Jun-Hee Jang

Power Control Scheme for Effective Serving Cell Selection in Relay Environment of 3GPP LTE-Advanced System

In this paper, we propose a power control scheme for effective serving cell selection in Relay environment of 3GPP (3rd Generation Partnership Project) LTE (Long Tenn Evolution)-Advanced system. A conventional serving cell selection scheme which does not use channel states of backhaul link has a problem that this scheme does not select serving cell supporting maximum throughput. Also, conventional proposed serving cell selection schemes that eNB or RN transmits channel states of backhaul link have problems that conventional schemes need to additional data transmission, serving cell selection process complexity is increased because UE considers channel states of backhaul link, and received signal is degraded because strong interference which is transmission signal from RN. Therefore, for solve these problems, we propose power control scheme that RN control transmission power according to received SINR (Signal to Interference plus Noise Ratio) of backhaul link. By extensive computer simulation, we verify that the power control Relay scheme is attractive and suitable for the Relay environment.

An improved complex field network coding scheme for wireless relay system in asymmetric link environment

In this paper, a complex field network coding (CFNC) scheme is presented, which focuses on improving transmission efficiency by reducing time slots that are needed to exchange data frames. Conventional CFNC scheme has the advantage of minimizing the number of time slots required for information exchanges. However, there exists serious performance deterioration because the transmit signals are interfered with each other. Also, considering a typical asymmetric link whose data traffic is biased in downlink such as orthogonal frequency division multiple access (OFDMA) system, conventional CFNC scheme cannot obtain sufficient coding gain due to frequency selective fading. In order to overcome these problems, we propose a repetition-based CFNC scheme as well as the corresponding maximum likelihood (ML) decision rule. The proposed repetition scheme is considered for obtaining frequency diversity gain, which is used to improve the decision accuracy in the proposed ML decision process. Simulation results verify that the proposed scheme has superior coded bit error rate (BER) performance compared with the conventional scheme.

DFT-based decision directed channel estimation algorithm for OFDM systems

An enhanced discrete Fourier transform (DFT)-based channel estimation for OFDM system is proposed. Conventional DFT-based channel estimations improve the performance by suppressing time domain noise. However, the disadvantage is that the effect of noise reduction is dependent upon the number of pilot signals. In order to overcome the disadvantage, the proposed algorithm improves performance by using the decision directed method for data subcarriers, and it is confirmed by computer simulation. Simulation results demonstrate that the proposed algorithm outperforms the conventional DFT-based estimation in terms of BER and MSE performance.

An enhanced interference mitigation scheme in the presence of asynchronous Inter-Cell Interference for MIMO-OFDMA

To mitigate the asynchronous ICI (Inter-Cell Interference), the CFR (Channel Frequency Response) and SCM (Spatial Covariance Matrix) should be accurately estimated. Therefore, the enhanced asynchronous ICI mitigation scheme which can effectively reduce the estimation error of both CFR and SCM, and improve performance for MIMO (Multiple-Input Multiple-Output)-OFDMA (Orthogonal Frequency Division Multiple Access) system is derived. In particular, the lower-bound performance is approached to within 1 dB, by using the proposed method for both MMSE (Minimum Mean Square Error) and ML (Maximum Likelihood) receiver in the presence of asynchronous ICI.

Beam-nulling based inter cell interference coordination for heterogeneous network

In 3rd generation partnership project (3GPP) long term evolution (LTE)-Advanced, heterogeneous network is considered as next generation network topology to improve radio link performance. However, in heterogeneous network, there are various kinds of low power nodes in macro cells coverage. Since they are sharing the same frequency spectrum, inter cell interference (ICI) is occurred. The main situation that experiences significant performance degradation due to the ICI is the macro user terminal which is close to the femto cell. In this case, the inter cell interference coordination (ICIC) method is necessary to achieve sufficient gain offered by heterogeneous network. Therefore, in this paper, we propose a new ICIC method that femto base station performs called beam-nulling that used position information of interfered user terminal in macro--femto deployment. By using computer simulation and mathematical analysis, we showed that the proposed method not only mitigates ICI but also improves resource utilization efficiency and overall throughput compared to the conventional methods.

The Partial Full Duplex Relay Scheme for Cell ID Detection of Type 1 Relay in 3GPP LTE-Advanced System

In this paper, we propose a partial full duplex relay scheme for 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution)-Advanced system using a Type 1 relay. The Type 1 relay as inband relay is prohibited to transmit and receive simultaneously because of self-interference. Therefore, the Type 1 relay cannot receive synchronization signals which are transmitted to eNB. To overcoming this problem, we propose the partial full duplex relay scheme which transmits to R-UE (Relay-User Equipment) and receives from eNB (evolved NodeB) simultaneously when eNB and the Type 1 relay transmit subframes which have synchronization signals. Additionally, for solving self-interference, the Type 1 relay transmitter and receiver antennas are sufficiently sufficiently isolated and self-interference cancellation is applied for the self-interference signal from the relay transmitter. Thus, the partial full duplex relay scheme can receive synchronization signals from eNB and solve the problems of conventional solutions and we propose the partial channel estimation scheme for partial full duplex relay scheme using SCI. By extensive computer simulation, we verify that the partial full duplex relay scheme is attractive and suitable for the Type 1 relay system.

A Study on Complex Field Network Coding Scheme for Wireless Relay System

In this paper, a CFNC (Complex Field Network Coding) scheme is presented, which focuses on improving transmission efficiency by reducing time slots that are needed to exchange data frames. Conventional CFNC scheme has the advantage of minimizing the number of time slots required for information exchanges. However, there exists serious performance deterioration because the transmit signals are interfered with each other. Moreover, when CFNC scheme is applied, the estimation and compensation performance of fine frequency offset severely deteriorates due to the multiple frequency offsets in received signal of relay node. In order to overcome these critical problems, we propose an improved CFNC scheme with modified maximum likelihood decision method which uses uplink transmit diversity. Also, we propose an enhanced fine frequency offset estimation method and a corresponding compensation method to deal with the multiple frequency offsets. Corresponding simulation results verify that the proposed methods are able to effectively solve the problems of CFNC scheme.

A Low-Complexity 2-D MMSE Channel Estimation for OFDM Systems

For OFDM (Orthogonal Frequency Division Multiplexing) systems, 2-D MMSE (2-Dimensional Minimum Mean Square Error) channel estimation provides optimal performance in frequency/time selective fading channel environment. However, the 2-D MMSE channel estimation has high computational complexity due to the large matrix size, because the 2-D MMSE channel estimation considers time as well as frequency axis for channel estimation. To reduce the computational complexity, we propose a modified 2-D MMSE channel estimator which is based on 1-D MMSE channel estimation with weighted sum. Furthermore, we consider RMS delay spread and Doppler frequency estimation for 2-D MMSE channel estimation. We show that the proposed method can significantly reduce computational complexity as well as that it can perform close to 2-D MMSE channel estimation.

An efficient ICI cancellation method for cooperative STBC-OFDM systems

In this paper, an efficient inter carrier interference (ICI) cancellation method for cooperative space time block coded orthogonal frequency division multiplexing (STBC-OFDM) system is presented. In cooperative STBC-OFDM system, ICI cancellation is needed because ICI due to the separated local oscillators always exists. To solve the complexity problem of matrix inversion operation in zero forcing method which shows the best performance, ICI cancellation method using sparse matrix decomposition (SMD) has been proposed. However, overall complexity is increased in proportion to the third order of required tap size which also increases in proportion to FFT size or carrier frequency offsets (CFOs). Considering implementation issue, the conventional method still has not sufficiently overcome the performance versus complexity trade-offs. Therefore, we propose an ICI cancellation method that focuses on solving practical complexity problem of conventional method. The proposed method adaptively decides the required tap size of each sparse matrix through signal to interference and noise ratio (SINR) measurement to reduce the complexity of conventional method, and we verified that the proposed method improves the performance versus complexity trade-offs compared with conventional method.

Enhanced channel estimation for OFDM systems with multiple transmit antennas

For OFDM systems with multiple transmit antennas, it is necessary to obtain a reliable CFR of each transmitted signal from the received signal which is the superposition of transmitted signals. Generally, pilot symbol aided frame structure is used for bandwidth efficient frame structure; hence interpolation method can be used to separate CIR of each transmitted signal for channel estimation. However, the performance of this method is seriously degraded by aliasing and interpolation error. Therefore, we propose an enhanced DFT-based estimation method based on the iterative algorithm which can estimate the CIR without aliasing and interpolation error. By using computer simulation, we show that the proposed method approaches the low bound in terms of MSE performance by reducing the estimation error of the CFR of each transmitted signal, and verify that the proposed method is attractive and suitable for implementation with stable operation.