Won-Gi Jeon

OFDMA uplink performance for interactive wireless broadcasting

A combination of orthogonal frequency division multiplexing (OFDM) with frequency-division multiple access (FDMA) called OFDMA is regarded as a promising solution for improving the performance of interactive wireless broadcasting systems. This work focuses on the performance of OFDM when used as a modulation and access technique for the uplink of an interactive wireless broadcasting system. In such an application, we present our investigations of the effects of symbol timing misalignments and peak-to-average power ratio (PAR). Under the condition of the same side information bits, the selective mapping (SLM) approach can be implemented by decimating OFDM samples, which is less complex compared to the ordinary SLM incurring a slight degradation of the PAR performance. Taking into account the effects of clipping operation and symbol timing misalignment, the bit error rate (BER) and throughput performances are investigated for a typical OFDMA uplink scenario.

An Enhanced DFT-Based Channel Estimation Using Virtual Interpolation with Guard Bands Prediction for OFDM

In this paper, we investigate the conventional windowed DFT-based channel estimation for OFDM systems with guard bands. The conventional method suffers from the edge effect and the pilot position mismatch. In order to solve the problems, we propose an enhanced DFT-based channel estimation using virtual interpolation with guard bands prediction, where linear minimum mean square error (LMMSE) smoothing/prediction and circular frequency shifting are employed. This technique can efficiently mitigate the edge effect caused by the DFT of the truncated channel impulse response and compensate the pilot position mismatch that pilot symbols are not allocated at the first and the last subcarriers in a useful band. Simulation results show that the proposed method has about 5 dB SNR gain at BER=10-3 compared to the conventional method and is as good as the frequency-domain LMMSE method

Analysis of Pilot-Aided Channel Estimation with Optimal Leakage Suppression for OFDM Systems

This letter analyzes the effect of the leakage on the mean square error (MSE) performance of the pilot-aided channel estimator using discrete Fourier transform (DFT)-based interpolation in orthogonal frequency division multiplexing (OFDM) systems with virtual carriers. The optimal linear estimator for leakage suppression is derived to minimize the MSE. Numerical results show that the pilot-aided channel estimator with optimal leakage suppression improves the MSE performance significantly over the conventional one.

DFT-based PSA channel estimation using linear prediction for OFDM systems with virtual carriers

This paper proposes an improved discrete Fourier transform-based and pilot-symbol-aided channel estimation (DFT-based PSA CE) for coherent orthogonal frequency division multiplexing (OFDM) systems. Conventional DFT-based PSA CE suffers from the dispersive distortion of an estimated channel impulse response (CIR) caused by virtual carriers. The proposed method estimates an equal-spaced channel frequency response (CFR) in a virtual carrier region by using forward and backward linear prediction (FBLP). Simulation results show that the proposed method outperforms the conventional one with slightly increased complexity and provides a performance similar to the frequency domain linear minimum mean square error (LMMSE) channel estimation with significantly reduced complexity.

Successive Interference Cancellation for STBC-OFDM systems in a fast fading channel

A time-varying multipath channel causes co-channel interference (CCI) as well as inter-carrier interference (ICI) in a space-time block coded orthogonal frequency-division multiplexing (STBC-OFDM) system using two transmit antennas and an conventional simplified decoding scheme (SDS). In this paper, we derive the power of the CCI as a function of normalized channel variation rate. Also, we propose a successive interference cancellation (SIC) technique to improve the performance of the STBC-OFDM by considering the CCI mechanism. The proposed SIC technique can compensate the CCI to some extend with slightly increased complexity compared to the SDS.

Performance evaluation of OFDM-CDMA with multiple antennas for broadband wireless access networks

An orthogonal frequency-division multiplexing (OFDM)-code-division multiple-access (CDMA) system employing various diversity schemes is considered as a possible candidate for broadband wireless access network applications. With an emphasis on a preamble design for multichannel separation, we address a channel estimation based on the time-domain windowing and its imperfectness in the space-time block-coded (STBC) OFDM-CDMA systems, respectively. Also, for a simple construction of the OFDM-CDMA system with the low peak-to-average power ratio (PAR) and reduced transmitter complexity, we investigate a simple PAR reduction scheme and its combination with both decimated selective mapping (SLM) and partial transmit sequence (PTS) methods. Taking into account the false side information introduced by the PAR reduction scheme, the effects of diversity techniques and PAR on the performance of the STBC OFDM-CDMA system are investigated and the maximum achievable diversity gain for a two-path Rayleigh-fading environment is evaluated. From the presented results, we find that the OFDM-CDMA signal with the low PAR is easily constructed by a simple phase transform and its combination version with the SLM and PTS approaches can be implemented to be less complex by decimating the OFDM samples. At the same time, the OFDM-CDMA system applying a space-time-frequency diversity with two and four transmit antennas can give the diversity of D=4 and D=8 for the multiuser case with half user capacity, respectively, even if encountered with the false side information.

Performance evaluation of V2X communications in practical small-scale fading models

This paper investigates the simulation performances of vehicle-to-anything (V2X) communications based on the IEEE 802.11p wireless access in vehicular environments (WAVE) system in practical small-scale fading models. The main contributions of this paper are as follows. First, we modify the experimental fading models reported by Georgia Institute of Technology to be applied to the computer simulation of V2X communications. Second, an enhanced linear minimum mean square error smoothing-aided decision directed (LSA-DD) channel estimator is proposed to track the time variation of fading channels and mitigate noise enhancement.

Timing synchronization for IEEE 802.15.3 WPAN applications

This letter derives and computes the detection probability of timing synchronization in a wireless personal area network (WPAN) system encountered with a multipath Rayleigh fading channel. The timing synchronization scheme using CAZAC training symbols and correlation techniques is adopted. For measuring system performance, the sync detection probability derived in the considered system is evaluated in a multipath Rayleigh fading channel.

Pilot and Data Aided Channel Estimation for OFDM systems in Rapidly Time-Varying Channels

In this paper, we propose an efficient estimation technique for time-varying multipath channels by exploiting two dimensional delay-time nature in pilot inserted orthogonal frequency division multiplexing (OFDM) systems. The proposed technique consists of multipath snapshot estimation at periodic channel observation instances over an OFDM block and the linear interpolation of two consecutive multipath snapshots. Two decision-directed data utilizing schemes, data removed and data added correlation schemes, are proposed to enhance the performance of the multipath snapshot estimation.

A Time-Domain ICI Canceller Using Modulation Order Increasing and Repetition Coding in OFDM

In this paper, an enhanced time-domain cancellation method is proposed for inter-carrier interference (ICI) reduction in OFDM systems. The conventional time-domain cancellation does not work well in deep fades. In order to supplement the conventional method, the simple repetition (de-)coding and the modulation order increasing are employed in the time-domain cancellation. The approximated repetition decoding provides reliable symbols in the regeneration operation, and the increased modulation order maintains the spectral efficiency. Simulation results indicate that the proposed method using 16QAM significantly improves the BER performance compared to the conventional method using QPSK, while maintains the spectral efficiency. Moreover, the proposed method using 64QAM concurrently improves both the BER performance and the spectral efficiency.