ITU-R REPORT BT.2137 PDF

Introduction

The implementation of DTTB services in parallel with existing analogue services in several countries has created the need to refine some of the traditional computer-based frequency planning techniques to enable a greater degree of accuracy in coverage prediction.

Whereas analogue systems fail rather gracefully, the "cliff-edge" failure characteristics of digital systems can mean that in some situations "holes" in DTTB coverage will result from the various factors that affect signal coverage. These include, but may not be restricted to, propagation characteristics of the bands used for DTTB transmissions, limits imposed on DTTB transmission power in order to protect the existing analogue services, terrain obstruction and man-made clutter.

Clearly the identification of geographic areas where such holes might be expected is important for coverage planning as well as for the receiver retail trade, where clear advice to potential viewers is essential.

It is for these reasons that improved coverage prediction methods have been introduced in a number of countries with considerable success, and that it is considered important that the new methods being developed are studied and documented by ITU with a view to achieving an appropriate degree of standardization worldwide.

This Report provides a brief outline of the results of comparisons between predicted and measured signal levels as reported by some administrations. These results show wide divergences between predicted and measured signal levels in terms of both mean error and standard deviation of errors. While these variations may have been acceptable in analogue television planning, the rapid failure of digital television signals means that a much closer match of predictions with measurements is required. An approach is discussed for predicting received field strength with particular discussion of profile extraction, radial prediction and the use of clutter data to take into account the effect of buildings and trees. Transmitter and population databases are also discussed.

It should be noted that in addition to the ongoing systems work described in this Report, Radiocommunication Working Party 3K is in the process of developing a text on a site-specific propagation model for terrestrial services from about 30 MHz to about 5 000 MHz. This deterministic model will include the effects of terrain features, ground covers and buildings. It will also include location and time variability, and multipath effects. As a first step towards the development of the above text, Working Party 3K is actively evaluating several existing site-specific propagation models.

The purpose of developing the improved prediction models is to produce consistent prediction results between related planning organizations while taking advantage of the availability of terrain and clutter data and improvements in computer power. To obtain this consistency the prediction model must specify the full sequence of processing steps.

Bearing in mind that most new DTTB services will be introduced in parallel with the existing analogue television services, using the existing antenna and down lead, a further point of considerable practical importance is that of providing an accurate model of typical domestic receiver/antenna installations and the impact of losses in this area on the required received field strength. Some initial work on this problem is reported below with the suggestion that typically, the required "implementation margin" may be quite considerable.