5.5.2.2 Digital links

Digital links display only a band of spectral occupancy visually similar to filtered white noise. It is not immediately evident from RF spectra whether any of the separate base-band channels are occupied or not. It is necessary to discuss the construction of these signals before any analysis is considered.

The source will typically be a single multiplexed digital signal as described by ITU-T Recommendation G.703. This describes the data and framing protocol of land line digital links. Since the transmitter has no control over the nature of the digital signal, it is exclusively ordered by a pseudo-random digital sequence generated within the system. The reason for this is to avoid situations of a no data microwave signal (spectral dispersion) and, in some cases, to provide sufficient digital transitions for clock recovery. There is no global specification for this randomizing sequence that differs with equipment type and manufacturer.

Similarly, digital modulation methods are equally varied, but may be consistent in any one band, depending on national spectrum management policy. QPSK or differential QPSK (D-QPSK) is common but various quadrature amplitude modulation (QAM) schemes are used representing various ways of enhancing spectral efficiency and system design.

5.5.2.3 Output spectrum

The analogue microwave radio (AMR) spectrum differs from the digital microwave radio (DMR) spectrum as shown in Fig. 5-39.

Notice how the DMR spectrum almost completely fills the band, whereas the AMR spectrum has the peak at the carrier frequency and sidebands gradually decreasing in amplitude at frequencies away from the carrier. It is this characteristic which makes AMR vulnerable to interference by DMR. If the frequency band of a DMR operates too close to that of an AMR, the large amplitude at the edges of the band of the DMR can swamp the low-amplitude sidebands of the AMR, causing interference so severe that the AMR fails. This point stresses the need for adequate IF and RF filtering of microwave radio signals whose bands are operating very close to each other on the same route.

PMC= Production technology Management

 Relations to Other Digital Broadcasting Systems

In addition to the European DAB system which is mainly covered by this book, several other digital sound broadcasting services exist which have been or are being developed and (partly) introduced. These systems differ in many aspects (specifications, service complexity, parameters) from the DAB concept. Some are focused more strongly on a single applicationor service (e.g. stationary reception) or provide lower audio quality levels (such as WorldSpace or DRM). Except for ISDB-T, which uses technologies very similar to DAB, all other systems are not expected to be able to comply with the audio quality and quality of service in mobile reception provided by DAB. Nevertheless the basic concepts and limitations of some of these systems will be introduced briefly. Also, there are a few cable-based digital radio services, which are mainly derived from existing satellite or terrestrial radio systems (for instance, ADR, DVB, etc.) so it may not be necessary to describe them separately. In general, those systems use QAM schemes in order to achieve high data rates in the limited bandwidth of the cable.

This is possible because of the high signal-to-noise ratio available. However, these services have only local importance, depending of the extension of the broadband cable distribution network used.

REFF.: 5.5.2.2&3.:from ITU Spectrum Monitoring 2002

Last : Digital Audio Broadcasting , WILEY, second edition