With TETRA, appearances can be deceiving
What is in this article?
More flexible, less expensive
There are other advantages to TETRA. One is that it employs 4-slot TDMA technology. As a result, four 6.25 kHz sub-channels can be carved out of a 25 kHz-wide channel, which dramatically increases capacity compared with analog, and even surpasses P25 Phase 2 and digital mobile radio systems, which employ two-slot TDMA.
"Let’s say that you have a four-channel trunked analog system," Nielsen said. "You would only have three talk channels. But, if you installed TETRA, you would have 15 channels."
Another plus is that TETRA not only complies with the FCC’s mandate to narrowband all frequencies below 470 MHz — i.e., converting 25 kHz-wide channels to 12.5 kHz—but also would be an ideal solution should the commission ever mandate 6.25 kHz-wide channel operation.
TETRA also is more flexible than P25 — a big selling point for the utilities sector, which has huge data needs, according to Bender. Many utilities are using a phalanx of monitoring solutions in an attempt to predict when devices will fail. Also, a huge electrical-grid modernization is underway, centered on the deployment of smart-grid infrastructure in many parts of the country.
"What the UTC is finding when we do frequency coordination is that, as much as utilities understand that data applications on the UHF channels are secondary, they still would prefer to use frequencies below 500 MHz, rather than going up to frequencies where they can get exclusive use," Bender said.
That’s because a lot of utilities are in "aggressive terrain areas," where building out a system in a higher frequency band would require considerably more infrastructure, in terms of the number of base stations—perhaps four times as many, according to Bender.
"It’s not worth it," he said.
However, TETRA offers a potential solution to this dilemma, because it allows the simultaneous transmission of both voice and data over the same channel — something that P25 can’t do.
"This is important, because UHF channels under Part 90 are not primary for data," Bender said. "So utilities that seek UHF channels only for data are subject to secondary use; that means a frequency coordinator can coordinate right over top of the data system, because it’s not primary, and these critical communications of utilities are subject to harmful interference."
Given all of this, plus the fact that TETRA subscriber units generally are far less expensive than P25 devices — attributable to TETRA’s economies of scale generated by its much wider use — one easily can see why entities would be clamoring for TETRA.
So, why isn’t it happening?