Sangho Nam

Joint Doppler-Frequency Diversity for OFDM Systems Using Hybrid Interference Cancellation in Time-Varying Multipath Fading Channels

In time-varying fading channels, orthogonal frequency division multiplexing systems suffer severe performance degradation caused by interchannel interference (ICI). We utilize this ICI as a source of diversity by using a new ICI-cancellation algorithm and Wiener filter. The proposed algorithm can support joint Doppler-frequency diversity gain. In addition, both the computational complexity and total detection time are reduced.

A modified covariance-based mobile velocity estimation method for Rician fading channels

This paper contains our proposal for a novel velocity estimation method based on the auto-covariance of the received signal and that of the squared envelope. This modified covariance-based (MCOV) estimator exhibits unbiased characteristics in a Rician fading environment. Moreover, it requires no other channel information, such as Rician factors or the angle of arrival (AoA). The authors also propose a scaling technique which can improve the performance of the MCOV method in a Rayleigh fading environment. When coupled with the scaling technique, the new method demonstrates robust performance in both the Rayleigh and Rician fading environments.

Time-Varying Channel Estimation in OFDM Systems with Null Subcarriers

In this paper, we propose a robust channel estimation method in OFDM systems over fast fading channels. In consideration of the effect of null subcarriers, the proposed channel estimation is first performed in the frequency domain. By making efficient use of channel correlation and the property of ICI to reduce the number of needed parameters for channel estimation, robust channel estimation for ICI is accomplished in parallel, pilot cluster by pilot cluster. It is then transformed into the time domain to obtain a gain in noise reduction and to reduce interpolation error as well. Simulation results show that the proposed channel estimation has good performance compared to previous works.

Channel Estimation Based on Comb-Type Pilot Structure in OFDM System with Null Subcarriers

This paper analyzes the degradation effect of null subcarriers in orthogonal frequency division multiplexing (OFDM) systems on the time-domain maximum likelihood (ML) estimation performance. The analysis is used as the basis for a proposal for a new channel estimation method that can overcome performance degradation caused by null subcarriers. The accuracy of the proposed channel estimation is guaranteed by using an interpolation of the estimated channel in the frequency domain that comes before the channel estimation in the time domain.

Robust velocity estimation for non-isotropic and AWGN channel

The maximum Doppler frequency, or equivalently, the mobile speed is very useful information to improve the performance of future wireless communication systems. In practical systems, however, the accuracy of the maximum Doppler frequency estimation is decreased by additive noise. Moreover, the information of signal-to-noise ratio (SNR) is not easily obtainable. Previously, several methods have been proposed to reduce the distortion due to the additive noise. However, these methods are not suitable for the non-isotropic fading channels and cause the distortion of the estimation values. In this paper, we propose the SNR-independent velocity estimation method for the nonisotropic channel which uses the covariance values of received signal. The proposed method provides accurate estimatation values without SNR information. In addition, it shows robust performance in the non-isotropic fading channels.

Efficient Preamble Structures for MIMO OFDM-Based WLAN Systems

In this paper, we propose a new preamble structure for channel estimation in a MIMO OFDM-based WLAN system. Both backward compatibility with IEEE 802.11 a and low overhead are considered for the preamble design. Simulation results show that the proposed preamble has low overhead and good performance gain for channel estimation.

Adaptive MCS level selection scheme for HSDPA in W-CDMA system

In this paper, an effective method is proposed to compensate effect of Doppler spread on High Speed Downlink Packet Access (HSDPA). First, the effect of Doppler spread on channel estimation method is described. Next, the approximate function for estimating threshold shifts is proposed. Simulation results show that the proposed method can effectively estimate the threshold shifts caused by Doppler spread.

A New Multistage Whitener-Based Detector for MIMO Systems

We propose a new multistage whitener-based detector (MWD) to guarantee low complexity and high performance for multiple-input multiple-output systems. The proposed MWD reduces complexity by detecting two out of NT transmitted symbols in each stage of the whitening process. Furthermore, it can enhance performance by adopting a norm-based selection criterion that considers the instantaneous channel state and interference cancellation using Ci local solutions at the i-th stage of the whitening process. Accordingly, the proposed MWD can achieve the optimum maximum likelihood detector (MLD) while having a considerably reduced computational complexity compared with the optimal MLD.

Channel Estimation with a New Preamble Structure for a MIMO OFDM-Based WLAN System

In this letter, we propose a new preamble structure for channel estimation in a MIMO OFDM-based WLAN system. Both backward compatibility with IEEE 802.11a and low overhead are considered in designing the preamble. Simulation results show that the proposed preamble has low overhead and good performance gain for channel estimation.

Channel Estimation in Comb-Type Pilot Arrangements for OFDM Systems with Null Subcarriers

This letter analyzes the degradation effect of null subcarriers in orthogonal frequency domain multiplexing (OFDM) systems on the time-domain maximum likelihood (ML) estimation performance. The analysis is used as the basis for a proposal for a channel estimation method that can overcome performance degradation caused by null subcarriers. The accuracy of the proposed method is confirmed by the numerical analysis.