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Do you know the main advantages of DMR digital walkie-talkie technology?

Date:Feb 01, 2021

Because DMR is a completely open and open standard, supported by many vendors. Through this kind of users, both the security of supply and the advantage of continuous and competitive development. Open standards encourage a wide range of vendors to participate; there are many successful examples of technologies developed in this way. The DMR standard scores high on traditional analog systems and other digital methods. Let’s talk about the main advantages of the DMR digital system today:


Double the capacity of existing licensed channels


DMR enables a single 12.5 kHz channel to support two simultaneous and independent calls. This is achieved using TDMA, time division multiple access. Under TDMA, DMR reserves 12.5 kHz channel width and divides it into two alternating time slots A and B (as shown in Figure 1 below), where each time slot is used as a separate communication path. In Figure 1, radios 1 and 3 communicate on time slot 1, while radios 2 and 4 communicate on time slot 2.

1


Each communication path is effectively half of 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 called the efficiency of having one call path per 6.25kHz spectrum. However, for DMR, the entire channel maintains the same profile as the analog 12.5kHz signal. This means that DMR radios operate among licensees of existing 12.5 kHz or 25 kHz channels; therefore, there is no need to re-bundle 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 tried and tested very well. TETRA and GSM cellular mobile devices are one of the two most widely used radio communication technologies in the world. They are the TDMA system. The U.S. public safety radio standard P25 is currently developing its second phase specification into two-slot TDMA.

2


FDMA, frequency division multiple access, another way to increase capacity is to divide the 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 can squeeze two new channels side by side in the old 12.5 kHz channel.


Actual reality is insufficient. In many countries/regions, there is no specific 6.25 kHz license, and regulatory mechanisms do not allow license holders to operate two 6.25 kHz channels in an existing 12.5 kHz license. It is generally possible to operate with a single 6.25 kHz radio channel within a 12.5 kHz license, but this will not increase the user's capacity. This situation is shown in Figure 3 below.

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In the United States, the license holds 6.25 kHz channels, 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 the 6.25 kHz FDMA system, users must seek new 6.25 kHz licenses in other areas of the spectrum.


Difficulties remain in jurisdictions that allow users to compress two 6.25 kHz paths into existing licenses. As we all know, the use of two channels adjacent to each other in the frequency spectrum to operate the system at a site will cause interference risks. Therefore, for this reason, users are still likely to want to obtain a new license in another area of the spectrum to increase capacity using a 6.25 kHz FDMA solution (see Figure 4 below). In contrast, since the two TDMA paths of DMR are completely suitable for the existing channel structure, no new interference problems will be encountered when installing the DMR system.

4


In short, the FDMA and TDMA systems used in the digital PMR/LMR protocol are also spectrally efficient in theory, but the TDMA method used by DMR brings the advantage of being compatible with existing license systems around the world and will not introduce new interference problem.


One potential advantage of the FDMA 6.25 kHz method is that you do not need a transponder to coordinate TDMA time slots to provide the two independent call paths required by DMR. (The DMR system works well without repeaters and still offers 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, if there is no repeater, all radios need to be within range of each other at all times in order to obtain a predictable capacity multiplication through FDMA. Therefore, if the system requires additional range of repeaters, or covers problem areas now or in the future (for example, by moving a site or opening a new location), the value of this benefit of FDMA is limited. The DMR system also has the advantage that the 12.5kHz signal is more resistant to interference than the 6.25kHz signal.


The advantage of 6.25 kHz FDMA system without repeater for capacity increase is only advantageous in the following cases:


The site is small, and during the entire life cycle of the system, all user radios will be within the direct range of all other users


The required frequency has been obtained because dividing the existing license into multiple 6.25 kHz channels will not be an option for regulatory or interference reasons


The cost or availability of a more powerful 12.5 kHz channel license is an issue


No need to be compatible with traditional 12.5 kHz analog systems


DMR was developed from the very beginning in consideration of long-term business needs, without these limitations.


Backward spectrum compatibility with traditional systems


License holders may need to keep their existing licenses to ensure backward compatibility with their own traditional radios or external organizations’ analog systems. Since DMR uses 12.5 kHz channels, the required spectrum compatibility is built in. As shown in Figure 5 below.

5


Effective use of infrastructure equipment


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

6


FDMA requires a dedicated repeater for each channel, as well as expensive combination equipment, so that multiple frequencies share a single base station antenna. There may be further costs in getting a combined device to work with a 6.25 kHz signal, and when used in this way, signal quality and range are usually lost. This in turn causes a demand for the power amplifier shown in FIG. 6.


For the FDMA 6.25 kHz system, the error tolerance caused by the aging of the oscillator is small, and the transmitted radio causes the signal to deviate from the required center frequency. This makes the adjacent channel protection less robust and makes the system vulnerable to interference. Professional equipment; high stability oscillators can be introduced; but at a price. In contrast, dual-slot TDMA uses single-channel equipment to achieve stable dual-channel equivalents. No need for additional repeaters or combined equipment (and the air conditioning has a lower displacement and requires less backup power at the relay site).


Longer battery life and higher power efficiency


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


(The product data released by a manufacturer gives 9 hours of operating talk time in analog mode, but 13 hours of talk time in digital mode on the same radio). DMR digital devices can also include sleep and power management technologies that extend battery life.


Many factors affect the power consumption of a single device. When using published battery life data for the widely sold DMR and FDMA digital radio data, the data shows that for hourly use, TDMA requires a 19% to 34% reduction in battery capacity compared to FDMA models. Choosing technologies with lower energy consumption can provide greater flexibility and environmental benefits. As communication requirements grow (for example, greater data requirements), more battery capacity is required, and it seems logical to switch to technology that is more efficient and can support other functions. As mentioned above, the DMR infrastructure is also simpler than that required by the FDMA system, and therefore requires less energy.


Easy to use and create data applications


The end-to-end digital nature of DMR means that applications such as text messaging, GPS and telemetry can be easily added to radio equipment and systems. The DMR standard also supports wireless transmission of IP data so that standard applications can be easily developed. This provides a higher potential return on your investment. One of the key drivers for many switching to digital is the addition of business-enhanced data services and applications to the radio system.


The doubling of the channel capacity achieved by DMR is also the key to adding data applications. In order to maintain the existing voice service quality, additional data traffic capacity must be available. This is especially important for applications such as automatic vehicle positioning, where the system can generate a large number of messages to keep the location constantly updated. Although this may be a very valuable tool for business users, if it does not negatively affect voice services, it is likely that additional capacity needs to be provided. DMR implementation can simply and cleanly provide the required additional capacity.


(The product data released by a manufacturer gives 9 hours of operating talk time in analog mode, but 13 hours of talk time in digital mode on the same radio). DMR digital devices can also include sleep and power management technologies that extend battery life.


Many factors affect the power consumption of a single device. When using published battery life data for the widely sold DMR and FDMA digital radio data, the data shows that for hourly use, TDMA requires a 19% to 34% reduction in battery capacity compared to FDMA models. Choosing technologies with lower energy consumption can provide greater flexibility and environmental benefits. As communication requirements grow (for example, greater data requirements), more battery capacity is required, and it seems logical to switch to technology that is more efficient and can support other functions. As mentioned above, the DMR infrastructure is also simpler than that required by the FDMA system, and therefore requires less energy.


Easy to use and create data applications


The end-to-end digital nature of DMR means that applications such as text messaging, GPS and telemetry can be easily added to radio equipment and systems. The DMR standard also supports wireless transmission of IP data so that standard applications can be easily developed. This provides a higher potential return on your investment. One of the key drivers for many switching to digital is the addition of business-enhanced data services and applications to the radio system.


The doubling of the channel capacity achieved by DMR is also the key to adding data applications. In order to maintain the existing voice service quality, additional data traffic capacity must be available. This is especially important for applications such as automatic vehicle positioning, where the system can generate a large number of messages to keep the location constantly updated. Although this may be a very valuable tool for business users, if it does not negatively affect voice services, it is likely that additional capacity needs to be provided. DMR implementation can simply and cleanly provide the required additional capacity.


The future roadmap for dual-socket TDMA applications includes the ability to temporarily combine two slots to effectively double the data rate to 9.6kb/s, or use the two slots together to enable full duplex, telephone calls (such as Private call). FDMA radios cannot provide these functions without adding additional transceivers and using additional licensed channels. This is because there is a communication path in a single 6.25kHz FDMA channel; only humans can speak but not two, or you can transmit voice or data, but not both at the same time, and the data rate is limited to a single 4.8kb/s for 6.25 kHz channel.


Advanced control functions


The DMR standard allows the use of the second time slot for reverse channel signaling-that is, when the first channel is in a call, commands in the form of signaling are sent to the radio on the second time slot channel. This realizes priority call control, remote control of the sending radio or emergency call preemption, and provides precise control and flexibility for the operator of the radio system. FDMA systems cannot provide similar functions because they are limited to one path per spectrum channel.


Excellent audio performance


DMR digital technology can provide better noise suppression and maintain voice quality in a larger range than analog, especially at the farthest edge of the transmission range. This is because a lot of effort has been made to select forward error correction (FEC) and cyclic redundancy check (CRC) encoders when developing standards. By analyzing the bits to detect errors, these encoders receive radio detection and automatically correct transmission errors. The DMR standard specifies more than 14 different encoders, each of which is compatible with different types of traffic. Through the use of encoders and other techniques, digital processing can filter noise and reconstruct the signal from the degraded transmission.



There is controversy as to which digital system provides the best coverage; systems based on 12.5 kHz or 6.25 kHz channels. Both have advantages and disadvantages. The system based on 6.25 kHz is at a disadvantage because when you compress multiple high-power transmissions in the 6.25 kHz channel into the spectrum, you must very strictly limit the modulated signal of each transmission; technically reduce signal deviation; Cause interference in the next channel in the spectrum. This limitation of signal deviation means that the receiver is less able to distinguish whether it is sending one or zero when the signal is weak (that is, at the edge of the system range). Theoretically, this will affect the coverage of the 6.25 kHz system.


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