## 4 System Performance Discussion
### Parameter Discussion on System Performance
#### 4.2.3 Signal-to-Noise Ratio  
Signal-to-noise ratio (SNR) is an important indicator of performance for both MIMO radar and single RF link system. The SNR is the ratio of signal power to noise power in the channel:  
\[SNR=10\lg\frac{P_{S}}{P_{N}} \tag{29}\]  
For a single RF link communication system, SNR is positively correlated with channel capacity. In theory, when the SNR increases to a certain extent, its performance is the same as the traditional MIMO channel. For MIMO radar systems, targets with higher SNR are more likely to be detected. However, because of the introduction of the extra SPDT switch, the input two-tone third-order intercept point (IIP3) value of the switch will impose additional restrictions on the system performance. If it exceeds this limit, the switch will enter the nonlinear operating range during the receiver processing, resulting in the deviation of the result. Therefore, the SNR of single RF link MIMO radar system should be controlled in a certain range. When the signal power is predetermined, the SNR of the system will decrease with the increase of clutter energy. Therefore, in order to explain their relationship, the remaining conditions were set as follows. The insertion loss and isolation were 0.5 dB and 30 dB, respectively. The clutter amplitudes ranged from 8 V to 14 V with a step length of 1 V, and Monte Carlo method was used for 1000 simulations. The noise amplitude and clutter power were positively correlated. The influence of the clutter amplitude on the detection probability of the single RF MIMO radar system is shown in Table 5. When the clutter amplitude is smaller than 8 V, the detection probability of the target is higher than 99.8%, which is not listed in the table. From the data, we can conclude that the clutter amplitude has a great impact on the single RF link MIMO radar. The target detection probability decreases with increasing clutter amplitude and the speed of the detection probability reduction gradually accelerates. In this system, when the clutter amplitude is 11 V, the detection probability of the target begins to decrease greatly. When it reaches 14 V, the detection probability is reduced to the lowest value of 97.8% in the table. When the SNR is low, the single RF link MIMO radar system discussed in this paper is still ideal, which means the signal can still be recovered completely. When the single link RF system is used in an actual situation, the signal cannot be correctly demodulated by the low SNR condition because the \begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Clutter Amplitude** & **8 V** & **9 V** & **10 V** & **11 V** & **12 V** & **13 V** & **14 V** \\ \hline Detection Probability & 0.996 & 0.993 & 0.993 & 0.991 & 0.987 & 0.981 & 0.978 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Clutter influence on the single RF MIMO radar performance. signal will not be correctly matched with the bandpass filter, thus the signal detection probability will be reduced. So, the SNR is more important for the single RF link MIMO radar system than for the traditional MIMO radar system. In this work, only one kind of clutter was studied, but in a real situation the radar system is often disturbed by many types of clutter. At this point, the system performance will inevitably be influenced. To solve this problem, we can add matching filters at the receiver part. If the actual clutter power or noise power has seriously affected the reception of the target--that is, the detection probability is lower than the allowed limit of the system--the space-time adaptive processing (STAP) can be used to suppress the clutter and noise of the received signal. The schematic diagram of the STAP process carried out by conventional radar is shown in Figure 13.  
The transmitting signal passes through the radar target module and the radar shift module, which inserts the continuous changing phase in the time interval, while the other branch produces clutter interference. The two signals are superimposed in the time domain and transmitted through the MIMO channel.