Shin'ichi Tachikawa

Bandwidth efficiency improvement for multi-carrier systems

A novel multi-carrier transmission system named high compaction multi-carrier modulation (HC-MCM) is proposed to improve the spectral efficiency of OFDM. The HC-MCM can arbitrarily overlap the spectra of its subcarriers with an operation for the transmitting signal, whereby a fast transmission rate can easily be achieved. A DFT-based transmitter and receiver, spectral efficiency, the minimum Euclidean distance, and bit-error rate (BER) characteristics are elaborated. As a result, it is shown that a remarkable improvement in spectral efficiency can be obtained by the HC-MCM as the number of subcarriers increases. It is also shown that subcarrier spectra overlapping is sensitive to the minimum Euclidean distance, but large enough in comparison with OFDM under the condition that the transmission rate and the frequency spacing of subcarriers are identical to those in OFDM.

Asynchronous, Decentralized DS-CDMA Using Feedback-Controlled Spreading Sequences for Time-Dispersive Channels

We propose a novel asynchronous direct-sequence codedivision multiple access (DS-CDMA) using feedback-controlled spreading sequences (FCSSs) (FCSS/DS-CDMA). At the receiver of FCSS/DS-CDMA, the code-orthogonalizing filter (COF) produces a spreading sequence, and the receiver returns the spreading sequence to the transmitter. Then the transmitter uses the spreading sequence as its updated version. The performance of FCSS/DS-CDMA is evaluated over time-dispersive channels. The results indicate that FCSS/DS-CDMA greatly suppresses both the intersymbol interference (ISI) and multiple access interference (MAI) over time-invariant channels. FCSS/DS-CDMA is applicable to the decentralized multiple access.

Studies on Fundamental Properties of MMFSK Systems

To compare the transmission system suitability for power-line communications the spectral efficiency and the required transmission power are two important fundamental factors. In this paper these two factors are studied for our proposed MMFSK (M-ary multilevel FSK) system. Spectral efficiency eta of MMFSK system is a function of number of frequency levels n, number of time chips L, number of hopping patterns h. An equation connecting eta and n, L, h is derived and quantitative relationships between these parameters are studied. As a comparison spectral efficiency of existing systems such as OFDM, MFSK, multilevel FSK are also shown. Results show that compared to ordinary OFDM systems one tenth of spectral efficiency can be achieved by the proposed system which may not be attained by the other systems. The required transmission power for a fixed error rate directly relates to the radiation power. To obtain the required transmission power, relation between error rates and signal to noise ratio should be clarified. Transmission feature between the output of the transmitter and the input of the receiver for MMFSK is same as that for MFSK and the symbol error probability at the receiver input is same. Difference in bit error rate comes from difference of symbol to bit transformation process. A consideration on the relation between bit error probability and symbol error probability in MMFSK is given and a formula representing worst case probability of bit error is derived. A comparison between the worst case bit error probability and the bit error probability obtained by a simulation is shown. Using these results it is cleared that the radiation power of MMFSK is 20~50 times smaller than that of OFDM.

A Combination of Adaptive Equalizer and LMS-RAKE Combining Scheme for DS-UWB System

This paper proposes a combination of adaptive equalizer and Least Mean Square-RAKE (LMS-RAKE) combining scheme receiver system for Direct Sequence-Ultra Wideband (DS-UWB) multipath channel model. The main purpose of the proposed system is to overcome the performance degradation for UWB transmission due to the occurrence of Inter-Symbol Interference (ISI) during high speed transmission of ultra short pulses in a multipath channel. The proposed system improves the system performance by mitigating the multipath effect using LMS-RAKE receiver and suppressing the ISI effect with the adaptive equalizer. Simulation results verify that significant equalization gain can be obtained by the proposed system especially in UWB multipath channel models such as channel CM3 and channel CM4 that suffered severe ISI effect.

Error Rate Simulation for a MMFSK System Using Even-Odd Discrimination Method

Error rate simulations under AWGN environment are made for a MMFSK (M-ary multilevel FSK) system, which use a novel hopping pattern synthesis method named even-odd discrimination method. There can be found great many number of systems for a MMFSK system using even-odd discrimination method depending on the system parameters such as number of chips, number of levels, and the selection of basic hopping pattern configurations however in this paper as a typical example 6 chips, 16 levels, and 8 hopping patterns for each group A and B are assumed. Through the course of computer simulations comparisons are made between direct input and soft decision, hard decision methods results show the effectiveness of these decision methods. Results of error rate simulation show the effectiveness of proposed system

FH Pattern Likelihood Ratio Decision Method against CW Jamming or/and Burst Noise

This paper proposes an improvement method of a lump likelihood ratio calculation in detection of frequency hopping pattern. In a conventional lump likelihood calculation, a lump likelihood ratio is calculated from both time-axis and frequency-axis together. However, the conventional calculation is not enough against CW jamming or burst noise because it adopts a probability density function of AWGN and signal gain of a frequency selective fading channel. In this paper, a calculation using a probability density function of CW jamming or/and burst noise is proposed. Then the comparisons with the conventional lump likelihood calculation and the proposed calculation, and its effects are shown

Characteristics of OFDM/CDMA System Using Frequency and Time Domain Spreading

This paper presents BER performances of OFDM/CDMA using combined frequency and time domain spreading (abbreviated as FT-CDMA). In the system, we employ a different frequency interleaver for each user and adaptive filters to suppress multiple access interference in an asynchronous multiple access system. In conventional MC-CDMA and MC-DS/CDMA systems, the improvement of BER performance can be obtained from only frequency or time domain however, in the FT-CDMA, it can be obtained from both domains. The performance are evaluated in a multi-path fading channel (a close 6-path model). As a result of computer simulations, it is shown BER performance of FT-CDMA is superior to that of the others in the channel

Comparison of MMFSK and MMFSK-mc Systems using Limiters in a High Frequency Power Line

This paper presents BER performances of FH/M-ary multilevel FSK (abbr. MMFSK) and MMFSK multi-tone combination (abbr. MMFSK-mc) systems using limiters, and compares their properties in an AWGN channel and in a high frequency power line. The proposed both MMFSK (MMFSK and MMFSK-mc) systems are modified by using a parallel system (abbr. P/MMFSK). As a limiter, a soft clipper is adopted. In a high frequency power line, two models terminated by matched and opened impedance are employed. As a result of computer simulation, it is shown the BER performance of P/MMFSK-mc using limiters has about 1~3[dB] loss compared with that of P/MMFSK, but data rate of P/MMFSK-mc is twice. Also, in the case of a high frequency power line with opened terminals, the BER performances of P/MMFSK-mc employing some modifications are superior than that of P/MMFSK

A modified acquisition method with code-orthogonalizing filters in asynchronous DS/CDMA

We present synchronous acquisition methods using code-orthogonalizing filters (COFs) in asynchronous direct sequence (DS)/CDMA. In the systems, the tap coefficients of the COF are controlled to orthogonalize to all users sequences including the desired users spreading sequence. Several probable synchronous phases are selected by using the difference of correlation function value between the tap coefficients and the desired users sequence for each phase. Then, the most probable synchronous phase is decided from these phases by a previous proposed precise acquisition system. By computer simulations, we show the improvements of the average acquisition time under several conditions.

A novel interference suppression method using DFT in DS/CDMA communication systems

This paper proposes a novel interference suppression method using discrete Fourier Transform (DFT) in DS/CDMA communication systems. After despreading process, a desired signal has only DC component, while, the other multiple access interference (MAI) has many high frequency components. By using these components, MAI can be suppressed. The suppression effect is evaluated by improvement of bit error rate (BER).