Layered Division Multiplexing (LDM), which grew out from the Cloud-Txn concept, is a technology to enhance the capacity of the new generation broadcasting systems. It has been proposed as a Physical Layer technology to the ATSC 3.0 Next Generation Digital TV standard.
Layered Division Multiplexing is the result of a cooperation project between CRC Canada, ETRI Korea and EHU Spain.
In short, the main goal is to develop a terrestrial DTV PHY Layer that is:
- Simple to build
- Flexible and Efficient use of the spectrum
- With backward compatible future extension
Layered Division Multiplex: Concept
LDM uses spectrum overlay technology to transmit multiple data streams in one RF channel with different robustness and data capacity for different services and reception environments. Signal cancellation is used to retrieve the robust upper layer signal first, cancel it from the received signal, and then start the decoding of lower layer signal.
The major advantage with TDM-ed systems relays on the fact that 100% of RF bandwidth and 100% of the time are used to transmit the multi-layered signals (hierarchical spectrum re-use) for spectrum efficiency and flexible use of the spectrum
The upper layer (UL) needs to be ultra-robust (Cloud Transmission)
- SNR Threshold close to 0 or even negative.
- Decoded in practically any situation (deep indoor, harsh mobile, slow moving portable)
- Targeting portable and mobile reception
The lower layer (LL) will be a high data rate bearer.
- Required for multiple HD and UHD services to fixed or portable terminals
- Injected from 3 to 6 dB below the upper layer signal
- DVB-T2/NGH can be used as the lower layer system
Layered Division Multiplex: Requirements
There are two major requirements for LDM:
- A strong error correction code and error mitigation system for the Upper layer that can achieve a negative SNR value, closer to the Shannon limit, and save power. LDPC (Low Density Parity Check Codes) are the perfect candidate.
- A good signal cancelation scheme that can minimize the cancellation errors, which makes a high data rate lower layer viable. This implies Low-complex channel estimation and equalization algorithms.
Layered Division Multiplex: Capacity and Service Example with LDM
The table shows how LDM is more efficient than current TDM systems. The columns on the right represent the MODCOD, bitrate and SNR thresholds of an LDM system with two layers. The first upper layer MODCOD is QPSK 6/15 and there are three possible MODCOD configurations for the lower one, ranging from 17.7 Mbps to 32.9 Mbps.
The right columns of the table display numbers for an equivalent TDM configuration, where 25% of the frames are allocated to the mobile service. In this case, the threshold for a similar bitrate on the mobile service is higher than 9 dB, whereas the thresholds associated to the fixed service look quite similar.
LDM: 16K FFT, GI= 1/16, P12,2.
TDM: Fixed 32K FFT, GI = 1/32, P24,4; Mobile 8K FFT, GI = 1/8, P6,2.
All SNR power levels are referenced to the total RF in-band power (of all layers)
Another set of examples with similar numbers apply to 6 MHz channels. In this case, the time allocation varies (30% for mobile– 70% for fixed)