Jaewoon Kim

An Effective MIMO–OFDM System for IEEE 802.22 WRAN Channels

We propose a closed-loop multiple-input multiple-output orthogonal frequency-division multiplexing (MIMO-OFDM) transmission scheme appropriate for the IEEE 802.22 wireless regional area network (WRAN) channel environments. The proposed scheme utilizes the Grassmannian beamforming and an antenna selection algorithm to maintain bit-error-rate (BER) performance and to reduce feedback information. To further reduce the feedback information considering the channel property of the IEEE 802.22 WRAN, the proposed scheme employs an extension of the feedback period and subcarrier grouping. Simulation results in the IEEE 802.22 WRAN reference channel models indicate that the proposed MIMO-OFDM transmission scheme achieves superior BER performance and spectral efficiency than open-loop MIMO-OFDM system even considering the feedback overhead of the system.

A two-step search scheme for rapid and reliable UWB signal acquisition in multipath channels

We propose the two-step search scheme with linear search based second step (TSS-LS) by improving the conventional two-step search scheme with bit reversal search based second step (TSS-BS) for rapid and reliable acquisition of ultra wide band (UWB) signals in multipath channels. The proposed TSS-LS utilizes two different thresholds and search windows to achieve fast acquisition. Furthermore, unlike the TSS-BS in which the bit reversal algorithm is applied in both steps, the linear search is adopted for the second step in the proposed TSS-LS to correctly find the starting point in the range of effective delay spread of the multipath channels, and to obtain reliable bit error rate performance of the UWB systems.

A Cognitive Beamforming Scheme for Coexistence of Incumbent and Cognitive Radios

We propose a new cognitive beamforming scheme for coexistence of IR (Incumbent Radio) CR (Cognitive Radio). The proposed cognitive beamforming scheme does not cause any interference to the IRU (IR User), while the CRU (Cognitive Radio User) is served without additional radio resource consumption. Simulation results show that when using the proposed scheme the CRU does not interfere to the IRU and there is little bit error rate performance degradation of the CRU as compared to that of the ideal beamforming system which does not consider the IR priority.

A Robust Companding Scheme against Nonlinear Distortion of High Power Amplifiers in OFDM Systems

We propose a CCP (companding combined with predistorter) scheme that maintains BER (bit error rate) performance, while effectively reducing PAPR (peak-to-average power ratio) of OFDM (orthogonal frequency division multiplexing) signals. The proposed CCP scheme combines the conventional companding scheme proposed by the authors with the predistorter, in order to prevent degradation of BER performance caused by signal distortion from nonlinear property of the HPA (high power amplifier) and the compression property of the companding scheme. Hence, the proposed scheme effectively reduces the PAPR and simultaneously maintains the BER performance independent of nonlinear property of the HPAs. Simulation results show that the proposed scheme provides the lower total degradation for all the modulations, when compared with the conventional companding scheme proposed by Wang and the clipping scheme.

An improved ToA estimation in compressed sensing-based UWB systems

Since the first arrival path may not be the strongest path of UWB (Ultra Wide Band) multipath channels, this makes ToA (Time-of-Arrival) estimation becomes a challengeable issue. Furthermore, because of ultra bandwidth of received signal, CS (Compressed Sensing) theory is employed to reduce the complexity caused by very high Nyquist sampling rate in UWB receivers. In this paper, we propose a ToA estimation scheme provides precise estimation performance, while exploiting the benefits of CS-based UWB receivers. Simulation results show that the proposed scheme can outperform other low complexity schemes in a wide range of signal-to-noise ratios.

An Effective MIMO-OFDM Transmission Scheme for IEEE 802.22 WRAN Systems

In this paper, we propose a closed-loop MIMO-OFDM (Multiple Input Multiple Output-Orthogonal Frequency Division Multiplexing) transmission scheme that is appropriate for the IEEE 802.22 WRAN (Wireless Regional Area Network) channel environments. The proposed scheme utilizes the Grassmannian beamforming and an antenna selection algorithm proposed by the authors to maintain BER (Bit Error Rate) performance and to reduce feedback information. To further reduce the feedback information considering channel property of the IEEE 802.22 WRAN, the proposed scheme employs an extension of the feedback period. Simulation results in the IEEE 802.22 WRAN reference channel models indicate that the proposed MIMO-OFDM transmission scheme achieves superior BER performance and spectral efficiency than open-loop MIMOq-OFDM system even considering the feedback overhead of the system.

An Effective Selective Detection Scheme Based on Pulse Repetition for Noncoherent UWB Systems

We propose a selective detection scheme based on pulse repetition considering the bit-error rate (BER) performance and complexity of noncoherent ultra-wide-band (UWB) systems. To take system complexity into account, the proposed scheme transmits the UWB signals by simple pulse repetition at the transmitter, like conventional pulse repetition coding (PRC). However, to effectively improve the BER performance, the proposed scheme performs selective detection by estimating the signal-to-noise ratio of the received pulse-repeated signals at the UWB receiver. Hence, the proposed scheme effectively improves the BER performance of the noncoherent UWB systems without much increasing system complexity, as compared to the conventional PRC.

A Rapid and Reliable Two-Step Search Scheme for UWB Signal Acquisition in Multipath Channels

We propose the “Two-Step Search scheme with Linear search based Second step (TSS-LS)” by improving the conventional “Two-Step Search scheme with Bit reversal search based Second step (TSS-BS)” for reliable as well as rapid acquisition of Ultra Wide Band (UWB) signals in multipath channels. The proposed TSS-LS utilizes two different thresholds and search windows to achieve fast acquisition. Furthermore, unlike the TSS-BS in which the bit reversal algorithm is applied in both steps, the linear search is adopted for the second step in the proposed TSS-LS to correctly find the starting point in the range of effective delay spread of the multipath channels, and to obtain reliable bit error rate performance of the UWB systems.

Bit error rate improvement scheme for transmitted reference UWB systems

We propose a transceiver structure that can effectively improve BER (Bit Error Rate) performance for TR-UWB (Transmitted Reference-Ultra Wide Band) systems based on impulse radios. Unlike coherent UWB systems that are too complex for practical implementation while having good BER performances, the complexity of the TR-UWB systems is quite low since they transmit data with the corresponding reference signals and demodulate the data through correlation using these received signals. However, the BER performance in the conventional TR-UWB systems is affected by SNR (Signal-to-Noise Ratio) of the reference templates used in the correlator. To this end, we propose a receiver structure that can effectively improve the BER performance by increasing the SNR of reference templates. Simulation results reveal that the proposed scheme achieves significant BER improvement as compared to the conventional TR-UWB systems.

Pulse Repetition Based Selective Detection Scheme for Coherent IR-UWB Systems

We propose a selective detection scheme based on pulse repetition for coherent IR-UWB (Impulse Radio-Ultra Wide Band) systems. For signal modulation and demodulation, we consider bi-phase modulation and on-off keying that are widely used in the coherent IR-UWB systems. The proposed selective detection scheme transmits the UWB signals by pulse repetition at the UWB transmitter to take system complexity into account, like conventional PRC (Pulse Repetition Coding). However, the proposed scheme performs selective detection by estimating the SNR (Signal-to-Noise Ratio) of the received pulse-repeated signal to effectively improve BER (Bit Error Rate) performance at the UWB receiver, unlike the conventional PRC. Simulation results in the IEEE 802.15.4a channel models reveals that the proposed scheme effectively improves BER performance of the coherent IR-UWB systems without much increment of system complexity, as compared to the conventional PRC.