23 August 2015

R.Romania Int. - Tiganesti: a DRM-B waveform

This is a DRM B-version: OFDM 206-tone (+ 1 empty place) 46.8Hz channel step, baudrate 37.5Bd and k=1/4 as reported in the radioscanner.ru page for DRM-A (bottom of the page, values of the k parameter). The official DRM System reference publication is ETSI DRM System specifications

OFDM symbol parameters as measured

A DRM trasmsission super frame (TSF) consists of three channels, or three transmission frames:
  •  FAC, Fast Access Channel: The Fast Access Channel is used by the receiver to obtain information about the OFDM signal properties and the SDC/MSC configuration
  • SDC, Service Description Channel: The SDC contains the information needed for MSC decoding, like the multiplex frame structure, as well as other additional information
  • MSC, Main Service Channel: audio and data stream
Since one DRM frame is 400 mS, the duration of a TSF is always 1.2 seconds. 

According to the ETSI DRM definitions, the OFDM carriers are divided in two groups and indicated as follow:

Some tones within the OFDM transmission frame are modulated with known fixed phases and amplitudes. These care pilot tones for channel estimation and synchronization. The positions, amplitudes and phases of these tonse are carefully chosen to optimize the performance, especially the initial synchronization duration and reliability. These tones are used by the receiver to detect the presence of the received signal and to estimate its frequency offset. They may also be used for channel estimation and various tracking processes. 
In DRM-B there are three frequency references, which are 750 Hz, 2250 Hz and 3000 Hz as referenced to the DC carrier that is not used; the tone numebers are: 16, 48 and 64 according to ETSI numbering (tones 120, 152 and 168 in SA numbering).
Looking at these pilot tones, they seem to have multiple modulation outlines that depend on the number of sent symbols:

- PSK-2 (periodicity 2)
- PSK-4 (periodicity 3)
- unmodulated (periodicity 4)
pilot tone 120 (ETSI 16)
pilot tone 152 (ETSI 48)
pilot tone 168 (ETSI 64)
The two couples of tones indicated by the ETSI numbering as -103,101 and -101,103 (1,205 and 3,207 in SA numbering) belong to the set of "gain reference tones" (those tones have a power gain of 2) and those who are close to the band lower and upper edges are over-boosted by a further power gain of 2, as it is clearly visible below. The periodicity of the gain reference pattern is 3 symbols:

Modulation is QAM-64 in even symbols:

in odd symbols modulation is QAM-64 with the evidence of outer phases (service and control) that are sent each three symbols; note that in this picture the signal has been resampled at a different frequency:

The positions of the service and control 'cells' are shown in Annex-A "Pilot reference" of the ETSI DRM document and justify the behaviors seen above.

Juts two words about the software "DREAM".
DREAM is an Open-Source Software Implementation of a DRM (Digital Radio Mondiale) Receiver under the GNU General Public License (GPL), it works both on-line and in post-production by processing wave recordings. Below some screenshots of DREAM that is struggling with same file processed by SA.
The software provides many measurements in the Chart Selector menu, including the views of the three phase constellations of DRM channels.

Power Spectral Density
MSC-SDC-FAC constellations

DREAM links:

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