This paper investigates the use of an unmanned aerial vehicle (UAV) to assist covert communication between a low-Earth orbit (LEO) satellite and a ground user under the surveillance of a passive warden. The UAV simultaneously serves its own ground network and acts as a friendly jammer to enhance the covertness of satellite transmissions. We derive a closed-form lower bound on the warden's average minimum detection error probability which is then used to define the covert constraint. Building on this, we formulate an optimization problem to jointly design the UAV's 3D placement, its power allocation, and the satellite's transmit power to maximize the system's covert rate. To solve the resulting non-convex problem, we propose an algorithm based on the block coordinate descent (BCD) and successive convex approximation (SCA) techniques, and further develop a Dinkelbach's algorithm for a special case. Numerical results validate the tightness of the derived bound and demonstrate the effectiveness of the proposed algorithms in configuring optimal system parameters.

In a recent article, Baiet al. (2022) investigated the impact of outdated channel state information (CSI) on covert communication in wireless greedy relay networks. In this commentary, we identify a critical mathematical flaw in their derivation of the cumulative distribution function (CDF) of the outdated channel gain. The flaw fundamentally undermines the paper’s main results and conclusions regarding the impact of outdated CSI in such networks. To prevent future studies from building upon this incorrect foundation, we also provide a correction for the flaw and suggestions for future investigations into this problem. Beyond correcting a specific error, this commentary serves as a warning on the potential pitfalls in probabilistic modeling under outdated CSI.

Symmetric block-wise concatenated Bose-Chaudhuri-Hocquenghem (SBC-BCH) codes are known to provide strong error-correcting performance with an iterative hard-decision decoding (IHDD). This work shows that some properties of SBC-BCH codes allow one to readily increase the error-correcting capability of constituent BCH codes by one, i.e., from t to$t + 1$, which greatly improves the error-rate performance of SBC-BCH codes. To this end, we propose a new decoding algorithm that utilizes structural features of SBC-BCH codes in conjunction with an extension of the Berlekamp-Massey (BM) algorithm, namely the one-step-ahead (OSA) BM algorithm. This combination enables us to reduce the computational complexity and combat the inherent decoding ambiguity associated with the OSA-BM algorithm. Furthermore, we develop an analytical framework to evaluate the average number of candidate codewords returned by the OSA-BM algorithm and derive an upper bound on the probability of ambiguity. Then, we propose a decoding algorithm, called OSA-aided IHDD and conduct extensive performance evaluations and comparisons for error-correcting systems employing SBC-BCH codes with the proposed decoding algorithm. The performance evaluations show that the proposed OSA-aided IHDD considerably improves the error-rate performance in both the waterfall and error-floor regions.

This study explores an uplink multi-users covert communication system where certain users cooperate to hide messages from a covert user. A recent work investigated the multi-users covert communication system in which all the users are located at an equal distance from the legitimate receiver and a warden. In contrast, our work extends the existing work to an unequal distance scenario, where users are at unequal distances from the legitimate receiver. In particular, we establish that an on-off scheme, previously proven to be optimal in the equal distance scenario, remains optimal in the unequal distance scenario. The optimality of the on-off scheme enables us to derive expressions of the minimum detection error probability and outage probability in closed-forms. In addition, the closed-form expressions allow us to turn the parameter optimization into a simple one-dimensional search problem. Finally, the theoretical results developed in this work are validated by comparing extensive performance evaluations utilizing the theoretical results and Monte Carlo simulations.

In this work, we study a covert communication scheme in which some users are opportunistically selected to emit interference signals for the purpose of hiding the communication of a covert user. This work reveals interesting facts that the channel correlation is beneficial to the throughput of the covert communication but detrimental to the energy efficiency, which has never been discussed before. The study is conducted in a generic setup where the channels between pairs of entities in the scheme are correlated. For the setup, we discover that the optimal power profile of the interference signals from the selected users turns out to be the equal power transmission at their maximum transmit power level. In addition, we optimize system parameters of the scheme for maximizing throughput and energy efficiency utilizing$Q$-learning, which however is plagued with long learning time and large storage space when the dimension of state gets large and/or a fine resolution of reward function value is necessary. To resolve the technical challenge, we propose a scalable$Q$-learning which recursively narrows down the discretization level of the continuous state in an iterative fashion. To confirm the results in this work, the system parameters are evaluated with theoretical results for independent channels and compared with the ones from the proposed scalable$Q$-learning.

A different approach for constructing protograph-based spatially coupled low-density parity-check (SC-LDPC) codes is proposed. Instead of lifting multiple copies of an identical SC-LDPC protograph at the lifting stage of code construction, we divide the copies into fractions$\alpha \in [{0,1}]$SC-LDPC protographs and$\beta =1-\alpha $uncoupled sequence of block LDPC protographs. The permutations in the edge bundles formed by two different protograph structures follow the multi-edge-type framework. The rate loss associated with SC-LDPC codes is reduced, and we obtain code ensembles with flexible rates, by varying$\beta $. We call these codes partial spatially coupled (PSC) LDPC codes, and through density evolution, we show that for data transmission over the binary erasure channel, the gap between channel capacity and thresholds of these codes is smaller as compared to SC-LDPC codes. The main bottleneck in the practical implementation of SC-LDPC codes is the rate loss. Conventionally, the rate loss can be reduced by taking large coupling length$L$, which however increases the decoding complexity. The proposed PSC-LDPC codes mitigate this tradeoff. Furthermore, these codes can also be decoded by the windowed decoder (WD), and through Monte Carlo simulations, we show that the WD performance of PSC-LDPC codes is better than that of SC-LDPC codes.

This work studies a covert communication scheme for an uplink multi-user scenario in which some users are opportunistically selected to help a covert user. In particular, the selected users emit interfering signals via an orthogonal resource dedicated to the covert user together with signals for their own communications using orthogonal resources allocated to the selected users, which helps the covert user hide the presence of the covert communication. For the covert communication scheme, we carry out extensive analysis and find system parameters in closed forms. The analytic derivation for the system parameters allows one to find the optimal combination of system parameters by performing a simple one-dimensional search. In addition, the analytic results elucidate relations among the system parameters. In particular, it will be proved that the optimal strategy for the non-covert users is an on-off scheme with equal transmit power. The theoretical results derived in this work are confirmed by comparing them with numerical results obtained with exhaustive searches. Finally, we demonstrate that the results of work can be utilized in versatile ways by demonstrating a design of covert communication with energy efficiency into account.

This letter studies deep learning-based efficient ground vibration monitoring systems. In this work, artificial intelligence (AI) techniques are adopted to effectively deal with practical issues of data collection and classification. Specifically, we develop a novel energy-efficient data collection scheme by adopting deep Q-network-based reinforcement learning. Also, we propose an enhanced joint recurrent neural network (RNN) and convolutional neural network (CNN) approach for ground vibration classification. The performance of the proposed scheme is evaluated using real-world ground vibration data. The experimental results show that the proposed classification scheme outperforms the best existing scheme with CNN by more than 13% in terms of classification accuracy. It is also shown that the proposed energy management scheme can improve the accuracy of the proposed ground vibration monitoring system by 7.6% over the comparable scheme using equal power allocation.

In the low-error rate regime, message-passing (MP) decoding for low-density parity-check (LDPC) codes is known to have performance degradation due to trapping sets (TSs), which often limits the use of LDPC codes for applications with low target error rates like storage devices. This work proposes a novel deep-learning based decoding algorithm which is tailored for breaking TSs. In particular, when MP decoding fails due to TSs, there exist pairs of unsatisfied check nodes (CNs) which are connected through paths only with error variable nodes (VNs), i.e., VNs with erroneous hard-decision results. The proposed algorithm efficiently identifies the paths with error VNs between unsatisfied CNs with the aid of deep-learning techniques. Then, the decoding failures are resolved by repeating the MP decoding after re-initializing the channel outputs for the error VNs in the identified paths. In addition, by analyzing the behaviors of the deep-learning based algorithm, we propose a low-complexity algorithm, called adaptive-error-path (AEP) detector. Simulation results show that the proposed algorithms efficiently break the TSs and significantly improve the error-floor performance in the low error-rate regime.

This work studies the design of MET-LDPC code ensembles at low code rates with good threshold performances and exponentially few small weight codewords. There was a notable study on the condition, so called the ` t-value condition', for exponentially few small weight codewords. Meanwhile, it was shown that by introducing degree-one variable nodes, the threshold performances of MET-LDPC code ensembles of low rates can be significantly improved. However, the degree-one variable nodes may result in small weight codewords and thus must be carefully introduced to MET-LDPC code ensembles. Despite the importance, the existing t-value condition was developed only for MET-LDPC code ensembles without degree-one variable nodes. This work extends the t-value condition to MET-LDPC code ensembles with degree-one variable nodes, which includes the existing work as a special case. The extended t-value condition provides useful insights into the contributions of degree-one variable nodes to the distribution of small weight codewords. Thus, the results of this work allow us to design MET-LDPC code ensembles of low rates with both good threshold performances and exponentially few small weight codewords. In addition, it will be demonstrated that MET-LDPC codes at finite lengths based on the designed code ensembles have good error-rate performances both in the waterfall and low-error-rate regions.

Since multi-edge type low-density parity-check (MET-LDPC) codes were first proposed, the design of MET-LDPC codes has been extensively studied for various applications. However, the existing design rules assume that check node degrees are in the so-called concentrated form which enables one to conveniently find pairs of edge and node distributions satisfying the socket count equalities (SCEs). However, it in return makes the code design conducted in a limited search space, which may lead to a sub-optimal design. This paper proposes a novel design rule for MET-LDPC codes together with a scheme to efficiently sort out invalid distributions, i.e., ones not satisfying the SCEs, in the design process. The proposed design rule does not require check node degrees to be in the concentrated form and can find out optimized MET-LDPC codes without any restriction on the search space. Based on the proposed design rule, MET-LDPC codes are designed across a wide range of code rates on binary-erasure channel and binary-input additive-white Gaussian channel. The designed codes are compared with ones reported in the open literature, which confirms that MET-LDPC codes with the proposed design rule have better threshold performances regardless of channel and code rate even with sets of limited code parameters.

This paper studies a wireless powered relaying (WPR) system in which a source transmits an information signal to a destination with the help of an energy constrained relay in the presence of an eavesdropper. This work considers a scenario in the full-duplex WPR system where the information transmission/reception and the energy harvesting are performed simultaneously at the relay. We first derive an achievable secrecy rate of the system and formulate a joint optimization problem of artificial-noise-aided secure beamforming to maximize the achievable secrecy rate. Then, an efficient semi-definite relaxation (SDR) based algorithm is proposed to obtain the optimal beamforming. Furthermore, we also propose three suboptimal beamforming but low computational complexity schemes. Comparisons are carried out among the performances of the proposed optimal and suboptimal schemes.

This paper introduces a high rate error-correcting coding scheme called symmetric block-wise concatenated Bose-Chaudhuri-Hocquenghem (symmetric BC-BCH) codes tailored for storage devices with hard-decision outputs, e.g., storage devices based on NAND flash memory. It will be shown that a careful integration of the symmetry and 2-D block-wise concatenation is especially beneficial to achieve improvements of error-rate performance when an iterative hard-decision-based decoding (IHDD) is assumed. The claim is substantiated by proving that the proposed symmetric concatenation is optimal in terms of error-rate performance in the low error-rate regime over other 2-D block-wise concatenations. Besides, this paper proposes a novel way to design constituent codes, which enables us to enjoy advantages of primitive BCH codes and to efficiently break stopping sets associated with the IHDD in the low error-rate regime. We consider error-control systems made up of a symmetric BC-BCH code, the IHDD, and simple auxiliary decoders specifically targeting to break stopping sets caused in the IHDD. It will be shown that the auxiliary decoders significantly improve error-rate performance at a negligible amount of extra complexity. Performance comparisons are also carried out between error-control systems with the proposed and other coding schemes such as BCH codes, quasi-primitive BC-BCH codes, and low-density parity-check codes.

This paper studies a secure communication over multiple-input multiple-output multi-antenna eavesdropper (MIMOME) channels which consist of legitimate parities, namely a transmitter and receiver, equipped with multi-antennas and a passive eavesdropper with multi-antennas. For securing the communication between the legitimate parties, we consider an artificial noise (AN) scheme in which the legitimate transmitter sends its secret messages and AN signal together. Meanwhile, it is assumed that the legitimate receiver has a full-duplex capability which enables it to capture the secret message from the transmitter and simultaneously generate a jamming signal to strengthen security. While there have been studies on similar setups, most of them focus on the design of the jamming signal with the assumption of full channel-state information (CSI) and/or system parameter optimization based on numerical evaluations. On the contrary, in this work, we instead introduce a tight lower bound on an achievable ergodic secrecy rate as a versatile analytic tool and derive a closed-form expressions for the bound. To confirm the analytic results, we carry out numerical evaluations of the ergodic secrecy rate which are compared with the proposed lower bound.

Correlation across transmit antennas in multiple-input multiple-output (MIMO) systems has been studied in various scenarios and has been shown to be detrimental or provide benefits depending on the particular system and underlying assumptions. In this paper, we investigate the effect of transmit correlation on the capacity of the Gaussian MIMO broadcast channel, with a particular interest in the large-scale array (or massive MIMO) regime. To this end, we introduce a new type of diversity, referred to as transmit correlation diversity, which captures the fact that the channel vectors of different users may have different channel covariance matrices spanning often nearly mutually orthogonal subspaces. In particular, when taking the cost of downlink training properly into account, transmit correlation diversity can yield significant capacity gains in all regimes of interest. Our analysis shows that the system multiplexing gain can be increased by a factor up to ⌊M/r⌋, where M is the number of antennas and r ≤ M is the common rank of the users channel covariance matrices, with respect to standard schemes that are agnostic of the transmit correlation diversity and treat the channels as if they were isotropically distributed. Thus, this new form of diversity reveals itself as a valuable “new resource” in multiuser communications.

In this work, we study high-rate error-control systems with serial quasi-primitive block-wise concatenated Bose-Chaudhuri-Hocquenghem (BC-BCH) codes for storage devices using multi-level per cell (MLC) NAND flash memories. The system targets at achieving a strong error-correcting capability when only hard-decision channel outputs are available. Error-rate performance of the proposed system is compared with those of systems based on various coding schemes including LDPC codes.

Trapping sets strongly degrade performance of low-density parity check (LDPC) codes in the low-error-rate region. This creates significant difficulties for the deployment of LDPC codes to low-error-rate applications such as storage and wireless systems with no or limited retransmission options. We propose a novel technique for breaking trapping sets based on collaborative decoding that utilizes two different decoding modes. While the main decoding mode executes message passing based on the original parity check matrix of the corresponding LDPC code, the sub-decoding mode operates on a modified parity check matrix formed by removing a portion of check nodes in the factor graph representation of the given code. The modified parity check matrix is designed to promote a passing of correct information into erroneous variable nodes in the trapping set. Theoretical properties of the proposed trapping-set-breaking technique have been established based on the notion of the improved separation for the trapped variable nodes. Simulation results show that the proposed collaborative LDPC decoding scheme switching between the two decoding modes back and forth effectively breaks dominant trapping sets of various known types of regular and irregular LDPC codes.

In modern tape storage, user data are recorded and retrieved along multiple tracks of rapidly moving, flexible magnetic medium that give rise to a variety of channel impediments including occasional long erasures, more frequent amplitude fades as well as a large amount of random errors. This work considers reliable recovery of data from such tape channels using a novel concatenation of an inner Reed-Solomon (RS) code and an outer nonbinary low-density parity-check (LDPC) code. This particular concatenation scheme and a highly tailored iterative decoding algorithm are chosen to efficiently handle the assortment of the tape channel impediments while meeting the stringent target error rate constraint as well as key practical requirements of the mass tape storage system. Despite the use of a nonbinary LDPC code, the proposed scheme allows excellent performance-complexity tradeoffs. In stark contrast to any existing coding schemes that involve LDPC codes, the proposed concatenation strategy allows semianalytic error rate performance evaluation at rates below what is possible using modern computers, thus providing an ability to ensure satisfactory low-error-rate performance.

Biologically inspired pulse-coupled synchronization has received increasing attention as one of key techniques for developing decentralized wireless networks due to its inherent scalability and simplicity. While it has been actively studied in recent years, most of the previous works have focused on the synchronization only in fault/attack free networks. However, in reality, a network may have malfunctioning nodes and/or attackers, and thus robustness against these threats in the pulse- coupled synchronization must be an important practical issue on its path to realization. Motivated by this, we analyze the influence of attacker's behaviors in a network of pulse-coupled oscillators. The analysis shows that when the number of attackers is less than a certain number, i.e. a threshold, the network sustains its synchronization. Moreover, the threshold is proportional to both of coupling strengths among the pulse-coupled oscillators and the number of legitimate oscillators. Finally, numerical experiments are given to confirm the analytic results.

In this work, we propose a novel design rule of block-wise concatenated Bose-Chaudhuri-Hocquenghem (BC-BCH) codes for storage devices using multi-level per cell (MLC) NAND flash memories. BC-BCH codes designed in accordance with the proposed design rule are called quasi-primitive BC-BCH codes in which constituent BCH codes are deliberately chosen for their lengths to be as close to primitive BCH codes as possible. It will be shown that such quasi-primitive BC-BCH codes can achieve significant improvements of error-correcting capability over the existing BC-BCH codes when an iterative hard-decision based decoding (IHDD) is assumed. In addition, we propose a novel collaborative decoding algorithm which targets at resolving dominant error patterns associated with the IHDD. Error-rate performances of error-control systems with the proposed quasi-primitive BC-BCH and existing BC-BCH codes are compared. For more comprehensive performance comparisons, systems with a hypothetically long BCH code and a product code are also considered in the comparisons.

We present a secret key agreement (SKA) protocol for a multi-user time-division duplex system where a base-station (BS) with a large antenna array (LAA) shares secret keys with users in the presence of non-colluding eavesdroppers. In the system, when the BS transmits random sequences to legitimate users for sharing common randomness, the eavesdroppers can attempt the pilot contamination attack (PCA) in which each of eavesdroppers transmits its target user's training sequence in hopes of acquiring possible information leak by steering beam towards the eavesdropper. We show that there exists a crucial complementary relation between the received signal strengths at the eavesdropper and its target user. This relation tells us that the eavesdropper inevitably leaves a trace that enables us to devise a way of measuring the amount of information leakage to the eavesdropper even if PCA parameters are unknown. To this end, we derive an estimator for the channel gain from the BS to the eavesdropper and propose a rate-adaptation scheme for adjusting the length of secret key under the PCA. Extensive analysis and evaluations are carried out under various setups, which show that the proposed scheme adequately takes advantage of the LAA to establish the secret keys under the PCA.

In this work, we consider high-rate error-control systems based on block-wise concatenated Bose-Chaudhuri-Hocquenghem (BC-BCH) codes with iterative hard-decision decoding (IHDD) for storage devices using multi-level per cell (MLC) NAND flash memories. In particular, we propose a novel design rule of BC-BCH codes which consists of quasi-primitive BCH codes and block-wise concatenation of the constituent codes. Comprehensive performance comparisons are carried out among error-control systems with various coding schemes such as BC-BCH codes and LDPC codes.

Massive multiple-input multiple-output (MIMO) has been actively studied for next generation wireless systems since it can provide high data rate and an improved energy efficiency in multiuser systems using large antenna arrays at base stations (BSs). While massive MIMO has various advantages, its perfor- mance is known to be limited by pilot contamination. To mitigate the intercell interference resulting from pilot contamination, pilot contamination precoding (PCP) was proposed for downlink transmissions. For PCP, slow-fading coefficients need to be estimated. In this paper, we study the estimation of slow-fading coefficients. We show that slow-fading coefficients cannot be estimated using the conventional uplink training. Thus, we have proposed amplitude modulated pilot sequences, which allow us to estimate slow-fading coefficients without requiring additional orthogonal pilot sequences.

In this work, we consider high-rate error-control systems for storage devices using multi-level per cell (MLC) NAND flash memories. Aiming at achieving a strong error-correcting capability, we propose error-control systems using block-wise parallel/serial concatenations of short Bose-Chaudhuri-Hocquenghem (BCH) codes with two iterative decoding strategies, namely, iterative hard-decision decoding (IHDD) and iterative reliability based decoding (IRBD). It will be shown that a simple but very efficient IRBD is possible by taking advantage of a unique feature of the block-wise concatenation. For tractable performance analysis and design of IHDD and IRBD at very low error rates, we derive semi-analytic approaches. The proposed error-control systems are compared with various error-control systems with well-known coding schemes such as a product code, multiple BCH codes, a single long BCH code, and low-density parity-check codes in terms of page error rates, which confirms our claim: the proposed error-control systems achieve good tradeoffs between error-performance and complexity as compared to the traditional schemes and is also very favorable for implementation.

We consider a cooperation scenario between primary users and untrusted secondary users in cognitive radio networks. The secondary users are willing to help the primary users to relay the primary users' messages in reward for being allowed to share the primary users' spectrum bands. However, the primary users might be reluctant to accept this help, since the secondary users are untrustworthy and may try unauthorized decoding of the primary users' messages. This paper will answer the question: when is this cooperation mutually beneficial for primary and secondary users? Taking an approach of information-theoretic secrecy, such as coding techniques for wiretap channels, the primary users can allow the secondary users to sense and relay the message, while ensuring that the secondary users are ignorant of the primary users' messages. We characterize an achievable secrecy rate of primary users with a rate of the secondary users' communication. From the derived rate pairs, an optimization problem is formulated such that the secondary transmitter distributes its transmit power to maximize its data rate while providing a higher secrecy rate to the primary users. We demonstrate that this cooperation can provide a positive secrecy rate, even when a non-cooperative scheme achieves a zero secrecy rate.

We consider secret key agreement (SKA) over time division duplex for a multi-user wireless system, in which a base station (BS) equipped with a large antenna array remotely generates different secret keys for multiple users in the system. The BS can have the SKA over downlink broadcast channels with precoding based on uplink training from the multiple users. In this case, unfortunately, an eavesdropper can effectively perform a pilot contamination attack (PCA) by transmitting a targeted user's training sequence for the purpose to steer beam direction toward the eavesdropper. As the beam direction becomes under the eavesdroppers control, this PCA can result in information leakage from the BS to the targeted user. In order to fend off this PCA, we derive PCA detectors based on generalized likelihood ratio test and propose a countermeasure. For the performance analysis, we consider the outage probability and show that it decreases exponentially with the number of antennas at the BS, which cannot be achieved by the conventional SKA.

In the past decades, physical layer security has been extensively studied to exploit fundamental capabilities of physical layer such as randomness in wireless channels, signal-to-noise ratio gap, intended jamming, etc., for secure wireless communications. The notion of secrecy rate plays a crucial role in quantifying the transmission rate for secure communications in the presence of an eavesdropper. Wireless sensor networks (WSNs) often require secure communications as the transmissions from sensors to a fusion center are vulnerable to eavesdropping. Although the application of cryptography schemes has been considered in WSNs, most schemes would be too expensive for sensors in terms of computation and energy cost. Thus, it might be necessary to devise physical layer security schemes that exploit properties of wireless channels to avoid eavesdropping with much less computation and energy cost. In this paper, we discuss physical layer security techniques for the WSNs that perform distributed detection. Since the notion of secrecy rate may not be useful in WSNs, we employ the notion of the maximum equivocation in distributed detection to see whether or not perfect secrecy is achievable.

Energy delay tradeoff (EDT) is a fundamental tradeoff that plays a crucial role in understanding the energy efficiency of various transmission schemes. In particular, for hybrid automatic repeat request (HARQ) protocols, the EDT can be well defined. In this paper, in order to understand the EDT for a wireless multiuser system, we consider various downlink transmission schemes where the HARQ with incremental redundancy (HARQ-IR) protocol is employed for reliable transmissions to users. For a given total power, the impact of different transmission schemes in conjunction with power allocation on EDT is extensively studied. Among the transmission schemes, it is shown that the threshold-based transmission (TBT) scheme efficiently lowers the minimum energy per bit (EB) in spite of limited prior channel state information (CSI) feedback. The study is further extended to a more plausible scenario where users have different channel statistics. In doing so, we consider a proportional fairness scheduling based on relative channel gain that can be easily accommodated in the proposed TBT scheme, and study its EDT behaviors. Performance evaluations show that the schemes incorporating proportional fairness achieve good EDT performances while guaranteeing the fairness among users.

Multilevel flash memories store more than one bit per storage cell and are further characterized by large word (page) sizes and very low target error rates. In this paper, a high-rate error control scheme is presented that uses inner trellis-coded modulation (TCM) for storing two bits per cell with five possible charge levels. The coded subset-label bits and the uncoded signal-label bits of TCM are independently protected by separate outer Reed-Solomon (RS) codes. The resulting scheme permits multistage decoding. Errors made by the TCM decoder in the subset-label bits occur in bursts and are corrected by the associated first RS decoder prior to determining signal-label bits and correcting errors in those bits by the associated second RS decoder. The multi-stage decoding avoids the significant spread of errors from subset-label bits into the generally larger number of signal-label bits which is typical for conventional serial RS-TCM concatenation when the inner TCM system operates at relatively low SNR. The error performance of the proposed scheme is evaluated at low error rates by a mixed simulation-analytic method. It is shown that the proposed scheme exhibits highly favorable performance vs. complexity tradeoffs compared to the other schemes.

Spectrum sensing in a cognitive radio network is an essential technique that makes secondary users to detect the presence of primary users. Furthermore, the secondary users can make use of sensing results to help the primary users' transmission in a way to forward them in reward for allowing the spectrum access. However, for the primary users, the spectrum sensing can be considered eavesdropping in the sense that the secondary users may try to decode primary users' messages based on the sensing results. In this paper, by applying the notion of information-theoretic secrecy to the cognitive radio scenario, we propose a secure cooperative transmission scheme targeting at allowing the secondary users to sense and relay but making them ignorant of the primary users' message. Wiretap channel coding is applied to an encoding process of primary users, and the secondary users help the primary users' transmission in a way to forward the amplified sensing results combined with their own messages. We characterize an achievable secrecy rate and data rate pair that primary and secondary users can achieve and formulate three optimization problems from which the secondary transmitter carefully distributes transmit power between the relaying signal and its own message. The numerical results show that our scheme has a non-zero positive secrecy rate in the area where non-cooperative scheme achieves a zero secrecy rate.

In this work, we propose a scheme to extract soft information from hard-decision outputs in multi-level per cell (MLC) flash memory based on a cell-to-cell interference model. It will be shown that the soft information extracted in the form of log-likelihood ratio (LLR) by taking into account a dominant cell-to-cell interference term provides significant performance improvements when the error-control system is designed with an error-correcting code with iterative decoding algorithm, e.g. low-density parity-check (LDPC) code with the Belief-Propagation (BP) algorithm. To confirm the claims, we design error-control systems with a conventional Bose-Chaudhuri-Hocquenghem (BCH) code and an LDPC code with/without the soft information extraction. Performances of the systems are extensively evaluated and compared, which clearly shows that the soft information extraction based on the cell-to-cell interference model leads to a considerably better performance.

In this work, we consider designing high-rate error-control systems for storage devices using MLC NAND flash memories. Traditional systems designed with either a single BCH code or multiple short BCH codes may suffer from high decoding complexity or rate loss due to limited error-correcting capability, respectively. Aiming at achieving a stronger error-correcting capability with much reduced complexity, we propose an error-control system using a concatenation of short BCH codes with iterative decoding strategies. The performance of the proposed coding scheme is thoroughly analyzed and evaluated with computer simulations and a semi-analytic way at a target page-error rate, 10-14, which confirms our claims: the proposed coding scheme achieves good error-performance and complexity tradeoffs as compared to the traditional schemes and is very favorable for implementation.

In this paper, an iterative distributed approach is studied for the signal amplitude optimization when the distributed detection is carried out with a symmetric signaling constraint in a wireless sensor network (WSN) consisting of a fusion center (FC) and multiple sensors. In the conventional distributed detection, if sensors'' characteristics change due to aging problems or varying operating conditions, they have to be available at the FC for a best combining result. On the other hand, in the proposed approach, each sensor can adjust its signal amplitude using a type-based multiple access (TBMA) technique to provide a best performance at the FC through an iterative approach without sending their individual characteristics.

We consider an overlay mode of an orthogonal frequency division multiple access (OFDMA) based cellular system using orthogonal beamforming to provide broadband wireless access for rural users. In overlay mode, since the signals transmitted to rural users should not interfere with mobile users of higher priority in the OFDMA-based cellular system, the beamforming vectors to rural users have to be orthogonal to the channel vectors to mobile users, which becomes a beamforming constraint for rural users. It is shown that the signal to interference-plus-noise ratio (SINR) can be reasonably high for rural users in overlay mode despite the beamforming constraint and the proposed overlay mode can offer broadband access to rural users.

We introduce a binary erasure wiretap channel of type II in which the number of eavesdropped bits μ becomes available a posteriori. We aim at achieving perfect secrecy over such a channel model. The most appropriate application is a secret key agreement scheme. We present a secret key agreement scheme that adopts the formulation S = HX of Wyner-Ozarows's linear coset coding. The scheme is based on the following simple observation: even if some information on a secret message leaked out, I(S; Xμ) >; 0, where Xμis a binary sequence of length μ, it is still possible to have perfect secrecy I(SJ; Xμ) = 0 for some subsequence SJof S. Our secret key agreement scheme achieves perfect secrecy by taking only those subsequences SJthat are independent of the eavesdropped bits Xμ. Our secret key agreement scheme naturally leads to defining a security measure DH(μ) for parity-check matrices such that the eavesdropper gets zero information on SJas long as the length of SJis less than DH(μ). We study basic properties of DH(μ) and prove the perfect secrecy of our key agreement scheme. For parity-check matrices of small sizes, we perform an exhaustive search for matrices maximizing DH(μ).

Finding an optimal linear precoding transmission strategy still remains an open problem, since it cannot be transformed to a convex problem. In this paper, we introduce some condition under which a duality of the rate regions of MMO broadcast channel (BC) and MIMO multiple-access channel (MAC) with linear precoding is established. In particular, we convert the non-convex and untractable sum rate optimization problem at hand to a well-structured dual problem like the dirty-paper coding (DPC) strategy. We provide sum-rate analysis of the proposed linear precoding strategy, referred to as block triangularization (BT), and address its optimality at high SNR. Specifically, we show that not only BT, but also the more familiar block diagonalization (BD) strategy is asymptotically optimal at high SNR. However, BT is shown to be always at least as good as BD over the entire SNR range. A sum-rate optimal solution at any SNR for the proposed BT strategy is also found.

We consider data confidentiality in a distributed detection scenario with a type-based multiple-access (TBMA) protocol where a large set of sensors sends local measurements to an ally fusion center (FC) over an insecure wireless medium called the main channel. Then, the ally FC makes a final decision to the physical environment. Although many wireless sensor networks are mission-specific and need data confidentiality due to the broadcast nature of wireless transmission, it can be easily wiretapped by unauthorized enemy FCs through eavesdropping channels. We propose a novel TBMA protocol called secure TBMA which provides data confidentiality by taking advantage of inherent properties of wireless channels, namely randomness and independence of the main and eavesdropping channels. In particular, the secure TBMA activates sensors having strong and weak main channel gains and makes the sensors follow different reporting rules based on the magnitudes of their channel gains. The reporting rules are carefully designed to confuse the enemy FC. The proposed secure TBMA delivers unconditional/perfect secrecy and does not assume any superiority of the ally FC over the enemy FC in terms of computational capability, secret key, and so on. For Rayleigh fading channels, we analyze the performance of the secure TBMA at both enemy and ally FCs by investigating conditions for perfect secrecy and an error exponent of detection error probability, respectively. On the one hand, the analysis at the enemy FC provides a design criterion of the reporting rules to achieve perfect secrecy. On the other hand, the analysis of the error exponent carried out with a Gaussian approximation shows that perfect secrecy is achievable at a marginal cost in detection error performance. All our claims are also verified with simulation results which have good matches with the analysis.

This paper presents a coding scheme for the Gaussian wiretap channel based on low-density parity-check (LDPC) codes. The messages are transmitted over punctured bits to hide data from eavesdroppers. The proposed coding scheme is asymptotically effective in the sense that it yields a bit-error rate (BER) very close to 0.5 for an eavesdropper whose signal-to-noise ratio (SNR) is lower than the threshold SNRE, even if the eavesdropper has the ability to use a bitwise maximum a posteriori (MAP) decoder. Such codes also achieve high reliability for the friendly parties provided they have an SNR above a second threshold SNRB. It is shown how asymptotically optimized LDPC codes are designed with differential evolution where the goal is to achieve high reliability between friendly parties while keeping the security gap SNRB/SNREas small as possible to protect against passive eavesdroppers. The proposed coding scheme is encodable in linear time, applicable at finite block lengths, and can be combined with existing cryptographic schemes to deliver improved data security by taking advantage of the stochastic nature of many communication channels.

We investigate the secrecy capacity of an ergodic fading wiretap channel when the main and eavesdropper channels are correlated. Assuming that the transmitter knows the full channel state information (CSI) (i.e., the channel gains from the transmitter to the legitimate receiver and eavesdropper), we quantify the loss of the secrecy capacity due to the correlation and investigate the asymptotic behavior of the secrecy capacity in the high signal-to-noise ratio (SNR) regime. While the ergodic capacity of fading channels grows logarithmically with SNR in general, we have found that the secrecy capacity converges to an upper-bound (a closed-form expression is derived) that will be shown to be a function of two channel parameters; the correlation coefficient and the ratio of the main to eavesdropper channel gains. From this, we are able to see how the two channel parameters affect the secrecy capacity and conclude that the excessively large signal power does not help to improve the secrecy capacity and the loss due to the correlation could be significant especially when the ratio of the main to eavesdropper channel gains is low.

Energy saving is the one of the most critical issues in wireless sensor networks (WSNs) due to limited battery power of sensor nodes. In this paper, we propose a channel-aware type-based multiple access (TBMA) scheme for longer lifetime of WSNs. In an effort to minimize data transmission which is the most energy consuming operation in a sensor device, the proposed scheme allows a subset of sensors to be opportunistically activated in TBMA when their channel gains are higher than a threshold broadcasted by the fusion center to satisfy a given detection error performance (DEP). With the assumption of the channel reciprocity in a narrowband time-division duplexing system, the activated sensors by the threshold exploit channel state information and control their transmit power in a way to maximize an error exponent of the DEP. Here, the broadcasted threshold plays a role in minimizing the number of activated sensors, and thereby it is expected to prolong the lifetime of WSNs. We analyze the DEPs of the proposed and a random selection schemes, and our results show that the proposed scheme provides significant energy saving as compared to the random selection scheme especially in the low signal-to-noise ratio regime.

The problem of maximizing sum rate of a multiple-antenna Gaussian broadcast channel (BC) with dirty-paper coding (DPC) is well known to be efficiently solved via duality, which transforms this nonconvex problem into a well-structured convex multiple-access channel (MAC) problem. To date, however, finding an optimal transmission strategy when employing linear precoding is still open, since it could not be transformed to a convex problem. In this paper, we introduce some restrictive condition and establish a duality between sum rates of MIMO BC and MIMO MAC with linear precoding under that condition, which converts the nonconvex and untractable problem at hand to a well-structured one like the DPC strategy. By doing so, we can anticipate a computationally efficient optimization algorithm which converges to the global optimum.

Since wireless sensor networks (WSNs) are vulnerable to threats and attacks due to the nature of wireless communications between sensors and fusion center (FC), it is often necessary to secure transmissions from any possible eavesdropping. In this paper, we study the eavesdropping issue when sensors are over-deployed and the channels between sensors and the ally FC are modeled as time-varying Rayleigh fading channels. We propose a transmission scheme in which only the sensors of strong and weak channel gains report their decisions in a predetermined way based on type-based multiple access protocol. By taking advantage of random behaviors of wireless channels in the form of the multiuser diversity and carefully design different roles of sensors of strong and weak channel gains to the ally FC, the proposed scheme can confuse the enemy FC in making a decision. Eventually, it guarantees the perfect secrecy promised by an information theoretic measure. We also analyze detection error probability and equivocation at the ally and enemy FCs, respectively.

In distributed detection for wireless sensor networks, multiple access schemes and channel conditions affect the performance of data fusion at a fusion center together with communication constraints (bandwidth and transmission energy). For an efficient channel use, the type-based multiple access (TBMA) has been proposed by Mergen et al. Since the performance of TBMA is not satisfactory for zero-mean fading channels, it could be extended to exploit multichannel diversity for better performance, which is called TBMA with multichannel diversity (TBMA-MD) in this paper. We study the performance of TBMA-MD over Rayleigh fading channels and show that the error probability decays exponentially in the multichannel diversity gain (not in the number of sensors). Error exponents are also derived for binary hypotheses with closed-form expressions for some cases.

We propose a channel-aware type-based multiple access (TBMA) scheme for longer lifetime of WSNs. In an effort to minimize data transmission, the proposed scheme allows a set of sensors to be opportunistically activated in TBMA when their channel gains are higher than a threshold broadcasted by the fusion center. In each transmission, the activated sensors control their transmit power in a way to maximize an error exponent of the detection error performance. We analyze the DEPs of the proposed scheme and show that it provides significant energy saving as compared to the random selection scheme especially in the low signal-to-noise ratio regime where most of power-limited sensors are working.

This paper presents a coding scheme for the Gaussian wiretap channel based on low-density parity-check (LDPC) codes. The messages are transmitted over punctured bits to hide data from eavesdroppers. It is shown that this method is asymptotically effective in the sense that it yields a BER very close to 0.5 for an eavesdropper whose SNR is lower than the threshold SNRE, even if the eavesdropper has the ability to use a bitwise MAP decoder. Such codes also achieve high reliability for the friendly parties provided they have an SNR above a second threshold SNRB. It is shown how asymptotically optimized LDPC codes can be designed with differential evolution where the goal is to achieve high reliability between friendly parties and security against a passive eavesdropper while keeping the security gap SNRB/SNREas small as possible. The proposed coding scheme is applicable at finite block lengths and can be combined with existing cryptographic schemes to deliver improved data security by taking advantage of the stochastic nature of many communication channels.

This paper presents a modified turbo principle (MTP) for iterative detection and decoding in multiple-antenna systems whose detector uses the max-log-MAP algorithm. The modified turbo principle intensionally introduces dependency between the messages of the max-log-MAP detector and the decoder. Our simulation results show that the proposed MTP significantly reduces the performance loss due to the suboptimal detection algorithm, max-log-MAP. We also substantiate our claims by analyzing the proposed MTP from an information theoretic perspective.

This paper aims to improve the design of iterative detection and decoding (IDD) based on the soft interference cancellation with minimum mean squared error (SC-MMSE) detector, which shows low performance compared to the maximum a posteriori (MAP) detector. Such low performance is attributed to that the "pure" (original) turbo principle is not always best for IDD with SC-MMSE. Thus, we propose a new IDD architecture based on the SC-MMSE detector which uses new a priori information. Extrinsic information transfer (EXIT) chart analysis and simulation results show that the performance of the proposed IDD is very close to that of IDD based on the MAP detector.

We consider efficiently encodable rate-compatible (E2RC) punctured low-density parity-check (LDPC) codes and show that punctured LDPC codes of the E2RC structure have high error floors at their base code rates. Efficient type-II hybrid Automatic Repeat reQuest (ARQ) protocols are possible with punctured LDPC codes since punctured LDPC codes can support a wide range of code rates with incremental redundancy transmissions. However, such high error floors may result in excessive number of retransmission requests and end up with poor efficiency of hybrid ARQ protocols. We find that the problem of the E2RC structure stems from dispersive right degree distribution and high maximum right degree which is an indispensable element in the E2RC structure. In this paper, we propose a modification to E2RC structure which eliminates the error-floor problem without compromising any of important advantages of the E2RC structure such as linear-time encodability and good bit-error rate (BER) performance over a wide range of code rates. Our claims will be verified with BER and frame error rate (FER) simulation results.

The minimum mean square error with successive interference cancellation (MMSE-SIC) receiver is known to achieve the capacity of multiple-input multiple-output (MIMO) fast fading channels in the presence of knowledge of the channel at the receiver. This paper presents efficient and numerically stable Cholesky decomposition based detection algorithms for MMSE-SIC, exploiting a property of ordering of MMSE-SIC. The proposed algorithms are shown to significantly reduce the computational complexity of existing efficient algorithms for SIC in MIMO flat fading channels.

While rate-compatible punctured low-density parity-check (RCP-LDPC) codes offer high flexibility in terms of code rate at a relatively low cost in implementation complexity, they are reported to require more decoding iterations than unpunctured LDPC codes. In this paper1we consider layered belief propagation decoding and propose efficient check node layering that significantly accelerates the decoding convergence of RCP-LDPC codes. We show that the proposed layering outperforms both random layering and conventional BP decoding. The performance improvements become more distinctive at high rates and they come at no additional implementation cost.

Low-density parity-check (LDPC) codes have an inherent stopping criterion, parity-check constraints (equations). By testing the parity-check constraints, an LDPC decoder can detect successful decoding and stop their decoding, which is, however, not possible with turbo codes. In this paper, we propose a stopping criterion to predict decoding failure of LDPC codes, instead of detecting successful decoding. If the decoder predicts the decoding failure in advance, the receiver can more rapidly response to the transmitter and request for additional parity bits with an automatic repeat request (ARQ) protocol, which reduces overall system latency. The receiver can also save power consumption by avoiding unnecessary decoder iterations. The proposed stopping criterion makes use of the variations of the number of satisfied parity-check constraints in the belief-propagation (BP) decoding which is always tested in the conventional BP decoding to detect successful decoding. Thus, the proposed stopping criterion does not require any additional complexity. The counting of satisfied parity-check constraints shows behaviors of the BP decoding, which comes, otherwise, from the observations of changes of log-likelihood ratio (LLR) values in multi-bit resolution with additional complexity.

This paper observes the inter-carrier interference (ICI) by Doppler effect in time domain in orthogonal frequency division multiplexing systems. This observation allows us to propose a new time domain two-stage equalization scheme that iteratively cancels the ICI. The proposed scheme consists of the following two stages. First, we apply low-complexity linear minimum mean squared error (MMSE) employing partial rank of time impulse response matrix to shorten the ICI components into diagonal region. Second, we propose new parallel interference cancellation with hard decision feedback to remove the residual ICI. Performance evaluations and complexity analyses show that the bit error rate and the signal-to-interference-plus-noise ratio performances of the proposed scheme are very close to those of the classical frequency domain linear MMSE equalization scheme with much reduced complexity

We consider the problem of rate-compatible puncturing of low-density parity-check (LDPC) codes over additive white Gaussian noise (AWGN) channels. In previous work, it was shown that "good" puncturing distributions exist for LDPC codes but the code length was large. In this correspondence, we give a procedure for determining the puncturing distributions for LDPC codes with short block lengths (a few thousand bits) and show that careful puncturing can produce good performance. We compare the performance of the rate-compatible punctured LDPC codes with dedicated LDPC codes across a range of rates and see that the rate-compatible codes have a favorable complexity/performance tradeoff.

In this paper we treat rate-compatible punctured low-density parity-check (RCP-LDPC) codes. We show how a mother code can be efficiently punctured to achieve good performance over a wide range of rates and discuss the design of a mother code. While this approach can be applied to virtually any time-varying channel, we apply it to the ultra wide band (UWB) channel, e.g. 802.15.3a, and we show that RCP-LDPC codes perform well as compared to a selection of fixed rate unpunctured codes. They reduce the implementation complexity and enable the use of retransmission protocols based on incremental redundancy to increase the throughput

In this paper we study and propose an algorithm to puncture finite length low density parity check (LDPC) codes (Ha, J, et al., 2002). The introduced puncturing criterion results in good performance (for 1024 and 4096 bits) when compared with both random puncturing and dedicated LDPC codes, i.e. unpunctured codes designed for a given rate. The comparison also shows that the proposed punctured LDPC codes have better block-error rates than the dedicated codes because of longer effective block lengths of the high-rate puncturing. Although we apply the idea for regular LDPC codes, we can easily modify the idea for irregular LDPC codes.

In this correspondence, we consider puncturing of low-density parity-check (LDPC) codes for additive white Gaussian noise (AWGN) channels. We show that good puncturing patterns exist and that the puncturing can be performed in a rate-compatible fashion. Furthermore, rate-compatible puncturing results in a small loss of performance with respect to threshold, namely, the punctured code is good (in terms of threshold) across a range of rates when compared with the optimal codes for each rate. This allows one to implement a single "mother" encoder and decoder that is good across a wide range of rates.

In this paper, we consider rate compatible puncturing of low density parity check (LDPC) codes. We present a general density evolution-based procedure which finds the optimal puncturing of a based code. We show that puncturing can be performed across a range of rates and code lengths in a manner that produces punctured codes with good thresholds. This allows one to implement a single optimal LDPC code of a low rate that can be punctured across a wide range of rates without loss of threshold performance. Simulation results show that the error floors of the codes do not degrade after puncturing.

We consider low-density parity-check code (LDPCC) design for additive white Gaussian noise (AWGN) channels with erasures. This model, for example, represents a common situation in magnetic and optical recording where defects or thermal asperities in the system are detected and presented to the decoder as erasures. We give thresholds of regular and irregular LDPCCs and discuss practical code design over the mixed Gaussian/erasures channel. The analysis is an extension of the Gaussian approximation work of Chung et al. In the two limiting cases of no erasures and large signal-to-noise ratio (SNR), the analysis tends to the results of Chung et al. (see ibid., vol. 47, p.657-670, Feb. 2001) and Luby et al. (1997), respectively, giving a general tool for a class of mixed channels. We derive a steady-state equation which gives a graphical interpretation of decoder convergence. This allows one to estimate the maximum erasure capability on the mixture channel, or conversely, to estimate the additional signal power required to compensate for the loss due to erasures. We see that a good (capacity-approaching) LDPCC over an AWGN channel is also good over the mixed channel up to a moderate erasure probability. We also investigate practical issues such as the maximum number of iterations of message-passing decoders, the coded block length, and types of erasure patterns (random/block erasures). Finally, we design an optimized LDPCC for the mixed channel, which shows better performance if the erasure probability is larger than a certain value (0.1 in our simulation) at the expense of performance degradation at unerased (AWGN channel) and lower erasure probability regions (less than 0.1 in our simulation).