Youngok Kim

Orthonormal pulses for high data rate communications in indoor UWB systems

In this letter, we propose novel pulses which not only meet the power spectral mask of the Federal Communications Commission (FCC) for indoor Ultra-wideband (UWB) systems but also preserve orthogonality at the correlation receiver. The proposed pulses are derived from a parametric closed-form solution. Thus, multiple orthonormal pulses that comply with the FCC mask without additional frequency shifting or bandpass filters can be generated for high data rate communications or multiple access schemes. Simulation results demonstrate that high data rate communication is achieved by the proposed orthonormal pulses via pulse shape modulation and pulse position modulation.

A 2-D Subcarrier Allocation Scheme for Capacity Enhancement in a Clustered OFDM System

An adaptive subcarrier allocation (SA) algorithm is proposed for both the enhancement of system capacity and the practical implementation in a clustered OFDM system. The proposed algorithm is based on the two dimensional comparison of the channel gain in both rows and columns of the channel matrix to achieve higher system capacity. Simulation results demonstrate that the proposed algorithm outperforms the SA algorithm based on only one dimensional comparison in terms of system capacity, and furthermore, it performs as well as the optimal SA algorithm at relatively low computational cost.

A novel orthogonal space-time-frequency block code for OFDM systems

In this letter, we propose an orthogonal spacetime- frequency block code for orthogonal frequency division multiplexing (STFBC-OFDM) systems, newly designed to be robust against both time- and frequency-selective fading. The performance of the proposed system is analyzed in spatially uncorrelated time-varying multipath Rayleigh-fading channels. The analytic results, which closely match the numerical results, show that the proposed system outperforms STBC-OFDM in time-varying channels and SFBC-OFDM in frequency-selective fading channels.

A Weighted OFDM Signal Scheme for Peak-to-Average Power Ratio Reduction of OFDM Signals

In this paper, a peak-to-average-ratio (PAPR) reduction scheme based on a weighted orthogonal frequency-division multiplexing (OFDM) signal is proposed to reduce the PAPR without distortion in removing the weight at the receiver side. In the proposed scheme, a weight is imposed on each discrete OFDM signal via a certain kind of a bandlimited signal, and an OFDM signal formed with the weighted discrete data is then considered before a high power amplifier (HPA), whereas the original signal can be recovered completely at the receiver side. Meanwhile, the time duration needed to transmit the weighted OFDM signal is the same as the time duration for the original OFDM signal. The effectiveness of the proposed scheme is evaluated with computer simulations. According to numerical results, the PAPR of the weighted OFDM signal is smaller than that of the clipping and filtering (C&F) method, and the bit-error-rate (BER) performance of the weighted OFDM system is improved compared with the C&F method. Here, the proposed method is simpler than the C&F method.

A novel location finding system for 3GPP LTE

We propose a novel location finding system exploiting the downlink reference signal of the Long Term Evolution (LTE) because of the absence of an efficient location finding system for the LTE. The proposed system is based on the correlation method and Chans method for location sensing and positioning process, respectively. The conventional correlation method, however, is not matched to the LTE since the orthogonal gold sequence used for reference signal is assigned in frequency domain. Therefore, we estimate Time Difference Of Arrival (TDOA) data by cross-correlating the transmitted signal and received signal in the frequency domain. Simulation results show that the proposed system meets the Federal Communication Commission (FCC) requirement for typical Signal-to-Noise Ratio (SNR) region, and the performance can be further improved as more antennas are implemented in transmitter or receiver.

An Adaptive Clipping and Filtering Technique for PAPR Reduction of OFDM Signals

In recent years, many peak-to-average power ratio (PAPR) reduction techniques have been proposed for orthogonal frequency division multiplexing (OFDM) signals. Among various techniques, the iterative clipping and filtering (ICAF) technique has been considered as a practical scheme, and widely used owing to its non-expansion of bandwidth, low computational complexity, and simplicity in implementation without receiver-side cooperation. However, the performance of conventional ICAF technique is degraded, because the same signals are iteratively clipped with a fixed clipping threshold (CT) in every clipping operation. In this paper, we analyze the performance of conventional ICAF technique, and then propose an adaptive ICAF scheme, which clips the signal with an adaptively modified CT in every clipping operation to achieve enhanced PAPR reduction of OFDM signals. Simulation results show that the proposed scheme significantly outperforms the conventional scheme, in PAPR reduction of OFDM signals at the same number of iterations.

Performance Analysis of the Clipping Scheme with SLM Technique for PAPR Reduction of OFDM Signals in Fading Channels

Since one of major problems of OFDM-based systems is high peak-to-average-power ratio (PAPR) of its transmitted signal, many PAPR reduction techniques and combined schemes with individual techniques have recently been developed. Among various techniques, the clipping technique has been widely used as a practical scheme owing to its low computational complexity and simplicity in implementation, while the selected mapping (SLM) technique is known to provide good PAPR reduction performance without signal distortion. Generally, the combined scheme of two PAPR reduction techniques, which are the clipping and the SLM, is expected to provide the enhanced performance of PAPR reduction, because the clipping noise of combined scheme would be less than that of single clipping technique, when the SLM technique is employed before clipping. However, the performance of clipping scheme with SLM technique has not been evaluated for practical systems over fading channels. In this paper, the performance of the clipping scheme with the SLM technique is theoretically analyzed and compared with simulation results over fading channels. The performance of combined scheme is analyzed with various clipping ratios, phase sets for SLM, and modulation schemes over flat and frequency selective fading channels. In addition, the effects of the clipping at the receiver and the oversampling on the BER performance are discussed. Based on the results of analysis, therefore, one can design the effective clipping scheme with the SLM technique for the PAPR reduction of OFDM-based systems.

An adaptive grouped-subcarrier allocation algorithm using comparative superiority

In this paper, an adaptive grouped-subcarrier allocation algorithm using comparative superiority is proposed for the enhancement of system capacity and its simple implementation in a multiuser OFDM system. Since the computational complexity of the optimal subcarrier allocation algorithm based on the each subcarrier is extremely high, various sub-optimal algorithms have been developed for the realization of adaptive subcarrier allocation algorithm with the reduced complexity. Some of the promising sub-optimal algorithms are the blockwise and the decentralized allocation algorithms, which allocate subcarriers in groups instead of each subcarrier. The proposed algorithm is similar to the blockwise or the decentralized subcarrier allocation algorithm but all subcarriers are grouped according to the coherence bandwidth for the enhancement of system capacity. In addition, the proposed algorithm provides a simple solution for the conflict problem among users by reallocating only the conflicted groups and unassigned groups instead of reallocating entire groups. Furthermore, the comparative superiority concept, which swaps the groups between users if the system capacity is increased, is adopted in the re-allocation process for the enhancement of system performance. Simulation results demonstrate that the proposed algorithm increases the system capacity effectively over a static, an adaptive blockwise, and a decentralized subcarrier allocation algorithms

Orthogonal Pulse Design in Consideration of FCC and IEEE 802.15.4a Constraints

In Ultra-wideband (UWB) system mainly coexisting with other narrow band systems, a judicious pulse shape design is necessary, under the IEEE 802.15.4a standard and the Federal Communications Commission (FCC) spectral mask constraints. In this letter, a strategy to generate multiple orthogonal pulses while complying with both constraints is presented. The performance analysis is also presented with mathematical derivations and numerical simulations to show that the proposed pulses can be utilized for higher data rate or lower error rate.

A low-complexity single-carrier frequency-division multiple access transmitter

A framework for a computationally efficient single-carrier frequency-division multiple access (SC-FDMA) transmitter is proposed in this paper. Compared with a wide system bandwidth, the uplink allocation for each user is supposed to be relatively small because of multiple user access, which makes each users signal vector to be sparse. When the localized subcarrier allocation is used for SC-FDMA, the inverse fast fourier transform can take advantage of the sparse user input vector to reduce its complexity. The analytical and simulation results show that the proposed framework can provide a significant complexity reduction compared with the conventional SC-FDMA transmitter. Copyright