- COFDM -
drm007002.jpg
Coded Orthogonal Frequency Division Multiplex
10 kHz mode B DRM spectrum centred on 12 kHz IF.
                                           

COFDM is a spectrum efficient digital modulation scheme employing many 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 (DAB) 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.

In DTT 8K mode there is a total of 6,817 carriers, 6,048 of these are data carriers.

With DTT-T2 (high definition) there are 27,265 carriers spaced 279 Hz (8K mode)

 


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.

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).

For example Mode A               1                      = 41.66 Hz
 [Tu -Tg]                         (26.66 - 2.66) mSec

where :

T = time
Tu = useful
Tg = guard
 

Each carrier can be either a data or pilot or reference carrier. 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.
 
 

 

Mode

 

 

Carrier spacing

 

 

Number of carriers
9 kHz

Number of carriers
10 kHz

Number of carriers
18 kHz

Number of carriers
20 kHz

Number of carriers

96 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

 

E

444.44 Hz

 

 

 

 

213

  

 

Symbol timing summary of the five DRM modes.

 

Cyclic prefix/ guard interval   (Tg)

mSec

useful part of symbol (Tu)

mSec

total symbol (Ts)

mSec

symbols per frame

Mode A  - minor fading.

2.66

24.00

26.66

15

Mode B  - as A but with time and frequency fading.

5.33

 

21.33 

 

26.66

 

15

Mode C - as B but with higher Doppler spread.

5.33

 

14.66

 

20.00

 

20

Mode D - as C but with severe delay and Doppler spread.

7.33

 

9.33

 

16.66

 

24

Mode E - DRM+

0.25

2.25

2.5

 

 

Coded (OFDM)
 
The carrier systematically changes function for each symbol/carrier and this frequency and 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 (no cross-talk) 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.

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. For Mode B transmitted in 10 kHz short wave channel there are 206 equally spaced carriers with approximately half are encoded with audio data at any one time.

 
 

Cyclic Prefix
 
In order to avoid reception problems when receiving DRM radio signals by multi-path, each OFDM symbol is extended by a 'cyclic prefix'. At the transmitter the last part of each symbol is inserted at the start of the same OFDM symbol. At the receiver the data contained in the cyclic prefix of the OFDM symbol is ignored after synchronisation.

If two different signals are received due to multi-path reception then the switch between two consecutive symbols in the delayed signal should occur within the cyclic prefix and this does not cause a problem.

As a result multi-path signals cannot cause Inter Symbol Interference (ISI). Different band radio propagation conditions require a set of different DRM modes so that the broadcaster can choose the most suitable mode for any given frequency/radio path to the target area.

This cyclic prefix (or guard interval) slightly reduces the effective data throughput as this duplicates data already present but the result is a robust signal that is immune to data errors caused by multi-path reception. This also explains why OFDM can be used in a Single Frequency Network where there are many transmitter broadcasting the same digital data on the same frequency. Something not possible with analogue broadcast as any overlap between signals from different transmitter cause considerable mutual interference.
 
Boosted Carriers

According to the DRM specification the 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 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 as the carriers are always present.

 

DRM background
COFDM
FAC
SFN
MSC
SDC data 1
Multimedia
Bandplan
DRM future
DRM audio
FAC sync
DRM multiplex
DRM modes
SDC data 2