DRM COFDM

The DRM radio signal consists of data transmitted using COFDM (Coded Orthogonal Frequency Division Multiplex). COFDM is a spectrum efficient digital modulation scheme employing equally spaced carriers and has become the standard method for digital broadcasting. The number of carriers, carrier modulation and spacing is different for different broadcast systems; as is the way synchronization is achieved.

Digital Audio Broadcasting has 1536 carriers spaced 1 kHz apart.
Digital Terrestrial Television (DTT) in 2K mode has 1705 carriers spaced 4.464 kHz apart, with 1512 carriers carry data and the remainder are pilots.

The actual number of DRM carriers depends on the mode and channel bandwidth (9 kHz for long or medium wave, and 10 kHz for short wave).

DRM mode	carrier spacing	number of carriers  9 kHz	number of carriers  10 kHz	number of carriers  18 kHz	number of carriers  20 kHz
A	41.66 Hz	204	228	412	460
B	46.88 Hz	182	206	366	410
C	68.18 Hz	*	138	*	280
D	107.14 Hz	*	88	*	178

* mode not specified for long/medium wave broadcast.

DRM mode	carrier spacing	duration of symbol	duration of guard interval	number of symbols per frame
A	41.66 Hz	26.66 mSec	2.66 mSec	15
B	46.88 Hz	26.66 mSec	5.33 mSec	15
C	68.18 Hz	20.00 mSec	5.33 mSec	20
D	107.14 Hz	16.66 mSec	7.33 mSec	24

DRM COFDM Carriers

Individual COFDM carriers are spread across the radio channel but not all of the carriers transmit data. The carrier spacing is equal to the inverse of the symbol duration.

For example Mode A             1             = 41.66 Hz
                                     (26.66 - 2.66) mSec

Each carrier can be either a data or pilot or reference. The pilot (or gain) carriers are used for channel estimation for use in fading channels. Pilot carriers are broadcast with a known amplitude and phase sequence.

The carrier systematically changes function for each symbol/carrier and this frequency/time interleaving of the data makes the signal more robust and improves reception in the presence of fading and interference.

QAM

Data to be transmitted is systematically spread across all these carriers and each carrier is modulated using QAM (Quadrature Amplitude Modulation). When viewed in the frequency domain the spectra of these carriers appear overlapped. However thanks to the principle of othogonality the carriers (once synchronized at the receiver) do not overlap and each individual carrier can be demodulated by software. This considerably increases the density of carriers and consequently increases the data rate available.

QAM is a mixture of fixed amplitude and phase modulation. DRM incorporates error protection so that selective fading, or interference, with the subsequent loss of some carriers does not affect the overall transmitted data.

Each transmission frame contains an FAC block (400 mSec). When more than one service is carried in the multiplex a number of FAC blocks are required to describe all the services.

Channel Estimation

With QAM modulation an amplitude reading of a particular carrier is meaningless as there may be selective fading on the signal. To determine the correct reading the decoding software looks at the amplitude of the nearest pilot carrier each side of the data carrier.

By interpolation with the known transmitted amplitude (assuming synchronization has already been done) of these pilots carriers an estimate of the channel response is derived which is then used as an offset correction to all the data carrier amplitudes.

This method simplifies DRM decoding as the effects of selective propagation fading can be measured using these pilots carriers and the received amplitude of the data carriers can be corrected in software. Using channel estimation enables coherent OFDM demodulation.

This is why Mode A can transmit more data than Mode B even though it has less bandwidth (9 kHz compared to 10 kHz). Mode A is used for medium wave and ground wave propagation where the channel fading characteristics are more benign than short wave. Consequently for Mode A more carriers are designated as data carriers rather then pilot carriers than for Mode B.

Boosted Carriers

According to the DRM specification two carriers close to the band upper and lower edge can be boosted by a factor of 4. Few DRM broadcaster do this because of the limited dynamic range available on the modified AM transmitters. The transmitter must operate as a ‘linear amplifier’ when broadcasting DRM. The broadcast signal is the sum of all the carriers, consequently DRM has a high peak-to-average power ratio.