Jae-Seon Yoon

Efficient techniques in OFDM system with channel evaluation

In this letter, we aim at the study of an OFDM system that employs adaptive modulation techniques to achieve efficient transmission service. In adaptive modulation techniques, a banded AM technique can reduce the number of adaptive modulation information bits and signaling overhead. We evaluate the banded AM technique and compare with Subcarrier AM technique. Moreover, to evaluate the channel condition, we apply the SNR estimation method. Furthermore, to increase the performance of OFDM system, frequency domain pre-equalization is applied. The simulation results have been shown that the OFDM system with adaptive schemes can achieve the efficient transmission for multi-media services.

Cooperative Transmission Technique Using Space Time Delay Code in OFDMA Uplink System

Multiple antennas are used to an essential part in order to support high link quality in wireless communication systems. The diversity techniques with multiple antennas provide better reliability than existing systems. But these techniques generally require more than one antenna at the transmitter or receiver. So, the system with multiple antennas has several limitations including size, cost, hardware complexity, and antenna space. In this letter, we propose a cooperative transmission technique that uses space time delay code in OFDMA uplink system; DVB-RCT. The proposed technique overcomes these limitations and gives the diversity gain of cooperative transmission in a concept of virtual MIMO with single transmit antenna.

AN ELABORATE FREQUENCY OFFSET ESTIMATION FOR OFDM SYSTEMS

In this paper, a carrier frequency offset estimation scheme in orthogonal frequency division multiplexing is proposed. We focus on increasing the correlation of each PN sequence code repeated in one preamble. We aim at getting an efficiency like using many preambles. The proposed method can improve the performance of the system by estimating fine frequency offset. Also, the proposed method enhances reliability by maximizing the number of correlations compared with the established method in time domain.

Cooperative OFDM system for high throughput in wireless personal area networks

Wireless Personal Area Networks (WPANs) is a promising application in future wireless communication. Multiple-input multiple-output - orthogonal frequency division multiplexing (MIMO-OFDM) systems have allowed the improvement of the system performance. However MIMOOFDM systems exist limitations of supporting high data rate and reliable transmission, as the size and cost of device become smaller and lower. Recently, cooperative OFDM systems are developed to be a solution. It makes that service providers supply both increased throughput and reliability. Although the performance of applications is developed, consumers still demand better performance. Therefore, we propose the novel cooperative OFDM system for high throughput with signal space diversity (SSD). Our proposed scheme adds the operation for rotation, superposition, and demodulation for the SSD signal on each user device before relaying. Simulation results show the throughput of our proposed cooperation scheme is increased by twice as compared with the conventional cooperative OFDM scheme.

Performance analysis of mutual decode-and-forward cooperative communication for OFDMA based mobile WiMAX system

A cooperative communication has been frequently studied because it provides reliability to the previous communication system without the addition of antenna. However, in a cooperative communication, the throughput of the communication system is decreased by half because each station cannot transmit own signal for relaying process. To overcome above limitation and provide more reliability, we propose and analyze the mutual decode-and-forward (DF) cooperative communication for orthogonal frequency-division multiple access (OFDMA) based mobile WiMAX system with single antenna in the Rayleigh fading channel environment. By adopting the proposed scheme, the performance of the mobile WiMAX system is improved in proportion as the quality of inter-user channel is increased.

Performance evaluation of cooperative communication for OFDMA based wireless communication system

Recently, the orthogonal frequency-division multiple access (OFDMA) based wireless communication system is usually used in interactive digital video broadcasting environments or the wireless metropolitan area network (WMAN). Therefore, the relative condition of the inter-user channel can be easily deteriorated. This paper proposes and evaluates the mutual amplify-and-forward (AF) cooperative communication between two users using modified space time block code (STBC) to provide the reliability for an OFDMA system with a single antenna in an inferior inter-user channel environment. As the results, an OFDMA system adopting the proposed cooperative communication is more robust to the inferior inter-user channel than the previous decode-and-forward (DF) cooperative communication.

Frequency offset mitigation of cooperative ofdm system in wireless digital broadcasting

Mobile digital broadcasting is expected to become a promising application in future wireless communication. To improve the system performance in limitation of size and cost, the OFDM and MIMO techniques are adopted. Recently, cooperative diversity techniques are developed to be a solution. In this paper, we propose the carrier frequency offset mitigation scheme in cooperative communication using multi-symbol encapsulated orthogonal frequency division multiplexing (MSE-OFDM), in which one cyclic prefix (CP) is used for multiple OFDM symbols. We use the FFT size-reduced MSE-OFDM system which can be used to reduce the peak-to-average power ratio (PAPR) and improve the robustness to carrier frequency offset. The cooperative communication uses not only direct path from source to destination but also indirect path via relay station. Each different relays require more precise frequency synchronization and lower PAPR, so we adopt the MSEOFDM to increase the system performance. The performance analysis of cooperative MSE-OFDM in multipath fading channels has been done and the effect of carrier frequency offset and PAPR have been studied.

EM-based low complexity channel estimation for OFDM system

The EM algorithm is a general method for ML channel estimation that arise in statistical estimation problems. However, this algorithm has two drawbacks which are slow convergence and large complexity. In order to reduce the complexity of the EM algorithm, we adapt quasi-Newton methods to the EM algorithm for channel estimation in frequency selective environments. The proposed methods and conventional EM algorithm are compared with each other in terms of complexity and the BER performance. The performance evaluations are based on the European digital television system DVB-H. Also, the outstanding performance of this proposed methods is shown and compared with other existing methods in DVB-H.

Design of MIMO-OFDM multi-hop relaying with cooperative base station

Basically, in cellular network systems, the relay station (RS) is used to increase the cell coverage of mobile stations (MS) at a cell edge. But, the system performance due to the use of RS is interfered by the neighboring cells and decreased by the low power of received signals, generally. In order to overcome these disadvantages and get a diversity gain or a high data rate, we design and propose the multiple-input multiple-output (MIMO) multi-hop relaying system structure using the BS cooperation scheme, and evaluate the performance of the proposed system in this letter. Therefore, the MSs can effectively receive the signals through the proposed design which uses the RS in the overlapping regions.

Cooperative hybrid cyclic delay diversity scheme in dual-hop wireless networks

Cooperative communication has been widely investigated to improve performance of wireless networks. In cooperative wireless system, various MIMO scheme such as OSTBC, CDD and V-BLAST can be easily applied and obtain diversity or multiplexing gain. These schemes cannot obtain diversity and multiplexing gain simultaneously. To guarantee both high throughput and reliable transmission, hybrid STBC can be applied. However, this typical hybrid scheme requires complex structure at the destination and generates a rate loss according to increasing the number of transmit antennas. Moreover, in cooperative wireless system, hybrid STBC is inflexible to choose relays because relays require specific STBC encoder. Therefore, in this paper, we propose the cooperative hybrid CDD scheme which can be easily applied with arbitrary number of relays without any rate loss. Furthermore, a modification is not necessary at the destination although the number of relays is changed.