Kyungchul Kwak

A note on effect of multiple antennas at the source on outage probability for amplify-and-forward relaying systems

The amplify-and-forward (AF) relaying system considered here consists of a source node with M transmit antennas, and relay and destination nodes with a single antenna each. The closed-form expressions for the outage probabilities for maximum-ratio-transmission (MRT) at the source are presented for some specific values of M. Numerical results show that increasing M from 1 to 2 can achieve roughly a 3 dB SNR gain, guaranteeing the same outage probability. However, we discover that for M ges 3, the additional gain compared to M = 2 decreases quickly as the SNR increases. Therefore, increasing M above 3 does not achieve significantly greater gain over M = 2 at high SNR.This paper analyses the estimation of residual frequency offset in MIMO OFDM systems using pilot subcarriers embedded in the data payload. A closed-form expression for the MSE is derived and it shows how the frequency and spatial correlation influence performance. It is also shown that an orthogonal design ensures robust performance, which is independent of the frequency correlation of the channel.

OFDM Channel Estimation With Jammed Pilot Detector Under Narrow-Band Jamming

This paper presents a channel estimation scheme using a new jammed pilot detection algorithm for orthogonal frequency-division multiplexing (OFDM) systems under narrow-band jamming (NBJ).signal-to-jamming ratio The jammed pilot subcarrier can be detected and excised by using the proposed algorithm after least squares estimation. The average mean square error (mse) on one OFDM symbol both under a jammed and a removed pilot subcarrier are derived analytically. Moreover, the symbol error rate (SER) performance for the channel estimation scheme using the proposed algorithm is evaluated by simulation. From the results, we can show that the channel estimator with the proposed algorithm improves the mse and SER performances at a low signal-to-jamming ratio.

New OFDM Channel Estimation with Dual-ICI Cancellation in Highly Mobile Channel

The number of coefficients for estimating time-varying channel is generally much larger than the maximum number of observable data. Linearly time-varying channel (LTV) models have been widely used as a way of reducing the number of these channel parameters. This approach allows estimation using the channel slope and channel frequency response between symbols. A fatal problem with this approach, however, is that before it is even initiated, the estimated channel in the frequency domain has already been degraded by inter-carrier interference (ICI). We demonstrate this by analyzing the channel estimation mean square error (MSE) and signal to interference and noise ratio (SINR) after ICI cancellation with respect to the LTV model. We then propose a new channel estimation technique using dual-ICI cancellation which resolves this problem by pre-canceling ICI for the channel estimation using the channel of the previous symbol and post-canceling ICI for symbol detection with the more accurate channel information. The process is initialized using the first symbol as a preamble to estimate the channel in the time domain. Performance comparisons of MSE and SINR show that the proposed method is better suited to fast-fading channels than the conventional method, where ICI dominates the interference and noise power.

A New Duplex without Loss of Data Rate and Utilizing Selection Diversity

This paper presents a new duplex approach which does not require guard resources suitable for indoor 2times2 MIMO environments. In the proposed duplex, two virtual channels in the spatial domain are generated by precoding and postcoding MIMO channels, not to use guard resources in either the time or frequency domain. In addition, this system can achieve selection diversity since the generation of virtual channels is not unique for the given channels information. We will show the generation of the virtual channel, and that the capacity and reliability of the communication links improves when the problem of the guard resource is addressed and selection diversity is utilized.

Channel estimation approach with variable pilot density to mitigate interference over time-selective cellular OFDM systems

We propose a new channel estimation technique to mitigate interference with the aid of pilot density discrepancy in a cellular OFDM system, especially for 3GPP systems. The proposed estimator applies interference mitigation to pilot symbols, using repeated interference features according to the pilot subcarrier distance. In order to use this property, the proposed estimator employs a specific pilot structure which consists of two types of pilot symbols with different pilot density. The combination of interference alleviation and pilot rearrangement not only makes the channel estimation robust to the time-selectivity of the channel but also reduces the number of pilot subcarriers needed to estimate the channel. To clarify the advantages of the proposed method, the average mean square errors (MSE) of the frequency channel estimate are derived for the proposed estimator with unequal pilot density, and it is compared with general estimators with equal pilot density. Numerical analysis and simulation results confirm that the proposed estimator with unequal density relationship outperforms the estimators over practical time-varying environments in terms of MSE performance and pilot overhead efficiency.

Performance analysis of DSTTD based on diversity-multiplexing trade-off

In this paper, the performance analysis of double space time transmit diversity (DSTTD) achieving both diversity and multiplexing gain is introduced. The performance analysis of DSTTD is based on diversity-multiplexing trade-off. In addition, It is compared with space time block code (STBC) and vertical bell labs layered space-time code (V-BLAST) which aim diversity gain and multiplexing gain, respectively. Additionally, symbol error rate (SER) is considered. As a result of analysis and simulation, it is proved that pursuing both diversity gain and multiplexing gain is most preferable.

On the design of ZF and MMSE Tomlinson-Harashima precoding in multiuser MIMO amplify-and-forward relay system

In this paper, we design a zero-forcing (ZF) Tomlinson-Harashima precoding (THP) and a minimum mean square error (MMSE) THP for multiuser MIMO relay system with direct link. Based on these precoding structures, we propose the optimal combining weights of the ZF THP and the MMSE THP in half duplex relay system, and the sub-optimal weights of MMSE THP to reduce feedback channel state information (CSI). Through the Monte Carlo simulation, we verify that the optimal weights combining method outperforms the constant weights combining method. In addition, we confirm that the performance of the sub-optimal weights combining method is close to the optimal method, while reducing the feedback CSI simultaneously.

A New Timing Offset Estimation Algorithm Using Phase Difference Between Adjacent Subcarriers in Interleaved OFDMA Uplink Systems

A new timing estimator using phase difference between adjacent subcarriers is proposed for interleaved orthogonal frequency-division multiple access (OFDMA) uplink systems. The phase difference of adjacent subcarriers is represented as the summation of the information of timing offset which we want to estimate and the phase difference of channel frequency responses which can cause performance degradation. Thus, a method to reduce the phase difference of channel frequency responses is applied in time domain, and then timing offset information is estimated from phase difference in frequency domain. Simulation results confirm that the proposed estimator can decrease the mean square error (MSE) compared with channel impulse response (CIR) based method when the information of channel length is unknown.

A Robust Timing Recovery Algorithm for OFDM Systems

We propose a new robust timing recovery algorithm with small symbol timing jitter for orthogonal frequency-division multiplexing (OFDM) systems. The concept of coarse FFT window selection and adaptive reference pilot generation are employed in this paper. This proposed scheme avoids inter-symbol interference (ISI) as well as delayed locked loop (DLL) tracking failure over frequency selective time-varying channels. In addition, the proposed algorithm has much smaller and smoother steady state DLL tracking error than the conventional algorithm.

Positioning the relay to achieve full diversity and minimize error probability in cooperative decode-and-forward relay systems

Error propagation of source-relay (S-R) link limits the performance of decode-and-forward (DF) relay system and prohibits cooperative DF relay system from achieving full diversity gain. In order to resolve these problems, we first derive the criterion to achieve full diversity. It explains why the SNR of S-R link would be better than other links. Using the criterion, the proposed strategy of positioning the relay achieves full diversity and minimizes the error probability of cooperative DF relay system.