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.

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.

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.

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.

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.

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.

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.

Passive RFID tags share a common channel and they use back-scattering modulation because of no self battery. It causes reader collision problems when signal from a reader interferes with signal from other readers. The evolving standards use CSMA based protocol called 'Listen Before Talk (LBT)' and FDMA based protocol called 'Frequency Hopping Spread Spectrum (FHSS)' to reduce reader collision problems. Moreover, these standards are not considered with real industry environment that a stationary reader and a mobility reader coexist with different operation methods and purposes in the same place. Therefore, we describe novel protocols to reduce reader collision problems by RSSI signal. They are mixed protocols between CSMA and TDMA based between a stationary reader and a mobility reader. Simulation results show that the proposed protocols improve RFID system performances.

In recent years, cognitive radio (CR) has received a great attention due to the ability to improve the spectrum utilization. It allows the secondary user (SU) in the CR network to opportunistically access the licensed spectrum of the primary user (PU). For this end, quality of service (QoS) requirements for both the PU and the SU should be guaranteed at the same time. These QoSs can be respectively translated into the interference temperature at the primary receiver and the received signal- to-interference-plus-noise-ratio (SINR) of the secondary receiver. In such a CR network, a power control can increase the energy efficiency by keeping the transmission power of the SU as low as possible within the QoS requirements. In this paper, we propose an autonomous distributed power control scheme for CR networks that considers the QoS requirements of the PU and the SU simultaneously. Since the transmission power of each SU is constrained so that the interference temperature at the primary receiver caused by all SUs does not exceed the interference tolerance of the PU, the QoS requirement for the PU is always guaranteed. Through the simulation results, we demonstrate that the proposed scheme never exceed the interference tolerance of the PU.

RFID is a generic term for technologies which use RF waves to identify, track, or categorize any object. One of the research areas in RFID systems is a tag anti-collision protocol; how to reduce identification time with a given number of tags in the field of an RFID reader. There are two types of tag anti-collision protocols for RFID systems: tree based algorithms and slotted aloha based algorithms. Since the tree based tag anti-collision protocols achieve 100% read rate, we consider how to improve the performances of the tree based RFID tag anti- collision protocols. This paper proposes bi-slotted tree based tag anti-collision protocols, bi-slotted query tree algorithm (BSQTA) and bi-slotted collision tracking tree algorithm (BSCTTA), which reduce both prefix overhead and iteration overhead by time divided responses depending on whether the collided bit is '0' or '1'. According to the simulation results, the bi-slotted tree based RFID tag anti-collision protocols require less time consumption for tag identification than the present tag anti-collision protocols.

The closed-loop multiple-input multiple-output (MIMO) systems can provide considerably improved system performances by taking advantage of channel information at the transmitter. In this paper, we propose an efficient transmit power allocation utilizing a fraction of channel feedback for the sorted QR decomposition (SQRD) based vertical Bell labs layered space-time (V-BLAST) systems. With the proposed transmit power allocation, the bit error rate (BER) performance can be improved significantly compare to the conventional open-loop SQRD based V-BLAST systems. Furthermore, the feedback overhead of the system can be reduced efficiently by employing the feedback reduction technique. Simulation results with some numerical analysis demonstrate the validity of the proposed algorithm.

In this paper, we propose an efficient transmit strategy utilizing the partial channel information feedback for the QRD based V-BLAST systems. In the proposed scheme, after QR decomposition (QRD) of the channel matrix, positive real-valued diagonal elements of R are forwarded to the transmitter through the feedback channel. Based on the feedback information, the transmitter allocates the optimum transmit power to minimize the bit error rate (BER) of the system. With small feedback overhead, the proposed scheme shows significantly improved BER performance compare to the conventional sorted QR decomposition (SQRD) based V-BLAST systems. Moreover, we can reduce the receiver complexity by removing the unnecessary sorting process at the receiver.

In this paper, we propose a novel detection algorithm using log-likelihood ratio (LLR) to detect signals for V-BLAST systems. This algorithm utilizes the sorting QR decomposition (SQRD) of the channel matrix, and applies LLR to determine the order of detection. Simulation results show that the proposed LLR-based MMSE-SQRD algorithm provides a better performance than the conventional SNR-based MMSE-SQRD algorithm with a few additional computations. Approximately, the average BER performance of our algorithm is better than that of the conventional SNR-based MMSE-SQRD algorithm by 4 dB at 10-4target BER.

As the wireless communication services grow quickly, the seriousness of spectrum scarcity has been on the rise gradually. In this context, an emerging technology, cognitive radio (CR) has been come out to solve today's spectrum scarcity problem. Among its fundamental functions, the most important function is spectrum sensing which requires precise accuracy and low complexity. Thus, in this paper, we propose the fast spectrum sensing algorithm using the discrete wavelet packet transform (DWPT) and infinite impulse response (IIR) polyphase filtering schemes. By simulations and complexity analysis, we verify our algorithm.

MIMO-OFDM system is one of the promising schemes for achieving high data rate in communication systems. In this paper, channel estimation techniques based on loosely synchronous (LS) codes are proposed for MIMO-OFDM systems. Since LS codes have perfect auto-correlation and cross-correlation functions within certain vicinity of the zero shifts, multiple antenna and multipath signal interference can be reduced substantially. Through simulations, it is shown that the proposed methods outperform both of least square and linear minimum mean square error (LMMSE) estimators.