Junyeong Bok

High Power Efficiency and Low Nonlinear Distortion for Wireless Visible Light Communication

High power efficiency and low nonlinear distortion is very important for wireless visible light communication system. OFDM (orthogonal frequency division multiplexing) system has been studied a lot as the method to transmit high speed data in wireless visible light communication system. However, OFDM system has drawbacks which are non-linear distortion by high PAPR characteristic and ICI problem occurred by RF impairments. In wireless visible light communication system, output power of visible light which is generated by driven current of LED (light emission diode) has non-linear characteristic. Therefore, transmitted signal is distorted by it. Researches for receive performance of visible light output which is emitted by non-linear LED transfer function and OFDM signal have been studied. In this paper, we suggest a new PAPR reduction technique to reduce non-linear distortion of LED for reducing the PAPR. Also, we propose an adaptive ICI suppression algorithm in order to improve receive performance through enhanced SNR (signal to noise power ratio) by an adaptive channel estimation method of proposed algorithm. Improved performance of proposed system is verified through computer simulation.

Performance evaluation of DCSK system with chaotic maps

In this paper, we analyze the performance in differential chaos shift keying (DCSK) schemes using different chaotic maps such as Gauss map, Tent map, Gingerbread man map, and Henon map. As technologies of communications develop, the need for data security is also significantly improved. DCSK modulation technique has advantages of traditional spread spectrum system and also provides enhanced data security. We ensure that the performance with Henon map is better than the other chaotic maps with typical spreading factors and at reasonable SNR levels.

Analysis of Performance of Digital Retrodirective Antenna Technology in High-Speed Rail

Fast tracking is important for high-speed data transmission in high-speed mobile environment such as high speed rail and vehicular. Digital retrodirective array antenna is possible to do automatically beam tracking because it can control the phase information of the output signal toward opposite direction to input signal without no a priori knowledge of the arrival direction. Also, Digital retrodirective array antennas has merit that it is easy to upgrade and modify compare with analogue retrodirective array antennas. In this paper, we analyze the BER performance of digital retrodirective array antenna under AWGN environment and multipath signal. Simulation results show correct phase estimation and conjugation of retrodirective array antenna by using phase detector block. Also, phase conjugation technique has better BER performance about 1 dB at source than that of without phase conjugation when phase lag is in AWGN environment. This paper also discusses effect of the presence of multipath signal. Phase and amplitude error about direction of direct signal occurs when retrodirective array system is affected by interference and multipath signal in the presence of multipath signal.

Path loss model considering Doppler shift for high speed railroad communication

In this paper, we propose the tuned free-space path loss modelling in high speed railroad considering Doppler effect. We use tuned free space path loss model which is utilized for measurement results at high speed railroad. The environment of high speed rail is mostly at viaduct and tintinnii over than 50 percent. The purpose of this paper is analyzing Doppler shift effect at viaduct and plain by using modified path loss model. Simulation results show that proposed path loss model considering estimated Doppler shift coincides with the free space loss model.

Analysis and suppression of phase noise and IQ imbalance for dual-band MIMO-OFDM

Carrier aggregation (CA) schemes and multiple antenna techniques are used for the orthogonal frequency division multiplexing (OFDM) in order to highly improve the communication capacity in LTE-advanced. In order to implement the multi-band system based on multiple-input multiple-output (MIMO) technique, a wideband frequency synthesizer is required. But, when using broadband frequency synthesizer, serious phase noise problem occurs at the transceiver. In addition, when using MIMO scheme, IQ imbalance is a critical impairment as it causes inter carrier interferences (ICI), that results in severe degradation of the receive performance. Therefore, a joint phase noise and IQ imbalance compensation technique is needed at the receiver for LTE-advanced system with CA and MIMO. In this article, we analyze the effects of the phase noise and IQ imbalance in dual band MIMO-OFDM system, and propose a novel compensation scheme in order to enhance the system performance. From the simulation results, it can be seen that phase noise and IQ imbalance cause serious performance degradation of dual-band MIMO-OFDM system, and the proposed schemes has significantly improved BER performance by concurrent suppression of ICI due to phase noise and IQ imbalance.

SNR Estimation in OFDM System by the Correlation of Decision Feedback Signal

In the channel-varying environment, it is very important to estimate the signal to noise ratio (SNR) of received signal and to transmit the signal effectively for the modern communication system. The performance of existing non-data-aided SNR estimation methods are substantially degraded for high level modulation scheme such as M-ary amplitude and phase shift keying or quadrature amplitude modulation. In this paper, we propose a SNR estimation method which uses zero point auto-correlation of received signal per block and auto/cross-correlation of decision feedback signal in orthogonal frequency division multiplexing (OFDM) system. Proposed method can be studied into two types; Type 1 can estimate SNR by zero point auto-correlation of decision feedback signal based on the second moment property. Type 2 uses both zero point auto-correlation and cross-correlation based on the fourth moment property. In block-by-block reception of OFDM system, these two SNR estimation methods can be possible for the practical implementation due to correlation based the estimation method and they show more stable estimation performance than the previous SNR estimation methods. Also, we mathematically derive the SNR estimation expression according to computational difference of auto/cross-correlation. Finally, Monte Carlo simulations are used to verify the proposed method.

Analysis and suppression of effects of CFO and phase noise in WFMT modulation for vehicular communications

We investigate the WFMT (wavelet filtered multitone) modulation scheme for vehicular system and describe the effect of the carrier frequency offset and phase noise on the performance of this scheme. WFMT modulation scheme is based on the wavelet theory and complex filter banks for synthesis and analysis of multi-channel signal. WFMT modulation scheme keeps on the advantage of filter banks system and decreases the system complexity and can be easily possible for the implementation of filter banks. In this paper, we compare ISI (inter symbol interference) and ICI distortions of the WFMT and vehicular OFDM system due to the CFO (carrier frequency offset) and phase noise. And we analyze the PAPR performance of these systems and show the BER performance curve in the HPA and ICI situation owing to the Doppler frequency shift and the frequency offset. Also, the performance of WFMT system versus vehicular OFDM is calculated to find the SNR degradation on DMT (discrete multitone)/OFDM and WFMT system with number of FFT points in the presence of ICI, ISI, phase noise, PAPR and Doppler Effect. In the simulation results, the PAPR performance of WFMT system becomes worse because of characteristic of wavelet component coefficients.

Wireless multimedia acoustic transmission with MIMO-OFDM

Acoustic communications are able to transmit multimedia data and sound simultaneously. But, there is a problem about low data throughput because of narrow frequency band of audible sound in acoustic communication system In this paper, we propose an acoustic multimedia data transmission system with multiple-input multiple-output orthogonal frequency division multiplexing (MIMO-OFDM) for high data rate in acoustic communication system. We compare the receive performance of the proposed system with the existing system with single-input single-output (SISO) by experiment. The data rates of SISO and MIMO system are 4.9 kb/s and 9.1kb/s respectively, with QPSK modulation, no channel coding, and about 3.5 KHz data frequency bandwidth. We ensure that the data throughput of the proposed system is increasing approximately 80% than SISO system. But the proposed systems need more Pilot SNR about 2dB than SISO system to satisfy the BER performance of 10e-3. In other words, we can dramatically increase data throughput of acoustic communication system with MIMO but receive performance is a little decrease.

Performance Analysis of Digital Retrodirective Array Antenna System in Presence of Frequency Offset

In this paper, we design and analyze a digital retrodirective array antenna (RDA) system based on bandpass sampling for wireless communication. The proposed system has low power consumption thanks to increased signal to interference noise ratio (SINR) because digital RDA can automatically make beam toward source with no information about the direction of incoming signal. Also, this paper presents a robust communications system to frequency offset due to digital PLL. Digital PLL can automatically compensate for frequency offset which occurs because of different frequency between transceivers. Simulation results show that the proposed scheme has better BER performance about 5 dB than that of without phase conjugation when the array elements are three.

Chaotic Wireless Communication System Using Retrodirective Array Antenna for Advanced High Security

In this paper, we propose chaotic wireless communication system using digital retrodirective array antenna (RDA) for improving security and receive performance at receiver. Chaotic modulation schemes have studied for reducing the probability of interception and interrupt. As a result, chaotic communication systems have enhanced security. But the receive performance at receiver is degraded. We propose digital RDA system based on chaotic modulation for improving the reception performance while maintaining the security. Simulation results show that to overcome degradation of reception performance in CDSK, digital RDA with array elements up to five is required compared to BPSK modulation scheme.