HuiKyu Lee

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.

ICI mitigation in concurrent multi-band receiver due to the phase noise and IQ imbalance

For the next generation long-term evolution (LTE) advanced mobile communication system, 100 MHz bandwidth and 1 Gbit/s data speed are needed. However, there is not enough and wide vacant frequency band. Therefore, spectrum aggregation method has been studied to extend available frequency bands. Frequency synthesiser and power amplifier of transceiver should cover this wide bandwidth. The phase noise and In-phase and quadrature (IQ) imbalance would increase, which would be a serious problem in this transceiver. Also, signal-to-noise ratio becomes degraded because of nonlinearity and the quantisation noises of the Analog-to-digital conversion (ADC) in the receiver. Uplink of LTE-advanced uses Aggregated DFT-spread (NxDFT-S) orthogonal frequency division multiplexing (OFDM) signals. Since the effect of the phase noise and IQ imbalance are more serious in the multi-band Discrete Fourier transform (DFT)-spreading OFDM system, we like to analyse the effect of inter-carrier interference in frequency domain of receiver and the degradation of bit error rate (BER) performance. Also, by the channel response in frequency domain of the uplink system, we separate phase noise and IQ imbalance effect. Finally, we like to propose a compensation method that estimates the channel exactly and removes IQ imbalance and phase noise. Simulation result shows that the proposed method achieves the 2 dB performance gain of BER = 10−4.

Analysis and Compensation of STO Effects in the Multi-band OFDM Communication System of TDM Reception Method

For the 4th generation mobile communication, LTE-advanced system needs the broad frequency band up to 100MHz for providing the data rate of maximum 1Gpbs. However, it is very difficult to secure the broad frequency band in the current frequency allocation situation. So, carrier aggregation was proposed as the solution, in which several fragmented frequency bands are used at the same time. Basically, multiple parallel receivers are required to get the information data from the different frequency bands but this conventional multi-chain receiver system is very inefficient. Therefore, in this paper, we like to study the single chain system that is able to receive the multi-band signals in a single receiver based on the time division multiplexing (TDM) reception method. This proposed TDM receiver efficiently manage to receive the multi-band signals in time domain and handle the baseband signals with one DSP board. However, the serious distortion could be generated by the sampling timing offset (STO) in the TDM-based system. Therefore, we like to analyze STO effects in the TDM-based system and propose a compensation method using estimated STO. Finally, it is shown by simulation that the proposed method is appropriate for the single chain receiver and show good compensation performance.

An efficient correction of carrier frequency offset and sample timing offset by feedback method

Orthogonal frequency division multiplexing (OFDM) is effective method for transmitting high speed data at frequency selective channel. But OFDM is sensitive to the carrier frequency offset (CFO) and sample timing offset (STO). As these effects, inter-carrier interference (ICI), inter-symbol interference (ISI) and common phase noise error (CPE) occur. And these effects cause degradation of performance. In this paper, we estimate STO and CFO by using block type pilot and then send estimated data to time domain for compensation. Especially, STO is considered as integer and fractional type STO. Each type of STO is analyzed and compensated. And we confirm system performance through simulation. As a result of simulation, we ensure that STO and CFO can be compensated without equalizer through feedback.

Group-oriented multiuser beamforming-OFDM for different QoS requirement

In this paper, we consider the group oriented beamforming taking into account the quality of service (QoS) requirement for multimedia applications in multiuser/multi-rate OFDM system. The aim of this paper is to find the scheme for maximizing the system throughput while guarantee the QoS requirements and fairness for all services. The simulation result indicates that this scheme proposed in this paper has a higher improvement for the system performance and power utilization regarding to BER and throughput.

Compensation of Timing Offset and Frequency Offset in the Multi-Band Receiver with Sub-Sampling Method

Software defined radio(SDR) has a goal that places the analog-to-digital converter(ADC) as near the antenna as possible. But current technique actually can`t do analog-to-digital converting about RF band signals. So one method is studying that samples RF band signals to IF band. One of the ways Sub-Sampling technique can convert signals from RF band to IF band without oscillator. If Sub-Sampling technique is used, over 2 bands can convert signals from RF band to IF band. But due to the filter performance in RF band, it is possible to generate interference between signals that is converted in low frequency band. The effect degrades performance. In this paper, we propose one method that uses time division multiplexing(TDM) method as a solution to avoid interference between signals. By doing TDM and Sub-Sampling at the same time that method can get signals without large changes of structures.

Efficient Estimation and Compensation of CFO and STO in Multi-carrier Communication System

Sample timing offset (STO) and carrier frequency offset (CFO) are caused by inter-symbol interference (ISI), inter-carrier interference (ICI) and phase error in orthogonal frequency division multiplexing (OFDM) system. OFDM characteristic is sensitive about STO and CFO. So when ICI occurs, compensation is hard and complex equalizer is needed. In this paper, we propose an effective correction method using feedback process with pilot and synchronization symbol. After feedback with estimated value in frequency domain, STO and CFO are corrected by control sample & and holder and oscillator. As a result of simulation, we confirm that STO and CFO can be corrected without equalizer through feedback.