CN113938200A - Digital mobile forward transmission method and device based on Delta-Sigma modulation - Google Patents

Digital mobile forward transmission method and device based on Delta-Sigma modulation Download PDF

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CN113938200A
CN113938200A CN202111040960.7A CN202111040960A CN113938200A CN 113938200 A CN113938200 A CN 113938200A CN 202111040960 A CN202111040960 A CN 202111040960A CN 113938200 A CN113938200 A CN 113938200A
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CN113938200B (en
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杨奇
邹阳
钟林晟
戴潇潇
程孟凡
邓磊
刘德明
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Huazhong University of Science and Technology
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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
    • H04B10/25—Arrangements specific to fibre transmission
    • H04B10/2575—Radio-over-fibre, e.g. radio frequency signal modulated onto an optical carrier
    • H04B10/25752—Optical arrangements for wireless networks
    • H04B10/25753—Distribution optical network, e.g. between a base station and a plurality of remote units
    • 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
    • H04B10/524—Pulse modulation
    • 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
    • H04B10/54—Intensity modulation
    • H04B10/541—Digital intensity or amplitude modulation
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04L—TRANSMISSION OF DIGITAL INFORMATION, e.g. TELEGRAPHIC COMMUNICATION
    • H04L27/00—Modulated-carrier systems
    • H04L27/02—Amplitude-modulated carrier systems, e.g. using on-off keying; Single sideband or vestigial sideband modulation
    • H04L27/08—Amplitude regulation arrangements
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    • Y02D—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN INFORMATION AND COMMUNICATION TECHNOLOGIES [ICT], I.E. INFORMATION AND COMMUNICATION TECHNOLOGIES AIMING AT THE REDUCTION OF THEIR OWN ENERGY USE
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Abstract

The invention discloses a digital mobile forward transmission method and a device based on Delta-Sigma modulation, belonging to the technical field of optical fiber wireless fusion access, comprising the following steps: converting two paths of analog signals into digital bit streams through Delta-Sigma modulation, respectively recording the digital bit streams as a first OOK signal and a second OOK signal, respectively serving as MSB and LSB to be combined to generate PAM4 signals, and then outputting radio frequency; after the radio frequency signal is subjected to electro-optical conversion, the radio frequency signal is transmitted through an optical fiber link by using a single wavelength and is reduced into a radio frequency signal through the electro-optical conversion; at the RRU end, separating MSB and LSB in PAM4 signal to restore first and second OOK signal; filtering out-of-band quantization noise in the two OOK signals respectively to recover two analog signals, and transmitting the two analog signals to two RRUs for wireless transmission; before the combination, the signal-to-noise ratio distribution of the two paths of signals is realized by adjusting the amplitudes of the two paths of OOK signals. The invention can expand the number of the access users by one time without increasing the number of the wavelengths, and the signal-to-noise ratio of the RRU end can be flexibly configured.

Description

Digital mobile forward transmission method and device based on Delta-Sigma modulation
Technical Field
The invention belongs to the technical field of optical fiber wireless fusion access, and particularly relates to a digital mobile forward transmission method and device based on Delta-Sigma modulation.
Background
In recent years, with the popularization of internet of everything and cloud services, the data volume on digital applications is rapidly increasing, so higher requirements are put on the throughput of a base station and the capacity of a fronthaul network, mainly in the aspects of spectrum efficiency, transmission signal-to-noise ratio, flexibility, cost and the like of the fronthaul network. Therefore, the optimization and technical innovation of the existing fronthaul network structure become the problems to be solved by the 5G development.
The digital forwarding scheme can better resist the influence of noise and distortion of a channel on a signal, but the bandwidth required by a link is far higher than the actual bandwidth of a radio frequency signal. Therefore, with the rapid increase of data volume, considering the influence of dispersion and the requirement of MIMO (multiple input multiple output) application, the conventional CPRI (Common Public Radio Interface) will become the bottleneck of forward transmission. The all-Digital transmitter based on Delta-Sigma modulation can not only improve the capacity of a forward interface, but also replace an expensive ADC (Analog-to-Digital Converter) device at the RRU (Remote Radio Unit) end with a low-cost filter, thereby having good commercial prospect. There are difficulties faced in actually laying out the wireless-over-fiber forwarding network.
On one hand, the existing optical fiber wireless forward transmission network basic optical device unit is an optical module component, the generated signal mode is relatively limited, and when the forward transmission requirement of large capacity and multiple users is met, the capacity expansion mode of the transmission link is relatively single, namely, the wavelength division multiplexing technology is adopted, but after the wavelength is increased to full load, the requirement of capacity increase cannot be further realized by the existing means. On the other hand, the existing optical fiber wireless forwarding network is constructed based on fixed optical module components, and the relationship between the format of the transmission signal and the distance is also fixed. In reality, however, the link conditions and capacity requirements between multiple RRUs and DUs (distribution units) are different.
Generally, the existing optical fiber wireless forwarding network has limited capacity and cannot realize dynamic adjustment of the signal-to-noise ratio.
Disclosure of Invention
Aiming at the defects and improvement requirements of the prior art, the invention provides a digital mobile forward transmission method and a digital mobile forward transmission device based on Delta-Sigma modulation, and aims to realize optical fiber wireless forward transmission with high capacity, low cost and flexible configuration.
To achieve the above object, according to one aspect of the present invention, there is provided a digital mobile forward transmission method based on Delta Sigma modulation, including:
respectively carrying out Delta-Sigma modulation on the two paths of analog signals to convert the two paths of analog signals into digital bit streams which are respectively marked as a first OOK signal and a second OOK signal; combining the first OOK signal and the second OOK signal respectively as MSB and LSB to generate PAM4 signal, and converting the PAM4 signal into radio frequency signal for output;
after the radio frequency signal is subjected to electro-optical conversion, the radio frequency signal is transmitted from one end of the optical fiber link to the other end of the optical fiber link by using a single wavelength, and is reduced into a radio frequency signal through the electro-optical conversion;
at the RRU end, separating MSB and LSB in the restored radio frequency signal to restore a first OOK signal and a second OOK signal; and respectively filtering out the out-of-band quantization noise in the two OOK signals to recover the original two analog signals, and respectively transmitting the two analog signals to the two RRUs for wireless transmission by the RRUs.
Further, before combining the first OOK signal and the second OOK signal as the MSB and the LSB, respectively, to generate the PAM4 signal, the method further includes:
adjusting the amplitude alpha of the first OOK signal and/or the amplitude beta of the second OOK signal according to the signal quality requirements of two paths of analog signals at the RRU end in wireless transmission;
when the amplitude value alpha of the first OOK signal is larger, the signal-to-noise ratio of the LSB signal is transferred to the MSB signal; when the amplitude β of the second OOK signal is large, the signal-to-noise ratio of the MSB signal shifts to the LSB signal.
Further, gray coding is adopted when the first OOK signal and the second OOK signal are respectively taken as MSB and LSB to be combined;
and, separating the MSB and LSB in the restored radio frequency signal, including:
for MSB signals, when the detected signal level value is greater than level V1Sum level V2When the average value is equal to the first OOK signal, outputting +1, otherwise, outputting-1, and thus restoring to obtain the first OOK signal;
for LSB signals, when the detected signal level value is less than the level V2Sum level V3Is greater than the level V0Sum level V1When the average value is smaller than the first threshold value, outputting +1, otherwise, outputting-1, and thus restoring to obtain a second OOK signal;
wherein, V0～V3Is the average value of each level sample in the PAM4 signal, and V0<V1<V2<V3。
According to another aspect of the present invention, there is provided a digital mobile forwarding device based on Delta Sigma modulation, comprising: the system comprises a centralized BBU pool, an optical fiber transmission module and an RRU end;
the centralized BBU pool comprises a first Delta-Sigma modulator, a second Delta-Sigma modulator, a layered modulator and a radio frequency output module; the first Delta-Sigma modulator and the second Delta-Sigma modulator are used for respectively carrying out Delta-Sigma modulation on the two paths of analog signals so as to convert the two paths of analog signals into digital bit streams which are respectively marked as a first OOK signal and a second OOK signal; the two input ends of the hierarchical modulator are respectively connected to the output ends of the first Delta-Sigma modulator and the second Delta-Sigma modulator, and the hierarchical modulator is used for combining the first OOK signal and the second OOK signal respectively as an MSB and an LSB to generate a PAM4 signal; the input end of the radio frequency output module is connected to the output end of the layered modulator, and the radio frequency output module is used for converting the PAM4 signal into a radio frequency signal to be output;
the optical fiber transmission module comprises an electro-optical conversion unit, an optical fiber link and a photoelectric conversion unit; the input end of the electro-optical conversion unit is connected to the output end of the radio frequency output module and is used for performing electro-optical conversion on the radio frequency signal to obtain an optical signal; the optical fiber link is connected with the electro-optical conversion unit and the photoelectric conversion unit at two ends respectively and is used for transmitting an optical signal to the photoelectric conversion unit by using a single wavelength; the photoelectric conversion unit is used for carrying out photoelectric conversion on the optical signal transmitted by the optical fiber link so as to restore the optical signal into a radio frequency signal;
the RRU end comprises a judgment module, a first filter and an RRU connected with the first filter, and a second filter and an RRU connected with the second filter; the input end of the judgment module is connected to the output end of the photoelectric conversion unit and is used for separating the MSB and the LSB in the restored radio frequency signal so as to restore a first OOK signal and a second OOK signal; the first filter and the second filter are respectively connected with two output ends of the decision module, and are used for filtering out the out-of-band quantization noise of the two OOK signals to recover the original two analog signals, and respectively transmitting the two analog signals to the two RRUs for wireless transmission by the RRUs.
Further, the layered modulator is further configured to adjust an amplitude α of the first OOK signal and/or an amplitude β of the second OOK signal according to a signal quality requirement of two analog signals at the RRU end in wireless transmission before combining the first OOK signal and the second OOK signal as an MSB and an LSB, respectively, to generate a PAM4 signal;
when the amplitude value alpha of the first OOK signal is larger, the signal-to-noise ratio of the LSB signal is transferred to the MSB signal; when the amplitude β of the second OOK signal is large, the signal-to-noise ratio of the MSB signal shifts to the LSB signal.
Further, the layered modulator combines the first OOK signal and the second OOK signal as MSB and LSB, respectively, to generate a PAM4 signal, using gray coding;
moreover, the manner of separating the MSB and the LSB of the restored rf signal by the decision module includes:
for MSB signals, when the detected signal level value is greater than level V1Sum level V2When the average value is equal to the first OOK signal, outputting +1, otherwise, outputting-1, and thus restoring to obtain the first OOK signal;
for LSB signals, when the detected signal level value is less than the level V2Sum level V3Is greater than the level V0Sum level V1When the average value is smaller than the first threshold value, outputting +1, otherwise, outputting-1, and thus restoring to obtain a second OOK signal;
wherein, V0～V3Is the average value of each level sample in the PAM4 signal, and V0<V1<V2<V3。
Furthermore, the centralized BBU pool is realized by an FPGA, and the radio frequency output module is an external I/O port of the FPGA.
Further, in the first Delta Sigma modulator and the second Delta Sigma modulator, the transfer function NTF of the feedback structure for noise shaping in the z-domain is:
Figure BDA0003249185150000051
where a to f are coefficients of each node in the feedback structure, and a is-0.69, b is-3.33, c is-1.51, d is 0.17, e is-3.01, and f is-0.79.
Further, the centralized BBU pool further includes an analog signal processing unit configured to perform baseband and carrier modulation on the original signal to obtain an analog signal to be transmitted.
Generally, by the above technical solution conceived by the present invention, the following beneficial effects can be obtained:
(1) the digital mobile forward transmission method and device based on Delta-Sigma modulation provided by the invention have the advantages that two paths of analog signals to be transmitted are subjected to Delta-Sigma modulation, the two paths of analog signals are converted into digital signals, and the two paths of digital signals are combined into one path of PAM4 signals, compared with the existing optical fiber forward transmission network, the number of access users is doubled by using two paths of analog signals carried by two wavelengths, the high-order modulation format is formed by utilizing Delta-Sigma modulation, namely, the two paths of analog signals are converted into two paths of OOK signals through Delta-Sigma modulation and then are synthesized into one path of PAM4 signals, so that the two paths of analog signals can share the same wavelength and are transmitted in a light link, and therefore, the invention can expand the number of access users by one time without increasing the number of wavelengths.
(2) According to the digital mobile forward transmission method and device based on Delta-Sigma modulation, before two paths of OOK signals are combined, the amplitude alpha of a first OOK signal and/or the amplitude beta of a second OOK signal are adjusted at a BBU end according to the signal quality requirements of two paths of analog signals at an RRU end in wireless transmission, so that the signal-to-noise ratio is transferred between the two paths of OOK signals at the RRU end as required, and different signal-to-noise ratio requirements are met.
(3) According to the digital mobile fronthaul device based on Delta-Sigma modulation provided by the invention, the centralized BBU pool is realized by the FPGA, and the PAM4 signal output by the layered modulator is directly output by radio frequency through the external I/O port of the FPGA, so that an expensive DAC device is avoided, on the receiving side, the original analog signal can be recovered by only one filter and sent to RRU for wireless transmission, an expensive ADC device is also avoided, and the realization cost of the device is effectively reduced.
(4) According to the digital mobile forward transmission device based on Delta-Sigma modulation, parameters of a feedback structure in a Delta-Sigma modulator are optimized, and based on optimized parameter combination, on one hand, quantization noise can be pushed out of a signal frequency band in the noise shaping process, so that the quality of a signal recovered by an RRU end is high; on the other hand, the Delta-Sigma modulator is enabled to be used at a center frequency of 3.5GHz, so that the apparatus is applicable to 5G communication.
Drawings
Fig. 1 is a schematic diagram of a digital mobile forwarding method and device based on Delta-Sigma modulation according to an embodiment of the present invention;
fig. 2 (a) is a schematic diagram illustrating generation and decision of a layered modulation PAM4 signal according to an embodiment of the present invention;
fig. 2 (b) is a schematic diagram illustrating the relationship between the signal-to-noise ratio transfer of the MSB and the LSB in the layered modulation PAM4 signal according to the embodiment of the present invention;
FIG. 3 shows different R provided in the embodiment of the present inventionLMUnder the value, the relation graph of the bit error rate and the received optical power of the MSB branch and the LSB branch is shown;
fig. 4 is a schematic diagram of a Delta Sigma modulator feedback structure and signal modulation and recovery provided by the embodiment of the invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.
In the present application, the terms "first," "second," and the like (if any) in the description and the drawings are used for distinguishing between similar elements and not necessarily for describing a particular sequential or chronological order.
In order to solve the technical problems that the existing optical fiber wireless forward transmission network has limited capacity and cannot realize the dynamic adjustment of the signal to noise ratio, the invention provides a digital mobile forward transmission method and a digital mobile forward transmission device based on Delta-Sigma modulation, and the overall thought is as follows: after two paths of analog signals are converted into digital signals, two paths of digital signals are combined to form a path of PAM4 signal, and radio frequency output is carried out, so that two paths of signals can share one wavelength for transmission in an optical fiber link, and the number of access users is doubled without increasing the number of wavelengths; on the basis, according to the quality requirements of the two paths of signals in optical fiber and wireless transmission, when the two paths of digital signals are combined, the amplitude values of the two paths of digital signals are adjusted, so that the signal-to-noise ratio is transferred between the two paths of digital signals, and the flexible configuration of the signal-to-noise ratio is realized.
The following are examples.
Example 1:
a digital mobile forward method based on Delta Sigma modulation, as shown in fig. 1, includes:
respectively carrying out Delta-Sigma modulation on the two paths of analog signals to convert the two paths of analog signals into digital bit streams which are respectively marked as a first OOK signal and a second OOK signal; combining the first OOK signal and the second OOK signal respectively as MSB and LSB to generate PAM4 signal, and converting the PAM4 signal into radio frequency signal for output; the PAM4 signal generated in this embodiment is formed by combining two OOK signals as MSB and LSB, respectively, and thus may be regarded as a PAM4 signal in layered modulation, where the MSB and LSB may be regarded as a first layer and a second layer in the layered modulation, respectively;
after the radio frequency signal is subjected to electro-optical conversion, the radio frequency signal is transmitted from one end of the optical fiber link to the other end of the optical fiber link and is reduced into a radio frequency signal through the electro-optical conversion; because the PAM4 signals simultaneously carry two OOK signals, when the PAM4 signals are transmitted in an optical link, the two OOK signals can share the same wavelength for transmission, so that the number of access users is doubled without increasing the number of wavelengths;
at the RRU end, separating MSB and LSB in the restored radio frequency signal to restore a first OOK signal and a second OOK signal; filtering out-of-band quantization noise in the two OOK signals respectively to recover the original two analog signals, and transmitting the two analog signals to two RRUs respectively for wireless transmission by the RRUs;
in order to match different requirements to improve wireless transmission performance, as a preferred embodiment, in this embodiment, before combining the first OOK signal and the second OOK signal as MSB and LSB respectively to generate the PAM4 signal, the method further includes:
adjusting the amplitude alpha of the first OOK signal and/or the amplitude beta of the second OOK signal according to the signal quality requirements of two paths of analog signals at the RRU end in wireless transmission;
when the amplitude value alpha of the first OOK signal is larger, the signal-to-noise ratio of the LSB signal is transferred to the MSB signal; when the amplitude beta of the second OOK signal is larger, the signal-to-noise ratio of the MSB signal is transferred to the LSB signal;
r of PAM4 signalLMThe value is used for measuring the uniformity degree of the level interval of the PAM4 signal, the maximum value is 1, and the level intervals of the PAM4 signals are equal; in this example, RLMWhen 1, the performance of the MSB and LSB branches behaves or provides almost the same signal-to-noise ratio; in this embodiment, the amplitudes α and β of the two OOK signals are adjusted before combining, and when the amplitudes α and β are no longer equal to each other, R is adjustedLMThe value of (a) is less than 1, then the PAM4 signal with non-uniform level distribution is obtained by combining, and the signal-to-noise ratio changes correspondingly between the MSB and the LSB; rLMThe calculation method of (c) is as follows:
Figure BDA0003249185150000081
Figure BDA0003249185150000082
Figure BDA0003249185150000091
RLM＝min{(3×ES1)，(3×ES2)，(2-3×ES1)，(2-3×ES2)}
wherein, V0～V3Is the average value of each level sample in the PAM4 signal, and V0<V1<V2<V3As shown in (a) of FIG. 2ES1 and ES2 are shown as intermediate variables; as shown in fig. 2 (a), it can be easily found that when the amplitude α of the first OOK signal is increased, RLMValue decrease, V1、V2The distance between the levels is increased, namely the Euclidean distance between constellation points of the '3' level and the '1' level and between the '-3' level and the '-1' level of the PAM4 signal is reduced, namely the opening degree of the 'eyes' at the edge of the layered modulation PAM4 signal is reduced; the euclidean distance between the corresponding "+ 1" "1" level constellation points is increased, so that the decision margin of the MSB is increased, and the error judgment probability of the LSB is increased, that is, the signal-to-noise ratio of the LSB shifts to the MSB, as shown in (b) in fig. 2, thereby achieving the purpose of hierarchical modulation; the process reflects the characteristics of hierarchical modulation, namely the R of the PAM4 signal can be adjusted by adjusting the amplitudes alpha and beta of two layers of OOK signalsLMThe signal-to-noise ratio or the signal quality which the two-layer signal can ultimately provide to the user is correspondingly changed;
conversely, by increasing the amplitude β of the second OOK signal, RLMThe value will also decrease when the signal-to-noise ratio of the MSB shifts towards the LSB.
The embodiment also detects that R is caused to be equal by increasing the amplitude alpha of the first OOK signalLMWhen the values are decreased, the Received Optical Powers (ROP) of the MSB and the LSB branches are shown in fig. 3, and the difference between the Received Optical Powers (ROP) of the MSB and the LSB branches under the same bit error rate threshold is shown in table 1.
TABLE 1
Figure BDA0003249185150000092
From the results shown in FIG. 3, it can be seen that when R is presentLMWhen decreasing from 1 to 0.6, the ROP required by the MSB decreases by 1.5dB, while the ROP required by the LSB increases by 1.45dB, i.e., the signal-to-noise ratio of the LSB shifts toward the MSB.
It should be noted that the above description is only exemplary, and should not be construed as the only limitation of the present invention, and in some other embodiments of the present invention, when other parameters such as modulation parameters are changed, different R's are differentLMThe value of ROP required by the MSB and LSB branches may also vary accordingly, but between the MSB and LSB branchesThe trend of the signal-to-noise ratio shift is consistent with the present embodiment.
As shown in fig. 2 (a), in this embodiment, gray coding is adopted when the first OOK signal and the second OOK signal are combined as MSB and LSB, respectively;
accordingly, the MSB of the recovered RF signal can be determined by subtracting V from V1And V2The signal level is obtained by performing one-time judgment between the levels when the signal level value is more than V1And V2When the average value of the levels is obtained, outputting +1, otherwise outputting-1, thereby restoring the original OOK signal corresponding to the MSB branch; then at V2And V3After the electric levels are judged and inverted, the sum of the electric levels and the V0、V1The sum of the decision values between the levels, i.e. the LSB, is obtained when the level value lies at V2、V3Average value of electric level and V0、V1When the average values of the levels are within the range, outputting +1, otherwise outputting-1, wherein the specific logic relationship is as follows: firstly, judge whether the signal is higher than V2、V3The average value of the levels is 1 if the average value is higher than the preset value and is 0 after inversion, and the average value of the levels is V0、V1The structural logic phase of the level mean value judgment is always 0 after the logical phase is judged, and-1 is output. If the signal level is lower than the preset value, 0 is taken, and the inverted value is 1, and whether the signal level is at V or not is judged at the moment0、V1If the average value of the levels is higher, the other bit takes 1, the phase of the two bits is 1, and +1 is output; if not, the other bit takes 0, and the phase is 0 after the phase comparison, and-1 is output.
Example 2:
a digital mobile forwarding device based on Delta Sigma modulation, as shown in fig. 1, comprising: the system comprises a centralized BBU pool, an optical fiber transmission module and an RRU end;
the centralized BBU pool comprises a first Delta-Sigma modulator, a second Delta-Sigma modulator, a layered modulator and a radio frequency output module; the first Delta-Sigma modulator and the second Delta-Sigma modulator are used for respectively carrying out Delta-Sigma modulation on the two paths of analog signals so as to convert the two paths of analog signals into digital bit streams which are respectively marked as a first OOK signal and a second OOK signal; the two input ends of the hierarchical modulator are respectively connected to the output ends of the first Delta-Sigma modulator and the second Delta-Sigma modulator, and the hierarchical modulator is used for combining the first OOK signal and the second OOK signal respectively as an MSB and an LSB to generate a PAM4 signal; the input end of the radio frequency output module is connected to the output end of the layered modulator, and the radio frequency output module is used for converting the PAM4 signal into a radio frequency signal to be output;
the optical fiber transmission module comprises an electro-optical conversion unit, an optical fiber link and a photoelectric conversion unit; the input end of the electro-optical conversion unit is connected to the output end of the radio frequency output module and is used for performing electro-optical conversion on the radio frequency signal to obtain an optical signal; the optical fiber link is connected with the electro-optical conversion unit and the photoelectric conversion unit at two ends respectively and is used for transmitting an optical signal to the photoelectric conversion unit by using a single wavelength; the photoelectric conversion unit is used for carrying out photoelectric conversion on the optical signal transmitted by the optical fiber link so as to restore the optical signal into a radio frequency signal;
the RRU end comprises a judgment module, a first filter and an RRU connected with the first filter, and a second filter and an RRU connected with the second filter; the input end of the judgment module is connected to the output end of the photoelectric conversion unit and is used for separating the MSB and the LSB in the restored radio frequency signal so as to restore a first OOK signal and a second OOK signal; the first filter and the second filter are respectively connected with two output ends of the decision module and used for filtering out the out-of-band quantization noise of the two OOK signals so as to recover the original two analog signals, and the two analog signals are respectively transmitted to two RRUs for wireless transmission by the RRUs;
in this embodiment, the layered modulator is further configured to adjust an amplitude α of the first OOK signal and/or an amplitude β of the second OOK signal according to a signal quality requirement of two analog signals at an RRU end in wireless transmission before combining the first OOK signal and the second OOK signal as an MSB and an LSB, respectively, to generate a PAM4 signal;
when the amplitude value alpha of the first OOK signal is larger, the signal-to-noise ratio of the LSB signal is transferred to the MSB signal; when the amplitude beta of the second OOK signal is larger, the signal-to-noise ratio of the MSB signal is transferred to the LSB signal;
optionally, in this embodiment, the layered modulator combines the first OOK signal and the second OOK signal as MSB and LSB, respectively, to generate a PAM4 signal, and uses gray coding;
moreover, the manner of separating the MSB and the LSB of the restored rf signal by the decision module includes:
for MSB signals, when the detected signal level value is greater than level V1Sum level V2When the average value is equal to the first OOK signal, outputting +1, otherwise, outputting-1, and thus restoring to obtain the first OOK signal;
for LSB signals, when the detected signal level value is less than the level V2Sum level V3Is greater than the level V0Sum level V1When the average value is equal to the first OOK signal, outputting +1, otherwise, outputting-1, and thus obtaining a second OOK signal;
wherein, V0～V3Is the average value of each level sample in the PAM4 signal, and V0<V1<V2<V3。
In order to further reduce the implementation cost of the device, as an optional implementation manner, in this embodiment, the centralized BBU pool is implemented by an FPGA, and the radio frequency output module is an external I/O port of the FPGA, so that an expensive DAC device is avoided, and the implementation cost of the device is effectively reduced.
When the Delta Sigma modulator pair converts an analog signal into a digital signal, the Delta Sigma modulator pair specifically comprises three parts of oversampling, noise shaping and noise quantization, as shown in fig. 4; correspondingly, when Delta-Sigma modulation is performed, firstly, a higher sampling rate is used to convert an original analog signal waveform into a discrete level value, that is, oversampling is achieved, and optionally, the sampling rate used in the embodiment is 10 times of the nyquist sampling rate; the level value obtained by each sampling is calculated through a feedback structure of a Delta-Sigma modulator to realize noise shaping; the noise-shaped level value is converted into a bit stream of ± 1 by outputting a corresponding value, which is 1 or-1 in case of one-bit quantization, through a quantizer. In this embodiment, the feedback structure of the Delta Sigma modulator used is shown in fig. 4, and the expression of the transfer function (NTF) in the z-domain is:
further, in the first Delta Sigma modulator and the second Delta Sigma modulator, the transfer function NTF of the feedback structure for noise shaping in the z-domain is:
Figure BDA0003249185150000121
wherein a-f are coefficients of each node in the feedback structure;
the decision module is generally realized by a simple operational amplifier circuit and a logic circuit; the signal separated by the decision module corresponds to the original Delta Sigma modulation signal quantized by one bit, wherein the frequency distribution of the modulated analog signal is not changed, the distribution of the quantization noise is consistent with the Noise Transfer Function (NTF) of the Delta Sigma modulator, the quantization noise outside the signal band is filtered by a filter (usually a band-pass type), and the frequency component of the signal is reserved to restore the analog waveform.
In order to further improve the signal quality and enable the forwarding apparatus to be applicable to 5G communication, in this embodiment, parameters of a feedback structure of the Delta-Sigma modulator are optimized, and in this embodiment, coefficients in the feedback structure are respectively: a-0.69, b-3.33, c-1.51, d-0.17, e-3.01, f-0.79; based on the optimized parameter combination, on one hand, the quantization noise can be pushed out of a signal frequency band in the noise shaping process, so that the quality of a signal recovered by the RRU end is higher; on the other hand, the Delta-Sigma modulator is enabled to be used at a center frequency of 3.5GHz, so that the apparatus is applicable to 5G communication.
It should be noted that the present invention can also be applied to communication in other frequency bands, such as 4G communication, and at this time, the coefficients in the feedback structure of the Delta Sigma modulator are adjusted and optimized correspondingly in combination with the frequency band to which the specific communication application belongs.
As shown in fig. 1, in this embodiment, the centralized BBU pool further includes an analog signal processing unit, configured to perform baseband and carrier modulation on an original signal to obtain an analog signal to be transmitted.
It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims (9)

1. A digital mobile forward transmission method based on Delta-Sigma modulation is characterized by comprising the following steps:
respectively carrying out Delta-Sigma modulation on the two paths of analog signals to convert the two paths of analog signals into digital bit streams which are respectively marked as a first OOK signal and a second OOK signal; combining the first OOK signal and the second OOK signal respectively as MSB and LSB to generate PAM4 signal, and converting the PAM4 signal into radio frequency signal for output;
after the radio frequency signal is subjected to electro-optical conversion, the radio frequency signal is transmitted from one end of an optical fiber link to the other end of the optical fiber link by using a single wavelength, and is reduced into a radio frequency signal through the electro-optical conversion;
at the RRU end, separating MSB and LSB in the restored radio frequency signal to restore a first OOK signal and a second OOK signal; and respectively filtering out the out-of-band quantization noise in the two OOK signals to recover the original two analog signals, and respectively transmitting the two analog signals to the two RRUs for wireless transmission by the RRUs.
2. The Delta-Sigma modulation based digital mobile forward method of claim 1, wherein before combining the first OOK signal and the second OOK signal as MSB and LSB, respectively, to generate the PAM4 signal, further comprising:
adjusting the amplitude alpha of the first OOK signal and/or the amplitude beta of the second OOK signal according to the signal quality requirements of two paths of analog signals at the RRU end in wireless transmission;
when the amplitude value alpha of the first OOK signal is larger, the signal-to-noise ratio of the LSB signal is transferred to the MSB signal; when the amplitude β of the second OOK signal is large, the signal-to-noise ratio of the MSB signal shifts to the LSB signal.
3. The digital mobile forward method based on Delta-Sigma modulation according to claim 1 or 2, characterized in that Gray coding is adopted when the first OOK signal and the second OOK signal are respectively taken as MSB and LSB to be combined;
and, separating the MSB and LSB in the restored radio frequency signal, including:
for MSB signals, when the detected signal level value is greater than level V1Sum level V2When the average value is equal to the first OOK signal, outputting +1, otherwise, outputting-1, and thus restoring to obtain the first OOK signal;
for LSB signals, when the detected signal level value is less than the level V2Sum level V3Is greater than the level V0Sum level V1When the average value is smaller than the first threshold value, outputting +1, otherwise, outputting-1, and thus restoring to obtain a second OOK signal;
wherein, V0～V3Is the average value of each level sample in the PAM4 signal, and V0<V1<V2<V3。
4. A digital mobile forwarding device based on Delta Sigma modulation, comprising: the system comprises a centralized BBU pool, an optical fiber transmission module and an RRU end;
the centralized BBU pool comprises a first Delta-Sigma modulator, a second Delta-Sigma modulator, a layered modulator and a radio frequency output module; the first Delta-Sigma modulator and the second Delta-Sigma modulator are used for respectively carrying out Delta-Sigma modulation on the two paths of analog signals so as to convert the two paths of analog signals into digital bit streams which are respectively marked as a first OOK signal and a second OOK signal; the two input ends of the hierarchical modulator are respectively connected to the output ends of the first Delta-Sigma modulator and the second Delta-Sigma modulator, and the hierarchical modulator is used for combining the first OOK signal and the second OOK signal respectively as MSB and LSB to generate a PAM4 signal; the input end of the radio frequency output module is connected to the output end of the layered modulator, and the radio frequency output module is used for converting the PAM4 signal into a radio frequency signal to be output;
the optical fiber transmission module comprises an electro-optical conversion unit, an optical fiber link and a photoelectric conversion unit; the input end of the electro-optical conversion unit is connected to the output end of the radio frequency output module, and the electro-optical conversion unit is used for performing electro-optical conversion on the radio frequency signal to obtain an optical signal; the two ends of the optical fiber link are respectively connected with the electro-optical conversion unit and the photoelectric conversion unit, and the optical fiber link is used for transmitting the optical signal to the photoelectric conversion unit by using a single wavelength; the photoelectric conversion unit is used for performing photoelectric conversion on the optical signal transmitted by the optical fiber link so as to restore the optical signal into a radio frequency signal;
the RRU end comprises a judgment module, a first filter and an RRU connected with the first filter, and a second filter and an RRU connected with the second filter; the input end of the judgment module is connected to the output end of the photoelectric conversion unit, and the judgment module is used for separating the MSB and the LSB in the restored radio frequency signal to restore a first OOK signal and a second OOK signal; the first filter and the second filter are respectively connected with two output ends of the decision module, and are used for filtering out-of-band quantization noise of the two OOK signals to recover the original two analog signals, and respectively transmitting the two analog signals to two RRUs for wireless transmission by the RRUs.
5. The Delta-Sigma modulation-based digital mobile fronthaul device of claim 4, wherein the layered modulator is further configured to adjust the amplitude α of the first OOK signal and/or the amplitude β of the second OOK signal according to the signal quality requirement of the two analog signals at the RRU end in wireless transmission before combining the first OOK signal and the second OOK signal as the MSB and the LSB, respectively, to generate the PAM4 signal;
when the amplitude value alpha of the first OOK signal is larger, the signal-to-noise ratio of the LSB signal is transferred to the MSB signal; when the amplitude β of the second OOK signal is large, the signal-to-noise ratio of the MSB signal shifts to the LSB signal.
6. The digital mobile forward-transmission device based on Delta-Sigma modulation according to claim 4 or 5, characterized in that the layered modulator combines the first OOK signal and the second OOK signal as MSB and LSB respectively to generate PAM4 signal by Gray coding;
and, the manner of separating the MSB and the LSB of the restored rf signal by the decision module includes:
for MSB signals, when the detected signal level value is greater than level V1Sum level V2When the average value is equal to the first OOK signal, outputting +1, otherwise, outputting-1, and thus restoring to obtain the first OOK signal;
for LSB signals, when the detected signal level value is less than the level V2Sum level V3Is greater than the level V0Sum level V1When the average value is smaller than the first threshold value, outputting +1, otherwise, outputting-1, and thus restoring to obtain a second OOK signal;
wherein, V0～V3Is the average value of each level sample in the PAM4 signal, and V0<V1<V2<V3。
7. The digital mobile fronthaul device according to claim 4 or 5, wherein the centralized BBU pool is implemented by an FPGA, and the RF output module is an external I/O port of the FPGA.
8. The digital mobile forward-transfer device based on Delta-Sigma modulation according to claim 4 or 5, characterized in that in the first Delta-Sigma modulator and the second Delta-Sigma modulator, the transfer function NTF of the feedback structure for noise shaping in the z-domain is:
Figure FDA0003249185140000041
where a to f are coefficients of each node in the feedback structure, and a is 0.69, b is-3.33, c is-1.51, d is 0.17, e is-3.01, and f is-0.79.
9. The digital mobile fronthaul device according to claim 4 or 5, wherein the centralized BBU pool further comprises an analog signal processing unit for performing baseband and carrier modulation on the original signal to obtain an analog signal to be transmitted.
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