Transmission Frame

How to transmit bits into the air (the DAB/DAB+ transmit interface)?

Before sending an item from point A to B, you first will pack the item into a box or envelope. It will be no other in digital wireless communications.

When sending digital data, it is best to pack your bits in the correct sequence into a box. This box, in telecommunications, is called a Frame.

The current page describes the organization of the frame for DAB/DAB+ transmission.

The DAB/DAB+ Transmission Frame

The DAB/DAB+ transmitter will have a continuous wireless transmit rate of 2400 kilobits per second (2.4 Mbit/s). This rate consists of consecutive (one after the other without interruption) transmission Frames sent every 96ms. See figure,

Fig. 1 – DAB Mode-I Transmission

The relationship between the figures is simple, 230400 bits/s divided by 0,096 (96 ms) seconds, resulting in 2400 kbits/s.

Further explanation is easier by dividing the frame into 77 (0-76) subdivisions (time frames). And giving them a name following the function they fulfill.

  • We call the NULL symbol the first subdivision, slightly longer than all the following symbols of equal time frame.
  • We call the Synchronisation channel the second subdivision.
  • We call the Fast Information Channel (FIC) the next 3 subdivisions
  • And we call The Main Service Channel (MSC) the last 72 subdivisions

The NULL symbol

It is the first subdivision of the frame that will be transmitted. However, transmitted may be the wrong word because it pauses the transmission (silence). The short silence of the transmitter will function as a separation of the consecutive frames. The receiver will use this as the frame synchronization signal of 96ms. We will elaborate on this type of frame synchronization later on.

Each frame will start with silence (no transmission). During the NULL symbol, there is no DAB/DAB+ information transmitted.
The NULL symbol is solely to give the DAB/DAB+receiver a chance to detect the start of each frame and to keep the 96ms synchronisation for the transmission frame on the receiver site alive.

Fig. 2 – DAB Mode I Transmission with NULL symbols
Introducing a small pause between the frames (white space)

The Synchronisation Channel

The next subdivision of the frame contains a synchronization sequence to synchronize the QPSK demodulators, which will detect the next 75 subdivisions with DAB/DAB+ information.

There will be no DAB(+) information or audio bits transmission during the synchronization symbol.

The Fast Information Channel (FIC)

The next 3 subdivisions form the FIC channel. The FIC channel contains bits with details about the EMUX. The bitrate of a subchannel, name of the subchannel, ID of the subchannel, the EMUX name, etc.

There is no audio in this channel, and the 3 subdivisions forming the FIC channel it as a bitrate of 96 kb/s.

The 230400 bits per frame spread over the remaining 75 subdivisions results in 230400/75 equals 3072 bits per subdivision. The FIC channel uses 3 subdivisions or 9216 bits per 96 ms.
Calculating this for 1-second results in an FIC channel capacity of 9216/0,096 = 96 kb/s

The Main Service Channel (MSC)

The next 72 subdivisions form the MSC channel.

Here is the audio transmitted that the radio receiver’s loudspeaker will reproduce. Remember, the audio has been convolutional coded for error correction.

The 72 subdivisions will contain 221184 bits per 96 ms or a speed of  2304 kb/s.

The complete DAB/DAB+ Transmission Frame

The DAB/DAB+ transmission frame is shown in literature often as a one-dimensional frame presentation in time.

Fig. – One-dimensional time representation of the transmission frame

Although the frame is in frequency space spread over the 1536 carriers, it isn’t easy to represent these two dimensions. However, we did give it a try in the following diagram.

Fig. – 2-dimensional table of the transmission frame

From where comes the 77 (0-76) subdivision?

We know that a DAB/DAB+ transmitter uses not one carrier but 1536 carriers. Each of our subdivisions exists on 1536 carriers. We do not say that we have 1536 transmission frames, but we consider only one transmission frame. Every 77 (0-76) frame divisions will transmit a symbol of 2  bits over each of the 1536 carriers.
The result is 3072 bits per division in the transmission frame. Taking the 3 divisions for the FIC and the 73 divisions for the MSC in total 75 divisions, we conclude that we have 75 divisions of 3072 bits or 230400 bits per 96ms.

The calculations

Only the FIC and MSC channels transmit DAB/DAB+ related bits.

Indeed 96 kb/s for the FIC channel and 2304 kb/s for the MSC channel results in a total DAB/DAB+ transmit capacity of  2400 kb/s.

The division in 96 ms is also correct:

FIC contains 9216 bits per 96ms

MSC has 221184 bits per 96ms

9216 + 221184 equals 230400 bits per 96 ms transmission frame.

230400 / 0,096 equals the transmission rate of 2400 kb/s

To better understand the capacity of a transmission frame, we suggest remaking the calculation bottom-up and bottom-down. And to verify the results.