A method for assisting blind frame synchronization of a polar code with bit assistance

Abstract

The application provides a method for assisting bit-assisted polar code blind frame synchronization, comprising the following steps: S1. before polar encoding, A bit auxiliary bits [a0, a1, …, a K‑1 ] are inserted into the front end of original K bit information bits [m0, m1, m2, …, m A‑1 ] to form K+A bit to-be-encoded sequence, and then the sequence is subjected to polar encoding; S2. at the receiving end, N long sliding windows are used to cut N long candidate sequences from the receiving sequence bit by bit, and N candidate sequences are cut for one time of blind frame synchronization; all the candidate sequences are subjected to decoding through SC or SCL decoder; S3. the original information bits are continuously decoded to obtain estimation, and the synchronization metric of the candidate sequence is calculated; the synchronization metric is used for representing the similarity between a sequence and a polar code permitted code word; S4. the first bit position of the sequence with the minimum synchronization metric SM t [N] in the candidate sequence which is not stopped early is selected as the estimated synchronization position.

Description

Technical Field

[0001] This invention belongs to the field of communications, and specifically relates to a polar code blind frame synchronization method with auxiliary bits. Background Technology

[0002] In digital communication systems, data is typically transmitted in the form of "frames." Frame synchronization is an essential step at the receiving end, its function being to accurately locate the start and end positions of a frame within the received bitstream, enabling the subsequent decoder to correctly decode the information contained within the frame. Traditional frame synchronization is achieved by inserting pilot sequences or frame synchronization words into the transmitting sequence and detecting them at the receiving end. To improve synchronization performance, one approach is to increase the length of the pilot sequence or frame synchronization word. The drawback of this method is that it leads to increased overhead, which may offset some of the gains from channel coding. Another method is to utilize the error correction capabilities of channel coding to assist frame synchronization. This method does not require the insertion of pilot sequences or frame synchronization words and is therefore also known as blind frame synchronization. While blind frame synchronization can reduce spectral overhead, this comes at the cost of higher complexity.

[0003] Currently, research on blind frame synchronization combining Turbo codes and LDPC codes is extensive, while research on combining polar codes is scarce. The first proposed method for blind frame synchronization using polar codes (Feng Z, Liu Y, Zhang S, et al. Polar-Coding-Assisted Blind Frame Synchronization Based on Soft Information of Frozen Bits. IEEE Commun. Letters, 2023, 27(10):2563-2567.) utilizes information from the frozen bits of the polar codes to calculate the probability that the current position is the start of a frame and proposes a synchronization metric. However, this method requires a large number of polar code decoders, resulting in high complexity. Therefore, optimizing existing blind frame synchronization algorithms for polar codes is meaningful. Summary of the Invention

[0004] This invention proposes a polar code blind frame synchronization method with auxiliary bits. This method replaces some frozen bits of the polar code with specific auxiliary bits. During decoding, the decoding results of these bits are detected, and candidate sequences whose decoding results do not match the set auxiliary bits are directly eliminated, thereby reducing computational complexity and improving synchronization performance.

[0005] The auxiliary bit-assisted polar code blind frame synchronization method of the present invention includes the following steps:

[0006] S1. Before polarization coding, in the original K information bits [m0, m1, m2, ..., m K-1 Insert the specified auxiliary bit A at the very beginning of the sequence [a0, a1, ..., a...]. A-1 This forms a (K+A) bit sequence to be encoded, and then polar coding is performed on this sequence to form an N-length polar code.

[0007] S2. At the receiving end, a candidate sequence of length N is truncated bit by bit from the received sequence using an N-length sliding window. A total of N candidate sequences are truncated in one blind frame synchronization. All truncated candidate sequences are decoded using an SC or SCL decoder. Due to the serial nature of the SC and SCL decoders, the A-bit auxiliary bits of all candidate sequences are decoded first, and the SC decoder obtains an estimate of the auxiliary bits. in, Let represent the estimate of the i-th auxiliary bit by the SC decoder, where i ∈ [0, A-1]; each decoding path of the SCL decoder will yield an estimate of the auxiliary bit. in, Let represent the estimated value of the i-th auxiliary bit for the k-th decoding path of the SCL decoder, where k∈[0, l-1]. The value can be 0 or 1. For SC decoders, if... If the decoding process is terminated early, otherwise step S3 is executed; for the SCL decoder, if If the decoding process is terminated prematurely, then proceed to step S3.

[0008] S3. Continue decoding to obtain an estimate of the original information bits. The synchronization metric of the candidate sequences is then calculated. The formula for calculating the synchronization metric is:

[0009]

[0010] Among them, SM t [i] represents the synchronization metric for decoding the t-th candidate sequence to the i-th bit, l t，i Let represent the log-likelihood ratio of the i-th leaf node of the t-th candidate sequence. This represents the set of frozen bits in a polar code. The synchronization metric characterizes the similarity of a sequence to a permissible codeword of a polar code; a smaller synchronization metric indicates that the position is more likely to be a synchronization position.

[0011] S4. Select the synchronization metric SM from the candidate sequences that have not stopped prematurely. t The position of the first bit of the smallest [N] sequence is used as the estimated synchronization position, i.e.

[0012] Define early stopping performance EP = 1 - P ret , where Pret The EP (Early Stop) is the proportion of candidate sequences that have completed the entire decoding process out of all candidate sequences. A higher EP indicates a higher proportion of candidate sequences that are stopped early, and better early stopping performance.

[0013] Preferably, the auxiliary bits in step S1 are selected according to the following principles:

[0014] S1.1. For two auxiliary bits in the same 2-length R1 node, set the first auxiliary bit to 1, and the second auxiliary bit can be arbitrarily selected as 0 or 1.

[0015] S1.2. For R1 nodes with a length greater than 2, decompose them into a series of R1 nodes with a length of 2, and then select auxiliary bits according to step S1.1.

[0016] S1.3. For a single auxiliary bit, if it is not the last auxiliary bit, it must be located in the Rep node, and its value should be set to 1.

[0017] S1.4. For auxiliary bits located on SPC nodes, decompose the SPC nodes into REP nodes and a series of R1 nodes, and then select them according to steps S1.1 to S1.3.

[0018] S1.5. The last auxiliary bit can be arbitrarily chosen as 0 or 1.

[0019] With the same length of auxiliary bits, steps S1.1 to S1.5 enable step S2 to achieve optimal early stopping performance and reduce complexity the most.

[0020] Preferably, given a certain code length and code rate, step S1 should select the auxiliary bit length that maximizes the reduction in overall complexity. The overall reduction in complexity for blind frame synchronization is defined as C - = EP × (C o -C ep ), C o The complexity of decoding an entire N-length codeword using SC or SCL is C. ep The complexity of early-stopped decoding can be estimated by the total number of f and g operations in the decoding process. Let the last auxiliary bit be the D-th bit of the codeword, then... C o And needs to be calculated based on the specific code length and code rate, C ep It needs to be calculated based on the code length, code rate, and auxiliary bit length.

[0021] Under the auxiliary bit selection principles in steps S1.1 to S1.5, the above-mentioned auxiliary bit length selection scheme can implement the method of the present invention with the lowest complexity.

[0022] The advantages and positive effects of this invention are as follows:

[0023] (1) The method of the present invention greatly reduces the complexity of existing polar code blind frame synchronization, which is beneficial to the implementation of hardware platforms.

[0024] (2) The method of the present invention improves the synchronization performance of polar code blind frame synchronization. Attached Figure Description

[0025] Figure 1 This is a schematic diagram of the method for inserting auxiliary bits in the present invention.

[0026] Figure 2 This is a schematic diagram of extracting candidate sequences from the received sequence.

[0027] Figure 3 This is the complexity reduction achieved by the method of the present invention under different auxiliary bit lengths.

[0028] Figure 4 This invention compares the frame error synchronization rate of the method under different auxiliary bit lengths. Detailed Implementation

[0029] The following will be combined with the appendix Figure 1-4 The present invention will be further described in detail with reference to the embodiments.

[0030] Taking a polar code with code length N=512, K=170, and A=6 auxiliary bits as an example, the execution process of auxiliary bit auxiliary polar code blind frame synchronization is explained. The frozen set is determined according to the 5G control channel polar code reliability sequence table provided by 3GPP.

[0031] Step 1: As Figure 1 As shown, before channel coding, a set 6 auxiliary bits are inserted at the beginning of the 170 information bits to form a total of 176 new information bits. These are then mapped to a 512-bit input vector u through a reliability sequence table, and then polar-coded into a 512-bit codeword x, which is then modulated and transmitted through the channel.

[0032] Step Two: As Figure 2 As shown, [...] m-1，1 x m-1，2 , ..., x m-1，N x m，1 [, ...] represents the codeword sequence to be sent, where x m，k This represents the k-th bit in the m-th codeword transmitted. After carrier synchronization and demodulation, the receiver obtains the received sequence [...]. m-1，1 y m-1，2 , ..., y m-1，N y m，1 [,...]. A total of 512 candidate sequences were extracted from the received sequence using a sliding window of length N bits. This represents the i-th N-length candidate sequence that has been truncated.

[0033] Step 3: Perform SCL2 decoding and synchronization metric calculation on all candidate sequences. When the 192nd bit of each candidate sequence has been decoded, as can be seen from the encoding method in Step 1, the decoding of 6 auxiliary bits has been completed. Compare these 6 auxiliary bits with the set value [1 0 1 1 1 0]. If they match the set value, the candidate sequence continues to be decoded and the synchronization metric calculation continues until the decoding of the sequence is completed; otherwise, the decoding of the candidate sequence is stopped, and the synchronization metric of the sequence is set to the quantizable maximum value.

[0034] Step 4: Take the position of the sequence with the smallest synchronization metric among the 512 candidate sequences as the estimated synchronization position to complete the blind frame synchronization of the polar code.

[0035] Table 1 shows the node configurations for the 6-bit auxiliary bits under the conditions of N=512, K=170, and A=6.

[0036] Table 1. Distribution of auxiliary bits

[0037] Auxiliary bit number node 1～2 2 long R1 nodes 3 4 long Rep nodes 4～6 4 long SPC nodes

[0038] According to a preferred embodiment of the present invention, the first, third, fourth and fifth auxiliary bits are selected as 1, and the other auxiliary bits can be arbitrarily selected. That is, selecting the 6 auxiliary bits as [1 0 1 1 1 0], [1 1 1 1 1 0], [1 0 1 1 1 1] or [1 1 1 1 11] can achieve the optimal early stopping performance under 6 auxiliary bits.

[0039] Figure 3 The results show the average complexity reduction per decoder in a single blind frame synchronization under the conditions of N=512, K=170, and different auxiliary bit lengths, achieving their respective optimal early stopping performance. The vertical axis represents the total reduction in the number of f and g operations, the horizontal axis represents the length of the auxiliary bits, and the red line at the top represents the number of f and g operations per decoder without the method of this invention. The results show that, under this selected polar code, the scheme with an auxiliary bit length of 6 can achieve polar code blind frame synchronization with the lowest complexity.

[0040] Figure 4 The paper presents a comparison between the synchronization error rate of the method of the present invention and the existing polar code blind frame synchronization method under different auxiliary bit lengths, with N=512 and K=170. The comparison results show that the synchronization error rate of the method of the present invention is lower than that of the existing polar code blind frame synchronization method when the auxiliary bit length is less than or equal to 7, and it has better synchronization performance.

Claims

1. A polar code blind frame synchronization method assisted by auxiliary bits, characterized in that, Includes the following steps: S1. Before polar coding, in the original K information bits Insert the set A-bit auxiliary bit at the very beginning This forms a K+A bit sequence to be encoded, and then polar encoding is performed on this sequence to form an N-length polar code. S2. At the receiving end, a candidate sequence of length N is truncated bit by bit from the received sequence through an N-length sliding window, and a total of N candidate sequences are truncated in one blind frame synchronization; all truncated candidate sequences are decoded by an SC or SCL decoder. S3. Continue decoding to obtain an estimate of the original information bits. And calculate the synchronization metric of the candidate sequences; The synchronization metric is used to characterize the similarity between a sequence and a polar code allowed codeword. The smaller the synchronization metric, the more likely the position of the candidate sequence is to be a synchronization position. S4. Select a synchronization metric from the candidate sequences that have not stopped prematurely. The position of the first bit of the smallest sequence is used as the estimated synchronization position, i.e. ; Define early stopping performance ,in, The EP (Early Stop) is the proportion of candidate sequences that have completed the entire decoding process out of all candidate sequences. A higher EP indicates a higher proportion of candidate sequences that are stopped early, and better early stopping performance.

2. The method for blind frame synchronization of polar codes with auxiliary bits as described in claim 1, characterized in that: The auxiliary bits in step S1 are selected according to the following principles: S1.

1. For two auxiliary bits in the same R1 node of length 2, set the first auxiliary bit to 1, and the second auxiliary bit can be arbitrarily selected as 0 or 1; S1.

2. For R1 nodes with a length greater than 2, decompose them into a series of R1 nodes with a length of 2, and then select auxiliary bits according to step S1.

1. S1.

3. For a single auxiliary bit, if it is not the last auxiliary bit, then it must be located in the Rep node, and its value should be set to 1. S1.

4. For auxiliary bits located on SPC nodes, decompose the SPC nodes into REP nodes and a series of R1 nodes, and then select them according to steps S1.1~S1.3; S1.

5. The last auxiliary bit can be arbitrarily chosen as 0 or 1.

3. A polar code blind frame synchronization method with auxiliary bit assistance according to claim 1 or 2, characterized in that: Given a certain code length and code rate, step S1 should select the auxiliary bit length that reduces the overall complexity the most; where the overall reduction in complexity for blind frame synchronization is defined as... , The complexity of decoding an entire N-length codeword for SC or SCL. The complexity of early-stopped decoding is estimated using the total number of f and g operations during the decoding process. Let the last auxiliary bit be in the codeword's... Position, then It needs to be calculated based on the specific code length and code rate. It needs to be calculated based on the code length, code rate, and auxiliary bit length.

4. The polar code blind frame synchronization method with auxiliary bit assistance according to claim 1, characterized in that: In step S2, the A-bit auxiliary bits of all candidate sequences are decoded first, and the SC decoder obtains an estimate of the auxiliary bits. ;in, This indicates that the SC decoder is for the first... The estimated value of the auxiliary bit, .

5. The polar code blind frame synchronization method with auxiliary bit assistance according to claim 1, characterized in that: Each decoding path of the SCL decoder obtains an estimate of an auxiliary bit. ,in, This indicates the first step of the SCL decoder. The decoding path for the number The estimated value of the auxiliary bit, , The value can be 0 or 1.

6. The polar code blind frame synchronization method with auxiliary bit assistance according to claim 4, characterized in that: For SC decoders, if If the decoding process fails, the decoding process will be terminated early; otherwise, step S3 will be executed.

7. The method for blind frame synchronization of polar codes with auxiliary bits as described in claim 5, characterized in that: For an SCL decoder, if If the decoding process fails, the decoding process will be terminated early; otherwise, step S3 will be executed.

8. The method for blind frame synchronization of polar codes with auxiliary bits as described in claim 1, characterized in that: In step S3, the formula for calculating the synchronization metric is: ； in, Indicates the first The candidate sequence is decoded to the... Synchronization measure of bits Indicates the first The first candidate sequence The log-likelihood ratio of the leaf nodes This represents the set of frozen bits in a polar code.

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