Jong-Ho Lee

Joint carrier frequency synchronization and channel estimation for OFDM systems via the EM algorithm

A joint carrier frequency synchronization and channel estimation scheme is proposed for orthogonal frequency-division multiplexing (OFDM) system. In the proposed scheme, carrier frequency synchronization and channel estimation are performed iteratively via the expectation-maximization (EM) algorithm using an OFDM preamble symbol. Moreover, we analytically investigate the effect of frequency offset error on the mean square error (MSE) performance of channel estimator. Simulation results present that the proposed scheme achieves almost ideal performance for both channel and frequency offset estimation.

Time and Frequency Synchronization for OFDMA Uplink System using the SAGE Algorithm

In this letter, we propose an iterative time and frequency synchronization scheme for orthogonal frequency-division multiple access (OFDMA) uplink system using the space-alternating generalized expectation-maximization (SAGE) algorithm. In the proposed scheme, the expectation step of the SAGE algorithm is intended to remove the multiple-access interference (MAI) caused by the other asynchronous users. Then, the maximization step is utilized in the timing and frequency offset estimation of each asynchronous user using the MAI-canceled signals. Simulation results present that the proposed scheme shows performance enhancement in the presence of multiple asynchronous users

Joint channel estimation and phase noise suppression for OFDM systems

A joint channel estimation and phase noise suppression scheme for orthogonal frequency-division multiplexing (OFDM) systems is proposed for a case where channel estimation is needed symbol by symbol. In the proposed scheme, channel estimation and phase noise suppression are performed iteratively via the expectation-maximization (EM) algorithm. The proposed algorithm mitigates the performance degradation due to phase noise effectively while providing an accurate channel estimate with comparatively few pilot subcarriers, so that the spectral efficiency of the OFDM system is improved.

Detection of Interleaved OFDMA Uplink Signals in the Presence of Residual Frequency Offset Using the SAGE Algorithm

In this paper, we propose an iterative detection scheme in the presence of residual frequency offsets (RFOs) using the space-alternating generalized expectation-maximization algorithm for an orthogonal-frequency-division-multiple-access uplink system. In the proposed scheme, the expectation step intends to remove intercarrier interference (ICI) due to the RFOs of simultaneous users in the received signals. Then, the maximization step is utilized to estimate the required parameters for each user (i.e., RFO, data symbols, and channel state information) employing the ICI-cancelled signals. Simulation results show that the proposed scheme achieves almost ideal bit-error-rate performance as long as the magnitude of the normalized RFO value is within 0.2

Residual frequency offset compensation using the approximate SAGE algorithm for OFDM system

In this letter, we propose an iterative detection scheme in the presence of residual frequency offset (RFO) for orthogonal frequency-division multiplexing systems using an approximate application of the space-alternating generalized expectation-maximization (SAGE) algorithm. In the proposed scheme, the expectation step intends to divide the received signal into the desired signal and the interference signal. In the maximization step, the desired signal is used to estimate required parameters (i.e., RFO, data symbols, and channel state information) sequentially. Simulation results present that the proposed scheme shows almost ideal performance as long as the normalized RFO value is within 0.2.

Joint Common Phase Error and Channel Estimation for OFDM-based WLANs in the Presence of Wiener Phase Noise and Residual Frequency Offset

In orthogonal frequency-division multiplexing (OFDM)-based wireless local area networks (WLANs), the phase noise (PHN) and a residual frequency offset (RFO) can cause the common phase error (CPE) and the inter-carrier interferences (ICI), which seriously degrade the performance of systems. In this paper, we propose a combined pilot symbol assisted and decision-directed scheme based on the least-squares (LS) and maximum-likelihood (ML) algorithms. In the proposed scheme, the CPE estimator is derived using the pilot symbols embedded in each OFDM symbol. Then, data symbols and channel response including the CPE are estimated using the CPE estimator. Simulation results present that the proposed scheme significantly improves the performance of OFDM-based WLANs.

Channel optimized predistortion for self-heterodyne DCT-based OFDM systems

This paper presents a novel technique to compensate intermodulation distortion of a self-heterodyne direct conversion OFDM system in multipath propagation environments. A self-heterodyne direct conversion system has an advantage that simple receivers can he built that are completely immune to any phase noise or frequency offset. This system, however, has a disadvantage that the nonlinear detector at the receiver of the self-heterodyne direct conversion system gives rise to intermodulation distortion. In this study, channel estimation is performed using a training sequence and then the predistortion coefficients with regard to estimated channel parameters are derived to compensate the receiver nonlinearity. Transmit power allocation technique is employed to overcome multipath fading channels as well. Computer simulation demonstrates that the proposed approach improves BER performance of the self-heterodyne direct conversion system in a multipath fading channel. This scheme gives advantage to multi-carrier systems, such as OFDM, that are much more sensitive to frequency and phase error than single-carrier systems.

Pilot symbol initiated iterative channel estimation and decoding for QAM modulated OFDM signals

In this paper, we consider the iterative channel estimation and decoding algorithms for quadrature amplitude modulation (QAM) modulated orthogonal frequency division multiplexing (OFDM) signals. We first describe the optimal sequence estimation based on the expectation-maximization (EM) algorithm. Using some approximations, we propose a sub-optimal iterative channel estimation and decoding algorithm, which is shown to have reduced computational complexity. The bit error rate (BER) performances of the proposed algorithm are evaluated using computer simulations. The results show that the proposed algorithm performs nearly as well as the optimal EM algorithm, and outperforms the conventional minimum mean square error (MMSE) estimator.

A residual frequency offset compensation scheme for OFDM system via SAGE algorithm

In this paper, we propose an iterative detection scheme in the presence of residual frequency offset (RFO) for orthogonal frequency-division multiplexing (OFDM) system using the space-alternating generalized expectation-maximization (SAGE) algorithm. In the proposed algorithm, the expectation step intends to remove the inter-carrier interferences (ICI) due to RFO in the received signals. Then, the maximization step is utilized to estimate the required parameters (i.e. RFO, data symbols and channel state information) using the ICI canceled signals. Simulation results present that the proposed algorithm shows almost ideal performance as long as the normalized RFO value is within 0.2.

Nonlinear distortion cancellation in self-heterodyne OFDM receivers over multipath fading channels

A digital signal processing technique for canceling a nonlinear distortion is derived with an emphasis on self-heterodyne direct conversion architectures. To cancel out the nonlinear distortion arisen at the self-mixer output of a receiver, data symbols are generated and consecutively transmitted in pairs, same amplitude but out of phase with each other. After passing through the self-mixer at the receiver, two received signals are combined according to the operation of subtraction from the received signal of the first symbol to that of the second symbol. This proposed scheme leads to cancel out the nonlinear distortion in multipath propagation environments.