Investigations for a joint FMCW radar and wireless communication system in FDM taking into account the power amplifier characteristics

The airborne circular synthetic aperture radar (CSAR) can be used for the continuous and daylight independent surveillance. In case of small flying platforms, it is attractive to use a broadband frequency-modulated continuous wave (FMCW) radar and to transmit the sensor data to a ground station inside the measurement area for CSAR signal processing to save on-board resources. Thus, a joint FMCW radar and wireless communication system is needed. We propose and investigate frequency division multiplex (FDM) of broadband FMCW radar and single-carrier quadrature phase-shift keying (QPSK) communication using a single high power amplifier (HPA) for saving weight, size, power consumption and costs. Because of the HPA characteristics it is necessary to insert a frequency gap between the broadband linear chirp and the communication bandwidth. In case of an example scenario, the increase in overall bandwidth is only around 2.5 % of the radar bandwidth at 500 m flight altitude. It is shown that the transmit power of the communication part can be reduced to 13.4 dBm compared to the radar part with 30 dBm for maximal 500 m altitude. This enables power balancing for reducing intermodulation products of the HPA. Therefore, the reduction of the overall bandwidth by using a 70 % smaller frequency gap between the two frequency bands is possible. By using this power balancing the spectral efficiency is increased from 0.67 bps/Hz to 1.03 bps/Hz, but the power efficiency of the HPA is decreased by only 2.41 %. Therefore, it shows better performance for such applications than the linearity-hungry orthogonal FDM technique.