###### Abstract  
In this paper, we propose tag sensor using multi-antennas in a Wi-Fi backscatter system, which results in an improved data rate or reliability of the signal transmitted from a tag sensor to a reader. The existing power level modulation method, which is proposed to improve data rate in a Wi-Fi backscatter system, has low reliability due to the reduced distance between symbols. To address this problem, we propose a Wi-Fi backscatter system that obtains channel diversity by applying multiple antennas. Two backscatter methods are described for improving the data rate or reliability in the proposed system. In addition, we propose three low complexity demodulation methods to address the high computational complexity problem caused by multiple antennas: (1) SET (subcarrier energy-based threshold) method, (2) TCST (tag's channel state-based threshold) method, and (3) SED (similar Euclidean distance) method. In order to verify the performance of the proposed backscatter method and low complexity demodulation schemes, the 802.11 TGn (task group n) channel model was utilized in simulation. In this paper, the proposed tag sensor structure was compared with existing methods using only sub-channels with a large difference in received CSI (channel state information) values or adopting power-level modulation. The proposed scheme showed about 10 dB better bit error rate (BER) performance and throughput. Also, proposed low complexity demodulation schemes were similar in BER performance with a difference of up to 1 dB and the computational complexity was reduced by up to 60% compared to the existing Euclidean distance method. Wi-Fi; Wi-Fi backscatter; multiple antennas; Internet of Things; energy harvesting 2020202  
(NFC). Moreover, the Wi-Fi backscatter system has a significant advantage of utilizing the existing Wi-Fi infrastructure. The existing Wi-Fi backscatter technology, however, has a low speed of 1 kbps for uplink communication and a small coverage of two meters. In order to solve these issues, various studies were carried out. A Wi-Fi passive backscatter technology can decode backscattered signals using commercial Wi-Fi radios, but requires a dedicated continuous wave signal generator [10]. A BackFi technology can transmit data at the speed of 5 Mbps and 1 Mbps in the region of one meter and five meters, respectively [11]. Since the RFID-based full-duplex backscatter technology was used, however, RF modules of Wi-Fi devices were supposed to be modified, which made it difficult to apply the technology to the existing Wi-Fi infrastructure. A power-level modulation technique used multiple attenuators in a tag sensor to control the reflection strength of Wi-Fi packets and can increase the data rate in the existing Wi-Fi infrastructure [12]. However, the bit error rate (BER) performance deteriorated since the Euclidean distance between symbols decreased as the number of modulation power levels increased. In this paper, multiple antennas were deployed to the tag sensor in order to improve the throughput or BER performance of uplink communication in Wi-Fi backscatter systems. Unlike the existing algorithm, which only utilize the impedance of a single antenna to increase the data rate, the proposed backscatter method can achieve better performance by controlling the impedance of multiple antennas independently. In general, symbols utilized in Wi-Fi backscatter communications are modulated onto a carrier by the influence of a tag sensor on the channel. In Wi-Fi backscatter communications, the variations on a channel by a tag sensor deeply depends on the demodulation performance. Therefore, the proposed algorithm employing multiple antennas can increase the performance compared to existing methods by easily affecting channel characteristics due to the antenna diversity. However, as the number of antennas used in Wi-Fi backscatter system increases, computational complexity required to demodulate increases.