TW202303625A - Method of selecting decoding strategy for data storage system comprising comparing a syndrome sum with thresholds of a number of decoders in order to select one of the decoders - Google Patents

Abstract

The present invention discloses a method of selecting a decoding strategy for a data storage system. In the method, a number of decoders are first provided. Next, a received word is read. Then, a syndrome sum of the received word is calculated. Afterwards, based on the syndrome sum, one decoder is selected from the decoders, and a parameter thereof is set. Finally, the selected decoder is applied to decode the received word. Each of the number of decoders has a corresponding threshold, and selecting the one decoder comprises selecting the decoder by comparing the syndrome sum with the thresholds. The thresholds are compared, in sequence from small to large, with the syndrome sum, and if the syndrome sum is smaller than the current threshold, the decoder corresponding to the current threshold is selected, otherwise the syndrome sum is compared with a next one of the thresholds. If the syndrome sum is not smaller than a maximum one of the thresholds, then the decoder corresponding to the maximum threshold is selected.

Description

Method for Selecting Decoding Strategy in Data Storage System

The present invention relates to NAND flash memory, especially applied to error control codes using Low Density Parity Check (LDPC) codes, especially a method for selecting a decoding strategy for a data storage system.

The applications of various storage devices, such as mobile phones, driving recorders, digital cameras, and network monitors, have grown rapidly in recent years. The rewritable non-volatile memory module (rewritable non-volatile memory module) has the characteristics of non-volatility, power saving, small size and fast reading and writing, so it is widely used in these devices.

In order to ensure the correctness of the stored data in the rewritable non-volatile memory module, before the data is written into the module, it will be encoded with an error control code to generate a series of check sequences, which will be stored together with the data in the In the rewritable non-volatile memory module.

However, factors inside or outside the device cause bit errors when reading data from the rewritable non-volatile memory module. Therefore, the original data is restored through the decoding mechanism.

The most commonly used error control codes in current NAND Flash are Low Density Parity Check code (Low Density Parity Check code: "LDPC") and Bose-Chaudhuri-Hocquenghem (Bose-Chaudhuri-Hocquenghem: "BCH") code. LDPC decoding module The formula covers hard decision decoding (Hard Decoding) and soft decision decoding (Soft Decoding).

The hard-decision decoding method only decodes the log-likelihood ratio (log-likelihood ratio: "LLR") corresponding to multiple memory packets read based on one read potential.

Soft-decision decoding needs to simultaneously read multiple bits from multiple memory packets by shifting the read potential multiple times. Each memory package will correspond to a binary sequence after multiple reads of different displacement potentials. These sequences correspond to the LLR values required for soft-decision decoding. The more accurate the LLR value of each bit is obtained by reading the potential shifts more times, the higher the success ratio of decoding is usually, but the decoding delay is caused by multiple readings.

According to the customary decoding process, after reading the data, enter the hard decision decoding mode to decode. Once the hard-decision decoding fails, the hard-decision decoding is performed after reading the data repeatedly, or the soft-decision is performed after one or several readings have obtained a more accurate LLR value per bit. This mode not only causes delay but also consumes power.

Therefore, how to determine how many times to read or which decoding mode to use is extremely important to the speed and accuracy of data reading.

The main purpose of the present invention is to provide a method for selecting a decoding strategy for a data storage system, so as to overcome the lack of prior art.

To achieve this, in the method, first, several decoders are provided. Next, a received word is read. Next, calculate the sum of the symptom values of the received word. Then, according to the sum of the symptom values, a decoder is selected from the decoders, and its parameters are set. Finally, the received word is decoded with the selected decoder.

1: Read control module

2: storage unit

3: Decoding configuration control module

4: Decoding module

5: Decoding decision control module

6: Command generation module

7: Output demultiplexer

8: Input multiplexer

D: Deviation value

D1-D4: Decoder

S10~S34: Steps

[ FIG. 1 ] is a schematic diagram of frame error rate curves of four decoders according to an embodiment of the present invention.

[Fig. 2] is a schematic diagram of the total number of error bits corresponding to the sum of symptom values.

[FIG. 3] is a block diagram of a device for selecting a decoding strategy in a data storage system according to an embodiment of the present invention.

[FIG. 4] is a flowchart of a method for selecting a decoding strategy in a data storage system according to an embodiment of the present invention.

A method for selecting a decoding strategy in a data storage system according to an embodiment of the present invention is described below with reference to related figures. A "decoding strategy" means decoding a received word with an appropriate decoder. In other words, a decoding strategy is a decoding procedure. To avoid confusion, "decoding strategy" is used to indicate the decoding procedure, and "method" is used to indicate the procedure of selecting one decoding strategy from several decoding strategies.

Various objects in this embodiment are drawn according to the proportion, size, deformation or displacement suitable for the description, and are not drawn according to the scale of the actual components, and are described first. Elements of the same and symmetrical arrangement in this embodiment are denoted by the same numerals. In addition, if directional terms such as "front, back, left, right, up, down, inside, outside" are used to describe this embodiment, they are indicated according to the specified viewing direction, and are not interpreted as limiting the present invention.

A data storage device includes at least one decoder. The type of error control code used, the parity check matrix, the decoding algorithm used to design the decoder, the parameter settings of the decoder itself, the accuracy of the LLR value of the received word, etc., will affect its frame error rate ( frame error rate: "FER"), decoding speed and energy consumption. Therefore, according to the error control code, parity check matrix, decoder, decoder setting parameters and LLR accuracy adopted by the error control module in the device, several decoding modules can be combined. Then, Simulate for multiple decoding modules to obtain FER under different raw error bits.

Under the condition that FER is 0.002, let FER_REQ=0.002, and FER_REQ is the required FER (required FER). Referring to Figure 1, the intersection of the horizontal line FER_REQ=2*10 ^-3 and the FER curves of the four decoders, it can be seen that when the number of error bits is less than or equal to 125 bits, almost all of the four decoders can meet the required FER. When the error is greater than 125 bits and less than or equal to 259 bits, decoders 2, 3, and 4 can almost all meet the required FER. When the error is greater than 259 bits and less than or equal to 380 bits, decoders 3 and 4 can almost achieve the required FER. When the error is greater than 380 bits and less than or equal to 440 bits, decoder 4 can almost always meet the required FER. When the error is larger than 440 bits, the four decoders can hardly achieve the required FER. Therefore, under the condition of FER_REQ=2*10 ^{−3 ,} the critical decoding capabilities of the four decoders in this embodiment are 125, 259, 380, and 440 bits respectively.

synd_sum_i<500，重新排序後E_i範圍在110到170個錯誤位元數。將A

synd_sum_i<B標示為(A+B)/2。 Referring to FIG. 2 , based on the characteristics of the LDPC matrix, the present invention estimates the number of error bits (error bits) of the received word from the sum of the syndrome values (syndrome sum: "synd_sum"). For example, the error control system adopts a parity-check matrix H of a low-density parity-check code, and the length of a legal code-word C (code-word) is N. In this embodiment, an equal number of 10 to 910 error bits is generated in advance, that is, the length of an error vector (error vector: "E") is N (length (E)=N) and the sum of the error vectors is 10 to 910 910. Use the formula synd_sum=sum(mod(H*E'), 2) to calculate the sum of its symptom values, "E'" represents the transposition matrix (transportation) of the vector E. In this way, many sequence pairs (synd_sum_i, sum(E_i)), i=0,1,2,...,1000000000 can be obtained. These sequence pairs are divided into intervals of 50 according to synd_sum, and the E_i range of the sequence pairs where synd_sum_i falls in this interval is counted. such as 450

synd_sum_i<500, E_i ranges from 110 to 170 error bits after reordering. Will A

synd_sum_i<B is denoted as (A+B)/2.

The error bit critical values of the four decoders in this embodiment are respectively 125, 259, 380, and 440 bits under the condition of FER_REQ=2*10 ⁻³ . As shown in Fig. 2, the sequence pair with synd_sum_i>500 has sum(E_i)>125. The order pair of synd_sum_i>950, its sum(E_i)>259. The sequence pair with synd_sum_i>1150 has sum(E_i)>380. For the sequence pair of synd_sum_i>1250, its sum(E_i)>440.

1150=synd_sum_thr_3的序對中，sum(E_i)

380。因此，第三解碼器為最佳解碼器。如此，計算收到的字的症狀值總和後，即可參考表1選擇適當解碼器以進行解碼策略(decoding strategy)。 According to Figures 1 and 2, the following table 1 can be deduced. The total threshold (synd_sum_thr) of the symptom value of each decoder is the lower limit of the sum range of the symptom value in FIG. 2 corresponding to the critical value of the number of error bits. For example, the words are received, and the sum of their symptom values is calculated to be 1000, and then, the decoder considering the least operation time and power consumption. The error bit threshold of decoder 3 is 380 bits. As shown in Figure 2, in synd_sum_i

1150=In the sequence pair of synd_sum_thr_3, sum(E_i)

380. Therefore, the third decoder is the best decoder. In this way, after calculating the sum of the symptom values of the received words, an appropriate decoder can be selected with reference to Table 1 to implement a decoding strategy.

Table 1

As shown in Figure 3, according to a preferred embodiment of the present invention, a device for selecting a decoding strategy for a data storage system includes a read control module 1, a storage Unit 2 , a decoding configuration control module 3 , a decoding module 4 , a decoding decision control module 5 , a command generation module 6 , an output demultiplexer 7 and an input multiplexer 8 .

The read control module 1 receives a read command, and analyzes its contents such as a read voltage, a position, and an encoder configuration.

The storage unit 2 is a memory, and is connected to the reading control module 1 . The storage unit 2 receives the read voltage, the position information, etc. from the read control module 1, and correspondingly reads a piece of data from a specific position in the memory. In the future, this data will be called "received word".

The decoding configuration control module 3 is connected to the reading control module 1 . The operation of the decoding configuration control module 3 will be described in detail later.

One end of the output demultiplexer 7 is connected to the storage unit 2 , and the other end is connected to the decoding configuration control module 3 . The decoding configuration control module 3 controls the output demultiplexer 7 to select a decoder. The function of the output demultiplexer 7 can be easily understood by those familiar with this technical field, so it will not be described in detail.

The decoding module 4 is connected to the output demultiplexer 7 . In other words, the decoding module 4 is connected to the storage unit 2 and the decoding configuration control module 3 through the output demultiplexer 7 . The decoding module 4 has several decoders, and the total number of these decoders is m. According to this example, m is 4. In other words, the decoding module 4 has four decoders D1, D2, D3 and D4. In other embodiments, the decoding module 4 may have less or more decoders.

The input multiplexer 8 is connected to the decoders D1, D2, D3 and D4. The input multiplexer 8 can be regarded as a part of the decoding module 4 .

During operation, the decoding configuration control module 3 receives the encoder configuration information from the reading control module 1 and generates a number of messages for controlling the decoding module 4 and the demultiplexer 7 . These messages include decoder-specific selection and parameter settings and Correspondence between received word and decoder. Therefore, in operation, according to the configuration of the decoding configuration control module 3, the decoder D1, D2, D3 or D4 is allocated to the received word, and the received word is output for decoding. In other words, each received word matches a decoder. However, the device can process multiple received words simultaneously.

The decoding decision control module 5 is connected to the decoding module 4 . During operation, the decoding decision control module 5 receives synd_sum, diff_0_1 and decoding status (decoding failure or success) from the decoding module 4 as the basis for decoding decision. The decoding decision control module 5 judges the information and determines the best decoding strategy, including the selection of the decoder and setting parameters of the decoder, such as the maximum number of decoding iterations (maximal iteration, iter_max).

The command generation module 6 is respectively connected to the reading control module 1 and the decoding decision control module 5 . During operation, the decoding decision control module 5 triggers the command generation module 6 to selectively output a re-read command (different read voltage or new encoder configuration message) according to the decoding strategy, and presents it in a specific format, For the reading control module 1 to use. In this way, the reading control module 1 is made to read again, and the decoding configuration control module 3 is made to provide new encoder configuration information.

During implementation, we can define diff_0_1 as the absolute value of the difference between the numbers of "0" and "1" in the received word. We are used to performing exclusive OR operation (XOR) with the random sequence before the original data is written. Therefore, under the condition that the numbers of "0" and "1" in the received word are nearly evenly distributed, the larger the value, the more error bits caused by the operation read voltage (read Vth). Therefore, when the amount of error bits caused by the reading voltage deviation is greater than the decoding capability of the decoding module 4, the decoding decision control module 5 outputs a decoding strategy, triggers the command generation module 6, and requires the reading control module Group 1 reads data from the storage unit 2 again according to different displacement reading voltages.

When the diff_0_1 of the word received is less than a preset deviation value D, and the sum of its symptom values (synd_sum) is calculated to be 800, it can be judged from Table 1 that the decoder D1 in the decoding module 4 is likely to decode successfully, so The decoding decision control module 5 outputs a A kind of decoding strategy triggers the command generation module 6, requires the reading control module 1, makes the decoding configuration control module 3, and uses the decoder D1 in the decoding module 4 to perform the received word Decode, and output the decoding result and status.

If the sum of the symptom values is calculated to be 1500, it can be seen that the decoding capability of the decoding module 4 is exceeded, so a decoding strategy is output to trigger the command generation module 6, requiring the reading control module 1 to use different reading voltages , re-read data from the storage unit 2, or adjust the setting parameters of the decoders.

As shown in FIG. 4, a method of selecting a decoding strategy using the above-mentioned apparatus will be described below.

At S10, the received word is read. Specifically, according to a preset read command, or using the command generation module 6, the read control module 1 can read one or more read voltages from a specific position in the storage unit 2 once or more read data. Each bit provides a binary sequence. Then, pass the binary sequence to the decoding module 4 .

At S12, several LLR values are generated. In detail, for each binary sequence obtained at S10, an LLR value is provided. Then, the LLR values are transmitted to the decoding module 4 through the output multiplexer 7 . The decoding module 4 receives a series of binary sequences from the output multiplexer 7 and maps them to a series of LLR values for the decoding module 4 to decode.

In S14, the sum of the symptom values of the received word and diff_0_1 are calculated. As mentioned above, diff_0_1 is the absolute value of the difference between the numbers of "0" and "1" in the received word. The decoding module 4 receives the received word from the output demultiplexer 7, and calculates the sum of symptoms and diff_0_1 of the received word.

In S16, it is judged whether the ratio of diff_0_1 (diff_0_1 divided by the statistical length) is greater than a deviation value D. If diff_0_1 is greater than or equal to the deviation value, go to S18, otherwise go to S20. In this preferred embodiment, the deviation D is calculated to be 2%.

In S18, the decoding decision-making control module 5 generates the module 6 via the command, and sends Out-of-position read voltage re-read command. Then, return to S10, and read data from the storage unit 2 according to the displacement read voltage.

If diff_0_1 is less than 2% of the statistical length, select an appropriate decoder based on the sum of symptom values and Table 1, and set parameters for the selected decoder. The method of the present invention will be described below by taking 5 scenarios as examples.

In the first scenario, the sum of symptom values is less than the sum of symptom values threshold of one.

In S20, set n to 1.

In S22, it is judged that the sum of symptom values is less than the sum threshold of symptom values 1, and the process goes to S28.

In S28, the decoder D1 is selected and its parameters are set.

Next, at S30, the received word is decoded by the decoder D1 according to its parameters and the LLR values.

In the second scenario, the sum of symptom values is between sum of symptom values threshold 1 and sum of symptom values threshold 2 .

In S20, set n to 1.

In S22, it is judged that the sum of symptom values is not less than the threshold value 1 of the sum of symptom values, and it goes to S24.

In S24, judge that 1 is less than 4, and go to S26.

At S26, n+1 is set. In other words, n becomes 2. Then, walk back to S22.

In S22, it is judged that the sum of symptom values is less than the threshold 2 of the sum of symptom values, and it goes to S28.

In S28, the decoder D2 is selected and its parameters are set.

Next, at S30, the received word is decoded by the decoder D2 according to its parameters and the LLR values.

In the third scenario, the sum of symptom values is between sum of symptom values threshold 2 and sum of symptom values threshold 3 .

In S20, set n to 1.

In S22, it is judged that the sum of symptom values is not less than the threshold value 1 of the sum of symptom values, and it goes to S24.

In S24, judge that 1 is less than 4, and go to S26.

At S26, n+1 is set. In other words, n becomes 2. Then, walk back to S22.

In S22, it is judged that the sum of symptom values is not less than the threshold value 2 of the sum of symptom values, and it goes to S24.

In S24, judge that 2 is less than 4, and go to S26.

At S26, n+1 is set. In other words, n becomes 3. Then, walk back to S22.

In S22, it is judged that the sum of symptom values is less than the sum threshold of symptom values 3, and it goes to S28.

In S28, the decoder D3 is selected and its parameters are set.

Next, at S30, the received word is decoded by the decoder D3 according to its parameters and the LLR values.

In a fourth scenario, the sum of symptom values is between the sum of symptom values threshold 3 and the sum of symptom values threshold 4 .

In S20, set n to 1.

In S22, it is judged that the sum of symptom values is not less than the threshold value 1 of the sum of symptom values, and it goes to S24.

In S24, judge that 1 is less than 4, and go to S26.

At S26, n+1 is set. In other words, n becomes 2. Then, walk back to S22.

In S22, it is judged that the sum of symptom values is not less than the threshold value 2 of the sum of symptom values, and it goes to S24.

In S24, judge that 2 is less than 4, and go to S26.

At S26, n+1 is set. In other words, n becomes 3. Then, walk back to S22.

In S22, it is judged that the sum of symptom values is not less than the sum threshold of symptom values 3, and Go to S24.

In S24, judge that 3 is less than 4, and go to S26.

At S26, n+1 is set. In other words, n becomes 4. Then, walk back to S22.

In S22, it is judged that the sum of symptom values is less than the threshold 4 of the sum of symptom values, and it goes to S28.

In S28, the decoder D4 is selected and its parameters are set.

Next, at S30, the received word is decoded by the decoder D4 according to its parameters and the LLR values.

In the fifth scenario, the sum of symptom values is greater than the sum of symptom values threshold 4.

In S20, set n to 1.

In S22, it is judged that the sum of symptom values is not less than the threshold value 1 of the sum of symptom values, and it goes to S24.

In S24, judge that 1 is less than 4, and go to S26.

At S26, n+1 is set. In other words, n becomes 2. Then, walk back to S22.

In S22, it is judged that the sum of symptom values is not less than the threshold value 2 of the sum of symptom values, and it goes to S24.

In S24, judge that 2 is less than 4, and go to S26.

At S26, n+1 is set. In other words, n becomes 3. Then, walk back to S22.

In S22, it is judged that the sum of symptom values is not less than the threshold value 1 of the sum of symptom values, and it goes to S24.

In S24, judge that 3 is less than 4, and go to S26.

At S26, n+1 is set. In other words, n becomes 4. Then, walk back to S22.

In S22, it is judged that the sum of symptom values is not less than the sum threshold of symptom values 4, and it goes to S24.

In S24, judge that 4 is not less than 4, and go to S28.

In S28, the decoder D4 is selected and its parameters are set.

Next, at S30, the received word is decoded by the decoder D4 according to its parameters and the LLR values.

No matter in that situation, it will go from S30 to S32.

In S32, it is judged whether the decoding is successful. If the decoding is successful, then end, otherwise go to S34.

In S34, it is judged whether n is smaller than 4 or not. If n is 1 or 2 or 3, return to S26, otherwise end.

The above description is only one embodiment of the present invention, which is intended to clarify the characteristics of the present invention, and is not intended to limit the scope of the embodiments of the present invention. Those of ordinary skill in the art will make equivalent changes according to the present invention. And changes well known to those skilled in the art should still fall within the scope of the present invention.

S10~S34: Steps

Claims (8)

A method for selecting a decoding strategy for a data storage system, comprising the following steps: Several decoders are provided; Read a received word (S10); Calculate the sum of the symptoms of the received word (S14); select a decoder from among the decoders and set its parameters according to the sum of symptom values; and The received word is decoded with the selected decoder (S30). The method for selecting a decoding strategy for a data storage system as described in Claim 1, wherein each decoder has a corresponding threshold, wherein the step of selecting a decoder includes the following steps: A decoder is selected by comparing the sum of symptom values with the thresholds. The method for selecting a decoding strategy for a data storage system as described in Claim 2, wherein the step of selecting a decoder includes the following steps: Compare the thresholds, from small to large, with the sum of the symptom values one by one; If the sum of the symptom values is less than the current threshold, select the decoder corresponding to the current threshold, otherwise, compare the sum of the symptom values with the next threshold. The method for selecting a decoding strategy for a data storage system as described in Claim 3, wherein the step of selecting a decoder includes the following steps: If the sum of the symptom values is not less than a maximum threshold, the decoder corresponding to the maximum threshold is selected. The method for selecting a decoding strategy in the data storage system as described in Claim 4 method, including the following steps: Judging whether the decoding is successful (S32); and If the decoding is successful, then end, otherwise judge whether the selected decoder is the last decoder. The method for selecting a decoding strategy for a data storage system as described in Claim 5 further includes the following steps: If the decoding is unsuccessful, it is judged whether the selected decoder is the last decoder (S34); and If the selected decoder is the last one, then end, otherwise compare the sum of the symptom value with the threshold of the next decoder. The method for selecting a decoding strategy for a data storage system as described in Claim 1 further includes the following steps: Calculate the sum of the symptom values of the received word and the absolute value of the difference between the number of "0" and "1" of the received word (S14); Divide the absolute value of the number difference by a statistical length to obtain a number difference ratio; Judging whether the number difference ratio is greater than or equal to a deviation value (S16); If the number difference ratio is smaller than the deviation value, then select a decoder from the decoders according to the sum of the symptom values (S20); and If the number difference ratio is greater than or equal to the deviation value, the step of re-reading a received word (S10). The method for selecting a decoding strategy for a data storage system as described in Claim 1, further comprising a step of generating a number of LLR values, wherein in the step of decoding the received word, according to the parameters of the selected decoder and the LLR values Decode the received word.

Images