CN101895346B - Transmitter of OOFDM system and method for pre-compensating delay caused by optical fiber dispersion - Google Patents

Abstract

The invention discloses a transmitter of an OOFDM system and a method for pre-compensating delay caused by optical fiber dispersion of the transmitter. The pre-compensation method comprises receiving a plurality of pre-compensation values corresponding to the sub-carriers; and transmitting the subcarrier after delaying the time corresponding to the pre-compensation value. By estimating the delay time between each sub-carrier at the receiver end and setting the pre-compensation value of the transmitter according to the delay time. The transmitter delays the pre-compensation values when the sub-carriers are transmitted. Therefore, each sub-carrier can reach the receiver at the nearly consistent time, and the purpose of pre-compensating the delay caused by the fiber dispersion is achieved.

Description

Transmitter of OOFDM system and method for pre-compensating delay caused by optical fiber dispersion

technical field

The present invention relates to a transmitter and method for pre-compensating delay caused by optical fiber dispersion in a system using Optical Orthogonal Frequency-division Multiplexing (OOFDM) technology.

Background technique

The Orthogonal Frequency-division Multiplexing (OFDM) system is a Frequency-division Multiplexing (FDM) system that adopts a digital multi-carrier modulation (Digital multi-carrier modulation) method. Multiple orthogonal sub-carriers (orthogonal sub-carriers, also referred to as sub-bands or sub-carriers) are used to transmit data. These data are cut into multiple parallel data streams (data streams) or channels (Channels) corresponding to each sub-carrier. Each subcarrier is transmitted using a quadrature modulation technique with a low symbol rate. In this way, more total data rates (total data rates) than conventional single-carriers can be obtained within the same bandwidth.

Please refer to FIG. 1A and FIG. 1B , which are schematic diagrams showing the spectrum distribution comparisons of conventional direct transmission and OFDM transmission respectively. The biggest difference between direct transmission and OFDM transmission lies in the distribution of bandwidth. FIG. 1A shows that the bandwidth occupied by direct transmission is f ₀ . If the bandwidth f ₀ is subdivided into equal-width quintiles in the manner of OFDM. Each sub-band (ie, the aforementioned sub-carriers) is orthogonal to each other, and the new spectrum distribution will be as shown in FIG. 1B . In OFDM transmission, as long as there are enough sub-frequency bands, basically for each sub-frequency band, the frequency response of the frequency band can be roughly regarded as flat. That is to say, only one equalizer with a single coefficient is needed for each sub-band to overcome the attenuation and phase distortion of each sub-channel. In addition, since the data rate transmitted by each sub-band is much lower than the original data rate transmitted directly, the operating frequency of the equalizer is naturally reduced in proportion.

Orthogonal frequency division multitasking technology is applied in the field of wireless communication, and the problem often encountered is the multi-path effect (multi-path effect). The multipath effect will lead to time delay spreading (time-spreading) and inter-symbol interference (inter-symbol interference, ISI) problems. This is the so-called Frequency-selective channel. This frequency selectivity problem is usually solved by adding a guard interval (Guard interval) to each OFDM symbol (or code, Symbol). This will increase the symbol period and occupy the bandwidth used to transmit data.

When the OFDM technology is applied to the optical communication system, since the light is transmitted in the same optical fiber, the multipath effect of the OFDM system is not significant, but it will be caused by the optical fiber chromatic dispersion (Chromatic Dispersion) This phenomenon causes problems of inter-channel synchronization and inter-symbol interference similar to multiple paths when receiving signals at the receiving end.

Optical fiber dispersion makes the transmission speed of high-frequency optical signals slower than that of lower-frequency optical signals when optical signals are transmitted on optical fibers. In the OFDM technology, since each subcarrier is transmitted at a different frequency, although the transmitting end sends each subcarrier at the same time, the received subcarriers at the receiving end arrive at different times. . This is the so-called group delay (Group delay) phenomenon.

Regarding the group delay phenomenon, please refer to FIG. 2A and FIG. 2B , which are schematic diagrams of signals transmitted and received by the transmitting end and the receiving end of the OFDM system, respectively. 2A and 2B are schematic diagrams showing OFDM signals in the frequency domain. The horizontal axis in the figure is time, and the vertical axis is frequency. Each subcarrier (subband) uses one frequency band. Taking Figure 2A as an example, each subcarrier uses the bandwidth of df in the illustration, the frequency band (band) of the first subcarrier is M*df, the next one is (M+1)*df, and so on. Shown in the figure is the data of only one symbol on each subcarrier. In actual transmission, consecutive symbols are sent and received. FIG. 2A is a schematic diagram of symbols transmitted by the transmitter. It can be seen from the figure that the start time of each symbol at the transmitting end is the same. Since the duration of each symbol is the same, the end time of each symbol at the transmitting end is also the same. Next, please refer to FIG. 2B , which is a schematic diagram of the signal received by the receiving end after the OFDM signal is transmitted through the optical fiber. It can be seen from the figure that the subcarriers with lower frequencies (i.e. the subcarriers located higher on the frequency axis) are received earlier, while the subcarriers with higher frequencies (i.e. the subcarriers located lower on the frequency axis) are received earlier. Arrived late. This is the so-called Group Delay's Variation/Dispersion mentioned above.

In order to solve the problem of group delay variation, operators add a guard interval (Guard Interval) to each symbol of a subcarrier. The guard interval can be a cyclic prefix or a cyclic postfix. With the setting of the guard interval, when the receiving end retrieves data within the same time interval (ie, the original symbol length plus the length of the guard interval), the data of each subcarrier will be complete. Only after judging the starting point, it can be decoded.

Although the guard interval can solve the problem of group delay variation caused by fiber dispersion, the more serious the fiber dispersion is, the longer the additional guard interval is required. However, the longer the protection interval is, the more bandwidth it occupies, which means that the bandwidth available for data transmission is reduced. The group delay caused by dispersion is proportional to the distance transmitted in the fiber and the frequency difference between subcarriers. That is to say, when the total bandwidth is larger or the transmission distance is longer, the group delay variation is larger and the protection interval is longer, so that this problem can be solved.

Communications usually have at least 6% of the communication bandwidth in the additional protection zone due to optical fiber dispersion. If the communication bandwidth is deducted from the bandwidth occupied by the pilot carrier (scatter pilot, preamble or mid-amble) used as channel response (channel response) estimation and the bandwidth of the control signal, it can be used to transmit data The bandwidth will be further reduced.

Contents of the invention

In view of the fact that the aforementioned protection interval needs to be increased due to the increase of the transmission distance, thereby occupying the communication bandwidth, the present invention proposes a transmitter of an OOFDM system and a method for pre-compensating the delay caused by optical fiber dispersion to effectively shorten the protection interval , to increase the bandwidth that can transmit data.

The method for pre-compensating the delay caused by optical fiber dispersion proposed by the invention is suitable for optical orthogonal frequency division multiple task transmitters. The transmitter transmits an optical signal with multiple subcarriers. Each subcarrier has a carrier frequency and the carrier frequency of the subcarrier is different, the method includes: receiving a plurality of precompensation values, the precompensation value is corresponding to the subcarrier; and delaying the subcarrier corresponding to the subcarrier The precompensation value is transmitted after the time.

The multi-subcarrier signal generator proposed by the present invention is suitable for a transmitter of an optical OFDM system. A serial-to-parallel component of the transmitter converts and maps a digital serial signal into multiple parallel signals. The generator includes multiple time-domain signal modulators, multiple guard interval additional elements, multiple delay units, and a combiner. The time-domain signal modulators correspond to the parallel signals in a one-to-one relationship. And each of the time-domain signal modulators generates a time-domain signal according to the parallel signal corresponding thereto. Guard interval add-ons correspond to the time-domain signal modulators in a one-to-one relationship. Each guard interval adding element respectively adds a guard interval to the time-domain signal generated by the corresponding time-domain signal modulator to become an added signal. Delay cells correspond to the guard interval additional elements in a one-to-one relationship. The delay units respectively have a predetermined delay value. And each of the delay units transmits the added signal generated by the corresponding guard interval additional element after delaying the predetermined delay value. The combiner combines the added signal transmitted by the delay unit to output a combined signal.

The transmitter of the optical OFDM system proposed by the present invention converts a digital sequence signal into an optical signal and then transmits it. The transmitter includes a serial-to-parallel component, a multi-subcarrier signal generator, a digital-to-analog component, and an electrical-to-optical component. The serial-to-parallel component converts and maps the digital serial signal into multiple parallel signals. The multi-subcarrier signal generator generates a plurality of corresponding time-domain signals according to the parallel signal, and the multi-subcarrier signal generator further delays the time-domain signal according to a plurality of predetermined delay values corresponding to the parallel signal. The delayed time domain signals are combined into a combined signal. The digital-to-analog component converts the combined signal into an analog signal. The electro-optic element converts the analog signal into the optical signal.

By means of the precompensation method, multi-subcarrier signal generator, and optical OFDM transmitter proposed by the present invention, the delay caused by dispersion can be properly precompensated at the transmitter end, and each subcarrier After the carrier is transmitted through the optical fiber, it can reach the receiving end at a fairly consistent time.

The present invention will be described in detail below in conjunction with the accompanying drawings and specific embodiments, but not as a limitation of the present invention.

Description of drawings

FIG. 1A and FIG. 1B are schematic diagrams comparing spectrum distributions between existing direct transmission and OFDM transmission;

2A and 2B are schematic diagrams of signals transmitted and received by the transmitting end and the receiving end of the existing optical OFDM system, respectively;

3 is a schematic diagram of a system architecture of a transmitter and a receiver of an optical orthogonal multitasking system according to an embodiment of the present invention;

4 is a schematic circuit block diagram of a multi-subcarrier signal generator of a transmitter of an optical OFDM system according to an embodiment of the present invention;

FIG. 5A is a frequency domain schematic diagram of delayed time domain signals transmitted from each delay unit of the transmitter according to an embodiment of the present invention;

FIG. 5B is a frequency domain schematic diagram of the delayed time domain signal of FIG. 5A being received by the receiver;

6 is a schematic flowchart of a method for precompensating delay caused by fiber dispersion according to an embodiment of the present invention;

Fig. 7 is a schematic flow chart of the method before the pre-compensation method of the embodiment of the present invention in Fig. 6;

FIG. 8 is a schematic diagram of a method flow in step S72 according to an exemplary embodiment of the present invention;

FIG. 9 is a schematic diagram of a method flow in step S722 according to an exemplary embodiment of the present invention;

FIG. 10A and FIG. 10B are schematic diagrams of signals at the receiving end without adopting the embodiment of the present invention and adopting the method of the embodiment of the present invention, respectively;

11 is a schematic circuit block diagram of another embodiment of the multi-subcarrier signal generator of the transmitter of the OFDM system according to the embodiment of the present invention.

Among them, reference signs

10 Teleporter

12 Serial-to-parallel components

14, 24 Multi-subcarrier signal generator

138a, 138b, 238a, 238b are additional signals

139a, 139b, 239a, 239b Protection zone additional elements

140, 141, 240, 241 Time domain signal modulator

142a, 242a Real part lookup table

142b, 242b Imaginary part look-up table

143, 144, 243, 244 time domain signal

145, 146, 245, 246 delay unit

147a, 247a The first multi-tasker

147b, 247b Second multiplexer

147c Phase Inverter

148 Combiner

149, 249 multitasking group

16 Digital to Analog Components

18 Electro-optical components

247c The third multi-tasker

247d Fourth multi-tasker

247e, 247f Phase Inverter

247g, 247h Signal Amplifier

247m Second coupler

248 The first coupler

60 receivers

62 optical receivers

64 Analog to digital components

66 synchronous components

68 Fast Fourier Transform Elements

69 Dispersion monitoring and equalization components

80 fiber

82 optical amplifier

84 control channels

90 digital sequence signal

91 parallel signal

910 real part

912 imaginary part

92 Merge signals

93 Analog signal

94 optical signal

95 Analog electrical signals

96 first digital signal

97 Second digital signal

98 third digital signal

99 Decoded signal

Detailed ways

First, please refer to FIG. 3 , which is a schematic diagram of the system architecture of the transmitter 10 and the receiver 60 of the optical orthogonal multiplexing system according to an embodiment of the present invention. It can be seen from the figure that the transmitter 10 converts a digital sequence signal 90 into an optical signal 94 and then transmits it. The optical signal 94 is received by the receiver 60 after being transmitted by the optical fiber 80 and amplified by the optical amplifier 82 .

Wherein, the optical signal 94 is an OFDM optical signal 94 . The aforementioned optical amplifier 82 may be, but not limited to, an erbium-doped fiber amplifier. The aforementioned optical fiber 80 is but not limited to a single-mode optical fiber (Singlemode fiber).

After receiving the optical signal 94 , the receiver 60 estimates the total delay time of the group delay caused by the dispersion of the optical signal 94 , and then sends the total delay time back to the transmitter 10 . The total delay time can be sent back to the transmitter 10 via the control channel 84 (Control channel) used for communication between the transmitter 10 and the receiver 60 . Or send it back to the transmitter 10 in other ways. Although the aforementioned control channel 84 is illustrated in a different manner from the optical fiber 80 in the figure, it may actually be a channel in optical fiber communication. In addition, a manual setting method can also be used, for example, after the total delay time of the group delay is measured or estimated at the receiver 60, the total delay time is manually set at the transmitter 10 .

The aforementioned transmitter 10 includes a serial-to-parallel component 12 , a multi-subcarrier signal generator 14 , a digital-to-analog component 16 , and an electrical-to-optical component 18 . The serial-to-parallel component 12 converts and maps the digital serial signal 90 into a plurality of parallel signals 91 . The aforementioned total delay time is converted into a plurality of predetermined delay values. Each predetermined delay value corresponds to the aforementioned parallel signal 91 . How to convert the total delay time into a predetermined delay value will be described in detail later.

The multi-subcarrier signal generator 14 generates a plurality of time-domain signals 143, 144 (see FIG. 4 ) respectively corresponding to each subcarrier according to the parallel signal 91. The multi-subcarrier signal generator 14 is also based on the parallel signal 91 A plurality of predetermined delay values (or called pre-compensation values) corresponding to the signal 91 delay the time domain signals 143 , 144 and combine the delayed time domain signals 143 , 144 into a combined signal 92 . The digital-to-analog component 16 converts the combined signal 92 into an analog signal 93 . The electro-optic element 18 converts this analog signal 93 into the aforementioned optical signal 94 . The electro-optic element 18 may be, but not limited to, a laser (Laser or DML, directly-modulated laser).

The serial-to-parallel component 12 first cuts the digital serial signal 90 to be transmitted into multiple parallel data streams. Each parallel data stream is then converted into the aforementioned parallel signal 91 by a low symbol rate modulation technique. The aforementioned modulation technology may be, but not limited to, Quadrature Amplitude Modulation (QAM, Quadrature Amplitude Modulation) or Phase Shift Modulation (also known as Phase Shift Keying, PSK, Phase Shift Keying).

The multi-subcarrier signal generator 14 delays the start time of each time domain signal 143 , 144 being sent according to the predetermined delay value of the parallel signal 91 . Please refer to FIG. 4 for the architecture of the multi-subcarrier signal generator 14 . It is a schematic circuit block diagram of the multi-subcarrier signal generator 14 of the transmitter 10 of the OFDM system according to an embodiment of the present invention.

It can be seen from the figure that the multi-subcarrier signal generator 14 includes a plurality of time domain signal modulators (timedomain modulated waveform generator) 140, 141, a plurality of guard interval additional elements (Guard Interval Adding Element) 139a, 139b, a plurality of delay Units 145 , 146 , and an adder 148 . The time-domain signal modulators 140 , 141 correspond to the parallel signal 91 in a one-to-one relationship. The time-domain signal modulators 140 and 141 convert the parallel signal 91 belonging to the frequency domain into time-domain signals 143 and 144 belonging to the time domain. Thereafter, the guard interval addition elements 139a, 139b correspond to the time-domain signal modulators 140, 141 and the delay units 145, 146 in a one-to-one relationship. The guard interval adding elements 139a, 139b add guard intervals to the time domain signals 143, 144 to become GI-added signals 138a, 138b. Each delay unit 145, 146 transmits the added signal 138a, 138b generated by the corresponding guard interval attachment element 139a, 139b after delaying the predetermined delay value. In this way, although each parallel signal 91 is simultaneously received by the multi-subcarrier signal generator 14, after being adjusted by the multi-subcarrier signal generator 14, the delay units 145, 146 will be delayed one by one by the additional signals 138a, 138b It is transmitted after a predetermined delay value. The combiner 148 combines the added signals 138 a , 138 b transmitted by the delay units 145 , 146 to output the combined signal 92 .

The aforementioned delay unit 145, 146 may be a digital delay unit. It may be but not limited to adjustable digital delay (variable digital delay), FIFO-based first-in-first-out based variable digital delay (FIFO-based first-in-first-out based variable digital delay).

For the purpose and calculation of the group delay and the predetermined delay value, please refer to FIG. 2A again. It can be seen from experiments that when the frequency difference between each subcarrier (group) is a fixed value (that is, the difference df bandwidth as shown in the figure), and the transmission environment of the optical signal 94 does not change, the aforementioned group Group latency is linear. An example is given below to illustrate. Assuming that the OFDM signal in Example 1 has 8 subcarriers in total, the total delay time (caused by dispersion) from the first subcarrier received by the receiver 60 to the last subcarrier (caused by dispersion) is 0.7 ns (nanoseconds) , 1x10 ^-9 sec). Under this condition, the group delay between each sub-carrier is 700 ps (1×10 ⁻¹² seconds) divided by (8-1) (since there are seven carrier intervals in total), which equals 100 ps. That is to say, although each subcarrier (group) is transmitted simultaneously, adjacent subcarriers (groups) are received sequentially with an interval of 100 ps, as shown in FIG. 2B .

In order to solve this problem, in the embodiment of the present invention, before the transmitter 10 transmits the optical signal 94 , each subcarrier is pre-compensated according to its expected delay time. When each subcarrier (group) is transmitted, the transmission is delayed according to the predetermined delay time. Subcarriers with adjacent frequencies will be transmitted sequentially. The time interval of the sequential transmission is equal to the group delay difference time between the above subcarriers (continuing from the first example above, which is 100 ps). In this way, the receiver 60 will receive the subcarriers arriving almost simultaneously. Solve the problem of group delay.

Due to dispersion, the optical signal 94 with a higher frequency is transmitted slower than the optical signal 94 with a lower frequency in the optical fiber 80 , therefore, the subcarriers with a higher frequency will be transmitted first. That is, subcarriers with higher frequencies are transmitted earlier. Please refer to FIG. 5A , which is a frequency domain schematic diagram of the delayed time domain signals 143 , 144 transmitted from the delay units 145 , 146 of the transmitter 10 according to the present invention. The horizontal axis in the graph is time and the vertical axis is frequency. The lower the vertical axis is on the graph, the higher the frequency. FIG. 5A takes four subcarriers as an example. It can be seen from the figure that the subcarriers with higher frequency are transmitted earlier. Subcarriers with lower frequencies are transmitted later. There is a unit delay time between adjacent subcarriers.

The above unit delay time is preferably an integral multiple of the unit sampling time (Sampling time). Continuing with the above example 1, it is assumed that the unit sampling time is 40 ps (equal to a sampling rate of 25 GHz). Each sampling point is called a point. The aforementioned unit sampling time (40 ps) is separated between the sampling points. The first subcarrier has the lowest carrier frequency and the fastest transmission speed. The eighth subcarrier has the highest carrier frequency and the slowest transmission speed. The frequency band width between each sub-carrier is the same (that is, the aforementioned frequency difference is the same). Therefore, the delay time for pre-compensation at the transmitting end 10 according to the received delay difference between subcarriers before compensation is as follows (in this example, the number of unit delay points is equal to the unit sampling time, which is 40 ps). The field of "before compensation, delay time when receiving" in the table below refers to that the transmitter 10 transmits each subcarrier at the same time, and when the receiver 60 receives each subcarrier, it compares the received time of each subcarrier with the The resulting delay time compared to the first received subcarrier. Taking Table 1 as an example, the first subcarrier will be received first, and the rest will be received sequentially, so this column is the delay time obtained by comparing other subcarriers with the first subcarrier.

Table 1 Delay schedule for pre-compensation

subcarrier number Before compensation, the delay time when receiving Predetermined delay value (precompensation value) Delay points (precompensation) The first subcarrier 0ps 720ps 18:00 The second subcarrier 100ps 600ps 15 o'clock The third subcarrier 200ps 520ps 13:00 The fourth subcarrier 300ps 400ps 10 O'Clock The fifth subcarrier 400ps 320ps 8 o'clock The sixth subcarrier 500ps 200ps 5 o'clock The seventh subcarrier 600ps 120ps 3 points

The eighth subcarrier 700ps 0ps 0 points

It can be known from Table 1 that the precompensation values of the second, fourth, sixth, and eighth subcarriers are set as the subcarrier and the eighth subcarrier (the eighth subcarrier is the subcarrier with the highest carrier frequency) reach the receiver 60 time difference, and the first, third, fifth and seventh subcarriers, because their "time difference between subcarriers" cannot be divisible by the unit sampling time, it is the multiple of the unit sampling time closest to the time difference between the subcarriers . For example, although the time difference between the first subcarrier and the eighth subcarrier is 700 ps, it is not divisible by the unit sampling time, so 720 ps is used as the delay time for delayed transmission. Of course, 680ps can also be used. And so on for the rest.

Taking the example in Table 1, the second, fourth, sixth and eighth subcarriers will be received by the receiver 60 at the same time, while the first, third, fifth and seventh subcarriers will be received 20 ps later. Therefore, the protection interval can be reduced from 700ps without using the technology of the present invention to 40ps using the present invention (because 20ps is less than 1 sampling point, the receiver 60 decodes with sampling points, so 20ps also uses 1 sampling point count). Significantly reduces the bandwidth occupied by the guard zone. If the time length of each symbol in this example one is 128 points (i.e. 128x40ps=5120ps time length), then the traditional protection interval is delayed by 18 points, and the traditional protection interval accounts for 12.8% of the sum of the symbol and the protection interval (18/(18 +128)=12.8%) bandwidth. If the technology of the present invention is used as an example, the guard interval of the present invention only accounts for 0.8% of the total bandwidth (1/(1+128)=0.77%).

From the above-mentioned precompensation value and the carrier frequency of each subcarrier, it can be known that the precompensation value of the subcarrier with a higher carrier frequency is smaller than the precompensation value of the subcarrier with a lower carrier frequency value.

In the foregoing example 1, the precompensation values of each subcarrier are different. But in actual application, it is not limited to this. If the delayed arrival time between the three subcarriers is less than one sampling point (time), the three subcarriers need to be delayed by the same number of points.

In addition, the above description of FIG. 5A is continued. The delayed time domain signals 143 , 144 of FIG. 5A after being combined and sent out as an optical signal 94 . The optical signal 94 travels through the optical fiber 80 to the receiver 60 . After the signal received by the receiver 60 undergoes fast Fourier transform (details will be described later), please refer to FIG. 5B for a schematic diagram of the frequency domain of the transformed signal. It can be seen from the figure that since the delay caused by dispersion has been pre-compensated by the transmitter 10 before transmission, the subcarriers of the signal received by the icon arrive almost simultaneously without delay. Simultaneous arrival here is not a limiting condition, it only means that the small time difference of arrival of each subcarrier will not cause decoding problems relative to the system.

With regard to the technology for precompensating the group delay phenomenon caused by the fiber optic dispersion proposed in the embodiment of the present invention, the aforementioned precompensation value can be set when the system construction is completed. Among them, unless the hardware is changed or the optical fiber path is replaced, the variation of the dispersion is not large, that is to say, this pre-compensation method is quite suitable for the optical orthogonal frequency division multitasking system.

The sub-carriers in the aforementioned example 1 take the same frequency band and spacing as an example. If the frequency band spacing is different, the present invention can also be used, as long as the delay time required for each sub-carrier is adjusted separately.

Please refer to FIG. 4 again for the detailed structure example of the time-domain signal modulators 140 and 141 . As can be seen in the figure, each time-domain signal modulator 140, 141 includes a real part look-up table 142a (look up table for real part), an imaginary part look-up table 142b (look up table for imaginary part), and a multitasking group 149. The real part look-up table 142a has a plurality of real part basic waveforms. The imaginary part look-up table 142b has a plurality of phase values. The multiplexer group 149 looks up the time domain signals 143 , 144 according to the parallel signal 91 corresponding to the time domain signal modulators 140 , 141 to the real part lookup table 142 a and the imaginary part lookup table 142 b.

The aforementioned real part look-up table 142a stores a plurality of basic waveforms, while the imaginary part look-up table 142b stores a plurality of phases. Each of the aforementioned parallel signals 91 has a real part 910 (real part) and an imaginary part 912 (imaginary part). According to the real part 910 and the imaginary part 912 of the parallel signal 91 , the multiplexer group 149 looks up the corresponding basic waveform and phase in the real part look-up table 142 a and the imaginary part look-up table 142 b corresponding to the parallel signal 91 .

The time domain signal modulators 140 and 141 in FIG. 4 are quadrature phase shift modulation (QPSK, QuadraturePSK) as an example. The quadrature phase offset modulated signals include four types: (0,0), (0,1), (1,1) and (1,0). That is to say, the first number in the brackets is the real part 910 of the parallel signal 91 (generally represented by I), and the second number is the imaginary part 912 of the parallel signal 91 (generally represented by Q). The multiplexer group 149 receives the real part 910 and the imaginary part 912 of the parallel signal 91, and selects the corresponding look-up table 142a, 142b accordingly. The multiplexer group 149 looks up a corresponding basic waveform according to the real part 910 of the parallel signal 91 to the real part look-up table 142a. The multiplexer group 149 looks up a corresponding phase value according to the imaginary part 912 of the parallel signal 91 to the imaginary part look-up table 142b. Then, the multiplexer group 149 outputs the time-domain signal according to the detected basic waveform and phase value.

The multiplexer group 149 has a first multiplexer 147a, a second multiplexer 147b and a phase inverter 147c (Inverter). The first multiplexer 147a receives the real part 910 of the parallel signal 91 and is used to select whether to output from the real part look-up table or the imaginary part look-up table. If the value of the real part 910 of the parallel signal 91 is 1, then the imaginary part look-up table 142b is used for output. On the contrary, if the value of the real part 910 of the parallel signal 91 is 0, then the real part look-up table 142a will make an output. Next, the output of the first multiplexer 147a is divided into two signals, one of which is connected to the input end of the second multiplexer 147b representing "0" after passing through the aforementioned phase inverter 147c. And the other signal is directly connected to the input representing "1" of the second multiplexer 147b.

In this implementation example, the phase inverter 147c is used as a phase conversion element, but if it is applied to a 16QAM system, the phase inverter 147c needs to be replaced with a phase converter (Phase Converter) or a signal amplifier, and other circuits Then modify it accordingly.

The second multiplexer 147b receives the imaginary part 912 of the parallel signal 91 and is used to select one of the two signals branched from the first multiplexer 147a as the output of the second multiplexer 147b. If the value of the imaginary part 912 of the parallel signal 91 is 1, the output of the first multiplexer 147a directly becomes the output of the second multiplexer 147b. Conversely, if the value of the imaginary part 912 of the parallel signal 91 is 0, the output of the second multiplexer 147b is the output signal of the phase inverter 147c. In this way, the time-domain signal modulators 140 and 141 can properly convert the parallel signal 91 into a basic waveform with a phase according to the real part 910 and the imaginary part 912 .

The aforementioned basic waveforms with phases are output to delay units 145 , 146 . As mentioned above, each delay unit 145, 146 is set with a pre-compensation value respectively. One precompensation value corresponds to one delay unit 145,146. For the setting of the pre-compensation value, take the digital delay unit as an example, and use the aforementioned points as the calculation unit. That is to say, it is set by delaying N sampling points. The delay units 145, 146 may include a plurality of shift registers (Shift Registers, such as shift registers with a length of N) and a controller. When the delay unit 145, 146 needs to be delayed by a point, the controller is set so that when the delay unit 145, 146 is triggered once, the signal input to the delay unit is moved into a shift register, and then moved out when triggered again. Similarly, if it is necessary to delay by two points, the controller is set so that the signals input to the delay units 145 and 146 are shifted out after passing through two shift registers, thereby achieving the purpose of delaying by two points.

Therefore, the aforementioned basic waveforms with phases are output to the delay units 145, 146, and the delay units 145, 146 delay the corresponding "basic waveforms with phases" according to the set pre-compensation value before outputting. The combiner 148 combines the signals output by the delay units 145 , 146 into the combined signal 92 .

The aforementioned time-domain signal modulators 140 and 141 are quadrature phase shift modulation (QPSK) as an example, but not limited thereto. If the signal to be converted by the time-domain signal modulator is quadrature amplitude modulation (QAM), the multiplexer group 149 will include and select elements for amplifying the amplitude. That is to say, the time-domain signal modulators 140 and 141 can select the corresponding basic waveform, phase and amplitude according to the parallel signal 91 .

The above correspondences between the time-domain signal modulators 140 and 141 , the multiplexer group 149 , the parallel signal 91 , the precompensation value and the subcarrier are all one-to-one. That is, each subcarrier corresponds to a parallel signal 91 , a multiplexer group 149 , a precompensation value, and a time domain signal modulator 140 . The real part look-up table 142 a and the imaginary part look-up table 142 b of each time-domain signal modulator 140 are respectively corresponding to the real part 910 and the imaginary part 912 of the parallel signal 91 .

Please refer to Figure 3 again. The aforementioned receiver 60 includes an optical receiver 62 (optical sensor), an analog to digital converter 64 (analog to digital converter), a synchronous element 66 (Synchronizer), a fast Fourier transform element 68 (Fast Fourier Transferring element), and a dispersion monitor and the like Component 69 (Dispersion monitor and equalizer QAM demodulator).

The optical receiver 62 receives the aforementioned optical signal 94 and converts it into an analog electrical signal 95 . The analog electrical signal 95 is converted into a first digital signal 96 via the analog-to-digital component 64 . Next, the synchronization element 66 estimates the symbol boundary of the first digital signal 96 and removes the guard interval of the first digital signal 96 to form the second digital signal 97 . The fast Fourier transform element 68 performs fast Fourier transform on the second digital signal 97 to become a parallel third digital signal 98 belonging to the frequency domain. The dispersion monitoring and equalization component 69 estimates whether there is still a dispersion delay between the subcarriers in the third digital signal 98, and if there is still a dispersion delay, it can be sent back to the transmitter 10 by using the control channel 84 as shown in the figure for further processing. compensate. Then the dispersion monitoring and equalization element 69 decodes the third digital signal 98 according to the frequency band of each sub-carrier and sends out a decoded signal 99 . In this way, the data transmitted from the transmitter 10 can be successfully decoded.

Again, please refer to Figure 6. It is a schematic flowchart of a method for pre-compensating delay caused by optical fiber dispersion according to an embodiment of the present invention. The precompensation method is suitable for an optical OFDM transmitter. The transmitter transmits an optical signal with multiple subcarriers. Each subcarrier has a carrier frequency and the carrier frequencies of the subcarriers are different. This precompensation method consists of:

Step S74: receiving a plurality of precompensation values corresponding to the subcarrier; and

Step S76: Delaying the subcarrier by the time corresponding to the precompensation value, and then sending it out.

The foregoing pre-compensation method is performed by the OFDM transmitter 10 . The carrier frequency of the aforementioned sub-carriers is the frequency band used by each of the aforementioned sub-carriers, that is, M*df or (M+1)*df as shown in FIG. 2A . The carrier frequency used by each subcarrier is different.

The pre-compensation value in step S74 may be the delay time corresponding to each subcarrier transmitted back from the receiver 60 via the control channel 84 in real time. It can also be manually set to the transmitter 10 after querying the receiver 60 . The transmitter 10 can execute the aforementioned method after receiving the pre-compensation value.

Step S76 is to delay each sub-carrier by its corresponding pre-compensation value (compensate for the delay time) as in the aforementioned manner of FIG. 5A . In this way, a signal as shown in FIG. 5B will be received at the receiver 60 .

For the pre-compensation method of the embodiment of the present invention, please refer to FIG. 7, before step S74, it may also include:

Step S71: receiving a delay value caused by fiber dispersion; and

Step S72: Calculate a plurality of precompensation values corresponding to the subcarrier according to the delay value.

The addition of these two steps takes into account the total delay time transmitted back from the receiver 60 which may be caused by fiber dispersion. That is, the total delay time between the highest frequency subcarrier and the lowest frequency subcarrier, rather than the delay time received by each subcarrier. Therefore, the transmitter 10 needs to perform the above steps to convert the total delay time into precompensation values corresponding to each subcarrier.

After step S71 receives the delay value sent back by the receiver 60 or the delay value manually input, step S72 converts the delay value into the pre-compensation value corresponding to each subcarrier. This delay value is the total delay time between the latest arriving subcarrier and the earliest arriving subcarrier.

See Figure 8. Step S72 further includes: step S720: estimating a delay time of the subcarrier respectively according to the delay value; and step S722: converting the delay time of the subcarrier into the precompensation value.

For the estimation of the aforementioned step S720, if the above example 1 is taken as an example, the delay value received by the transmitter 10 is 700 ps. Since the frequency spacing (or frequency band width, or carrier frequency spacing) between each subcarrier is the same, estimating a delay time of the subcarrier according to the delay value is to divide the delay value by the number of subcarrier intervals. For example 1, it means to divide 700ps by (8-1) to get 100ps. Next, the subcarrier with the lowest frequency (the first subcarrier) arrives first, so the delay time of the subcarrier with the lowest frequency is 0 ps. The delay time of each sub-carrier is calculated sequentially (from low frequency to high frequency) with an arithmetic progression. The delay times from the second to the eighth subcarriers are 100, 200, 300, 400, 500, 600, and 700 ps in sequence.

Next, step S722 is executed: converting the delay time of the subcarrier into the pre-compensation value. See Figure 9. This step S722 includes: S726: converting the delay time into a plurality of compensation times; and step S728: comparing the compensation time with an integer multiple of a sampling time, and setting the value closest to the integer multiple of the delay time as the precompensation value.

Step S726 is to convert the delay time into compensation time (ie, the time to delay sending). In the present invention, the subcarrier with the highest frequency is transmitted first, so as an example, the eighth subcarrier does not need to be delayed, and the first subcarrier needs to be delayed by 700 ps. That is to say, the compensation time required by each subcarrier is the total delay time (700 ps) minus the respective delay time. Therefore, the respective backoff times from the first subcarrier to the eighth subcarrier are 700, 600, 500, 400, 300, 200, 100 ps, respectively.

Then, considering that this is a digital delay, it needs to be synchronized with the sampling time, so step S728 needs to be executed to make the compensation time consistent with the multiple of the sampling time, that is, the value closest to the integer multiple of the delay time is set as the pre-compensation value. Taking the seventh subcarrier as an example, the compensation time is 100 ps, which is not an integer multiple of the sampling time, so 80 ps or 120 ps can be considered. The example in the above table is 120p.

By means of the above method, the delay caused by dispersion can be effectively pre-compensated at the transmitter 10, so that after the optical signal 94 is transmitted through the optical fiber 80, the subcarriers received at the receiver 60 are close to synchronization , and reduce the necessity for the receiver 60 to perform estimation.

Finally, in addition to the above example 1, the following example 2 is provided below to illustrate the improvement of the present invention in terms of efficacy compared with the prior art. The dispersion constant of the optical fiber used in Example 2 is 17 ps/nm/km (that is, the transmission of each nanometer wavelength per kilometer produces a dispersion delay of 17 ps). The frequency bandwidth is 25GHz, the sampling rate is 25GHz, and the points (size) of the fast Fourier transform are 128 points. The transmission distance is 1000 kilometers (km). Under this test condition, using the traditional method, since the wavelength difference of 1nm is approximately equal to the frequency difference of 125GHz, 25GHz is approximately equal to 0.2nm (ie 25/125). After 1000km of transmission, the group delay time of the highest and lowest frequencies is 3400ps (17ps/nm/km×1000km×0.2nm=3400ps). The sampling rate of 25GHz is 40ps/point. The protection interval will require 85 sampling points (3400ps/40ps=85points). This will increase the duration of each rune from 128 to 213 (85+128). In this way, the total transmission bandwidth occupied by the guard interval (also called overhead here) is 85/(85+128)=40%. That is to say, in the bandwidth of 25 GHz, 10 GHz is in the transmission protection interval (such as the cyclic prefix). If 100Gps (10 ⁹ bit/second) Ethernet Link is used in the future, 40Gps will be in the protection interval of transmission repetition.

On the contrary, using the method of the embodiment of the present invention in the second example above, it can be understood from the calculation description in the above table that only one point is needed for the protection interval to solve the problem of group delay variation. If a conservative approach is adopted, the protection interval is set to two points, so that the protection interval accounts for only about 1.5% of the total bandwidth (2/(2+128)=1.5%). Compared with the prior art, considerable bandwidth is saved.

Next, please refer to FIG. 10A and FIG. 10B , which are schematic diagrams of signals at the receiving end without using the present invention and using the method of the present invention, respectively. The test bandwidth of both is 10GHz (10x10 ⁹ Hz). Single-mode optical fiber transmission over a test distance of 1000 km. These two diagrams are the constellation diagrams of 16QAM received by the receiver 60 . The horizontal axis is the real part (I, also called sine), and the vertical axis is the imaginary part (Q, also called cosine). FIG. 10A is a constellation diagram without using the pre-compensation method of the present invention. Fig. 10B is a constellation diagram using the pre-compensation method of the present invention. It can be seen from FIG. 10A that each point area of the receiving end 60 on the constellation diagram is quite divergent (spread out), which will greatly increase the error rate after decoding. In contrast, using FIG. 10B of the embodiment example of the present invention, each dot area is quite concentrated, and it is obvious that the present invention has improved efficacy compared with the prior art.

Please refer to FIG. 11 for a circuit block diagram of another embodiment of the multi-subcarrier signal generator of the transmitter of the OFDM system according to the embodiment of the present invention. The multi-subcarrier signal generator 24 is adapted to generate 16QAM time domain signals 243,244.

The multi-subcarrier signal generator 24 of Fig. 11 generates a plurality of time-domain signals 243, 244 respectively corresponding to each subcarrier according to the parallel signal 91, and the multi-subcarrier signal generator 24 is also based on the corresponding signal 91 corresponding to the parallel signal 91. After delaying the time domain signals 243 , 244 , the delayed time domain signals 243 , 244 are combined into a combined signal 92 .

The multi-subcarrier signal generator 24 includes a plurality of time-domain signal modulators 240 , 241 , a plurality of guard interval additional elements 239 a , 239 b , a plurality of delay units 245 , 246 , and a first combiner 248 . The time-domain signal modulators 240 , 241 correspond to the parallel signal 91 in a one-to-one relationship. The time-domain signal modulators 240 and 241 convert the parallel signal 91 belonging to the frequency domain into time-domain signals 243 and 244 belonging to the time domain. Thereafter, the guard interval attachment elements 239a, 239b correspond to the time-domain signal modulators 240, 241 and the delay units 245, 246 in a one-to-one relationship. Guard interval addition elements 239a, 239b add guard intervals to time domain signals 243, 244 to become appended signals 238a, 238b. Each delay unit 245, 246 transmits the added signal 238a, 238b generated by the corresponding guard interval attachment element 239a, 239b after delaying the predetermined delay value. In this way, although each parallel signal 91 is simultaneously received by the multi-subcarrier signal generator 24, after being adjusted by the multi-subcarrier signal generator 24, the delay units 245, 246 delay the time-domain signals 243, 244 correspondingly It is transmitted after a predetermined delay value. The first combiner 248 combines the added signals 243 , 244 transmitted by the delay units 245 , 246 to output the combined signal 92 .

The time-domain signal modulators 240 , 241 include a real part look-up table 242 a , an imaginary part look-up table 242 b , and a multiplexer group 249 . The multiplexer group 249 looks up and converts the time domain signals 243 , 244 according to the parallel signal 91 corresponding to the time domain signal modulators 240 , 241 to the real part lookup table 242 a and the imaginary part lookup table 242 b.

Multiplexer group 249 comprises a first multiplexer 247a, a second multiplexer 247b, a third multiplexer 247c, a fourth multiplexer 247d, two phase inverters 247e, 247f, two signal amplifiers 247g, 247h, and a second coupler 247m. Wherein, the signal amplifiers 247g and 247h amplify the received signal. In this embodiment, the signal amplifiers 247g and 247h amplify the received signal three times before outputting.

The first multiplexer 247a and the second multiplexer 247b corresponding to the real part look-up table 242a combine the phase inverter 247e and the signal amplifier 247g, and look up the parallel signal 91 from the real part look-up table 242a according to the parallel signal 91 corresponding output signal. The output signal may be formed by looking up the real part look-up table 242a, then passing through the phase inverter 247e and the signal amplifier 247g, or may not pass through the phase inverter 247e and the signal amplifier 247g. It depends on the signals output from the parallel signal 91 to the first multiplexer 247a and the second multiplexer 247b. Similarly, the third multiplexer 247c and the fourth multiplexer 247d corresponding to the imaginary part look-up table 242b combine the phase inverter 247f and the signal amplifier 247h, and look up the imaginary part look-up table 242b according to the parallel signal 91. An output signal corresponding to the parallel signal 91 .

The second combiner 247m combines the output signals from the second multiplexer 247b and the fourth multiplexer 247d (action similar to addition) to form the aforementioned time domain signal 243 .

The parallel signal 91 in the frequency domain can be converted into 16QAM time domain signals 243 and 244 by the multi-subcarrier signal generator 24 in FIG. 11 .

Certainly, the present invention also can have other multiple embodiments, without departing from the spirit and essence of the present invention, those skilled in the art can make various corresponding changes and deformations according to the present invention, but these corresponding Changes and deformations should belong to the scope of protection of the appended claims of the present invention.

Claims (11)

1. the method for the caused delay of precompensation optical fiber dispersion, it is characterized in that, be applicable to an optical orthogonal frequency-division multitask conveyer, this conveyer is the light signal that transmission one has a plurality of subcarriers, this carrier frequency that each subcarrier has a carrier frequency and this subcarrier is different, and the method comprises:

Receive a plurality of pre-compensation value, this pre-compensation value is to should subcarrier;

This subcarrier is postponed send out after time of this corresponding with it pre-compensation value;

Wherein comprise before the step that receives a plurality of pre-compensation value:

The length of delay that reception is caused by optical fiber dispersion;

Calculate this pre-compensation value that should subcarrier according to this length of delay;

The step of wherein calculating this pre-compensation value that should subcarrier according to this length of delay comprises:

Estimate respectively a time of delay of this subcarrier according to this length of delay;

Be this pre-compensation value this time of delay of changing this subcarrier.

2. the method for the caused delay of precompensation optical fiber dispersion according to claim 1, is characterized in that, be that the step of this pre-compensation value comprises this time of delay of wherein changing this subcarrier:

Changing this time of delay is a plurality of make-up times;

Respectively should the make-up time and the integral multiple of a sample time compare, be set as this pre-compensation value with the integral multiple near sample time of this make-up time.

3. the method for the caused delay of precompensation optical fiber dispersion according to claim 2, it is characterized in that, this pre-compensation value that wherein has this subcarrier of this higher carrier frequency is this pre-compensation value less than this subcarrier with this lower carrier frequency.

4. multi-subcarrier signal generator, it is characterized in that, it is the conveyer that is suitable for an optical orthogonal frequency-division multitask system, this conveyer a string turns and element is that conversion and mapping one numerical sequence signal are a plurality of and column signal, respectively also column signal comprises a real part and an imaginary part, and this generator comprises:

A plurality of time-domain signal modulators, be with one-one relationship to signal side by side, respectively this time-domain signal modulator be according to it corresponding should and column signal produce a time domain signal;

Add ons between a plurality of guard plots, be with one-one relationship to should the time-domain signal modulator, respectively between this guard plot, add ons is to add respectively between a guard plot this time-domain signal that produces in this corresponding with it time-domain signal modulator and become one by additional signal;

A plurality of delay cells, to add ons between should the guard plot with one-one relationship, this delay cell has respectively a predetermined delay value, respectively this delay cell be postpone to transmit after this predetermined delay value between this corresponding with it guard plot, add ons produces this by additional signal;

One colligator is that this that transmit in conjunction with this delay cell exported a combined signal by additional signal.

5. multi-subcarrier signal generator according to claim 4, is characterized in that, wherein respectively this time-domain signal modulator comprises:

One real part look-up table has a plurality of real part basic waveforms;

One imaginary part look-up table has a plurality of phase values;

One multiplexer group is to consult out this time-domain signal according to also column signal to this real part look-up table and this imaginary part look-up table corresponding with this time-domain signal modulator.

6. multi-subcarrier signal generator according to claim 5, it is characterized in that, wherein this multiplexer group is to consult out this basic waveform of a correspondence to this real part look-up table according to this real part of this and column signal, this multiplexer group is consulted out the phase value of a correspondence according to this imaginary part of this and column signal to this imaginary part look-up table, and this multiplexer group is this basic waveform of finding according to this quilt and phase value and export this time-domain signal.

7. the conveyer of an optical orthogonal frequency-division multitask system, is characterized in that, this conveyer is to send out after a numerical sequence signal is converted to a light signal, and this conveyer comprises:

A string turning and element is that conversion and this numerical sequence signal of mapping are a plurality of and column signal;

One multi-subcarrier signal generator, be according to should and column signal and produce corresponding a plurality of time-domain signals, this multi-subcarrier signal generator another according to this and the corresponding a plurality of predetermined delay values of column signal, postpone after this time-domain signal, this time-domain signal that is delayed to be merged into a combined signal;

One digital revolving die is intended element, is to transfer this combined signal to an analog signal;

One electricity turns optical element, is this light signal with this analog signal conversion.

8. the conveyer of optical orthogonal frequency-division multitask according to claim 7 system, is characterized in that, wherein to turn optical element be a laser to this electricity.

9. the conveyer of optical orthogonal frequency-division multitask according to claim 7 system, is characterized in that, wherein this multi-subcarrier signal generator comprises:

A plurality of time-domain signal modulators, be with one-one relationship to signal side by side, respectively this time-domain signal modulator be according to it corresponding should and column signal produce this time-domain signal;

A plurality of delay cells, be with one-one relationship to should the time-domain signal modulator, respectively this delay cell is to postpone to transmit this time-domain signal that this corresponding with it time-domain signal modulator produces after this predetermined delay value;

One colligator is this time-domain signal that transmits in conjunction with this delay cell and export this combined signal.

10. the conveyer of optical orthogonal frequency-division multitask according to claim 9 system, is characterized in that, wherein respectively this time-domain signal modulator comprises:

One real part look-up table has a plurality of real part basic waveforms;

One imaginary part look-up table has a plurality of phase values;

One multiplexer group is to consult out this time-domain signal according to also column signal to this real part look-up table and this imaginary part look-up table corresponding with this time-domain signal modulator.

11. the conveyer of optical orthogonal frequency-division multitask according to claim 10 system, it is characterized in that, wherein respectively being somebody's turn to do also, column signal comprises a real part and an imaginary part, this multiplexer group is to consult out this basic waveform of a correspondence to this real part look-up table according to this real part of this and column signal, this multiplexer group is consulted out the phase value of a correspondence according to this imaginary part of this and column signal to this imaginary part look-up table, and this multiplexer group is this basic waveform of finding according to this quilt and phase value and export this time-domain signal.

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