Digitalization of Japan's Terrestrial Broadcasting

Japan's frequency congestion: 50 times greater than the United States

Figure 1. Concept of MFN and SFN

Terrestrial television broadcasting in Japan has been considered as a basic medium closely related to people's lives. Presently in Japan, analog terrestrial broadcasting is available in almost the entire country. Analog terrestrial broadcasting utilizes MFN (Multi-Frequency Network), a transmission scheme that uses a different transmitting frequency in each service area (Figure 1). MFN requires the use of many transmitting stations to reach a national audience due to radio wave interference in each area covered by multiple radio signals. Approximately 15,000 transmitting stations were established throughout Japan and almost all of the available frequencies for television broadcasting have to be allotted to analog broadcasting services. The degree of frequency congestion in Japan is approximately 50 times that of the United States and twice that of Britain (estimated by the number of broadcasting stations compared to landmass). In terms of sound broadcasting, the frequency bands allocated to analog AM/FM broadcasting are in full use, making it difficult to spare channels for digital sound broadcasting.

OFDM OFDM is one of the digital data radio transmission systems that operate by compiling several groups of digital data (comprised of 1s and 0s), and then sending each group out as one signal. OFDM inserts a lengthy signal partition called a "guard interval"* between each signal. A superior characteristic of this arrangement is the elimination of multipath signal interference (Figure 2). This characteristic gives the system strong mobile reception capability, allowing possible future application to TV and sound service for train/subway/bus/taxi passengers or for an ITS (Intelligent Transportation System). An SFN (Single Frequency Network) is also technically feasible. *Guard interval: When a receiver detects synchronized signals from multiple transmission facilities, the varying distances between the receiver and each broadcasting facility produce time differences among the identical signals reaching the receiver, creating obstructive radio waves that prevent clear reception. In contrast, by using OFDM, the same types of signals can be properly received during a detection period as long as any delay is within the specified guard interval.

Figure 2. Relationship between OFDM desired waves and obstructive waves

The ISDB-T television broadcasting scheme is using segmented BST (Band Segmented Transmission)-OFDM (Orthogonal Frequency Division Multiplexing). It can flexibly adjust to terrestrial broadcasting channel environments such as communization with SFN (Single Frequency Networks) and sound broadcasting. OFDM will technically enable repetitive use of a single frequency (SFN) and effective utilization of available frequencies.
Our laboratories have consistently contributed to the system's development from system proposals through field trials, in developing schemes based on a common technology for both digital terrestrial television broadcasting and digital terrestrial sound broadcasting.

In order to achieve a smooth transition to digital broadcasting while efficiently utilizing precious frequency resources and to provide a sufficient period of parallel broadcasting (simultaneous broadcasts) with analog broadcasting, it is important to ensure the establishment of a frequency use plan (Channel Plan) in addition to adopting technology to utilize available frequencies effectively.
The draft Channel Plan for digital terrestrial broadcasting, compiled by the MPT, was released in December 1998, and the Joint Committee Concerning Digital Terrestrial Broadcasting, consisting of the MPT, commercial broadcasters, and NHK, was established in September 1999. The Committee will conduct channel simulations gradually, starting with master stations, progressing to large-scale relay stations, and finally to small-scale relay stations.
On April 26, it completed a report on the digital terrestrial broadcasting master station Channel Plan. The report also covered the expenses involved with analog broadcasting channel changes (the so-called analog-analog conversion) cost, and their implementation.

Preparations are being conducted for the early realization of digital terrestrial broadcasting, technical system field trials, application experiments, and nationwide regional characteristics experiments.