COFDM Cyclic Prefix

In order to avoid reception problems when receiving multi-path DRM radio signals, 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 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.

DRM broadcast mode		Cyclic prefix/ guard interval   (Tg) mSec	useful part of symbol (Tu) mSec	total symbol (Ts) mSec	symbols per frame
A	minor fading.	2.66	24.00	26.66	15
B	as mode A but with time and frequency fading, and longer delay.	5.33	21.33	26.66	15
C	as mode B but with higher Doppler spread.	5.33	14.66	20.00	20
D	as mode C but with severe delay and Doppler spread.	7.33	9.33	16.66	24

symbol timing summary of the four DRM modes.

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.