DMR digital walkie-talkie technology

With the walkie-talkie, do you understand the main advantages of DMR digital walkie-talkie technology?

Since DMR is a fully open and open standard, it is supported by a number of vendors. Through this kind of user has both the security of supply and the advantage of sustainable and competitive development. Open standards encourage a wide range of suppliers to participate; there are many examples of successful technologies developed in this way. The DMR standard scores high on traditional analog systems and other digital methods. Today Xiaobian will talk to you about the main advantages of the DMR digital system:

Double the capacity of existing licensing channels

DMR enables two simultaneous and independent calls on a single 12.5 kHz channel. This is done using TDMA, time division multiple access. Under TDMA, the DMR retains a 12.5 kHz channel width and divides it into two alternating time slots A and B (as shown in Figure 2 below), with each time slot acting as a separate communication path. In Figure 1, radios 1 and 3 communicate on time slot 1 and radios 2 and 4 make a call on time slot 2.

Figure 1

Each communication path is half effective in the 12.5 kHz bandwidth, and each communication path uses an equivalent bandwidth of half x 12.5 kHz or 6.25 kHz. This is referred to as the efficiency of having one call path per 6.25 kHz spectrum. However, for DMR, the entire channel maintains the same profile as the analog 12.5 kHz signal. This means that the DMR radio operates in the existing license holders of the 12.5 kHz or 25 kHz channel; therefore, there is no need to re-bind or re-license, but double the channel capacity. This is illustrated in Figure 2 below.

This TDMA method of increasing call capacity within a given bandwidth has been well tried and tested. TETRA and GSM cellular mobile devices are among the two most widely used radio communication technologies in the world, and they are TDMA systems. The US Public Safety Radio Standard P25 is also currently developing its second-stage specification as a two-slot TDMA.

Figure 2 : Analog to digital migration using DMR systems

FDMA, Frequency Division Multiple Access, another way to increase capacity is to split a 12.5 kHz or 25 kHz channel into two or more cautious 6.25 kHz channels. The theoretical radio operating in 6.25 kHz FDMA is capable of squeezing two new channels side by side in the old 12.5 kHz channel.

The actual reality is not enough. In many countries, there is no specific 6.25 kHz license and the regulatory mechanism does not allow licensees to run two 6.25 kHz channels in an existing 12.5 kHz license. A single 6.25 kHz radio channel can usually be operated within a 12.5 kHz license, but this does not increase the user’s capacity. This situation is shown in Figure 3.

Figure 3 : Analog to digital migration with a 6.25 kHz digital FDMA system

In the United States, the license holds a 6.25 kHz channel and the licensee is not allowed to subdivide the existing 12.5 kHz license into multiple 6.25 kHz channels. In order to increase the capacity of a 6.25 kHz FDMA system, users must seek a new 6.25 kHz license in other areas of the spectrum.

There are still difficulties in allowing users to compress two 6.25 kHz paths into the jurisdiction of an existing license. It is well known that the use of two channels adjacent to each other in the spectrum poses a risk of interference in a site operating system. Therefore, for this reason, users are still likely to want new licenses in another area of the spectrum to increase capacity using the 6.25 kHz FDMA solution (see Figure 4 below). In contrast, since the two TDMA paths of the DMR are fully suitable for the existing channel structure, no new interference problems are encountered when installing the DMR system.

Figure 4 : Analog to digital migration using 6.25 kHz FDMA

In summary, the FDMA and TDMA systems used in the digital PMR / LMR protocol are theoretically equally spectrally efficient, but the TDMA method used by the DMR brings the advantages of compatibility with existing licensing systems around the world and does not introduce new interference. problem.

One potential advantage of the FDMA 6.25 kHz approach is that you do not need a repeater to coordinate TDMA time slots to provide the two separate talk paths required by the DMR. (The DMR system works well without repeaters and still provides many of the benefits inherent in DMR systems, such as reverse channel signaling, but not two completely independent channels per 12.5 kHz spectrum). However, without a repeater, all radios need to be within range of each other in order to achieve predictable capacity multiplication by FDMA. Therefore, the value of this benefit of FDMA is limited if the system requires an additional range of repeaters, or if the problem area is now or will be covered (eg, by site movement or opening a new location). The DMR system also has the advantage that the 12.5 kHz signal is more resistant to interference than the 6.25 kHz signal.

The 6.25 kHz FDMA system for capacity-increasing non-repeater advantages has advantages only in the following situations:

● The site is small and all user radios will be within the direct reach of all other users throughout the life of the system

● The required frequency has been obtained because dividing the existing license into multiple 6.25 kHz channels will not be the cause of regulatory or interference reasons

● The cost or availability of a more powerful 12.5 kHz channel license is a problem

● No need to be compatible with traditional 12.5 kHz analog systems

DMR was developed from the outset with consideration of long-term business needs, without these limitations.

Backward spectrum compatibility with legacy systems

Licensees may need to retain existing licenses to ensure backward compatibility with their own legacy radio or external organization’s analog systems. Since the DMR uses a 12.5 kHz channel, the required spectrum compatibility is built in. As shown in Figure 5 below.

Figure 5 : Compatibility of DMR spectrum channels with traditional analog systems

Effective use of infrastructure equipment

With DMR TDMA, you can get two communication channels with one repeater, one antenna and one simple duplexer. Compared to FDMA solutions, dual-slot TDMA allows you to achieve 6.25 kHz efficiency while minimizing the investment in repeaters and combo devices. The required equipment for the two methods of the simple system is shown in Figure 6 below.

Figure 6 : Equipment required for dual channel FDMA and TDMA systems

FDMA requires dedicated repeaters for each channel, as well as expensive combined devices to allow multiple frequencies to share a single base station antenna. Making the combined device work with a 6.25 kHz signal can incur further costs, and when used in this manner, signal quality and range are often lost. This in turn causes a need for the power amplifier shown in Figure 6.

For FDMA 6.25 kHz systems, the error tolerance caused by oscillator aging is small and the transmitting radio causes the signal to deviate from the desired center frequency. This results in less robust adjacent channel protection and makes the system susceptible to interference. Professional equipment; high stability oscillators can be introduced; but at a price. In contrast, dual-slot TDMA uses a single-channel device to achieve a stable two-channel equivalent. No additional repeaters or combination equipment is required (and the air conditioning has a low displacement and requires less backup power at the relay site).

Longer battery life and higher power efficiency

Maximizing battery life has been one of the biggest challenges for mobile devices, and these devices have limited options for increasing talk time on a single charge. Since a single call on a dual-slot TDMA uses only one of two time slots, it only requires half the transmitter capacity. The transmitter is idle for half of the time; whenever it is a “turn” of an unused time period. Using a typical duty cycle example of 5% transmission, 5% reception, and 90% idle, transmission time accounts for a large percentage of the radio battery power consumption. By halving the effective transmission time, the two-socket TDMA can be increased by 40% over analog radio.

(A product profile published by a manufacturer gives a 9-hour operational talk time in analog mode, but a digital mode talk time on the same radio is 13 hours). DMR digital devices can also include sleep and power management techniques that extend battery life.

Many factors can affect the power consumption of a single device. When published battery life data is used for widely sold DMR and FDMA digital radio data, the data indicates that for hourly use, TDMA requires 19% to 34% less battery capacity than FDMA models. Choosing technologies with lower energy consumption can provide greater flexibility and environmental benefits. As communication needs grow (such as larger data demands), more battery capacity is needed, and it seems logical to turn the technology itself more efficiently and to support other functions. As mentioned above, the DMR infrastructure is also simpler than the infrastructure required for FDMA systems and therefore requires less energy.

Easy to use and create data applications

The end-to-end digital nature of DMR means applications such as text messaging, GPS and telemetry can be easily added to radios and systems. The DMR standard also supports wireless transmission of IP data, making it easy to develop standard applications. This provides a higher potential return for your investment. One of the key drivers for many switch-to-digital is the addition of service-enhanced data services and applications to the radio system.

Doubling the channel capacity of DMR is also key to adding data applications. In order to maintain the quality of existing voice services, additional data traffic capacity must be available. This is especially important for applications such as automated vehicle positioning, where the system can generate a large number of messages to keep the location up to date. While this may be a very valuable tool for business users, it is likely that additional capacity will be required if the voice service is not adversely affected. The DMR implementation can provide the extra capacity required simply and cleanly.

Figure 7 : Location-based service for use with DMR systems to track user location

System flexibility through simultaneous use of TDMA channels

When the voice uses the first time slot, then the second time slot can be used in the TDMA system to send application data such as text messages or location data in parallel. For example, this is useful in scheduling systems that provide verbal and visual scheduling instructions. This is an enhanced data capability that is becoming more and more important in a data-rich environment.

Future roadmaps for two-socket TDMA applications include the ability to temporarily combine two slots to effectively double the data rate to 9.6kb/s, or use two slots together to enable full-duplex, telephone calls (such as Private call). FDMA radios do not provide these features, without the need to add additional transceivers and use additional licensed channels. This is because there is a communication path in a single 6.25 kHz FDMA channel; only people can talk, but not two, or you can transmit voice or data, but not both, and the data rate can be compressed to a single 4.8 kb/s for the 6.25 kHz channel.

Advanced control functions

The DMR standard allows the use of a second time slot for reverse channel signaling – that is, when the first channel is in a call, instructions in the form of signaling are transmitted to the radio on the second time slot channel. This enables priority call control, remote control of the transmitting radio or emergency call preemption, and provides precise control and flexibility to the operator of the radio system. FDMA systems do not provide similar functionality because they are limited to one path per spectrum channel.

Superior audio performance

DMR digital technology provides better noise rejection and maintains speech quality over a larger range than analog, especially at the farthest edge of the transmission range. This is because a lot of effort has been put into the selection of forward error correction (FEC) and cyclic redundancy check (CRC) encoders when developing standards. Errors are detected by analyzing bits that receive radio detection and automatically correct transmission errors. The DMR standard specifies more than 14 different encoders, each matching a different type of flow. By using encoders and other techniques, digital processing can filter noise and reconstruct signals from degraded transmissions.

Figure 8 : Improved range of DMR compared to simulation

There is controversy about which digital system provides the best coverage; systems based on 12.5 kHz or 6.25 kHz channels. Both have advantages and disadvantages. Systems based on 6.25 kHz are at a disadvantage because when you compress multiple high-power transmissions in a 6.25 kHz channel into the spectrum, each transmitted modulated signal must be very tightly constrained; technically reducing signal skew; causing spectrum Interference from the next channel. This limitation of signal skew means that when the signal is weak (ie at the edge of the system), the receiver is less able to distinguish whether it is transmitting one or zero. In theory, this will affect the coverage of the 6.25 kHz system.

Some regulators also limit the power of the repeaters used in the 6.25 kHz FDMA system to 50% of the available power of the 12.5 kHz DMR system, where the user wishes to operate two 6.25 kHz in a given 12.5 kHz spectrum. Repeater. This is done to ensure that the total power level is maintained per unit spectrum. These restrictions may also affect the scope. DMR systems also benefit from the superior implementation of forward error correction protocols. However, the FDMA system does benefit from the fact that for a 6.25 kHz channel, the noise floor is lower than the noise floor with a wider 12.5 kHz channel.

Through the above description, do you have a deeper understanding of the main advantages of DMR digital walkie-talkie technology? Welcome message~

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