Non-synchronized single frequency network

Unlike in the case of conventional analogue sound broadcasting transmission where the service was provided by a single high power transmitter typically located at the centre of the service area and some possible re-broadcast stations using different channels to resolve the problem areas, a new concept called "distributed transmission" is proposed to provide the required field strength over the entire service area by a number of transmitters operating on the same channel. This concept

provides the best performance when a COFDM modulation scheme is used since it allows for a constructive power addition of echoes produced by these various on-channel transmitters.

One way to implement this concept is by means of a single frequency network (SFN) which is closely related to the use of a regular lattice of synchronized on-channel transmitters. The concept described here is slightly different in that the transmitters need not to be synchronized on the same timing, therefore no parallel transmission infrastructure will be needed to bring the signal to the on- channel repeaters. In particular, this concept is proposed to extend the coverage of a main transmitter by the use of on-channel repeaters fed over the air. It can also be used to tailor the coverage by selec- ting sites where these repeaters are to be located. Because of the reduced extent of coverage for each transmitter and the nature of the propagation, the power needed at each transmitter is much reduced, the availability of the signal at the receiver at such smaller distance can also be improved by a margi- nal increase in transmit power and, further, a certain level of redundancy in receiving the signal from the multiple transmitters allows for a further improvement in signal availability (i.e., network gain).

Normally, the repeater would pick-up the signal off-air and re-transmit it without any delay. Some further adjustment of the delay may be useful to improve the coverage. Using negative delays relative to the propagation delay, such as in the case of a synchronized SFN, means that the signal needs to be brought to the re-transmitter sites through a parallel infrastructure (e.g., satellite, optical fibre, micro-wave links, etc.). Positive delays mean using memory lines at the repeater to delay the signal further after pick-up from off-air.

In the case of active echoes produced by on-channel repeaters and the main transmitter, depending on where the receiver is located, some active echoes can be received either before or after the main signal. In fact, at a specific location, two active echoes can be received at exactly the same power and depending whether the receiver is moved towards one repeater or the other (one can be the main transmitter), one echo will be stronger than the other. Each of these active echoes will also be received with passive echoes generated by the receiver surroundings. The presence of active echoes produced by on-channel repeaters will therefore result, in most locations, in apparently more severe multipath conditions over a wider time window at the receivers and unless the receivers can take advantage of these conditions as in the case of a COFDM modulation, the reception would be made more difficult.

In the case of COFDM, the increase in symbol period to cover for both, the active and passive echoes result in an increase in the number of orthogonal carriers in the channel bandwidth. This increase has three effects: a) the reception becomes more susceptible to degradations caused by the Doppler shift in the case of a moving vehicle (there is a linear relationship between the maximum speed at whichproper reception in a vehicle is possible and the symbol period, and thus the guard interval for a given spectrum efficiency); b) the tolerance on the phase noise of the receiver local oscillator will need to be tighter for proper signal demodulation; and c) the complexity of the real- time FFT used for the multi-carrier demodulation increases (as a function of Nlog2N where N is the

number of carriers).

A proper trade-off needs to be found between the size of the guard interval, and therefore the flexibility in locating the on-channel repeaters up to a given distance from the main transmitter, and the susceptibility of the transmission to the Doppler shift and to the receiver local oscillator phase noise as well as the complexity of the receivers. Such trade-off which involves technical as well as overall system aspects needs to be made before the modulation parameters are set. The computer program described in the following section should be instrumental in such trade-off analysis.

Computer simulations of this concept are included in Annex 1-D, which illustrates the advantages and constraints of using this approach.

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The conventional way to cover a service area, that is a single transmitter usually located at the centre of the area, has been used extensively for conventional sound broadcasting. With the advent of digital modulation schemes to be used in digital sound broadcasting, a new approach, called the "distributed transmission concept", has been developed. This concept is quite effective in decreasing the total transmit power required and can provide a better service availability up to the edge of the service area.

The distributed transmission concept can be used to its best advantage when a COFDM type modulation scheme is used. This scheme allows for the constructive power addition of the active echoes generated by the on-channel re-transmitters and therefore improves the signal availability. Best use of this concept is achieved by the use of omnidirectional antennas at all transmitters but this imposes an additional constraint on the width of the guard interval (typically 142 μsec is required to locate the re-transmitters at 50 km from the main transmitter for an extent of coverage of 70 km radius).

Another important aspect of the distributed transmission concept is the fact that the coverage area can be carefully shaped to reduce the power requirement and also to produce a sharper signal roll-off at the edge of the coverage area. This allows for a reduction of the separation distance between adjacent coverage areas that use the same frequency and therefore an increase in the overall spectrum efficiency.

ANNEX 1-A

Digital audio broadcasting (DAB) system description (Digital System A)

1. Introduction

Digital System A is designed to provide high-quality, multi-service digital radio

broadcasting for reception by vehicular, portable and fixed receivers. It is designed to operate at any frequency up to 3 000 MHz for terrestrial, satellite, hybrid (satellite and terrestrial), and cable broadcast delivery. The system is also designed as a flexible, general-purpose Integrated Services Digital Broadcasting (ISDB) system which can support a wide range of source and channel coding options, sound-programme associated data and independent data services, in conformity with the flexible and broad-ranging service and system requirements given in Recommendations

ITU-R BO.789 and ITU-R BS.774, supported by this Report and Report ITU-R BO.955. The system is a rugged, yet highly spectrum and power-efficient sound and data

broadcasting system. It uses advanced digital techniques to remove redundancy and perceptually irrelevant information from the audio source signal, then it applies closely-controlled redundancy to the transmitted signal for error correction. The transmitted information is then spread in both the frequency and time domains so that a high quality signal is obtained in the receiver, even when working in conditions of severe multipath propagation, whether stationary or mobile. Efficient spectrum utilization is achieved by interleaving multiple programme signals and a special feature of frequency reuse permits broadcasting networks to be extended, virtually without limit, using

additional transmitters all operating on the same radiated frequency.

A conceptual diagram of the emission part of the system is shown in Fig. 17.

Digital System A has been developed by the Eureka 147 (DAB) Consortium and is known as the Eureka DAB System. It has been actively supported by the EBU in view of introducing digital sound-broadcasting services in Europe in 1995. Since 1988, the system has been successfully

demonstrated and extensively tested in Europe, Canada, the United States and in other countries worldwide. In this annex, Digital System A is referred to as "the System". The full system specification will be available as a European Telecommunications Standard.