4.4.2 TX Beamforming Signal Processing

This section provides an example signal processing chain in TX beamforming mode. Users might have their own preferred signal chain depending on the application. This example signal processing chain is used to generate an azimuth/range heat map by stitching multiple TX beamforming scans together.

Figure 17 shows the block diagram of the example signal processing chain in TX beamforming mode. The desired steering angles are defined as β1 β2 ... βn. For each angle βi, the phase shifter value is calculated based on the TX phase calibration vector and βi with given TX antenna positions (see Section 3.3.3). The received ADC data from each RX channel goes through range and Doppler FFT, and only zero Doppler bin is selected for range/azimuth heat map generation assuming it is a static test scene. For moving test scene, user needs to change accordingly. For each range bin at zero Doppler, after phase compensation using calibration vector, RX beamforming is performed to steer the RX beam towards the same βi where the TX channels are focusing on. As a result, one range line is generated for each angle βi. By stitching all the lines generated with different βi angles, a range/azimuth heat map is generated.

Figure 18 shows the example heat map results. Two corner reflectors are separated by around 1.8 degrees at 6 m within the anechoic chamber. The anechoic chamber size is around 6 m in width and 12 m in length. The radar is put around 4 m away from one side. The top left and top right plots are the heat map results (top-down view) running in MIMO mode and stitched TX beamforming mode. The TX beam is steering within [–60 60] with step size of 0.5 degree. The rectangular shape corresponding to the anechoic chamber wall, and the two bright spots in the middle are corresponding to the two corner reflectors. The bottom left and bottom right plots are the heat map results in a 3D view, with the height indicates the reflection intensity. The separation of the two peaks is more visible in the 3D view. It is worth to note that the grating lobes caused by the wide distance of adjacent TX channels are no longer observed because of the cancellation in the RX beamforming process.

This experiment result proves that the receiver angle resolution in these two operation modes are equivalent since the effective aperture size is the same. Further, the signal SNR in the beamforming is much higher due to the coherent gain in TX beamforming mode. The SNR in MIMO mode can be improved by increasing the chirp integration time.