Digital Two-Way Radios: NXDN vs. DMR. What does it mean and why does it make a difference?

When considering migrating to digital radios, there is a crossroad.  For many years, it didn’t matter what manufacturer you chose, all radios worked together.  Say you have some Motorola radios but Kenwood has the latest hot product?  No problem!  Just program it to the same frequency and tone. Now it works with the others.  Not necessarily so anymore.

The FCC, in an attempt to find room for more radio users, forced existing users to “narrowband”. This meant that radios would use less bandwidth to transmit through.  As opposed to using 25 kHz of bandwidth, they would use 12.5 kHz.  Manufacturers made their equipment capable of doing this and users bought replacement radios to be compliant.  This ultimately created more “slots” available in between.  Think of it as radio users standing shoulder to shoulder suddenly turning sideways.  Now you can stick a new user in between two incumbent users.
The FCC also indicated that 12.5 kHz was simply a stepping stone on their way to the ultimate destination of 6.25 kHz.  This brought out a problem; you can’t transmit voice through that ultra narrow bandwidth.  But you can transmit data (digital transmissions).  This is where digital radios came in and a division was established.  Two technologies were born out of this.  One technology developed was based on TDMA (Time Division Multiple Access), the other FDMA (Frequency Division Multiple Access).  Both are compliant with the FCC and work but are completely incompatible with each other.

In TDMA, you still transmit with 12.5 kHz bandwidth but 2 groups can transmit at the same time (interleaved like a zipper into the same frequency) effectively creating 6.25 equivalence. In FDMA, the radios transmit with 6.25 mHz of bandwith.  Multiple transmissions are divided by being on different frequencies.

Again, both work, but what makes the most sense?  Manufacturers will argue vehemently about how their technology is better.  Kenwood and Icom chose FDMA; Hytera, Motorola, and Vertex/Standard chose TDMA.

The answer of what is better lies in what you want to accomplish.  If you have a need to connect a few individuals with radios, either technology will work.  If you have multiple groups wanting to communicate, TDMA is the clear cut winner.

Yes, FDMA accomplishes the same goal (6.25 mHz digital) but you need multiple frequencies to provide the same capacity.  In many instances this means you have to double your infrastructure costs (repeaters, antennas, etc.) and presents some limitations on how multiple groups are able to communicate.  One key difference is that if everyone is on the same frequency (as is the case in TDMA) making a call to multiple groups (all call) is more feasible.

Also, if you want to incorporate individual call, TDMA technology makes this realistic.  In FDMA, if there is a transmission on the frequency, no other transmission can go through.  In TDMA, utilizing Hytera’s patented Pseudotrunk technology, you can effectively utilize individual calls, group calls, and all calls with a single frequency providing room for concurrent transmissions.
While this can be extremely confusing, we can help you navigate the nuances of the different technologies and develop a plan to choose the appropriate platform for your digital migration.

For more information on Hytera’s Pseudotrunk, see our previous post about “The other (better) Mototrbo solution”.

 http://pwcmnblog.blogspot.com/2015/03/the-other-better-mototrbo-solution.html