Yuan-Hwui Chung

Joint estimation of I/Q imbalance, CFO and channel response for MIMO OFDM systems

In this paper, we study the joint estimation of inphase and quadrature-phase (I/Q) imbalance, carrier frequency offset (CFO), and channel response for multiple-input multipleoutput (MIMO) orthogonal frequency division multiplexing (OFDM) systems using training sequences. A new concept called channel residual energy (CRE) is introduced. We show that by minimizing the CRE, we can jointly estimate the I/Q imbalance and CFO without knowing the channel response. The proposed method needs only one OFDM block for training and the training symbols can be arbitrary. Moreover when the training block consists of two repeated sequences, a low complexity two-step approach is proposed to solve the joint estimation problem. Simulation results show that the mean-squared error (MSE) of the proposed method is close to the Cramer-Rao bound (CRB).

Joint estimation of I/Q imbalance, CFO and channel response for OFDM systems

In this paper, we study the joint estimation of I/Q imbalance, CFO and channel response for OFDM systems. Using two repeated OFDM blocks for training, we propose a new method to solve the joint estimation problem. The implementation complexity is low. Simulation results show that the proposed method has a good performance and its BER is very close to the ideal case where all the parameters are known perfectly at the receiver.

Frame synchronization and joint estimation of IQ imbalance and channel response for OFDM systems

In this paper, we first introduce a new method for joint estimation of IQ imbalance and channel response in OFDM systems under the assumption of perfect frame synchronization. The proposed method needs only one OFDM block for training and the solution is given in closed form. Then the method is applied to the scenario when the OFDM frame is not synchronized. A reduced complexity algorithm is derived. With a relatively low complexity, the proposed method can accurately estimate the frame starting position, the IQ imbalance and the channel response simultaneously. Simulation results show that the bit error rate (BER) performance of the proposed method is very close to the ideal case when all these parameters are known perfectly at the receiver.

Channel estimation in the presence of transmitter and receiver I/Q mismatches for OFDM systems

In this paper, we study the problem of channel estimation in the presence of the transmitter and receiver inphase and quadrature-phase (I/Q) mismatches for orthogonality frequency division multiplexing (OFDM) systems. A new concept called channel residual energy (CRE) is introduced. We show that by minimizing CRE, we can jointly estimate the transmitter and receiver I/Q mismatches without knowing the channel information. The optimal solution is given in closed form. Once we have the I/Q parameters, an estimate of the channel response can be obtained by simple substitution. The proposed method needs only one OFDM block for training and the training data can be arbitrary. Simulation results show that the proposed method can achieve a good performance.

Unitary Precoders for ST-OFDM Systems Using Alamouti STBC

This paper studies unitary precoders for space-time coded orthogonal frequency division multiplexing (ST-OFDM) systems using Alamouti space-time block codes (STBC). We consider two channel models: (1) the LTI case where the channels are invariant for two consecutive OFDM blocks; (2) the time-varying case where the channels are invariant within one OFDM block but they vary slightly between two OFDM blocks. For the LTI case, we derive the optimal precoder for QPSK modulation. It is shown that the optimal precoder is channel independent and can greatly enhance the performance of the ST-OFDM system. For the time-varying case, the channel variation will cause inter-carrier interference (ICI) in the ST-OFDM system and can result in serious performance degradation. We will show that by properly designing the precoder, we can average the output error variance and hence greatly alleviate the ICI effect. As a result, the bit error rate (BER) performance can be significantly improved.

Joint estimation of transmitter and receiver I/Q imbalances, CFO, and channel response for OFDM systems

In this paper, we study the problem of channel estimation in the presence of the transmitter and receiver in-phase and quadrature-phase (I/Q) imbalances and carrier frequency offset (CFO) for orthogonality frequency division multiplexing (OFDM) systems. A recently proposed concept called channel residual energy (CRE) is applied to solve the joint estimation problem. By minimizing CRE, we propose a new method to jointly estimate I/Q parameters, CFO and channel response using training sequences. The proposed method needs only one arbitrary OFDM block for training and can achieve a very good performance. When two repeated blocks are available for training, we propose a low complexity method to solve the joint estimation problem. Simulation results show that both the proposed methods provide a good performance.

Joint estimation of CFO and receiver I/Q imbalance using virtual subcarriers for OFDM systems

In this paper, we study the estimation of carrier frequency offset (CFO) in the presence of the receiver in-phase and quadrature-phase (I/Q) imbalance for orthogonal frequency division multiplexing (OFDM) systems. By minimizing the energy of the samples on the virtual subcarriers, our proposed algorithm can jointly estimate the CFO and I/Q parameters using one OFDM block. When the CFO is small, a closed-form solution can be obtained. Simulation results show that our proposed method can provide a good performance for both of the I/Q and CFO estimation and it compares favorably with a recent method.

Linearly precoded ST-OFDM systems in the presence of ISI

Recently it was shown that a linear precoder can greatly improve the performance of OFDM systems. In this paper, we study linear precoder for space-time coded OFDM (ST-OFDM) system for multiple antenna transmission. The effect of a linear precoder is analyzed when the ST-OFDM system suffers from intersymbol interference caused by undetected channel variations. For QPSK modulation, the optimal precoder that minimizes the bit error rate (BER) is derived. It is found that the optimal precoder is channel independent. Simulation results show that the optimal precoder can significantly lower the BER of ST-OFDM system