# MIMO Radio Frequency Identification: A Brief Survey
## 2 Estimation Techniques  
Both forward interrogation range (FIR) and RIR are considered when estimating the actual interrogation range of an RFID reader [23]. FIR calculated as the longest distance at which an RFID signal can turn on a tag and is the measure of average power absorbed at its voltage rectifier. RIR, on the other hand, is the longest distance at which the backscattered tag response signal can be accurately detected and is measured using the signal to noise ratio (SNR) that is strong enough for the tag's data to be read after demodulation. The RIR plays a significant role in evaluating tag reading capabilities of a reader compared to the FIR. The estimation techniques summarized in this work are listed in Table 1. Various estimators employed in the summarized works have been listed along with the frequency range, MIMO configuration and gain. The gain represents performance improvements over a SISO RFID configuration. [PERSON] et al. [24] studies the reverse-link interrogation range of MIMO UHF RFID readers with maximal-ration combining (MRC) and compare it against a SISO reader. The channel is assumed to be arbitrarily correlated to the Nakagami-\(m\) distribution. The interrogation range is one of the foremost parameters in evaluating the performance of an RFID reader. Figure 2 details the proposed system architecture of mono-static and bi-static readers. In Figure 2b, it can be observed that from \(M_{t}\) transmit antennas, \(M_{r}\) receive antennas and a single tag antenna, an \(M_{t}\times 1\times M_{r}\) pinhole channel matrix is obtained. Due to spatial multiplexing, a MIMO based multi receiver antenna-based setup can greatly enhance the BER performance at the receiver [25]. It is derived that using multiple antennas improves the reader SNR and hence improved average RIR. In uncorrelated Rayleigh channels, the gain of a \(2\times 2\) MIMO system is 36% higher, while for a \(3\times 3\) MIMO system is 60% higher compared to that of a SISO reader system. On the other hand, in correlated environment the gain is almost 26% for \(2\times 2\) MIMO and 44% for \(3\times 3\) MIMO. The complexity of the system will increase along with hardware and computational costs, but it is worth the improvement. The indoor channels are usually characterized by a combination of light-of-sight (LOS) component together with dense multipath components (DMC) [26; 27]. The authors of [28] propose a UHF MIMO RFID system for high accuracy positioning. A maximum likelihood (ML) direct positioning algorithm for coherent measurements is performed by nearby antennas in a closed setting and non-coherent measurements in long displaced settings. The proposed solution considers the LOS component together with DMCs. The presence of DMCs makes the process of precise positioning more difficult. Exploiting diversity and a higher bandwidth can improve positioning in such channels. An ML positioning technique is used to estimate the position, along with the channel estimates. A dual antenna tag is proposed that supports both 2.45 GHz and UHF bands. The proposed system is able to reduce the position errors below 0.15 m by 80%. Figure 2: (**a**) Mono-static RFID Reader (**b**) Bi-static RFID Reader. Reproduced from [24] with permission from IEEE. While the interest in in the microwave frequencies such as 2.45 GHz is growing rapidly, the authors of [29] make a case for blind signal estimation to detect multiple tags in a MIMO RFID system. The backscattering in RFID interrogation can be modeled as a fading channel, which makes the interrogation process more complex. A perspective of blind channel estimation is presented to the RFID reader receiver for multiuser detection. Multiple antennas have been explored to enhance multiple the reading capabilities. In the proposed blind estimation-based system, linear equalization techniques have been employed at the receiver including minimal mean square error (MMSE) and zero-forcing (ZF) to estimate the channel state.