Double your call capacity

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. 
 
DMR Association
Figure 1: 2 timeslot TDMA structure of DMR
 
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. This is illustrated in diagram 2 below. This 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. The US public safety radio standard, P25, is also currently evolving its Phase II specifications to two-time slot TDMA.

                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      

 

 
Figure 2: Analogue to digital migration with DMR systems.
 
The alternative approach to increasing capacity is to split 12.5kHz or 25kHz channels into two or more discreet 6.25kHz channels, known as Frequency Division Multiple Access or FDMA. Radios that are able to able to talk in 6.25kHz FDMA are then theoretically able to squeeze two new channels side by side in an old 12.5kHz channel. The practical reality is different. In many countries no specific 6.25 kHz licenses exist and the regulatory regime does not permit a license holder to operate two 6.25kHz channels in an existing 12.5 kHz licence. It is usually, however, possible to operate with a single 6.25kHz radio channel within a 12.5 kHz license but no increase in capacity is achieved for the user. This situation is illustrated in figure 3 below. In the United States, where 6.25kHz channels are licensed, licence holders have not been permitted to sub-divide existing 12.5kHz licenses into multiple 6.25kHz channels. So to increase capacity for 6.25kHz FDMA systems, users have to seek new 6.25kHz licences in other areas of the spectrum. Even in jurisdictions, where a user is allowed to squeeze two 6.25 kHz paths an existing licence this may cause problems. Operating a system at one site using two channels that are adjacent to each other in the spectrum is well known to create a risk of interference . So for this reason users would still most likely want to obtain a new licence in another area of the spectrum to increase capacity with a 6.25 kHz FDMA solution (see figure 4 below). In contrast because DMR’s two TDMA paths fit neatly into the existing channel structure, no new interference issues will be encountered when DMR systems are installed.
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                       
 
 
 
 

 

Figure 4: Analogue to digital migration with 6.25 kHz FDMA Vs TDMA
 

In summary both FDMA and TDMA systems used in digital PMR/LMR protocols are, in theory, equally spectrum efficient in that they can provide two talk paths in 12.5 kHz of spectrum but the TDMA approach used by DMR brings the advantages of compatibility with the existing licence regimes in place around the world and and does not introduce new interference issues.
One potential advantage of the FDMA 6.25 kHz approach is that you do not need a repeater to co-ordinate TDMA timeslots to deliver two independent talk paths as is necessary for DMR. (DMR systems do work well without repeaters and still deliver the many benefits inherent in DMR systems such as reverse channel signalling, but not two fully independent channels per 12.5 kHz of spectrum). Without a repeater, however, all radios need to be in range of each other at all times to get a predictable doubling of capacity with FDMA. So if the system needs a repeater for extra range, or to cover a problem area, now or in the future (e.g. with a site move or the opening of a new location) this benefit of FDMA is of limited value. DMR systems also have the advantage that 12.5 kHz signals are more robust to interference than 6.25 kHz signals. This means that in noisy environments a signal on a 12.5 kHz channel is less likely to be degraded than a 6.25 kHz signal and so will be more likely to give an acceptable level of service to a radio user. So the non-repeater advantage of 6.25 kHz FDMA systems for capacity increase is only beneficial if: a) You only have a small site where at all times for the lifetime of the system all users radios will be in direct range of all others users they might need to communicate with. b) You have been able to obtain the frequencies required, as splitting an existing license into multiple 6.25 kHz channels will not be an option for regulatory or interference reasons. c) Cost or availability of more robust 12.5 kHz channel licences is an issue. d) There is no need to have compatibility with legacy 12.5 kHz analogue systems (see below).
DMR, which was developed from the start with long term business needs in mind, does not have these constraints.