CN105245482B - A kind of OFDM-PON uplink temporal synchronization method - Google Patents

Abstract

The invention discloses an OFDM-PON uplink time synchronization method. (1) The ONU that needs to access the OFDM-PON system obtains the parameters related to the uplink transmission and the available synchronization code from the downlink frame information; (2) The ONU randomly selects a synchronization code, modulates it on the synchronization sub-channel and sends it to the OLT; (3 ) The OLT detects whether each received OFDM symbol contains a complete synchronization code; (4) The OLT calculates the time offset in the frequency domain after detecting the synchronization code; (5) After the ONU receives the synchronization response message sent by the OLT Adjust the time offset. The method can realize the time synchronization of multiple ONUs in the OFDM-PON, and the method can quickly and accurately realize the time synchronization and has low time complexity.

Description

A Time Synchronization Method for OFDM-PON Uplink

technical field

The invention relates to an OFDM-PON uplink time synchronization method, which realizes the time synchronization of multiple ONUs in the uplink and provides a solution for the synchronization problem of the next generation optical access network.

Background technique

With the development of Internet technology, multimedia services such as voice, video, and high-definition TV continue to emerge, people's demand for network bandwidth capacity increases sharply, and new types of bandwidth access requirements continue to emerge, leading to the emergence of bandwidth bottlenecks in access networks. Passive Optical Network (PON) technology has the advantages of low cost, large capacity, flexible networking, convenient equipment installation and simple expansion, and has been widely concerned by people in the industry, and has been identified as the core of the next-generation broadband access technology . As a low-cost technology to solve the problem of fiber-to-the-home, PON is the mainstream direction of current access network technology research.

Figure 1 is a typical PON system structure diagram, which is mainly composed of three parts: an optical line terminal (OpticalLine Terminal, OLT), multiple optical network units (Optical Network Unit, ONU) and optical distribution network (Optical Distribution Network, ODN) . The OLT acts as a switch or router of the access network to complete the forwarding of data packets, and it is located at the center office (CO) end. The ONU is located at the user end and is used to temporarily store the uplink data sent by the user and the downlink data sent by the OLT, and complete the data forwarding at an appropriate time. ODN is used to connect ONU and OLT, mainly composed of passive components such as passive optical splitter and passive optical coupler.

Orthogonal Frequency Division Multiplexing (OFDM) is a very mature technology in wireless communication. It is applied to standards such as IEEE802.11G and IEEE802.16. It is a modulation technology with the highest spectrum utilization rate at present. Its basic principle is to divide the high-speed data stream into several parallel low-speed data streams for simultaneous transmission in the time domain, divide the system bandwidth into mutually orthogonal subcarriers in the frequency domain, and use independent modulation on each subcarrier Format for transmission, effectively improving spectrum utilization and system transmission capacity.

The combination of OFDM technology and PON highlights the advantages of flexible access methods, high spectrum efficiency, support for wired and wireless converged access, and low system cost. OFDM-PON research has become the most competitive and representative for next-generation access networks. Program. OFDM-PON technology is a new type of optical access network technology that applies Orthogonal Frequency Division Multiple Access (OFDMA) to PON. Compared with traditional PON, it has many advantages:

1) Since OFDM is a multi-carrier modulation (MCM) system, the system spectrum utilization rate is high.

2) OFDM technology has a strong ability to resist chromatic dispersion and anti-polarizing film dispersion.

3) The cost-effectiveness is good, and the processing of the receiving end and the sending end are relatively simple to realize.

4) The bandwidth of OFDM-PON can be allocated more effectively and flexibly, has good bandwidth granularity, and supports dynamic

bandwidth allocation.

In the uplink of OFDM-PON, OFDM-PON adopts OFDMA technology as the multiple access technology. The signal received by OLT is the superposition of signals sent by different ONUs, and one OFDM symbol carries the data information of multiple ONUs. In addition to causing inter-carrier interference (Inter Carrier Interference, ICI) and inter-symbol interference (Inter Symbol Interference, ISI), the synchronization error will also cause inter-user multiple access interference (Multiple Access Interference, MAI). Therefore, the present invention proposes a method for OFDM-PON uplink time synchronization to realize time synchronization of each ONU.

Contents of the invention

The purpose of the present invention is to solve the time synchronization of a plurality of ONU signals in the OFDM-PON uplink, and propose a time synchronization method for the OFDM-PON uplink, so as to realize the alignment of the signals of each ONU at the OLT end and achieve accurate time synchronization.

The present invention is realized through the following technical solutions:

Through the process of initial synchronization, the present invention aligns the OFDM symbols of each ONU arriving at the OLT to ensure the orthogonality between uplink subcarriers, thereby realizing OFDM-PON uplink time synchronization.

Said method of the present invention comprises the steps:

Step 1. When an ONU that has not been synchronized needs to access the OFDM-PON system, it first performs downlink frame synchronization, and obtains parameters related to uplink transmission and usable synchronization codes and other information from the downlink frame information.

Step 2: The ONU randomly selects an available synchronization code and selects a synchronization time slot, modulates the synchronization code onto the synchronization sub-channel, and then sends the synchronization OFDM frame to the OLT.

Step 3: The OLT detects the synchronization code, and detects whether each received OFDM symbol contains a complete synchronization code.

Step 4: After the OLT detects the synchronization code, it calculates the time offset, and then broadcasts a synchronization response message, which includes the detected synchronization code and time offset information.

Step 5. After the ONU receives the synchronization response message, it compares it with the synchronization code sent by itself. If it matches, it adjusts it according to the time offset in the synchronization response message. After the adjustment, the OFDM symbols sent by the ONU are aligned at the OLT . If the synchronization process is unsuccessful, the ONU reselects a synchronization time slot to send the synchronization code.

The synchronization code described in the above step 1 is a constant envelope zero autocorrelation (Constant Amplitude ZeroAutocorrelation, CAZAC) sequence, and its expression is: Where j is the imaginary unit, π is pi, L is the length of the sequence, and r is a constant that is relatively prime to L. CAZAC sequences have ideal cyclic autocorrelation properties.

The synchronization sub-channel described in the above step 2 is specially reserved for time synchronization. Synchronized ONUs transmit data on the data sub-channel. A synchronous subchannel is a group of adjacent subcarriers with a length of L, and its number is marked as {i _r (n); n=0,1,...L-1}. The synchronization code is a CAZAC sequence of length L in the frequency domain, denoted as cr _r =[c(0),c(1),...,c(L-1)] ^T , and then insert NL zeros to form the length Signal X _r =[X _r (0), X _r (1),...X _r (N-1)] ^T for N signal, namely

The signal X _r undergoes the inverse discrete Fourier transform of N points to obtain the signal x _r . The signal x _r is repeated twice, and a cyclic prefix is added to obtain a synchronous OFDM frame. The length of a synchronous OFDM frame is 2N+N _g , in fact, N is the length of the FFT, and N _g is the length of the cyclic prefix. This synchronous OFDM frame structure can ensure that at the receiving end of the OLT, only one FFT window contains a complete synchronization code.

The detection of above-mentioned step three synchronization codes, its specific process is as follows:

The OFDM frame obtained at the OLT end, after removing the cyclic prefix, obtains the time-domain signal y=[y(0),y(1),...y(N-1)] ^T , after discrete Fourier transform Hereafter, the obtained frequency domain signal is denoted as Y=[Y(0), Y(1),...Y(N-1)] ^T . In order to detect whether the symbol contains a complete synchronization code, the signal Y and all possible synchronization codes are cross-correlated in the frequency domain, namely:

Then a threshold n is set to judge whether to include complete synchronization code, and the threshold n used is:

If E _g >η, and E _g is the maximum value of several adjacent symbols, it can be considered that the OFDM symbol contains a complete synchronization code.

The specific operation method of time offset calculation in step 4 is as follows

After the synchronization code is detected, the time offset needs to be calculated. The circular convolution is performed using the received time domain signal y and the signal _xr , and the time offset can be determined at the maximum point of the circular convolution. In order to reduce the time complexity of calculation, circular convolution in the time domain can be realized by multiplying in the frequency domain. on signal Perform inverse discrete Fourier transform to obtain z _r ={z _r (n):n=0,1,...N-1}. time offset by

Compared with the prior art, the present invention has the following prominent substantive features and remarkable advantages:

The present invention utilizes the zero autocorrelation characteristic of the CAZAC sequence, detects the synchronization code and calculates the time offset, can accurately realize the time synchronization of OFDM-PON, and the method of the present invention has relatively low time complexity, and is suitable as an interface Network access synchronization solution.

Description of drawings

FIG. 1 is a schematic diagram of a system structure of a PON according to the related art.

Fig. 2 is a schematic diagram of the OFDM-PON uplink time synchronization method of the present invention.

Fig. 3 is a schematic diagram of a synchronous OFDM frame of OFDM-PON according to the present invention.

Fig. 4 is the error rate of time offset estimation under different signal-to-noise ratios of the OFDM-PON uplink time synchronization method of the present invention.

Fig. 5 is the error rate of time offset estimation under the condition that different numbers of ONUs simultaneously perform the synchronization process in the OFDM-PON uplink time synchronization method of the present invention.

Detailed ways

Embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings.

In the OFDM-PON uplink, the signal received by the OLT is the superposition of signals sent by multiple ONUs. In order to obtain the transmitted signals of all ONUs through correct demodulation at the OLT end, the signals of each ONU need to be aligned at the OLT end to achieve precise time synchronization. The present invention provides a method for OFDM-PON uplink time synchronization, the specific steps of which are as follows:

Step 1. When an ONU that has not been synchronized needs to access the OFDM-PON system, it first performs downlink frame synchronization, and obtains parameters related to uplink transmission and usable synchronization codes and other information from the downlink frame information.

Described synchronization code is constant envelope zero autocorrelation (CAZAC) sequence, and its expression is: Where j is the imaginary unit, L is the length of the sequence, and r is a constant that is relatively prime to L. CAZAC sequences have ideal cyclic autocorrelation properties.

Step 2: The ONU randomly selects an available synchronization code and selects a synchronization time slot, modulates the synchronization code onto the synchronization sub-channel, and then sends the synchronization OFDM frame to the OLT.

The synchronization sub-channel is specially reserved for synchronization. Synchronized ONUs transmit data on the data sub-channel. As shown in Figure 2, in the OFDM-PON uplink, the synchronized ONU transmits data on the data sub-channel, and the unsynchronized ONU sends a synchronous OFDM frame on the synchronous sub-channel when it needs to access the system.

A synchronous subchannel is a group of adjacent subcarriers with a length of L, and its number is marked as {i _r (n); n=0,1,...L-1}. The synchronization code is a CAZAC sequence of length L in the frequency domain, denoted as cr _r =[c(0),c(1),...,c(L-1)] ^T , and then insert NL zeros to form the length Signal X _r =[X _r (0), X _r (1),...X _r (N-1)] ^T for N signal, namely

The signal X _r undergoes the inverse discrete Fourier transform of N points to obtain the signal x _r . The signal x _r is repeated twice, and a cyclic prefix is added to obtain a synchronous OFDM frame. The length of the synchronous OFDM frame is 2N+N _g , in fact, the length of N for FFT, and N _g is the length of the cyclic prefix. The structure of a synchronous OFDM frame is shown in FIG. 3 . This synchronous OFDM frame structure can ensure that at the receiving end of the OLT, only one FFT window contains a complete synchronization code.

Step 3: The OLT detects the synchronization code, and detects whether each received OFDM symbol contains a complete synchronization code.

The specific process of the described synchronous detection is as follows:

The OFDM frame obtained at the OLT end, after removing the cyclic prefix, obtains the time-domain signal y=[y(0),y(1),...y(N-1)] ^T , after discrete Fourier transform Hereafter, the obtained frequency domain signal is denoted as Y=[Y(0), Y(1),...Y(N-1)] ^T . In order to detect whether the symbol contains a complete synchronization code, the signal Y and all possible synchronization codes are cross-correlated in the frequency domain, namely:

Then a threshold n is set to judge whether to include complete synchronization code, and the threshold n used is:

If E _g >η, and E _g is the maximum value of several adjacent symbols, it can be considered that the OFDM symbol contains a complete synchronization code.

Step 4: After the OLT detects the synchronization code, it calculates the time offset, and then broadcasts a synchronization response message, which includes the detected synchronization code and time offset information.

The detection of the synchronization code and the calculation of the time offset, its specific process is as follows:

After the synchronization code is detected, the time offset needs to be calculated. The circular convolution is performed using the received time domain signal y and the signal _xr , and the time offset can be determined at the maximum point of the circular convolution. In order to reduce the time complexity of calculation, circular convolution in the time domain can be realized by multiplying in the frequency domain. on signal Perform inverse discrete Fourier transform to obtain z _r ={z _r (n):n=0,1,...N-1}. time offset by

Step 5. After the ONU receives the synchronization response message, it compares it with the synchronization code sent by itself. If it matches, it adjusts it according to the time offset in the synchronization response message. After the adjustment, the OFDM symbols sent by the ONU are aligned at the OLT . If the synchronization process is unsuccessful, the ONU reselects a synchronization time slot to send the synchronization code.

In the simulation process shown in Figure 4, an OFDM-PON application scenario with one OLT and four ONUs is designed. Among them, 3 ONUs have already been synchronized, and 1 ONU is in the process of synchronizing. The uplink channel is composed of N=512 subcarriers, the length of the cyclic prefix is Ng=32, and the number of subcarriers included in the synchronous subchannel is _L =50. On the data subcarrier, the modulation format of the data is 16QAM. Fig. 4 is the error rate of time offset estimation under different signal-to-noise ratios of the OFDM-PON uplink time synchronization method of the present invention. The present invention can estimate the time offset relatively accurately.

Fig. 5 is the result of off-line experiments conducted under the OFDM-PON platform with direct detection of intensity modulation. Use Matlab and arbitrary waveform generator (Arbitrary waveform generator, AWG) to generate the transmission signal, the signal bandwidth is 900MHz, and the clock frequency of AWG digital-to-analog conversion is 2Gb/s. Fiber Channel is 25km of single-mode fiber. The receiving end adopts a clock of 4Gb/s, and then performs offline processing. The parameter setting is the same as that of the simulation in Figure 4. Fig. 5 is the error rate of time offset estimation under the condition that different numbers of ONUs simultaneously perform the synchronization process in the OFDM-PON uplink time synchronization method of the present invention. It can be seen that when multiple ONUs are synchronizing simultaneously, the method of the present invention can still guarantee relatively high accuracy.

Claims (5)

1. a kind of OFDM-PON uplink temporal synchronization method, which is characterized in that specific steps are as follows:

Step 1: not yet synchronous ONU carries out downlink frame synchronization, from downlink frame when needing to access OFDM-PON system first The information such as the related parameter of uplink and the synchronous code being able to use are obtained in information；

Step 2: ONU randomly chooses a usable synchronous code, and a synchronization time slot is selected, synchronous code is modulated to synchronization In subchannel, synchronous OFDM frame is then sent to OLT；

Step 3: OLT synchronizes the detection of code, detect whether each OFDM symbol received includes complete synchronization Code；

Step 4: calculating time migration after OLT detects synchronous code, a synchronous response message, the message are then broadcasted Including the synchronous code and time migration information detected；

Step 5: be compared after ONU receives synchronous response message with the synchronous code oneself sent, if it does, then according to Time migration inside synchronous response message is adjusted, and the OFDM symbol that the ONU is sent after adjusting just is aligned at the end OLT If synchronizing process is unsuccessful, ONU if, reselects a synchronization time slot and sends synchronous code；

In OFDM-PON uplink, the signal that OLT is received is the superposition that multiple ONU send signal, is demodulated at the end OLT Signal to the transmission signal of ONU, each ONU is aligned at the end OLT, realizes OFDM-PON uplink temporal synchronization.

2. a kind of OFDM-PON uplink temporal synchronization method according to claim 1, which is characterized in that the step Synchronous code in one is permanent envelope zero auto-correlation sequence, i.e. CAZAC sequence, expression formula are as follows:Wherein j is imaginary unit, and π is pi, and L is the length of sequence, r It is the constant relatively prime with L, CAZAC sequence has ideal circulation autocorrelation performance.

3. a kind of OFDM-PON uplink temporal synchronization method according to claim 1, which is characterized in that the step The production method of synchronization OFDM frame in two is as follows: synchronization suchannel is specially to reserve to carry out time synchronization, together For the ONU of step in subchannel data transmitting data, synchronization suchannel is the adjacent sub-carrier that one group of length is L, and number is denoted as {i_r(n)；N=0,1 ... L-1 }；Synchronous code is denoted as c in the CAZAC sequence that frequency domain is exactly that a length is L_r=[c (0), c (1),...,c(L-1)]^T, it is inserted into the signal X that N-L zero formation lengths are N_r=[X_r(0),X_r(1),...X_r(N-1)]^T, I.e.

Signal X_rSignal x is obtained by the inverse discrete Fourier transform of N point_r；Signal x_rIt is repeated twice, adds cyclic prefix, obtain Length to synchronous OFDM frame, synchronous OFDM frame is 2N+N_g, wherein N is the length for carrying out FFT, N_gIt is the length of cyclic prefix； The structure of this synchronous OFDM frame guarantees in the receiving end of OLT, only one FFT window is to contain complete synchronous code.

4. a kind of OFDM-PON uplink temporal synchronization method according to claim 1, which is characterized in that the step The specific operation method is as follows for synchronous code detection in three:

In the OFDM frame that the end OLT obtains, remove obtain after cyclic prefix time-domain signal y=[y (0), y (1) ... y (N-1) ]^T, after discrete Fourier transform, obtained frequency-region signal is denoted as Y=[Y (0), Y (1) ... Y (N-1)]^T；In order to Whether detecting the symbol, to include complete synchronous code, signal Y and all possible synchronous code carry out computing cross-correlation in frequency domain, That is:

Then one thresholding η is set to determine whether comprising complete synchronous code, the thresholding η used are as follows:

If E_g> η, and E_gIt is the maximum value of adjacent several symbols, then it is assumed that the OFDM symbol contains complete synchronous code.

5. a kind of OFDM-PON uplink temporal synchronization method according to claim 1, which is characterized in that the step The specific operation method is as follows for time migration calculating in four:

It after detecting synchronous code, needs to calculate time migration, uses the time-domain signal y and signal x received_rCarry out circulation volume Product, cyclic convolution maximum of points determine time offset；In order to reduce the time complexity of calculating, the cyclic convolution of time domain passes through Product realization is carried out in frequency domain；To signalCarry out discrete fourier inversion Get z in return_r={ z_r(n): n=0,1 ... N-1 }；Time migration is