[in Japanese]

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We propose a digital transmission system for satellite broadcasting using the 12-GHz broadcasting satellite service (BSS) band. The system needs to provide high-quality and new digital services, which have not been possible in analog systems. In addition, it should take into account the improvement to the current satellite broadcasting system of the service grade, since the current system already has more than ten million receivers. The new system introduces trellis-coded 8PSK as a main modulation scheme to improve the spectrum efficiency for transmitting multi-channel HDTV and TMCC (Transmission Multiplex Configuration Control) signals so as to flexibly control the modulation scheme, It also introduces a hierarchical transmission to secure high service availability. Some necessary gate arrays for a suitable demodulator for the system have been developed.

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A lot of research has been done on digital terrestrial television systems using orthogonal frequency division multiplexing (OFDM). One of important points in achieving a robust system against noise and disturbance is an error-correcting code and its decoding technique. A punctured convolutional code with the constraint length of 7 has already been chosen as the inner error-correcting code of the European digital terrestrial TV system. This paper discusses the basic characteristics of the code when it is combined with QAM or DAPSK systems under additive white Gaussian noise (AWGN) channels. The bit error rates (BERs) of the hard and soft-decision methods for the QAM and DAPSK systems are compared. A few different types of soft-decision techniques in particular are numerically examined for the DAPSK systems. The results show that the punctured convolutional code attains the coding gains of 2-5 dB for the QAM systems at the BER of 2×10^-4. However, in some cases, the code can only achieve very small or even negative gains for the DAPSK systems at the BER.

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In an OFDM system, it is possible to improve the symbol error rate degradation from the multipath by using an equalizer. However, received power of specific carriers drops down and carrier to noise ratio becomes lower for such carriers when an affection of multipath becomes big. This limits the improvement of the symbol error rate by the equalizer. In this paper, an OFDM system that distributes data symbols to all carriers by a unitary transform is proposed. By distributing data symbols to all carriers, it is possible to recover symbol data even if some carriers are lost by the multipath or because of interference. Good symbol error characteristics against multipath and narrow band interference are confirmed by theoretical analyses and computer simulations.

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Orthogonal frequency division multiplexing (OFDM) is a suitable digital modulation method for terrestrial digital broadcasting systems. However, the phase noise in an OFDM receiver must be reduced, because the phase noise disturbs the orthogonality among the OFDM sub-carriers. This report describes the design of a wideband tunable local oscillator with low phase noise characteristics for OFDM receivers. We have developed a balanced oscillator with an L-C parallel resonance circuit between the transistor emitter ports, which has a wide band frequency range of 1.15-2.2 GHz and low phase noise performances of-66 dBc/Hz (1-kHz offset) or less. We also calculated the SNR at the output of the tuner of OFDM sub-carriers by using this balanced oscillator, and the results show that the SNRs of 33dB or more are achievable even for the 1-kHz sub-carrier spacing system.

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Terrestrial television broadcasting stations have been asked to use frequency bands more efficiently because of the shortage of frequencies. Digital transmission systems are therefore being extensively studied, particularly those using OFDM. We have developed an OFDM modem for use in digital terrestrial television broadcasting. Its characteristics are described in this paper. The modem has a function called graceful degradation to compensate for the cliff effect which is commonly seen in digital radio transmission systems. Of several graceful degradation technologies, we chose a method that uses multiple constellations. Computer simulations confirmed that the graceful degradation system made up of different constellations can compensate effectively for an insufficient D/U ratio and an extremely long excess delay time.

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Digital terrestrial television broadcasting has been studied for new TV services and efficient use of frequency. Its modulation method should be orthogonal frequency division multiplexing, because this method is robust against multipath interference. The method also enables single frequency network. This paper describes a theoretical study of bit error rate degradation by multipath waves. The result of experimental measurements are given. Bit error rate degradation in mobile reception under a single frequency network is also discussed.

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The Cable Television Conference working group conducted experiments on the 64 QAM signal transmission in a CATV system from 1995 to 1996. These experiments were based on the tentative specifications of the Telecommunications Technology Council of the Ministry of Posts and Telecommunications (MPT), to promote standardization of the Japanese digital CATV system. Experimental results showed that a C/N of more than 26 dB, a Composite Triple Beat (CTB) of less than-43 dB, and a single carrier interference of less than-30 dB were required to keep a BER of less than 1×10^-4 without forward error correction. Cable reflection within the conventional CATV standard for NTSC-AM signals was low enough for 64 QAM signals. Experiments on adjacent channel interference between 64 QAM and NTSC-AM signals indicated that the 64 QAM signal level needed to be lower than the NTSC-AM signal level by about 4-20 dB. Based upon these results, the Japanese standard was introduced by MPT on December 3, 1996.

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A hierarchical broadcasting system divides the digital data into several layers and transmits them with different reliability according to each layer's priority. Hierarchical broadcasting is effective for mobile receivers in which the channel quality is degraded by multi-path fading. This paper proposes a simple mobile receiving scheme to get only high priority data using simple differential detection. The differential detection not only simplifies the system, but also mitigates the degradation due to fading fluctuation. This paper also clarifies the degradation of system performance caused by simplification of the fading compensation scheme.

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In multi-channel cable transmission systems, the non-linearity of a laser diode or an amplifier causes a number of intermodulation (IM) distortions (CSO, CTB, etc). IM distortions and thermal noise impair the digital signal in cable transmission and degrade the BER of digital channels. In order to find out the relationship between the IM distortions and the BER, the authors measured the interference's amplitude distribution in a channel. Experimental results show that the amplitude distribution of interference, including the IM distortion and thermal noise, differs from a Gaussian distribution. The distributions depend not only on its power but also on the kind of IM (CSO or CTB), the number of components, and the CN ratio. We performed a computer simulation of the experimental setup. The results of the simulation matched well with the measured results from the experiment.

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This paper reports a new scheme of carrier recovery for multilevel QAM. First, we investigate the conventional LMS algorithm with the error estimation function. To achieve quick carrier recovery and simplify the circuit, apply the conventional LMS algorithm not only to the phase domain but also to the frequency domain. By defining the estimated frequency error to be the variation of the coefficient of phase compensation in the phase domain, we introduce the new algorithm. We apply this new algorithm to the carrier recovery circuit of a 64 QAM demodulator in an offset frequency condition. The computer simulation results show that the performance of the new algorithm is superior to that of the conventional LMS algorithm in terms of the recovery time and the enhanced range of the offset correction up to ± 1/2 symbol frequency.

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A digital microwave OB link system has been developed for use in program production, such as in broadcasting and in news material transmission. The system employs multi-level digital FM modulation, which allows conventional analog equipment to be used as the digital link by adding a digital interface unit. It provides spectrum compatibility, i.e., transmission bandwidth, adjacent channel protection ratios, etc., with conventional analog systems. Therefore, it can serve in the analog-digital mixed environment during the analog-to-digital transition period. An eight-level system was developed and examined in laboratory and field tests. The results showed that the system can transmit a bit rate of 45 Mbps over a conventional microwave channel of 17 MHz bandwidth.

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The OFDM (Orthogonal Frequency Division Multiplexing) modulation scheme is robust under multipath fading conditions. An experimental OFDM modem using the Differential Quadrature Phase Shift Keying (DQPSK) modulation scheme for each carrier has been developed to enable stable transmission of high-rate digital sources from moving vehicles. As one of the promising applications of the DQPSK-OFDM modulation scheme, a digital Field Pick-up Unit (FPU) is expected to be efficient in the case of mobile transmission. This paper describes the transmission characteristics of the DQPSK-OFDM modem in an urban area which transmits digital sources from vehicles in the UHF 800-MHz band, and also discusses the error protection scheme, the guard interval length, and the carrier spacing of the DQPSK-OFDM modem for mobile transmission.

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This paper describes the desired performance for digital microwave links (Field Pick-up Units in Japan) in the 7-GHz band and the simulated performance of an adaptive equalizer used for a digital multilevel QAM System. A requirement for the digital microwave links is that they must exceed analog systems in their performance, but must have reduced hardware complexity. The desired performance for the digital microwave links is : Echo DUR=10 dB and CNR=27 dB for standard transmission. However, echo DUR=0 dB with its delay time less than or equal to 30-50 ns for long-distant transmission.In addition, introduction of the reference signal makes it possible for a decision feedback equalizer to start its operation in the DUR=0 dB multipath environment.

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Various methods have been proposed for motion compensation for moving pictures. Each method has advantages and disadvantages regarding performance, but they have not necessarily been discussed under fair conditions. In this paper, we investigate four representative motion compensation methods : block matching, motion compensation using affine transformation, warping prediction, and global motion compensation. We evaluated the performance of these methods by applying MPEG-2 for the input of common natural video sequences. We show that motion compensation methods which have been proposed to improve the performance are not superior to the conventional block matching when they are used in conjunction with MPEG-2.

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Since picture quality in digital coding depends heavily on such picture characteristics as spatial detail and motion, statistical analysis is required to estimate picture quality in digital broadcasting systems. This paper proposes a new definition of “criticality”, a quantitative measure of difficulty for MPEG-2 video coding, to analyze the picture quality of television programs statistically. Equipment is developed to measure criticality and the relationship between criticality values and assessment test scores is examined for test sequences. The results show that the definition of criticality provides sufficient accuracy to estimate picture quality for a large number of pictures statistically.

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Recent years have seen the development of methods for motion-compensated television standards conversion (MC-TSC) in which motion information in the picture is utilized for conversion between TV standards that have different numbers of scanning lines and fields. Motion information used for standards conversion should contain true movement, which is not always a requirement in motion-compensated predictive coding. In conventional motion estimation methods for MC-TSC, however, the trueness has been insufficient, which has led to degradation of the converted picture. The authors therefore have improved the motion estimation scheme, utilizing a DOG (Difference of Gaussian) preprocessing filter for motion estimation, the iterative gradient method with improved high-precision, and the AOSF (Absolute Order Statistics Filter) for post-processing of motion vectors. A new high-quality motion-compensated television standards converter has been developed adopting the improved motion estimation scheme. This converter now makes it possible to greatly suppress degradation of the converted picture which has been difficult up until now.

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