The overall ERP allocated to a transmitting site is not changed whether the signal broadcasted is TDM or LDM.
LDM is based on multiplexing two or more layers on a single RF channel, which in terms of power means allocating different portions of the transmitter ERP to each one of the layers composing the LDM ensemble.
At the transmitter side, the LDM signal is created by arranging each layer’s signal on top of the other in a synchronous mode. Both (signals) layers will share:
- Guard Interval
- Pilot Carriers
- FFT Size
Figure shows a simplified block diagram of the transmitter. Each layer will have a different MODCOD configuration depending upon the robustness specifications for each broadcast scenario.
The injection level, D (dB) provides an additional degree of flexibility to the broadcaster. Adjusting the injection level, the broadcaster can adjust the coverage of each layer according to that specific commercial scenario: portable indoor + fixed reception, portable outdoor + fixed reception, mobile + fixed reception.
Next figure shows a closer look to an LDM system transmitter block diagram. This example assumes an OFDM –like structure, with a common interleaver for upper and lower layers.
Even not mandatory, a common interleaver simplifies the memory requirements at the receiver side.
LDM is compatible with other advanced techniques present in state-of-the-art standards or being researched at the moment for future broadcast systems: Time Frequency Slicing, Channel Bundling, MISO, MIMO, Multiple PLPs, etc.