What is the difference ?
Both systems are pretty identical in the way that they use the same transmission frames. It means the transport of bits between the encoder and transmitter (ETI-frame of 24ms) and further on to the receiver (transmission frames of 96ms) is identical for DAB and DAB+.
DAB+ is the result of introducing new audio compression techniques with much higher efficiencies. The difference is in using other encoders and error-correction codes for DAB+.
Instead of using the former audio compression system MPEG-1/2 layer II used in the first DAB systems with a bit rate of 128 ~ 256 kbps (stereo), DAB+ uses AAC (Advanced Audio Coding). AAC works with much lower bitrates for the same or even higher quality than MPEG-2. The overall result is that instead of 6 radio stations in a ‘classic’ DAB ensemble, now we can go up to 12 and even higher to 18 stereo radio channels, still with a reasonably good audio quality on DAB+.
So, DAB and DAB+ transmitters are identical in that they use the same bit transport system and can serve both systems. However, the headend (audio)encoders and error correction are different. One can even transmit a mix of DAB and DAB+ stations in one ensemble!
Older DAB receivers can not receive DAB+. The reason is that they do not have the DAB+ AAC decoders and error-correction features on board. They were built solely for MPEG-1/2 layer II and the unequal error correction which goes with that.
New DAB/DAB+ radio receivers have different audio decoders and error correction on board. They can decode both DAB/DAB+ signals.