Hyungu Hwang

Doppler Frequency Offset Estimation in OFDM systems

This paper presents frequency offset estimation algorithm in wireless OFDM cellular system such as 3G-LTE [1]. We first analyze the method of frequency offset estimation in general OFDM system. The analysis will show that the value of frequency offset estimation in cell boundary is inaccurate. Because doppler frequencies of signals transmitted from different base stations are different, the error of frequency offset estimation value is generated. We show how proposed algorithm can reduce the error of frequency offset estimation. Results of simulation show that the utilization of proposed algorithm can reduce the error of frequency offset estimation considerably. Proposed frequency offset estimation algorithm can increase overall system capacity because of performance improvement in cell boundary. Especially, 16-QAM or 64-QAM modulations for high data rate transmission is extremely sensitive to frequency offset, therefore proposed algorithm increases channel capacity for user in cell boundary.

Automatic gain control for ASM channel in maritime communication

This paper presents Automatic Gain Control (AGC) algorithm in maritime communication system using application specific message (ASM). In maritime communication, gain control is not easy than land mobile communication. Received signal is converted to digital signal by analog to digital converter (ADC). If the signal input power of ADC is smaller than adequate power then quantization noise increases. On the contrary, if the signal input power of ADC is large than adequate power then the output signal of ADC is saturated. This paper proposes essential algorithm that controls gain value effectively in case that received signal power and propagation delay are unknown. We first analyze the general method of AGC in maritime communication. The analysis will show that the time for gain convergence is needed and much longer than known case because the distance from transmitting ship to receiving ship is randomly distributed and unknown in each slot. The difference of gain values in each slot is as large as 100dB in maximum case. The larger a difference of gain values is, the longer gain convergence time is. This is because that any ship can transmit in any slot randomly, therefore, received signal power is not expectable. We show how proposed algorithm can effectively control the gain value in maritime communication environments. Result of simulation shows that the utilization of proposed algorithm can calculate the optimal gain value and regenerate optimal signal that is similar with original source. As a result, proposed AGC algorithm changes all gain values of total received signal into same gain value using back gain compensation. Especially, the gain value of received signal should be controlled in adequate because transmission randomly occurs in each slot in maritime communication. Proposed automatic gain control algorithm is essential and effective for stable demodulation performance in ASM maritime communication.

Automatic gain control for multi carrier in OFDM systems

This paper presents automatic gain control (AGC) algorithm in wireless orthogonal frequency division multiplexing (OFDM) system using carrier aggregation such as 3G-LTE. When analog signal of multiple frequency bands is converted to digital signal by analog to digital converter (ADC), proposed algorithm controls each band gain effectively. We first analyze the method of AGC using carrier aggregation in general OFDM system such as 3G-LTE. The analysis will show that the gain value is unstable because of wireless channel status or presence of packet or difference with losses for radio frequency (RF) front end in each band. This unstable fluctuation of gain is generated by the variation of received signal power that varies with channel status or quantity of received packet. We show how proposed algorithm can effectively control the gain value in carrier aggregation environments. Results of simulation show that the utilization of proposed algorithm can calculate the optimal gain value. As a result, proposed AGC algorithm can increase bit resolution of received signal because optimal gain values are calculated in each aggregation band. Especially, received signal can be easily saturated in ADC input because OFDM system or 16-QAM and 64-QAM modulation schemes for high data rate transmission have the characteristic of high peak to average power ratio (PAPR), therefore proposed AGC algorithm supports to keep system performance stable in carrier aggregation environments.

A timing synchronization design in OFDM systems

This paper presents time synchronization algorithm in OFDM system. We first analyze the impact of channel estimation performance when synchronization time drifts in general OFDM system. The analysis will show that channel estimation performance is degraded in transient region of time synchronization. When channel estimation process is initialized, channel estimation performance is degraded. This is because that channel estimation process should be initialized when synchronization time is changed. We then show how proposed algorithms can reduce the performance degradation of channel estimation. Results of simulation show that the utilization of proposed algorithms can reduce the performance degradation of channel estimation. Proposed algorithms can be utilized in both high speed and low speed OFDM system. Above all, the use of proposed algorithms prevents burst error that is harmful for general communication network system.

Development of an ASM2.0 RF system for maritime communications of high-data-rate in the VHF band

This paper presents a development of a RF system for the application specific message 2.0 (ASM2.0) which is a new maritime communication service in the VHF band. It contains a specification determination related to a RF system, some design points to support a complex modulation scheme for high-data-rate, and the implementation results. And, performances linked with a baseband (BB) system are also described.

Constellation analysis of PSK signal for diagnostic monitoring

This paper presents constellation analysis for phase-shift keying (PSK) signal in time region on some problems. Generally diagnostic monitoring (DM) equipment is used for development of communication systems. And DM equipment shows signal constellation to diagnose communication status. However it is not known what specific signal constellation shapes mean. In this paper we investigate that specific signal constellations originate from specific problems during development process. Simulation results are coincident with the results of target hardware board. In this paper we mainly analyze differential quadrature phase-shift keying (DQPSK) signals in time region because another modulation signals have similar features. Therefore this paper could support to easily find out the problems during development process by simple comparing DM constellation with specific signal constellations.

Automatic frequency control in packet based OFDM systems

This paper presents automatic frequency control (AFC) algorithm in packet based wireless orthogonal frequency division multiplexing (OFDM) system such as 3G-LTE [1]. We first analyze the method of general AFC using cyclic prefix (CP) in OFDM systems. The analysis will show that the compensated frequency value is unstable and related with presence of packet. This instability of frequency control is generated by the variation of accuracy of estimation value that varies with presence of packet. We show how proposed algorithm reduces the instability of frequency control. Results of simulation show that proposed algorithm reduces the instability of compensated frequency and improve tracking performance considerably. In addition, proposed algorithm has good performance in fading channel. This is because proposed algorithm can detect deep fade region and exclude it from frequency control. As a result, proposed AFC algorithm improves stability and performance of frequency control by decision of packet presence. Especially, 16-QAM and 64-QAM modulation schemes for high data rate transmission are sensitive to frequency offset, therefore proposed AFC algorithm supports to keep high data rate system performance stable.

Automatic gain control for interoperability in OFDM systems

This paper presents interoperable Automatic Gain Control (AGC) algorithm in wireless Orthogonal Frequency Division Multiplexing (OFDM) system such as WLAN, WMAN, 3G-LTE [1]. We first analyze the method of AGC using received power in general OFDM systems. The analysis will show that the gain value largely fluctuates with presence or structure of signals. This fluctuation of gain is generated by the variation of received signal power that varies with presence of packet. We show how proposed algorithm can reduce the fluctuation of gain value. Results of simulation show that the utilization of proposed algorithm can reduce the fluctuation of gain value considerably. As a result, proposed AGC algorithm that can be used various communication systems can increase bit resolution of received signal because of reduction of gain fluctuation. Especially, OFDM system with high Peak to Average Power Ratio (PAPR) or 16-QAM and 64-QAM modulation schemes for high data rate transmission is sensitive to bit resolution, therefore proposed AGC algorithm supports to keep system performance stable in various communication systems.