Do-Seob Ahn

Timing estimation for OFDM systems by using a correlation sequence of preamble

In this paper, we propose a novel timing estimation method for orthogonal frequency division multiplexing (OFDM) systems. The most common approaches in estimating timing use correlation by using a given preamble. Thus, the estimation performance is directly dependent on the specific preamble type. The proposed algorithm estimates the timing by using a simple circular shift operation of the given preamble at the receiver. For this reason, the proposed method can be used for any of the OFDM systems regardless of their preamble structures. The simulation results investigated in this paper reveal that the proposed method remarkably increases the estimation performance by more than 5 dB in terms of the mean squared error compared to conventional methods. We also consider the computational complexity of the methods and introduce a simple structure to reduce the computational burden of the proposed method.

Technology development for wireless communications system using stratospheric platform in Korea

Wireless communications systems using stratospheric platforms have become the focus of the worlds attention, since they would have many advantages over the conventional wireless communication infrastructure such as terrestrial and satellite communications systems. The research and development for putting the system to practical use is ongoing in some countries. This paper describes the results and the status of research on the technology for stratospheric communications system in Korea.

Capacity of the WCDMA System Using High Altitude Platform Stations

In this paper, we provide a method to estimate the capacity of the wideband code division multiple access (WCDMA) scheme using High Altitude Platform Stations (HAPS), and demonstrate the results. This is to validate that the HAPS system is one of the most effective solutions to provide high quality wireless communication services, such as IMT-2000 services. We use a practical system model which describes the HAPS system most appropriately, and this leads to realistic estimation results. We estimate the reverse and forward link capacity of the HAPS WCDMA system and compare it to that of the terrestrial system. The estimation results show that the capacity of the HAPS WCDMA system is about 1.2–1.67 times larger than that of the terrestrial system.

Evaluation of interference effect into cellular system from high altitude platform station to provide IMT-2000 service

HAPS (high altitude platform station) is allowed to be used as a base station to provide IMT-2000 service. ITU-R has been requested to conduct studies, as a matter of urgency, on the feasibility of utilizing HAPS system to provide IMT-2000 service. One of the most important parts of the above feasibility studies is interference analysis, since HAPS system as a base station should not cause harmful interference to the cellular IMT-2000 system. On the sharing problems between two systems, the self interference from the adjacent cells in cellular system must be considered as well as the interference from HAPS system. In this paper, we estimated the interference to cellular mobile station from HAPS system by interference power, and finally the carrier to interference power ratio (C/I) with various system parameters, such as the number of HAPS users per cell and the multibeam cell radius of HAPS. With the estimation results, separation distance between a cellular system coverage and HAPS coverage for sharing between two systems can be determined.

A satellite core network system for emergency management and disaster recovery

In general, the terrestrial communication resources are limited when infra structure is damaged or excessive traffic. But it is essentially required that an emergency communications should be reliable and flexible for disaster mitigation and relief operation. All users wish emergency communications is working perfectly everywhere and every time in any circumstances. Moreover the need for broadband communications during disaster or emergency times is gradually increasing recently. Based upon radio links, satellite based systems have several advantages in terms of reliable, available, robust, quickly deployable, and accessible from any locations. In this paper a satellite communications system is introduced as a core network for emergency management and disaster recovery including satellite network topologies, service configuration, and recommended technical specifications.

A fast LUT predistorter for power amplifier in OFDM systems

We proposed a new predistortion scheme that result in a last convergence performance by using a gradual quantization on input signal value from rough to fine. We compare the performance of the proposed scheme to conventional methods. Simulation results of the OFDM system on an AWGN channel reveal that the proposed method show improved convergence performance without any loss in accuracy.

A Study on the Separation Distance for Frequency Sharing between GSO Network and Terrestrial Network in Ka Band

Co-existence of various wireless systems may cause frequency interference problems. For this reason, frequency sharing studies play a very important role in order to use limited spectrum resources efficiently. Because a satellite system covers a global area in nature, the frequency sharing between the satellite and terrestrial systems is essential. For satellite systems using Ka bands, we can increase spectrum efficiency by considering off- axis angles of the earth station antennas. In this paper, we demonstrate the analysis results of the separation distance between the satellite earth station and a terrestrial station with random height. By using these results, we can design and implement Ka band wireless systems with the optimum separation distances, and this will lead efficient use of limited frequency spectrum resources.

Is HAPS viable for the next-generation telecommunication platform in Korea?

HAPS is a promising technology capable of providing broadband multimedia services as an alternative to the satellite communication system or terrestrial network. In this study, economic aspects of HAPS service are analyzed, HAPS services are defined, and revenues from the defined services are forecasted assuming nine scenarios. Capital expenditure as well as operating expenditure is estimated. To evaluate the profitability of HAPS service, the net present value (NPV), payback period, and the internal rate of return (IRR) are calculated. The results show that HAPS is economically justifiable in all the scenarios. Assuming that ARPU for the service is

Performance Evaluation for A closed loop power control using An Efficient Channel Estimation in SAT-CDMA

35 per month in the average scenario, NPV is calculated as

Forward link capacity of the WCDMA system using high altitude platform stations

2964 million, IRR becomes 31.9%, and payback occurs in 2017, which implies that HAPS service is profitable in Korea. In addition, the results of the sensitivity analysis show that the results are fairly robust.