# mmHSE: A Two-Stage Framework for Human Skeleton Estimation Using mmWave FMCW Radar Signals
## 3 Radar Data Acquisition and Processing
### Experimental Setup and Data Acquisition  
#### 3.2.1 Data Acquisition Platforms  
In this study, we develop a multi-branch data acquisition system comprising the following components:  
* MmWave FMCW Radar Sensing: As illustrated in Figure 3a,b, the system integrates two commercial mmWave FMCW radars from Texas Instruments--the IWR1642 BOOST and IWR6843 ISK--as primary sensing devices. These radars are connected to the DCA1000 EVM data capture module, enabling efficient acquisition and real-time transmission of raw radar signals. Two mmWave FMCW radars with different operating frequencies (60 GHz and 77 GHz) are employed to capture complementary spatial information. The higher-frequency radar provides finer angular resolution due to its narrower beamwidth, while the lower-frequency radar offers more robust sensing under challenging conditions. This setup enhances azimuthal discrimination by capturing scene information from distinct frequency-dependent perspectives.
* Optical Ground Truth Annotation: As shown in Figure 3c, the system employs the Microsoft Kinect V2 optical sensor as the ground truth annotation device. This sensor provides high-precision three-dimensional coordinates for 21 skeletal joints, as depicted in Figure 3d. Compared to the commonly used 17 keypoints, which focus on major joints such as the head, shoulders, elbows, and knees, the 21 keypoints offer a more detailed representation of the human body by including additional keypoints like the wrists, ankles, and facial landmarks. This extended set provides a finer level of detail, particularly for modeling more complex body movements and interactions. These 21 keypoints serve as a reliable reference for subsequent algorithm training, offering a balance between anatomical coverage and computational efficiency.
* Synchronization and Control System: Two high-performance computers function as the control center to manage the entire system. Time synchronization across multiple data sources is achieved using the network time protocol, enabling millisecond-level timestamp alignment and ensuring temporal consistency among different modalities.
* Radar Configuration: The core parameters of the two radars as shown in Table 1. The IWR1642 BOOST operates in the 77 GHz mmWave band with a two-transmitter, four-receiver (2 Tx-4 Rx) MIMO antenna array, forming eight equivalent channels in the azimuth dimension through MIMO virtual aperture technology. Similarly, the IWR6843 ISK operates at 60 GHz with a three-transmitter, four-receiver (3 Tx-4 Rx) configuration. Both configurations utilize all available antennas on the respective radar modules, with the difference in the number of transmitters being due to the hardware specifications of the radar modules, rather than a specific performance consideration. By using all available receive antennas, both radar configurations ensure maximum spatial resolution and improved angular accuracy, which enhances the ability to resolve azimuth and elevation angles. These setups maximize the number of virtual channels formed through MIMO, providing better angle-of-arrival estimation with minimal computational complexity. Other parameters are consistent across both devices: an effective bandwidth of 3.072 GHz, 256 samples per frame. The frame period was empirically set to 50 ms to balance temporal resolution and system complexity. This setting also enables reliable temporal synchronization among the two mmWave FMCW radars and the Kinect V2 camera, ensuring consistent multi-sensor alignment during data collection.