • the light emitted by a single fiber transceiver port is refracted by a lens or reflected by a mirror, and a light beam with a certain angle expansion is generated in a certain direction.
  • the light emitted by different fiber transceiver ports is refracted or reflected to different directions.
  • the optical fiber transceiver port array and lenses or mirrors are used to generate multiple optical beams in different directions to achieve full beam coverage of the communication area, that is, the optical beams corresponding to all optical fiber transceiver ports can cover the entire communication area.
  • the received light rays in different directions are refracted by lenses or reflected by mirrors to different optical fiber transceiver ports, coupled into the optical fiber and transmitted to the optical transceiver link.
  • the optical transceiver link realizes the mutual conversion between optical and electrical signals, and its system architecture is shown in Figure 2.
  • the electric signal is added with a bias current to drive the laser diode (LD, laser diode) to generate an optical signal corresponding to the electric signal.
  • An external modulator can also be used to combine the optical signal generated by the laser source with
  • the electrical signal is input to the external modulator, and an optical signal corresponding to the electrical signal is generated.
  • the optical signal passes through an optical amplifier, such as an erbium-doped fiber amplifier (EDFA, erbium-doped fiber amplifier), and is amplified and transmitted to an optical antenna for transmission through an optical fiber.
  • EDFA erbium-doped fiber amplifier
  • the optical signal received by the optical antenna is transmitted to the optical transceiver link through the optical fiber.
  • the optical detector such as avalanche photodiode (APD, avalanche photodiode)
  • APD avalanche photodiode
  • the optical signal such as its intensity, Converted to the corresponding electrical signal.
  • an optical circulator OC is used to separate optical signals in different directions.
  • the optical antenna and the optical transceiver link are directly connected through an optical fiber, or through an optical switching unit.
  • each optical transceiver link corresponds to a fiber transceiver port, and the number of optical transceiver links is the same as the number of fiber transceiver ports.
  • the optical switching unit is used to switch the correspondence between the optical transceiver link and the optical fiber transceiver port, The signal in the link corresponds to the beam generated by the optical antenna.
  • the baseband signal processing unit is used to implement functions including user scheduling and processing of receiving and sending signals.
  • the embodiment of the present invention discloses two baseband signal processing methods.
  • the baseband signal processing unit includes A/D and D/A modules and a digital baseband processing and control module.
  • the base station and the user terminal implement multiple-input multiple-output or Massive MIMO optical wireless communication.
  • the digital baseband processing and control module on the base station side is used to realize user scheduling and multi-user precoding transmission, and generate the transmission signal of each user terminal.
  • the D/A module is used to generate the digital baseband processing and control module.
  • the transmitted signal is converted into an analog signal and input into the optical transceiver link; the user terminal side A/D module is used to convert the electrical signal output by the optical transceiver link into a digital signal, and the digital baseband processing and control module is used to detect the received signal, Resume the transmission signal of the base station.
  • the digital baseband processing and control module on the user terminal side is used to realize precoding transmission, the D/A module is used to convert the generated transmission signal into an analog signal and input into the optical transceiver link; the base station side A/D module is used In order to convert the electrical signal output by the optical transceiver link into a digital signal, the digital baseband processing and control module is used to detect the multi-user received signal and restore the transmitted signal of each user terminal.
  • the baseband signal processing unit does not have A/D and D/A modules, and includes baseband modulation and baseband demodulation modules (such as OOK (on-off keying) modulation and demodulation) and digital baseband processing
  • baseband modulation and baseband demodulation modules such as OOK (on-off keying) modulation and demodulation
  • digital baseband processing With the control module, the base station and the user terminal realize the beam division multiple access (BDMA, multiple access) optical wireless communication of multiple users or a large number of users.
  • BDMA beam division multiple access
  • the digital baseband processing and control module on the base station side is used to allocate non-overlapping beam sets for different user terminals to generate digital baseband signals sent to each user terminal.
  • the baseband modulation module is used to generate The sent analog baseband signal is transmitted to the corresponding optical transceiver link and sent using the corresponding optical transceiver port; the user terminal side baseband demodulation module is used to demodulate the analog signal output by the optical transceiver link to generate a digital baseband signal , The digital baseband processing and control module is used to select the fiber transceiver port corresponding to the base station, and restore the transmitted signal on the base station side according to the received digital baseband signal.
  • the digital baseband processing and control module on the user terminal side is used to generate the uplink digital baseband transmission signal
  • the baseband modulation module is used to generate the analog baseband transmission signal, which is transmitted to the optical transceiver link
  • the optical fiber corresponding to the base station is used to transmit and receive
  • the baseband demodulation module on the base station side is used to demodulate the analog baseband received signal output by the optical transceiver link to generate a digital baseband signal
  • the digital baseband processing and control module is used for the result of beam allocation and each user terminal
  • the digital baseband signal on the corresponding beam restores the transmitted signal of each user terminal.
  • the optical antenna is composed of a fiber transceiver port array and a lens or a mirror.
  • This embodiment takes the fiber transceiver port array and lens as an example.
  • the structure is shown in FIG. 3, and an optical antenna equipped with a mirror can be similarly obtained.
  • the corresponding design method can be applied to other fiber structures.
  • the light intensity emitted by the single-mode fiber port approximately obeys the Gaussian distribution.
  • the light intensity distribution can be modeled as
  • I 0 (z) represents the strongest light intensity on the cross section at a distance of z
  • ⁇ is the wavelength of light.
  • the light emitted from the fiber port is mainly concentrated in the range of the light spot ⁇ 1 (z).
  • a microlens is configured at the optical fiber port to form an optical fiber transceiver port.
  • the focal length of the microlens is f and the distance between the microlens and the fiber port is d 1 , the intensity distribution of the light emitted by the fiber transceiver port can be expressed as
  • ⁇ 2 (z) is the spot size of the light emitted by the optical fiber transceiver port at position z
  • ⁇ C describes the angle range of the optical fiber transceiver port to send and receive light.
  • the angle range of the optical fiber transceiver port to generate the beam can be changed.
  • the distance d 1 should be reduced, that is, the micro lens is as close to the fiber port as possible.
  • ⁇ is the polarization angle relative to the z-axis
  • U( ⁇ ) is the unit step function
  • the optical antenna includes a fiber optic transceiver port array and a lens.
  • M optical fiber transceiver ports form a square array, a circular array, or a hexagonal array, etc.
  • a single lens covers the entire fiber transceiver port array, or multiple lenses cover the entire fiber transceiver port array, and different lenses cover different fiber transceiver ports.
  • the angle between the light emitted by the i-th optical fiber transceiver port and the vertical direction is After being refracted by the lens, the direction is
  • F is the focal length of the lens.
  • the light from a single optical fiber transceiver port can be refracted by a lens to generate a light beam with a certain angle range in a certain direction.
  • ⁇ 1-d 2 /F
  • the intensity distribution of the light with the angle (relative refraction angle) ⁇ i with the central light can be expressed as
  • T lens represents the lens gain.
  • the optical fiber transceiver port array and lens are used to design the horizontal and vertical positions of the optical fiber transceiver port array to generate multiple or a large number of optical beams in different directions to cover the entire communication area.
  • the maximum angular range of the communication coverage area is ⁇ .
  • the adjacent beams overlap at the position where the maximum power is attenuated by half, then the optical fiber transceiver port array and the lens The distance between
  • ⁇ 1/2 is the angular position at which the maximum power is attenuated to half. From this, the coordinates of the i-th optical fiber transceiver port can be obtained as
  • d a is the distance between adjacent ports optical transceivers, satisfying m 1 and m 2 represent the positions of m 1 row and m 2 column in the square array, satisfying
  • the fiber optic transceiver ports can also be arranged in a circular array or a hexagonal array. When they are arranged in a circular array, the fiber optic transceiver ports are evenly arranged at the center of the circle and on the circle with a radius of r c , 2r c, etc., by adjusting the radius of the circle and the fiber
  • the interval between the transceiver ports makes the light from the fiber optic transceiver ports refracted by the lens to achieve full beam coverage of the receiving plane; when arranged in a hexagonal array, at the center, and the hexagonal side length is rh, which is a honeycomb structure that expands outward
  • the optical fiber transceiver ports are arranged in positions, and the light from the optical fiber transceiver ports is refracted by the lens to achieve full beam coverage by adjusting the side length of the hexagon.
  • the full beam coverage of the communication area can also be achieved through an array of optical fiber transceiver ports and mirrors.
  • a single reflector covers the entire optical fiber transceiver port array, or multiple reflectors collectively cover the entire optical fiber transceiver port array.
  • the light intensity distribution model (6) of a single optical fiber transceiver port the light emitted from the optical fiber transceiver port at the position (x i , y i , z i ) established by geometric optics is reflected by the mirror to obtain The direction and angle range of the beam generated by the fiber optic transceiver ports at different positions after being reflected by the reflector.
  • the position of the optical fiber transceiver port is designed so that the light emitted by the optical fiber transceiver port is reflected by the reflector and covers the entire communication area.
  • the distance d 2 between the fiber transceiver port array and the lens tends to the focal length F of the lens, and the fiber transceiver port array is located at the focal plane of the lens.
  • the light emitted from different optical fiber transceiver ports is refracted to different directions by the lens, and illuminates different areas.
  • the light from a single optical fiber transceiver port reaches the asymptotically orthogonal light intensity to the two user terminals.
  • a single fiber transceiver port can transmit signals from one user at most, and different user terminals receive optical signals from different fiber transceiver ports.
  • Figure 4 shows the beam pattern generated by the 8 ⁇ 8 fiber optic transceiver port array on the receiving plane.
  • the base station In a 5m ⁇ 5m communication scene with a height of 3m, the base station is located at the center of the scene.
  • Figure 4 (a) shows the beam pattern generated by a single fiber transceiver port in the fiber transceiver port array, that is, the distribution of light intensity on the receiving plane. The light from a single fiber transceiver port is refracted by the lens and converges to a certain area to form A beam;
  • Figure 4 (b) shows the beam pattern generated by the fiber optic transceiver port array. Different fiber optic transceiver ports generate beams in different directions, thereby using 8 ⁇ 8 beams to achieve full beam coverage of the entire communication area.
  • the link budget of single link transmission is considered, and a complete channel model for downlink transmission and uplink transmission is established.
  • First take the following transmission process as an example to calculate the link budget from the base station to the user terminal, including four parts: electrical-optical conversion at the transmitting end, optical wireless channel gain, photoelectric conversion at the receiving end, and electrical noise at the receiving end.
  • electrical-optical conversion at the transmitting end including four parts: electrical-optical conversion at the transmitting end, optical wireless channel gain, photoelectric conversion at the receiving end, and electrical noise at the receiving end.
  • the uplink transmission process is similar to the downlink transmission process, and the uplink transmission channel model can be similarly established.
  • the electrical-optical conversion converts the electrical signal into the corresponding optical signal, which is achieved by directly changing the drive current of the laser, or by an external modulator.
  • directly changing the drive current of the laser when the drive current is higher than the threshold I th , there is a linear correspondence between the output optical power and the input current. Therefore, adding a bias current I B (I B > I th ) to the electrical signal x carrying information and inputting the laser, the output light intensity in the linear range can be expressed as
  • P LD,0 is the output optical power driven by the bias current
  • m is the conversion coefficient between the optical intensity and the input current.
  • the optical signal output by the laser is amplified by the optical amplifier and then sent. Let the gain of the optical amplifier be G, then the output light intensity is
  • Optical wireless channel gain describes the gain of the wireless transmission channel from the fiber port on the base station side to the fiber port on the user terminal side, including beam modeling at the transmitter, free space transmission channel gain, the received power ratio of the fiber transceiver port at the receiver, and the coupling efficiency of the fiber port. Parts.
  • the beam modeling of the transmitting end has been discussed in the optical antenna design.
  • the k-th user terminal and the i-th optical fiber transceiver port of the base station have an angle of When the light intensity distribution of the optical fiber port light is given by (8) Given.
  • Ak represents the area of the k-th user terminal side lens
  • d k is the distance from the base station to the user terminal
  • ⁇ d (d k ) describes the channel attenuation caused by the transmission distance d k , and can be calculated according to the law of conservation of light transmission energy
  • the channel attenuation is inversely proportional to the square of the transmission distance, and also inversely proportional to ⁇ 2 , ⁇ describes the angular expansion change of the beam after being refracted by the lens.
  • ⁇ is small, the beam is concentrated in a smaller angular range, and the channel gain is larger .
  • the received light is refracted by the lens and then received by the optical fiber transceiver port.
  • the incident angle of the incident light is The incident position is When it is refracted by the lens and transmitted in free space After the distance, the angle and position at the receiving plane are
  • the power of the light received by a single optical fiber transceiver port is proportional to the irradiated area of the received light on the receiving plane and the common area of the optical fiber transceiver port. Let the area of the j-th optical fiber transceiver port be A j , then the j-th optical fiber transceiver port receives light The power ratio is
  • the optical antenna includes a fiber optic transceiver port array and a reflector
  • the power of the light received by a single fiber transceiver port and the irradiation area of the received light on the receiving plane are proportional to the common area of the fiber transceiver port.
  • the fiber port is equipped with a microlens to expand the beam angle ⁇ C. According to the reversibility of light transmission, light within the angle ⁇ C can be coupled into the fiber for transmission, then the receiving angle of the fiber port is
  • ⁇ represents the receiving angle in the horizontal direction
  • ⁇ represents the receiving angle in the vertical direction
  • Coupling coefficient I the ratio of the volume of the common area of ⁇ f and ⁇ s to the area of ⁇ s, which can be expressed as
  • the gain of the wireless optical transmission channel from the i-th fiber port on the base station side to the j-th fiber port of the k-th user terminal can be obtained as
  • the received optical signal is first amplified by an optical amplifier, and then a photodetector is used to convert the optical signal into a corresponding electrical signal.
  • the received optical signal power is P 1 and the gain of the optical amplifier is G
  • the output power of the optical amplifier is GP 1 .
  • the optical signal output by the optical amplifier is converted into a corresponding electrical signal by a photodetector (such as an avalanche diode).
  • the amplification factor of the photodetector is M p
  • the responsivity of the photodetector is R
  • the output electrical signal of the photodetector is M p RGP 1 .
  • the electrical signal transmission channel gain between the i-th optical fiber transceiver port of the base station and the j-th optical fiber transceiver port of the k-th user terminal can be expressed as
  • the i-th optical fiber transceiver port of the base station sends electrical signals
  • the relationship between the electrical signal y kj received by the j-th optical fiber transceiver port to the k-th user terminal is
  • n p is noise, which mainly includes the shot noise of the photodetector and the beat noise mixed with the amplified spontaneous emission noise.
  • the variance can be approximately expressed as
  • q is the electronic charge
  • B e is the electrical signal bandwidth
  • F(M p ) is the noise figure of the photodetector
  • NF is the noise figure of the optical amplifier. Therefore, the received signal-to-noise ratio of a single link can be calculated as
  • the complete channel model from the base station to the user terminal is considered below.
  • the base station is configured with M fiber optic transceiver ports
  • the user terminal is configured with N fiber optic transceiver ports
  • the electrical signal transmission channel matrix from the base station to the k-th user terminal is Its (j,i)th element is By (23), the optical wireless transmission channel gain Can be decomposed into
  • the channel matrix Can be decomposed into in [ ⁇ ] T represents the transposition operation, and the result shows that the rank of the channel matrix is 1, showing low-rank characteristics.
  • the channel model of uplink transmission is similar to the channel model of downlink transmission.
  • the electrical-optical conversion and photoelectric conversion processes of the uplink transmission and the downlink transmission are the same, only the transmission power of the user terminal and the gain of the optical wireless transmission channel are different .
  • the transmit power on the user terminal side is
  • the intensity distribution of light emitted by the j-th fiber optic transceiver port of the k-th user terminal can be similarly modeled as in Is the relative refraction angle from the user to the base station, then the channel gain from the j-th fiber transceiver port of the k-th user terminal to the base station side lens can be modeled as
  • A is the area of the base station side lens
  • ⁇ u (d k ) represents the channel attenuation caused by the transmission distance d k
  • the channel path of uplink transmission and the channel path of downlink transmission are reciprocal, that is, the k-th user terminal in the downlink transmission receives the signal of the i-th optical fiber transceiver port of the base station.
  • the i-th fiber transceiver port of the base station receives the transmitted signal of the k-th user terminal; also on the user terminal side, the user's j-th fiber transceiver port receives the signal from the base station in the downlink transmission, and the base station receives the j-th signal in the uplink transmission.
  • the signal from the fiber optic transceiver port is not limited to the signal of the fiber optic transceiver port.
  • a base station uses multiple (several to tens) or a large number (hundreds to thousands) of beams to communicate with multiple or a large number of user terminals in two directions.
  • the transceiver end is configured with A/D and D/A modules corresponding to the optical transceiver link, its digital baseband processing can achieve uplink and downlink multi-user MIMO (MU-MIMO, multi-user MIMO) transmission, and then achieve Multi-user MIMO optical wireless communication.
  • MU-MIMO multi-user MIMO
  • the base station can communicate with a large number of user terminals at the same time to realize massive MIMO (massive MIMO) optical wireless communication.
  • massive MIMO massive MIMO
  • the multi-user MIMO/mass MIMO communication process includes the following four steps: synchronization, channel sounding, downlink transmission, and uplink transmission. 1)
  • the base station broadcasts a downlink synchronization signal, and the user terminal uses the received signal to establish and maintain synchronization with the base station. 2)
  • the user terminal sends an uplink sounding signal, and the base station estimates the channel information of each user terminal based on the received sounding signal.
  • the base station uses the channel information of each user terminal and the low-rank characteristics of the channel to perform precoding transmission, and simultaneously transmits signals of all user terminals, including pilot signals and data signals. Each user terminal estimates channel information based on the received pilot signals. And use the channel information to recover the data signal. 4)
  • the user terminal uses precoding transmission to simultaneously send signals to the base station, including pilot signals and data signals.
  • the base station receives the signal superimposition of all user terminals, estimates the channel information of each user terminal based on the pilot signal, and uses the low rank of the channel
  • the feature performs receiving processing and restores the data signal of each user terminal. The following will specifically introduce the downlink transmission and uplink transmission process.
  • the base station sends signals from K user terminals at the same time, and the signal sent by the i-th optical fiber transceiver port to the k-th user terminal is recorded as Then the signal sent by the base station to the k-th user terminal is recorded as The received signal of the kth user terminal can be expressed as
  • n k is Gaussian noise
  • its mean value is 0
  • the covariance matrix is a diagonal matrix ⁇ k
  • the (j,j)th diagonal element is
  • the base station estimates the uplink channel information according to the received sounding signal, and uses the reciprocity of the downlink channel and the uplink channel to obtain the downlink channel matrix.
  • the rank of the channel matrix from the base station to a single user terminal is 1, and the channel matrix It can be decomposed into the form of the product of column vector and row vector, namely
  • the signal sent by the base station to the k-th user terminal Is composed of independent and identically distributed data symbols Linearly precoded generate.
  • the k-th user terminal adopts a linear receiver according to the received signal Detect the data signal, namely Then in downlink transmission, the received signal-to-interference and noise ratio of the k-th user terminal is
  • e i [0,...,0,1,0,...,0] T is the unit vector, only the i-th element is 1, and the remaining elements are 0, p d is the base station side single fiber transceiver The power constraint of the port.
  • the diagonal matrix D is an auxiliary matrix, so that the precoding vector satisfies the constraint condition.
  • the optimal linear precoding can be obtained by the following algorithm:
  • Step 2 Calculate the coefficients a j and b k according to formula (35).
  • Step 3 Update the precoding vector according to formula (34) Calculate the sum rate (R d ) (i) .
  • Step 4 Calculate the difference between the power constraint Among them, diag( ⁇ ) represents a column vector composed of diagonal elements.
  • Step 5 If
  • the base station uses the precoding vector Send a signal to the k-th user terminal, and the k-th user terminal uses a linear reception vector Detect the received signal and recover the data signal sent by the base station.
  • the user terminal uses the channel reciprocity to obtain the uplink channel information through downlink channel estimation, and designs linear precoding to send independent signals, and the base station receives the superposition of the signals sent by the user terminal.
  • the uplink signal sent for the kth user terminal can be expressed as in Is the user-side precoding vector, It is the transmission signal of the k-th user terminal.
  • the received signal at the base station side can be expressed as
  • I the gain of the uplink transmission electrical signal from the kth user terminal to the base station. Since the rank of the channel matrix is 1, it can be decomposed into the form of the product of the column vector and the row vector, namely z is the electrical signal noise on the base station side, its mean value is 0, the covariance matrix is a diagonal matrix ⁇ , and the (i,i)th element is
  • the base station uses a linear receiver according to the received signal Detect the transmitted signal of the k-th user terminal, namely
  • the uplink transmission system and rate are the uplink transmission system and rate.
  • p u is the power constraint of each user terminal in uplink transmission.
  • the optimal precoding vector for maximizing the uplink transmission system and rate is
  • the optimal linear receiver is the optimal linear receiver.
  • c k is an auxiliary variable related to the transmission power of the k-th user terminal.
  • the optimal linear precoding and linear receiver can be obtained by the following algorithm:
  • Step 2 Given the value of c i of other user terminals, calculate the value of c k according to formula (43), and update the precoding vector and receiver vector according to formulas (41) and (42), using formula (39) Calculate the uplink transmission and rate R (k) .
  • the optimal c k value of each user terminal can be obtained, and the precoding vector can be calculated according to formulas (41) and (42) And the receiver vector
  • the k-th user terminal uses the precoding vector Send a signal to the base station, the base station uses a linear reception vector Detect the signal of the k-th user terminal.
  • the above designing the optimal linear precoding and optimal linear receiver with the goal of maximizing the system and rate is an example to illustrate.
  • the base station can also use the maximum ratio transmission (MRT, maximum ratio transmission) or maximum ratio transmission according to the row vector of the channel matrix decomposition.
  • MRT maximum ratio transmission
  • Precoding methods such as regularized zero forcing (RZF, regularized zero forcing) are used for precoding design, and the maximum ratio combining (MRC, maximal ratio combining) receiver can also be used according to the column vector decomposed by the channel matrix of each user.
  • RZF regularized zero forcing
  • MRC maximal ratio combining
  • BDMA Beam Division Multiple Access
  • the transceiver end of the FE-OWC system may not be equipped with broadband A/D and broadband D/A modules corresponding to the optical transceiver link.
  • the base station can share multiple user terminals or a large number of user terminals.
  • BDMA beam division multiple access
  • the fiber-enabled beam division multiple access optical wireless communication method is: the base station allocates non-overlapping beams to different user terminals according to the channel information of each user terminal, and each beam sends and receives signals from at most one user terminal, using different directions
  • the optical beam communicates with the user terminal in both directions at the same time; the user terminal selects the beam in the corresponding direction of the base station according to the channel information, and uses a single beam to communicate with a single base station.
  • the specific communication process includes the following four steps: synchronization, channel detection, downlink transmission, and uplink transmission. 1)
  • the base station broadcasts a downlink synchronization signal, and the user terminal uses the received signal to establish and maintain synchronization with the base station.
  • the user terminal sends an uplink sounding signal, and the base station allocates a beam to each user terminal according to the received sounding signal. For users communicating with the base station on the same time-frequency resource, the allocated beams do not overlap each other and each user is allocated only one beam.
  • the base station sends an independent signal on the beam allocated by each user terminal according to the beam allocation result, and each user terminal selects the beam corresponding to the base station for reception detection based on the received signal.
  • each user terminal sends a signal on the beam corresponding to the base station, and the base station receives and detects the transmitted signal of each user terminal in the beam allocated by each user terminal according to the result of beam allocation.
  • the downlink transmission and uplink transmission process are specifically as follows:
  • the base station allocates beams that do not overlap with each other for different user terminals, and uses different beams to send signals to different user terminals.
  • the precoding vector of the signal sent by the k-th user terminal is Where e i is a unit vector, the i-th element is 1, and the remaining elements are 0. Since the beams allocated by the base station to different user terminals do not overlap each other, the precoding vectors of different user terminals are orthogonal to each other, that is, In this case, the base station in the first I k th beam, sends a signal to the power p to the k-th user terminal.
  • the user terminal selects the beam corresponding to the base station to receive according to the received signal strength, that is, the receiving vector is Where j k represents the fiber transceiver port corresponding to the base station on the k-th user terminal side.
  • the base station selects different transmitting beams to send signals from different user terminals, and the user terminal selects the corresponding beam to receive signals from the base station, so the multi-user downlink transmission link can be decomposed into multiple parallel single-user chains road.
  • the base station can use baseband modulation (such as OOK modulation) to generate the user's analog baseband transmission signal.
  • the user terminal selects the beam corresponding to the base station according to the received signal strength, and uses baseband demodulation (such as OOK demodulation) on the corresponding beam to generate the digital baseband Signal. Therefore, base stations and user terminals may not need to use A/D and D/A devices, which greatly reduces the implementation complexity of ultra-high-speed wireless transmission systems.
  • the k-th user terminal uses the j k- th fiber port to send a signal, that is, the precoding vector for uplink transmission is
  • the base station according to the received signals, on the second detector I k k-th beam signal receiving user terminal, i.e. the base station detects the received vector of the k th user terminal is Since the beams assigned by the base station to different user terminals do not overlap each other, the base station uses different fiber transceiver ports to receive and detect signals from different user terminals, so the multi-user uplink transmission link can also be decomposed into multiple parallel single-user links.
  • Each user terminal uses baseband modulation (such as OOK modulation) to generate an analog baseband signal on the beam corresponding to the base station.
  • the base station uses baseband demodulation (such as OOK demodulation) on the beam allocated by each user terminal according to the result of beam allocation. ) Method for demodulation to generate digital baseband signals for each user terminal, which can avoid the use of A/D and D/A devices and reduce the complexity of system implementation.
  • Figure 5 shows a schematic diagram of the increase in system throughput with the number of fiber optic transceiver ports in a large-scale scene with an area of 16m ⁇ 16m and a height of 8m (such as a terminal and a gymnasium). There are 300 user terminals in this scene.
  • the multi-user MIMO/massive MIMO transmission method (Optimal in the figure) and the performance of beam division multiple access (BDMA) transmission are combined with the maximum ratio transmission (MRT, maximum ratio transmission) and regularized zero-forcing precoding (RZF, regularized zeroforcing) is compared, the precoding vectors of maximum ratio transmission and regularized zero-forcing precoding are respectively
  • FIG. 5 (a) is a schematic diagram of downlink transmission performance.
  • the optimal design scheme and the performance of beam division multiple access (BDMA) transmission are compared with the maximum ratio combining (MRC, maximal ratio combining).
  • the received vector of the maximum ratio combining is
  • Figure 5 (b) shows a schematic diagram of uplink transmission performance. It can be seen that as the number of fiber optic transceiver ports increases, the performance of beam division multiple access transmission approaches the optimal transmission, which is better than that of MRC. In addition, when the number of optical fiber transceiver ports is large, the system implementation complexity of beam division multiple access downlink transmission and uplink transmission is very low.

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Abstract

本发明公开了光纤使能光无线通信(FE-OWC)系统及方法。本发明采用由光纤收发端口阵列以及透镜或反射镜构成的光天线，生成不同方向的光波束，实现通信区域的全波束覆盖，光天线通过光纤与光收发链路连接，光收发链路实现光信号与电信号的相互转换，与光收发链路电信号连接的基带信号处理单元实现用户调度、收发信号处理等。基站与用户终端之间利用光波束实施多用户MIMO或大规模MIMO或波束分多址光无线通信。本发明所提出的FE-OWC系统及方法可以支持超高速率的用户数据传输及系统吞吐量，且复杂度低，可用于构建巨容量热点覆盖和各种特殊应用的光无线通信系统，并用于光纤通信网络向无线覆盖延展，实现移动光通信和支持移动性的全光通信。

Description

光纤使能光无线通信系统及方法 技术领域
本发明涉及一种光无线通信技术，具体涉及光天线、光纤使能光无线通信(FE-OWC，fiber enabled optical wireless communication)装置、FE-OWC系统及方法，属于移动通信技术领域。
背景技术
随着移动通信的发展，移动设备的数量以及无线传输速率的需求迅猛增长，虚拟现实、增强现实等新兴应用需要极高的传输速率。在后5G以及6G移动通信系统中，单用户传输速率需求高达10Gbps，系统吞吐量需求达到1Tbps。然而传统低频段的频谱资源严重短缺，无法支持超高速率的移动通信，需要利用更高频段的频谱资源。光无线通信利用光波段提供非常丰富的频谱资源，支持高速数据传输，是一种极具潜力的无线通信方式。相比于高频段的射频无线通信，光无线通信还具有收发信号处理复杂度低、通信器件和设备成熟等优点。
可见光通信是光无线通信中的一个研究方向，将信号调制到可见光的幅度上，利用LED提供照明的同时向用户终端传输数据信号。为了满足照明的需求，LED生成宽波束覆盖整个通信区域，同时由于大多数场景仅考虑直达径传输情况，因此光无线通信的信道系数具有高度的相关性。单个光发送节点发送全向信号，通常视为单个发送天线，只能传输单路数据信号，系统同时服务的用户终端个数受限。另外，由于LED的调制带宽约为20MHz，不能充分利用光无线通信中丰富的频谱资源，系统的传输速率较低，不能满足超高速率数据传输的需求。此外，目前的可见光无线通信系统仅考虑基站到用户终端的下行传输，不能支持双向通信。
另一个研究方向是红外光无线通信，通过生成具有方向性的窄波束，将光信号能量集中到用户终端处，极大提升接收能量，同时利用激光器产生光信号，支持数十Gbps的传输速率。然而，由于激光器产生的红外光波束具有极强的方向性，因此收发器件需要精确对准，在用户移动等场景下，极大增加了系统的复杂度，而且大多数红外光无线光通信系统仅考虑点对点单链路传输，无法支持多个或大量用户终端同时通信。
发明内容
发明目的：针对现有光无线通信面临着服务用户终端个数受限、覆盖范围小、方向校准复杂度高、以及单向传输的瓶颈问题，本发明目的在于提供一种光天线，以及基于该光天线的FE-OWC装置、FE-OWC系统及方法，以能充分利用光波段丰富的频谱资源，实现通信区域内信号全覆盖并支持终端移动性的高速率双向通信，满足未来移动通信应用需求。
技术方案：为实现上述发明目的，本发明所述的光天线，包括光纤收发端口阵列以及透镜或反射镜；在发送信号的过程中，单个光纤收发端口发出的光经过透镜折射或反射镜反射后在某一方向生成具有一定角度范围的光波束，不同光纤收发端口发出的光折射或反射到不同方向；在接收信号的过程中，来自不同方向的接收光线经过透镜折射或反射镜反射后耦合进不同的光纤收发端口进行接收，不同光纤收发端口接收不同方向的光信号。
进一步地，所述光天线中的光纤收发端口包括光纤端口和微透镜；在发送信号的过程中，单个光纤端口发出的光信号经过微透镜折射后，生成具有一定角度范围的光波束；在接收信号的过程中，微透镜将一定角度范围内的光信号耦合进光纤端口。
进一步地，所述光天线利用光纤收发端口阵列以及透镜或反射镜生成不同方向的光波束，不同光波束覆盖不同区域，光纤收发端口阵列生成的所有光波束覆盖整个通信区域，实现通信区域全波束覆盖。
本发明所述的FE-OWC装置，包括上述的光天线以及光收发链路；光天线用于发送和接收不同方向的光信号；光天线与光收发链路通过光纤直接连接，或通过光交换单元连接；光 收发链路用于实现光信号与电信号的相互转换；单个FE-OWC装置与单个或一组FE-OWC装置进行无线通信。
进一步地，所述光收发链路用于实现光信号与电信号的相互转换；在发送信号的过程中，电信号加上偏置电流后驱动激光器，生成与电信号对应的光信号，或采用外调制器方式，将激光源产生的光信号与电信号输入外调制器，生成对应的光信号，光信号经过光放大器放大后通过光纤传输到光天线；在接收信号的过程中，光天线接收的信号通过光纤传输到光收发链路，经过光放大器放大后，利用光探测器检测光信号，转换为对应的电信号。
进一步地，当光天线与光收发链路直接连接时，每个光收发链路对应一个光纤收发端口，光收发链路个数与光纤收发端口个数相同；当光天线与光收发链路通过光交换单元连接时，光收发链路个数小于等于光纤收发端口个数，光交换单元用于切换光收发链路与光纤收发端口的对应关系，将光收发链路中的信号与光天线生成的波束相对应。
进一步地，基站侧FE-OWC装置还包括基带信号处理单元，所述基带信号处理单元包含A/D和D/A模块以及数字基带处理与控制模块；下行传输过程中，基站侧的数字基带处理与控制模块用于实现用户调度以及多用户预编码传输，生成每个用户终端的发送信号，D/A模块用于将数字基带处理与控制模块生成的发送信号转化为模拟信号输入光收发链路；上行传输过程中，A/D模块用于将基站侧光收发链路输出的电信号转化为数字信号，数字基带处理与控制模块用于对多用户接收信号进行检测，恢复每个用户终端的发送信号。
进一步地，用户终端侧FE-OWC装置还包括基带信号处理单元，所述基带信号处理单元包含A/D和D/A模块以及数字基带处理与控制模块；下行传输过程中，A/D模块用于将用户终端侧光收发链路输出的电信号转化为数字信号，数字基带处理与控制模块用于对接收信号进行检测，恢复基站的发送信号；上行传输过程中，用户终端侧的数字基带处理与控制模块用于实现预编码传输，D/A模块用于将生成的发送信号转化为模拟信号输入光收发链路。
进一步地，基站侧FE-OWC装置还包括基带信号处理单元，所述基带信号处理单元包含基带调制与基带解调模块以及数字基带处理与控制模块；下行传输过程中，基站侧的数字基带处理与控制模块用于为不同用户终端分配互不重叠的波束集合，生成向各用户终端发送的数字基带信号，基带调制模块用于生成向各用户终端发送的模拟基带信号，传输到对应的光收发链路，利用相应的光纤收发端口进行发送；上行传输过程中，基带解调模块用于将基站侧光收发链路输出的模拟基带接收信号进行解调，生成数字基带信号，数字基带处理与控制模块用于根据波束分配的结果以及每个用户终端对应波束上的数字基带信号恢复各用户终端的发送信号。
进一步地，用户终端侧FE-OWC装置还包括基带信号处理单元，所述基带信号处理单元包含基带调制与基带解调模块以及数字基带处理与控制模块；下行传输过程中，用户终端侧基带解调模块用于将光收发链路输出的模拟信号进行解调，生成数字基带信号，数字基带处理与控制模块用于挑选基站对应的光纤收发端口，根据其接收数字基带信号恢复基站侧的发送信号；上行传输过程中，用户终端侧的数字基带处理与控制模块用于生成上行数字基带发送信号，基带调制模块用于生成模拟基带发送信号，传输到光收发链路，利用与基站对应的光纤收发端口进行发送。
本发明所述的FE-OWC系统，该系统的基站和用户终端配置均配置上述FE-OWC装置。基站与用户终端之间实现多用户MIMO或大规模MIMO光无线通信或波束分多址BDMA光无线通信。
本发明所述的FE-OWC方法，该方法基于所述的FE-OWC系统，计算单链路传输的链路预算并建立收发端电信号传输信道模型；链路预算包括发送端电光转换、光无线信道增益、接收端光电转换以及接收端电噪声；发送端电光转换部分根据电光转换器件的光电特性，建立发送端输出的光功率与输入电信号之间的对应关系；光无线信道增益为发送端光纤收发端口到接收端光纤收发端口之间的无线信道增益；接收端光电转换部分考虑接收光信号经过光 放大器放大以及光探测器检测两个过程，建立接收端输入光信号与输出电信号之间的转换关系；接收端电噪声包括光放大器和光探测器引入的电噪声，建立单个链路接收电信号中信号功率与噪声功率之间的关系；在单个链路预算的基础上，建立完整的从发送端到接收端电信号传输的信道模型；基于该信道模型，基站和用户终端之间实施多用户MIMO或大规模MIMO或波束分多址光无线通信方法。
进一步地，所述光无线信道增益描述从发送端到接收端光无线传输的信道增益，包括发送端的波束建模、自由空间传输信道增益、接收端光纤收发端口接收功率比率以及光纤端口的耦合效率四个部分；发送端的波束建模描述光纤收发端口发出的光线经过透镜折射或反射镜反射后生成单个波束的光强分布，随着发送端光纤收发端口个数的增加，单个光纤收发端口到不同用户终端的光强渐进正交；自由空间传输信道增益考虑光波束经过自由空间从发送端到接收端的传输过程，其与传输距离的平方成反比；接收端光纤收发端口接收功率比率描述单个光纤收发端口接收的光功率占用户总接收功率的比率，其与接收平面上接收光线投影和光纤收发端口的公共面积成正比；光纤端口的耦合效率为光纤端口处的接收光线能够耦合进入光纤的比率，其与入射光线在光纤端口处的角度范围和光纤端口的接收角的公共区域成正比。
另一方面，本发明所述的FE-OWC方法，为基于所述的FE-OWC系统实现的多用户MIMO或大规模MIMO光无线通信方法，具体通信过程包含如下步骤：
同步：基站广播下行同步信号，用户终端利用接收信号建立并保持与基站同步；
信道探测：用户终端发送上行探测信号，基站根据接收的探测信号估计各用户终端的信道信息；
下行传输：基站利用各用户终端的信道信息以及信道的低秩特性进行预编码传输，同时发送所有用户终端的信号，包括导频信号和数据信号，各用户终端根据接收的导频信号估计信道信息，并利用信道信息恢复数据信号；
上行传输：用户终端利用预编码传输同时向基站发送信号，包括导频信号和数据信号，基站接收到所有用户终端的信号叠加，根据导频信号估计各用户终端的信道信息，并利用信道的低秩特性进行接收处理，恢复各用户终端的数据信号。
进一步地，下行传输过程中，基站通过上行探测过程估计每个用户终端到基站的上行信道，利用信道的互易性获得下行信道，将信道矩阵分解为列向量与行向量的乘积，并计算各用户终端的接收信干噪比以及可达和速率，在单个光纤收发端口功率约束条件下，以最大化系统和速率为目标设计最优线性预编码；或者基站根据信道矩阵分解的行向量采用最大比发射MRT或正则化迫零RZF预编码方法，利用预编码向量同时发送所有用户终端的信号，包括导频信号和数据信号；上行传输过程中，基站接收到所有用户终端的信号叠加，根据接收的导频信号估计各用户终端的上行信道矩阵，并将其分解为列向量与行向量的乘积，以最大化系统和速率为目标设计最优线性接收机；或者基站根据各用户信道矩阵分解的列向量采用最大比合并MRC接收机，利用线性接收机对接收的数据信号进行检测，恢复各用户终端的发送信号。
进一步地，下行传输过程中，各用户终端根据接收的导频信号估计下行传输信道矩阵，并将其分解为列向量与行向量的乘积，以最大化系统和速率为目标设计最优线性接收机，用户终端利用最优线性接收机对接收的数据信号进行检测；上行传输过程中，各用户终端利用信道的互易性，根据下行信道估计获得上行信道信息，并将信道矩阵分解为列向量与行向量的乘积，在总功率约束的条件下，以最大化系统和速率为目标设计最优预编码向量，用户终端利用预编码向量同时发送上行信号，包括导频信号和数据信号。
另一方面，本发明所述的FE-OWC方法，为基于所述的FE-OWC系统实现的波束分多址光无线通信方法，所述波束分多址光无线通信方法为基站根据各用户终端的信道信息为不同的用户终端分配互不重叠的波束，每个波束发送和接收最多一个用户终端的信号，利用不 同方向的光波束同时与用户终端双向通信；用户终端根据信道信息挑选基站对应方向的波束，利用单个波束与单个基站进行通信；具体通信过程包含如下步骤：
同步：基站广播下行同步信号，用户终端利用接收信号建立并保持与基站同步；
信道探测：用户终端发送上行探测信号，基站根据接收的探测信号为每个用户终端分配波束，同一时频资源上与基站通信的用户，所分配的波束互不重叠且每个用户仅分配一个波束；
下行传输：基站根据波束分配结果，在每个用户终端分配的波束上发送独立信号，各用户终端根据接收到的信号，挑选与基站对应的波束进行接收检测；
上行传输：各用户终端在基站对应的波束上发送信号，基站根据波束分配的结果，在每个用户终端分配的波束上接收检测各用户终端的发送信号。
进一步地，下行传输过程中，基站根据波束分配的结果，在每个用户终端分配的波束上向其发送信号，不同波束发送不同用户终端的信号，多用户下行传输链路分解为多个并行的单用户链路，基站采用基带调制方式生成用户模拟基带发送信号；上行传输过程中，基站根据波束分配的结果，在每个用户终端分配的波束上接收检测该用户终端的发送信号，基站利用不同的波束接收检测不同用户终端的信号，多用户上行传输链路分解为多个并行的单用户链路，基站采用基带解调方式生成各用户终端的数字基带信号。
进一步地，下行传输过程中，各用户终端根据接收信号的强度，挑选基站对应的波束，在对应的波束上采用基带解调方式生成数字基带信号；上行传输过程中，每个用户终端在基站对应的波束上，采用基带调制方式生成模拟基带信号。
有益效果：与现有技术相比，本发明具有如下有益效果：
1.本发明中，基站利用光纤收发端口阵列以及透镜或反射镜构成的光天线生成多个或大量不同方向的波束，不同波束覆盖不同区域，基站利用不同波束同时与多个或大量用户终端通信，大幅提升光无线通信支持的用户终端个数。
2.本发明利用光纤支持的超高速率数据传输，显著提升每个用户终端链路的传输速率以及系统吞吐量。
3.基站和用户终端利用光纤收发端口发送和接收信号，可以实现双向通信，解决光无线通信中单向传输的局限。
4.基站利用光天线生成多个或大量不同方向的波束，实现通信区域的全波束覆盖。用户终端在移动的过程中，从一个波束区域移动到另一个波束区域，基站仅需要切换用户终端对应的波束，不需要复杂的跟踪系统。
5.基站利用不同的波束与不同的用户终端进行通信，将多用户传输链路分解为多个单用户传输链路，每个单用户传输链路可以采用光调制与光解调(如OOK调制与解调)方式，可以不需要A/D和D/A设备，显著降低超高速光无线通信系统的实现复杂度。
6.光天线与光收发链路通过光纤连接，由于光纤传输过程中的损耗极低，光天线可以灵活布设，而且光天线为无源系统，成本较低，由此可以大幅降低系统构建成本。
7.基站和用户终端的光天线将接收到的光信号输入到光纤，利用光纤放大器进行放大之后再进行光接收处理，可以大幅提高接收能力，由此降低发送端的发射功率，或增加通信距离。
8.所提出的光纤使能光无线通信方法，可以利用成熟的光纤通信技术、器件和设备，高效地构建高速光无线通信系统，满足未来5G之后移动通信对传输速率和系统容量量级或多量级提升的需求。
9.所提出的光纤使能光无线通信系统，也可以方便地与光纤通信网络对接，实现光纤通信网络向无线覆盖延展，由此实现移动光通信及支持终端移动性的全光通信。
附图说明
图1为光纤使能光无线通信系统架构示意图；
图2为光收发链路示意图；
图3为光天线结构示意图；
图4为波束图样示意图，(a)单个光纤收发端口，(b)光纤收发端口阵列；
图5为系统吞吐量性能示意图，(a)下行传输，(b)上行传输。
具体实施方式
为了使本技术领域的人员更好地理解本发明方案，下面将结合本发明实施例中的附图，对本发明实施例中的技术方案进行清楚、完整的描述，显然，所描述的实施例仅是本发明一部分实施例，而不是全部的实施例。基于本发明中的实施例，本领域普通技术人员在没有做出创造性劳动前提下所获得的所有其他实施例，都应当属于本发明保护的范围。
(1)系统构成
本发明公开了光纤使能光无线通信(FE-OWC,fiber enabled optical wireless communication)系统，其系统架构如图1所示，单个基站(BS,base station)同时服务K个用户终端(UT,user terminal)，基站和用户终端均配置FE-OWC装置，FE-OWC装置包括光天线、光收发链路以及基带信号处理单元。光天线与光收发链路通过光纤连接，由于光纤传输过程中的损耗极低，光天线可以灵活布设。
光天线用于发送和接收光信号，包括光纤收发端口阵列以及透镜或反射镜。在发送信号的过程中，单个光纤收发端口发出的光经过透镜折射或反射镜反射后，在某一方向生成具有一定角度扩展的光波束，不同光纤收发端口发出的光折射或反射到不同方向，利用光纤收发端口阵列以及透镜或反射镜生成多个不同方向的光波束，实现通信区域的全波束覆盖，即所有光纤收发端口对应的光波束能够覆盖整个通信区域。在接收信号的过程中，不同方向的接收光线经过透镜折射或反射镜反射到不同的光纤收发端口，耦合进光纤传输到光收发链路。
光收发链路实现光信号与电信号的相互转换，其系统架构如图2所示。在发送信号的过程中，电信号加上偏置电流后驱动激光器(LD,laser diode)，生成与电信号对应的光信号，也可以采用外调制器的方式，将激光源产生的光信号与电信号输入外调制器，生成与电信号对应的光信号。光信号经过光放大器，如掺铒光纤放大器(EDFA,erbium-doped fiber amplifier)，放大后通过光纤传输到光天线进行发送。在接收信号的过程中，光天线接收的光信号通过光纤传输到光收发链路，经过光放大器放大后，利用光探测器，如雪崩二极管(APD,avalanche photodiode)，检测光信号，如其强度，转换为对应的电信号。另外，在双向传输过程中，光环形器(OC,optical circulator)用于分隔不同方向的光信号。
光天线与光收发链路通过光纤直接连接，或者通过光交换单元连接。当光天线与光收发链路直接连接时，每个光收发链路对应一个光纤收发端口，光收发链路个数与光纤收发端口个数相同。当光天线与光收发链路通过光交换单元连接时，光收发链路个数小于等于光纤收发端口个数，光交换单元用于切换光收发链路与光纤收发端口的对应关系，将光收发链路中的信号与光天线生成的波束相对应。
基带信号处理单元用于实现包括用户调度、收发信号处理等功能。具体而言，本发明实施例公开两种基带信号处理方法。在第一种基带信号处理方法中，基带信号处理单元包含A/D和D/A模块以及数字基带处理与控制模块，基站与用户终端之间实现多用户MIMO(multiple-input multiple-output)或大规模MIMO光无线通信。在下行传输过程中，基站侧的数字基带处理与控制模块用于实现用户调度以及多用户预编码传输，生成每个用户终端的发送信号，D/A模块用于将数字基带处理与控制模块生成的发送信号转化为模拟信号输入光收发链路；用户终端侧A/D模块用于将光收发链路输出的电信号转化为数字信号，数字基带处理与控制模块用于对接收信号进行检测，恢复基站的发送信号。在上行传输过程中，用户终端侧数字基带处理与控制模块用于实现预编码传输，D/A模块用于将生成的发送信号转化为模拟信号输入光收发链路；基站侧A/D模块用于将光收发链路输出的电信号转化为数字信号，数字基带处理与控制模块用于对多用户接收信号进行检测，恢复每个用户终端的发送信号。
在第二种基带信号处理方法中，基带信号处理单元没有A/D和D/A模块，包含基带调制与基带解调模块(如OOK(on-off keying)调制与解调)以及数字基带处理与控制模块，基站与用户终端之间实现多用户或大量用户的波束分多址(BDMA,beam division multiple access)光无线通信。在下行传输过程中，基站侧的数字基带处理与控制模块用于为不同用户终端分配互不重叠的波束集合，生成向各用户终端发送的数字基带信号，基带调制模块用于生成向各用户终端发送的模拟基带信号，传输到对应的光收发链路，利用相应的光纤收发端口进行发送；用户终端侧基带解调模块用于将光收发链路输出的模拟信号进行解调，生成数字基带信号，数字基带处理与控制模块用于挑选与基站对应的光纤收发端口，根据其接收数字基带信号恢复基站侧的发送信号。在上行传输过程中，用户终端侧的数字基带处理与控制模块用于生成上行数字基带发送信号，基带调制模块用于生成模拟基带发送信号，传输到光收发链路，利用与基站对应的光纤收发端口进行发送；基站侧基带解调模块用于将光收发链路输出的模拟基带接收信号进行解调，生成数字基带信号，数字基带处理与控制模块用于根据波束分配的结果以及每个用户终端对应波束上的数字基带信号恢复各用户终端的发送信号。
(2)光天线设计
光天线由光纤收发端口阵列以及透镜或反射镜构成，本实施例以光纤收发端口阵列和透镜构成为例，其结构如图3所示，配置反射镜的光天线可以类似地得到。以单模光纤为例，相应的设计方法可以应用于其它光纤结构中。单模光纤端口发出的光强近似服从高斯分布，在传播距离为z的横截面上距离中心半径为r的位置，光强分布可以建模为
Figure PCTCN2020086078-appb-000001
其中，I 0(z)表示在距离为z的横截面上的最强光强，
Figure PCTCN2020086078-appb-000002
表示在距离为z的横截面上的光斑大小，ω 0为束腰宽度(z＝0)，
Figure PCTCN2020086078-appb-000003
为瑞利距离，λ为光的波长。光纤端口发出的光线主要集中在光斑ω 1(z)的范围内。
为扩大光纤端口发出和接收光线的角度范围，在光纤端口配置微透镜构成光纤收发端口。当微透镜的焦距为f，微透镜与光纤端口之间的距离为d 1时，光纤收发端口发出光线的强度分布可以表达为
Figure PCTCN2020086078-appb-000004
其中，ω 2(z)为光纤收发端口发出的光在位置z处的光斑大小，
Figure PCTCN2020086078-appb-000005
当传输距离z远大于透镜焦距f和d 1时，光纤收发端口发出光线的角度扩展为
Figure PCTCN2020086078-appb-000006
其中，ω 3为
Figure PCTCN2020086078-appb-000007
上式中，θ C描述了光纤收发端口发送和接收光线的角度范围，通过调节微透镜的焦距f以及微透镜与光纤端口之间的距离d 1可以改变光纤收发端口生成波束的角度范围。为了扩大波束的角度扩展，应当减少距离d 1，即微透镜尽可能靠近光纤端口。从而，在角度θ C范围内，单个光纤收发端口生成的波束远场强度分布可以建模为
Figure PCTCN2020086078-appb-000008
其中，θ为相对于z轴的极化角，U(·)为单位阶跃函数。
光天线包括光纤收发端口阵列以及透镜。例如，M个光纤收发端口构成方形阵列或者圆形阵列或者六边形阵列等，单个透镜覆盖整个光纤收发端口阵列，或多个透镜共同覆盖整个光纤收发端口阵列，不同透镜覆盖不同的光纤收发端口，光纤收发端口阵列与透镜之间的距离为d 2。以透镜的中心为原点，建立直角坐标系，记第i个光纤收发端口的坐标为(x i,y i,z i)，则水平距离为
Figure PCTCN2020086078-appb-000009
垂直坐标为z i＝-d 2。第i个光纤收发端口发出的光线，其与垂直方向夹角为
Figure PCTCN2020086078-appb-000010
经过透镜折射后，方向为
Figure PCTCN2020086078-appb-000011
其中，F为透镜的焦距。
当
Figure PCTCN2020086078-appb-000012
时，表示光纤收发端口发出的中心光线，其经过透镜折射后方向为-r i/F，仅与光纤收发端口的水平位置有关；
Figure PCTCN2020086078-appb-000013
表示与垂直方向夹角为
Figure PCTCN2020086078-appb-000014
的光线经过透镜折射后与折射后的中心光线的相对夹角，其与光纤收发端口的垂直位置有关。由(6)式可知单个光纤收发端口发出的光线生成角度扩展为θ C的光波束，经过透镜折射后，在-r i/F方向上生成一个角度范围为(1-d 2/F)θ C的光波束。因此，单个光纤收发端口发出的光线经过透镜折射可以在某一方向生成具有一定角度范围的光波束。令α＝1-d 2/F，则与中心光线夹角(相对折射角)为ψ i的光线强度分布可以表示为
I i(ψ i)＝T lensI(α -1ψ i,d 2)            (8)
其中，T lens表示透镜增益。
为了实现通信区域的全波束覆盖，利用光纤收发端口阵列以及透镜，通过设计光纤收发端口阵列的水平位置和垂直位置，生成不同方向的多个或大量光波束，覆盖整个通信区域。以
Figure PCTCN2020086078-appb-000015
个光纤收发端口构成方形阵列为例，在x轴和y轴方向上，通信覆盖区域的最大角度范围为Θ，相邻波束在最大功率衰减到一半的位置重叠，则光纤收发端口阵列与透镜之间的距离为
Figure PCTCN2020086078-appb-000016
其中，θ 1/2为最大功率衰减到一半的角度位置。由此可以得到第i个光纤收发端口的坐标为
Figure PCTCN2020086078-appb-000017
其中，d a为相邻光纤收发端口之间的距离，满足
Figure PCTCN2020086078-appb-000018
m 1和m 2表示在方形阵列中第m 1行和第m 2列位置，满足
Figure PCTCN2020086078-appb-000019
光纤收发端口也可以排列成圆形阵列或六边形阵列，当排成圆形阵列时，在圆心处以及半径为r c、2r c等圆周上均匀排列光纤收发端口，通过调整圆周半径和光纤收发端口间隔使得光纤收发端口发出的光经过透镜折射实现接收平面的全波束覆盖；当排列成六边形阵列时， 在中心处，以及六边形边长为r h呈蜂窝结构向外扩展的位置排列光纤收发端口，通过调整六边形的边长使得光纤收发端口发出的光经过透镜折射实现全波束覆盖。
通信区域的全波束覆盖也可以通过光纤收发端口阵列以及反射镜实现。单个反射镜覆盖整个光纤收发端口阵列，或多个反射镜共同覆盖整个光纤收发端口阵列。在单个光纤收发端口的光强分布模型(6)式基础上，利用几何光学建立在位置(x i,y i,z i)处的光纤收发端口发出的光经过反射镜反射的过程，从而得到不同位置的光纤收发端口经过反射镜反射后生成波束的方向和角度范围。根据反射镜反射的物理规律以及通信区域全覆盖的需求，设计光纤收发端口的位置，使得光纤收发端口发出的光经过反射镜反射后覆盖整个通信区域。
当光纤收发端口个数较大时，光纤收发端口阵列与透镜之间的距离d 2趋向于透镜的焦距F，光纤收发端口阵列位于透镜的焦平面处。不同光纤收发端口发出的光线经过透镜折射到不同方向，照射不同区域。当用户终端位于不同区域时，单个光纤收发端口发出的光到两个用户终端的光强渐近正交，
Figure PCTCN2020086078-appb-000020
其中，
Figure PCTCN2020086078-appb-000021
和
Figure PCTCN2020086078-appb-000022
为用户1和用户2关于第i个光纤收发端口的相对折射角。单个光纤收发端口最多发送一个用户的信号，不同用户终端接收不同光纤收发端口发出的光信号。
图4为8×8光纤收发端口阵列在接收平面上生成的波束图样，在一个5m×5m高为3m的通信场景，基站位于该场景的中心。图4中(a)展示了光纤收发端口阵列中单个光纤收发端口生成的波束图样，即在接收平面上光强的分布，单个光纤收发端口发出的光经过透镜折射后，汇聚到某一区域形成一个波束；图4中(b)展示了光纤收发端口阵列生成的波束图样，不同的光纤收发端口生成不同方向的波束，从而利用8×8的波束实现整个通信区域的全波束覆盖。
(3)链路预算和信道模型
基于上述的FE-OWC系统，考虑单链路传输的链路预算，并建立完整的下行传输和上行传输的信道模型。首先，以下行传输过程为例，计算从基站到用户终端的链路预算，包括发送端电光转换、光无线信道增益、接收端光电转换以及接收端电噪声四个部分。在此基础上，建立下行传输信道模型，并分析其信道特性，上行传输过程与下行传输过程相似，可以类似地建立上行传输信道模型。
在发送端，电光转换将电信号转换为对应的光信号，通过直接改变激光器的驱动电流实现，或通过外调制器来实现。采用直接改变激光器的驱动电流时，当驱动电流高于门限I th时，输出光功率与输入电流之间存在线性对应关系。因而，在承载信息的电信号x上添加偏置电流I B(I B＞I th)后输入激光器，则在线性范围内输出光强可以表达为
P LD＝P LD,0(1+mx)             (12)
其中，P LD,0为偏置电流驱动下输出的光功率，m为光强与输入电流的转换系数。激光器输出的光信号，经过光放大器放大后进行发送。令光放大器的增益为G，则输出光强为
Figure PCTCN2020086078-appb-000023
其中，
Figure PCTCN2020086078-appb-000024
光无线信道增益描述从基站侧光纤端口到用户终端侧光纤端口的无线传输信道增益，包括发送端的波束建模、自由空间传输信道增益、接收端光纤收发端口接收功率比率以及光纤端口的耦合效率四个部分。发送端的波束建模已在光天线设计中进行了讨论，当第k个用户终端与基站第i个光纤收发端口的中心光线夹角为
Figure PCTCN2020086078-appb-000025
时，光纤端口光发出的光线强度分布由 (8)式
Figure PCTCN2020086078-appb-000026
给出。从基站侧第i个光纤收发端口到第k个用户终端透镜处的无线信道增益
Figure PCTCN2020086078-appb-000027
为
Figure PCTCN2020086078-appb-000028
其中，A k表示第k个用户终端侧透镜的面积，
Figure PCTCN2020086078-appb-000029
为用户终端侧光线的入射角，d k为基站到用户终端的距离，β d(d k)描述由传输距离d k引起的信道衰减，根据光线传输能量守恒定律，可以计算得到
Figure PCTCN2020086078-appb-000030
信道衰减与传输距离的平方成反比，同时也反比于α 2，α描述了经过透镜折射后波束的角度扩展变化，当α较小时，即波束集中在较小的角度范围内，信道增益较大。
在用户终端侧，接收光线经过透镜折射后被光纤收发端口接收。当入射光线的入射角为
Figure PCTCN2020086078-appb-000031
入射位置为
Figure PCTCN2020086078-appb-000032
时，经过透镜折射并在自由空间传输
Figure PCTCN2020086078-appb-000033
距离后，在接收平面处的角度和位置为
Figure PCTCN2020086078-appb-000034
其中，
Figure PCTCN2020086078-appb-000035
为用户终端侧透镜的焦距。入射角为
Figure PCTCN2020086078-appb-000036
的入射光线在接收平面上的照射区域为
Figure PCTCN2020086078-appb-000037
其中，
Figure PCTCN2020086078-appb-000038
为用户终端侧透镜的半径，
Figure PCTCN2020086078-appb-000039
由下式计算得到
Figure PCTCN2020086078-appb-000040
单个光纤收发端口接收光线的功率与接收光线在接收平面上的照射区域与光纤收发端口的公共区域成正比，令第j个光纤收发端口的区域为A j，则第j个光纤收发端口接收光线功率的比率为
Figure PCTCN2020086078-appb-000041
若光天线包括光纤收发端口阵列以及反射镜，利用几何光学原理分析入射光线以角度
Figure PCTCN2020086078-appb-000042
入射反射镜时，经过反射镜反射后，在接收平面上的照射区域，单个光纤收发端口接收光线的功率与接收光线在接收平面上的照射区域与光纤收发端口的公共区域成正比。
考虑光纤端口的耦合效率，光纤端口安装了微透镜扩大波束角度为θ C，根据光线传输的可逆性，在角度为θ C之内的光线可以耦合进入光纤进行传输，则光纤端口的接收角为
Ω f＝{0≤Ξ≤2π,0≤θ≤θ C}             (20)
其中，Ξ表示水平方向接收角，θ表示垂直方向接收角度。经过透镜折射后，入射光线在光纤端口处的角度范围Ω s可以计算为
Figure PCTCN2020086078-appb-000043
耦合系数
Figure PCTCN2020086078-appb-000044
为Ω f与Ω s的公共区域与Ω s区域的体积之比，其可以表示为
Figure PCTCN2020086078-appb-000045
综合上述四部分，可以得到从基站侧第i个光纤端口到第k个用户终端的第j个光纤端口的无线光传输信道增益为
Figure PCTCN2020086078-appb-000046
接下来考虑接收端的光电转换过程，接收的光信号首先经过光放大器放大，之后利用光电探测器将光信号转换为对应的电信号。当接收的光信号功率为P 1，光放大器的增益为G时，光放大器的输出功率为GP 1。光放大器输出的光信号被光电探测器(如雪崩二极管)转换为对应的电信号，光电探测器的放大因子为M p，光电探测器的响应度为R，则光电探测器的输出电信号为M pRGP 1。
结合电光转换、光无线信道增益以及接收端光电转换，基站第i个光纤收发端口到第k个用户终端的第j个光纤收发端口之间的电信号传输信道增益可以表达为
Figure PCTCN2020086078-appb-000047
基站第i个光纤收发端口发送电信号
Figure PCTCN2020086078-appb-000048
到第k个用户终端的第j个光纤收发端口接收的电信号y kj之间的关系为
Figure PCTCN2020086078-appb-000049
其中，n p为噪声，其主要包含光电探测器的散粒噪声以及信号与放大的自发辐射噪声混合的差拍噪声，其方差可以近似表达为
Figure PCTCN2020086078-appb-000050
其中，q为电子电荷，B e为电信号带宽，F(M p)为光电探测器的噪声系数，NF为光放大器的噪声系数。因而，单链路的接收信噪比可以计算为
Figure PCTCN2020086078-appb-000051
其中，
Figure PCTCN2020086078-appb-000052
为发射信号的功率。
在建立了从基站到用户终端单链路传输信道模型以及单链路信噪比之后，下面考虑从基站到用户终端完整的信道模型。基站配置M个光纤收发端口，用户终端配置N个光纤收发端口，则基站到第k个用户终端的电信号传输信道矩阵为
Figure PCTCN2020086078-appb-000053
其第(j,i)个元素为
Figure PCTCN2020086078-appb-000054
由(23)式，光无线传输信道增益
Figure PCTCN2020086078-appb-000055
可以分解为
Figure PCTCN2020086078-appb-000056
因而信道矩阵
Figure PCTCN2020086078-appb-000057
可以分解为
Figure PCTCN2020086078-appb-000058
其中
Figure PCTCN2020086078-appb-000059
[·] T表示转置运算，该结果表明信道矩阵的秩为1，呈现低秩的特性。
上行传输的信道模型与下行传输的信道模型相似。当基站和用户终端在上行传输和下行传输的过程中使用相同的收发设备，上行传输和下行传输的电光转换以及光电转换过程是一致的，仅有用户终端的发送功率以及光无线传输信道增益不同。在上行传输过程中，用户终端侧的发送功率为
Figure PCTCN2020086078-appb-000060
第k个用户终端的第j个光纤收发端口发出的光线强度分布可以类似地建模为
Figure PCTCN2020086078-appb-000061
其中
Figure PCTCN2020086078-appb-000062
为用户到基站的相对折射角，则从第k个用户终端的第j个光纤收发端口到基站侧透镜的信道增益可以建模为
Figure PCTCN2020086078-appb-000063
其中，A为基站侧透镜的面积，β u(d k)表示由传输距离d k引起的信道衰减，
Figure PCTCN2020086078-appb-000064
为基站侧的入射角。当上行传输中第k个用户的信号耦合进基站第i个光纤收发端口的功率系数为
Figure PCTCN2020086078-appb-000065
进入光纤端口的耦合效率为
Figure PCTCN2020086078-appb-000066
则光传输信道增益为
Figure PCTCN2020086078-appb-000067
因此，从第k个用户终端到基站完整的电信号传输信道矩阵为
Figure PCTCN2020086078-appb-000068
其第(i,j)个元素为
Figure PCTCN2020086078-appb-000069
同样，上行传输信道矩阵
Figure PCTCN2020086078-appb-000070
可以分解为
Figure PCTCN2020086078-appb-000071
其中，
Figure PCTCN2020086078-appb-000072
Figure PCTCN2020086078-appb-000073
因此，上行传输到信道矩阵的秩为1。
由于光线传输的可逆性，上行传输的信道路径和下行传输的信道路径是互易的，即下行传输中第k个用户终端接收到基站第i个光纤收发端口的信号，在上行传输过程中，基站的第i个光纤收发端口接收第k个用户终端的发送信号；同样在用户终端侧，下行传输中用户的第j个光纤收发端口接收基站的信号，在上行传输中基站接收到第j个光纤收发端口发出的信号。
(4)多用户MIMO/大规模MIMO光无线通信方法
在FE-OWC系统中，基站利用多个(数个至数十个)或大量(数百至数千个)波束与多个或大量用户终端双向通信。当收发端均配置与光收发链路相对应的A/D和D/A模块的情况下，其数字基带处理可以实现上下行多用户MIMO(MU-MIMO,multi-user MIMO)传输，进而实现多用户MIMO光无线通信。进一步地，当基站侧配置大量光收发链路和光纤收发端口的情况下，基站可以同大量用户终端同时通信，实现大规模MIMO(massiveMIMO)光无线通信。从数字基带处理角度，多用户MIMO/大规模MIMO通信过程包含如下四个步骤：同步、信道探测、下行传输以及上行传输。1)基站广播下行同步信号，用户终端利用接收信号建立并保持与基站同步。2)用户终端发送上行探测信号，基站根据接收的探测信号估计各用户终端的信道信息。3)基站利用各用户终端的信道信息以及信道的低秩特性进行预编码传输，同时发送所有用户终端的信号，包括导频信号和数据信号，各用户终端根据接收的导频信号估计信道信息，并利用信道信息恢复数据信号。4)用户终端利用预编码传输同时向基站发送信号，包括导频信号和数据信号，基站接收到所有用户终端的信号叠加，根据导频信号估计各用户终端的信道信息，并利用信道的低秩特性进行接收处理，恢复各用户终端的数据信号。下面将具体介绍下行传输和上行传输过程。
a)下行传输
在下行传输过程中，基站同时发送K个用户终端的信号，第i个光纤收发端口向第k个用户终端发送的信号记为
Figure PCTCN2020086078-appb-000074
则基站向第k个用户终端发送的信号记为
Figure PCTCN2020086078-appb-000075
第k个用户终端的接收信号可以表示为
Figure PCTCN2020086078-appb-000076
其中，
Figure PCTCN2020086078-appb-000077
表示从基站到第k个用户终端的电信号传输信道增益，n k为高斯噪声，其均值为0，协方差矩阵为对角阵Λ k，第(j,j)个对角线元素为
Figure PCTCN2020086078-appb-000078
基站在信道探测过程中根据接收的探测信号估计上行信道信息，利用下行信道和上行信 道的互易性，获得下行信道矩阵。基站到单个用户终端的信道矩阵秩为1，信道矩阵
Figure PCTCN2020086078-appb-000079
可以分解为列向量与行向量乘积的形式，即
Figure PCTCN2020086078-appb-000080
基站向第k个用户终端发送的信号
Figure PCTCN2020086078-appb-000081
是由独立同分布的数据符号
Figure PCTCN2020086078-appb-000082
经过线性预编码
Figure PCTCN2020086078-appb-000083
生成。第k个用户终端根据接收的信号，采用线性接收机
Figure PCTCN2020086078-appb-000084
检测数据信号，即
Figure PCTCN2020086078-appb-000085
则下行传输中，第k个用户终端的接收信干噪比为
Figure PCTCN2020086078-appb-000086
当发送信号
Figure PCTCN2020086078-appb-000087
为零均值单位方差的均匀分布时，可达和速率表示为
Figure PCTCN2020086078-appb-000088
其中，κ＝6/(πe)。在单个光纤收发端口功率约束条件下，最大化系统和速率的问题可以表示为
Figure PCTCN2020086078-appb-000089
其中，e i＝[0,...,0,1,0,...,0] T为单位向量，只有第i个元素为1，其余元素为0，p d为基站侧单个光纤收发端口的功率约束。
上述最大化系统和速率的最优线性接收机为
Figure PCTCN2020086078-appb-000090
最优线性预编码为
Figure PCTCN2020086078-appb-000091
其中，
Figure PCTCN2020086078-appb-000092
系数a j和b k分别为
Figure PCTCN2020086078-appb-000093
对角阵D是辅助矩阵，使得预编码向量满足约束条件。最优线性预编码可以通过如下算法获得：
步骤1，初始化预编码向量
Figure PCTCN2020086078-appb-000094
以及对角阵D (0)，根据公式(31)计算和速率初始值(R d) (0)，设置迭代指示i＝1。
步骤2，根据公式(35)计算系数a j以及b k。
步骤3，根据公式(34)更新预编码向量
Figure PCTCN2020086078-appb-000095
计算和速率(R d) (i)。
步骤4，计算与功率约束之间的差值
Figure PCTCN2020086078-appb-000096
其中diag(·)表示由对角线元素构成的列向量。
步骤5，如果||Δ||≤ò，ò为误差门限，则终止迭代；否则，更新D (i)＝D (i-1)+tΔ，其中t为更新步长，设置i＝i+1，返回步骤2。
上述迭代算法终止时，可以得到最优预编码向量
Figure PCTCN2020086078-appb-000097
因此，在光纤使能光无线通信下行传输中，基站利用预编码向量
Figure PCTCN2020086078-appb-000098
向第k个用户终端发送信号，第k个用户终端利用线性接收向量
Figure PCTCN2020086078-appb-000099
对接收的信号进行检测，恢复基站发送的数据信号。
b)上行传输
在光纤使能光无线通信上行传输过程中，用户终端利用信道的互易性通过下行信道估计获得上行信道信息，并设计线性预编码发送独立的信号，基站接收到用户终端发送信号的叠加。记
Figure PCTCN2020086078-appb-000100
为第k个用户终端发送的上行信号，其可以表示为
Figure PCTCN2020086078-appb-000101
其中
Figure PCTCN2020086078-appb-000102
为用户侧预编码向量，
Figure PCTCN2020086078-appb-000103
为第k个用户终端的发送信号。则基站侧接收信号可以表达为
Figure PCTCN2020086078-appb-000104
其中，
Figure PCTCN2020086078-appb-000105
为第k个用户终端到基站的上行传输电信号增益，由于信道矩阵的秩为1，其可以分解为列向量与行向量乘积的形式，即
Figure PCTCN2020086078-appb-000106
z为基站侧电信号噪声，其均值为0，协方差矩阵为对角阵Λ，第(i,i)个元素为
Figure PCTCN2020086078-appb-000107
基站根据接收的信号，采用线性接收机
Figure PCTCN2020086078-appb-000108
检测第k个用户终端的发送信号，即
Figure PCTCN2020086078-appb-000109
则第k个用户终端的接收信干噪比为
Figure PCTCN2020086078-appb-000110
上行传输系统和速率为
Figure PCTCN2020086078-appb-000111
在总功率约束的条件下，设计线性预编码以及线性接收机最大化上行传输系统和速率，该问题可以表示为
Figure PCTCN2020086078-appb-000112
其中，p u为上行传输中每个用户终端的功率约束。满足上述功率约束条件下，最大化上行传输系统和速率的最优预编码向量为
Figure PCTCN2020086078-appb-000113
最优线性接收机为
Figure PCTCN2020086078-appb-000114
其中，c k为与第k个用户终端发送功率相关的辅助变量。当c k≠0或者
Figure PCTCN2020086078-appb-000115
时，c k满足
Figure PCTCN2020086078-appb-000116
最优线性预编码和线性接收机可以通过如下算法获得：
步骤1，初始化
Figure PCTCN2020086078-appb-000117
根据公式(41)和(42)计算预编码向量以及接收机向量，并利用公式(39)计算上行传输和速率R u，设置k＝1。
步骤2，在给定其他用户终端c i值的情况下，根据公式(43)计算c k值，并根据公式(41)和(42)更新预编码向量以及接收机向量，利用公式(39)计算上行传输和速率R (k)。
步骤3，如果R (k)＞R u，更新R u＝R (k)，设置c k为根据公式(43)计算的结果；否则，保持c k不变。
步骤4，设置k＝k+1，如果k≤K，返回步骤2；否则，终止迭代。
通过上述算法，可以获得各用户终端最优c k值，根据公式(41)和(42)计算预编码向量
Figure PCTCN2020086078-appb-000118
以及接收机向量
Figure PCTCN2020086078-appb-000119
在光纤使能光无线通信上行传输中，第k个用户终端利用预编码向量
Figure PCTCN2020086078-appb-000120
向基站发送信号，基站利用线性接收向量
Figure PCTCN2020086078-appb-000121
检测第k个用户终端的信号。
上述以最大化系统和速率为目标设计最优线性预编码和最优线性接收机为例做示例性说明，基站也可以根据信道矩阵分解的行向量采用最大比发射(MRT,maximal ratio transmission)或正则化迫零(RZF,regularized zero forcing)等预编码方法进行预编码设计，也可以根据各用户信道矩阵分解的列向量采用最大比合并(MRC,maximal ratio combining)接收机，此处不再一一赘述。
(5)波束分多址(BDMA)光无线通信方法
为降低系统构建成本，FE-OWC系统收发端可以不配置与光收发链路相对应的宽带A/D和宽带D/A模块，在此情况下，基站可以同多个用户终端或大量用户终端实现波束分多址(BDMA)通信。值得注意的是，即使收发端不配置宽带A/D和宽带D/A模块，为实现通信过程的有效控制，系统中可以配置适当带宽的A/D和D/A模块，同时即使收发端配置宽带A/D和宽带D/A模块，仍然可以使用BDMA通信方法，来降低数字基带处理的复杂度。光纤使能波束分多址光无线通信方法为：基站根据各用户终端的信道信息为不同的用户终端分配互不重叠的波束，每个波束发送和接收最多一个用户终端的信号，利用不同方向的光波束同时与用户终端双向通信；用户终端根据信道信息挑选基站对应方向的波束，利用单个波束 与单个基站进行通信。具体通信过程包含如下四个步骤：同步、信道探测、下行传输以及上行传输。1)基站广播下行同步信号，用户终端利用接收信号建立并保持与基站同步。2)用户终端发送上行探测信号，基站根据接收的探测信号为每个用户终端分配波束，同一时频资源上与基站通信的用户，所分配的波束互不重叠且每个用户仅分配一个波束。3)下行传输过程中，基站根据波束分配结果，在每个用户终端分配的波束上发送独立信号，各用户终端根据接收到的信号，挑选基站对应的波束进行接收检测。4)上行传输过程中，各用户终端在基站对应的波束上发送信号，基站根据波束分配的结果，在每个用户终端分配的波束接收检测各用户终端的发送信号。下行传输和上行传输过程具体为：
a)下行传输
在光纤使能波束分多址下行传输过程中，基站为不同的用户终端分配互不重叠的波束，利用不同的波束向不同用户终端发送信号。当基站分配给第k个用户终端的波束为i k时，第k个用户终端发送信号的预编码向量为
Figure PCTCN2020086078-appb-000122
其中e i为单位向量，其第i个元素为1，其余元素为0。由于基站分配给不同用户终端的波束互不重叠，因而不同用户终端的预编码向量相互正交，即
Figure PCTCN2020086078-appb-000123
在这种情况下，基站在第i k个波束上，以功率p向第k个用户终端发送信号。用户终端根据接收信号强度挑选基站对应的波束进行接收，即接收向量为
Figure PCTCN2020086078-appb-000124
其中j k表示在第k个用户终端侧基站对应的光纤收发端口。
由于在波束分多址下行传输中，基站挑选不同的发送波束发送不同用户终端的信号，用户终端挑选基站对应的波束接收信号，因而多用户下行传输链路可以分解为多个并行的单用户链路。基站可以采用基带调制(如OOK调制)方式生成用户模拟基带发送信号，用户终端根据接收信号强度，挑选基站对应的波束，在对应的波束上采用基带解调(如OOK解调)方式生成数字基带信号。因而，基站和用户终端可以不需要使用A/D和D/A器件，极大降低了超高速无线传输系统的实现复杂度。
b)上行传输
在光纤使能波束分多址上行传输过程中，根据光线传输路径的可逆性，第k个用户终端利用第j k个光纤端口发送信号，即上行传输的预编码向量为
Figure PCTCN2020086078-appb-000125
基站根据接收信号，在第i k个波束上接收检测第k个用户终端的信号，即基站检测第k个用户终端的接收向量为
Figure PCTCN2020086078-appb-000126
由于基站分配给不同用户终端的波束互不重叠，基站利用不同的光纤收发端口接收检测不同用户终端的信号，因此多用户上行传输链路也可以分解为多个并行的单用户链路。每个用户终端在基站对应的波束上，采用基带调制(如OOK调制)方式生成模拟基带信号，基站根据波束分配的结果，在每个用户终端分配的波束上采用基带解调(如OOK解调)方式进行解调，生成各用户终端的数字基带信号，可以避免使用A/D和D/A器件，降低系统实现复杂度。
图5展示了在一个面积为16m×16m高为8m的大型场景(如候机厅、体育馆)中，系统吞吐量随着光纤收发端口个数增长的示意图，该场景中有300个用户终端。在下行传输过程中，将多用户MIMO/大规模MIMO传输方法(图中Optimal)以及波束分多址(BDMA)传输的性能与最大比发射(MRT,maximal ratio transmission)和正则化迫零预编码(RZF,regularized zeroforcing)进行了对比，最大比发射和正则化迫零预编码的预编码向量分别为
Figure PCTCN2020086078-appb-000127
其中，β MRT和β RZF为功率系数满足功率约束条件，α是正则化因子。图5中(a)为下行传输性能示意图，随着光纤收发端口个数的增长，波束分多址传输的性能优于RZF以及MRT 逼近最优性能，并且系统吞吐量超过1Tbps。在上行传输过程中，将最优设计方案以及波束分多址(BDMA)传输的性能与最大比合并(MRC,maximal ratio combining)进行对比，最大比合并的接收向量为
Figure PCTCN2020086078-appb-000128
图5中(b)展示了上行传输性能示意图，可以看出随着光纤收发端口个数的增加，波束分多址传输的性能逼近最优传输，优于MRC的性能。另外，在光纤收发端口个数较大的情况下，波束分多址下行传输和上行传输的系统实现复杂度非常低。

Claims (20)

  1. 一种光天线，其特征在于：包括光纤收发端口阵列以及透镜或反射镜；在发送信号的过程中，单个光纤收发端口发出的光经过透镜折射或反射镜反射后在某一方向生成具有一定角度范围的光波束，不同光纤收发端口发出的光折射或反射到不同方向；在接收信号的过程中，来自不同方向的接收光线经过透镜折射或反射镜反射后耦合进不同的光纤收发端口进行接收，不同光纤收发端口接收不同方向的光信号。
  2. 根据权利要求1所述的光天线，其特征在于：所述光天线中的光纤收发端口包括光纤端口和微透镜；在发送信号的过程中，单个光纤端口发出的光信号经过微透镜折射后，生成具有一定角度范围的光波束；在接收信号的过程中，微透镜将一定角度范围内的光信号耦合进光纤端口。
  3. 根据权利要求1所述的光天线，其特征在于：利用光纤收发端口阵列以及透镜或反射镜生成不同方向的光波束，不同光波束覆盖不同区域，光纤收发端口阵列生成的所有光波束覆盖整个通信区域，实现通信区域全波束覆盖。
  4. 光纤使能光无线通信FE-OWC装置，其特征在于：包括根据权利要求1所述的光天线以及光收发链路；光天线用于发送和接收不同方向的光信号；光天线与光收发链路通过光纤直接连接，或通过光交换单元连接；光收发链路用于实现光信号与电信号的相互转换；单个FE-OWC装置与单个或一组FE-OWC装置进行无线通信。
  5. 根据权利要求4所述的FE-OWC装置，其特征在于：所述光收发链路用于实现光信号与电信号的相互转换；在发送信号的过程中，电信号加上偏置电流后驱动激光器，生成与电信号对应的光信号，或采用外调制器方式，将激光源产生的光信号与电信号输入外调制器，生成对应的光信号，光信号经过光放大器放大后通过光纤传输到光天线；在接收信号的过程中，光天线接收的信号通过光纤传输到光收发链路，经过光放大器放大后，利用光探测器检测光信号，转换为对应的电信号。
  6. 根据权利要求4所述的FE-OWC装置，其特征在于：当光天线与光收发链路直接连接时，每个光收发链路对应一个光纤收发端口，光收发链路个数与光纤收发端口个数相同；当光天线与光收发链路通过光交换单元连接时，光收发链路个数小于等于光纤收发端口个数，光交换单元用于切换光收发链路与光纤收发端口的对应关系，将光收发链路中的信号与光天线生成的波束相对应。
  7. 根据权利要求4所述的FE-OWC装置，其特征在于：基站侧FE-OWC装置还包括基带信号处理单元，所述基带信号处理单元包含A/D和D/A模块以及数字基带处理与控制模块；下行传输过程中，基站侧的数字基带处理与控制模块用于实现用户调度以及多用户预编码传输，生成每个用户终端的发送信号，D/A模块用于将数字基带处理与控制模块生成的发送信号转化为模拟信号输入光收发链路；上行传输过程中，A/D模块用于将基站侧光收发链路输出的电信号转化为数字信号，数字基带处理与控制模块用于对多用户接收信号进行检测，恢复每个用户终端的发送信号。
  8. 根据权利要求4所述的FE-OWC装置，其特征在于：用户终端侧FE-OWC装置还包括基带信号处理单元，所述基带信号处理单元包含A/D和D/A模块以及数字基带处理与控制模块；下行传输过程中，A/D模块用于将用户终端侧光收发链路输出的电信号转化为数字信号，数字基带处理与控制模块用于对接收信号进行检测，恢复基站的发送信号；上行传输过程中，用户终端侧的数字基带处理与控制模块用于实现预编码传输，D/A模块用于将生成的发送信号转化为模拟信号输入光收发链路。
  9. 根据权利要求4所述的FE-OWC装置，其特征在于：基站侧FE-OWC装置还包括基带信号处理单元，所述基带信号处理单元包含基带调制与基带解调模块以及数字基带处理与控制模块；下行传输过程中，基站侧的数字基带处理与控制模块用于为不同用户终端分配互不重叠的波束集合，生成向各用户终端发送的数字基带信号，基带调制模块用于生成向各用 户终端发送的模拟基带信号，传输到对应的光收发链路，利用相应的光纤收发端口进行发送；上行传输过程中，基带解调模块用于将基站侧光收发链路输出的模拟基带接收信号进行解调，生成数字基带信号，数字基带处理与控制模块用于根据波束分配的结果以及每个用户终端对应波束上的数字基带信号恢复各用户终端的发送信号。
  10. 根据权利要求4所述的FE-OWC装置，其特征在于：用户终端侧FE-OWC装置还包括基带信号处理单元，所述基带信号处理单元包含基带调制与基带解调模块以及数字基带处理与控制模块；下行传输过程中，用户终端侧基带解调模块用于将光收发链路输出的模拟信号进行解调，生成数字基带信号，数字基带处理与控制模块用于挑选基站对应的光纤收发端口，根据其接收数字基带信号恢复基站侧的发送信号；上行传输过程中，用户终端侧的数字基带处理与控制模块用于生成上行数字基带发送信号，基带调制模块用于生成模拟基带发送信号，传输到光收发链路，利用与基站对应的光纤收发端口进行发送。
  11. 光纤使能光无线通信FE-OWC系统，其特征在于：所述FE-OWC系统的基站和用户终端配置均配置根据权利要求4-6任一项所述的FE-OWC装置；或者基站配置权利要求7所述的FE-OWC装置，用户终端配置根据权利要求8所述的FE-OWC装置；或者基站配置权利要求9所述的FE-OWC装置，用户终端配置根据权利要求10所述的FE-OWC装置。
  12. 光纤使能光无线通信FE-OWC系统，其特征在于：所述FE-OWC系统的基站配置根据权利要求4-7任一项所述的FE-OWC装置，用户终端配置根据权利要求4-6、8任一项所述的FE-OWC装置，基站与用户终端之间实现多用户MIMO或大规模MIMO光无线通信；或者基站配置根据权利要求4-6、9任一项所述的FE-OWC装置，用户终端配置根据权利要求4-6、10任一项所述的FE-OWC装置，基站与用户终端之间实现波束分多址BDMA光无线通信。
  13. 光纤使能光无线通信FE-OWC方法，其特征在于：所述通信方法基于根据权利要求12所述的FE-OWC系统，计算单链路传输的链路预算并建立收发端电信号传输信道模型；链路预算包括发送端电光转换、光无线信道增益、接收端光电转换以及接收端电噪声；发送端电光转换部分根据电光转换器件的光电特性，建立发送端输出的光功率与输入电信号之间的对应关系；光无线信道增益为发送端光纤收发端口到接收端光纤收发端口之间的无线信道增益；接收端光电转换部分考虑接收光信号经过光放大器放大以及光探测器检测两个过程，建立接收端输入光信号与输出电信号之间的转换关系；接收端电噪声包括光放大器和光探测器引入的电噪声，建立单个链路接收电信号中信号功率与噪声功率之间的关系；在单个链路预算的基础上，建立完整的从发送端到接收端电信号传输的信道模型；基于该信道模型，基站和用户终端之间实施多用户MIMO或大规模MIMO或波束分多址光无线通信方法。
  14. 根据权利要求13所述的FE-OWC方法，其特征在于：所述光无线信道增益描述从发送端到接收端光无线传输的信道增益，包括发送端的波束建模、自由空间传输信道增益、接收端光纤收发端口接收功率比率以及光纤端口的耦合效率四个部分；发送端的波束建模描述光纤收发端口发出的光线经过透镜折射或反射镜反射后生成单个波束的光强分布，随着发送端光纤收发端口个数的增加，单个光纤收发端口到不同用户终端的光强渐进正交；自由空间传输信道增益考虑光波束经过自由空间从发送端到接收端的传输过程，其与传输距离的平方成反比；接收端光纤收发端口接收功率比率描述单个光纤收发端口接收的光功率占用户总接收功率的比率，其与接收平面上接收光线投影和光纤收发端口的公共面积成正比；光纤端口的耦合效率为光纤端口处的接收光线能够耦合进入光纤的比率，其与入射光线在光纤端口处的角度范围和光纤端口的接收角的公共区域成正比。
  15. 光纤使能光无线通信FE-OWC方法，其特征在于：所述通信方法为基于根据权利要求12所述的FE-OWC系统实现的多用户MIMO或大规模MIMO光无线通信方法，具体通信过程包含如下步骤：
    同步：基站广播下行同步信号，用户终端利用接收信号建立并保持与基站同步；
    信道探测：用户终端发送上行探测信号，基站根据接收的探测信号估计各用户终端的信道信息；
    下行传输：基站利用各用户终端的信道信息以及信道的低秩特性进行预编码传输，同时发送所有用户终端的信号，包括导频信号和数据信号，各用户终端根据接收的导频信号估计信道信息，并利用信道信息恢复数据信号；
    上行传输：用户终端利用预编码传输同时向基站发送信号，包括导频信号和数据信号，基站接收到所有用户终端的信号叠加，根据导频信号估计各用户终端的信道信息，并利用信道的低秩特性进行接收处理，恢复各用户终端的数据信号。
  16. 根据权利要求13或15所述的FE-OWC方法，其特征在于：下行传输过程中，基站通过上行探测过程估计每个用户终端到基站的上行信道，利用信道的互易性获得下行信道，将信道矩阵分解为列向量与行向量的乘积，并计算各用户终端的接收信干噪比以及可达和速率，在单个光纤收发端口功率约束条件下，以最大化系统和速率为目标设计最优线性预编码；或者基站根据信道矩阵分解的行向量采用最大比发射MRT或正则化迫零RZF预编码方法，利用预编码向量同时发送所有用户终端的信号，包括导频信号和数据信号；上行传输过程中，基站接收到所有用户终端的信号叠加，根据接收的导频信号估计各用户终端的上行信道矩阵，并将其分解为列向量与行向量的乘积，以最大化系统和速率为目标设计最优线性接收机；或者基站根据各用户信道矩阵分解的列向量采用最大比合并MRC接收机，利用线性接收机对接收的数据信号进行检测，恢复各用户终端的发送信号。
  17. 根据权利要求13或15所述的FE-OWC方法，其特征在于：下行传输过程中，各用户终端根据接收的导频信号估计下行传输信道矩阵，并将其分解为列向量与行向量的乘积，以最大化系统和速率为目标设计最优线性接收机，用户终端利用最优线性接收机对接收的数据信号进行检测；上行传输过程中，各用户终端利用信道的互易性，根据下行信道估计获得上行信道信息，并将信道矩阵分解为列向量与行向量的乘积，在总功率约束的条件下，以最大化系统和速率为目标设计最优预编码向量，用户终端利用预编码向量同时发送上行信号，包括导频信号和数据信号。
  18. 光纤使能光无线通信FE-OWC方法，其特征在于：所述通信方法为基于根据权利要求12所述的FE-OWC系统实现的波束分多址光无线通信方法，所述波束分多址光无线通信方法为基站根据各用户终端的信道信息为不同的用户终端分配互不重叠的波束，每个波束发送和接收最多一个用户终端的信号，利用不同方向的光波束同时与用户终端双向通信；用户终端根据信道信息挑选基站对应方向的波束，利用单个波束与单个基站进行通信；具体通信过程包含如下步骤：
    同步：基站广播下行同步信号，用户终端利用接收信号建立并保持与基站同步；
    信道探测：用户终端发送上行探测信号，基站根据接收的探测信号为每个用户终端分配波束，同一时频资源上与基站通信的用户，所分配的波束互不重叠且每个用户仅分配一个波束；
    下行传输：基站根据波束分配结果，在每个用户终端分配的波束上发送独立信号，各用户终端根据接收到的信号，挑选与基站对应的波束进行接收检测；
    上行传输：各用户终端在基站对应的波束上发送信号，基站根据波束分配的结果，在每个用户终端分配的波束上接收检测各用户终端的发送信号。
  19. 根据权利要求13或18所述的FE-OWC方法，其特征在于：下行传输过程中，基站根据波束分配的结果，在每个用户终端分配的波束上向其发送信号，不同波束发送不同用户终端的信号，多用户下行传输链路分解为多个并行的单用户链路，基站采用基带调制方式生成用户模拟基带发送信号；上行传输过程中，基站根据波束分配的结果，在每个用户终端分配的波束上接收检测该用户终端的发送信号，基站利用不同的波束接收检测不同用户终端的信号，多用户上行传输链路分解为多个并行的单用户链路，基站采用基带解调方式生成各用 户终端的数字基带信号。
  20. 根据权利要求13或18所述的FE-OWC方法，其特征在于：下行传输过程中，各用户终端根据接收信号的强度，挑选基站对应的波束，在对应的波束上采用基带解调方式生成数字基带信号；上行传输过程中，每个用户终端在基站对应的波束上，采用基带调制方式生成模拟基带信号。
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