# A Circularly Polarized Millimeter Wave Radar for Wind Turbine Sensing
## 2 Radar Architecture Design
### Link Budget Analysis  
The clearance radar presented in this paper utilizes the AWR2243 Texas Instruments as its foundation [18]. This chip boasts many benefits, including a wide frequency range of 76-81 GHz, a high transmitting signal power of 13 dBm, and a low noise figure of 12 dB. Additionally, it features a \(3\times 4\) MIMO (multiple-input multiple-output) radar configuration and allows for flexibility in combining all three transmitting channels into a single path to realize a \(1\times 4\) SIMO (single-input multiple-output) radar configuration. Combining three transmitting channels into one path via a combiner can enhance the SNR of a weak return signal, which means the transmitting signal power is \(13+4.7=17.7\) dBm. The radar system's block diagram is included in Figure 3. The stm32 generates the control RF signal, and the RF circuit transmits the electromagnetic wave through the transmitting antenna. After the four receiving antennas receive the signal, the electromagnetic wave is sent to the FPGA by the RF link for processing.  
For the link budget analysis, receiver sensitivity \(S_{\text{imin}}\) is the minimum signal strength required for a receiver to detect and process a signal. The target can only be detected when the received power \(P_{\text{r}}\) is higher than the sensitivity [19]. At room temperature, receiver  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**ID** & **Index Name** & **K-Band Radar** & **W-Band Radar** \\ \hline
1 & Frequency range & 24–24.25 GHz & 76–81 GHz \\
2 & Bandwidth & 250 MHz & 5 GHz \\
3 & Range resolution & Minimum 60 cm & Minimum 3.75 cm \\
4 & Velocity resolution & Low & Three times the former \\
5 & Antenna size & Large & A third of the former \\ \hline \hline \end{tabular}
\end{table}
Table 2: Parameter comparison of millimeter-wave radars.  
Figure 3: System block diagram.  
sensitivity consists mainly of ambient thermal noise, noise figure NF, receiver echo signal bandwidth B, and demodulation threshold SNR. The formula is as follows.  
\[\mathrm{S_{imin}}=-174+\mathrm{NF}+10\mathrm{lgB}+\mathrm{SNR} \tag{1}\]  
In addition, the intensity of the echo signal \(\mathrm{P_{r}}\) is given by the following formula as  
\[\mathrm{P_{r}}=\frac{\mathrm{P_{t}}\mathrm{G_{r}}\mathrm{G_{r}}\mathrm{A}^{2} \sigma}{(4\pi)^{3}\mathrm{R}^{4}} \tag{2}\]  
where \(\mathrm{P_{t}}\) is the transmitter output power of 17.7 dBm, \(\mathrm{G_{t}}\) and \(\mathrm{G_{r}}\) represent transmitting and receiving antenna gain, \(\sigma\) is the radar cross section, and R is the radar range. Since the blade tip is a weakly reflected signal with a cross-sectional area of about \(-12.5\) dBsm, this point should be seriously considered when designing the radar architecture. The transceiver antenna gains \(\mathrm{G}\)t and \(\mathrm{G}\)r increase as the number of units increases. After carefully considering the radar system's need and available space, the optimal transmitting and receiving antenna arrays are determined to be 8 \(\times\) 14 and 3 \(\times\) 14 cells, respectively. These arrays have an antenna gain of approximately 20 dBi.