Myung-Soon Kim

Design and prototype development of MIMO-OFDM for next generation wireless LAN

Multiple input multiple output-orthogonal frequency division multiplexing (MIMO-OFDM) is regarded as a promising technology enabling higher data rate wireless communications in frequency selective fading channels. This paper proposes the next generation wireless local area network (WLAN) physical (PHY) layer transmission technology using dual-band and MIMO-OFDM schemes and discusses test results of MIMO-OFDM prototype system implemented with field programmable gate arrays (FPGA). This design targets over 200 Mbps of the maximum PHY data rate in 40 MHz bandwidth and compatibility with the legacy IEEE 802.11a. For a cost-effective implementation and improved packet error rate (PER) performance, 2 transmit and 3 receive antennas and 40 MHz channel are used to achieve the desired PER and throughput performance in the 5 GHz band.

R2²SDF FFT Implementation with Coefficient Memory Reduction Scheme

Fast Fourier transform (FFT) is a key building block for orthogonal frequency division multiplexing (OFDM) systems. Due to the development of wireless portable devices, it is important to minimize the size and power of a FFT processor. One of the methods to satisfy such demands is reducing the size of twiddle coefficient memory. This paper presents an effective coefficient memory reduction scheme for a R22SDF FFT implementation. When applying a conventional method to an N- point R22SDF FFT, the number of twiddle coefficients is 3N/4. However, the proposed scheme requires only (N/8+1) coefficients and its additional hardware architecture is very simple. The effectiveness of the proposed method is verified by implementation results on a FPGA.

Transmit antenna selection for MIMO systems with V-BLAST type detection

In an attempt to improve the spectral efficiency of wireless communication systems, MIMO spatial multiplexing architectures have been introduced. V-BLAST is proposed as an efficient receiver structure in terms of performance and computational complexity. We introduce a transmit antenna selection scheme for a V-BLAST type receiver with an assumption that channel information is available at the transmit side. With a given MIMO channel matrix, we derive a simplified channel capacity and vector symbol error probability in V-BLAST. Using these results, the paper provides the optimal criteria for transmit antenna selection in terms of capacity and error probability. These criteria are based on tap weight vectors in V-BLAST detection. Additionally, we propose sub-optimal methods that achieve near-optimal performance with low computational load. To verify the efficiency of the proposed schemes, computer simulation results are presented.

Equalization scheme for OFDM systems in long delay spread channels

A decision feedback equalizer (DFE) type of equalization scheme is proposed to mitigate the effect of delay spread for orthogonal frequency division multiplexing (OFDM) systems. The performance degradation is severe in case that the delay spread of wireless channels is longer than a cyclic prefix (CP) due to the effect of inter-symbol interference (ISI) and inter-carrier interference (ICI). The effect of ICI and ISI is analytically described in matrix equations which includes signal-to-noise ratio (SNR) degradation for a given channel condition. The proposed scheme improves the performance using decision feedback mechanism without increasing the length of CP, which preserves the system efficiency for the OFDM.

MIMO detection methods considering AGC effects

Multiple input multiple output (MIMO) detection methods are derived with an assumption that the noise power of all receiving paths are equal. This assumption is not valid in practical systems performing automatic gain control (AGC) which adjusts the signal level to the dynamic range of analog to digital converters (ADC). If the independent AGC scheme with the different gain for each receiving path to minimize the quantization noise is used, the output noise of AGC is not white in the spatial domain, i.e. the noise variances of receiving paths are not identical. Therefore, this spatially colored noise effects should be taken into consideration for the optimal detection.

Channel estimation and equalization for high speed mobile OFDM systems

In this paper, we propose new channel estimation and equalization techniques for OFDM (orthogonal frequency division multiplexing) systems in mobile environment. In the mobile OFDM system, the estimation of time varying channel is the most important issue. In the proposed method, we assume that wireless channels are varying in the linear fusion during a certain internal, and estimate the initial channel coefficients and their corresponding slopes. After such estimation, we compensate the estimated channel and ICI (inter-carrier interference) effect due to time selectivity. We verify that this scheme shows better performance than conventional methods using computer simulations.

Optimal detection considering AGC effects in multiple antenna systems

Most detection methods for multiple input multiple output (MIMO) systems are derived with an assumption that the noise from multiple receive paths is white. However, This assumption cannot be valid according to automatic gain control (AGC) schemes. If the independent AGC scheme which has difference gain for each receive path to minimize the quantization noise is employed, the AGC output noise is not white any more. Therefore, the colored noise effects due to AGC should be considered for the optimal detection. In detail, such colored noise effects should be included in the maximum likelihood (ML) detection criterion and the log likelihood ratio (LLR) calculation.

Implementation of ZigBee-VLC system to support light control network configuration

In this paper, ZigBee-VLC Transmitter and Receiver are designed, implemented and tested. By utilizing the ZigBee-VLC Transmitter and Receiver, commissioning procedures for light control network configuration are simplified and commissioning time is drastically reduced. With this configuration, lighting control network configured to use a maximum of 216 lighting is possible. As a result of this research, the transmitter is complete with ZigBee-VLC features implemented in the Single MCU without rising production costs and the 1-board solution receiver including a ZigBee and VLC functions are implemented. In addition, as a result of the test work using the light control app, dramatically shortening commissioning time, easy lighting control is possible was confirmed.

Multiple-Decision-Feedback Detection for STBC over Time-Selective Fading Channels

This paper introduces an efficient multiple-decision- feedback (MDF) detection for space time block code (STBC) over time-selective fading channels. The MDF detection performs several decision-feedback (DF) detections for the most probable points obtained by a log-likelihood ratio (LLR) based bit flipping scheme. Simulation results show that, with reasonable complexity, the MDF detection provides near maximum likelihood (ML) performance by achieving the considerable SNR gain, compared with the existing DF detections.