DVB-T2 (Second Generation Terrestrial) - PAPR Recommendation
Recommendation on interpretation of PAPR techniques in DVB-T2
In 2019, some questions to DVB highlighted that some aspects of the DVB-T2 specification may benefit from clarification: in particular, the two PAPR reduction techniques and especially the Active Constellation Extension (ACE) feature.
The PAPR clauses were added late in the drafting process and are written in such a way that they “stand alone”, not depending on symbols defined elsewhere in the specification. They also make use of symbols that are used with different meanings elsewhere. Internally to each clause, the symbols are used consistently, but there is a risk that implementers might confuse the symbols in those clauses with similar ones used elsewhere.
Furthermore, both clauses also describe the techniques in the context of an intermediate signal that, whilst well known to implementers, does not appear explicitly in the rest of the standard. This document aims to provide additional guidance on the interpretation of the relevant parts of the DVB-T2 specification.
To DVB’s knowledge, the Active Constellation Extension (ACE) technique has not yet been deployed in real DVB-T2 networks, and it was not originally covered by the extensive Validation and Verification (V&V) work carried out by DVB. However, there is at least one commercially available implementation of the technique and it has also been independently implemented in the DVB Common Simulation Platform (CSP). Subsequent V&V comparisons have shown that these two implementations interpret the ACE algorithm in the same way. Nevertheless, implementing the algorithm based on the specification alone requires several steps of deduction and inference, and the present document aims to spell out those steps in detail in order to make implementation easier and avoid the risk of misinterpretation.
The use of ACE has been shown to allow potentially significant transmitter power savings (which increase with reducing order of modulation). Whilst network operators could regard this as a significant benefit, its use implies a small theoretical loss of link performance and, like any other previously unused feature, may give rise to compatibility issues in receiver populations deployed before ACE was brought into use. Accordingly, operators considering using ACE in a network should carry out appropriate receiver testing to ensure that receivers likely to be used in the network continue to work without problems and to determine the appropriate ACE parameters.