## 3 Experimental Verification of the Proposed UHF MIMO RFID System Performance  
A dual antenna UHF MIMO RFID tag operating at 925 MHz built in our laboratory was used for the experiments. Figure 2a shows the UHF RFID MIMO tag circuit architecture and a photograph of it is shown in Figure 2b which is designed with reference to WISP tag [7]. The part of the front-end antenna is composed of Antenna 1 and Antenna 2 with their polarizations orthogonal to each other. As previously mentioned in this study, this design allows the tag to exhibit uncorrelated channel characteristics when transmitting or receiving signals, achieving the effect of polarization diversity. In the forward link, there are two sets of symmetric modules transmitting signals to the backend processor MSP430. This processor uses selective combining techniques to select the SNR. In order to achieve the passive effect of the tag, the ADG918 component [19] was used in the front-end RF switcher. The reader is as specified in the previous section. In NI-5644R from National Instruments the LabView is installed and was used to control the field-programmable gate array (FPGA) that controls the vector signal transceiver (VST). The experiments were taken at the Anechoic Chamber of National University of Science and Technology (NTUST), whereas good approximation to the Gaussian channels could be reached. The reader transmitting and receiving antennas, Tx and Rx, respectively in Figure 8, are both unidirectional patch antennas with a gain of 6 dBi. The antennas use circular polarization. Figure 8: Experimental setup. Figure 7: Miler decoder, UID code. The tag antennas are omnidirectional (approximated). More details about the tag can be found in [14]. It is worth mentioning that the tag built in our laboratory and used for the experiments is unique, in the sense that it performs backscatter modulation with a similar principle to that proposed in [15], but with its particular advantages earlier and well introduced by our laboratory group in [14]. As can be seen in the Figures 8 and 9, the separation between the tag and the reader antennas was set to \(d=1.8\) m (meters). The interference can be ignored inside the chamber. Assuming that the noise in the non-reflective environment is very small, the only considered and measured noise is the thermal noise generated by the instrument. As a strong LoS component is present, Rician better represents the chamber room environment. As used in theory, \(K_{f}\) and \(K_{b}\) represent the Rician channel parameters, which are for forward and backward channels, respectively [20]. To obtain the simulation results, we set the reader with a carrier frequency of \(f_{c}=925\) MHz, the number of backscatter bits as 1,000,000 in total at each SNR level. The number of Monte Carlo simulations is \(N=50\),000. To perform this experiment, in the first place, the reader is allowed to repeatedly transmit query instructions 50,000 times, so that the tag continuously returns the backscattered signal and decodes it. The decoding part is divided into maximum likelihood detection decoding and Miller decoding. The error accumulation is the sum of the symbol errors, latter the sum of bit errors, which are then used to calculate the SER and BER, respectively. To obtain different values of SNR, we added FAT-AM5 AF5G6G2W3 attenuators at the receiver side (connecting to the VST), with a gap of 3 dB in each experiment trial. We used a 50 \(\Omega\) terminator to obtain the thermal noise for the SNR reference value. For the channel variance, we used 0 dB. Each antenna of the tag uses the BPSK modulation as in the mathematical description. The key parameter \(K_{f}\) and \(K_{b}\) are illustrated later. When the values of \(K_{f}\) and \(K_{b}\) are less than one, the worse channel case is obtained. When \(K_{f}=K_{b}=1\), the equivalent to a Rayleigh channel can be obtained.