In this paper, we introduce the new concept of temporal diversity utilization based on asymmetric transmission to minimize network interference in wireless ad-hoc networks with a two-hop half-duplex relaying (HDR) protocol. Asymmetric transmission is an interference-aware backoff technique, in which each communication session (source-relay-destination link) adaptively chooses a certain subset of spectrally-orthogonal data streaming which should be delayed by the duration of one time-slot (i.e., half of one subframe). We design the problem in the HDR scenario by applying the concept of asymmetric transmission, and evaluate the game-theoretical algorithm, called asymmetric transmission game (ATG), to derive the suboptimal solution. We show that ATG is an exact potential game, and derive its convergence and optimality properties. Furthermore, we develop an approximated version of ATG (termed A-ATG) in order to reduce signaling and computational complexity. Numerical results verify that two algorithms proposed show significant synergistic effects when collaborating with the conventional methods in terms of interference coordination. Ultimately, the energy consumption to satisfy the rate requirement is reduced by up to 17.4% compared to the conventional schemes alone.

In this paper, we propose a received signal interpolation method for enhancing the performance of multiple signal classification (MUSIC) algorithm. In general, the performance of the conventional MUSIC algorithm is very sensitive to signal-to-noise ratio (SNR) of the received signal. When array elements receive the signals with nonuniform SNR values, the resolution performance is degraded compared to elements receiving the signals with uniform SNR values. Hence, we propose a signal calibration technique for improving the resolution of the algorithm. First, based on original signals, rough direction of arrival (DOA) estimation is conducted. In this stage, using frequency-domain received signals, SNR values of each antenna element in the array are estimated. Then, a deteriorated element that has a relatively lower SNR value than those of the other elements is selected by our proposed scheme. Next, the received signal of the selected element is spatially interpolated based on the signals received from the neighboring elements and the DOA information extracted from the rough estimation. Finally, fine DOA estimation is performed again with the calibrated signal. Simulation results show that the angular resolution of the proposed method is better than that of the conventional MUSIC algorithm. Also, we apply the proposed scheme to actual data measured in the testing ground, and it gives us more enhanced DOA estimation result.

In cellular network environments, where users are not evenly distributed across cells, overloaded base stations handling many users have difficulties in providing effective and fair services with their limited resources. Additionally, users at the cell edge may suffer from the potential problems resulting from low signal-to-interference ratio owing to the incessant interference from adjacent cells. In this paper, we propose a relay-assisted load balancing scheme to resolve these traffic imbalance. The proposed scheme can improve the performance of the overall network by utilizing relay stations to divert heavy traffic to other cells, and by adopting a partial frequency-reuse scheme to mitigate inter-cell interference. Each user and relay station calculates its own utility influence in the neighboring candidates for reassociation and decides whether to stay or move to another cell presenting the maximum total network utility increment. Simulation results show that the proposed scheme improves the overall network fairness to users by improving the performance of cell boundary users without degrading the total network throughput. We achieve a system performance gain of 16 ∼ 35% when compared with conventional schemes, while ensuring fairness among users.

A carrier frequency synchronization scheme is proposed for orthogonal frequency-division multiplexing (OFDM) systems in the presence of phase noise (PHN). In the proposed scheme, carrier frequency synchronization is performed based on the maximum-likelihood (ML) algorithm using an OFDM preamble symbol. The proposed scheme is compared with conventional methods. Simulation results are presented to illustrate the effectiveness of the proposed scheme in the presence of PHN.

In this paper, we propose an efficient clutter suppression algorithm for automotive radar systems in iron-tunnel environments. In general, the clutters in iron tunnels makes it highly likely that automotive radar systems will fail to detect targets. In order to overcome this drawback, we first analyze the cepstral characteristic of the iron tunnel clutter to determine the periodic properties of the clutters in the frequency domain. Based on this observation, we suggest for removing the periodic components induced by the clutters in iron tunnels in the cepstral domain by using the cepstrum editing process. To verify the clutter suppression of the proposed method experimentally, we performed measurements by using 77GHz frequency modulated continuous waveform radar sensors for an adaptive cruise control (ACC) system. Experimental results show that the proposed method is effective to suppress the clutters in iron-tunnel environments in the sense that it improves the early target detection performance for ACC significantly.

With extensive use of automotive radars, mutual interference between radars has become a crucial issue, since it increases the noise floor in the frequency domain triggering frequent false alarms and unsafe decision. This paper introduces a mathematical model for a frequency-modulated continuous-wave (FMCW) radar in interfering environments. In addition, this paper proposes a time-domain interference suppression method to provide anti-interference capability regardless of the signal-to-interference ratio. Numerical results are presented to verify the performance of a 77GHz FMCW radar systme with the proposed method in interference-rich environments.

In this letter, a precoding design for a multiple-input multiple-output (MIMO) full-duplex relay (FDR) system is proposed. For this system, mitigating the self-interference imposed by the transmit antennas on the receive antennas in the same relay station is crucial for improving the performance of the FDR system. The precoding scheme designed in this study uses block-diagonalization (BD). Using this precoding scheme, FDR capacity analysis is performed in the MIMO downlink relay system. Numerical results on system performance in terms of capacity are shown and discussed.

This paper presents a novel technique to compensate intermodulation distortion of a self-heterodyne direct conversion OFDM system in multipath propagation environments. A self-heterodyne direct conversion system has an advantage that simple receivers can be built that are completely immune to any phase noise or frequency offset. This system, however, has a disadvantage that the nonlinear square-law detector at the receiver of the self-heterodyne direct conversion system gives rise to second order intermodulation distortion. In this study, channel estimation is performed using a training sequence and then the predistortion coefficients with regard to estimated channel parameters are derived to compensate the receiver nonlinearity. Transmit power distribution is employed to overcome multipath fading channels as well. Computer simulation demonstrates that the proposed approach improves the BER performance of the self-heterodyne direct conversion OFDM system in a multipath fading channel. This scheme gives advantage to multi-carrier systems that are much more sensitive to frequency and phase error than single-carrier systems.

This letter corrects some errors on a previous letter concerning the derivation of the covariance matrix of phase noise. This derivation doesn't affect the results of the previous letter.

This letter presents the variation of localization error to network parameters, the number of range estimation results from anchor nodes (ANs) and average distance between ANs in centralized Wireless Sensor Network (WSN). In sensor network, ANs estimate the relative range to Target Node (TN) using Time-Of-Arrival (TOA) information of Ultra WideBand (UWB) radio and a fusion center determines the final localization of TN based on estimation results reported. From simulation results, the variation of localization error, which is defined as the difference between localization result of TN and its actual location, is represented as the function of number of estimation results to average distance between ANs. The distribution of localization error is matched to the Rician distribution whose K-factor value is given by the proposed formula as well. Finally, the normalized error function for the efficient localization network design is characterized.

In this paper, we propose an analog cancellation scheme for multipath self-interference channels in full-duplex wireless orthogonal frequency-division multiplexing (OFDM) systems. The conventional approaches emulate the radio-frequency (RF) self-interference signals by passing the RF transmit signals through delay lines and programmable attenuators. By contrast, our proposed scheme computes the phase-rotated and weighted versions of the baseband transmit signals in the baseband domain, which are simply upconverted to obtain the emulated RF self-interference signals. Numerical results are presented to verify the suppression performance of the proposed scheme.

This paper deals with the small-scale fading distribution for UWB channels in the absence and presence of human bodies in indoor line-of-sight (LOS) environments and performance analysis of UWB systems considering the small-scale fading distribution. To obtain small-scale fading statistics, the channel measurements are performed in five representative environments that have different structure and size while locating the receiver (Rx) antenna on 49 (7×7 grid) local points with a fixed transmitter (Tx) antenna in each environment. The measured channel data are processed by a vector network analyzer and the target frequency bands range from 3 to 4.6GHz. From the measured data, we find the best fitted channel model among several typical theoretical distribution models such as Lognormal, Nakagami, and Weibull distributions, showing good agreement with the empirical channel data. We analyze the amplitude variation of the small-scale fading distribution in the absence and presence of human bodies. The results show that the small-scale fading statistics are best described by Weibull distribution and the two parameters of the distribution that determine the shape and the scale of the distribution depend on whether or not human bodies exist. We modeled and analyzed two parameters at different excess delays for all environments. Based on the measured small-scale fading distribution, this paper deals with the performance of UWB system using Rake receivers and also compares the performance with the existing channel model. The results suggest that the small-scale fading distribution in the absence and the presence of human bodies in indoor LOS environments should be considered when assessing the performance of UWB systems.

In this paper, the virtual antenna technique is applied to a single input multiple output (SIMO) radar system to enhance the performance of the conventional beamforming direction of arrival (DOA) estimation method. Combining the virtual array generated by the interpolated array technique and the real array, the angular resolution of the DOA estimation algorithm is improved owing to the extended number of antennas and aperture size. Based on the proposed interpolation technique, we transform the position of the antenna elements in a uniform linear array (ULA) to the arbitrary positions to suppress the grating lobe and side lobe levels. In simulations, the pseudo spectrum of the Bartlett algorithm and the root mean square error (RMSE) of the DOA estimation with the signal-to-noise ratio (SNR) are analyzed for the real array and the proposed virtually extended array. Simulation results show that the angular resolution of the proposed array is better than that of the real array using the same aperture size of array and the number of antennas. The proposed technique is verified with the practical data from commercialized radar system.

Modification of ITU-R P.1411 model to enhance the prediction accuracy in urban environments having variable heights of buildings is proposed in this paper by introducing two kinds of novel correction factors. One is considering the relationship of the highest building height and the transmitter (Tx) antenna height, and the other is considering the effect of receiver (Rx) position on crossroads. After introducing two correction factors, the prediction accuracy is shown to be improved.

In this letter, the cumulative distribution function (CDF) for the maxima of the OSTBC-MIMO channel capacity in a temporal interval is estimated using the first-order Rice series approximation. As the estimation of the maxima distribution using the Rice series is applicable only to Gaussian random processes, the Gaussian-approximated probability density function (PDF) for the OSTBC-MIMO channel capacity is derived from existing exact PDF (non-Gaussian). The resulting CDF for the maxima capacity is useful to design OSTBC-MIMO systems.

In this letter, iterative sequence estimation technique based on expectation-maximization (EM) algorithm is considered for quadrature amplitude modulation (QAM)-orthogonal frequency division multiplexing (OFDM) signals. For QAM-OFDM signaling, the optimal EM algorithm requires high computational complexity due to the inversion of complex matrix executed at each iteration. To avoid this problem, we propose a sub-optimal iterative sequence estimation algorithm with some approximations, which results in reduced computational complexity for QAM-OFDM signals. Moreover, we use two different approaches to obtain initial estimate for beginning iteration of proposed algorithm. One is for less time-dispersive but fast fading channel and the other is for highly time-dispersive but relatively slow fading channel. The bit error rate (BER) performances of the proposed algorithm are evaluated using computer simulations. The results show that the proposed algorithm performs nearly as well as the optimal EM algorithm.

Phase noise (PHN) can cause the common phase error (CPE) and the inter-carrier interference (ICI), both of which impair the accurate channel estimation in orthogonal frequency division multiplexing (OFDM) systems. In this letter, we build a new signal model parameterized by the channel impulse response, the CPE and the ICI. Based on this model, we derive the maximum likelihood estimator (MLE) and the minimum mean square error estimator (MMSEE). Simulation results show that the proposed schemes significantly improve the performance of OFDM systems in the presence of PHN.

This letter proposes an Ultra WideBand (UWB) channel gain estimation algorithm that is based on frequency response correlation. From data measured in indoor and outdoor environments, the frequency correlation statistics of the UWB channel are characterized by correlation coefficients, which are represented as functions of the frequency separation according to the propagation environments. We use empirically established correlation coefficient models to develop the proposed channel gain estimation algorithm. In this algorithm, unknown channel gains of intermediate frequencies are estimated from known channel gains of neighboring reference frequencies. The performance of the proposed algorithm is evaluated against various configurations of reference frequencies and compared with that of a conventional estimation algorithm using a linear interpolation scheme.

In orthogonal frequency-division multiplexing (OFDM)-based wireless local area networks (WLANs), phase noise (PHN) and residual frequency offset (RFO) can cause the common phase error (CPE) and the inter-carrier interferences (ICI), which seriously degrade the performance of systems. In this letter, we propose a combined pilot symbol assisted and decision-directed channel estimation scheme based on the least-squares (LS) and the maximum-likelihood (ML) algorithms. Simulation results present that the proposed scheme significantly improves the performance of OFDM-based WLANs.

This paper presents an interference suppression and signal restoration technique that can create the clean signals required by automotive frequency-modulated continuous wave radar systems. When a radar signal from another radar system interferes with own transmitted radar signal, the target detection performance is degraded. This is because the beat frequency corresponding to the target cannot be estimated owing to the increase in the noise floor. In this case, advanced weighted-envelope normalization or wavelet denoising can be used to mitigate the effect of the interference; however, these methods can also lead to the loss of the desired signal containing the range and velocity information of the target. Therefore, we propose a method based on an autoregressive model to restore a signal damaged by mutual interference. The method uses signals that are not influenced by the interference to restore the signal. In experiments conducted using two different automotive radar systems, our proposed method is demonstrated to effectively suppress the interference and restore the desired signal. As a result, the noise floor resulting from the mutual interference was lowered and the beat frequency corresponding to the desired target was accurately estimated.