суббота, 5 ноября 2011 г.


The trunking system for VHF, Br - 8000, mode GMSK

Author: SergUA6 
Band Width ~9000-1000 Hz
Baud Rate 8000 Hz
ACF  20 ms
RX mode  source I/Q record

pic.1 General view

pic.2 Manipulation speed

pic.3 The graphical spectrum

pic.4 Allocation of the channeles

pic. 5 Phase constellation

 pic.6  Transitions in the constellation

pic.7 The signal's ACF

TETRAPOL - an old enough system of digital trunking communication/connection, which had gotten wide enough spreading. The standard is developed in the mid-eighties the last century. Usage of GMSK modulation with the shaping/forming filter of the Gauss with BT = 0.25 is very interesting. I Will remind, GSM uses the filter of the Gauss with BT = 0.3 and has compact enough spectrum, reduction of this parameter allows to narrow further the spectrum, but as a whole, it does already noticeably reduce interference immunity or restricts/reduces the zone of confident communication. It Was unexpected enough to find out, that on the signals sent by foreign colleagues BT is even less than 0.25, that even more worses interference immunity.

Nevertheless, in TETRAPOL standard it is marked, that it is possible use both channeles as with band 12.5 kHz (most likely it is with BT = 0.25), so in case of need, with a band 10 kHz, and it already implies smaller BT values.

Considering that the standard isn't oriented on commercial networks, and intended for the organization of communication services of public safety, then some lowering of a confident cover zone isn't serious lack according to the developers opinion.