11 June 2015

Rohde & Schwarz GM2100 HF modem, "Signal Format"

(ANgazu, I56578)
Besides the well-known standards PSK-8 signals with carrier at 1800 Hz and 2400 Baud, there are variants and proprietary systems developed by several manufacturers either to meet specific needs or to maintain the prestige of the brand proprietary systems.
One of these proprietary signals is the R&S Signal Format developed by Rohde & Schwarz and included in several of its modems. The documentation available is limited, so this short study could fit into some not correct parameters.

Signal was heard using Twente-SDR on 8132 USB, this frequency allows to get the correct ALE 2G center frequency (1625 Hz). The user is the German Police, as clearly visible by decoding the ALE (MIL 188-141A) segment. More likely, the GM2100 "signal format" waveform is also used by the Italian GdF (Guardia di Finanza).

initial ALE segment, link between BPLEZS and BP2

The signal parameters are:

PSK-8 modulation at 2400 Bd
carrier frequency 1500 Hz In this recording)
ACF: 133.33 mS (superframe)

Frame: 26.66 mS (64 symbols, 48 data + 16 sync)
Superframe: 133.33 mS (5 frames)

As shown in the manufacturer's documentation, the preamble consists of three frames of 64 symbols, followed by the data blocks (48 + 16 symbols) and ending in another frame of 64 symbols:
ACF measurement

structure of the superframe
The superframe (due to the legth of the interleaver or scrambler) consists of five frames and originates the 133.33 msec ACF. As said, each frame is composed of 48 unknown + 16 known symbols (48/16). The initial preamble symbols (192) and the end of message symbols (64) are formed by specific sequences.
Feature of the R&S GM2100 HF modem can be seen here:

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