Axell Wireless specializes in devices for extending the coverage of radio networks, and at PMRExpo it showed a new TETRA repeater featuring the latest evolution of its software-defined radio technology. This adds frequency-shifting functionality to the basic repeater concept.

“We have some quite exciting results from rural area coverage tests done in various countries in Europe”, said Håkan Samuelsson, of Axell. “We’ve been able to extend the coverage of one existing base station up to the maximum distance of 57 km. That’s as far as TETRA goes, due to the time-slot timing issue.

“With frequency-shifting technology, where the input frequency and the output frequency are different, there is no risk of the repeater oscillating, which means that you can have the antennas – both the pick-up and the radiating antenna – rather close to each other. You don’t have to watch out for antenna isolation.

“That also means that we can have a high-gain repeater with limited antenna isolation.”

Cost savings

Needing no microwave links to connect it, and being much cheaper than a standard TETRA base station, the repeater offers an economical way of reaching shadow areas or remote locations such as offshore oil rigs – although it does so at the cost of robbing some traffic capacity from the base station which serves it.

“It’s an interesting concept which, by the way, has been used in GSM networks for many, many years”, Mr Samuelsson said. “It’s a proven technology and now we have for the first time introduced it into TETRA.”

The frequency shift, he explained, is executed in software. “We use one channel in the TETRA band as backhaul. We take the base station signal and shift it to an unused channel, shoot that channel over to the remote site and convert it back to the original base station frequency.

“There is a signal in the air in the allocated frequency band, so the mobiles could pick it up and consider it to be a usable signal. But if they try to place a call, it’s on the wrong frequency! It doesn’t reach back to the base station. To avoid that, we did another trick: we actually invert the spectrum on that link, which we can do also with software – so you can say that the link frequency is scrambled to TETRA. We convert it back at the other end and it suddenly becomes a readable TETRA signal again.”

Slot by slot

A subtle feature of Axell’s software-defined radio technology is that the TETRA time-slots can be manipulated individually. “For instance”, Mr Samuelsson continued, “if you have several repeaters around a base station feeding signals back to the base station, and on a certain channel there is no traffic, then the repeater is running noise back only. The channel with a high-gain repeater becomes a little bit noisy. If you have several repeaters around the base station, the sensitivity of the base station goes down because all these repeaters are emitting noise.

“With this one, we can measure when one single time slot in the channel is not in use and then we mute the repeater. Then, if there is a mobile in the area of the repeater, the repeater opens up. And the action is so quick that no analogue ALC loop would be able to cope with that without over-swinging.”