We received an interesting question the other day which we thought would be of interest to our readers. Here is the question and the answer. Hope you find this information useful.
Question: Is it possible for two different groups of digital radio users on the same frequency to conduct private simultaneous conversations?
Answer: One would think that this may be possible since typical digital operation is based on either time slot (TDMA) or frequency division technology (FDMA). Unfortunately, this is not the case. A “frequency” is a frequency and it cannot be divided into separate and distinct segments. If this were possible, each segment would in fact become a “frequency”.
An apparent exception is the Motorola MotoTRBO radio which is advertised to be capable of handling two separate communications functions (generally voice and data) at the same time on a single channel 12.5 kHz repeater. The truth is that a MotoTRBO repeater can handle two separate communications functions operating simultaneously. This is accomplished by splitting the 12.5 kHz channel into two separate 6.25 kHz segments or “channels”. The MotoTRBO system is based on TDMA technology. See www.info4u.us/TDMA-White-Paper.pdf for additional information on TDMA.
Neither NXDN (also known as IDAS by ICOM) nor P25 Phase 1(the federal standard) currently use TDMA. P25 Phase 2 radios will be based on TDMA technology, but from what we have seen, these new radios are beyond the price range of all but very large agencies. The pros and cons of the competing technologies is a subject for another time. Suffice it to say for now that a MotoTRBO repeater is the only repeater offering the ability to handle two communications activities at the same time. It is an excellent value, in particular, for those desiring to use both voice and data (GPS and Messaging) on the same 12.5 kHz “channel”
Conversely, the original XPR8300 series MotoTRBO repeater is not capable of mixed-mode operation (i.e. capable of handling calls from both analog and digital radios simultaneously). Both NXDN and P25 are mixed-mode capable which is a very attractive feature for those who are migrating from analog to digital over a period of time, rather than totally replacing an existing system all at once.
For those that don’t need mixed-mode operation, the MotoTRBO repeater system would be a good choice since the repeater costs considerably less than a comparable Kenwood NXDN repeater. ICOM offers a dual repeater package that costs only slightly more that the MotoTRBO repeater. In essence, the ICOM “dual” repeater provides the same functions as the MotoTRBO (i.e. two 6.25 kHz channels operating simultaneously) PLUS multi-mode operation. Additional information is available at www.icom.co.jp/world/idas/index.html. Current XPR8300 series can be upgraded to mixed mode operation, but the price is somewhat prohibitive as compared to the original models.
In conclusion, if you want two communications functions (voice or data) operating simultaneously, your choice is either MotoTRBO with no multi-mode capability or an ICOM dual repeater package. If you want three or more communications functions operating simultaneously, you will need a “trunking” system with compatible portable or mobile radios. That is a subject for another time. For now, p
