1. Properties
  2. Frame Structure
  3. References
  4. 4FSK
  5. Guard Slot
  6. DMRDecode





Frame Structure

The Frame:

The 27.5-ms frame consists of a total of 264-bits; 108-bit payload, 48-bit SYNC or embedded signaling, and a second 108-bit payload for a total of 216-bits of payload per frame.

The vocoder must compress 60-ms of audio with FEC (forward error correction) into 216-bits of data for transmission.

The 2.5ms-gap is used for guard time to allow PA ramping and propagation delay

Base Station Frame Structure:

Mobile Station Frame Structure:

Single Burst Structure:



Electronics Notes - How does DMR Mobile Radio Work:

Robert M0NVQ Blog - Digital Mobile Radio (DMR):



Dibit Frequency Encoding
01carrier - deviation
00carrier - (1/3)*deviation
10carrier + (1/3)*deviation
11carrier + deviation

4FSK modulation can be expressed using following equation.

S(t) = Ac*Cos[2*π(Fc+Δf)*t]

Where in 4 different Δf as shown in the table are used to represent 4 diferent binary dibits in 4FSK to achieve corresponding outputs.

The figure depicts a 4FSK power spectrum. There are 4 carrier peaks in 4FSK modulation. This is due to the fact that data are represented by 4 carrier phase shifts in the 4FSK.



Guard Slot

DMR has a 2.5mS Guard Slot between the time slots that different transmitters can use. The guard slot is the gap between the end of one time slot and the beginning of another.

1.5mS of the Guard Slot is used for ramp up at the beginning of a slot, and ramp down at the end, leaving only 1mS true quiet time. There is also the issue that the clock frequencies running the Digital Signal Processors (DSP) in Transmitter A and Transmitter B may be different, so the respective transmissions could start slightly earlier or later than they ideally should. Typical DSP Clock accuracy accounts for another 0.5mS, so the true guaranteed quiet period between adjacent DMR TDMA slots is only 0.5mS.

If one transmitter more distant, the propagation delay is longer. Eventually the signal from a closer transmitter will be received before the delayed signal from the distant transmitter has finished and as the signal from closer transmitter is much stronger, the end of the transmission from more distant transmitter is lost. All things being equal it is generally assumed that this occurs at aout 110 to 140 Km




Some abbreviations you might see while decoding with utilities like DMRDecode

Glenn Lyons VK4PK
Ver:gnl20220131 - pre published v0.9