Articles

The Doppler frequency offset, target distance, and angles of arrival (THETA/PHI) are defined in a data file and vary over time. These parameters are used to define the target model. The clutter magnitude distribution is set to Rayleigh and the clutter power spectrum is formed as Weibull. The antenna radiation patterns (Figure 4) for both the transmit and receive antennas are based on file-based data from a separate EM simulation, but could also be similarly modeled with measured data. The receiver filters the incoming reflected signal prior to amplification via a low-noise amplifier (LNA), which is then down-converted through a mixer and further filtered before input into the coupled correlator. The correlator performs correlation of the downconverted reflected signal with a coupled signal representing the input to the RF transmitter.

Figure 4. Subcircuit modeling both the TX and RX antennas and target model, including RCS model, multi-path channel, and RF path delay. Click here for full-size image.

Radar searching, tracking, and other operations are usually carried out over a specified range (receive) window and defined by the difference between the radar maximum and minimum range. Reflected signals from all targets within the receive window are collected and passed through a matched filter circuitry to perform pulse compression. The correlation processor is often performed digitally using the fast Fourier transform (FFT).

To detect the moving object more effectively, MTD, which is based on a high-performance signal processing algorithm for PD radar, is used. A bank of Doppler filters or FFT operators cover all possible expected target Doppler shifts. The output of the MTD is used for the CFAR processing. Measurements for the detection and false alarm rate are provided. The MTI is used to remove stationary objects, the MTD is used to identify the remaining moving target with the FFT size set to 64, and the CFAR performs a sliding average to ensure that the detected signal is greater than a set threshold.

Simulation Results

Figure 5. Plots representing various simulation results and system definitions such as antenna radiation pattern.

Under these settings, the simulation results are displayed in Figure 5. The radar signal waveform is measured in the time domain at the receiver input. Because the target return signal is often blocked by clutter, jamming, and noise, detection in the time domain is not possible and an MTD is used to perform the Doppler and range detection in the frequency domain. In the MTD model, the data are grouped for corresponding target range and Doppler frequency. Afterwards, a CFAR processor is used to set the decision threshold based on the required probabilities of detection and false alarm.

Chirp waveform: The time-domain graph shows the transmitted pulse, received pulse, and the pulse after the transmit/receive correlation. The correlator output is used in the baseband-received signal processing blocks to turn it into useful target information.

Antenna pattern: The radial plot shows the combined transmit and receive antenna pattern. When the simulation is run for the first time, the antenna parameters PHI and THETA are swept to obtain this data (see also antenna pattern VSS diagram for the swept variable setting).

MTI output: The time-domain plot shows the output of the MTI, which uses a second-order delay line canceler to remove effects of stationary clutter and leave Doppler information in the signal.

The graph shows the system metrics, including the detected speed, Doppler, probability of detection (PoD), radar cross section (RCS), and distance across multiple pulses.

Conclusion

This application example illustrates how key models and simulation capabilities within VSS enable practical radar design. Since much of the simulation control setup and radar system details (operating conditions) have been parameterized, this project can be used as a template for different PD applications. The radar signal is a function of PRF, power, and pulse width (duty cycle) and these parameters can be modified for different cases. In the simulation, the radar signal also can be replaced by any defined signal through the data file reader in which the recorded or other custom data source can be easily used.

This article was written by David Vye, Technical Marketing Director, National Instruments (Austin, TX). For more information, visit here.