Sekchin Chang

A simplified predistorter for compensation of nonlinear distortion in OFDM systems

This paper presents a new simplified predistortion scheme to compensate nonlinear distortion introduced by the high power amplifier (HPA) in OFDM systems. OFDM systems are susceptible to HPA nonlinearities in particular because of their large peak-to-average power ratios. Previously, a Volterra-based predistorter was suggested for compensation of HPAs nonlinear distortion in OFDM systems. In contrast to this earlier work, our adaptive predistorter utilizes a more simplified structure. It is shown that the new simplified structure exhibits faster convergence than the previous Volterra-based predistorter. In addition, the performance of the new predistortion scheme for OFDM systems is evaluated in terms of total degradation, optimal output backoff, and BER. Simulation results also show that our predistortion scheme is more efficient in compensating the nonlinear distortion of HPAs in OFDM systems than the previous scheme.

A Beamforming Approach to Smart Grid Systems Based on Cloud Cognitive Radio

In this paper, we desire to use cognitive radio (CR) channels for communication among a wireless network of smart meters. However, self-interference critically limits the performance of CR systems. This is due to the coexistence of many unplanned systems simultaneously accessing the same signaling bands in an uncoordinated manner. To solve this problem, we show a beamforming approach that effectively mitigates the self-interference effects of the smart meter channel. The beamforming approach is based on minimum mean squared error (MMSE) method in smart meter systems. The MMSE beamformer usually requires accurate channel estimates and noise-plus-interference power estimates for effective mitigation of self-interference in CR systems. In this paper, we propose novel channel estimation and noise-plus-interference power estimation methodologies that efficiently exploit the preamble feature of the IEEE802.22 wireless regional area network (WRAN). Our framework is premised upon the utilization of a cloud computing smart grid infrastructure that hosts the IEEE 802.22 WRAN CR standard. The simulation results for a smart grid system with the MMSE beamformer illustrate significant improvements in system capacity and BER.

An Efficient Time Synchronization Scheme for Broadband Two-Way Relaying Networks Based on Physical-Layer Network Coding

We present an efficient time synchronization scheme for broadband two-way relaying networks based on two-phase physical layer network coding. Especially, a preamble structure is proposed in this letter for the synchronization. The synchronization approach exploits the preamble in frequency domain and time domain to effectively separate the mixed signals, and jointly estimate timing-offsets and channel parameters, respectively. Numerical results confirm that the suggested method is superior to the conventional scheme, and is very suitable for the synchronization in broadband two-way relaying networks based on two-phase physical layer network coding.

A compensation scheme for nonlinear distortion in OFDM systems

This paper presents an adaptive predistorter to compensate nonlinear distortion introduced by the high power amplifier (HPA) in OFDM systems. OFDM systems are susceptible to HPA nonlinearities in particular because of their large peak-to-average power ratios. In contrast to earlier work, the adaptive predistorter in this paper is based on both the direct and indirect learning architectures, and utilizes a complex Volterra series which is known to be suitable for compensating nonlinear phenomena. It is shown that the new predistorter scheme exhibits faster convergence than the previous method based on the indirect learning architecture only. In addition, the performance of the predistorter for OFDM systems is evaluated in terms of total degradation, optimal output backoff, and BER.

An Intelligent Positioning Scheme for Mobile Agents in Ubiquitous Networks for U-City

Various advanced schemes have been proposed for the establishment of u-city. Especially, most of the schemes are based on ubiquitous networks. In this paper, it is assumed that the ubiquitous networks include some mobile agents. Therefore, an intelligent positioning scheme is presented for the efficient positioning of the mobile agents in the ubiquitous networks. The approach consists of location detection and location tracking. Simulation results indicate that using the algorithm, the excellent detection and tracking performance can be achieved in ubiquitous networks for u-city.

An efficient multiplexing method of T-DMB and cell broadcast service in emergency alert systems

This paper presents a novel multiplexing method of T-DMB and cell broadcast service for robust emergency alert service in mobile stations. In the method, the cell broadcast service is used as a primary device for detection of an emergency alert message. If a detection outage occurs, the T-DMB is enabled as a secondary device for detection of the alert message, which guarantees more reliable alert message delivery. For better detection performance of emergency alert messages in the T-DMB, a channel estimation scheme is proposed, which is based on a divide-and-conquer algorithm. The presented estimation approach effectively exploits the phase reference symbol of TDMB. The simulation results confirm that the multiplexing and channel estimation approaches produce better alert delivery performance in an emergency alert system.

An efficient compensation of TWTA's nonlinear distortion in wideband OFDM systems

A multi-stage approach is presented for efficient compensation of TWTAs nonlinear distortion (which belongs to Wiener systems) in wideband OFDM systems. In stages 1 and 2, TWTAs memory effects and memoryless nonlinear distortion are separately modeled, which leads to very fast modeling convergence. Then, TWTAs nonlinear distortion is completely compensated based on the models in stage 3.

A signaling emergency alert system multiplexed with T-DMB channel for emergency alert service

Mobile broadcasting services are considered effective methods for emergency alert delivery. Terrestrial digital multimedia broadcasting (T-DMB) offers an emergency alert service called automatic emergency alert service (AEAS). However, the conventional AEAS is inefficient in the aspects of alert service channel, disabled T-DMB receiver, and location-based service. Therefore, this paper proposes a novel signaling method in order to overcome the drawbacks of the AEAS. The presented alert system exploits an additional alert channel, which is multiplexed with the conventional T-DMB channel. This paper also addresses the multiplexing approach, which effectively eliminates the interference between the additional alert channel and the TDMB channel. The simulation results confirm that the proposed signaling method effectively offers the emergency alert services in T-DMB1.

A Synchronous Cooperative Communication for Emergency Alert Broadcast Based on Cellular Systems

The CBS methodology has been utilized in order to make a rapid broadcast of emergency alert based on cellular systems. We present a synchronous cooperative communication method for the CBS. Especially, we suggest a synchronization scheme and a data recovery approach for high-rate cooperative communications. For the high-rate transfer of emergency alert, the cyclic prefix is added to the preamble for the synchronization. For the data recovery, the Alamouti technique is utilized on frequency domain, which is similar to SC-FDE. The simulation results confirm that our proposed scheme is very suitable for the CBS.

LP-WPAN based on a novel SC-FDE for wideband wireless sensor networks

An efficient LR-WPAN scheme is proposed for wideband wireless sensor networks. The LR-WPAN is based on a novel SC-FDE for efficient suppression of the channel effects in wideband environments. The presented SC-FDE exploits the estimation features of IEEE802.15.4 LR-WPAN to reduce the complexity of the conventional SC-FDE and enhance the receiver performance of the conventional SCFDE. In addition, a multi-hop scheme based on the novel SC-FDE is suggested for reliable ad-hoc communications in wideband channels. The simulation results show that the performance of the proposed SC-FDE is better than that of the conventional SC-FDE in wideband channels.