Digital Microwave Radio
Codan 8800 series
Introduction to Digital Microwave Radio Technology
1. Introduction to DMR
Point-to-point digital microwave radio (DMR), as the name implies, is a digital transmission
technology that provides a wireless radio link operating at microwave frequencies between two
points. A terminal at one end of the link communicates exclusively with a complementary terminal
at the other end of the link. Each terminal is fitted to a parabolic dish antenna and communication
is by line-of-sight beams between the dishes.
DMR is very flexible and does not depend on other elements such as satellite, cable, or optical
fibre). Communication distances can be as short as a few meters (eg across the street between
buildings in the city) or very long (up to 80km) in the country. To achieve line-of-sight, antennas
and at least a portion of the terminal are typically mounted on rooftops, on hills or on towers. Links
can also be daisy-chained to avoid major obstructions or to cover virtually endless communications
distances.
DMR links can be used to carry a wide variety of traffic. In the telecommunications industry, they
are used to carry data, voice, fax while in the broadcast industry they carry video and audio signals.
In the wireless data communications market, DMR links carry Ethernet traffic between Local Area
Network (LAN) sites. Other applications include security, telemetry, monitor and control and many
other applications requiring transport of digitised information.
Radio spectrum usage and data transmission standards are subject to regulatory frameworks
throughout the world, in the interests of efficient spectrum usage and interoperability. The
European Telecommunications Standards Institute (ETSI) incorporates International
Telecommunications Union (ITU) recommendations into the European regulatory framework, and
these are followed in much of the rest of the world. The Federal Communications Commission
(FCC) oversees radio spectrum usage in the US, where American National Standards Institute
(ANSI) data standards are typically used.
For telecommunications, the traffic usually carried by DMR is structured in a hierarchy of data rates
and formats known collectively as Plesiochronous Digital Hierarchy (PDH) according to standards
set by the ITU and ANSI.
The microwave operating frequencies and the structure of the actual frequencies and bandwidths
used are standardised by the ITU and FCC into operating bands. The frequency chosen for a
particular link will depend upon many factors including the region (higher frequencies are
attenuated by rain and cannot be used in tropical environments) and the service (there are more
frequency allocations available at higher frequencies and they are used in areas of higher traffic
density, such as cities).
The products offered by Codan have operating frequencies ranging from 7GHz to 38GHz with data
interfaces allowing flexible combinations of PDH data streams and Ethernet traffic. The maximum
aggregate data rate that can be carried is 52Mbs, depending on the data standards and spectrum
licencing arrangements in the country of use.
A substantial driver for the development of the DMR industry in recent times has been deregulation
of the telecommunications industry in many countries. Today, fixed and mobile network operators
and even private users can establish their own networks throughout the world, with the right to
provide the transmission infrastructure independently of the dominant carriers. DMR allows private
voice and data networks and cellular networks to be established very quickly, efficiently and at
substantially lower cost than cable systems.
As more countries deregulate their telecommunications infrastructure, as communications services
are extended to more regions of the world, and as the demand for ever higher data rate capacities
expands, the market for communications by digital microwave radio is expected to expand rapidly.
Let us now focus on the applications of digital microwave radio and its main users, while also trying
to explain the reasons for an explosive growth in demand for such products.