Telecommunication system and a main station for use in such a system

Telecommunication systems such as PON-FITL-Systems are known having a TDMA frame structure and comprising a main station and a number of substations. The known systems can comprise apparatus for coarse ranging and for fine ranging. A Pseudo Noise Sequence based coarse ranging technique employs coarse ranging at a fixed position within the upstream multiframe reserved for ranging, preferably for fine ranging. By applying a low power Pseudo Noise Sequence for coarse ranging and sampling the range in the main station only within a fine ranging window the position of which is known to the main station, a more accurate coarse ranging is achieved. Within the fine ranging window normally no data are present, and thus the signal-to-interference ratio is an order of magnitude better than in the other part of the frame.