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Published on Aug 18, 2011
The DMR standard has many benefits both in comparison to legacy analogue systems and to other digital approaches.
One of the principle benefits of DMR is that it enables a single 12.5kHz channel to support two simultaneous and independent calls, achieved using TDMA Under the DMR standard, Time Division Multiple Access (TDMA) retains the 12.5kHz channel width and divides it into two alternating timeslots A and B (illustrated in figure 1 below) where each timeslot acts as a separate communication path. In figure 1 Radios 1 and 3 are talking on time slot 1 and Radios 2 and 4 are talking on timeslot 2. In this arrangement each communication path is active for half of the time in 12.5 kHz of bandwidth and so each uses an equivalent bandwidth of half x 12.5 kHz or 6.25 kHz . This is known as having an efficiency of one talk path per 6.25kHz of spectrum, but with DMR the channel as a whole maintains the same profile as an analogue 12.5kHz signal. This enables DMR radios to operate in a licence holders existing 12.5 kHz or 25 kHz channels meaning there is no need for re-banding or re-licensing but at the same time doubling the capacity of the channel. his TDMA approach to increasing call capacity in a given bandwidth is very well tried and tested. TETRA and GSM cellular mobile -- two of the world's most widely adopted two-way radio communication technologies -- are TDMA systems.
DMR delivers backwards spectrum compatibility with legacy analogue systems It also may be important for license holders to keep hold of existing licenses to ensure backwards compatibility with their own legacy radios or with an external organisation's analogue system (for example an on site contractor). As DMR uses 12.5kHz channels the required spectrum compatibility is built in.
Another advantage of the DMR TDMA approach is that you get two communications channels with one repeater, one antenna and a simple duplexer. Compared to FDMA solutions, two-slot TDMA allows you to achieve 6.25 kHz efficiency while minimising investments in repeaters and combining equipment.
One of the biggest challenges with mobile devices has always been battery life. In the past, there have been limited options for increasing the talk time on a single battery charge. Two-slot TDMA, however, offers a good way forward. Since an individual call uses only one of the two timeslots, it requires only half of the transmitter's capacity. The transmitter is idle half of the time — that is, whenever it's the unused timeslot's "turn".
DMR digital technology provides better noise rejection and preserves voice quality over a greater range than analogue, especially at the farthest edges of the transmission range. One of the reasons that DMR has an excellent range performance is that a great deal of effort was put into selecting Forward Error Correction (FEC) and Cyclic Redundancy Check (CRC) coders when developing the standard. These coders enable receiving radios to detect and automatically correct transmission errors by analysing bits inserted into messages that enable the receiving radio to tell if there is an error. The DMR standard specifies over 14 different coders to be used, each matched to different types of traffic that is being transmitted.
As DMR is a fully public open standard backed by a wide variety of vendors, buyers can be assured of continuity of supply. There are many examples of the success of technologies incorporated into open standards because standards encourage wide ranging supplier participation. More suppliers results in more choice for users, more rapid product development and lower prices from competitive pressures.
Watch this virtual set studio video to learn more.
DMR Association Category 1 Members: CML Microcircuits, Eastern Communications, Funkwerk, Fylde Micro, Hytera ( HYT ), ICOM, Kenwood, Kirisun, Motorola Solutions, Radio Activity, Radiodata, Selex Elsag, Sepura, Tait Radio Communications, Team Simoco, Vertex Standard