Patent Publication Number: US-2023162803-A1

Title: System and method for non-parametric optimal read threshold estimation using deep neural network

Description:
BACKGROUND 
     1. Field 
     Embodiments of the present disclosure relate to a scheme for determining an optimal read threshold voltage in a memory system. 
     2. Description of the Related Art 
     The computer environment paradigm has shifted to ubiquitous computing systems that can be used anytime and anywhere. As a result, the use of portable electronic devices such as mobile phones, digital cameras, and notebook computers has rapidly increased. These portable electronic devices generally use a memory system having memory device(s), that is, data storage device(s). The data storage device is used as a main memory device or an auxiliary memory device of the portable electronic devices. 
     Memory systems using memory devices provide excellent stability, durability, high information access speed, and low power consumption, since the memory devices have no moving parts. Examples of memory systems having such advantages include universal serial bus (USB) memory devices, memory cards having various interfaces such as a universal flash storage (UFS), and solid state drives (SSDs). Memory systems may determine an optimal read threshold voltage among read threshold voltages according to various schemes. 
     SUMMARY 
     Aspects of the present invention include a system and a method for non-parametric optimal read threshold estimation using a deep neural network. 
     In one aspect of the present invention, a memory system includes a memory device including a plurality of pages and a controller including a neural network. The controller is configured to: perform one or more read operations on a page selected from among the plurality of pages using a read threshold set including a plurality of read threshold voltages; obtain the read threshold set, a checksum value and an asymmetric ratio of ones count and zeros count which are associated with decoding of the selected page according to each of the read operations; provide the obtained read threshold set, the checksum value and the asymmetric ratio as input information to the neural network; and estimate, by the neural network, an optimal read threshold voltage based on the input information and weights for the input information, the weights including a combination of multiple matrices and bias vectors. 
     In another aspect of the present invention, a method for operating a memory system, which includes a memory device including a plurality of pages and a controller including a neural network, includes: performing one or more read operations on a page selected from among the plurality of pages using a read threshold set including a plurality of read threshold voltages; obtaining the read threshold set, a checksum value and an asymmetric ratio of ones count and zeros count which are associated with decoding of the selected page according to each of the read operations; providing the obtained read threshold set, the checksum value and the asymmetric ratio as input information to the neural network; and estimating, by the neural network, an optimal read threshold voltage based on the input information and weights for the input information, the weights including a combination of multiple matrices and bias vectors. 
     Additional aspects of the present invention will become apparent from the following description. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a block diagram illustrating a data processing system. 
         FIG.  2    is a block diagram illustrating a memory system. 
         FIG.  3    is a circuit diagram illustrating a memory block of a memory device. 
         FIG.  4    is a diagram illustrating distributions of states for different types of cells of a memory device. 
         FIG.  5 A  is a diagram illustrating an example of Gray coding for a multi-level cell (MLC). 
         FIG.  5 B  is a diagram illustrating state distributions for pages of a multi-level cell (MLC). 
         FIG.  6 A  is a diagram illustrating one example of Gray coding for a triple-level cell (TLC). 
         FIG.  6 B  is a diagram illustrating state distributions for pages of a triple-level cell (TLC). 
         FIG.  7    is a diagram illustrating a flow of an error recovery algorithm in a memory system. 
         FIG.  8    is a diagram illustrating a distribution of cell levels for a particular page of a triple-level cell (TLC) flash memory device. 
         FIG.  9    is a diagram illustrating a memory system in accordance with one embodiment of the present invention. 
         FIG.  10    is a diagram illustrating a neural network in accordance with one embodiment of the present invention. 
         FIG.  11    is a diagram illustrating an optimal read threshold estimator in accordance with one embodiment of the present invention. 
         FIG.  12    is a diagram illustrating a training operation of an optimal read threshold estimator in accordance with one embodiment of the present invention. 
         FIG.  13    is a flowchart illustrating an optimal read threshold estimation operation in accordance with one embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     Various embodiments of the present invention are described below in more detail with reference to the accompanying drawings. The present invention may, however, be embodied in different forms and thus should not be construed as limited to the embodiments set forth herein. Rather, these embodiments are provided so that this disclosure conveys the scope of the present invention to those skilled in the art. Moreover, reference herein to “an embodiment,” “another embodiment,” or the like is not necessarily to only one embodiment, and different references to any such phrase are not necessarily to the same embodiment(s). The term “embodiments” as used herein does not necessarily refer to all embodiments. Throughout the disclosure, like reference numerals refer to like parts in the figures and embodiments of the present invention. 
     The present invention can be implemented in numerous ways, including as a process; an apparatus; a system; a computer program product embodied on a computer-readable storage medium; and/or a processor, such as a processor suitable for executing instructions stored on and/or provided by a memory coupled to the processor. In this specification, these implementations, or any other form that the present invention may take, may be referred to as techniques. In general, the order of the operations of disclosed processes may be altered within the scope of the present invention. Unless stated otherwise, a component such as a processor or a memory described as being suitable for performing a task may be implemented as a general device or circuit component that is configured or otherwise programmed to perform the task at a given time or as a specific device or as a circuit component that is manufactured or pre-configured or pre-programmed to perform the task. As used herein, the term ‘processor’ or the like refers to one or more devices, circuits, and/or processing cores suitable for processing data, such as computer program instructions. 
     The methods, processes, and/or operations described herein may be performed by code or instructions to be executed by a computer, processor, controller, or other signal processing device. The computer, processor, controller, or other signal processing device may be those described herein or one in addition to the elements described herein. Because the algorithms that form the basis of the methods (or operations of the computer, processor, controller, or other signal processing device) are described herein, the code or instructions for implementing the operations of the method embodiments may transform the computer, processor, controller, or other signal processing device into a special-purpose processor for performing any one of the methods herein. 
     If implemented at least partially in software, the controllers, processors, devices, modules, units, multiplexers, generators, logic, interfaces, decoders, drivers, generators and other signal generating and signal processing features may include, for example, a memory or other storage device for storing code or instructions to be executed, for example, by a computer, processor, microprocessor, controller, or other signal processing device. 
     A detailed description of various embodiments of the present invention is provided below along with accompanying figures that illustrate aspects of the present invention. The present invention is described in connection with such embodiments, but the present invention is not limited to any specific embodiment. The present invention encompasses numerous alternatives, modifications and equivalents. Numerous specific details are set forth in the following description in order to provide a thorough understanding of the present invention. These details are provided for the purpose of example; the present invention may be practiced without some or all of these specific details. For clarity, technical material that is known in technical fields related to the present invention has not been described in detail so that the present invention is not unnecessarily obscured. 
       FIG.  1    is a block diagram illustrating a data processing system  2  in accordance with one embodiment of the present invention. 
     Referring  FIG.  1   , the data processing system  2  may include a host device  5  and a memory system  10 . The memory system  10  may receive a request from the host device  5  and operate in response to the received request. For example, the memory system  10  may store data to be accessed by the host device  5 . 
     The host device  5  may be implemented with any of various types of electronic devices. In various embodiments, the host device  5  may be an electronic device such as for example a desktop computer, a workstation, a three-dimensional (3D) television, a smart television, a digital audio recorder, a digital audio player, a digital picture recorder, a digital picture player, and/or a digital video recorder and a digital video player. In various embodiments, the host device  5  may be a portable electronic device such as for example a mobile phone, a smart phone, an e-book, an MP3 player, a portable multimedia player (PMP), and/or a portable game player. 
     The memory system  10  may be implemented with any of various types of storage devices such as a solid state drive (SSD) and a memory card. In various embodiments, the memory system  10  may be provided as one of various components in an electronic device such as for example a computer, an ultra-mobile personal computer (PC) (UMPC), a workstation, a net-book computer, a personal digital assistant (PDA), a portable computer, a web tablet PC, a wireless phone, a mobile phone, a smart phone, an e-book reader, a portable multimedia player (PMP), a portable game device, a navigation device, a black box, a digital camera, a digital multimedia broadcasting (DMB) player, a 3-dimensional television, a smart television, a digital audio recorder, a digital audio player, a digital picture recorder, a digital picture player, a digital video recorder, a digital video player, a storage device of a data center, a device capable of receiving and transmitting information in a wireless environment, a radio-frequency identification (RFID) device, as well as one of various electronic devices of a home network, one of various electronic devices of a computer network, one of electronic devices of a telematics network, or one of various components of a computing system. 
     The memory system  10  may include a memory controller  100  and a semiconductor memory device  200 . The memory controller  100  may control overall operations of the semiconductor memory device  200 . 
     The semiconductor memory device  200  may perform one or more erase, program, and read operations under the control of the memory controller  100 . The semiconductor memory device  200  may receive through input/output lines a command CMD, an address ADDR and data DATA. The semiconductor memory device  200  may receive power PWR through a power line and a control signal CTRL through a control line. The control signal CTRL may include for example a command latch enable signal, an address latch enable signal, a chip enable signal, a write enable signal, a read enable signal, as well as other operational signals depending on design and configuration of the memory system  10 . 
     The memory controller  100  and the semiconductor memory device  200  may be integrated in a single semiconductor device such as a solid state drive (SSD). The SSD may include a storage device for storing data therein. In one embodiment of the invention, where the semiconductor memory system  10  is used in an SSD, operation speed of a host device (e.g., host device  5  of  FIG.  1   ) coupled to the memory system  10  may remarkably improve. 
     The memory controller  100  and the semiconductor memory device  200  may be integrated in a single semiconductor device such as a memory card. For example, the memory controller  100  and the semiconductor memory device  200  may be integrated to configure a personal computer (PC) card of personal computer memory card international association (PCMCIA), a compact flash (CF) card, a smart media (SM) card, a memory stick, a multimedia card (MMC), a reduced-size multimedia card (RS-MMC), a micro-size version of MMC (MMCmicro), a secure digital (SD) card, a mini secure digital (miniSD) card, a micro secure digital (microSD) card, a secure digital high capacity (SDHC), and/or a universal flash storage (UFS). 
       FIG.  2    is a block diagram illustrating a memory system in accordance with one embodiment of the present invention. For example, the memory system of  FIG.  2    may depict the memory system  10  shown in  FIG.  1   . 
     Referring to  FIG.  2   , the memory system  10  may include a memory controller  100  and a semiconductor memory device  200 . The memory system  10  may operate in response to a request from a host device (e.g., a request from host device  5  of  FIG.  1   ), and in particular, store data to be accessed by the host device. 
     The memory device  200  may store data to be accessed by the host device. 
     The memory device  200  may be implemented with a volatile memory device such as for example a dynamic random access memory (DRAM) and/or a static random access memory (SRAM) or a non-volatile memory device such as for example a read only memory (ROM), a mask ROM (MROM), a programmable ROM (PROM), an erasable programmable ROM (EPROM), an electrically erasable programmable ROM (EEPROM), a ferroelectric random access memory (FRAM), a phase change RAM (PRAM), a magnetoresistive RAM (MRAM), and/or a resistive RAM (RRAM). 
     The controller  100  may control storage of data in the memory device  200 . For example, the controller  100  may control the memory device  200  in response to a request from the host device. The controller  100  may provide data read from the memory device  200  to the host device, and may store data provided from the host device into the memory device  200 . 
     The controller  100  may include a storage  110 , a control component  120 , which may be implemented as a processor such as for example a central processing unit (CPU), an error correction code (ECC) component  130 , a host interface (I/F)  140  and a memory interface (I/F)  150 , which are coupled through a bus  160 . 
     The storage  110  may serve as a working memory of the memory system  10  and the controller  100 , and storage  110  may store data for driving the memory system  10  and the controller  100 . For example, when the controller  100  controls operations of the memory device  200 , the storage  110  may store data used by the controller  100  and the memory device  200  for such operations as read, write, program and erase operations. 
     The storage  110  may be implemented with a volatile memory such as a static random access memory (SRAM) or a dynamic random access memory (DRAM). As described above, the storage  110  may store data used by the host device in the memory device  200  for the read and write operations. To store the data, the storage  110  may include a program memory, a data memory, a write buffer, a read buffer, a map buffer, and the like. 
     The control component  120  may control general operations of the memory system  10 , and a write operation or a read operation for the memory device  200  in response to a write request or a read request from the host device. The control component  120  may drive firmware or other program instructions, which can be referred to as a flash translation layer (FTL), to control operations of the memory system  10 . For example, the FTL may perform operations such as logical-to-physical (L2P) mapping, wear leveling, garbage collection, and/or bad block handling. The L2P mapping is known as logical block addressing (LBA). 
     The ECC component  130  may detect and correct errors in the data read from the memory device  200  during a read operation. In one embodiment, the ECC component  130  may not correct error bits when the number of the error bits is greater than or equal to a threshold number of correctable error bits, but instead may output an error correction fail signal indicating failure in correcting the error bits. 
     In various embodiments, the ECC component  130  may perform an error correction operation based on a coded modulation such as for example a low density parity check (LDPC) code, a Bose-Chaudhuri-Hocquenghem (BCH) code, a turbo code, a turbo product code (TPC), a Reed-Solomon (RS) code, a convolution code, a recursive systematic code (RSC), a trellis-coded modulation (TCM), or a Block coded modulation (BCM). However, error correction is not limited to these techniques. As such, the ECC component  130  may include any and all circuits, systems or devices suitable for error correction operation. 
     The host interface  140  may communicate with the host device through one or more of various communication standards or interfaces such as for example a universal serial bus (USB), a multi-media card (MMC), a peripheral component interconnect express (PCI-e or PCIe), a small computer system interface (SCSI), a serial-attached SCSI (SAS), a serial advanced technology attachment (SATA), a parallel advanced technology attachment (PATA), an enhanced small disk interface (ESDI), and an integrated drive electronics (IDE). 
     The memory interface  150  may provide an interface between the controller  100  and the memory device  200  to allow the controller  100  to control the memory device  200  in response to a request from the host device. The memory interface  150  may generate control signals for the memory device  200  and process data under the control of the control component  120 . In one embodiment where the memory device  200  is a flash memory such as a NAND flash memory, the memory interface  150  may generate control signals for the memory and process data under the control of the control component  120 . 
     The memory device  200  as shown for example in  FIG.  2    may include a memory cell array  210 , a control circuit  220 , a voltage generation circuit  230 , a row decoder  240 , a page buffer  250  which may be in the form of an array of page buffers, a column decoder  260 , and an input and output (input/output) circuit  270 . The memory cell array  210  may include a plurality of memory blocks  211  which may store data. The voltage generation circuit  230 , the row decoder  240 , the page buffer array  250 , the column decoder  260  and the input/output circuit  270  may form a peripheral circuit for the memory cell array  210 . The peripheral circuit may perform program, read, or erase operations of the memory cell array  210 . The control circuit  220  may control the peripheral circuit. 
     The voltage generation circuit  230  may generate operational voltages of various levels. For example, in an erase operation, the voltage generation circuit  230  may generate operational voltages of various levels such as for example an erase voltage and a pass voltage. 
     The row decoder  240  may be in electrical communication with the voltage generation circuit  230 , and the plurality of memory blocks  211 . The row decoder  240  may select at least one memory block among the plurality of memory blocks  211  in response to a row address generated by the control circuit  220 , and transmit operation voltages supplied from the voltage generation circuit  230  to the selected memory blocks. 
     The page buffer  250  may be coupled with the memory cell array  210  through bit lines BL (shown in  FIG.  3   ). The page buffer  250  may precharge the bit lines BL with a positive voltage, transmit data to, and receive data from, a selected memory block in program and read operations, or temporarily store transmitted data in response to page buffer control signal(s) generated by the control circuit  220 . 
     The column decoder  260  may transmit data to, and receive data from, the page buffer  250  or may transmit and receive data to and from the input/output circuit  270 . 
     The input/output circuit  270  may transmit to the control circuit  220  a command and an address, received from an external device (e.g., the memory controller  100  of  FIG.  1   ), transmit data from the external device to the column decoder  260 , or output data from the column decoder  260  to the external device. 
     The control circuit  220  may control the peripheral circuit in response to the command and the address. 
       FIG.  3    is a circuit diagram illustrating a memory block of a semiconductor memory device in accordance with an embodiment of the present invention. For example, the memory block of  FIG.  3    may be any of the memory blocks  211  of the memory cell array  210  shown in  FIG.  2   . 
     Referring to  FIG.  3   , the memory block  211  may include a plurality of word lines WL 0  to WLn−1, a drain select line DSL and a source select line SSL coupled to the row decoder  240 . These lines may be arranged in parallel, with the plurality of word lines between the DSL and SSL. 
     The memory block  211  may further include a plurality of cell strings  221  respectively coupled to bit lines BL 0  to BLm−1. The cell string of each column may include one or more drain selection transistors DST and one or more source selection transistors SST. In the illustrated embodiment, each cell string has one DST and one SST. In a cell string, a plurality of memory cells or memory cell transistors MC 0  to MCn−1 may be serially coupled between the selection transistors DST and SST. Each of the memory cells may be formed as a multiple level cell. For example, each of the memory cells may be formed as a single level cell (SLC) storing 1 bit of data. Each of the memory cells may be formed as a multi-level cell (MLC) storing 2 bits of data. Each of the memory cells may be formed as a triple-level cell (TLC) storing 3 bits of data. Each of the memory cells may be formed as a quadruple-level cell (QLC) storing 4 bits of data. 
     The source of the SST in each cell string may be coupled to a common source line CSL, and the drain of each DST may be coupled to the corresponding bit line. Gates of the SSTs in the cell strings may be coupled to the SSL, and gates of the DSTs in the cell strings may be coupled to the DSL. Gates of the memory cells across the cell strings may be coupled to respective word lines. That is, the gates of memory cells MC 0  are coupled to corresponding word line WL 0 , the gates of memory cells MC 1  are coupled to corresponding word line WL 1 , etc. The group of memory cells coupled to a particular word line may be referred to as a physical page. Therefore, the number of physical pages in the memory block  211  may correspond to the number of word lines. 
     The page buffer array  250  may include a plurality of page buffers  251  that are coupled to the bit lines BL 0  to BLm−1. The page buffers  251  may operate in response to page buffer control signals. For example, the page buffers  251  my temporarily store data received through the bit lines BL 0  to BLm−1 or sense voltages or currents of the bit lines during a read or verify operation. 
     In various embodiments of the present invention, the memory blocks  211  may include a NAND-type flash memory cell. However, the memory blocks  211  are not limited to such cell type, and may include NOR-type flash memory cell(s). Memory cell array  210  may be implemented as a hybrid flash memory in which two or more types of memory cells are combined, or one-NAND flash memory in which a controller is embedded inside a memory chip. 
       FIG.  4    is a diagram illustrating distributions of states or program voltage (PV) levels for different types of cells of a memory device. 
     Referring to  FIG.  4   , each of memory cells may be implemented with a specific type of cell, for example, a single level cell (SLC) storing 1 bit of data, a multi-level cell (MLC) storing 2 bits of data, a triple-level cell (TLC) storing 3 bits of data, or a quadruple-level cell (QLC) storing 4 bits of data. Usually, all memory cells in a particular memory device are of the same type, but that is not a requirement. 
     An SLC may include two states P 0  and P 1 . P 0  may indicate an erase state, and P 1  may indicate a program state. Since the SLC can be set in one of two different states, each SLC may program or store 1 bit according to a set coding method. An MLC may include four states P 0 , P 1 , P 2  and P 3 . Among these states, P 0  may indicate an erase state, and P 1  to P 3  may indicate program states. Since the MLC can be set in one of four different states, each MLC may program or store two bits according to a set coding method. A TLC may include eight states P 0  to P 7 . Among these states, P 0  may indicate an erase state, and P 1  to P 7  may indicate program states. Since the TLC can be set in one of eight different states, each TLC may program or store three bits according to a set coding method. A QLC may include 16 states P 0  to P 15 . Among these states, P 0  may indicate an erase state, and P 1  to P 15  may indicate program states. Since the QLC can be set in one of sixteen different states, each QLC may program or store four bits according to a set coding method. 
     Referring back to  FIGS.  2  and  3   , the memory device  200  may include a plurality of memory cells (e.g., NAND flash memory cells). The memory cells are arranged in an array of rows and columns as shown in  FIG.  3   . The cells in each row are connected to a word line (e.g., WL 0 ), while the cells in each column are coupled to a bit line (e.g., BL 0 ). These word and bit lines are used for read and write operations. During a write operation, the data to be written (‘1’ or ‘0’) is provided at the bit line while the word line is addressed. During a read operation, the word line is again addressed, and the threshold voltage of each cell can then be acquired from the bit line. Multiple pages may share the memory cells that belong to (i.e., are coupled to) the same word line. When the memory cells are implemented with MLCs, the multiple pages include a most significant bit (MSB) page and a least significant bit (LSB) page. When the memory cells are implemented with TLCs, the multiple pages include an MSB page, a center significant bit (CSB) page and an LSB page. When the memory cells are implemented with QLCs, the multiple pages include an MSB page, a center most significant bit (CMSB) page, a center least significant bit (CLSB) page and an LSB page. The memory cells may be programmed for example using a coding scheme (e.g., Gray coding) in order to increase the capacity of the memory system  10  such as SSD. 
       FIG.  5 A  is a diagram illustrating an example of coding for a multi-level cell (MLC). 
     Referring to  FIG.  5 A , an MLC may be programmed using a set type of coding. An MLC may have 4 program states, which include an erased state E (or PV 0 ) and a first program state PV 1  to a third program state PV 3 . The erased state E (or PV 0 ) may correspond to “11.” The first program state PV 1  may correspond to “10.” The second program state PV 2  may correspond to “00.” The third program state PV 3  may correspond to “01.” 
     In the MLC, as shown in  FIG.  5 B , there are 2 types of pages including LSB and MSB pages. 1 or 2 thresholds may be applied in order to retrieve data from the MLC. For an MSB page, the single threshold value is VT 1 . VT 1  distinguishes between the first program state PV 1  and the second program state PV 2 . For an LSB page, 2 thresholds include a threshold value VT 0  and a threshold value VT 2 . VT 0  distinguishes between the erased state E and the first program state PV 1 . VT 2  distinguishes between the second program state PV 2  and the third program state PV 3 . 
       FIG.  6 A  is a diagram illustrating an example of Gray coding for a triple-level cell (TLC). 
     Referring to  FIG.  6 A , a TLC may be programmed using Gray coding. A TLC may have 8 program states, which include an erased state E (or PV 0 ) and a first program state PV 1  to a seventh program state PV 7 . The erased state E (or PV 0 ) may correspond to “111.” The first program state PV 1  may correspond to “011.” The second program state PV 2  may correspond to “001.” The third program state PV 3  may correspond to “000.” The fourth program state PV 4  may correspond to “010.” The fifth program state PV 5  may correspond to “110.” The sixth program state PV 6  may correspond to “100.” The seventh program state PV 7  may correspond to “101.” 
     In the TLC, as shown in  FIG.  6 B , there are 3 types of pages including LSB, CSB and MSB pages. 2 or 3 thresholds may be applied in order to retrieve data from the TLC. For an MSB page, 2 thresholds include a threshold value VT 0  that distinguishes between an erase state E and a first program state PV 1  and a threshold value VT 4  that distinguishes between a fourth program state PV 4  and a fifth program state PV 5 . For a CSB page, 3 thresholds include VT 1 , VT 3  and VT 5 . VT 1  distinguishes between a first program state PV 1  and a second program state PV 2 . VT 3  distinguishes between a third program state PV 3  and the fourth program state PV 4 . VT 5  distinguishes between the fourth program state PV 5  and the sixth program state PV 6 . For an LSB page, 2 thresholds include VT 2  and VT 6 . VT 2  distinguishes between the second program state PV 2  and the third program state PV 3 . VT 6  distinguishes between the sixth program state PV 6  and a seventh program state PV 7 . 
     After a memory array including a plurality of memory cells is programmed as described in  FIGS.  5 A and  6 A , when a read operation is performed on the memory array using a reference voltage such as a read threshold voltage (also called “read voltage level” or “read threshold”), the electrical charge levels of the memory cells (e.g., threshold voltage levels of transistors of memory cells) are compared to one or more reference voltages to determine the state of individual memory cells. When a specific read threshold is applied to the memory array, those memory cells that have threshold voltage levels higher than the reference voltage are turned on and detected as “on” cell, whereas those memory cells that have threshold voltage levels lower than the reference voltage are turned off and detected as “off” cell, for example. Therefore, each read threshold is arranged between neighboring threshold voltage distribution windows corresponding to different programmed states so that each read threshold can distinguish such programmed states by turning on or off the memory cell transistors. 
     When a read operation is performed on memory cells in a data storage device using MLC technology, the threshold voltage levels of the memory cells are compared to more than one read threshold level to determine the state of individual memory cells. Read errors can be caused by distorted or overlapped threshold voltage distributions. An ideal memory cell threshold voltage distribution can be significantly distorted or overlapped due to, e.g., program and erase (P/E) cycles, cell-to-cell interference, and/or data retention errors. For example, as program/erase cycles increase, the margin between neighboring threshold voltage distributions of different programmed states decreases and eventually the distributions overlap. As a result, the memory cells with threshold voltages that fall within the overlapping region of the neighboring distributions may be read as being programmed to a value other than the original targeted value and thus cause read errors. Such read errors may be managed in many situations by using error correction codes (ECC). When the number of bit errors on a read operation exceeds the ECC correction capability of the data storage, the read operation using a set read threshold voltage fails. The set read threshold voltage may be a previously used read threshold voltage (i.e., a historical read threshold voltage). The historical read threshold voltage may be the read threshold voltage used in the last successful decoding, that is, a read voltage used in a read-passed read operation performed before read retry operations. When the read operation using the set read threshold voltage failed, the controller  120  may control an error recovery algorithm such as for example the algorithm shown in  FIG.  7   . 
     Referring to  FIG.  7   , the controller  120  may perform one or more read retry operations for the memory cells using one or more read threshold voltages applied in a set order (S 100 ). For example, the read threshold voltages may include N (e.g., N is 5 or 10) read threshold voltages (or read voltage levels) including a first read threshold voltage to an Nth read threshold voltage. The first read threshold voltage may be a previously used read threshold voltage (i.e., history read threshold voltage). The historical read threshold voltage may be the read threshold voltage used in the last successful decoding, that is, a read voltage used in a read-passed read operation performed before the read retry operations. The controller  120  may perform the read retry operations until it is determined that decoding associated with a corresponding read retry operation is successful. 
     When all read retry operations using the read threshold voltages have failed, the controller  120  may perform additional recovery operations. For example, the additional recovery operations may include an optimal read threshold voltage search (S 200 ), a soft decoding using an error correction code (ECC) (S 300 ) and/or a redundant array of independent disks (RAID) recovery (S 400 ). 
     As noted above, data retrieved from a memory device (e.g., a NAND flash device) in a data storage device (e.g., a solid state drive (SSD)) usually contains many bit errors due to various noise sources. To protect the data, it is encoded by an error correction code before being written to the memory device (i.e., before being written to a storage medium). A decoder may correct all bit errors by using this error correction code. As illustrated in  FIG.  7    by the “Read Retries” shown there, in one embodiment of the present invention, when the number of errors surpasses the capability of the error correction code, the data storage device may invoke a particular defense algorithm (e.g., a read retry operation) to read the storage media multiple times with better parameters than the original read operation until the data can be recovered by the error correction code. As detailed below, different read thresholds can be used to better read pages of data. 
     For a solid state drive, one source of increased bit errors is the use of sub-optimal read thresholds during the read operation as shown in  FIG.  8   . In  FIG.  8   , OptVt represents an optimal read threshold voltage and Rd 0  to Rd 2  represent sub-optimal read threshold voltages during the read operation for a particular page (e.g., an LSB page) of a triple-level cell (TLC) NAND flash memory device. One method (e.g., an eBoost algorithm) to estimate an optimal read threshold performs several additional read operations with different read thresholds on the same page, and estimates the optimal read threshold that minimizes the bit errors in the retrieved data. These additional read operations increase the latency of the read operation and degrades a quality of service (QoS) of the data storage device (or memory system). Accordingly, embodiments of the present invention provide a scheme to estimate an optimal read threshold without necessarily performing additional read operations. 
       FIG.  9    is a diagram illustrating a memory system  10  in accordance with an embodiment of the present invention. 
     Referring to  FIG.  9   , the memory system  10  may include a controller  100  and a memory device  200 . The memory device  200  may include a plurality of memory cells (e.g., NAND flash memory cells)  210 . The memory cells are arranged in an array of rows and columns such as shown in  FIG.  3   . The cells in each row are connected to a word line (e.g., WL 0 ), while the cells in each column are coupled to a bit line (e.g., BL 0 ). These word and bit lines are used for read and write operations. During a write operation, the data to be written (‘1’ or ‘0’) is provided at the bit line while the word line is addressed. During a read operation, the word line is again addressed, and the threshold voltage of each cell can then be acquired from the bit line. Multiple pages may share the memory cells that belong to (i.e., are coupled to) the same word line. When the memory cells are implemented with MLCs, the multiple pages include a most significant bit (MSB) page and a least significant bit (LSB) page. When the memory cells are implemented with TLCs, the multiple pages include an MSB page, a center significant bit (CSB) page and an LSB page. When the memory cells are implemented with QLCs, the multiple pages include an MSB page, a center most significant bit (CMSB) page, a center least significant bit (CLSB) page and an LSB page. The memory cells may be programmed using a coding scheme (e.g., Gray coding) in order to increase the capacity of the memory system  10  such as an SSD. 
     The controller  100  may include a read processor  910 , a decoder  920  and an optimal read threshold determiner  930 . Although it is illustrated that components of the controller  100  are implemented separately, these components may be implemented with an internal component (i.e., firmware (FW)) of the control component  120  in  FIG.  2   . Although not shown in  FIG.  10   , the controller  100  and the memory device  200  may include various other components such as those shown in  FIG.  2   . 
     The read processor  910  may control one or more read operations for the memory device  200  in response to a read request from a host (e.g., the host  5  of  FIG.  1   ). The read processor  910  may control the read operations using various read thresholds. The decoder  920  may decode data associated with the read operations. 
     In various embodiments of the present invention, the read processor  910  may control a read operation for the memory cells using a select read threshold from a set read level table. In various embodiments, the read level table may include multiple read thresholds and the select read threshold may include a default read threshold. When the read operation is performed for the MSB page of TLC, the select read threshold may include a pair of first and second read thresholds [VT 0 , VT 4 ] as shown in  FIG.  6 B . The first read threshold value VT 0  is used to distinguish between an erase state (i.e., E) and a first program state (i.e., PV 1 ), and the second read threshold value VT 4  is used to distinguish between a fourth program state (i.e., PV 4 ) and a fifth program state (i.e., PV 5 ). When the read operation is performed for the LSB page of TLC, the select read threshold may include a pair of first and second read thresholds [VT 2 , VT 6 ] such as shown in  FIG.  6 B . The first read threshold value VT 2  is used to distinguish between a second program state (i.e., PV 2 ) and a third program state (i.e., PV 3 ), and the second read threshold value VT 6  is used to distinguish between a sixth program state (i.e., PV 6 ) and a seventh program state (i.e., PV 7 ). 
     In one embodiment of the present invention, it is determined whether the read operation using a read threshold selected from a read threshold set succeeded or failed, depending on the decoding result of the decoder  920 . When the read operation using the selected read threshold failed, the read processor  910  may control one or more read retry operations for the memory cells using read retry entries, such as shown in  FIG.  7   . 
     The optimal read threshold determiner  930  may provide a scheme of an optimal read threshold voltage (Vt) estimation (or prediction). The optimal read threshold determiner  930  may be implemented with one or more deep neural networks (DNNs). In various embodiments of the present invention, the optimal read threshold determiner  930  may include a training component  930 A and an inference component  930 B. The operations for the training component  930 A and the inference component  930 B is described below. 
       FIG.  10    is a diagram illustrating one example of a neural network  1000  (representative of a training component) in accordance with one embodiment of the present invention. In various embodiments of the present invention, for an optimal read threshold estimation or prediction, the neural network  1000  may be included in the controller  100  of the memory system  10  in  FIG.  9   . That is, the neural network  1000  may be implemented for the optimal read threshold determiner  1030  in  FIG.  9   . 
     Referring to  FIG.  10   , a feature map  1002  associated with one or more input conditions may be input to the neural network  1000 . In turn, the neural network  1000  may output information  1004 . As illustrated, the neural network  1000  includes an input layer  1010 , one or more hidden layers  1020  and an output layer  1030 . Features from the feature map  1002  may be connected to input nodes in the input layer  1010 . The information  1004  may be generated from an output node of the output layer  1030 . One or more hidden layers  1020  may exist between the input layer  1010  and the output layer  1030 . The neural network  1000  may be pre-trained to process the features from the feature map  1002  through the different layers  1010 ,  1020 , and  1030  in order to output the information  1004 . 
     The neural network  1000  may be a multi-layer neural network that represents a network of interconnected nodes, such as an artificial deep neural network, where knowledge about the nodes (e.g., information about specific features represented by the nodes) is shared across layers and knowledge specific to each layer is also retained. Each node represents a piece of information. Knowledge may be exchanged between nodes through node-to-node interconnections. Input to the neural network  1000  may activate a set of nodes. In turn, this set of nodes may activate other nodes, thereby propagating knowledge about the input. This activation process may be repeated across other nodes until nodes in the output layer  1030  are selected and activated. 
     As illustrated, the neural network  1000  includes a hierarchy of layers representing a hierarchy of nodes interconnected in a feed-forward way. The input layer  1010  may exist at the lowest hierarchy level. The input layer  1010  may include a set of nodes that are referred to herein as input nodes. When the feature map  1002  is input to the neural network  1000 , each of the input nodes of the input layer  1010  may be connected to each feature of the feature map  1002 . Each of the connections may have a weight. These weights may be one set of parameters that are derived from the learning/training of the neural network  1000 . The input nodes may transform the features by applying an activation function to these features. The information derived from the transformation may be passed to the nodes at a higher level of the hierarchy. 
     The output layer  1030  may exist at the highest hierarchy level. The output layer  1030  may include one or more output nodes. Each output node may provide a specific value of the output information  1004 . The number of output nodes may depend on the needed number of the output information  1004 . In other words, in one embodiment, there is a one-to-one relationship or mapping between the number of output nodes and the number of the output information  1004 . 
     The hidden layer(s)  1020  may exist between the input layer  1010  and the output layer  1030 . The hidden layer(s)  1020  may include “N” number of hidden layers, where “N” is an integer greater than or equal to one. Each of the hidden layers  1020  may include a set of nodes that are referred to herein as hidden nodes. Example hidden layers may include up-sampling, convolutional, fully connected layers, and/or data transformation layers. 
     At the lowest level of the hidden layer(s)  1020 , hidden nodes of that layer may be interconnected to the input nodes. At the highest level of the hidden layer(s)  1020 , hidden nodes of that level may be interconnected to the output node. The input nodes may be not directly interconnected to the output node(s). If multiple hidden layers exist, the input nodes are interconnected to hidden nodes of the lowest hidden layer. In turn, these hidden nodes are interconnected to the hidden nodes of the next hidden layer. An interconnection may represent a piece of information learned about the two interconnected nodes. The interconnection may have a numeric weight that can be tuned (e.g., based on a training dataset), rendering the neural network  1000  adaptive to inputs and capable of learning. 
     Generally, the hidden layer(s)  1020  may allow knowledge about the input nodes of the input layer  1010  to be shared among the output nodes of the output layer  1030 . To do so, a transformation ƒ may be applied to the input nodes through the hidden layer  1020 . In one example, the transformation ƒ can be non-linear. Different non-linear transformations ƒ are available including, for instance, a rectifier function ƒ(x)=max(0,x). In another example, a particular non-linear transformation ƒ can be selected based on a cross-validation. For example, given known example pairs (x,y), where x∈X and y∈Y, a function ƒ: X→Y can be selected when such a function results in the best matches. 
     For example, the neural network  1000  may be a deep learning neural network used for a memory system including a NAND flash memory device. The deep leaning neural network may be created with “K” input nodes and an output node, where “K” is the number of factors (e.g., features) that define input conditions for the memory system. The output node(s) may be used to perform an activation function for a combination of input conditions. The number of layers and size of each layer in the neural network  1000  may depend on the NAND flash memory device and on an amount of the data that this memory can store. 
     As noted above, the neural network  1000  may be implemented for the optimal read threshold determiner  1030  in  FIG.  9   . In various embodiments of the present invention, the implementations of a neural network described in the following patent applications can be used: U.S. patent application Ser. No. 17/157,495, filed on Jan. 25, 2021, entitled “DEEP LEARNING BASED PROGRAM-VERIFY MODELING AND VOLTAGE ESTIMATION FOR MEMORY DEVICES;” U.S. patent application Ser. No. 17/374,750, filed on Jul. 13, 2021, entitled “SYSTEMS AND METHODS FOR NON-PARAMETRIC PV-LEVEL MODELING AND READ THRESHOLD VOLTAGE ESTIMATION;” U.S. patent application Ser. No. 17/443,755, filed on Jul. 27, 2021, entitled “SYSTEMS AND METHODS FOR MODELESS READ THRESHOLD VOLTAGE ESTIMATION;” and U.S. patent application Ser. No. 17/443,726, filed on Jul. 27, 2021, entitled “READ THRESHOLD VOLTAGE ESTIMATION SYSTEMS AND METHODS FOR PARAMETRIC PV-LEVEL MODELING,” which are all incorporated by reference herein in their entirety. 
       FIG.  11    is a diagram illustrating an optimal read threshold estimator  1100  (representative of an inference component) in accordance with one embodiment of the present invention. For example, the optimal read threshold estimator  1100  may correspond to the inference component  930 D of the optimal read threshold determiner  930  in  FIG.  9   . 
     Referring to  FIG.  11   , the optimal read threshold estimator  1100  may include a deep neural network (DNN). DNN  1100  may be implemented in a system on a chip (SoC) or firmware (FW) depending on the size of the DNN used. In various embodiments of the present invention, DNN  1600  may be used for a particular page (e.g., an MSB, CSB or LSB page) of a triple-level cell (TLC) NAND flash memory device. 
     The optimal read threshold estimator  1100  may estimate optimal read thresholds for a failed read page based on information collected from previous read attempts of the failed read page. That is, DNN  1100  may estimate optimal read thresholds without necessarily any additional sensing operations and any intermediate parameter estimation, which are required in the implementations above mentioned. 
     As shown in  FIGS.  11  and  12   , DNN  1100  may receive information associated with previous read operations as inputs. The inputs may include a read threshold set (Vt read0 , Vt read1 , . . . ), checksums (CS read0 , CS read1 , . . . ) and an asymmetric ratio/ones counts/zeros counts set (AR read0 , AR read1 , . . . ). As shown in  FIG.  12   , DNN  1100  outputs an estimated threshold Vt DNN  which is output to Loss Function  1150 . Loss Function  1150  receives the Vt Opt  signal and compares the estimated threshold Vt DNN  to the Vt Opt  signal. As shown in  FIG.  12   , the comparison is feedback into DNN  100 , and estimates of Vt DNN  may continue until the comparison (difference value) provided back to DNN  100  is sufficiently low. 
     The read threshold set Vt in  FIG.  11    may include multiple values for each read of a particular page. For example, as shown in  FIG.  6 B , the read threshold set Vt for reading an LSB page of a TLC memory device may include two read threshold voltage values VT 2  and VT 6 . VT 2  distinguishes between the second program state PV 2  and the third program state PV 3 . VT 6  distinguishes between the sixth program state PV 6  and a seventh program state PV 7 . For a CSB page of a TLC memory device, three read threshold voltage values (VT 1 , VT 3 , VT 5 ) may be estimated as shown in  FIG.  6 B . For an MSB page of a TLC memory device, two read threshold voltage values (VT 0 , VT 4 ) may be estimated as shown in  FIG.  6 B . 
     Each checksum may be an approximation for the number of bit errors, which indicates a noise level in an optimal read threshold voltage estimation. When low density parity check (LDPC) codes are used for decoding a read page as an error correction code (ECC), a syndrome weight may correspond to the number of unsatisfied check (USC) node(s) of a codeword and may be used as checksum. 
     Each asymmetric ratio/ones count/zeros count (i.e., an asymmetric ratio of ones count and zeros count) may be any one of the three metrics: ones count; zeros count and an asymmetric ratio of ones count and zeros count. Ones count may be the number of bit  1 &#39;s in a codeword. Zeros count may be the number of bit  0 &#39;s in a codeword. The asymmetric ratio AR may be defined as a ratio of the number of a first binary value to the number of a second binary value in a codeword. For example, the asymmetric ratio AR may be a value representing the number of 1&#39;s (i.e., ones count) divided by the number of 0&#39;s (i.e., zeros count) in decoded raw data. In one embodiment in which reverse logic convention is used, AR may be the number of 0&#39;s (i.e., zeros count) divided by the number of 1&#39;s (i.e., ones count). In one embodiment, a read threshold optimization system in the present invention may use any of the asymmetric ratios described in U.S. patent application Ser. No. 16/927,567, filed on Jul. 13, 2020, entitled “READ THRESHOLD OPTIMIZATION SYSTEMS AND METHODS USING DOMAIN TRANSFORMATION,” the entire contents of which are incorporated herein by reference. 
     The DNN  1100  may be a single fully-connected network with multiple layers with a set function (e.g., a rectified linear unit (ReLu) function) as non-linearity or any other appropriate DNN structure. The DNN  1100  may estimate (i.e., predict) an optimal read threshold set OptVt based on the inputs, i.e., a read threshold set (Vt read0 , Vt read1 , . . . ), checksums (CS read0 , CS read1 , . . . ) and an asymmetric ratio/ones counts/zeros counts set (AR read0 , AR read1 , . . . ), as shown in  FIG.  11   . The predicted optimal read threshold set OptVt may be used for a next read operation and subsequent reading operations until a successful recovery of the data read from a particular NAND flash page is realized. For this estimation, the DNN  1100  may be trained as shown in  FIG.  12   . 
     DNN  1100  may be trained on a particular dataset that includes read threshold set (Vt read0 , Vt read1 , . . . ), checksums (CS read0 , CS read1 , . . . ) and an asymmetric ratio/ones counts/zeros counts set (AR read0 , AR read1 , . . . ) as input features and the predicted optimal read threshold voltage as the output. In some embodiments, DNN  1100  may be trained offline on measurements collected from pages of a real NAND flash memory device or a suitable channel model. 
     For the training of DNN  1100 , a set loss function  1150  (e.g., a differentiable loss function) may be defined to measure an error between the predicted optimal read threshold voltage Vt DNN , which is the output of DNN  1100 , and the expected optimal read threshold voltage Vt Opt  of data points in the dataset. For example, the mean square error function (Vt DNN −Vt Opt ) 2 /n (where n is the number of data points in a subset of training data of the dataset) may be used as the loss function  1150 . The loss function  1150  may be not restricted to the mean square error function only. 
     The DNN  1100  may receive the result from the loss function  1150 . Further, the DNN  1100  may be trained to find the predicted optimal read threshold voltage Vt DNN  based on the result of the loss function such that the difference between the true (actual) optimal read threshold voltage Vt Opt  and the predicted optimal read threshold voltage Vt DNN  from DNN  1100  is minimized. 
     In the illustrated example of  FIG.  10   , the neural network  1000  (i.e., DNN  1100  of  FIG.  11   ) may be pre-trained to process the features (i.e., read threshold, checksums and an asymmetric ratio/ones counts/zeros counts set) from the feature map  1002  through the different layers  1010 ,  1020 , and  1030  in order to output the optimal read threshold voltage  1004 . As such, DNN  1100  may be trained to refine the optimal read threshold voltage such that the difference (or error) between an actual optimal read threshold voltage Vt Opt  and a predicted optimal read threshold voltage Vt DNN  from DNN  1100  is minimized. 
     In various embodiments of the present invention, weights of DNN  1100  may be obtained. For example, a stochastic gradient descent (SGD) process, its variants and/or other algorithms may be used to obtain weights of DNN  1100  such that the loss function is minimized. The obtained weights (weight values) may be then used to process the input features (vectors) to acquire the optimal read threshold voltage. In various embodiments, input features may have read thresholds, checksums and an asymmetric ratio. For example, DNN&#39;s obtained weights may be in the form of multiple matrices A DNN,0 , A DNN,1 , . . . A DNN,N  and bias vectors b DNN,0 , b DNN,1 , . . . , b DNN,N . That is, DNN&#39;s obtained weights may include a combination of multiple matrices A DNN,0 , A DNN,1 , . . . , A DNN,N  and bias vectors b DNN,0 , b DNN,1 , . . . , b DNN,N . 
     DNN  1100  may receive inputs (i.e., a read threshold set, checksums and an asymmetric ratio/ones counts/zeros counts set) and estimate an optimal read threshold voltage OptVt based on the received inputs and weights. One example of estimation of the optimal read threshold set by the DNN  1100  is described below. 
     In one implementation, three reads are used to collect input information such as the checksum and asymmetric ratio/ones count/zeros count. As noted above, since two read threshold voltages are used for the LSB page in the TLC NAND flash memory device, these three reads correspond to six read threshold voltages. As a result, the read threshold set Vt, the checksum CS and the asymmetric ratio/ones count/zeros count AR are represented as below: 
         Vt =[ Vt   read0   L   ,Vt   read1   L   ,Vt   read2   L   ,Vt   read0   R   ,Vt   read1   R   ,Vt   read2   R ]; 
         CS =[ CS   read0   ,CS   read1   ,CS   read2 ]; and 
         AR =[ AR   read0   ,AR   read1   ,AR   read2 ]. 
     In the equations above, Vt read0   L  represents a first read threshold voltage (e.g., VT 2  of  FIG.  6 B ) for a zeroth read of the particular page, Vt read1   L  represents the first read threshold voltage for a first read of the particular page and Vt read2   L  represents the first read threshold voltage for a second read of the particular page. Vt read0   R  represents a second read threshold voltage (e.g., VT 6  of  FIG.  6 B ) for the zeroth read of the particular page, Vt read1   R  represents the second read threshold voltage for the first read of the particular page and Vt read2   R  represents the second read threshold voltage for the second read of the particular page. CS read0  represents a checksum associated with a zeroth read of the particular page, CS read1  represents a checksum associated with a first read of the particular page and CS read2  represents a checksum associated with a second read of the particular page. AR read0  represents a checksum associated with a zeroth read of the particular page, AR read1  represents a checksum associated with a first read of the particular page and AR read2  represents a checksum associated with a second read of the particular page. 
     In one embodiment of the invention, the DNN  1100  shown in  FIG.  12    may receive input information obtained from three reads (i.e., a read threshold set, checksums and an asymmetric ratio/ones counts/zeros counts set). Further, DNN  1100  may estimate an optimal read threshold voltage OptVt based on the received input information and weights, which are represented as below: 
       Opt Vt=A   DNN,2 ×( R ( A   DNN,1   ×R ( A   DNN,0 ×[ Vt,CS,AR ]+ b   DNN,0 )+ b   DNN,1 ))+ b   DNN,N2 .
 
     The equation above for three (3) previous reads may be generalized for N previous reads as below: 
       Opt Vt=A   DNN,N   ×R ( A   DNN,N-1   ×R ( A   DNN,N-2   ×R ( . . .  R ( A   DNN,0 ×[ Vt,CS,AR ]+ b   DNN,0 ) . . . )+ b   DNN,N-2 )+ b   DNN,N-1 )+ b   DNN,N .
 
     In the equations above, R represents a set activation function. In various embodiments of the present invention, the set activation function may be a rectified linear unit (ReLU) activation function. Other activation functions may be applicable. As noted above, the DNN&#39;s weights may be in the form of multiple matrices A DNN,0 , A DNN,1 , . . . A DNN,N  and bias vectors b DNN,0 , b DNN,1 , . . . , b DNN,N . 
       FIG.  13    is a flowchart illustrating an optimal read threshold estimation operation  1300  in accordance with one embodiment of the present invention. The optimal read threshold estimation operation  1300  may be performed by the optimal read threshold determiner  930  of the controller  100  in  FIG.  9   , which may be implemented with the neural network (i.e., DNN)  1100  in  FIG.  11   . 
     Referring to  FIG.  13   , at operation  1310 , the optimal read threshold determiner  930  may perform one or more read operations on a page selected from among the plurality of pages using a read threshold set including a plurality of read threshold voltages. 
     At operation  1320 , the optimal read threshold determiner  930  may obtain the read threshold set, a checksum value and an asymmetric ratio of ones count and zeros count which are associated with decoding of the selected page according to each read operation. 
     At operation  1330 , the optimal read threshold determiner  930  may provide the obtained read threshold set, checksum value and asymmetric ratio as input information of the neural network. 
     At operation  1340 , the optimal read threshold determiner  930  may estimate, by the neural network  1100 , an optimal read threshold voltage based on the input information and weights for the input information, the weights including a combination of multiple matrices and bias vectors. 
     In various embodiments of the present invention, the neural network  1100  may be trained to obtain the weights for the input information such that a set loss function is minimized. In this embodiment, the set loss function may include a differentiable loss function to measure an error between outputs of the neural network and the optimal read threshold voltage. 
     In other embodiments, the neural network  1100  may estimate the optimal read threshold voltage based on a combination of the input information, the multiple matrices, the bias vectors and a particular activation function. In this embodiment, the optimal read threshold voltage may be estimated based on Equation above: OptVt=A DNN,N ×R(A DNN,N-1 ×R (A DNN,N-2 ×R( . . . R(A DNN,0 ×[Vt, CS, AR]+b DNN,0 ) . . . )+b DNN,N-2 )+b DNN,N-1 )+b DNN,N , where OptVt represents the optimal read threshold voltage, Vt represents the read threshold set, CS represents the checksum values, AR represents the asymmetric ratio of ones count to zeros count, A DNN,N  represents a (N+1)th matrix, b DNN,N  represents a (N+1)th bias vector, and R represents the activation function. In this embodiment, the activation function can include a rectified linear unit (ReLU) activation function. 
     In various embodiments of the present invention, the selected page may include a least significant bit (LSB) page of a triple level cell (TLC). In this embodiment, the read threshold voltage set may include a first read threshold voltage for distinguishing an erased state and a first program state and a second read threshold voltage for distinguishing a second program state and a third program state. 
     As described above, embodiments of the present invention can provide a scheme for estimating an optimal read threshold voltage in a memory system (e.g., SSD). Embodiments of the present invention can estimate an optimal read threshold voltage using information obtained from previous read attempts through a neural network without any additional sensing operations and any intermediated parameter estimations. Thus, these embodiments can improve quality of service (QoS) and reliability of the memory system. 
     Although the foregoing embodiments have been illustrated and described in some detail for purposes of clarity and understanding, the present invention is not limited to the details provided. There are many alternative ways of implementing the invention, as one skilled in the art will appreciate in light of the foregoing disclosure. The disclosed embodiments are thus illustrative, not restrictive. The present invention is intended to embrace all modifications and alternatives of the disclosed embodiment. Furthermore, the disclosed embodiments may be combined to form additional embodiments.