Transmitter Considerations, Over the air

DIGITAL RADIO GUIDE

TERRESTRIAL TRANSMISSION SYSTEMS - DRM

QAM is used for the modulation that is impressed upon the subcarriers to convey the information. Two primary QAM constellations are used: 64-QAM and 16-QAM. The former provides the highest audio quality, but is less robust than the latter. In addition, a 4-QAM (QPSK) signal, which is very robust, is used for some of the signalling (but not for the MSC).

The interleaver time span (applied to the MSC) for HF transmission is around 2.4 seconds to cope with time and frequency selective fading by protecting the audio and data from rapid fades during the natural sequence of speech and music. Owing to the less difficult propagation conditions for the LF and MF bands, a shorter interleaver of around 0.8 seconds can be used.

The multi-level convolutional coding scheme uses code rates in the range between

0.5 and 0.8, with the lower rate being associated with the difficult HF propagation conditions. A 0.5 code rate means that only half the transmitted bits within the overall coded block are used for the actual services in the multiplex, whereas a 0.8 rate means 80% are.

4.1.3Transmitter Considerations

Beyond the modulator box in Figure 4.1 is the transmitter exciter. The DRM system exciter can be used to impress signals on either linear or non-linear transmitters. It is expected that high-powered non-linear transmitters will be the more usual way of transmitting, much as is done now with analogue modulation. However, there are broadcasting service situations where very low powered linear transmissions could be the best way to serve the public.

With respect to non-linear amplification (Class C operation), the incoming DRM signal needs to be split into its amplitude and phase components prior to final amplification. Using QAM modulation, there is a small discrete set of possible amplitudes and phases. The amplitude component is passed via the anode circuitry; the phase component is passed through the grid circuitry. These are then combined with the appropriate time synchronization to form the output of the transmitter.

Measurements of the output spectra show the following: the energy of the digital signal is more or less evenly spread across the 9 or 10 kHz channel, the shoulders are steep at the channel edges, and drop rapidly to 40 dB or so below the spectral density level within the assigned channel, and the power spectral density levels continue to decrease beyond the 4.5 or 5 kHz from the central frequency of the assigned channel with a rapidity that permits conformance to the ITU-R mask for the use of the channels.

(1)Over the air

The digital phase/amplitude information on the RF signal is corrupted to different degrees as the RF signal propagates. Some of the HF channels provide challenging situations of fairly rapid flat fading, multipath interference that produces frequency-selective fading within a channel and large path delay spreads of a few milliseconds or more, and ionospherically induced high levels of Doppler spreads on the order of 1 or more hertz.

The error protection and error correction incorporated in the DRM system design mitigates these effects to a great degree. This permits the receiver to accurately decode the transmitted signal information.

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