- Fast Access Channel Reference Carriers -
drm009001.jpg
 
 
 
Fast Access Channel (FAC) Synchronisation

The above show a spectrum plot of a strong Mode B DRM signal (Deutsche Welle), in reality the spectrum rarely shows this perfect flat top shape as the radio signal may suffer selective fading. To create this DRM spectrum plot I used Spectrum Lab freeware program (written by DL4YHF) with a FFT of 16k and averaged the result over several seconds to enhance the FAC reference pilots. The spectrum also shows that there is no OFDM carrier broadcast where the carrier would be if it was an analogue AM double side band signal.

 

FAC reference carriers

The three reference pilots (or carriers) are used by the DRM receiver software to help detect and synchronise the DRM signal. The three FAC reference carriers are transmitted at baseband 750 Hz, 2250 Hz, 3000 Hz (marked as F1, F2, F3).

Unlike all the other DRM carriers, they are dedicated reference carriers whereas all the other carriers constantly switch between either carrying data or used as a pilot (to enable channel estimation).

 

These FAC reference carriers have their amplitude boosted relative to carriers carrying data by a power gain of 2 so that the DRM software can recognize these

carriers.

 

Baseband frequency

Mode A
carrier

Mode B
carrier

Mode C
carrier

Mode D carrier

 

 

750 Hz

18

16

11

7

F1

2250 Hz

54

48

33

21

F2

3000 Hz

72

64

44

28

F3

 

These carriers are transmitted at the same frequency irrespective of the DRM mode being used. FAC reference carriers are used to calculate the initial coarse frequency offset of the received DRM signal compared to the receiver’s local oscillator. This is the first part of the COFDM synchronisation process required before the received DRM signal can be properly decoded. After this is done then the decoding software needs to decode the FAC to find out what mode and other parameters of the DRM broadcast.

 

The DRM specification numbers the OFDM carriers with 0 being where the AM carrier would be. Carriers are numbered negatively if they are to the left of AM carrier (i.e. lower sideband if we are still thinking in terms of AM), and positively (upper sideband) if they are to the right.

 

decoding DRM
 

It is not possible for the decoding software to directly look for these FAC frequencies in the data as there will be an unknown frequency offset caused by the radio’s local oscillator not being exactly on frequency. For successful OFDM decoding the transmitter and receiver need to be exactly on the same frequency.

 

Although the DRM decoding software doesn’t know exactly where these carriers are in the frequency domain data, it does know the relative carrier frequency spacing will always be the same and will not change. The recognition software uses three windows (±150 Hz) appropriately spaced to move through the frequency domain data created by the FFT summing the value of all the individual carriers.

 

At a certain point there will be a correlation of the carrier amplitude that should coincide with the three boosted FAC carriers. From this peak the actual frequency can be determined and the offset can be calculated, this gives a coarse frequency offset. This is a necessary first part of the DRM synchronisation process.

 

 
 
FAC reference carrier correlation for DRM RTL broadcast

 

The above plot shows the FAC correlation for a RTL short wave broadcast when they were broadcasting on 6095 kHz (49 meter band). The X axis is the frequency offset centred on FFT bin 50 (12 kHz), the correlation peak is at FFT bin 65, giving a frequency offset of (65 – 50) x FFT resolution = 15 x 2.93 = 43.95 Hz.

 

Internally a DRM decoding software will compensate for this frequency offset and will also have a tracking frequency routine for any frequency drift within the DRM receiver.

 

 


 

DRM background
COFDM
FAC
SFN
MSC
SDC data 1
Multimedia
Bandplan
DRM future
DRM audio
FAC sync
DRM multiplex
DRM modes
SDC data 2