Tollers make way as NextNav muscles into 902-928MHz spectrum

Toll operators and Progeny trade claim and counter claim about the potential ramifications of operating in the 902-928MHz spectrum, as Jon Masters finds out.

Two months after the 2115 Federal Communications Commission (FCC) determined that Progeny can start commercial operation of its NextNav location finding service, the dust has begun to settle. The tolling industry has had a chance to reflect on how this may impact its operations, in the knowledge that NextNav will share the 902-928MHz frequency band with RFID communication between toll tags and readers.

Superficially this appears to be nothing but bad news for the US tolling industry, presenting what seems to be the very real likelihood of radio interference and loss of data transmission between vehicle tags and readers at toll plazas. Could revenue losses amount to a complete breakdown in toll operators’ business models that rely on a 99% accuracy rate from their free flow electronic tolling technology?

Most probably not to this level it seems. Delve a little deeper and the background and explanation behind the Communications Commission’s determination becomes apparent. Progeny and the tolling industry are actually quite evenly balanced neighbours, in terms of their rights to operate and relative positions in the 902-928 spectrum.

All RFID toll transmissions occur at 902-904MHz or 911-920MHz. According to Progeny, its NextNav system will be restricted to operating only within a 4MHz bandwidth above 920MHz.

Significant concern

Nonetheless, there are enough unknowns and unanswered questions to justify significant concern among the tolling industry. Prominent among those that have expressed their unease is E-ZPass executive director PJ Wilkins. He wrote to NextNav CEO Gary Parsons at the beginning of May this year, inviting Progeny to engage in joint testing of NextNav alongside the E-ZPass electronic tolling system.

As Wilkins points out in his letter, seen by 1846 ITS International, E-ZPass tolling transactions occur in milliseconds at traffic speed with 99.9% accuracy. This has led to very high levels of customer satisfaction, improved safety and reduced congestion, as well as environmental benefits, he writes. The 4981 E-ZPass Group – an association of 25 toll agencies operating across 15 states – is concerned that interference may have dire consequences for safety and traffic flow, as well as implications for the revenue of its operations.

“I did receive a reply from Progeny, but this said they feel sufficient testing has already been carried out,” Wilkins told

ITS International. “We are disappointed with the FCC’s determination. Many in the tolling industry have reached out to legislative offices, but to no avail. We have no choice but to continue to monitor the situation and carry out our own checks for interference from NextNav.”
Central to the concerns of E-ZPass and others is the possible impact of NextNav operations on toll tag batteries. “It’s unknown what the implications for battery life might be. If this is reduced down to three to four years there will be a huge impact on cost and customer service,” Wilkins says. “Interference can drain the batteries in transponders. As an example, an indoor location system in Colorado was found to be draining transponders within 30 days because they were close to the system’s antenna.”

NextNav explained

NextNav, a multilateration-location monitoring service (M-LMS) is essentially a system for locating the positions of users’ cellphones. It is being marketed as a ‘mobile E911 indoor service’, an emergency response system capable of pinpointing cellphone location to a high level of horizontal and vertical accuracy. This enables responders to find stricken individuals within buildings where GPS signals are often blocked or fail to give an accurate location.

Progeny reports that around 70% of E911 calls are now made from mobile devices and that 70-80% of voice and data transmissions from cellphones are now made from inside buildings.

NextNav uses beacons placed in urban environments to augment GPS location by taking turns to send a location signal every 10th of a second. With a GPS chipset upgrade, cellphones receive signals from multiple beacons and use these to triangulate and fix their position in much the same way as with use of GPS.

Progeny first obtained licenses to operate within the 902-928MHz bandwidth in 1999 – permission that was conditional on Progeny not causing interference to other licensed users operating in the same band. Crucially, as technology and the NextNav system developed, Progeny needed a waiver of two M-LMS rules: one would allow it to provide services to all devices – not just vehicles – and the second permitted one-way communication to reduce transmissions and the likelihood of interference.

Interference testing

The Communications Commission granted Progeny its waiver in December 2011, conditional again on the same rules of limiting interference. In particular the recent order in favour of Progeny states that emphasis was placed on the requirement for demonstration that NextNav would not cause unacceptable levels of interference to unlicensed devices.

Unlicensed devices are not afforded protection rights under the Part 15 regulatory rules that they operate, but it is the possibility of interference for unlicensed devices that has drawn the most protest, judging by the wording of the Communications Commission’s order. The determination goes into a long explanation behind why it has rejected claims that Progeny’s testing with Part 15 devices has been insufficient. Principally it asserts that the intention was always to minimise rather than eradicate interference and claiming that no representative test can be developed for all Part 15 devices.

“We began testing two years ago on a variety of different devices and worked with the Communications Commission to identify all different transmission types that might be affected,” NextNav’s Parsons told ITS International. “Manufacturers of Part 15 devices are specifically told in their certifications that they are offered no protection against interference and should design their devices accordingly. All bounce around the 26 different frequencies, by channel hopping, or signal spreading to avoid the possibility of using the same bandwidth as other nearby devices.

“Some toll operators consider themselves unlicensed because they use similar techniques, but toll collection groups are licensed to use the same frequency band so they are promised protection against interference by Part 15. Nearly all toll collection groups operate at 915MHz and when the FCC looked specifically at equipment of the tolling industry it found no impact from our signal down to 915MHz. It’s useful to understand that we’ve been operating in a 1000 square mile area of California shared with toll collection agencies for three years and received no reports of interference.”

White paper

NextNav’s likely impact on neighbouring frequency bands has been comprehensively researched, Parsons says, and the findings reported in a Progeny white paper on the subject, which says ‘nearly all’ tolling equipment signals occupy a 2MHz bandwidth across 914-916MHz. The lower of Progeny’s two signals, at 919.75-921.80MHz, operates at a minimum of 3.75MHz above those of toll systems, the report says.

The report also acknowledges, however, that non-multilateral equipment, such as toll transponders, use a form of modulation that results in relatively high ‘out of band’ emissions. No coded data is modulated within these sidebands of the signal, so crossover or interference with this sideband energy will not cause loss of communication, the report says. Additionally, it points out that the NextNav beacons are ‘placed at the highest available points, often on existing broadcast, paging or cellular towers, which will further create isolation and separation from the wireless toll collection systems’.

Some in the tolling industry are satisfied that NextNav will not cause them any difficulty. In a statement from 139 Transcore, the company’s chief technology officer Kelly Gravelle says: “TransCore has been following the Communications Commission proceedings related to Progeny. We have advised our ETC system customers that we are not anticipating any adverse impact to their operations resulting from the Progeny proposal as presented in the current filings.”

Schneider Electric’s director of electronic tolling solutions Darby Swank says: “There is the possibility of interference from NextNav, but it is highly unlikely and when we are looking at potential tolling sites we always carry out a study of radio frequency activity in the area. If NextNav signals are present we will be able to see them and work around them because the system is constant and predictable. It is likely we will see NextNav signals because most tolling operations are centred around cities, but providing it’s consistent we can work around it.”

That is with regard to new installations of tolling systems. For existing toll collection networks there is less certainty, partly because not all systems are the same. Testing in the one area in California is not enough some say. “Demonstration that NextNav does not cause interference in California does not merit the assumption that there will be no interference with all toll systems,” says Randy Roebuck, advanced technology specialist in RFID for toll equipment manufacturer 4080 3M.

“There are different wake up and response mechanisms and some systems both read and write between toll tag and reader at traffic speed. The write transaction will be more sensitive to interference as it involves transfer of more data. Interference may not be an issue, but we just do not know for sure. The biggest question is, if NextNav does cause a problem, what can be done about it? As a stakeholder, we are here to help if a solution is needed.”