LDPC Coding Scheme for Improving the Reliability of Multi-Level-Cell NAND Flash Memory in Radiation Environments

Utilizing commercial off-the-shelf(COTS) components in satellites has received much attention due to the low cost. However, commercial memories suffer severe reliability problems in radiation environments. This paper studies the low-density parity-check(LDPC) coding scheme for improving the reliability of multi-level-cell(MLC) NAND Flash memory in radiation environments. Firstly, based on existing physical experiment works, we introduce a new error model for heavyion irradiations; secondly, we explore the optimization of writing voltage allocation to maximize the capacity of the storage channel; thirdly, we design the degree distribution of LDPC codes that is specially suitable for the proposed model; finally, we propose a joint detection-decoding scheme based on LDPC codes, which estimates the storage channel state and executes an adaptive log-likelihood ratio(LLR) calculation to achieve better performance. Simulation results show that, compared with the conventional LDPC coding scheme, the proposed scheme may almost double the lifetime of the MLC NAND Flash memory in radiation environments.

Measurement-device-independent quantum secure direct communication

Quantum secure direct communication(QSDC)is a unique technique,which supports the secure transmission of confidential information directly through a quantum channel without the need for a secret key and for ciphertext.Hence this secure communication protocol fundamentally differs from its conventional counterparts.In this article,we report the first measurement-deviceindependent(MDI)QSDC protocol relying on sequences of entangled photon pairs and single photons.Explicitly,it eliminates the security loopholes associated with the measurement device.Additionally,this MDI technique is capable of doubling the communication distance of its conventional counterpart operating without using our MDI technique.We also conceive a protocol associated with linear optical Bell-basis measurements,where only two of the four Bell-basis states could be measured.When the number of qubits in a sequence reduces to 1,the MDI-QSDC protocol degenerates to a deterministic MDI quantum key distribution protocol.

Implementation and security analysis of practical quantum secure direct communication

Rapid development of supercomputers and the prospect of quantum computers are posing increasingly serious threats to the security of communication.Using the principles of quantum mechanics,quantum communication offers provable security of communication and is a promising solution to counter such threats.Quantum secure direct communication(QSDC)is one important branch of quantum communication.In contrast to other branches of quantum communication,it transmits secret information directly.Recently,remarkable progress has been made in proof-of-principle experimental demonstrations of QSDC.However,it remains a technical feat to bring QSDC into a practical application.Here,we report the implementation of a practical quantum secure communication system.The security is analyzed in the Wyner wiretap channel theory.The system uses a coding scheme of concatenation of lowdensity parity-check(LDPC)codes and works in a regime with a realistic environment of high noise and high loss.The present system operates with a repetition rate of 1 MHz at a distance of 1.5 kilometers.The secure communication rate is 50 bps,sufficient to effectively send text messages and reasonably sized files of images and sounds.

Realization of quantum secure direct communication over 100 km fiber with time-bin and phase quantum states

Rapid progress has been made in quantum secure direct communication in recent years.For practical application,it is important to improve the performances,such as the secure information rate and the communication distance.In this paper,we report an elaborate physical system design and protocol with much enhanced performance.This design increased the secrecy capacity greatly by achieving an ultra-low quantum bit error rate of<0.1%,one order of magnitude smaller than that of existing systems.Compared to previous systems,the proposed scheme uses photonic time-bin and phase states,operating at 50 MHz of repetition rate,which can be easily upgraded to over 1 GHz using current on-the-shelf technology.The results of our experimentation demonstrate that the proposed system can tolerate more channel loss,from 5.1 dB,which is about 28.3 km in fiber in the previous scheme,to 18.4 dB,which corresponds to fiber length of 102.2 km.Thus,the experiment shows that intercity quantum secure direct communication through fiber is feasible with present-day technology.

Experimental free-space quantum secure direct communication and its security analysis

We report an experimental implementation of free-space quantum secure direct communication based on single photons.The quantum communication scheme uses phase encoding,and the asymmetric Mach–Zehnder interferometer is optimized so as to automatically compensate phase drift of the photons during their transitions over the free-space medium.At a 16 MHz pulse repetition frequency,an information transmission rate of 500 bps over a 10 m free space with a mean quantum bit error rate of 0.49%±0.27%is achieved.The security is analyzed under the scenario that Eve performs the collective attack for single-photon state and the photon number splitting attack for multi-photon state in the depolarizing channel.Our results show that quantum secure direct communication is feasible in free space.

Generalized Low-Density Parity-Check Coding Scheme with Partial-Band Jamming

In this study, a class of Generalized Low-Density Parity-Check （GLDPC） codes is designed for data transmission over a Partial-Band Jamming （PBJ） environment. The GLDPC codes are constructed by replacing parity-check code constraints with those of nonsystematic Bose-Chaudhuri-Hocquenghem （BCH）, referred to as Low-Density Parity-Check （LDPC）-BCH codes. The rate of an LDPC-BCH code is adjusted by selecting the transmission length of the nonsystematic BCH code, and a low-complexity decoding algorithm based on message- passing is presented that employs A Posteriori Probability （APP） fast BCH transform for decoding the BCH check nodes at each decoding iteration. Simulation results show that the LDPC-BCH codes with a code rate of 1/8.5 have a bit error rate performance of 1 x10-8 at signal-noise-ratios of -6.97 dB, -4.63 dB, and 2.48 dB when the fractions of the band jammed are 30%, 50%, and 70%, respectively.

Design and Efficient Hardware Implementation Schemes for Non-Quasi-Cyclic LDPC Codes

The design of a high-speed decoder using traditional partly parallel architecture for Non-Quasi-Cyclic（NQC） Low-Density Parity-Check（LDPC） codes is a challenging problem due to its high memory-block cost and low hardware utilization efficiency. In this paper, we present efficient hardware implementation schemes for NQCLDPC codes. First, we propose an implementation-oriented construction scheme for NQC-LDPC codes to avoid memory-access conflict in the partly parallel decoder. Then, we propose a Modified Overlapped Message-Passing（MOMP） algorithm for the hardware implementation of NQC-LDPC codes. This algorithm doubles the hardware utilization efficiency and supports a higher degree of parallelism than that used in the Overlapped Message Passing（OMP） technique proposed in previous works. We also present single-core and multi-core decoder architectures in the proposed MOMP algorithm to reduce memory cost and improve circuit efficiency. Moreover, we introduce a technique called the cycle bus to further reduce the number of block RAMs in multi-core decoders. Using numerical examples, we show that, for a rate-2/3, length-15360 NQC-LDPC code with 8.43-d B coding gain for Binary PhaseShift Keying（BPSK） in an Additive White Gaussian Noise（AWGN） channel, the decoder with the proposed scheme achieves a 23.8%–52.6% reduction in logic utilization per Mbps and a 29.0%–90.0% reduction in message-memory bits per Mbps.

Efficient Helicopter-Satellite Communication Scheme Based on Check-Hybrid LDPC Coding

When implementing helicopter-satellite communications, periodical interruption of the received signal is a challenging problem because the communication antenna is intermittently blocked by the rotating blades of the helicopter. The helicopter-satellite channel model and the Forward Error Control（FEC） coding countermeasure are presented in this paper. On the basis of this model, Check-Hybrid（CH） Low-Density Parity-Check（LDPC）codes are designed to mitigate the periodical blockage over the helicopter-satellite channels. The CH-LDPC code is derived by replacing part of single parity-check code constraints in a Quasi-Cyclic LDPC（QC-LDPC） code by using more powerful linear block code constraints. In particular, a method of optimizing the CH-LDPC code ensemble by searching the best matching component code among a variety of linear block codes using extrinsic information transfer charts is proposed. Simulation results show that, the CH-LDPC coding scheme designed for the helicopter-satellite channels in this paper achieves more than 25% bandwidth efficiency improvement, compared with the FEC scheme that uses QC-LDPC codes.

Quantum secure direct communication with entanglement source and single-photon measurement

Quantum secure direct communication(QSDC)transmits information directly over a quantum channel.In addition to security in transmission,it avoids loopholes of key loss and prevents the eavesdropper from getting ciphertext.In this article,we propose a QSDC protocol using entangled photon pairs.This protocol differs from existing entanglement-based QSDC protocols because it does not perform Bell-state measurement,and one photon of the entangled pair is measured after the entanglement distribution.It has the advantage of high signal-to-noise ratio due to the heralding function of entanglement pairs,and it also has the relative ease in performing single-photon measurement.The protocol can use a practical entanglement source from spontaneous parametric down-conversion(SPDC);Gottesman-Lo-Lu¨tkenhaus-Preskill theory and the decoy state method give a better estimate of the error rate.Security analysis is completed with Wyner’s wiretap channel theory,and the lower bound of the secrecy capacity is estimated.Numerical simulations were carried out to study the performance of the protocol.These simulations demonstrated that the protocol with a practical SPDC entanglement source performed well and was close to the case with an ideal entanglement source.

Generalized sparse codes for non-Gaussian channels:Code design,algorithms,and applications

In this paper,generalized sparse(GS)codes are proposed to support reliable and efficient transmission over non-Gaussian channels.Specifically,by expanding the single-parity check(SPC)code constraints with powerful algebraic codes,GS codes generalize conventional sparse codes with enhanced error-correcting capability,as well as better code design flexibility by covering a wide range of block-lengths and coding rates with reduced encoding/decoding complexity.Moreover,by introducing a universal communication channel model,a general framework for performance analysis and code design of GS codes is formulated,by which the coding parameters can be optimized for different target channel conditions.Finally,example codes are constructed for several critical application scenarios with non-Gaussian channels.Numerical simulations are performed to demonstrate the superiority of the proposed GS coding scheme to traditional channel coding schemes.