We propose an autocorrelation model of SF (shadow fading) in LOS (line-of-sight) rural macro environments. The proposed model is based on the empirical autocorrelations obtained from wideband MIMO (multiple input multiple output) channel measurements at 2.38 and 3.705 GHz. It was found that the SF depends on frequency and so do the empirical autocorrelations. The proposed model provides good matches to empirical autocorrelations in the individual measurement routes.

This paper considers a dynamic spectrum allocation (DSA) for efficient distribution of the spectrum resource by allocating the spectrum to be responsive to demand variations. In this DSA model, a spectrum broker (SB) is responsible for the allocation of the spectrum with the regional license inside the region of responsibility and the coordination of the wireless interference inside the region for quality of service (QoS) provisioning over the allocated spectrum. In this paper, we address the interference constrained DSA problem and propose an optimal DSA scheme by applying the binary integer programming (BIP) approach to the DSA. Within the interference graph based interference constraint, the proposed scheme obtains a optimal set of binary decisions on whether to allocate or not allocate the channels to the access points within the region in accordance with the objective (i.e., maximization of the revenue of the SB, maximization of the spectrum utilization inside the region, and multi-objective optimization compromising the revenue and spectrum utilization maximization). Simulation results demonstrate that the proposed scheme efficiently distributes the spectrum resource in accordance with the corresponding objective and the multi-objective optimization obtains the compromise solution depending on the preference between the revenue and spectrum utilization.

Recently, as a variety of cellular systems are coexisted and the use of mobiles in a confined space increases, interference problems between terminals become more important. In this paper, we derive the closed-form probability density function (PDF) of the downlink signal-to-interference ratio (SIR) due to uplink transmission of the adjacent terminal station of the other system for the coexistence environment of two cellular systems. And it is shown that the formula of the PDF is exactly the same with Monte-Carlo simulation. This probability distribution model of the SIR is useful in that it is very simple and can be applicable to general propagation models including shadow fading. With this model, interference analysis for coexisting LTE systems in 900 MHz band of South Korea is suggested.

Based on the non line-of-sight (NLOS) indoor measurements performed in the corridors on the third floor of the Electronics and Telecommunications Research Institute (ETRI) building in Daejeon city, Republic of Korea, we investigated the distribution of the eigenvalues of HH*, where H denotes a 2×2 multiple-input multiple-output (MIMO) channel matrix. Using the observation that the distribution of the measured eigenvalues matches well with the Gamma distribution, we propose a model of eigenvalues as Gamma distributed random variables that prominently feature both transmitting and receiving correlations. Using the model with positive integer k_i, i=1, 2, which is the shape parameter of a Gamma distribution, we derive the closed-form ergodic capacity of the 2×2 MIMO channel. Validation results show that the proposed model enables the evaluation of the outage and ergodic capacities of the correlated 2×2 MIMO channel in the NLOS indoor corridor environment.

The performance of MIMO-OFDM beamforming systems with maximal ratio combining receivers is considered based on the extensive wideband 2 × 8 (i.e., 2 transmitting and 8 receiving antennas) MIMO channel measurement data in non line-of-sight urban macrocell environments. Using the experimental observation that the probability density function (PDF) of the measured maximum eigenvalue for H†mHm, where Hmand superscript † denote a MIMO channel frequency response matrix and conjugation and transposition, respectively, matches the Gamma distribution well, we propose the maximum eigenvalue model as Gamma distributed random variables whose parameters are both transmitting and receiving correlations and the number of receiving antennas. Using the model, the performances of MIMO-OFDM beamforming systems, such as the PDF of the output signal-to-noise ratio, outage probability, symbol error rate, and achievable diversity order, are analyzed to demonstrate the effect of spatial correlation in the realistic 2 × n (downlink) and n × 2 (uplink) MIMO channels.

This paper considers a dynamic spectrum allocation (DSA) model that a spectrum broker (SB) coordinates the allocation of the spectrum with the regional license inside the region of responsibility. It has a potential to leverage the spectrum utilization but requires a sophisticated approach to manage the allocation-dependent interference effect. Thus, the SB is responsible to manage the wireless interference between base stations (BSs) within the region for quality of service (QoS) provisioning over the allocated channels. In this paper, we address the interference constrained DSA problem and propose an interference management scheme that collaboratively works with the spectrum allocation algorithm to implement the DSA. By the allocation-aware interference management based on the received signal-to-interference-plus-noise-ratio (SINR), the proposed scheme can reflect the context of the SB's decisions at the allocation process. Simulation results demonstrate that the proposed scheme efficiently distributes the spectrum resource while guaranteeing the QoS requirements of all BSs with allocated channels.

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.

In this paper, we investigate the distribution of eigenvalues of HH†where the H and superscript†denote the MIMO (multiple input multiple output) channel matrix measured in urban microcell and conjugation and transposition. Our channel sounding system is the BECS (band exploration and channel sounding) 2006, which works at a carrier frequency of 3.705 GHz with 100 MHz bandwidth. From the measured data, It is observed that the distribution of eigenvalues of HH†almost fits with Gamma distribution. We derive simple expressions of mean and variance of eigenvalues, which are functions of the absolute values of Tx (transmit) and Rx (receive) antenna correlation coefficients and the number of Rx antennas. Using the expressions of mean and variance, we then model the eigenvalues as Gamma distributed random variables for 2×N (i.e., 2 Tx and N Rx antennas) MIMO channel capacity. Monte Carlo simulations are used to generate eigenvalue realizations according to the model. The results are compared with the measure data and good agreements are found.

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 aims at the eigenvalue analysis and its modeling for 2×2 (i.e., 2 transmit and 2 receive antennas) Multiple Input Multiple Output (MIMO) channel capacity based on Non Line-Of-Sight (NLOS) wideband indoor measurements. Our channel sounding system is the portable Band Exploration and Channel Sounding (pBECS) System, which works at a carrier frequency of 2.58GHz with 20MHz bandwidth. From the measured data, we investigate Transmit Antenna Correlation Coefficient (TxACC), Receive Antenna Correlation Coefficient (RxACC), channel capacity and eigenvalues of HH* where H and (*) are channel frequency response matrix and conjugate transpose. Based on the eigenvalue characteristics, we derive mean and variance of eigenvalues which are a function of the absolute values of TxACC and RxACC, |TxACC| and |RxACC|, and then model the eigenvalues for 2×2 MIMO channel capacity. Monte Carlo simulations are used to generate the eigenvalue realizations according to the model. The results are compared with the measured data and good agreements are found.

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.

This paper aims at modeling of eigenvalues for 2timesN (i.e., 2 transmit and N receive antennas) channel capacity based on non line-of-sight (NLOS) wideband outdoor measurements. Our channel sounding system is the Band Exploration and Channel Sounding System (BECS) 2005, which works at a carrier frequency of 3.7 GHz with 100 MHz bandwidth. From the measured data, we investigate transmit antenna correlation coefficient (TxACC), receive antenna correlation coefficient (RxACC), channel capacity and eigenvalues of HH* where H and (*) are channel frequency response matrix and conjugate transpose. Based on eigenvalue characteristics, we derive mean and variance of eigenvalue which are a function of the absolute value of TxACC, |TxACC|, the absolute value of RxACC, |RxACC|, and the number of Rx antennas, and then model eigenvalues for 2timesN channel capacity. Monte Carlo simulations are used to generate eigenvalue realizations according to the model. The results are compared with the measure data and good agreements are found.

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.

Code division multiplexing (CDM) based MIMO channel sounder architecture is excellent at measuring fast fading channel. In this paper, loosely synchronous (LS), Kasami and chaotic sequences are suggested as candidates for the code that will be used in CDM architecture. Among those sequences, then, the best suitable sequence for channel sounding is examined through simulation. And by proposing the combined scheme of CDM and TDM when the number of transmit antennas are more than 4, the decrease of dynamic range due to interferences between different sequences can be mitigated.

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.

MIMO-OFDM system is a promising way to achieve high data-rate wireless transmission. In this paper, loosely synchronous (LS) codes assisted multipath channel interference canceller (MCIC) is proposed for the channel estimation of MIMO-OFDM systems. Since LS codes have perfect autocorrelation and cross-correlation functions within certain vicinity of the zero shifts, LS codes assisted MCIC can substantially reduce multipath channel interference. Simulation results show that the MCIC estimator outperforms least square estimator and has slightly inferior performance as compared with linear minimum mean square error (LMMSE) estimator. Moreover the MCIC estimator has the advantage of not requiring any information about the channel statistics like auto-correlation of channels.

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.

A sub-optimal but computationally efficient modified Fano detection (MFD) algorithm for DSTTD system is presented. The proposed algorithm utilizes the sequential detection scheme based on tree searching in order to find the optimal symbol sequence. For more reliable signal detection and complexity reduction, the decoder is designed to move backward for the specified value at the end of the tree and to compute the reduced branch metrics. Simulation results show that the performance of MFD is comparable to that of ML detector while reducing the computational effort of ML method significantly.

This paper introduces a structure of delivering traveler information service including point of interest information through the terrestrial multimedia broadcasting system, in which, the receiver is connected with cellular communication terminal to enable the interactivity of the service. By combining real time traffic information and point of interest information, we could get an efficient traffic and traveler information service system that could be used for many location based applications.

In this paper, the code-division multiplexing (CDM) based MIMO channel sounder with loosely synchronous (LS) codes and Kasami codes is presented for real time measurement of MIMO radio channel. Since this scheme has drawback of reduced dynamic range depending on the number of transmit antennas, it is important that low correlation codes should be utilized to get reliable performance. This paper adopts two efficient codes, LS codes and Kasami codes, of which performances are investigated through Monte-Carlo simulation in 2 x 8 and 4 x 8 MIMO channel environment, respectively. Basically, LS codes are noise-limited and Kasami codes are interference-limited in relation with cross-correlation characteristic. According to the simulation, LS codes are the best in 2 x 8 MIMO channel measurement and modified LS codes approach to this performance of LS codes in both 2 x 8 and 4 x 8 MIMO channel measurement. Kasami codes achieve somewhat less performance than the others, but are not nearly affected by the number of transmit antennas. Thus, Kasami codes are effective when transmit antennas are more than 4.

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.

This paper introduces the development of a remultiplexer which multiplexes the MPEG-4 stream into the digital multimedia broadcasting transmission signal. The remultiplexer also has additional channel coding functions which are essential for mobile reception of moving pictures through the digital multimedia broadcasting signal.