CN114598391A - Far-end true delay beamforming implementation method based on few-mode optical fiber - Google Patents
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Abstract
The invention discloses a method for realizing far-end real time delay beam forming based on few-mode optical fibers, which comprises the following steps: 1) adjusting the bandwidth of an ASE light source according to the signal-to-noise ratio requirement of a radio frequency signal to be transmitted, and taking the adjusted ASE light source as an optical carrier; 2) modulating the optical carrier by the radio frequency signal to be transmitted to obtain an optical carrier radio frequency signal, dividing the optical carrier radio frequency signal into N paths of time delay copy signals, and injecting the signals into a corresponding mode in the at least mode optical fiber link through the mode multiplexer to be transmitted to the mode demultiplexer at the far end; 3) the mode demultiplexer demultiplexes the N paths of time delay copy signals and couples the N paths of time delay copy signals into N paths of single-mode tail fibers; the length of the ith single-mode tail fiber meets the requirement that the time delay difference between the ith time delay copy signal and the intermode crosstalk is far larger than the coherence time of an ASE light source; 4) and converting and amplifying the N paths of single-mode tail fiber output signals, and injecting the signals into N units of the antenna array for emission.
Description
Technical Field
The invention relates to the field of microwave photonics, in particular to a remote real-time delay beam forming method based on few-mode optical fibers, which eliminates coherence between signals and inter-mode crosstalk by using an amplified spontaneous emission light source (hereinafter referred to as ASE light source) as an optical carrier and avoids the influence of inter-mode interference on time delay jitter.
Background
Beamforming is a key technology in 5G mobile communication, and can effectively reduce electromagnetic interference in a wireless link and greatly improve radiation gain in a specific direction. The beam shaper realized by using the optical true time delay can avoid the problem of beam squint, thereby supporting the access of broadband radio frequency signals. Due to this advantage, a number of local beamforming schemes have been proposed in recent years. However, in a 5G mobile communication system, dense and complex local base stations will inevitably increase the cost and power consumption of the radio access network; and the real time delay signal is generated in the central office and is transmitted to the remote wireless unit through the optical fiber, so that the structure of the wireless base station can be greatly simplified, and a large-scale mobile network is powerfully supported. With the development of Space Division Multiplexing (SDM) technology, weakly coupled few-mode fibers and multi-core fibers have strong competitiveness in the transmission of real time delay signals.
Several Multi-core fiber-based remote real-time delay transmission schemes have been proposed, such as the Multi-core fiber-based multibeam-shapers proposed by m.morant et al in [ m.morant, et al, "Multi-beam formed provided by dual-wavelength true time delay and multicore fiber," j.light.technol.38, 5311-5317 (2020) ]. However, the far-end beamforming scheme based on few-mode fiber is not easy to implement, and one important reason is mode crosstalk. For short-range (1km or less) weakly coupled few-mode fiber links, crosstalk is mainly introduced at the mode demultiplexer, which is typically at a level of-20 dB or higher, orders of magnitude higher than multi-core fibers. Due to the coherence between crosstalk and signals, interference between signals and crosstalk can occur, thereby seriously affecting the performance of the few-mode system. This interference effect is known as intermodal interference. For non-multiple input multiple output (MIMO-less) intensity modulation direct detection systems, the influence and solution of inter-mode interference have been discussed, including the suppression of multi-path interference by using Wavelength interleaving method [ y.tie, et al. "wavelet-interleaved mdm-wdm-orthogonal over wave-interleaved fmf," opt.express25, 16603-16617 (2017) ].
In a far-end real-time delay beam forming system based on few-mode optical fibers, each channel outputs a time delay copy signal, and the phase of the time delay copy signal is seriously influenced by the interference between the modes. Because the phase and the time delay have a conversion relation, the time delay difference between channels is also changed violently, thereby seriously influencing the directional stability of a far-end wireless wave beam and causing the problem of beam squint. When using a typical low crosstalk mode multiplexer/demultiplexer (crosstalk level-20 dB), there is a phase jitter of over 23 ° (corresponding to a 10GHz radio frequency signal, translating to a delay jitter of 6.4 ps). The influence of the inter-mode interference makes the far-end real-time delay beam forming system based on the few-mode optical fiber difficult to be applied.
Disclosure of Invention
In order to overcome the time delay jitter caused by the inter-mode interference, the invention provides a solution method which adopts an ASE light source as an optical carrier. Since ASE light sources have extremely short coherence times: for ASE sources with bandwidths of the order of 1nm, the coherence time is of the order of ps. Therefore, the delay difference between the optical carrier radio frequency signals transmitted by each mode in the few-mode optical fiber system and the intermode crosstalk is far larger than the coherence time of the ASE light source by modulating the radio frequency signals to be transmitted on the ASE light source. At this time, the optical carrier radio frequency signal (i.e. the ASE optical signal modulated by the radio frequency) and the crosstalk can be regarded as completely incoherent, thereby eliminating the inter-mode interference and avoiding the time delay jitter. The optical carrier can select ASE light sources with different bandwidths, and it is only necessary that the signals are irrelevant to crosstalk (it should be noted that the ASE light sources with different bandwidths have an influence on the signal-to-noise ratio of the radio-frequency signals after photoelectric detection). Two incoherent ASE signals are photographed to obtain a spontaneous radiation beat frequency noise bottom with twice ASE bandwidth. Therefore, when the ASE light source is adopted as the optical carrier, the intermode interference becomes broadband noise bottom after photoelectric detection, the interference with the same frequency of the radio frequency signal to be transmitted does not exist any more, and the time delay jitter caused by the interference can be eliminated (if the traditional coherent light source is adopted as the optical carrier, the frequency of the obtained radio frequency signal and the radio frequency signal to be transmitted is the same frequency after the intermode interference is detected by the photoelectric detector, the phase and the time delay of the signal to be transmitted can be influenced, and the ASE light source is adopted as the optical carrier, so that the problem can be solved. Through signal-to-noise ratio analysis, when 7% redundant forward error correction coding is used, signal-to-noise ratio deterioration caused by ASE light source beat frequency noise bottom cannot influence error-free transmission of 1GHz bandwidth multi-system radio frequency signals. Therefore, the far-end real-time-delay beam forming system based on the few-mode optical fiber can be applied.
The technical scheme adopted by the invention for solving the technical problems is as follows:
a method for realizing far-end real time delay beam forming based on few-mode optical fiber comprises the following steps:
1) adjusting the bandwidth of the ASE light source according to the signal-to-noise ratio requirement of the radio-frequency signal to be transmitted, and taking the adjusted ASE light source as an optical carrier;
2) modulating the optical carrier by the radio frequency signal to be transmitted to obtain an optical carrier radio frequency signal and dividing the optical carrier radio frequency signal into N paths of time delay copy signals; the ith path of time delay copy signal is injected into the ith mode in the at least mode optical fiber link through the mode multiplexer and is transmitted to the mode demultiplexer at the far end, and i is 1-N;
3) the mode demultiplexer demultiplexes the N paths of time delay copy signals and couples the N paths of time delay copy signals into N paths of single-mode tail fibers; the length of the ith single-mode tail fiber meets the requirement that the time delay difference between the ith time delay copy signal and the intermode crosstalk is far larger than the coherence time of an ASE light source;
4) the N single-mode tail fiber output signals are received by a photoelectric detector and amplified by an electric amplifier and then injected into N units of an antenna array to realize the emission of a far-end beam forming signal.
Further, an ASE light source is used as a light source based on the few-mode fiber beam forming system, and the coherence length of the light source and the length of each matched single-mode tail fiber are determined according to the bandwidth of the ASE light source.
Further, for the radio frequency signal to be transmitted with the bandwidth not greater than 1GHz and the modulation format not greater than 16QAM, the bandwidth of the ASE light source is 1nm or greater, and the corresponding coherence time is tauASENot more than 5ps, coherence length LASE≤1mm。
Further, the optical carrier is modulated and amplified by the radio frequency signal to be transmitted, and then is divided into N paths of time delay copy signals through a 1 x N coupler.
Further, the output power, the time delay and the dispersion compensation of each path of time delay copy signals are controlled.
Furthermore, for the ith path of time delay copy signal, the output power of the ith path of time delay copy signal is controlled by connecting an adjustable attenuator in series, the time delay of the ith path of time delay copy signal is controlled by connecting an adjustable time delay line in series, and the dispersion introduced to the ith path of time delay copy signal by the ith mode of the few-mode fiber is compensated by connecting a chirped grating in series.
In the central office of the remote real-time delay beam forming system, an ASE light source filtered by a narrow-band optical filter is used as an optical carrier. The bandwidth of the light source can be adjusted according to the signal-to-noise ratio requirement of the radio-frequency signal to be transmitted. The higher the signal-to-noise ratio requirement, the larger the required ASE light source bandwidth. For the radio frequency signal to be transmitted with the bandwidth not more than 1GHz and the modulation format not more than 16QAM, the ASE light source bandwidth of 1nm can meet the signal-to-noise ratio requirement, and the corresponding coherence time is tauASE5ps, coherence length LASE1mm standard single mode fiber. After being modulated and amplified by a radio frequency signal to be transmitted, the narrow-band ASE optical carrier is divided into N paths of time delay copy signals through a 1 x N coupler. For each path, controlling the output power by connecting an adjustable attenuator in series; serially connecting adjustable delay lines to control the delay difference among all paths; the chirped grating is connected in series to compensate the chromatic dispersion introduced by each mode of the few-mode optical fiber. Through the mode multiplexer, N paths of delay copy signals are respectively injected into N modes of the weak coupling few-mode optical fiber and transmitted in a few-mode optical fiber link. At the far-end wireless unit, the N paths of time delay copy signals are demultiplexed through a mode demultiplexer and coupled into N paths of single-mode tail fibers. By reasonable designThe length of the N single-mode tail fibers enables the time delay difference between signals and crosstalk of each mode to be far larger than the coherence time of an ASE light source at the position where the mode demultiplexer introduces crosstalk. For ASE light source with bandwidth of 1nm, the length of N-path single-mode tail fiber can be dL-10LASEThe requirement can be met by increasing the distance to 1cm and forming an arithmetic progression. The N paths of output signals are received by the photoelectric detector and amplified by the electric amplifier and then injected into N units of the antenna array to realize the emission of the far-end beam forming signal. For ASE light sources with the bandwidth of 1nm and above, the length difference of the N single-mode tail fibers only needs to be more than or equal to about 1 cm; as for the specific used number of cm of tail fibers, whether the distances between the tail fibers are equal or not, the system performance is not influenced.
The invention has the beneficial effects that:
1. a method for eliminating intermodal interference is provided, namely an ASE light source is adopted as an optical carrier. By using the ASE light source as the optical carrier, the phase and delay jitter caused by inter-mode interference is eliminated, so that a far-end real-time delay beam forming system based on few-mode optical fibers is realized, and the complexity and the deployment cost of a wireless access network can be simplified in the future.
2. The use of an ASE light source as the optical carrier greatly reduces the need for a carrier light source, thereby further reducing the cost of the central office.
Drawings
FIG. 1 is a schematic diagram of a far-end real-time-delay beamforming system based on few-mode optical fibers according to the present invention;
FIG. 2 illustrates the maximum SNR that can be achieved when ASE sources of different bandwidths are used in the present invention;
FIG. 3 shows the delay jitter performance of the present invention (FMF) compared to the single mode fiber back-to-back (BTB).
Detailed Description
The embodiments of the present invention will be described in further detail with reference to the accompanying drawings.
The principle of the solution of the invention is shown in fig. 1. The embodiments herein take the example of a two element beamforming system that employs LP01 and LP02 modes of transmission. At the central office, the ASE light source is narrowband filtered with an optical filter. With narrow-band filteringThe ASE light source is amplified by an optical amplifier and injected into an intensity modulator. Modulating the radio frequency signal to be modulated on an ASE optical carrier, amplifying the ASE optical carrier again through an optical amplifier, and splitting the ASE optical carrier into a first time delay copy signal and a second time delay copy signal by using a 1-x 2 coupler. After the first time delay copy signal passes through the variable optical attenuator and the variable optical delay line, injecting an LP01 mode of a few-mode optical fiber by adopting a mode multiplexer, and transmitting the signal to a far end; the second time-delayed copy signal is injected into the LP02 mode of the few-mode fiber directly through the mode multiplexer and transmitted to the far end. The LP01 and LP02 modes, where few-mode fibers are used, have the same dispersion coefficient. In practical applications, the central office also needs to add a dispersion ratio of-D to the mode nnL, where D is the chirp gratingnThe dispersion coefficient for mode n, L is the few-mode fiber link length. At the far-end wireless unit, the first time delay copy signal and the second time delay copy signal are demultiplexed through a mode demultiplexer, and are respectively injected into a photoelectric detector through a single-mode tail fiber. After photoelectric detection, the first time delay copy signal and the second time delay copy signal are amplified by an electric amplifier and are respectively emitted through the two unit antenna arrays.
Setting the path time delay of the first time delay copy signal in the central office as tauaThe time delay introduced by the adjustable time delay line is tauv(ii) a The path delay of the second time-delay copy signal in the central office is taub(ii) a Group time delays introduced by the first time delay copy signal and the second time delay copy signal in the few-mode optical fiber link are respectively tau01And τ02(ii) a The path time delay introduced by the first time delay copy signal and the second time delay copy signal in the remote wireless unit is taucAnd τdTherefore, the delay difference of two signals is τ, which is:
τ＝(τa+τv+τ01+τc)-(τb+τ02+τd).
at the demultiplexer, for a first time-delayed replica signal, the time delay difference of the crosstalk compared to the signal is:
(τa+τv+τ01)-(τb+τ02)＝τ+(τc-τd)；
for the second time-delayed replica signal, the time delay difference of the crosstalk compared with the signal is:
(τb+τ02)-(τa+τv+τ01)＝-τ-(τc-τd).
the length of the single-mode tail fiber of the two units at the far-end wireless unit is reasonably designed, so that the tau + (tau)c-τd)＞＞τASEHere τASEIs the coherence time of the ASE light source. For example, for 1nm ASE light source, the coherence time is about 5ps, so the difference of the lengths of the tail fibers of the two units at the far end is only required to be 1cm (corresponding to tau)c-τd50ps) level, i.e., the signal and crosstalk can be made incoherent. In this embodiment, the difference in the length of the pigtails at the distal ends of the two units is 20 cm. For N-path system, the tail fiber lengths of the units at the far end can be arranged according to an arithmetic progression, and the difference between every two tail fibers is 10 tauASETherefore, the incoherence between each path of signal and crosstalk can be satisfied.
After the ASE light source is used as an optical carrier, the signal-to-noise ratio of the received radio frequency signal is reduced due to the influence of ASE self-timer noise. The degradation of the signal-to-noise ratio can be reduced by increasing the bandwidth of the ASE light source. This is because the wider the ASE light source bandwidth, the smaller the power spectral density of the ASE beat noise, and the lower the noise floor per bandwidth for optical carriers of the same power. As shown in FIG. 2, the present invention provides the maximum SNR corresponding to different ASE light source bandwidths, where the bandwidth of the modulated RF signal is 1 GHz. For ASE light source with 1nm bandwidth, the signal-to-noise ratio is larger than 15dB, and after 7% redundancy forward error correction coding is used, the error-free transmission of OOK, QPSK and 16QAM signals with 1GHz bandwidth can be supported; for 5nm bandwidth ASE sources, the SNR is greater than 20dB, and after forward error correction coding with 7% redundancy, it can support error-free transmission of 1GHz bandwidth 64QAM signals [ R.A. Shafik, et al. "On the extended positions amplitude EVM, BER and SNR as performance metrics," in 2006International conference On electric Computer Engineering, (2006), pp.408-411 ].
As shown in FIG. 3, the present invention provides a far-end beam forming system based on few-mode fiber using ASE light source as optical carrierThe delay jitter performance of the system (FMF) was compared to that of a single mode fiber back-to-back system (BTB). The bandwidth of the ASE light source is 1nm, and the modulated radio frequency signal is a single tone signal of 9.2 GHz; the waveform of the radio frequency first time delay copy signal and the waveform of the radio frequency second time delay copy signal output by the far end are collected through the oscilloscope, and the standard deviation of the time delay difference between the first time delay copy signal and the second time delay copy signal can be obtained. Here at time delay differences of 0 and + -27.15 ps (corresponding to phase differences of 0 deg. and
) Are tested. The direct sampling mode and the 16-time average sampling mode of the oscilloscope are used for testing. The reason for using the latter is to avoid the influence of thermal noise, shot noise, and quantization noise of the oscilloscope. Fig. 3 shows that the delay jitter performance of the present invention is completely equivalent to that of a single-mode fiber back-to-back system (BTB), and has no significant degradation. Under the direct sampling mode, the time delay jitter of the two is about 0.7 ps; in the 16-time average sampling mode, the delay jitter of the two is about 0.2 ps. Therefore, the invention solves the problem of time delay jitter caused by intermode interference in the few-mode optical fiber by adopting the ASE light source as the optical carrier, and realizes the far-end real time delay beam forming system based on the few-mode optical fiber.
The above embodiments are only intended to illustrate the technical solution of the present invention and not to limit the same, and a person skilled in the art can modify the technical solution of the present invention or substitute the same without departing from the spirit and scope of the present invention, and the scope of the present invention should be determined by the claims.
Claims (6)
1. A method for realizing far-end real time delay beam forming based on few-mode optical fiber comprises the following steps:
1) adjusting the bandwidth of the ASE light source according to the signal-to-noise ratio requirement of the radio-frequency signal to be transmitted, and taking the adjusted ASE light source as an optical carrier;
2) modulating the optical carrier by the radio frequency signal to be transmitted to obtain an optical carrier radio frequency signal and dividing the optical carrier radio frequency signal into N paths of time delay copy signals; the ith path of time delay copy signal is injected into the ith mode in the at least mode optical fiber link through the mode multiplexer and is transmitted to the mode demultiplexer at the far end, and i is 1-N;
3) the mode demultiplexer demultiplexes the N paths of time delay copy signals and couples the N paths of time delay copy signals into N paths of single-mode tail fibers; the length of the ith single-mode tail fiber meets the requirement that the time delay difference between the ith time delay copy signal and the intermode crosstalk is far larger than the coherence time of an ASE light source;
4) the N single-mode tail fiber output signals are received by a photoelectric detector and amplified by an electric amplifier and then injected into N units of an antenna array to realize the emission of a far-end beam forming signal.
2. The method as claimed in claim 1, wherein an ASE light source is used as a light source based on the few-mode fiber beam forming system, and the coherence length of the light source and the length of each matched single-mode pigtail are determined according to the bandwidth of the ASE light source.
3. The method of claim 1, wherein for a radio frequency signal to be transmitted having a bandwidth of not more than 1GHz and a modulation format of not more than 16QAM, the bandwidth of the ASE light source is 1nm or more, corresponding to a coherence time τASENot more than 5ps, coherence length LASE≤1mm。
4. The method of claim 1, wherein the optical carrier is modulated and amplified by the RF signal to be transmitted, and then divided into N paths of time-delayed copy signals by a 1 x N coupler.
5. The method of claim 1 wherein the output power, delay and dispersion compensation of each path of the delayed replica signal are controlled.
6. The method of claim 1 or 5, wherein for the ith delay copy signal, the output power of the ith delay copy signal is controlled by connecting an adjustable attenuator in series, the delay of the ith delay copy signal is controlled by connecting an adjustable delay line in series, and the dispersion introduced by the ith mode of the few-mode fiber to the ith delay copy signal is compensated by connecting a chirped grating in series.
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| CN103095373A (en) * | 2013-01-31 | 2013-05-08 | 华中科技大学 | Self-correlation optical fiber communication system based on mode division multiplexing |
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