dPMR Technology Benefits
ETSI Compliant | True Digital Narrowband | Multi-Vendor | Open Standard | Proven
dPMR is a digital radio protocol specifically targeting highly functional solutions by using low cost and low complexity technology. dPMR is a narrowband (6,25 kHz) FDMA technology that is 100% digital and offers many forms of voice and/or data applications. The FDMA protocol specified in both TS102 490 and TS102 658 ETSI standards complies with the European Harmonised Standard EN301 166-2 for use in 6,25 kHz channels.
dPMR exists in a range of functional levels:
dPMR 446 - This is the licence-free product for use in peer to peer operations without base stations or repeaters in the 446 MHz UHF band. This technology is covered by the ETSI standard TS102 490.
This is dPMR in its simplest form; a peer to peer only operation without base stations or repeaters. As this equipment operates under licence-free status, it has to follow similar limitations as for the analogue equivalent PMR446. These limitations are that RF power is limited to 0,5 watt and only handheld equipment is permitted. The frequency allocation is just above that used by PMR446 and operates on 446,100 to 446,200 MHz. Naturally, because of the 6,25 kHz channelling, dPMR446 offers double the number or channels as PMR446.
dPMR446 equipment is capable of both voice and data modes of operation using a simplified addressing system that can be considered analogous with CTCSS use in PMR446 or with an extended addressing system as used by the fully functional TS102 658 type dPMR radios. As well as offering voice and data, dPMR446 protocol also supports combined voice+data so it is possible to embed data into a voice call or automatically append it at the end of a call. This means that dPMR446 can offer all the usual voice services plus text messaging (SMS), status information texts, embedded data such as GPS position etc.
dPMR Mode 1 - This is the general purpose application of dPMR for all forms of licenced PMR use. Mode 1 is part of the ETSI standard TS102 658
This is the peer to peer mode of dPMR (without repeaters or infrastructure) but without the limitations of the licence-free counterpart. It can operate all typical licenced PMR frequency bands and without the RF power limits of dPMR446. As well as offering voice and data, dPMR446 Mode 1 also supports combined voice+data so it is possible to embed data into a voice call or automatically append it at the end of a call. dPMR Mode 1 can be considered as an advanced version of dPMR446.
Certainly by programming a dPMR Mode 1 radio with the same frequencies, colour codes and address mode as a dPMR446 radio it would be possible for the two radios to communicate. Just as for dPMR446, the dPMR Mode 1 protocol also supports combined voice+data so it is possible to embed data into a voice call or automatically append it at the end of a call. This means that dPMR446 can offer all the usual voice services plus text messaging (SMS), status information texts, embedded data such as GPS position etc. The removal of licence-free limitations means that dPMR Mode 1 can also offer such functions as priority and emergency calls and break-in.
dPMR Mode 2 - This is the standard licenced PMR version that includes all base station and repeater functionality and allows interfaces via gateways (Telephone, IP etc) Mode 2 is part of the ETSI standard TS102 658.
In the dPMR Mode 2 level of functionality, repeaters and infrastructure are added. This brings extra functionality such as network interfaces which can be IP based or simply analogue. Coverage areas are greatly increased, even more so when multiple repeaters are used.
Such multiple repeaters can be managed by dynamic channel selection or they can be co-channel wide area using that specific function from the dPMR Mode 2 protocol. Again, dPMR Mode 2 can offer all the basic functions of the Mode 1 protocol with the extra benefit of being able to interface beyond the wireless part of the network. IP connectivity will permit user groups to include PC based terminals from other offices, areas or even countries. The same interface could provide remote control of a base station or repeater from a fixed connection.
dPMR Mode 3 - This is the full functionality of dPMR that can be offered by managed access, multi-site, complex systems including all the same interfaces and gateways as Mode 2.
dPMR Mode 3 is the final step where all the possible functionality of the protocol is available. Mode 3 can offer multi-channel, multi-site radio networks that are fully managed by specific beacon channels at each radio site. This ensures optimum use of spectrum and optimum density of radio traffic. Management of the radio network starts from the authentication of radios that wish to connect.
Calls are set up by the infrastructure when both parties have responded to the call request, ensuring optimum use of the radio resource. Calls may be diverted to other radios, landline numbers or even IP addresses. The infrastructure managing these beacon channels would be capable of placing a call to another radio, whether that radio is using the same site or another site within the network. As the allocation of communication channels is made dynamically, the system can further optimise traffic by dynamically modifying permitted call times. Radios that fail authentication can be blocked, either temporarily or permanently.
The system will allow radios with the authority to make priority or emergency calls to pre-empt the use of channels from users making non-priority calls. Where call requests exceed capacity, these calls can be queued by the system until resources become available. All the services and functions available in Mode 1 and 2 are possible with a few exceptions, as well as several extra functions implemented by the beacon channel infrastructure.
The difference between FDMA and TDMA
Without getting too technical, the basic difference between FDMA (Frequency Divided Multiple Access) and TDMA (Time Divided Multiple Access) is the definition of a channel and how it is used (accessed). In FDMA, a particular bandwidth (E.g. 6.25kHz) at a particular frequency (E.g. 150.000MHz) is used to define a channel in essentially the same way channels have been allocated for decades. In TDMA, the same principle applies regarding bandwidth and frequency but the signal is divided into time slots that allow the channel to have 'extra' capacity in the same bandwidth E.g. Two 6.25kHz 'equivalent' channels in a 12.5kHz channel. See the diagram below for a graphical explanation.
Until now, TDMA was more spectrum efficient at wider channel spacing like 25kHz as, for example, two or three users could access the same bandwidth as one FDMA channel user. However, in the case of the newly developed narrowband 6.25kHz FDMA technology like dPMR, both this and 2-slot 12.5kHz TDMA technology achieve the same result as far as spectrum efficiency is concerned.
dPMR, in common with other fully digital radio systems, does not transmit analogue voice signals. The voice signals transmitted have to be converted to digital and this process is accomplished by the vocoder. In order to ensure that all types and modes of dPMR will interoperate, it is clearly essential that use is made of the same vocoder. For this reason, the dPMR association has evaluated suitable vocoder technologies and has selected the standard vocoder to be used by all dPMR radios. Users can now be assured that radio products which carry the dPMR approved trademark will interoperate for all types of voice call. It is of course possible, using the dPMR protocol, to implement radio equipment with more than one vocoder type for specific applications. Provided such equipment also offers the standard vocoder, it may also be marked with the dPMR approved trademark.
Both TDMA and FDMA technologies achieve the same 6.25kHz narrowband capability via different methods. The difference is that the FDMA system is a 'true' 6.25kHz channel and the TDMA system provides 6.25kHz channel 'equivalence' via the time slots in 12.5kHz band-width. From the perspective that 12.5kHz is considered the current narrowband standard channel spacing, then both systems achieve so called "double capacity". The difference is that the FDMA system is ALWAYS double capacity whether it is used with or without infrastructure. For TDMA, double capacity is ONLY achieved when a repeater is synchronising the time slots and when two users are in the same geographical area, accessing the same repeater at the same time.
FDMA Digital Mode
FDMA is always double capacity, whether in peer to peer or via a repeater
TDMA - Digital
TDMA using a repeater in digital mode
TDMA - peer to peer in digital mode
Voice call 1 (Time slot 1) occupies the whole channel, so time slot 2 stations cannot communicate.
In theory, in identical conditions, the narrower channel width of the FDMA system would allow the signal to achieve better coverage than the12.5kHz TDMA (or FDMA) system when transmitted at the same output power. This is because the noise floor of any receiver is proportional to the filter bandwidth. Therefore the smaller the bandwidth, the smaller the signals that can be received.
In real world use, various factors such as topography, antenna height of base stations and surrounding buildings etc. all affect coverage, so without specific comparison tests, either system cannot claim to be better than the other. What can be said is that when compared to an analogue FM signal, digital easily out-performs analog at the fringes of the communication range, thus providing more reliable audio over a greater total area, even if the coverage footprint is the same as analogue.
Longer Battery Life
The manufacturer of a competing TDMA system claims 40% improved battery life in digital mode, as the radio is transmitting only half the time (i.e. one time slot).Whilst only market feedback will prove this true or false, to date we have not been able to find any figures on actual transmit power consumption in public literature to make any accurate judgments on this claim. As such, the user cannot calculate if there really is any improvement in digital mode battery life. As explained in “Coverage”, in the FDMA system, reduced noise components with the narrower channel bandwidth improves receiver sensitivity. Therefore, it could be possible to transmit at reduced power, which in turn conserves battery life and thus can prolong radio use time.