C.Y. Ahn’s research while affiliated with Yonsei University and other places

Multi-Code CDMA (MC-CDMA) can not only be integrated easily with a conventional CDMA system, but also achieve good spectral efficiency and high processing gain. MC-CDMA requires a linear amplifier due to the increasing value of its peak-to-average power ratio (PAPR) as the number of codes increases. As such, a multi-phased MC-CDMA (MP-CDMA) system is proposed to provide a variable rate service that is not susceptible to the nonlinear characteristics of an amplifier. A clipping technique is used between the output of a multi-code modulator and the input of an MPSK modulator to improve the performance of the MPSK chip demodulator and reduce the system's complexity. System performance is analyzed and compared with the number of codes and clipping levels. The optimum clipping level is also evaluated for the number of codes on both AWGN and flat fading channels.

Transmit adaptive array requires the forward link channel state for evaluating the optimum transmit weight in which a feedback channel transports the forward link channel state to the base station. Since the feedback information limits the transmission rate of the reverse link traffic, it is necessary to keep the number of feedback bits to a minimum. This paper presents a system in which the N transmit antennas are extended to the 2N transmit antennas while the feedback channel is limited as that of N-transmit antenna system. The increased antennas can give additional diversity gain but requires higher rate of feedback bits. The limited feedback channel increases the quantization error of feedback information since the number of feedback bits assigned to each antenna is reduced. In order to overcome the limited rate of feedback channel problem, this paper proposes the transmit antenna selection schemes which can effectively use the limited feedback bits, reduce the computational complexity at the mobile station, and eventually achieve diversity gain. System performances are investigated for the case of N = 4 for the various antenna selection schemes on both flat fading and multi-path fading channels.