Digital Predistortion for Phased-Array Radar

In modern phased array radar, each channel has a transmit/receive module (TRM) connected to antenna or radiating element. In this kind of TRM, one of the key components is the high power amplifier (HPA) which functions as the high power signal source in the transmitter(s). The HPA is operated in saturation to generate maximum power with optimal efficiency. Operating the HPA in saturation results in non-linear output behavior, which causes phase and amplitude errors in the transmitted waveform. A common approach used to avoid non-linear operation is to reduce (back-off) amplifier drive. In this case, amplifier power efficiency quickly will drop and output power will be greatly reduced. Digital pre-distortion (DPD) is a powerful linearization technique which allows the HPA to operate in the saturated region with much improved linearity, power efficiency and output power. DPD is especially important for phased array radar systems. A distorted transmit signal that radiates through the phased array antenna system, results in degradation of antenna directivity, sidelobe level, null depth, and null direction. Additionally, transmit waveform distortion results in increased range-time sidelobes. Applying DPD will improve transmitter and antenna performance for phased array radar.

In an effort to study the potential of such a phased array, NCAR is developing DPD enabled phased array system for demonstration as an advanced radar platform. Memory polynomial DPD is adapted to improve the linearity, power efficiency, and antenna beam pattern. The memory polynomial used is the well known Volterra series. The HPA output is sampled using external loopback to an observational port. The algorithm characterizes and calculates the coefficients of a polynomial which is the inverse of the HPA’s nonlinearity. Applying this polynomial to the transmit waveform in the digital domain is known as DPD. DPD model can be applied to existing TRM in phased array radars or in conventional radars which use a solid state power amplifier (SSPA) for a transmitter. Applications include spaceborne, airborne and ground based radar systems.

In this paper, the design of a DPD enabled transmitter will be discussed. Simulation results depicting the effects of DPD on antenna directivity, scanning capability and sidelobe level will be shown. In addition, measured results of DPD applied to an 8 watt, C-Band HPA will be presented.