CN1283053C - A Method for Obtaining Rough Synchronization of OFDM System Using PN Code Synchronization Channel - Google Patents

Abstract

The present invention relates to a method which adopts a PN (a pseudorandom sequence) code synchronization channel to obtain OFDM (orthogonal frequency division multiplexing modulation) coarse synchronization, which belongs to the field of mobile communication OFDM modulation demodulation technology. The method is characterized in that a PN code with good self correlativity is used as data of the synchronization channel, a kind of PN code configuration is set to select patterns, and the maximal correlated peak output is obtained through the match and the calculation of a correlator in a period of time. Compared with the patterns of the PN code, frame synchronizing signals for receiving the data are obtained, and the ambiguity of time slots is effectively restrained. Meanwhile, a synchronized state interpreter is adopted, the time range is calculated according to an output result of the correlator and the correlator, and coarse synchronization signals are effectively determined to provide an accurate basis for the next time-frequency accurate synchronous calculation. The method overcomes the defect that CP (a cyclic prefix) is simply used to carry out the coarse synchronization.

Description

A Method for Obtaining Rough Synchronization of OFDM System Using PN Code Synchronization Channel

technical field

It is used in the next generation mobile communication OFDM modulation and demodulation system to improve coarse synchronization performance, overcome time slot ambiguity, obtain frame synchronization signal, and make a more accurate basis for fine synchronization.

Background technique

The goal of the next generation of mobile communication is to provide higher transmission rates and higher spectrum utilization. Based on this goal and the successful application of OFDM technology in existing communication systems, OFDM technology is selected as the next generation of mobile communication. Core modem technology.

In the OFDM modulation and demodulation system, the acquisition of synchronization is very important. In the wireless transmission link, under the conditions of dispersion fading and fast moving channel, the demodulator needs to complete a series of operations such as FFT operation and channel estimation on the premise of knowing accurate synchronization. Synchronization technology is usually divided into two steps: coarse synchronization and fine synchronization. If the communication system uses data frames to solve the problem of multi-user and multi-point access, the synchronization technology should be able to achieve frame synchronization and overcome time slot ambiguity. Provide a certain basis for judgment for fine synchronization.

The previous coarse synchronization technology is based on CP, the synchronization speed is slow, and the synchronization timing range is relatively rough, and the anti-noise performance is relatively poor. The method of simply adding a PN sequence in front of the slot (time slot) only achieves slot synchronization, and it is difficult to achieve frame synchronization. However, the cellular mobile communication system is still used in the next generation of mobile communication, and its data system requires a certain data frame structure. Therefore, the method of simply increasing the PN sequence synchronization codeword cannot overcome the slot (time slot) ambiguity. Based on the above two considerations, a new coarse synchronization algorithm that guides the synchronization channel according to a certain PN sequence allocation pattern is proposed.

Contents of the invention

The content of the present invention is to provide a method for obtaining coarse synchronization of an OFDM system by using a PN code synchronization channel, which can simultaneously realize frame synchronization and time slot synchronization.

The present invention is characterized in that:

(1) Use the PN code correlation matching algorithm to obtain slot (time slot) synchronization, and then use the unique distribution pattern of PN codes in different time slots in each frame to obtain frame synchronization, eliminate the ambiguity of time slot synchronization, and construct a synchronization state transition machine To ensure synchronization reliability;

(2) The first kind of PN code is represented by "0", and the second kind is represented by "1" so as to form a linear code word;

(3) The processing result of resisting multipath fading and eliminating white noise interference as much as possible is obtained by dividing the correlation calculation results of step (1) by the instant average power within this period and performing normalization ;

(4) The synchronous state transition machine judges whether the position reaches the agreed rough timing starting point by whether the same sampling position has obtained the minimum code distance for 3 consecutive times.

The present invention constructs a coarse synchronous transmission and reception system scheme, the transmission scheme is mainly designed as OFDM synchronous channel frame format, and the reception scheme is based on the coarse synchronization algorithm description of the frame format, which is used to ensure reliable synchronization signals and overcome time slot ambiguity.

In the designed OFDM synchronous channel frame format (Fig. 1), a time slot period is T, S time slots are a frame, and F frames constitute a superframe. There are K samples in total in each time slot, and each time slot includes N OFDM symbols in total. Within a time slot, there are one or more time-frequency pilot sequences inserted between OFDM symbols. There may be several OFDM symbols in one time slot, and known frequency-domain pilot symbols are inserted at certain intervals.

Based on such a synchronous channel structure, it is considered to use a unique long PN sequence code as a PN sequence for synchronous search. In this frame structure, each time slot has a PN code as the time slot guide sequence (the length is set to P sampling points), and through the good autocorrelation performance of the PN sequence, the coarse synchronization of the frame timing can be obtained through the guide sequence result. In the scheme, there are S PN code groups in a complete frame, using two different PN codes (PN1 and PN2), and the two PN codes are distributed in a frame signal with a certain sequence pattern (illustrated in Figure 2 A kind of PN code distribution pattern, wherein 0 represents the first PN code, 1 represents the second PN code), according to the sequence pattern, it can be known that the time slot where the PN code is located is which time slot in the frame, and also This pattern information can be used to obtain a frame synchronization signal, which is conducive to accelerating the completion of rough synchronization and reducing the probability of losing data frames during the synchronization process.

The basic idea of the coarse synchronization algorithm is to first calculate the correlation matcher of the two PN codes respectively. The correlation matcher is to multiply and accumulate the two sequences and output them. According to the autocorrelation characteristics of the PN sequence, if the received signal is synchronized with the local PN sequence , when the starting point is the same, then multiplying and accumulating them will output a very high peak value, otherwise it will only output a very low amplitude. The output result of the correlator needs to be normalized by power, so that the influence of multipath and Gaussian white noise can be smoothed, and then the threshold value setting (for example, the threshold value can be set according to half of the maximum peak value) and hard judgment (greater than The correlation peak of the threshold keeps recording its position, and the correlation peak smaller than the threshold can be discarded), so as to obtain the distribution pattern of the two PN codes at the slot head of each time slot, which may be the result pattern of the strongest path (at this time it should be The best synchronization result), it may also be the result pattern of other multipath (due to the randomness of multipath, this kind of synchronization result is not reliable).

Accompanying drawing 2 is the distribution configuration scheme of two kinds of PN codes in ten time slot headers of one frame. 0 represents the first type of PN code, and 1 represents the second type of PN code; that is to say, in Figure 2, there are 10 grids of time slots, and each grid represents the synchronization data of a time slot. If the time slot Marked as 0, we insert the first type of PN code as the synchronization data of the time slot, if the time slot is marked as 1, we insert the second type of PN code as the synchronization data of the time slot. After setting in this way, the PN code configuration pattern of 10 time slots in one frame can be regarded as a code word (that is, according to the corresponding relationship between the two PN sequences and 0 and 1 agreed in Figure 2), this pre-set The determined codeword is compared with the codeword obtained by synchronous search and detection, and the Hamming code distance between the two is calculated (the Hamming code distance is the unequal number of corresponding position data of the two codewords), and the code distance is the smallest ( It should be 0 under ideal synchronization conditions) In that case, it is a sign of obtaining synchronization status.

Compared with simply using CP (Cyclic Prefix), it has a more accurate estimated range of coarse synchronization, and also effectively overcomes the problem of time slot ambiguity, and provides a coarse synchronization signal of the data frame.

Description of drawings

Figure 1 shows the OFDM data structure, subdivided from the superframe to the final synchronous PN code;

Figure 2. Configuration pattern of two PN codes in ten different time slot headers in one frame, 0 means that the first type of scrambling code is inserted into this time slot, and 1 means that the second type of scrambling code is inserted into this time slot;

Figure 3 block diagram of coarse synchronization algorithm;

Fig. 4 is a schematic diagram of the state transition of the synchronous state transition machine, 0 represents the out-of-synchronization signal input to the state machine, and 1 represents the synchronous signal;

Fig. 5 is an example of synchronous channel frame format parameters;

Figure 6 PN code correlation result pattern explanation.

Figure 7 PN code generator block diagram, wherein No. 1 PN code and No. 100 PN code are the first PN code and the second PN code in this patent illustration

Detailed ways

The specific calculation steps (accompanying drawing 3) are:

(1) Correlate the input signal with the local PN code, and obtain the cumulative correlation results temp1 and temp2 with the two PN codes (PN1 and PN2) respectively;

(2) Outputting the power average value within a period of time (such as within several sampling points) while the signal sequence correlation operation is performed;

(3) Divide the results temp1 and temp2 of the correlation operation by the real-time average power within this period of time, and perform normalization processing to obtain tt1 and tt2. The purpose of this processing is to resist multipath fading and minimize Eliminate white noise interference;

(4) Set the corresponding threshold based on the strongest peak detected. Generally, the strongest peak means obtaining slot synchronization, considering the influence of multipath and time-varying fading, and other strong peaks exceeding the threshold should also be considered. A PN code configuration pattern within a frame-long time range can be obtained by making a hard judgment on tt1 and tt2 according to the threshold.

(5) The pattern of the PN code is regarded as a kind of linear code, and the two PN code configuration pattern sequences after the hard decision are used to calculate the code distance respectively, and the code distance obtained by the two sequences is added up to obtain the code The distance from the minimum point (the point that should be 0 in theory) is the correct frame coarse timing starting point.

(6) Input the result of step 5 into the synchronous state transition machine, and the synchronous state transition machine judges whether the system has reached the coarse synchronous state.

The synchronous state transition machine is a state transition algorithm designed to ensure the timing of coarse synchronization in a relatively long period of time (figure 4). It is divided into synchronous state and tracking state, three consecutive out-of-synchronization (or acquisition of synchronization) is the transition condition from synchronous state to out-of-synchronization state (or from out-of-synchronization state to synchronous state). When the PN pattern in a frame obtained by the detection result of a frame time is different from the known PN code configuration pattern (that is, the code distance is not equal to 0), the input of the state conversion machine is 0, which means that the frame PN code pattern does not match, Frame out of synchronization; when the PN pattern in the detection result is identical to the known PN code configuration pattern (that is, the code distance is equal to 0), the state switch input is 1, which means that the frame PN code pattern matches and the frame is synchronized. In Figure 4, T0, T1, and T2 are three tracking states, T0 represents the initial state; T1 is the state of obtaining a frame synchronization, when the input is 1, it can be transferred from the T0 state to the T1 state, and when the input is 0 , the T1 state is transferred to the T0 state; T2 is the state of obtaining frame synchronization twice in a row, that is to say, when the input is 1, the state can be transferred from the T1 state to the T2 state; similarly, when the input is 0, the state is transferred from the T2 state to the T1 state . S0, S1, and S2 are three synchronous states, and S0 represents a synchronous stable state. When 1 is input on the basis of the T2 state, it can be transferred from the T2 state to S0. At this time, the system is marked to enter the synchronous state, and the coarse synchronization at this time should be followed. The signal undergoes follow-up demodulation and other processing; when the input is 0 on the basis of S0, it means that the system temporarily loses synchronization and shifts to the S1 state. If the next input is 1, it means that the system returns to the synchronization state S0, and the system is still in normal synchronization; When 0 is input on the basis of S1, it means that the system loses synchronization again, and it needs to switch to S2 state, and then inputting 0 in S2 state means that the system loses synchronization for 3 consecutive times, the system is no longer stable and synchronized, and enters T0 tracking state to continue Perform a rough synchronization search, if you input 1, you can return to the synchronization state of S0. This mechanism of three consecutive synchronizations or out-of-synchronizations as the state transition between the synchronization state and the tracking state effectively ensures the maintenance of the system synchronization state and avoids the interference of the system by unexpected situations. This section uses a practical example to illustrate how the coarse synchronization algorithm works.

First, set the frame format parameters of the sync channel at the sending end. In the frame format, a time slot period is 1 ms, 10 time slots are a frame, and 256 frames form a super frame. There are a total of 10240 samples in each time slot, and each time slot includes a total of 8 OFDM symbols (Fig. 5). Within a time slot, there are one or more time-frequency pilot sequences inserted between OFDM symbols. The selection of the PN code is based on the principle of the pseudo-random sequence generator to generate two kinds of scrambling codes. For example, we use a PN code generator used in the WCDMA downlink as an example. The structure diagram of the PN code generator is as follows Shown in accompanying drawing 7. Among the PN codes generated by the PN code generator, the No. 1 and No. 100 PN codes are selected as the PN codes of the two configurations in our rough synchronization algorithm. The first PN code (No. 1 PN code) corresponds to 0 in the accompanying drawing 2, that is to say we insert the first PN code as synchronization data on the time slot marked 0 in the accompanying drawing 2; the second PN code ( No. 100 PN code) corresponds to 1 in the accompanying drawing 3, that is to say we insert the second kind of PN code as synchronous data on the time slot marked 1 in the accompanying drawing 2.

Figure 6 illustrates how the receiving end judges coarse synchronization. In Figure 6, a represents a correlation result pattern searched within a certain period of time (usually a frame or an integer multiple of a frame), and this correlation result is a matching correlation result of two PN codes. If the correlation result of the first PN code A long and narrow peak appears, indicating that the PN code has been synchronized. Similarly, a long and narrow peak in the correlation result of the second type of PN code indicates that the second type of PN code has obtained synchronization. When using the first type of PN code for correlation to obtain the peak value, it can be considered that the time slot is configured with the first type of PN code, that is to say, the time slot is marked with "0"; when the second type of PN code is used to obtain the peak value, it can be considered that the time slot The slot is configured with the second type of PN code, that is to say, the slot is marked with "1". After such an operation, a code word of a PN code configuration pattern in one frame can be obtained. In Figure 6, the code word obtained from the correlation result of a is "0000110010". Comparing this code word with the pre-set code word (attached to Figure 2), the original code word is "0010110011", which can be seen to be different The number of digits is 2, that is to say, the Hamming code distance is 2, indicating that the correlation result is not well synchronized and does not match, which may be an erroneous result caused by multipath or noise interference. And b in accompanying drawing 6 according to above-mentioned processing method, the obtained code word is " 0010110011 ", compared with the pre-set code word (accompanying drawing 2), the Hamming code distance is 0, shows that this correlation result obtains synchronization . Obtaining such a rough synchronization result at the same sampling position three times in a row indicates that the position is in a synchronous state.

Claims (4)

1. utilize pseudo random sequence PN sign indicating number synchronizing channel to obtain the thick synchronous method of OFDM, it is characterized in that: utilize PN sign indicating number correlation matching algorithm to obtain slot synchronization, utilize the distribution pattern of different time-gap PN sign indicating number uniqueness in every frame to obtain frame synchronization then, eliminate the ambiguity of slot synchronization, construct the synchronous regime interpreter simultaneously and guarantee synchronization dependability, it contains following steps successively:

(1) at transmitting terminal, adopt PN sign indicating number synchronous channel structure, promptly in frame structure PN sign indicating number of each time slot allocation as the time slot homing sequence, length is P sampled point, in a complete frame, be divided into S the PN code character that equates with timeslot number, adopt a kind of in two kinds of different PN sign indicating numbers respectively, form certain sequence pattern and be distributed in the frame signal;

(2) while carries out anti-multipath decline processing to above-mentioned related operation result respectively and eliminates white noise as far as possible and disturb processing;

(3) set corresponding threshold value according to detected each time slot highest peak in the above-mentioned processing procedure, according to threshold value above-mentioned result is declared firmly respectively, promptly if first kind of PN sign indicating number relevant peaks surpasses threshold value then to this time slot note " 0 ", if second kind of PN sign indicating number relevant peaks surpasses threshold value then to this time slot note " 1 ", obtain the interior PN sign indicating number configuration pattern of time range of a frame length;

(4) the PN sequence pattern that transmitting terminal is set respectively with declare firmly after two PN configuration patterns ask the computing of code distance, and, judge that frame by frame the code distance smallest point that obtains is the correct thick timing of frame starting point with the code distance addition that two sequences are tried to achieve;

(5) result of step (4) is input to synchronous state machine and judges whether the thick synchronization timing that reaches stable.

2. the PN sign indicating number synchronizing channel of utilizing according to claim 1 obtains OFDM coarse synchronization signal method, it is characterized in that: described first kind of PN sign indicating number be with " 0 " expression, second kind with " 1 " thus expression constitutes a linear code word;

3. the PN sign indicating number synchronizing channel of utilizing according to claim 1 obtains OFDM coarse synchronization signal method, and it is characterized in that: the result that described opposing multipath fading is also eliminated the white noise interference as far as possible is by the related operation result of step (1) is obtained divided by the instant average power within this period and as normalized respectively;

4. the PN sign indicating number synchronizing channel of utilizing according to claim 1 obtains OFDM coarse synchronization signal method, it is characterized in that: described synchronous regime interpreter obtains to arrive when code distance is minimum judges this position thick synchronous state continuous 3 times by same position, if do not have continuous 3 acquisition code distance minimums at same position, so just judge not arrive thick synchronous state.

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