Weather Radars and Radio Frequencies Interference: Overview and Impact on Observations

The evolution of telecommunication systems is demanding new frequencies bands, close to the ones allocated for remote sensing users, and for sharing other portions of the electromagnetic spectrum. Due to the limited availability of dedicated frequency bands, decisions have been made in past years to allocate telecommunication systems and remote sensing instruments in the same band. Since the World Radiocommunication Conference in 2003 (WRC-03), the primary allocation for Wireless Access Systems (WAS), including Radio Local Area Network (RLAN) and Wireless Local Area Network (WLAN), was set in the bands 5.150 - 5.350 and 5.4750 - 5.725 GHz. Since then, weather radars and WAS are expected to coexist in the same frequency band, so these last are required to implement the Dynamic Frequency Selection (DFS) function. This algorithm should be able to detect radar signals in the WAS operating frequency. However, as reported in literature, there are several issues that affect the DFS capability to detect radar signals: first of all, weather radars transmit short pulses (e.g., 0.5μs) and different Pulse Repetition Frequency (PRF), which are difficult to be detected by the DFS.

Our team conducted an extensive set of experiments dedicated to identifying and documenting RFI and weather radars as well as developing techniques and technologies for mitigation. We conducted experiments with special receivers and decoders to identify Interfering sources and decoding the information carried in the service set identifier (SSID) and in the basic service set identifier (BSSID) transmitted by WLANs. Cross-checking the retrieved information of the interfering signals and the data available in the regional database of electromagnetic sources of North-West Italy, it was possible to easily understand where the antenna that generates the interference was located. Thus, field investigation was carried out in collaboration with the National Regulatory Authority to verify the standards compliance in the WLAN antennas.

RFI may also be generated by spurious or out-of-band emissions of devices which are not operating in the same band of a remote sensing instrument. It has been reported that weather radars operating at 9.3GHz, which is the X-band, are also affected by interference. Field investigations have been conducted and their results show that interfering signals may be caused by electronic components of commercial devices.

Weather observations and post processing algorithms, such as Quantitative Precipitation Estimation, Hydrometeor Classification and nowcasting may be affected by interference. C-band radars deployed in North-West Italy have been experiencing this. The amount of interference nowadays received at the lowest elevation scans is a significant issue affecting the data quality. In the past, simple filter thresholds have been used to mitigate the effects of RFI, but weak weather echoes are also filtered. Therefore, we have developed alternate techniques to separate the interference and weather echoes.