Digital transmission signals are distorted by multi-path fading in a mobile radio channel. This is a severe problem, especially when a digital field pick-up unit transmits high-bit-rate data such as HDTV digital signals. The OFDM (orthogonal-frequency-division-multiplexing) modulation scheme mitigates the adverse effects of multi-path fading by distributing the transmission data into a large number of carriers with low-bit-rate modulation. To reduce the degradation due to multi-path fading in high-bit-rate transmission, we have developed an experimental OFDM modem that uses multi-mode modulation : 16/32 DAPSK (differential amplitude phase shift keying). Transmission experiments showed that the a 16/32 DAPSK-OFDM modem can transmit data at 23.7 Mbps by 16 DAPSK modulation and at 29.7 Mbps by 32 DAPSK modulation within a bandwidth of 13.5 MHz in a frequency-selective Rayleigh fading channel.

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An OFDM signal is easily degraded by amplifier nonlinearity. A method for measuring such nonlinearity has been developed and tested in two ways : vector measurement using the two-tone method and comparison between a sinc-shaped input signal and the output signal. Four amplifier characteristics were measured with the methods and represented as AM-AM/AM-PM conversion characteristics. Using these results, computer simulation was run to determine the effect of nonlinearity. OFDM signals were generated and the CN ratios were measured. The results agreed with those of the experiments. Testing of the predistortion in the amplifier with the most remarkable AM-PM characteristic confirmed the simulated predistortion.

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We have studied frequency-and time-domain equalization techniques under fading conditions for the European DVB-T OFDM system. We first describe a frequency-domain equalizer using simplified linear interpolation in the symbol direction that estimates the channel response of receiving-data carriers by using the pilot-carrier channel response. We calculated the signal-to-interference ratio (SIR) and the bit error rate (BER) over one-path and two-path Rayleigh-fading channels. We found that an SIR of about 20 dB and a BER of less than 1.5 × 10^-3 (QPSK and 16 QAM) are obtained even for a maximum doppler frequency of 50 Hz in a 2-k FFT system. We next describe a simplified time-domain equalizer using step interpolation over the OFDM useful period. A simulated BER of 2 × 10^-3 (QPSK) over one-path Rayleigh-fading channels (fd =50 Hz) was attained for an 8-k FFT system, when the new time-domain and frequency-domain equalizer techniques were applied.

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Reception characteristics of OFDM signals using array antennas have been analyzed. In OFDM systems, equalizers are used to compensate for the amplitudes and phases of received symbols distorted by multipath propagation. For simplicity, DQPSK modulation, which does not require equalization, is also used. However, in single-antenna systems, degradation of the signal-to-noise ratio of specific carriers due to frequency selective fading can not be compensated for. Array antennas were used to improve the characteristics of carriers whose signal-to-noise ratio was degraded by multipath propagation. To achieve an optimal combination of signals from each antenna, the channel transfer functions were estimated by using scattered pilot symbols. The bit error rate characteristics for DQPSK, PSK and 16QAM were examined using computer simulation and were found to be much better than those of single-antenna receivers.

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The draft standard for Digital Terrestrial Television Broadcasting (DTTB) was approved in the Telecommunication and Technology Council (TTC) in September 1997. Practical field trials using Tokyo tower were carried out from November 1998 to March 1999. The transmission scheme of DTTB was fixed in May 1999. This paper describes the fixed reception performance of the practical field trials using one transmitter and two SFN transmitters. The results using one transmitter show that a 35.5 dB (pV) receiver input level is needed for a correct reception rate of 99% at all measured points in the case of 64 QAM-OFDM modulation with a 7/8 coding rate. In the SFN field trials using two transmitters, preferable results are obtained when the transmitters have optimum transmission timing. But if the OFDM demodulator can not adjust the FFT window position, there will be measuring points at which correct reception can not be achieved.

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The draft standard for the Digital Terrestrial Television Broadcasting (DTTB) system of Japan using Orthogonal Frequency Division Multiplexing (OFDM) digital transmission was approved in 1998. DTTB services are provided along with analog television services, so that DTTB signals can be interfered with by analog signals.Various ratios for protecting DTTB signals against analog television signals, have been tested. The results show that co-channel protection ratios of 25 dB, and adjacent-channel protection ratios of no more than-20 dB are required when the DTTB signals are modulated by 64 QAM-OFDM and convolutionally coded at a rate of 7/8. The BER performance for co-channel interference was improved by using soft decision decoding with erasure.

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[in Japanese]

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[in Japanese]

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[in Japanese]

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The aim of this research is to locate a face in an input image, then extract the contour lines of the face and the facial features, such as the eyes, mouth, eyebrows and nose in a standard office environment. In this paper we describe a system for extracting the facial features required to reconstruct a 3D model of a face from a 2D image. The technique for extracting the eye and mouth contour lines is based on the combination of a quadratic approximation of the edges and an active contour model. The technique for face outline contour line relys on the hybridization of a symmetrical active contour model and an elastic contour model with a standard facial shape model. We applied this system to 46 faces and compared the results with manually detected contours : 90% of the eye and mouth contours and 80% of the jaw contours and pupil positions matched the hand-prepared contours closely.

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A neural network canceller has been applied to the PR4ML channel with partial erasure. First, a model of the read/write channel with partial erasure and the neural network canceller are described. Then, the bit-error rate of the PR4ML channel is evaluated by computer simulation and the performance is compared with that of a conventional neural network equalizer and a Nyquist equalizer. The results show that the neural network canceller exhibits better performance than the conventional neural network equalizer and Nyquist equalizer.

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Human walking and posture are determined by joint movement, which is controlled by muscle force. Muscle force working as an internal force is the most important factor determining human movement and posture. In this paper, we describe a system for human walking, which is generated by joint movement based on muscle force. We define a joint muscle force function based on the biomechanical characteristics of muscles (their length and contraction velocity). By using this function, we can control joint movement within the maximum joint force which depends on the joint angular values (velocity, acceleration). we propose a human walking system and a method for controlling the internal and external force based on goal-oriented control, which is based on the periodic characteristic of human walking.

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[in Japanese]

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[in Japanese]

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