Tampa (THEA) Connected Vehicle Pilot Detected and Resolved Wireless Interference

The Tampa Hillsborough Expressway Authority (THEA) Connected Vehicle Pilot provides the ability to link equipped vehicles with infrastructure via roadside units (RSUs) installed at downtown intersections and other key deployment areas. One such area is at the end of the Selmon Reversible Express Lanes (REL) where the THEA expressway facility meets the downtown Tampa street network. Equipped vehicles and RSUs in this area will communicate to both monitor real-time traffic conditions and provide alerts to drivers of the need to slow down as they approach potential queued traffic at the end of the REL. In addition, the vehicle-infrastructure communications will be used for supporting services, such as downloading over-the-air (OTA) software updates to vehicle devices and transferring log files for analysis and performance measurement.

The supporting wireless communications capabilities between vehicles and infrastructure have been established using wireless spectrum in the 5.9 GHz range licensed by the Federal Communication Commission (FCC) to provide reliable performance for both safety and operational efficiency. The Florida Department of Transportation (FDOT) serves as the licensee for infrastructure RSUs, as it manages transportation-related communications licensing and frequency coordination for the public agencies within Florida.

In 2017, during the preparations for deployment of the infrastructure RSUs, the THEA CV Pilot team observed that there were other apparent communications on some of the wireless channels being used in the pilot deployment. Specifically, while testing the THEA CV Pilot devices, Internet Protocol (IPv6) radio traffic was observed on some of the Dedicated Short-Range Communications (DSRC) channels planned for use in the pilot. The THEA team procured a wireless network capture device to investigate further, and captured samples of the packets observed on these channels for further analysis.

In analyzing the captured IPv6 packets, the identifier “HamWAN” was frequently observed, and the Media Access Control (MAC) addresses and an amateur license identifier were consistently observed. The THEA team was able to associate the identifiers to a group known as the Florida Simulcast Group (FSG), which is a local Florida group associated with HamWAN. HamWAN is a moniker associated with a technique used by amateur (Ham) radio operators that employ wireless radios in the amateur radio frequencies to provide Internet Protocol communications over a region. Radios used in HamWAN have a range of over 50 km and are using frequencies/channels that are also in the 5.9 GHz band used by DSRC. However, HamWAN radios are not designed to co-exist with DSRC communications and hence pose an interference hazard.

Amateur Radio has a secondary allocation in the 5.9 GHz spectrum, meaning that secondary users are responsible not to harmfully interfere with the primary user (DSRC) in the spectrum. During a THEA live demonstration of interactions between vehicles and roadside infrastructure in early 2018, it was observed that some applications’ performance was significantly degraded as compared to prior off-site testing in a different location. The sample from the wireless capture device was subsequently found to have significant non-THEA traffic on the channels being used by these applications and appeared to show that the secondary use was causing harmful interference.

However, this data was not accepted by the secondary user as definitive proof that the interference was associated with their activities. Despite several discussions, the THEA team was not able to get the FSG to voluntarily cease or cooperatively eliminate the interference with the DSRC channels being used in the CV Pilot. As a result, the THEA team, working with FDOT, pursued a strategy to define and document evidence of HamWAN interference.

Since there was no pre-defined process for documenting interference to DSRC communications, the THEA team developed a test plan that leveraged an existing capability developed by the CV pilot system to transmit OTA updates from RSUs to vehicle devices. This update capability allows for software updates to be wirelessly received by vehicle devices without needing to visit a garage or other facility, by using RSUs installed in the area of the Selmon REL where participants are expected to drive by regularly. To control for other factors, a vehicle with an on-board unit (OBU) installed was parked at a fixed location approximately 100 m from an established RSU location, with clear line of sight. The devices were then configured to transmit (from the RSU) and receive (on the OBU) an over-the-air update of a known fixed size, utilizing different channels on two separate days. A wireless network monitoring device was also used to identify if non-DSRC (HamWAN) transmissions were present during testing. When HamWAN traffic was present, the median time to complete the OTA update reception was 78 seconds, as compared to 51 seconds when HamWAN communication was not detected. FDOT reviewed the test results and sent a “secure your equipment” notice to FSG in late 2018, asserting that the secondary licensee was causing interference to the primary licensee. Subsequently THEA has not observed any HamWAN transmissions.

HamWAN radio antenna

The above pictures document the position of the vehicle as well as the OBU antenna position on the vehicle. Further the line of sight to the HamWAN radio antenna (red arrow) is shown. (Source: THEA)

The THEA team continues to monitor for potential interference by means of random wireless monitoring (sniffer) surveys in the CV Pilot area. In addition, the THEA team has recommended to other CV deployers that an early and complete radio spectral analysis should be conducted for areas for deployment, to include the full DSRC spectrum and adjacent channels. It is important to note that because HamWAN uses related but incompatible IEEE 802.11, identification of the interfering traffic was made easier by virtue of the MAC and other identifiers present in the logged packets. For other types of radio interference, a complete spectral analysis may be required to identify signals and manually track the source(s).