# MIMO Radio Frequency Identification: A Brief Survey
## 2 Estimation Techniques
The channel is assumed to be an indoor one and the proposed blind estimation technique independence component analysis (ICA) is employed for estimating the CIR. The proposed technique outperforms traditional linear estimation schemes when the number of tags is less. The domain of theoretical modeling of MIMO RFID systems offers a lot of opportunities to study various MIMO techniques in RFID context [33]. Multi-antenna tags enable the possibility of having uncorrelated channel characteristics during the transmission and reception of signals realizing polarization diversity. [PERSON] et al. [30] have evaluated the performance of a \(1\times 2\times 1\) MIMO UHF RFID system and propose a new UHF MIMO passive tag prototype and its corresponding reader system. The proposed system consists of a reader with one transmit and receive antenna, whereas the tag has two antennas and is based on a polarization-time coded backscatter diversity scheme. For validation, the experimental BER of the proposed scheme is compared against BER for simulated AWGN, Rayleigh, and Rician models while altering different parameter. For forward and backward channels, a Rician fading model has been used to simulate the chamber room environment. One of the key challenges is to keep the design of the RFID tag simple, cutting cost and power requirement when employing anti-collision schemes [34]. [PERSON] et al. [31] have proposed a novel channel estimation technique employing concealed pilots for a multi-antenna RFID system. The proposed solution does not use additional data bits and reduces the number of tag interrogation requests and improves channel estimation performance. The technique is based on hidden pilots consisting of a predetermined sequence summed with the data symbols. Since the suggested technique is bandwidth efficient, it enables quick detection of multiple tags. Figure 3 details the proposed architecture with a single transmit and multiple receive antennas. A regularized LS estimation scheme is applied on the preamble part of the tag's signal that contains the concealed pilots that is same for all tags. The authors also derived solution for the optimization problem and a closed form expression considering the mean square error (MSE) for the given technique. The proposed technique outperforms slotted aloha and post-preamble-based estimation in terms of reading large number of tags. \begin{table}
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In the past decade, various multiaccess algorithms such as slotted ALOHA [35; 36] have been proposed to tackle the issue of collision where multiple tags exist in the interrogation range of a reader. To enhance the interrogation throughput, the frame size must be the same length as the number of tags in the interrogation range. However, in a real-life environment, the number of tags in the interrogation range of the reader vary significantly and several works such as [37] have investigated the estimation of tags in the range of an RFID reader. [PERSON] et al. [32] have proposed a multi-antenna RFID system that employs blind adaptive beamforming (BABF) algorithm to improve the interrogation range and the error rate performance in half- and full-duplex modes. Figure 4[32] depicts the proposed MIMO RFID reader in half duplex and full duplex modes having M antennas. In Figure 4a, both the transmitter and receiver use common antennas leading to full duplex communication, whereas in Figure 4b, separate transmit and receive antennas are used. In a bistatic reader system, both the forward and reverse links undergo the same channel variations, and the channel coefficients are same, whereas in monostatic systems, they observe independent channel coefficients [38]. The authors have quantified the gains related to reader interrogation range while employing multiple antennas and then used a BABF scheme to improve both the interrogation range and data transmission. The idea is to estimate the antenna weight vectors, by sending a continuous wave to probe the tags, in order to maximize the interrogation range.