Hiromitsu Wakana

An adaptive power distribution algorithm for improving spectral efficiency in OFDM

In this paper, we propose an adaptive power distribution algorithm which aims at improving the spectral efficiency in orthogonal frequency division multiplexing (OFDM) under the condition that the target bit error rate (BER) should be maintained and the transmit power threshold should not be exceeded. This algorithm is based on the iterative mechanism. In every iteration, the power and the corresponding number of bits are loaded to the subcarrier at which there is a modulation mode demanding the minimum average incremental transmit power per incremental bit to guarantee the pre-designated bit error rate (BER) performance. The iterative procedure terminates once the transmit power threshold is reached. The performance investigations show the proposed adaptive power distribution algorithm always outperforms the scheme of equal power distribution in a large dynamic range of transmit power threshold. Moreover, the performance improvement is very large when the transmit power threshold is required to be low.

Analysis of the Combined Effects of Nonlinear Distortion and Phase Noise on OFDM Systems

In orthogonal frequency-division multiplexing (OFDM) transmission schemes, nonlinear distortion from high power amplifiers and phase noise due to oscillator instability are potentially serious problems, especially in millimeter-wave transmission systems. In this paper, we investigate and compare the combined effects on uncoded and coded OFDM signals of amplifier nonlinearity and phase noise from silicon-based devices. An analytical parametric formulation of system performance is presented. The theoretical results were in complete agreement with the computer simulation results. Total degradation was greatly reduced by applying a suppression scheme to compensate for the constant phase rotation in coded OFDM systems. In addition, the total coding gain defined for systems with different high-level M-QAM modulation was shown to increase with M.

LMS adaptive beamforming based on pre-FFT combining for ultra high-data-rate OFDM system

In this paper, we propose four adaptive beamforming schemes for the ultra high-data-rate orthogonal frequency division multiplexing (OFDM) system which is capable of supporting beyond 100 Mbit/s data rate. These schemes are based on the least mean square (LMS) algorithm and pre-FFT combining. Their differences result from two factors: one is the pilot arrangement in each OFDM block and the other is either the frequency-domain or time-domain pilot vector used in the beamforming. These schemes are compared under the condition that the pilot-to-data ratio is the same. The performance investigation shows that the different schemes present different performance in the convergence behavior and estimation accuracy. Provided that enough time is allowed to guarantee the accomplishment of the antenna weight convergence, all proposed adaptive beamforming schemes can decrease the mean square error (MSE) to very low values. Therefore, their BER performance is very similar.

COMETS 21-GHz advanced satellite broadcasting experiments-evaluation of trellis-coded 8-PSK performance

An advanced satellite broadcasting system using 21 GHz has been studied by using the high-power (200 W) and wideband (120 MHz) transponders on the Communication and Broadcasting Engineering Test Satellite (COMETS), which was launched 21 February 1998. A highly effective trellis-coded eight-phase-shift-keying (TC8PSK) modem was developed for the COMETS satellite broadcasting experiments, and this paper describes the degradation in the bit error performance of the TC8PSK modem due to the nonlinear amplification (AM/AM distortion and AM/PM conversion effects), group-delay distortion, and amplitude-frequency characteristics of the satellite transponder. The results of the COMETS experiments and computer simulations indicate that the performance degradation is due primarily to the nonlinear distortion and the amplitude-frequency characteristics of the traveling wave tube amplifiers in the transponder.

Subband Selection and Handover of OFDMA System over Frequency Selective Channel

Multipath propagation of radio signal introduces frequency selectivity. OFDMA systems greatly suffer from frequency selective fading. It is an important limit factor of performance of OFDMA systems, especially in subband based multiple user access scehems. In this paper, we propose the method of subband selection and handover to improve the system performance over the frequency selective channel. Two subband selection algorithms are presented to accurately select the subband with high channel gain and avoid the channel notch. The random access procedure employing subband selection is presented as an example. The effects of the subband selection are also given. The selection effectively improves the performances of frame synchronization, frequency synchronization, channel estimation, and bit error rate (BER). The investigations show that the proposed scheme is promising to reliable communications over frequency selective fading channel.

Overall Architecture and Traffic Dynamic Control Method in the Satellite/Terrestrial Integrated Mobile Communication System

A hybrid satellite and terrestrial system can achieve robustness even in the case of disastrous situations and has been attracting an attention these days. The STICS (Satellite / Terrestrial Integrated mobile Communication System) is one of the most promising hybrid satellite and terrestrial systems. In this paper, we discuss the enhancement of the STICS. A novel STICS architecture, a trac dynamic channel assignment scheme, and a fast

Performance study on frequency-domain link adaptation in IEEE 802.11a wireless LAN

The link adaptation scheme in the current IEEE 802.11a technical specifications only consider channel variation in the time-domain, and assume all the data subcarriers use the same modulation mode and coding rate once the transmission data rate is determined. However, in the frequency-selective fading scenario, the BER performance is severely degraded by OFDM subcarriers where deep fading occurs. We propose a frequency-domain link adaptation scheme for IEEE 802.11a, which is based on a modified adaptive bit and power loading algorithm and a modified pilot pattern. Simulation results show our proposal can greatly improve performance when perfect channel information is obtainable. We also investigate the performance of the proposed scheme in two channel mismatch situations, and show it is capable of achieving large performance gain in the most favorable environment for IEEE 802.11a, the low mobile speed environment.

Coded frequency-domain link adaptation scheme for OFDM in TDD mode

The link adaptation scheme in current orthogonal frequency division multiplexing (OFDM) systems such as IEEE802.11a only considers the channel variation in the time-domain, and assumes all the data subcarriers use the same modulation mode and transmit power once the data rate is determined. However, in a frequency-selective fading environments, the error probability performance of OFDM is severely degraded by the subcarriers where deep fading occurs. In order to deal with this issue, we propose a coded frequency-domain link adaptation scheme for OFDM in the time division duplex (TDD) mode. This scheme is based on a concatenation of the channel coding and the adaptive bit and power loading. The simulation results show the proposed scheme can significantly improve the bit error rate (BER) performance compared with other schemes.

Combination of adaptive beamforming and adaptive loading for OFDM packet transmission

We propose a combinational scheme of adaptive beamforming and adaptive loading for orthogonal frequency division multiplexing (OFDM). Two adaptive beamforming schemes based on least mean square (LMS) algorithm and pre-FFT combining are used to obtain the space diversity gain. Their difference, which lies in either the frequency-domain or time-domain pilot vector is used in the beamforming. In order to counter the severe frequency-selective fading which is inevitable in broadband wireless communications, we use the frequency-domain adaptive loading to load the bits and transmit power to the individual subcarriers. The performance investigations show that the combinational scheme based on the adaptive beamforming and adaptive loading is very effective in improving the performance of OFDM packet transmission.

A coded frequency-domain link adaptation scheme for OFDM transmission in TDD mode

The link adaptation scheme in the current orthogonal frequency division multiplexing (OFDM) systems such as IEEE802.11a only consider the channel variation in the time-domain, and assume all the data subcarriers use the same modulation mode and transmit power once the data rate is determined. However, in the frequency-selective fading environments, the error probability performance of OFDM is severely degraded by the subcarriers where the deep fading occurs. In order to deal with this issue, we propose a coded frequency-domain link adaptation scheme for OFDM packet transmission in the time division duplex (TDD) mode. This scheme is based on the concatenation of the channel coding and the adaptive bit and power loading. The simulation results show the proposed scheme can significantly improve the packet error rate (PER) performance compared with other schemes. Further investigations also show that it can contribute to large performance gain even in the challenging high-speed environments.