Digital walkie-talkies are getting more and more attention with the
market demand and the demand for walkie-talkies. The Digital Mobile Radio
Standard (DMR) is a digital radio standard specially developed by the European
Telecommunications Standards Institute (ETSI) for professional mobile radio
(PMR) users. Approved in 2005.
Main advantage
DMR systems have many advantages over traditional analog systems and
other digital systems. In short, the DMR digital system brings the following
advantages:
• Existing licensed channel capacity doubled
One of the main advantages of DMR is the ability to support two
simultaneous and independent calls over a single 12.5 kHz channel via TDMA.
According to the DMR standard, TDMA reserves a 12.5 kHz channel width and divides
it into two alternating time slots A and B (as shown in Figure 1 below), each
time slot acting as a separate communication path. In Fig. 1, walkie-talkies 1
and 3 talk on time slot 1; walkie-talkies 2 and 4 talk on time slot 2.
• Backward compatible with traditional analog
systems
It is important for licensees to retain existing licenses to ensure
backward compatibility with their legacy radio or external systems such as
on-site contractors. Since the DMR uses a 12.5 kHz channel, the required spectrum
compatibility is built in.
• Efficient use of infrastructure equipment
Another advantage of the DMRTDMA approach is that two communication
channels are obtained with one repeater, one antenna and one simple duplexer.
Compared to the FDMA solution, dual-slot TDMA achieves an efficiency of 6.25
kHz while minimizing the investment in repeaters and combined equipment. The
equipment requirements for the two methods for a simple system are shown in
Figure 6 below.
• Longer battery life and greater efficiency
Battery life has always been one of the biggest challenges for mobile
devices. In the past, there was limited choice to increase the talk time for
one charge. However, dual-slot TDMA provides a good solution. Since a single
call uses only one of two time slots, only half of the transmitter capacity is
required. The transmitter is idle for half of the time, that is, it is an
unused time slot "round" at any time.
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