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

Table 49: Carrier numbers for each robustness mode

The DC carrier and certain carriers around DC are not used in some robustness modes, as detailed in table 50.

Table 50: Unused carriers according to robustness mode

8.3.2Simulcast transmission

For robustness modes A, B, C and D, the DRM signal is designed to work in the same broadcast bands as AM signals. Simulcast transmission of services using DRM and AM can be performed by the juxtaposition of the analogue AM signal (DSB or VSB or SSB) and a DRM digital signal. Many arrangements are possible and some are illustrated in annex K.

The spectrum occupancy number relates to the DRM signal. A broadcaster may choose to signal the presence of the AM simulcast by use of the Alternative frequency signalling: Other services data entity in the SDC (see

clause 6.4.3.12).

8.4Pilot cells

8.4.1Functions and derivation

Some cells within the OFDM transmission frame are modulated with known fixed phases and amplitudes.

These cells are pilot cells for channel estimation and synchronization. The positions, amplitudes and phases of these cells are carefully chosen to optimize the performance, especially the initial synchronization duration and reliability.

ϑ1024 [s, k] takes integer values and is either explicitly tabulated or derived using integer arithmetic, as defined in the following clauses (clause 8.4.2 to clause 8.4.5.2 inclusive).

8.4.2Frequency references

8.4.2.0Introduction

These cells are used by the receiver to detect the presence of the received signal and to estimate its frequency offset. They may also be used for channel estimation and various tracking processes.

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For robustness mode E, no frequency reference cells are defined.

8.4.2.1Cell positions

For robustness modes A, B, C and D, frequency references are located at frequencies which are common to these four robustness modes.

There are three frequency references, which are 750 Hz, 2 250 Hz and 3 000 Hz as referenced to the DC carrier, as defined in table 51.

Table 51: Carrier numbers for frequency references

They shall be present in all symbols of each transmission frame.

8.4.2.2Cell gains and phases

For robustness modes A, B, C and D, all frequency reference cells shall have a power gain of 2, i.e. as,k = 2 , in order

to optimize performances at low signal to noise ratio and be compatible when the same cell functions as both a frequency reference and a time reference.

The phases are defined as follows. For the first symbol in the frame (i.e. s = 0 ), the phases ϑ1024 [s, k] are given in table 52.

Table 52: Cell phases for frequency references

For subsequent symbols, the phases are chosen in order to ensure the tones are continuous, which is achieved by applying the following rules.

For robustness modes A, B and C, and carrier 28 only of robustness mode D:

ϑ1024[s, k] = ϑ1024 [0, k]

For robustness mode D, carriers 7 and 21:

ϑ1024[s, k] = ϑ1024 [0, k] , for even values of s, and

ϑ1024 [s, k ] = (ϑ1024[0,k]+512)mod 1024 , for odd values of s.

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