# MIMO Radio Frequency Identification: A Brief Survey
###### Abstract  
In this paper, we briefly look at the latest state-of-the-art in the domain of multi-input multi-output (MIMO) radio frequency identification (RFID) systems while detailing the work done in the domain of anti-collision, range enhancements, bit error rate (BER) improvements and security. Various passive ultra-high frequency (UHF) RFID implementations are considered that employ multiple antennas at the reader and single or multiple antennas at each tag. We look at several recent works those explored MIMO for RFID receivers. When using MIMO at the backscatter channel, significant improvements can be achieved in the BER as well as range extension. With the extra reliability and increased throughput, such systems can be deployed in many important applications like large tag reading scenarios and accurate tracking. Increased throughput is directly dependent on estimation of tag quantity in a bulk reading environment and usually estimators designed for single antenna systems under-perform in such settings causing low signal to noise ratio (SNR) when employed in MIMO systems where tag signal overlapping can happen more often. 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. We provide a brief survey in some of the recent developments related to MIMO RFID systems, the protocols and algorithms used, and improvements achieved. rfid; mimo; survey; multiple antenna; anti-collision; estimation; passive 2022  
[PERSON], [PERSON], [PERSON], [PERSON], F.  
forward and backscattering components and both channels can be represented via Rayleigh distribution [16]. However, the cascaded channel acts to degrade the transmission since it fades deeper compared to an ordinary Rayleigh channel [17]. Some of the parameters affecting the tag reading ability of the reader are the distance of the tag, its orientation with respect to the receiver, multipath fading, noisy channel, and interference. The interference can be both from multiple tags transmitting signals at the same time or from nearby RFID readers. An RFID system (Figure 1) is usually composed of a receiver with M transmit antennas, N receive antennas, and a tag having L antennas. The core principle behind backscatter communication is such that initially a reader will transmit a continuous wave (CW) to energize the tag or tags. This is followed by the tag sending back the CW signal to the receiver by modifying the impedance loads of its attached antennas according to a particular code prestored in the tag and known as identification data. This technique is referred to as backscatter modulation, whereby the tag transmits secret information to the reader which is modulated over the reflected CW signal. The reader will extract the required information by first estimating and then decoding the signal from the tag after backscatter modulation. The transmission and reception paths, analogous to radar systems, are divided into two categories: bistatic and monostatic [18]. In monostatic systems, a single or multiple antennas first transmit and then listen to the tag response and the channel is half-duplex. On the other hand, in bistatic systems, directional coupling is employed, whereby separate RF channels are used for transmission and reception to enable full-duplex communication. The reader continues to broadcast the CW signal throughout the process to keep the tag energized and at the same time receive the signal echoed by the tag when operating in a full-duplex mode. While such systems are faster, the interference at the receiver side from its own transmit antennas needs to be eliminated. There are various protocols and standards [19] that defines the communication process among the tag and the reader. For HF band, there is ISO 15693 and ISO 18000-3, and ISO 18000-6 and EPCglobal for UHF [20]. Additionally, the UHF frequency range is within 860-915 MHz and it is 13.56 MHz for HF. Figure 1: A typical MIMO RFID system.