Se-Bin Im

Robust OFDMA Frame Synchronization Algorithm on Inter-Cell Interference

Cellular OFDMA system is susceptive to inter-cell interference when a frequency reuse factor is one and all neighboring cells are fully loaded. At the cell edge, particularly, a severe synchronization error may occur due to decreased signal-to-interference ratio. In this paper, therefore, we propose a robust frame synchronization method in inter-cell interference for OFDMA systems. The proposed algorithm shows a clear peak at the correct frame timing and has low computational complexity

Improved Channel Estimation Method Based on Time-Domain Processing for OFDM Systems

Channel impulse response (CIR) can be estimated by time-domain processing (TDP) based on cyclic correlation in orthogonal frequency division multiplexing (OFDM) systems. We propose an improved generalized TDP method to reduce the estimation error by taking the average of different CIR values which are derived according to the different starting points of cyclic correlation. Moreover, an effective channel length estimation method which is performed by reverse detection processing to search the last effective CIR is also proposed. The whole proposed method is more efficient to OFDM systems with long cyclic prefix (CP) because the CP information is sufficiently utilized during the CIR estimation. The analysis and the simulation results verify that the mean square error (MSE) performance of the proposed method is 4-5 dB better than the conventional method under the same conditions.

A Robust Timing Synchronization Algorithm for OFDM Systems over Multipath Rayleigh Fading Channels

In this paper, we propose a robust timing synchronization algorithm for wireless MAN OFDM systems. The proposed algorithm is composed of frame synchronization and FFT timing synchronization. The conventional correlation scheme for estimation of timing offset has an ambiguity in the detection of accurate symbol timing. The proposed scheme, however, shows a clear peak at the correct timing by using the conjugate-symmetric characteristic of preamble and is able to control the FFT window position continuously with the 1-st order loop filter. Simulation results demonstrate the superiority of the proposed algorithm.

Pilot Structure for High Data Rate in OFDM/OQAM-IOTA System

In this paper, we introduce a novel pilot structure for OFDM/OQAM system which has high spectral efficiency by isotropic orthogonal transform algorithm (IOTA) function. In contrast with classical OFDM/QAM transmission scheme, the OFDM/OQAM-IOTA system requires a specific pilot structure for channel estimation because side-lobe components of the IOTA function cause an intrinsic interference. In case of conventional pilot structure, successive signal cancellation and null insertion schemes are considered for minimizing the channel estimation error. These schemes, however, reduce overall data transmission rate, especially for frequent pilot transmission, and it results in decreased throughput. For improving the data rate, we propose an additional data transmission scheme based on signal cancellation and an efficient 2-dimensional pilot block. Analysis and simulation results verify that the proposed pilot structure exhibit not only higher data rate but also more reliable channel estimation performance than the conventional pilot structure.

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.

A novel pilot mapping method for channel-quality estimation in SC-FDMA system

In this paper, we introduce a novel pilot mapping method for channel-quality (CQ) estimation of each user in single carrier-frequency division multiple access (SC-FDMA) system. In case of conventional code division multiplexing (CDM) method, it enables measurement of CQ levels over all sub-carriers. The method, however, has a limit on estimating the accurate channel frequency response for coherent demodulation in contrast with frequency division multiplexing (FDM) based on localized mapping. In order to overcome the structural problem, we combine two characteristic methods, CDM and FDM, and estimate the channel values depending on the purpose. Simulation results show that the proposed method can improve the demodulation performance with a similar CQ error rate compared with the conventional method.

Frequency Domain MMSE Equalization with Moving FFT for MBOK DS-UWB System

In this paper, we propose a frequency domaian(FD) MMSE equalizer for M-ary bi-orthogonal keying direct sequence UWB (MBOK DS-UWB) systems considered as a PHY proposal for high-speed wireless communication in IEEE 802.15.3a. The conventional FD MMSE equalization scheme has a structural limit due to insertion of the cyclic prefix (CP) in all transmit packets, but the proposed scheme is able to equalize the channel effect without CP. In order to overcome channel estimation error by multipath delay, we introduce a moving FFT and a moving average scheme. Compared with the traditional time-domain (TD) MMSE-RAKE receiver, the proposed FD MMSE equalizer has a better BER performance and we demonstrate this result by computer simulation.

An efficient tail-biting MAP decoder for convolutional turbo codes in OFDM systems

In this paper, we consider an efficient tail-biting MAP (Maximum a posteriori probability) decoding algorithm for CTC (Convolutional Turbo Codes) specified in IEEE 802.16a/e OFDM. For short block lengths, the CTC encoder uses tail-biting scheme instead of inserting tail bits for path termination. To process a tail-biting codeword, the decoder begins with estimating the circulation states. But this estimation procedure increases the computational complexity. We first describe the CTC decoding process based on the MAP algorithm of the nonbinary codes. Using a suboptimal iterative decoding method, we propose an efficient circulation states estimation algorithm. The BER (Bit Error Rate) performance of the proposed algorithm is evaluated using computer simulations. The results show that the proposed algorithm performs close to the ideal case with known circulation states information.

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.

Efficient OFDM channel estimation for realistic band-limited and non-sample-spaced channels

In this paper, we propose computationally efficient channel estimation methods to minimize energy leakage which is caused by non-sample-spaced channels as well as band-limited OFDM signal with virtual subcarriers. The energy leakage effect mainly resulted from conventional channel estimation methods which do not utilize channel statistics information for low complexity such as DFT-based estimation and correlation error cancellation (CEC). The proposed methods solve the problem through a modified non-ideal correlation function which is derived by expanded DFT. Simple analyses and simulation results verify that the proposed methods can achieve MSE performance close to that of optimal LMMSE estimation with sufficiently low complexity.