## 2 System Model
### Modulation and Demodulation
#### 2.2.2 UHF MIMO RFID Reader Architecture  
The National Instruments (NI) NI-5644R was used to build the UHF MIMO RFID reader. Two patch antennas are connected to the input and output RF ports. One antenna performs as the transmitter and the other as the receiver. We used the NI-5644R vector signal transceiver (VST) which includes the VSG and VSA, the vector signal analyzer and the vector signal acquisition, respectively. System Protocol  
For establishing communication, first the reader sends a query command and a continuous carrier wave. Considering the tag energy consumption problem, the query is set as a negative trigger signal. After the tag successfully recognizes the negative trigger, the backscatter signal is changed by changing the frontend impedance. The data format of the backscatter signal is as shown in Figure 4. \begin{table}
\begin{tabular}{c c c c} \hline \hline  & **UHF MIMO RFID TAG** & **WISP TAG** & **MIMO RFID TAG in [14]** \\ \hline Receiver & & & \\ \hline Transistor & HSMS-285C & HSMS-285C & No \\ \hline Regulator & NCP583 & NCP583 & Yes \\ \hline Bit translator & NLSV2T244 & NLSV1T244 & Yes \\ \hline  & S100C30N4T & & \\ \hline Matching & lamda/4 and R\&C tunning & R\&C and 50 Ohm \\  & & transmission line & R\&C tuning \\ \hline Super cap & Yes & Yes & No \\ \hline Temperature, accelator & & & \\ sensor,3D axis sensing & & & \\ \hline EEPROM & No & 24 AA08 & Yes \\ \hline Microcontroller & & & \\ \hline MCU & MSP430F1232 & MSP430 \(\times\) 21 \(\times\) 2 & MSP430F1232 \\ \hline Crystal & No & FC-135 & No \\ \hline \hline \end{tabular}
\end{table} 
Table 1: WISP and UHF MIMO RFID TAG RF design comparison. Figure 4: _Cont._The tag's backscatter signal is built as a response from the two tag antennas. As can be seen in Figure 4, there are the pilot and tag UID signals. The purpose of this backscatter signal is to let the reader synchronize the signal and estimate the channel state information (CSI). A more detailed explanation can be found in our previous study. The received signal strength indicator (RSSI) is cancelled out and the unique identification (UID) code is given by the six levels. In this case, the UID code was set to [011010]. The baseband signal processing architecture is shown in Figure 5. It is mainly divided into four blocks: the DC carrier estimation, synchronizer, channel estimator, and decoder. The DC Carrier Estimator is responsible for estimating the DC component of the carrier received by the reader receiving antenna and eliminating it to keep the tag signal of interest. Even when the tag backscatters the signal, the reader will continue transmitting the carrier in order to provide energy to the tag. Therefore, the RF signal received by the reader includes the carrier energy leaked from the transmitting antenna in addition to the tag backscatter signal. We use the method of time domain for estimation of carrier information to eliminate the carrier signal. The discrete I (in phase) and Q (quadrature) information are only extracted at the time of the DC component. Later, its average value is calculated, that is, the DC carrier signal is estimated, and the tag backscatter signal is deducted from the estimated DC signal. Channel Estimator  
The corresponding channel estimation result and the maximum ratio combining technique are used to obtain the demodulation signal. The purpose of channel estimation is to obtain channel information that could cause signal attenuation in a wireless communication environment, mostly unknown phase changes and amplitude variations. We assume that it is a non-time-varying channel environment and we use the least square method (LS) for estimation [18]. Since our tag is manually soldered it is susceptible to phase and gain errors. This effect is shown in Figure 6, where \(\Gamma_{L,i}\) indicates the \(i\)-th ideal reflection coefficient of  
Figure 4: MIMO tag backscatter signal format. Figure 5: Baseband signal processing architecture (reader).