A time delay compensation method for a terahertz optical fiber communication system

Abstract

The application discloses a time delay compensation method of a terahertz optical fiber communication system and belongs to the technical field of access communication. The application introduces an optical true time delay compensation network between digital precoding and analog precoding of a hybrid precoding structure. Uniform optical carriers generated by an optical comb wave generator are used for wavelength mapping, and then the properties of a dispersion medium are used to generate different time delays for different wavelengths to form a beam compensation mode. By introducing an optimal beam compensation mode in a radio frequency chain, the beams are aligned with the target direction at all frequencies, and the beam splitting effect is solved. By using passive wavelength multiplexers and wavelength demultiplexers, the application greatly reduces power consumption and deployment complexity. The beam compensation mode provided by the optical true time delay compensation network changes the signal time delay, and the time delay compensation can realize different phases for different frequencies to compensate the beam splitting effect. Different precision compensation can be provided according to actual deployment needs to match different application scenarios.

Description

Technical Field

[0001] This invention belongs to the field of access communication technology, and specifically relates to a delay compensation method for a terahertz fiber optic communication system. Background Technology

[0002] In 2020, the ITU officially launched research on next-generation wireless networks, 6G. 6G will achieve comprehensive wireless connectivity and coverage from indoor to outdoor scenarios. The "6G Overall Vision and Potential Key Technologies White Paper" points out that 6G networks will help achieve deep integration between the real physical world and the virtual digital world, building a new world of intelligent interconnection and digital twins. At the same time, it also places higher demands on access networks. Optical and wireless converged access networks combine the high capacity of optical access networks with the flexibility and mobility of wireless access networks, providing broadband users with higher data rates, lower access costs, wider coverage, better service quality, and more convenient service access. This will become a necessary information network architecture to meet the development needs of 6G networks. To achieve the 6G vision, peak data rates are expected to exceed 1Tbps. However, the bandwidth provided by the 5G millimeter-wave band cannot support such high data rates. Compared to millimeter waves, the terahertz band has abundant frequency resources and can support ultra-high communication rates. Therefore, terahertz communication is considered an important air interface technology alternative to achieve 6G terabits per second (Tbps) communication rates.

[0003] However, due to the extremely high frequency of terahertz signals, they suffer severe path loss, resulting in limited coverage for terahertz communication. Massive MIMO technology can utilize large antenna arrays to provide directional array gain. Specifically, by adjusting the phase of signals on numerous transmitting antennas, the signals are superimposed in the direction of the receiver, increasing the received signal strength. Because of the short wavelength of terahertz signals, the antenna spacing can be very close when deploying massive MIMO antennas. Therefore, terahertz systems support the use of ultra-large-scale antenna arrays to generate high-gain and highly directional beams to compensate for terahertz signal attenuation, making them suitable for terahertz communication systems. Traditional all-digital MIMO systems require a separate radio frequency chain for each antenna, leading to high complexity and high power consumption, making them difficult to deploy in practical applications. To reduce power consumption, a hybrid digital and analog beamforming method using a small number of radio frequency chains has been proposed. Its core idea is to divide traditional all-digital precoding into two parts: small-scale digital signal processing implemented with a small number of radio frequency chains and large-scale analog signal processing implemented with numerous phase shifters. Due to the sparsity of terahertz channels, a small number of radio frequency chains is sufficient to fully realize multiplexing gain in terahertz massive MIMO systems.

[0004] In hybrid precoding architectures, the phase shifters in analog beamformers are frequency-independent, meaning they adjust the phase of subcarriers at all frequencies to the same value. For narrowband systems, this frequency-independent phase shifter can meet beamforming requirements. However, in terahertz massive MIMO, due to the large bandwidth and numerous antennas, the beam generated by the frequency-independent phase shifter can only be aligned with the user on subcarriers near the center frequency, while subcarriers at other frequencies will deviate from the user's direction. Therefore, only the beam near the center frequency can achieve high array gain, while beams at other frequencies suffer severe array gain loss, even negating the rate gain brought by massive MIMO. The "Terahertz Communication Technology Research Report" points out that the beam splitting effect in terahertz ultra-large-scale antenna technology is a bottleneck problem for terahertz massive MIMO. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of the prior art and provide a delay compensation method for terahertz fiber optic communication systems. By introducing an optical true delay compensation network between digital precoding and analog precoding, corresponding phase compensation is performed on subcarriers at different frequencies, and it has the advantages of low complexity and low power consumption.

[0006] This invention provides a delay compensation method for a terahertz fiber optic communication system, comprising an analog precoding network F RF Optical Delay Compensation Network and digital precoding network F BB Delay compensation is achieved in terahertz fiber optic communication systems.

[0007] Obtain the number of radio frequency links in a terahertz fiber optic communication system The number of antenna subarrays W, the number of antennas Q and the number of transmit antennas N in each subarray. t ;

[0008] The analog precoding network F RF include Groups of phase shifters, each group of phase shifters includes N t A phase shifter independent of the frequency corresponding to the transmitting antenna, simulating the precoding network F. RF Forming by transmitting the phase shift value of each set of phase shifters to the transmitting antenna The beams with different target directions are used, and the phase shift value that needs to be compensated for due to beam splitting effect is calculated based on the analog precoding matrix composed of the output of the frequency-independent phase shifter. That is, by setting the maximum array gain point as the user's target direction, the phase shift value that the optical true delay compensation network needs to compensate is solved in reverse.

[0009] The optical true delay compensation network Includes an optical comb wave generator, an optical cross-connect network, and A set of optical true delay modules, wherein each set of optical true delay modules contains W optical true delay units, and each optical true delay unit contains an optical multiplexer and a photodetector;

[0010] An optical comb wave generator generates multiple equally spaced optical carriers (i.e., uniform optical carriers) and inputs them into an optical cross-connect network. The uniform optical carriers are then separated by a passive wavelength demultiplexer in the optical cross-connect network. The separated uniform optical carriers are combined into K groups containing W wavelengths according to a specific wavelength interval, and each wavelength group corresponds to a beam compensation mode, where K is a positive integer greater than 1.

[0011] The K groups of wavelengths are then combined using a passive wavelength multiplexer to determine the optimal beam compensation mode for each group based on the required phase shift value. The optimal beam compensation mode is then selected from the W beam compensation modes and assigned to each RF chain using an optical switch. The RF signal on the RF chain is modulated onto the optical carrier of the selected beam compensation mode using opto-modulation.

[0012] The modulated radio frequency signal is delayed by different time delays through a dispersive medium. Then, the wavelengths corresponding to each time delay are assigned to different optical multiplexers and photodetectors by a passive wavelength demultiplexer. After the delayed radio frequency signal is processed by electrical signal, it is transmitted to Q frequency-independent phase shifters connected to each optical true delay unit. After phase shifting by the phase shifter, a frequency-dependent beam is realized. At different subcarriers, the required different phase shift values ​​are compensated by frequency-dependent phase shifts to ensure that all subcarriers are aligned with the target direction.

[0013] In the beam-compensated mode, the time delay difference caused by the wavelength passing through the dispersive medium is Δt = DLΔλ, where D is the dispersion coefficient of the dispersive medium, L is the length of the dispersive medium, and Δλ is the wavelength difference; and the phase shift generated by the beam-compensated mode under different subcarriers (subcarriers generated in orthogonal frequency division multiplexing) is:

[0014]

[0015] in, f represents the phase shift value generated by the beam compensation mode under the m-th subcarrier. m The frequency of the subcarrier;

[0016] The digital precoding network F BB Used for baseband processing of radio frequency signals;

[0017] Singular value decomposition (SVD) is performed on the analog precoding matrix of the analog precoding network, the optical true delay compensation matrix of the optical true delay compensation network, and the channel matrix to obtain the digital precoding matrix. The data stream input into the terahertz fiber optic communication system passes through the digital precoding network F.BB After processing, the signals are distributed to each radio frequency chain, and the signals in the radio frequency chain are then passed through the analog precoding network F. RF Harmony Light True Delay Compensation Network After processing, the signal is transmitted to the transmitting antenna, and after being transmitted by the transmitting antenna, it forms a frequency-dependent beam.

[0018] The objective of this invention is achieved as follows:

[0019] This invention discloses a delay compensation method for terahertz fiber optic communication systems. It introduces a true optical delay compensation network between digital and analog precoding in a hybrid precoding structure. Wavelength mapping is performed using a uniform optical carrier generated by an OCG (Optical Coding System), and then different delays are generated for different wavelengths using the properties of the dispersive medium, forming a beam compensation mode. By introducing an optimal beam compensation mode into the radio frequency chain, beam alignment with the target direction is achieved at all frequencies, resolving beam splitting effects. The use of passive wavelength multiplexers and wavelength demultiplexers significantly reduces power consumption and deployment complexity.

[0020] The technical solution provided by this invention brings at least the following beneficial effects:

[0021] (1) In this invention, the optical true delay compensation network is located between digital precoding and analog precoding and can be connected to the existing hybrid precoding structure;

[0022] (2) The signal delay is changed by the beam compensation mode provided by the optical true time delay compensation network. The time delay compensation can be used to achieve different phases for different frequencies to compensate for the beam splitting effect.

[0023] (3) The beam compensation mode in the optical true time delay compensation network is formed using only one active OCG and a passive wavelength multiplexer and wavelength demultiplexer, which greatly reduces power consumption and complexity.

[0024] (4) The beam compensation mode generated by the optical true delay compensation network can be changed according to parameters such as antenna and bandwidth, and can provide compensation with different precision according to actual deployment needs, thereby matching different application scenarios. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the accompanying drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0026] Figure 1 This is a schematic diagram of the terahertz optical fiber communication system structure used in an embodiment of the present invention;

[0027] Figure 2 This is the terahertz optical true delay compensation network deployment scheme adopted in this embodiment of the invention;

[0028] Figure 3 yes Figure 2 A schematic diagram of the optical true time delay compensation network in the structure shown;

[0029] Figure 4 yes Figure 3 The beam compensation mode is generated in the optical true time delay compensation network shown.

[0030] Figure 5 This invention presents a comparison between optical true delay compensation network schemes under different bandwidths and traditional schemes in this embodiment.

[0031] Figure 6 In this embodiment of the invention, the optical true delay compensation network under different optical true delay units and beam compensation modes can achieve rate performance. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of the present invention clearer, the embodiments of the present invention will be described in further detail below with reference to the accompanying drawings.

[0033] For ease of description, the relevant technical terms appearing in the specific implementation methods will be explained first:

[0034] OCG (Optical Comb Generator): Optical Comb Wave Generator;

[0035] OXC (Optical Cross-Connect);

[0036] EOM (Electro-Optic Modulation);

[0037] DM (Dispersive Medium);

[0038] OM (Optical Multiplexer) is an optical multiplexer.

[0039] PD (Photodetector) photodetector;

[0040] ESP (Electrical Signal Processing) refers to the processing of electrical signals.

[0041] USOC (Uniformly-Spaced Optical Carrier) is a uniform optical carrier.

[0042] OTTDCN (Optical True Time Delay Compensation Network) is an optical true time delay compensation network.

[0043] OS (Optical Switch) is a type of optical switch.

[0044] To address beam splitting, low-frequency communication systems typically use time delays instead of phase shifters to implement analog beamforming matrices. Since this frequency offset is proportional to the carrier frequency, the time delay can be used to simultaneously apply the required beamforming weights to different frequencies, aligning the beams on different subcarriers with the user direction and thus eliminating beam splitting. However, in the terahertz band, adjustable time delays have extremely high hardware complexity. Furthermore, most existing hybrid analog-digital massive MIMO architectures assume ideal, infinitely precise adjustable phase shifters and time delays. To solve this problem, this invention proposes using a finite-precision optical true delay compensation network to achieve near-optimal performance while reducing complexity and power consumption.

[0045] like Figure 1 As shown, this invention adds a THz-based ultra-wideband wireless link between the baseband processing unit (BBU) and the radio remote head (RRH). One BBU can simultaneously connect to multiple remote RRHs via wireless transmission. High-speed data transmission using terahertz (THz) is achieved in areas where fiber optic cables cannot be deployed, such as mountains, deserts, and rivers, avoiding the need for laying fiber optic links, significantly reducing the cost of 6G network fiber optic deployment, and increasing network deployment flexibility.

[0046] In this embodiment, as Figure 2 As shown, an optical true delay compensation network is introduced into the traditional hybrid precoding structure, serving as a new coding layer between digital and analog precoding. Specifically, each RF chain is connected to W optical true delay units in the optical true delay compensation network, and each optical true delay unit is then sub-connected to Q frequency-independent phase shifters. Therefore, the signal on each RF chain can undergo frequency-dependent phase shifting through delay in the optical true delay compensation network, and then be connected to each antenna through frequency-independent phase shifters. Thus, the optical true delay compensation network transforms traditional beamforming controlled by frequency-independent phase shifters into frequency-dependent beamforming compensated by the optical true delay compensation network.

[0047] Among them, the physical implementation of the optical true delay compensation network is as follows: Figure 3 As shown; and Figure 3 The generation of the beam compensation mode of the optical true delay compensation network shown is as follows: Figure 4 As shown; the performance of adding an optical true delay compensation network at different bandwidths is compared with that of the traditional solution, for example. Figure 5As shown; for optical true time delay compensation networks, the comparison with the unconstrained optimal rate under different parameters is as follows: Figure 6 As shown.

[0048] As one possible implementation, the time delay compensation method for a terahertz fiber optic communication system provided in this embodiment of the invention includes:

[0049] Including analog precoding network F RF Optical Delay Compensation Network and digital precoding network F BB In terahertz fiber optic communication systems, analog precoding networks F RF include Groups of phase shifters, each group of phase shifters includes N t A phase shifter that is independent of the frequency corresponding to the antenna;

[0050] Analog precoding network F RF Forming by transmitting the phase shift value of each set of phase shifters to the antenna Beams from different target directions, simulating a precoding network F RF Because a frequency-independent phase shifter is used, a frequency-independent beam is still formed in the target direction. The phase shift value that needs to be compensated for for the beam splitting effect is calculated based on the analog precoding matrix (i.e., the mathematical representation of the phase shifter) formed by the output of the frequency-independent phase shifter.

[0051] Optical Delay Compensation Network Includes an OCG and an optical cross-connect network, and The optical true delay module group contains W optical true delay units, and each optical true delay unit contains an optical multiplexer and a photodetector.

[0052] OCG generates equally spaced optical carriers λ jLet j = 1, 2, ..., m, where m represents the number of optical carriers. The uniform optical carriers generated by the OCG are input into the optical cross-connect network and separated by a passive wavelength demultiplexer within the network. The separated uniform optical carriers are combined into K wavelength groups containing W wavelengths according to specific wavelength intervals. Each wavelength group corresponds to a beam compensation mode. The K wavelength groups are then combined using a passive wavelength multiplexer. The optimal beam compensation mode is calculated based on the required phase shift value: the optimal beam compensation mode (the compensation mode of the beam closest to the required phase shift value) is selected from the W beam compensation modes using an optical switch and assigned to each RF chain. The RF signal on the RF chain is modulated onto the selected beam compensation mode's optical carrier using opto-modulation. The modulated RF signal, after passing through a dispersive medium, achieves different time delays, and then the wavelengths are assigned to different optical multiplexers and photodetectors using a passive wavelength demultiplexer. The delayed RF signal, after electrical signal processing, is transmitted in a sub-connection manner to Q frequency-independent phase shifters connected to each delay unit. (Note: a delay unit connects only some, not all, phase shifters; i.e., one delay module connects to Q phase shifters, and there are a total of W delay modules, therefore W × Q = N.) t After phase shifting by a phase shifter, frequency-dependent beamforming is achieved. At different subcarriers, frequency-dependent phase shift compensation is used to ensure that all subcarriers are aligned with the target direction by compensating for the different phase shift values ​​required.

[0053] In the beam compensation mode generated by the OCG and the optical cross-connect network, the wavelength is: p k =[λ 1k ,λ 2k ,...,λ Wk ];

[0054] Where, λ ik Let represent the i-th wavelength of the k-th beam compensation mode, and let kΔλ be the difference between adjacent wavelengths in the k-th beam compensation mode.

[0055] The time delay difference caused by the wavelength passing through the dispersive medium in the beam compensation mode is Δt = DLΔλ, where D is the dispersion coefficient of the dispersive medium, L is the length of the dispersive medium, and Δλ is the wavelength difference. Different time delay differences can be achieved by selecting different beam compensation modes, and a set of time delay differences can compensate for beam splitting effects under different subcarriers. The phase shift generated by the beam compensation mode under different subcarriers is:

[0056]

[0057] in f represents the phase shift value generated by the beam compensation mode under the m-th subcarrier. mLet be the frequency of the subcarrier. According to formula (1), a set of time delay differences can be used for phase compensation for different subcarriers;

[0058] Digital precoding network F BB This is used for baseband processing of radio frequency (RF) signals. The digital precoding matrix is ​​obtained by performing singular value decomposition on the analog precoding matrix of the analog precoding network, the optical true delay compensation matrix of the optical true delay compensation network, and the channel matrix. The data stream input to the terahertz fiber optic communication system is processed by the digital precoding network and then distributed to each RF chain. The signals in the RF chains are then processed by the analog precoding network and the optical true delay compensation network before being transmitted to the antenna, where they form a frequency-dependent beam. In this application, the analog precoding matrix is ​​the mathematical expression of the analog precoding network, the optical true delay compensation matrix is ​​the mathematical expression of the optical true delay compensation network, and the digital precoding matrix is ​​the mathematical expression of the digital precoding network.

[0059] The following explains the process by which an optical true time delay compensation network selects the optimal beam compensation mode to achieve frequency-dependent beams for different frequency subcarrier signals:

[0060] The antenna array response is expressed as follows:

[0061]

[0062]

[0063] Where, θ l,m N represents the spatial direction on the l-th path at the m-th subcarrier. t Indicates the number of transmitting antennas, c is the speed of light, e is the natural base, and sinγ l Let c be the physical direction of the l-th path, and d be the antenna spacing, where d = c / (2f) c ). ψ l Indicates the physical direction of the antenna.

[0064] The l-th column of the analog precoding matrix can be represented as:

[0065]

[0066] in, f represents the spatial direction along the l-th path at the center frequency. c Indicates the center frequency. The l-th column of the analog precoding matrix is ​​used to generate a ψ-oriented... l Frequency-independent beam.

[0067] After introducing an optical true time delay compensation network, towards ψ l The beam vector in the direction can be expressed as:

[0068]

[0069]

[0070] in, t represents the beamforming vector generated by the Q frequency-independent phase shifters connected to the w-th optical true delay unit in the l-th RF chain. l,w This is the time delay generated by the w-th optical true delay unit in the l-th beam. The phase shift compensated by the optical true delay compensation network... At frequency f m The array gain facing the physical direction ψ (any physical direction) is:

[0071]

[0072] Among them, the sinc function Θ W (x)=(sinWπx) / (sinπx / 2), sinc function Θ Q (x)=(sinQπx) / (sinπx / 2), take ψ l,m =(ψ l +β l,m / Q) / ε m The maximum array gain can be obtained, where ε m =f m / f c It is a relative frequency. This is achieved by setting ψ. l,m =(ψ l +β / Q) / ε m =ψ l Let the direction of maximum gain of the l-th beam at the m-th subcarrier be from ψ l,m Change to target direction ψ l . β l,m β represents the phase shift value at the m-th subcarrier of l beams, i.e., the phase that the optical true delay compensation network needs to compensate for. l,m =(ε m -1)Qψ l .

[0073] The beam compensation matrix generated by the optical true time delay cell is:

[0074]

[0075] Where, ζ m,i This is the i-th (i = 1, ..., K) beam compensation mode at the m-th subcarrier. The beam compensation mode closest to the phase requiring compensation is selected through the optical true delay compensation network. This allows for the simultaneous implementation of the required beamforming weights for different frequencies.

[0076] Based on the above theory and analysis, the beam compensation mode generated by the optical true time delay compensation network can effectively solve the beam splitting problem in terahertz large-scale MIMO systems. Figure 5 Comparing the optical true delay compensation network scheme with traditional schemes under different bandwidths, it can be seen that the optical true delay compensation network scheme can even achieve twice the achievable rate of traditional schemes under large bandwidth conditions. Figure 6 This paper describes the achievable rate performance of optical true delay compensation networks with different delay units as a function of the number of beam compensation modes. Figure 6 It can be observed that the achievable speed increases with the increase of optical true delay units and beam compensation modes. However, beyond a certain threshold, deploying more optical true delay units and beam compensation modes results in slow growth in achievable speed, but requires more cost and power consumption. Selecting an appropriate number of optical true delay units and beam compensation modes (e.g., 16 optical true delay units and 16 beam compensation modes) can achieve a near-unconstrained optimal achievable speed. Therefore, optical true delay compensation networks can achieve near-unconstrained optimal achievable speeds with minimal complexity and cost.

[0077] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

[0078] The above descriptions are merely some embodiments of the present invention. Those skilled in the art can make various modifications and improvements without departing from the inventive concept of the present invention, and these all fall within the scope of protection of the present invention.

Claims

1.A time delay compensation method of a terahertz fiber optical communication system, introducing an optical true time delay compensation network between a digital precoding network and an analog precoding network, and realizing time delay compensation in the terahertz fiber optical communication system comprising the analog precoding network, the optical true time delay compensation network and the digital precoding network; Obtaining radio frequency link number of terahertz fiber communication system , number of antenna subarrays W, number of antennas Q included in each subarray and number of transmitting antennas ; The digital precoding network is used for baseband processing of radio frequency signals; The analog precoding matrix of the analog precoding network and the optical true time delay compensation matrix of the optical true time delay compensation network and the channel matrix are singular value decomposed to obtain a digital precoding matrix, and the data stream of the terahertz fiber optical communication system is distributed to each radio frequency chain after being processed by the digital precoding network, and the signal in the radio frequency chain is transmitted to the transmitting antenna after being processed by the analog precoding network and the optical true time delay compensation network, and the frequency-dependent beam is formed after being transmitted by the transmitting antenna; The optical true-time delay compensation network includes an optical comb generator, an optical cross-connect network, and A group optical true-time delay module, wherein, Each group of optical true time delay module contains W optical true time delay units, and each optical true time delay unit contains an optical multiplexer and an optical detector; wherein, one time delay unit is connected with Q phase shifters, and ; The optical comb wave generator generates a plurality of equidistant optical carriers, and inputs the plurality of optical carriers into the optical cross-connect network, and separates the plurality of optical carriers by the passive wavelength demultiplexer in the optical cross-connect network; The uniform optical carriers after separation are combined into K groups of wavelength groups containing W wavelengths according to a specific wavelength interval, and each wavelength group corresponds to a beam compensation mode, wherein K is a positive integer greater than 1; Each of the K groups of wavelength groups is combined by the passive wavelength multiplexer in the optical cross-connect network to determine the best beam compensation mode of each group according to the required phase shift value, and the optical switch in the optical cross-connect network selects the best beam compensation mode from the W beam compensation modes and distributes it to each radio frequency chain; The radio frequency signal on the radio frequency chain is modulated onto the optical carrier of the selected beam compensation mode by the optoelectronic modulation in the optical true time delay compensation network; wherein the best beam compensation mode refers to the beam compensation mode closest to the required phase shift value; The modulated radio frequency signal produces different time delays by the dispersive medium in the optical true time delay compensation network, and then the wavelengths corresponding to the time delays are distributed to different optical multiplexers and photodetectors by the passive wavelength demultiplexer, and the delayed radio frequency signal is transmitted to the Q frequency-independent phase shifters connected to each optical true time delay unit after electrical signal processing, and the frequency-dependent beam is realized after phase shifting by the phase shifters. The time delay difference of the wavelengths in the beam compensation mode passing through the dispersion medium is where D is the dispersion coefficient of the dispersion medium, L is the length of the dispersion medium, is the wavelength difference, indicating the difference between adjacent wavelengths in the current beam compensation mode; and the phase shift generated by the beam compensation mode under different subcarriers is: ； wherein, denotes the phase shift value resulting from the beam compensation pattern at the mth subcarrier, is the frequency of the subcarrier; The analog precoding network comprises a group of phase shifters, each group of phase shifters comprising a frequency-independent phase shifter corresponding to a transmitting antenna, the analog precoding network forming a beam of a different target direction by transmitting the phase shift value of each group of phase shifters to the transmitting antenna, and calculating the phase shift value required for compensating the beam splitting effect according to the analog precoding matrix composed of the outputs of the frequency-independent phase shifters, i.e. by setting the maximum array gain point as the target direction of the user, inversely solving the phase shift value required for compensating the optical true time delay compensation network.

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