6.6.5.3Receiver considerations

The availability of FEC packets in the packet stream is indicated by the presence of SDC data entity type 14 (see clause 6.4.3.15).

The configuration of the FEC scheme is signalled to the decoder by the SDC data entity type 14. This SDC data entity provides the parameters R and C along with the packet length L, so that the FEC decoding can start even before the first SDC data entity type 5 element has been received. Knowing the number of rows R and columns C of the Application Data Table, the decoder can reconstruct the FEC frame in memory along with the received Reed-Solomon parity bytes by applying the steps to create the FEC packets in reverse order.

The Reed-Solomon error correction mechanism can only take place after all packets belonging to the Application Data Packet Set and corresponding FEC Packet Set have been received. However, if the CRC check passes for a particular data packet, this packet may be used immediately. If the CRC check of a data packet or an FEC packet fails, the receiver may choose to inform the Reed-Solomon decoder of the potentially erroneous byte positions within the Reed-Solomon codeword to enhance the decoder's error correction performance.

The cache memory M required in the receiver to collect the received data and FEC packets into an FEC frame is limited to 3 072 bytes.

A receiver can evaluate Reed-Solomon parity information provided as FEC packets after its initial successful synchronization to the packet stream, even if the CRC check for FEC packets fails and therefore the packet headers of these packets cannot be evaluated. This functionality is achieved by inserting the FEC packets with identical FEC configuration and in identical order at equal distances within the packet stream.

Receivers without support for FEC decoding can extract and decode all data packets albeit without the enhanced error correction performance.

7 Channel coding and modulation

7.1Introduction

The DRM system consists of three different channels, the MSC, SDC and FAC. Because of the different needs of these channels different coding and mapping schemes shall be applied. An overview of the encoding process is shown in figure 18.

The coding is based on a multilevel coding scheme for which the principle is explained in clause 7.3. Due to different error protection needs within one service or for different services within one multiplex different mapping schemes and combinations of code rates are applicable: Unequal Error Protection (UEP) and Equal Error Protection (EEP) are available and can be combined with hierarchical modulation. Equal error protection uses a single code rate to protect all the data in a channel. EEP is mandatory for the FAC and SDC. Instead of EEP, unequal error protection can be used with two code rates to allow the data in the Main Service Channel to be assigned to the higher protected part and the lower protected part. When using hierarchical modulation three mapping strategies are applicable to the MSC: the Standard Mapping (SM), the symmetrical Hierarchical Mapping (HMsym) and a mixture of the previous two mappings (HMmix) that results in the real component of the constellation following a Hierarchical Mapping and the imaginary part following a standard one. The Hierarchical Mappings split the decodable data stream into two parts: a Very Strongly Protected Part (VSPP) and a Standard Protected Part (SPP). The SM method only consists of a SPP. In any case, up to two different overall code rates shall be applied to the SPP of the MSC. For the FAC and SDC only SM is allowed. The application of the coding to the different channels is described in clause 7.5.

ETSI