WO2012155762A1

WO2012155762A1 - Local oscillator synchronization method of pon system, onu and olt

Info

Publication number: WO2012155762A1
Application number: PCT/CN2012/074464
Authority: WO
Inventors: 陈宏伟; 梁云华; 陈明华; 谢世钟; 何子安; 苏婕; 张佩华
Original assignee: 中兴通讯股份有限公司
Priority date: 2011-07-29
Filing date: 2012-04-20
Publication date: 2012-11-22
Also published as: CN102264012A; CN102264012B

Abstract

Disclosed are a local oscillator synchronization method of a passive optical network (PON) system, an optical network unit (ONU) and an optical line termination (OLT). The method is respectively applied in the ONU and the OLT, comprising: the ONU sending an uplink spread spectrum signal to the OLT; the OLT de-spreading the uplink spread spectrum signal and estimating a local oscillator frequency of the ONU; generating a frequency control word according to a frequency offset value between the local oscillator frequency and a channel specified by the OLT; modulating the frequency control word into a downlink spread spectrum signal and sending the signal to the ONU; the ONU demodulating the downlink spread spectrum signal to obtain the frequency control word; and adjusting the local oscillator frequency according to the frequency control word. According to the method, the local oscillator frequency of each ONU is pulled to the channel specified by the OLT, thereby avoiding the overlap of signal spectrum after each ONU is started. Local oscillator synchronization of the PON system is realized without affecting the normal communication of in-transmitting users.

Description

Local oscillator synchronization method of PON system and ONU and OLT

TECHNICAL FIELD The present invention relates to the field of communications, and in particular, to a local oscillator synchronization method for a passive optical network (P0N) system, and an optical line terminal (0LT) and an optical network unit (ONU) for implementing the method. A Passive Optical Network (PON) is a point-to-multipoint optical access technology, which consists of an optical line termination (OLT) at the central office and an optical network unit on the user side. An optical network unit (ONU) and an optical distribution network (ODN) are used for single-fiber bidirectional transmission. There is no active electronic equipment in the optical distribution network. The transmission of information in the PON is downlink (OLT transmission), and the uplink (ONU) uses time-sharing transmission. The OLT authorizes an ONU to transmit data. At present, the access network can be roughly divided into two types: time division multiplexing (TDM) and wavelength division multiplexing (WDM). WDM technology can simultaneously transmit multiple different types of signals with a certain wavelength interval, which greatly expands the data of the optical fiber. Transmission rate and transmission capacity. Since the channel spacing is greater than 100 GHz and the single channel occupied bandwidth is less than 20 GHz, the frequency offset (in the order of GHz) of the ONU of the user-side ON is relatively small, so that cross-channel crosstalk and spectrum overlap do not occur, and the ONU釆Direct detection is used, so the system does not need to adjust the frequency of the light source. In addition, the ONU of the commercial access network uses direct detection to demodulate the downlink data. Therefore, the frequency difference between the OLT and the ONU does not affect the signal recovery. However, with the increasing market demand for communication bandwidth and the development of network communication technologies, the channel spacing of ultra-dense wavelength division multiplexing-passive optical networks (WDM-PON) is narrower and narrower, Dense Wavelength Division Multiplexing (DWDM) Evolution to Ultra Dense Wavelength Division Multiplexing (UDWDM) with channel spacing less than 5 GHz. In order to ensure that the single channel transmission rate is Gbit/s, communication means using coherent detection and complex signal modulation are needed to achieve the above system specifications. In a high-speed coherent system, the frequency difference directly affects the detected bit error rate or even communication failure. In addition, when the ONU is turned on, it is affected by the physical characteristics of the device, and the frequency of the light source is usually At the frequency of the last boot, the frequency of each ONU laser is unknown for the OLT. At this time, the system needs to solve the frequency synchronization problem between the OLT and multiple ONUs at the same time to avoid overlapping the signal spectrum of each ONU. Moreover, since the channel spacing of the UDWDM is less than 5 GHz, the commercial light source on the ONU end may deviate from the last power-on center frequency, and there is a frequency offset of the order of GHz. At this time, the center frequency of the uplink signals of the multiple ONU terminals may fall into some In the transmission channel, it is also necessary to ensure that the synchronization operation cannot affect the normal communication of the user in the communication state. These problems have not yet been solved in UDWDM-PON, so there is an urgent need for a method to solve the above problems.

SUMMARY OF THE INVENTION The technical problem to be solved by embodiments of the present invention is to provide a local oscillator synchronization method for a passive optical network (PON) system, and an optical line terminal (OLT) and an optical network unit (ONU) for implementing the method, so that the ONU is The vibration frequency is pulled to the channel designated by the OLT to avoid overlapping of the signal frequency of each ONU after power-on, and does not affect the normal communication of the transmitting user. In order to solve the above technical problem, an embodiment of the present invention provides a local oscillator synchronization method for a PON system, which is applied to an optical network unit (ONU), and includes: the ONU transmitting an uplink spread spectrum signal to an optical line terminal (OLT), Receiving, by the OLT, a downlink spread spectrum signal including a frequency control word, where the frequency control word is a frequency offset value between a local frequency of the ONU and a frequency of a channel specified by the OLT, and the downlink The spread spectrum signal is demodulated to obtain the frequency control word, and the local local oscillator frequency is adjusted according to the frequency control word. Optionally, the ONU includes one or more, and when the ONU is multiple, the uplink spread spectrum signals sent by the ONUs include different spreading codes. Optionally, in the step that the ONU sends the uplink spread spectrum signal to the optical line terminal, the ONU sends the uplink spread spectrum signal by using optical power lower than that during normal communication. Optionally, the ONU sends an uplink spread spectrum signal to the OLT according to the adjusted local oscillator frequency, and receives the downlink spread spectrum signal sent by the OLT, and determines whether the downlink spread spectrum signal carries the adjustment completion indication information or the Determining whether the second frequency control word demodulated in the downlink spread spectrum signal is within an allowable frequency offset range; if yes, the adjustment is completed, and if not, controlling the local local oscillator according to the second frequency Adjust the frequency and perform this step again. In order to solve the above technical problem, the embodiment of the present invention further provides a local oscillator synchronization method of a PON system, which is applied to an optical line terminal (OLT), and includes: the OLT receives an uplink extension transmitted by an optical network unit (ONU) a frequency signal, despreading the uplink spread spectrum signal, estimating a local oscillator frequency of the ONU, and generating a frequency control word according to a frequency offset value between the local oscillator frequency and a frequency of the OLT designated channel, The frequency control word modulation is sent to the ONU in a downlink spread spectrum signal.

Optionally, the OLT receives an uplink spread spectrum signal that is sent after the ONU adjusts the local oscillator frequency, estimates a local oscillator frequency of the ONU after despreading the frequency, and determines the local oscillator frequency and the OLT. Whether the frequency offset value between the frequencies of the designated channel is within the allowed frequency offset range, and if so, generating a downlink spread spectrum signal including the adjustment completion indication information to the ONU, and if not, according to the And generating, by the frequency offset value between the adjusted local oscillator frequency and the OLT designated channel, a second frequency control word, and modulating the second frequency control word in the downlink spread spectrum signal, sending the signal to the ONU, and executing again This step. Optionally, performing the following steps one or more times: the OLT receives the uplink spread spectrum signal sent by the ONU after adjusting the local oscillator frequency, and estimates the local oscillator frequency after the ONU adjustment according to the despreading frequency, according to Generating, by the frequency offset value between the adjusted local oscillator frequency and the frequency of the OLT designated channel, a second frequency control word, and transmitting the second frequency control word modulation to the ONU in a downlink spread spectrum signal . Optionally, a center frequency of the downlink spread spectrum signal is within a coherent reception frequency range of the ONU.

In order to solve the above technical problem, an embodiment of the present invention further provides an optical network unit (ONU), including a sending unit, a receiving demodulating unit, and a local oscillator frequency adjusting unit, where: the sending unit is configured to: send an uplink spread spectrum signal to An optical line terminal (OLT); the receiving and demodulating unit is configured to: receive a downlink spread spectrum signal that is sent by the OLT and includes a frequency control word, and despread and demodulate the spread spectrum signal to obtain the frequency control word, where Frequency control word is a frequency offset value between the local oscillator frequency of the ONU and the frequency of the channel specified by the OLT; sending the frequency control word to the local oscillator frequency adjusting unit; the local oscillator frequency adjusting unit is configured to: according to the frequency The control word adjusts the local local oscillator frequency. Optionally, the sending unit is configured to generate the uplink spread spectrum signal by using a spreading code different from the other ONUs when multiple ONUs send the uplink spread spectrum signal to the OLT. Optionally, the sending unit is configured to send the uplink spread spectrum signal by using optical power lower than normal communication. Optionally, the sending unit is further configured to send one or more uplink spread spectrum signals to the OLT according to the adjusted local oscillator frequency, until a local frequency of the ONU is between the frequency of the channel specified by the OLT and the frequency of the channel specified by the OLT. The frequency offset value is within the allowed frequency offset range; the receiving and demodulating unit is further configured to receive the downlink spread spectrum signal sent by the OLT, and determine whether the downlink spread spectrum signal carries the adjustment completion indication information or the slave Demodulating in the downlink spread spectrum signal to determine whether the second frequency control word is within the allowed frequency offset range, and if not, transmitting the second frequency control word to the local oscillator frequency adjustment unit; The local oscillator frequency adjusting unit is further configured to adjust the local local oscillator frequency according to the second frequency control word.

In order to solve the above technical problem, an embodiment of the present invention further provides an optical line terminal (OLT), including a receiving demodulation unit, a signal generating unit, and a transmitting unit, where: the receiving demodulating unit is configured to receive an optical network unit (ONU) Sending an uplink spread spectrum signal, despreading the uplink spread spectrum signal to estimate a local oscillator frequency of the ONU; and generating, by the signal generating unit, a frequency offset between the local oscillator frequency and the OLT designated channel The value generates a frequency control word, modulates the frequency control word in a downlink spread spectrum signal, and transmits the downlink spread spectrum signal to a transmitting unit; and the transmitting unit is configured to send the downlink spread spectrum signal to the ONU. Optionally, the method further includes: a frequency verification unit, where: the receiving and demodulating unit is further configured to receive an uplink spread spectrum signal that is sent after the ONU adjusts the local oscillator frequency, and estimate the ONU adjustment for the despreading frequency thereof. a frequency of the local oscillator; the frequency check unit is configured to determine whether the local oscillator frequency has been adjusted to a frequency offset range allowed by the channel specified by the OLT, and send the determination result to the signal generating unit; The signal generating unit is further configured to: when the determining result is no, generate a second frequency control word according to a frequency offset value between the adjusted local oscillator frequency and a frequency of the OLT designated channel, where Transmitting, by the second frequency control word, the downlink spread spectrum signal to the transmitting unit in the downlink spread spectrum signal; and when the determining result is yes, generating a downlink spread spectrum signal including the adjustment complete indication information to the ONU . Optionally, the receiving and demodulating unit is further configured to receive an uplink spread spectrum signal that is sent after the ONU adjusts a local oscillator frequency, and estimate a local oscillator frequency of the ONU after despreading; The generating unit is further configured to generate a second frequency control word according to the frequency offset value between the adjusted local oscillator frequency and the OLT designated channel, and modulate the second frequency control word in the downlink spread spectrum signal And transmitting the downlink spread spectrum signal to the sending unit. Optionally, a center frequency of the downlink spread spectrum signal is within a coherent reception frequency range of the ONU.

The center frequency of the line signal is synchronized, and the local oscillator frequency of each ONU is pulled to the channel designated by the OLT to avoid overlapping of the signal frequencies of the ONUs after the power-on, and the local oscillator synchronization of the PON system is not realized. Will affect the normal communication of the passing users. BRIEF abstract

1 is a flowchart of a local oscillator synchronization method applied to a PON system in an ONU in the first embodiment; FIG. 2 is a flowchart of a local oscillator synchronization method applied to a PON system in an OLT in the first embodiment; FIG. 3 is a flowchart in the embodiment. Schematic diagram of an ONU structure for realizing the local oscillator synchronization of a PON system; 4 is a schematic structural diagram of an OLT for realizing local oscillator synchronization of a PON system in an embodiment; FIG. 5 is a flowchart of a local oscillator synchronization method for a high-speed UDWDM-PON system according to Embodiment 2; FIG. 6 is a frequency-locked loop of the second embodiment. Schematic diagram of the structure of the FLL;

7 is a flow chart of a local oscillator synchronization method of a high speed UDWDM-PON system in an application example; FIG. 8 is a schematic diagram of simulation results in an application example.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the accompanying drawings. It should be noted that, in the case of no conflict, the features in the embodiments and the embodiments in the present application may be arbitrarily combined with each other. Embodiment 1:

As shown in FIG. 1 , an embodiment of the present invention provides a local oscillator synchronization method for a PON system, which is applied to an optical network unit (ONU), and includes the following steps:

S101: The ONU sends an uplink spread spectrum signal to the optical line terminal (OLT). The ONU may include one or more. When the ONU is multiple, the uplink spread spectrum signals sent by the ONUs include different spreading codes.

The ONU can transmit the uplink spread spectrum signal using optical power lower than that in normal communication, which can reduce interference to other on-the-go channels, thereby not affecting normal communication of other on-channels.

S102: The ONU receives the downlink spread spectrum signal that is sent by the OLT and includes the frequency control word, and demodulates the spread spectrum signal to obtain the frequency control word. The frequency control word is the local oscillator frequency of the ONU and the OLT designation. The frequency offset value between the channels. The frequency offset value can be an absolute value or a frequency offset value with positive and negative directions.

S103: The ONU adjusts the local local oscillator frequency according to the frequency control word. In addition, after performing the above steps, the ONU may perform multiple repetitions of the local oscillator frequency adjusted by the ONU until the frequency offset between the local oscillator frequency of the ONU and the channel specified by the OLT is Within the allowed frequency offset range, that is, within this frequency offset range, the local oscillator frequency of the ONU can be considered to be drawn to the channel designated by the OLT without overlapping with other users. In a specific implementation, if the frequency offset value is a value with positive and negative directions, a preferred manner is: the ONU sends an uplink spread spectrum signal to the OLT according to the adjusted local oscillator frequency, and receives the OLT. Sending a downlink spread spectrum signal, determining whether the downlink spread spectrum signal carries the adjustment completion indication information or whether the second frequency control word demodulated from the downlink spread spectrum signal is within the allowed frequency offset range If not, adjusting the local local oscillator frequency according to the second frequency control word; the above step is performed one or more times until the local frequency of the ONU and the frequency specified by the OLT are The offset value is within the allowable frequency offset range and the adjustment is completed. In the specific implementation, if the frequency offset value is an absolute value, it should be noted that there are two possibilities when the ONU terminal adjusts the frequency, one may be to double the frequency difference, and the other is Change the frequency difference to 0. Therefore, when determining the second frequency control word demodulated from the downlink spread spectrum signal, if the second frequency control word is within an allowable frequency offset range (for example, 0), it indicates that the ONU has Adjusting the local oscillator frequency to the channel specified by the OLT; if the second frequency control word exceeds the allowable frequency offset range, the ONU should adjust the local local oscillator frequency to the opposite direction from the previous adjustment according to the second frequency control word. Adjust to the channel specified by the OLT.

In this way, it is ensured that the local oscillator frequency of the ONU is properly pulled and is towed to the channel designated by the OLT.

In addition, as shown in FIG. 2, the embodiment further provides a local oscillator synchronization method of the PON system, which is applied to the OLT, and includes:

S201: The OLT receives an uplink spread spectrum signal sent by an optical network unit (ONU), and despreads the spread spectrum optical signal to estimate a local oscillation frequency of the ONU.

S202: The OLT generates a frequency control word according to a frequency offset value between the local oscillator frequency of the ONU and the designated channel of the OLT; the frequency offset value may be an absolute value or a frequency offset value with positive and negative directions.

S203: The OLT modulates the frequency control word to be sent to the ONU in the downlink spread spectrum signal. The center frequency of the downlink spread spectrum signal needs to be within the coherent reception frequency range of the ONU, and the ONU coherent reception mode has homodyne coherence and heterodyne coherence. Therefore, the center frequency of the downlink spread spectrum signal can be adjusted to the ONU. The local oscillator frequency can also be adjusted to a certain value that has a certain difference from the local oscillator frequency, as long as there is a corresponding relationship, thus ensuring that the ONU can receive the downlink sent by the OLT in its channel band. Spread spectrum signal. In addition, the OLT may also check whether the local oscillator frequency of the ONU has been adjusted to the frequency offset allowed by the channel specified by the OLT after receiving the uplink spread spectrum signal sent after the ONU adjusts the local oscillator frequency. In a specific implementation, if the frequency offset value is a frequency offset value with positive and negative directions, a preferred manner is:

Receiving, by the OLT, an uplink spread spectrum signal that is sent after the ONU adjusts the local oscillator frequency, estimating a local oscillator frequency of the ONU after despreading the frequency, and determining between the local oscillator frequency and the channel specified by the OLT. Whether the frequency offset value is within the allowed frequency offset range, and if not, generating a second frequency control word according to the frequency offset value between the adjusted local oscillator frequency and the OLT designated channel, Transmitting the second frequency control word modulation to the ONU in the downlink spread spectrum signal; the foregoing step is performed one or more times until the frequency offset value between the local oscillator frequency of the ONU and the channel specified by the OLT is Within the allowed frequency offset range, a downlink spread spectrum signal including adjustment completion indication information is generated to the ONU. In the specific implementation, if the frequency offset value is an absolute value, it should be noted that there are two possibilities when the ONU terminal adjusts the frequency, one may be to double the frequency difference, and the other is Change the frequency difference to 0. Therefore, the OLT receives the uplink spread spectrum signal sent by the ONU after adjusting the local oscillator frequency, and estimates the local oscillator frequency of the ONU after despreading the frequency, according to the frequency offset between the adjusted local oscillator frequency and the OLT designated channel. The second frequency control word is generated by shifting, and the second frequency control word is modulated and transmitted to the ONU in the downlink spread spectrum signal.

Correspondingly, as shown in FIG. 3, an embodiment of the present invention further provides an optical network unit (ONU) for implementing local oscillator synchronization of a PON system, including a transmitting unit, a receiving demodulating unit, and a local oscillator frequency adjusting unit, where The sending unit 301 is configured to send an uplink spread spectrum signal to the optical line terminal (OLT); when multiple ONUs send the uplink spread spectrum signal to the OLT, the sending unit is further configured to generate an uplink spread by using a different spreading code than other ONUs. Frequency signal. The transmitting unit transmits the uplink spread spectrum signal by using optical power lower than that during normal communication, so that interference to other on-channels can be reduced, so that normal communication of other on-channels is not affected. The receiving demodulation unit 302 is configured to receive a downlink spread spectrum signal that is sent by the OLT and includes a frequency control word, and despread and demodulate the spread spectrum signal to obtain a frequency control word, and send the frequency control word to the local oscillator frequency The adjusting unit; wherein the frequency control word is a frequency offset value between a local oscillator frequency of the ONU and a channel designated by the OLT. The frequency offset value can be an absolute value or a frequency offset value with positive and negative directions. The local oscillator frequency adjusting unit 303 is configured to adjust the local local oscillator frequency according to the frequency control word. In addition, the ONU may perform multiple iterations on the adjusted local oscillator frequency of the ONU to determine whether the frequency offset value between the local oscillator frequency of the ONU and the channel specified by the OLT is within an allowable frequency offset range. And adjusting, in time, the sending unit 301 is further configured to send one or more uplink spread spectrum signals to the OLT according to the adjusted local oscillator frequency, until the local oscillator frequency of the ONU is specified by the OLT The frequency offset value between the channels is within the allowable frequency offset range; the receiving demodulation unit 302 is further configured to receive the downlink spread spectrum signal sent by the OLT, and determine whether the downlink spread spectrum signal carries the adjustment Completing the indication information or demodulating from the downlink spread spectrum signal to determine whether the second frequency control word is within the allowed frequency offset range, and if not, transmitting the second frequency control word to the local oscillator frequency The adjusting unit; the local oscillator frequency adjusting unit 303 is further configured to adjust the local local oscillator frequency according to the second frequency control word.

Finally, the adjusted ONU, the frequency between the local oscillator frequency and the channel specified by the OLT The rate offset value will be within the allowed frequency offset range.

As shown in FIG. 4, the embodiment further provides an optical line terminal (OLT) for realizing the local oscillator synchronization of the PON system, comprising: a receiving and demodulating unit 401, configured to receive an uplink spread spectrum transmitted by an optical network unit (ONU). a signal, despreading the uplink spread spectrum signal to estimate a local oscillator frequency of the ONU; the signal generating unit 402 is configured to generate a frequency control word according to a frequency offset value between the local oscillator frequency of the ONU and the specified channel of the OLT, The frequency control word is modulated in the downlink spread spectrum signal, and the downlink spread spectrum signal is sent to the sending unit 403; the frequency offset value may be an absolute value, or may be a frequency offset with positive and negative directions. value. The sending unit 403 is configured to send the downlink spread spectrum signal to the ONU. The center frequency of the downlink spread spectrum signal needs to be within the coherent receiving frequency range of the ONU, and the ONU coherent receiving mode has homodyne coherence and heterodyne coherence. Therefore, the center frequency of the downlink spread spectrum signal can be adjusted to the ONU. The local oscillator frequency can also be adjusted to a certain value that has a certain difference from the local oscillator frequency, as long as there is a corresponding relationship, thus ensuring that the ONU can receive the downlink spread spectrum transmitted by the OLT in its channel band. In the signal. In addition, the OLT may further include a frequency check unit 404, after receiving the uplink spread spectrum signal sent by the demodulation unit after receiving the ONU adjusting the local oscillator frequency, and estimating the ONU adjusted local oscillator frequency by despreading the frequency; The check unit is configured to determine whether the frequency offset value between the local oscillator frequency and the channel specified by the OLT is within an allowable frequency offset range, and send the determination result to the signal generating unit; The unit 402 is further configured to: when the determining result is no, generate a second frequency control word according to the frequency offset value between the adjusted local oscillator frequency and the OLT designated channel, and modulate the second frequency control word And transmitting, in the downlink spread spectrum signal, the downlink spread spectrum signal to the transmitting unit, and when the determining result is YES, generating a downlink spread spectrum signal including the adjustment completion indication information to the ONU. Embodiment 2:

In a specific application scenario, taking the high-speed UDWDM-PON (Ultra Dense Wavelength Division Multiplexing - Passive Optical Network) system as an example, User 1 transmits data on channel 1, while another user 2 transmits wavelength when turning on the light source. Falling in adjacent channel 1, the center frequency of the user 2 signal needs to be pulled to the designated channel 2, so that the data communication of the two channels does not interfere with each other. However, the OLT terminal cannot directly extract the user 2 signal in channel 1. At this time, the transmission data of the user 2 can be spread-coded by the CDMA technology to obtain a certain coding gain, and the transmission optical power is reduced to be much lower than the user. At the power level of 1, the user 2 demodulates the frequency control data of the downlink low bit rate while ensuring that the signal transmission of the channel 1 is not affected by the spread spectrum signal of the user 2.

In a specific implementation, as shown in FIG. 5, this embodiment provides a local oscillator synchronization method for a high-speed UDWDM-PON system, including the following steps:

S501: After the user 2 0NU is powered on, a high-rate CDMA spreading sequence is generated, and the electro-optic modulation is transmitted into the spread spectrum optical signal to the OLT. In specific implementation, for example, the transmission rate and power of the channel 1 can be maintained. In the case of reducing the transmitted optical power of the channel 2, the attenuation is half of the above-mentioned CDMA spread spectrum coding gain (the attenuation of the signal power at the PD end is squared with the attenuation of the optical power), so that the optical power of the user 2 is reduced to a very low level. At the power level of the user 1, the optical power of the user 2 is reduced mainly to reduce the interference to the user 1, thereby ensuring that the OLT can simultaneously demodulate the data of the two channels, thus, the user 2 and the OLT perform wavelength control. The communication of data does not need to interrupt the communication of User 1, and the uplink and downlink data transmission of Channel 1 is not affected by the presence of Channel 2.

S502: After receiving the spread spectrum optical signal, the OLT despreads the spread spectrum signal by using a two-dimensional search technique, searches within a certain estimated frequency difference range, estimates the local oscillator frequency of the ONU, and generates and adjusts The frequency control word of the local oscillator frequency of the ONU; wherein the frequency control word refers to a frequency offset value between the local oscillator frequency of the ONU and the channel to be towed to the OLT, and the frequency offset value is positive and negative. value. The system OLT allocates an unoccupied channel to the user to be transmitted according to the spectrum occupancy of the channel. Therefore, the local oscillator frequency of the ONU needs to be pulled to the channel specified by the OLT, thereby ensuring that the channel spectrum of each user does not overlap. The above despreading process is the same as the despreading process of the conventional CDMA communication, the one-dimensional search is a CDMA chip search, and the other dimension search is a center frequency search. Since the ONU terminal's boot frequency is within a certain range, usually less than 20 GHz, the OLT terminal only needs to search within this range. In the specific implementation, the OLT performs two-dimensional search within a certain frequency difference range, and the search algorithm is as follows: S5021: The OLT first generates a spreading sequence with a carrier (the code rate is consistent with the ONU end), and the uplink signal after the sampling Multiply and accumulate to obtain the integral value (integral sum sum);

S5022: Calculate an integral value of a product of a local signal of a local I/Q carrier and a chip position of different frequencies and a received signal;

S5023: Find the maximum value of the integral (the detection probability is the largest, that is, the carrier and chip of the two signals are basically the same), and the frequency value and code phase value of the local carrier corresponding to the signal.

S5024: Set the chip step size and frequency step size of the 2D search to a smaller value, so that the OLT terminal can obtain a more accurate LOU local oscillator frequency value.

S503: The OLT generates a pseudo-random sequence that is the same as the uplink signal of the user 2 (the ONU end), and modulates the frequency control word into a downlink spread spectrum signal and transmits the signal to the ONU. The modulated data of the spread spectrum signal generated by the OLT is a frequency control word. The code rate is the same as the code rate of the spreading sequence of the user 2, and the center frequency is adjusted to the center frequency of the ONU uplink signal, that is, the estimated local frequency of the ONU. The center frequency of the downlink signal of User 2 generated by the OLT is equal to the OLT local oscillator frequency plus the frequency offset of the digital carrier. The center frequency of the OLT downlink signal needs to be within the coherent receiving frequency range of the ONU, and the ONU coherent receiving mode has homodyne coherence and heterodyne coherence. Therefore, the center frequency of the downlink spread spectrum signal can be adjusted to the ONU. The local oscillator frequency can also be adjusted to a certain value that has a certain difference from the local oscillator frequency. As long as there is a corresponding relationship, the center frequency of the OLT downlink signal and the center frequency of the ONU uplink signal will have one - The corresponding relationship is to ensure that the ONU can receive the downlink signal sent by the OLT. In addition, the pseudo-random sequence generated by the OLT needs to be consistent with the spreading sequence of the spread spectrum optical signal sent by the ONU to distinguish different users, because different users use different spreading sequences. S504: The ONU receives the signal sent by the OLT, despreads the frequency by using a two-dimensional search, and demodulates the frequency control word by using a frequency-locked loop FLL loop tracking method, and then adjusts the ONU according to the frequency control word. Local local oscillator frequency.

The working process of the above-mentioned ONU despreading is the same as that of the OLT, and is implemented by a two-dimensional search technique. The difference is that the ONU also needs the frequency-locked loop FLL loop tracking, so as to ensure the correct solution of the frequency control information. Tune. The specific structure of FLL is shown in Figure 6: Discriminator + Low Pass Filter + Loop Gain + Carrier NCO (Numerical Control Oscillator), the input data is a random sequence of the sampled signal and the local I/Q carrier and tracking code phase. The multiply-accumulate sum (I/Q integral energy), the carrier NOC is used for carrier control word generation. This is prior art and will not be repeated here.

It should be noted that the adjustment error may occur during the above-mentioned adjustment of the local oscillator frequency of the ONU. Therefore, in order to ensure more accurate adjustment of the local oscillator frequency of the ONU, the local oscillator frequency of the ONU may be checked and adjusted. The process of verifying the adjustment is as follows:

S601: The ONU of the user 2 sends an uplink spread spectrum signal to the OLT according to the adjusted local oscillator frequency;

S602: After receiving the uplink spread spectrum signal, the OLT estimates the local oscillator frequency after the ONU adjustment for the despreading frequency.

S603: The OLT determines whether the local oscillator frequency has been adjusted to the frequency offset range allowed by the channel specified by the OLT, and if yes, generates a downlink spread spectrum signal including the adjustment completion indication information to the ONU, and ends the process. The high speed data communication is started; if not, step S604 is performed.

S604: The OLT generates a second frequency control word according to the frequency offset between the adjusted local oscillator frequency and the OLT designated channel, and the second frequency control word modulation is sent to the downlink spread spectrum signal. ONU;

S605: The ONU receives the downlink spread spectrum signal and demodulates the second frequency control word, and adjusts a local local oscillator frequency according to the frequency control word.

The above steps S601~S605 may be repeated multiple times until the frequency offset value between the local oscillator frequency of the ONU and the channel specified by the OLT is within the allowable frequency offset range.

Finally, the local oscillator frequency of the ONU will be pulled to the channel specified by the OLT. The center frequency of the downlink signal generated by the OLT will eventually be adjusted to the position specified by the OLT, so that the local oscillator frequency of the ONU (the center frequency of the uplink signal of the user 2) ) complete with the center frequency of the downstream signal generated by the OLT Synchronization, the synchronization referred to here does not necessarily have to be exactly the same frequency. When the 0NU adjusted local oscillator frequency has a certain correspondence with the center frequency of the downlink signal generated by the OLT, it is allowed in the channel specified by the OLT. Within the frequency offset range, the synchronization effect has been achieved, thereby realizing the local oscillator synchronization of the high-speed UD WDM-PON system.

For the case of multiple users (multiple ONUs), it is only necessary to adjust the center frequency of each user's downlink signal within the coherent receiving frequency range of each user ONU, so that the local oscillator frequency of the final ONU of each user (the center frequency of the uplink signal) The center frequency of the downlink signal generated by the OLT is pulled to the channel designated by the system OLT for the user to achieve synchronization.

Application example: In an application example, considering the number of users of the system and the requirements for frequency synchronization time, the system parameters are set as follows: Here, the spreading period is selected to be 1023, and the code rate of the spreading sequence of the ONU to be frequency-driven is normal data. The transmission rate of communication, 30dB coding gain after spreading, reduces the transmitted optical power to 1/1023 (about -30dB) in normal communication, because of the attenuation of the electrical signal amplitude of the PD detection terminal and the attenuation of optical power. In a linear relationship, this parameter selection ensures that both the ONU and the OLT obtain the same bit error rate for both the spread communication and the normal high-speed communication. In this way, the OLT can correctly demodulate data of one channel and ten frequency traction channels simultaneously in the single channel spectrum (the ratio of the channel power to the total frequency of the 10 frequency traction channels is greater than 100, that is, the letter The dry ratio is 20dB, and this signal-to-interference ratio can satisfy the value required to receive the signal-to-noise ratio higher than the FECO.1% bit error rate). Generally, the ONU terminal's power-on frequency is less than 20 GHz from the last power-on. Therefore, for a UDWDM-PON system with a channel spacing of 5 GHz, at most 8 ONUs, the wavelength of the laser at the same time is at the same time. In the channel, the spread spectrum design with a code length of 1023 can meet the communication requirements of the 5 GHz channel spacing. When the channel spacing is less than 4 GHz, it only needs to increase the length of the code period to meet the requirement. Here, different users are assigned different spreading codes, so that simultaneous adjustment of the multi-user local oscillator can be realized. When the local oscillator frequency LO spectrum of multiple ONUs is in one channel, the number of such users is usually less than 4, the power-on frequency is less than 20 GHz, and the channel spacing is 5 GHz. The worst case for users of other channels when booting. The LO frequency LO of the adjacent left and right 4 channels will fall into the transmission channel, and the local oscillator frequency will be performed for the proposed ONU end of each of the 4 channels adjacent to the left and right of the transmission channel. Rate traction. The spread spectrum mechanism with a code length of 1023 can ensure that the total power of the adjacent ONU frequency control signal (interference with respect to the transmission channel) is still small compared with the power of the transmission channel, and the introduced interference does not affect other transmissions. The data communication of the channel can meet the requirements of the local oscillator frequency synchronization of multiple ONUs without affecting the normal communication of other transmission channels.

As shown in FIG. 7, when there are multiple users, in the specific implementation, the local oscillator synchronization method of the high-speed UDWDM-PON system includes the following steps:

S701: Each ONU generates a CDMA spreading sequence 歹' J with a code rate of 1 Gb/s and a period of 1023, and is modulated into a spread spectrum optical signal for uplink transmission to the OLT;

S702: After performing coherent detection by the OLT, the DSP performs a two-dimensional search (chip position + center frequency), estimates the local oscillator frequency of each ONU, and generates a frequency control word for adjusting the local oscillator frequency of each ONU;

S703: The OLT modulates the frequency control word into a downlink spread spectrum signal and transmits the signal to the ONU.

S704: Each ONU performs a two-dimensional code search + FLL loop tracking, demodulates the frequency control word, and then adjusts the local local oscillator frequency;

Whether the frequency offset value between the channel specified by the OLT is within the allowed frequency offset range; if yes, step S706 is performed, and if not, steps S701 to S705 are repeated until the local oscillator frequency of each ONU Whether the frequency offset value between the channel specified by the OLT is within the allowable frequency offset range, but each ONU generates a spreading sequence according to the adjusted local oscillator frequency, and the OLT demodulates the obtained code each time. The vibration frequency is the re-adjusted local oscillator frequency.

S706: Start high speed data communication. According to the above parameters for simulation, Figure 8 shows the error result of the frequency tracking of the ONU terminal under the condition that the signal-to-noise ratio of the PD received signal is 0 dB and the frequency difference is fixed (the FFT calculation first controls the frequency error within 500 Hz), and the ONU end passes The processing of the order of 100 ms can converge to the frequency of the LO at the OLT end, and demodulate the frequency control word to adjust its own light source. This means that every 5 kHz/s frequency step, the loop can converge, and the frequency control word is successfully demodulated. The loop bandwidth set here is 5 Hz. The actual frequency difference step may reach 5MHz/s or even larger. In this case, it is necessary to increase the code rate of the spread spectrum signal or to use a faster analog frequency-locked loop (AFLL) to achieve frequency tracking and feedback control of the local oscillator frequency. In another application example, the channel spacing is 5 GHz, and the power of the spread spectrum signal is -30 dB smaller than that of the non-spread spectrum signal. In order not to affect the high-speed data transmission of the adjacent channel (the received signal satisfies a certain signal-to-noise ratio requirement), The number of users performing frequency synchronization in the same channel must be limited. It is assumed that the SNR of the data communication BER is >20 dB, the code length is 1023, and the UDWDM-PON with a channel spacing of 5 GHz requires that the number of such users be less than 10. Under the condition, the laser start frequency difference of the ONU terminal does not exceed 50G, which is far greater than the maximum frequency offset of the commercial laser. If the system needs to tolerate 100 users when the local oscillator frequency is in the same channel, the code rate can be reduced to 1/10 or the spread spectrum period is set to 10 times, which makes the back-end demodulation gain 10 times. Reduce the system's local oscillator frequency offset requirement on the ONU side. In this application example, consider the number of users of the system (tolerate 100 users to boot) and the requirements for frequency synchronization time. Here, the spread spectrum period is 10230, the user's transmit power is reduced to 20 dB, and the code rate is unchanged (lGb/s). ), so that the system can tolerate 100 users when the local oscillator frequency is on the same channel.

As can be seen from the above embodiment, in the above embodiment, the spread spectrum signal of the ONU is despread by the OLT to estimate the local oscillator frequency of the ONU and generate a frequency control word, and the frequency control word is modulated into The downlink spread spectrum signal is transmitted to the ONU, and the ONU adjusts the local oscillator frequency of the ONU according to the frequency control word. Finally, the local oscillator frequency of the ONU is adjusted to the channel designated by the OLT, and the center frequency of the downlink signal generated by the OLT is finally adjusted. To the location specified by the OLT, the local oscillator frequency of the ONU is synchronized with the center frequency of the downlink signal generated by the OLT, and the local oscillator frequency of each ONU is pulled to the channel designated by the OLT to avoid the signal of each ONU after being turned on. The overlapping of the spectrums does not affect the normal communication of the transmitting users while realizing the local oscillator synchronization of the PON system. One of ordinary skill in the art will appreciate that all or a portion of the above steps may be accomplished by a program instructing the associated hardware, such as a read-only memory, a magnetic disk, or an optical disk. Alternatively, all or part of the steps of the above embodiments may also be implemented using one or more integrated circuits. Correspondingly, each module/unit in the above embodiment may be implemented in the form of hardware or in the form of a software function module. The invention is not limited to any specific form of combination of hardware and software.

The above description is only a preferred embodiment of the present invention, and is not intended to limit the protection of the present invention. Wai. In view of the present invention, various other modifications and changes can be made without departing from the spirit and scope of the invention, and any modifications, equivalents, improvements, etc. are made without departing from the spirit and scope of the invention. All should be included in the scope of protection of the present invention.

The industrial practicability is synchronized, and the local oscillator frequency of each ONU is pulled to the channel designated by the OLT to avoid overlapping of the signal frequency of each ONU after the power-on, and the local oscillator synchronization of the PON system is realized without affecting. The normal communication of the user is transmitted.

Claims

Claim

A local oscillator synchronization method for a passive optical network (PON) system is applied to an optical network unit (ONU), the method comprising: the ONU transmitting an uplink spread spectrum signal to an optical line terminal (OLT), and receiving a downlink spread spectrum signal including a frequency control word sent by the OLT, where the frequency control word is a frequency offset value between a local oscillator frequency of the ONU and a frequency of a channel specified by the OLT, and the downlink expansion The frequency signal is demodulated to obtain the frequency control word, and the local local oscillator frequency is adjusted according to the frequency control word.

2. The method according to claim 1, wherein: the ONU comprises one or more, and when the ONU is multiple, the uplink spread spectrum signal sent by each ONU includes different spreading codes.

3. The method according to claim 1, wherein: in the step of the ONU transmitting the uplink spread spectrum signal to the optical line terminal, the ONU transmits the uplink spread spectrum signal using an optical power lower than that during normal communication.

The method according to any one of claims 1 to 3, the method further comprising: the ONU transmitting an uplink spread spectrum signal to the OLT according to the adjusted local oscillator frequency, and receiving the

a downlink spread spectrum signal sent by the OLT, determining whether the downlink spread spectrum signal carries the adjustment completion indication information or whether the second frequency control word demodulated from the downlink spread spectrum signal is within an allowable frequency offset range If yes, the adjustment is completed. If not, the local local oscillator frequency is adjusted according to the second frequency control word, and the step is performed again.

5. A local oscillator synchronization method for a passive optical network (PON) system, applied to an optical line terminal

( OLT ), the method includes: the OLT receives an uplink spread spectrum signal sent by an optical network unit (ONU), and despreads the uplink spread spectrum signal to estimate a local oscillator frequency of the ONU, according to the The frequency offset value between the local oscillator frequency and the frequency of the OLT designated channel generates a frequency control word, and the frequency control word is modulated and transmitted to the ONU in a downlink spread spectrum signal.

6. The method of claim 5, the method further comprising: Receiving, by the OLT, an uplink spread spectrum signal that is sent after the ONU adjusts the local oscillator frequency, estimating a local oscillator frequency of the ONU after despreading the frequency, and determining the local oscillator frequency and the channel specified by the OLT. Whether the frequency offset value between the frequencies is within the allowable frequency offset range, and if so, generating a downlink spread spectrum signal including the adjustment completion indication information to the ONU, and if not, according to the adjusted version A frequency offset value between the vibration frequency and the OLT designated channel generates a second frequency control word, and the second frequency control word is modulated and transmitted to the ONU in a downlink spread spectrum signal, and the step is performed again.

The method of claim 5, the method further comprising: performing the following steps one or more times: the OLT receives the uplink spread spectrum signal sent by the ONU after adjusting the local oscillator frequency, and despreads the frequency Estimating the local oscillator frequency after the adjustment of the ONU, generating a second frequency control word according to the frequency offset value between the adjusted local oscillator frequency and the frequency of the OLT designated channel, and controlling the second frequency Word modulation is sent to the ONU in a downlink spread spectrum signal.

8. The method according to any one of claims 5 to 7, wherein: the center frequency of the downlink spread spectrum signal is within a coherent reception frequency range of the ONU.

An optical network unit (ONU), comprising: a transmitting unit, a receiving demodulating unit, and a local oscillator frequency adjusting unit, wherein: the sending unit is configured to: send an uplink spread spectrum signal to an optical line terminal (OLT); The receiving and demodulating unit is configured to: receive a downlink spread spectrum signal that is sent by the OLT and includes a frequency control word, and despread and demodulate the spread spectrum signal to obtain the frequency control word, where the frequency control word is a frequency offset value between a local oscillator frequency of the ONU and a channel specified by the OLT; transmitting the frequency control word to the local oscillator frequency adjusting unit; the local oscillator frequency adjusting unit is configured to: according to the frequency The control word adjusts the local local oscillator frequency.

The ONU according to claim 9, wherein: the sending unit is configured to generate the uplink expansion by using a spreading code different from other ONUs when a plurality of ONUs send uplink spreading signals to the OLT. Frequency signal.

11. The ONU of claim 9, wherein: the transmitting unit is configured to transmit the uplink spread spectrum signal using optical power lower than normal communication.

The ONU according to any one of claims 9 to 11, wherein: the sending unit is further configured to: send one or more uplink spread spectrum signals to the OLT according to the adjusted local oscillator frequency, until the ONU is The frequency offset value between the vibration frequency and the frequency of the channel specified by the OLT is within an allowable frequency offset range; the receiving and demodulating unit is further configured to: receive the downlink spread spectrum signal sent by the OLT, and determine Whether the downlink spread spectrum signal carries the adjustment completion indication information or demodulates from the downlink spread spectrum signal to determine whether the second frequency control word is within an allowable frequency offset range, and if not, the second The frequency control word is sent to the local oscillator frequency adjusting unit; the local oscillator frequency adjusting unit is further configured to adjust the local local oscillator frequency according to the second frequency control word.

An optical line terminal (OLT), comprising: a receiving demodulation unit, a signal generating unit, and a transmitting unit, wherein: the receiving and demodulating unit is configured to: receive an uplink spread spectrum signal sent by an optical network unit (ONU), Estimating a local oscillator frequency of the ONU by despreading the uplink spread spectrum signal; the signal generating unit is configured to: generate a frequency control according to a frequency offset value between the local oscillator frequency and the OLT designated channel a character, the frequency control word is modulated in a downlink spread spectrum signal, and the downlink spread spectrum signal is sent to the sending unit; the sending unit is configured to: send the downlink spread spectrum signal to the ONU.

The OLT according to claim 13, further comprising a frequency verification unit, wherein the receiving and demodulating unit is further configured to: receive an uplink spread spectrum signal sent by the ONU after adjusting the local oscillator frequency, and solve the solution Spreading frequency estimates the local oscillator frequency after the adjustment of the ONU; the frequency check unit is configured to: determine whether the local oscillator frequency has been adjusted to the frequency offset range allowed by the channel specified by the OLT, and determine The result is sent to the signal generating unit; The signal generating unit is further configured to: when the determining result is no, generate a second frequency control word according to a frequency offset value between the adjusted local oscillator frequency and a frequency of the OLT designated channel, The second frequency control word is modulated in the downlink spread spectrum signal, and the downlink spread spectrum signal is sent to the sending unit; when the determining result is yes, the downlink spread spectrum signal including the adjustment completion indication information is generated. To the sending unit.

The OLT according to claim 13, wherein the receiving and demodulating unit is further configured to: receive an uplink spread spectrum signal that is sent after the ONU adjusts a local oscillator frequency, and estimate the ONU adjustment for its despreading frequency The signal generating unit is further configured to: generate a second frequency control word according to a frequency offset value between the adjusted local oscillator frequency and a frequency of the OLT designated channel, The two frequency control word modulation transmits the downlink spread spectrum signal to the transmitting unit in the downlink spread spectrum signal.

The OLT according to any one of claims 13 to 15, wherein: a center frequency of said downlink spread spectrum signal is within a coherent reception frequency range of said ONU.