I. Introduction
Skywave over-the-horizon radar (OTHR) operates in the high frequency (HF) band (3–30 MHz) by reflecting transmitting signals from the ionosphere. It is able to detect the surface and airborne targets beyond the line of sight [1]–[4]. However, in the user-congested HF band, the skywave OTHR is easy to be interfered. One of the interferences is the radio frequency interference (RFI). The RFI is usually generated by shortwave communication, broadcast, and other operational equipment, which means that it is uncorrelated with the radar transmitting signal. Therefore, it will still occupy all the range cells in its duration after pulse compression. Moreover, comparing its duration with the coherent processing interval (CPI), the RFI can be either transientlike or nontransient interference. The transient RFI has much shorter duration than the CPI, and its Doppler spectrum will occupy many Doppler channels. For the nontransient case, the wider the RFI's bandwidth, the more Doppler channels it will occupy. As a result, the RFI in both cases will submerge potential targets in the range-Doppler map
Range-Doppler map refers to the 2-D spectrum consisting of radar echoes after pulse compression and coherent accumulation.
and severely degrade the target detection performance.