113 ETSI ES 201 980 V4.1.2 (2017-04)

Table 37: Code rate for the SDC with 4-QAM (robustness modes A, B, C and D)

Table 38: Code rate for the SDC with 4-QAM (robustness mode E)

Annex J provides the number of input bits per SDC block.

Error detection with a CRC is described in clause 6.

7.5.3Coding the FAC

The FAC shall use 4-QAM mapping with code rate 0,6 for robustness modes A, B, C and D or 4-QAM mapping with code rate 0,25 for robustness mode E. A fixed code rate shall be applied.

The number of input bits LFAC per FAC block is calculated as given in clause 7.2.

One of the code rates given in table 39 or table 40 shall be used.

Table 39: Code rate for the FAC (robustness modes A, B, C and D)

Table 40: Code rate for the FAC (robustness mode E)

Error detection with a CRC is described in clause 6.

7.6MSC cell interleaving

A cell-wise interleaving shall be applied to the QAM symbols (cells) of the MSC after multilevel encoding. For robustness modes A, B, C and D the possibility to choose low or high interleaving depth (denoted here as short or long interleaving) according to the predicted propagation conditions exists. For robustness mode E only one interleaver depth is available which corresponds to the algorithm for high interleaver depth. The basic interleaver parameters are adapted to the size of a multiplex frame which corresponds to NMUX cells.

For propagation channels below 30 MHz with moderate time-selective behaviour (typical ground wave propagation in LF and MF) the short interleaving provides sufficient timeand frequency diversity for proper operation of the decoding process in the receiver (spreading of error bursts).

The same block interleaving scheme as used for bit interleaving in the multilevel encoder (see clause 7.3.3) is always applied to the NMUX cells of a multiplex frame for all robustness modes.

ETSI

The input vector of the block interleaver corresponding to the NMUX QAM cells zn, i of multiplex frame n is given by:

Zn = (zn,0 , zn,1, zn,2 ,K, zn, NMUX −1 )

The output vector with the same number of cells or elements, respectively, is given by:

where the output elements are selected from the input elements according to:

zˆn, i = zn, Π(i) .

The permutation Π(i) is obtained from the following relations:

s = 2 log 2(NMUX ) , means round towards plus infinity;

q = s / 4 − 1 ;

t0 = 5 ;

Π(0) = 0 ;

for i = 1, 2,K, NMUX −1:

Π(i) = (t0 Π(i −1) + q)(mod s);

while Π(i) ≥ NMUX :

Π(i) = (t0 Π(i) + q)(mod s).

For severe timeand frequency-selective channels below 30 MHz as being typical for signal transmissions in the HF short wave frequency bands and for channels above 30 MHz a greater interleaving depth is provided by an additional convolutional interleaving scheme. For this the interleaving depth D is defined in integer multiples of multiplex frames. As a good trade-off between performance and processing delay a value of D = 5 for robustness modes A, B, C and D and D = 6 for robustness mode E has been chosen.

The output vector for long interleaving with NMUX cells carrying complex QAM symbols is computed in almost the same way as for short interleaving. The only exception is that the permutation is based not only on the current but also on the last D-1 multiplex frames. The permutation Π(i) as defined before is used again to determine the relation

between the indices within the output vector ˆ n and the D input vectors Zn , Zn−1,K, Zn−D+1 .

Z

The output elements are selected from the input elements according to:

zˆn, i = zn−Γ(i), Π(i)

Taking into consideration the transmission of the full content of a multiplex frame the overall delay of the pure interleaving/deinterleaving process is given by approximately 2 × 400 ms, i.e. 800 ms, for the short interleaving for robustness modes A, B, C and D. In the case of the long interleaving it corresponds to about 2,4 s for robustness modes A, B, C and D and 0,7 s for robustness mode E.

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7.7Mapping of MSC cells on the transmission super frame structure

The content of MTF consecutive interleaved multiplex frames (with NMUX QAM cells each) is mapped onto a transmission super frame, i.e. the corresponding number NSFU of useful MSC cells is fixed as an integer multiple of MTF. MTF = 3 for robustness modes A, B, C and D and MTF = 4 for robustness mode E. Due to the fact that the number of FAC and synchronization cells is varying from OFDM symbol to OFDM symbol a small loss NL of 1 or 2 cells can occur compared with the number of available cells in a transmission super frame which is given by:

NSFA= NSFU+NL =MTF×NMUX+NL

Tables 41 to 45 give the values for the different robustness modes and bandwidths.

Table 41: Number of QAM cells for MSC for robustness mode A

Table 42: Number of QAM cells for MSC for robustness mode B

Table 43: Number of QAM cells for MSC for robustness mode C

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