A Widely Linear MMSE Anti-Collision Method for Multi-Antenna RFID Readers
Abstract: In a multi-tag radio frequency identification (RFID) system, signals sent from different tags are likely to interfere with others, leading to degradation in the transmission quality. To solve this problem, a novel widely linear minimum-mean square- error (WLMMSE) anti-collision method is proposed by taking into account the improper second-order statistics of backscattered tag signals, which result from the quadrature nature of RFID readers. The proposed method provides an efficient means to separate multiple overlapping tag signals within a single time slot and can be easily integrated into multiantenna RFID systems to improve their throughput. It’s modeling advantages over conventional linear model-based signal recovery methods are illustrated by both the simulations and real-world experiments. Existing system: Recently, there has been a growing interest in resolving tag collision problem at the physical layer. For single antenna RFID systems, the work in used clustering and voltage-level-selection for collision detection and signal recovery, and in, a tag rate reduction was paid to recover tag signals shout knowledge of the channel state
information. Since the unprecedented growth of the tag population in recent years has imposed a higher throughput requirement on RFID systems, the multi-antenna technology becomes a preferable solution to the reader design because the increased spatial diversity it brings efficiently combats the effects of multipath propagation. Proposed system: The digital multi-beam forming technique was employed in [6] to fast the tag identification procedure. Moreover, in, both the zero-forcing (ZF) and linear minimum-mean-square-error (LMMSE) multi-antenna receivers were proposed for multiple-tag identification and their performance were quantified in theory for FSA-based RFID systems. Although the related works show that the multi-antenna RFID technology is a promising approach to solve the problem of tag anticollision, one key statistical property of collided signals has been systematically overlooked, that is, the real valued tag waveform can be regarded as a rectilinear signal with the maximal degree of impropriety, when observed from a quadrature receiver. While for improper complex-valued signals, additional performance gain can be achieved when both the received signal and its complex conjugate are jointly processed. Advantages: In this letter, we first consider the tag anti-collision problem in the context of multi-antenna RFID systems as a multiuser detection model mathematically. Furthermore, a novel tag signal recovery method is proposed based on the widely linear minimum-mean-square-error (WLMMSE) criterion to use the full second order statistics of collided tag signals. Numerical simulations in the MATLAB programming environment and real-world experiments built upon multi-antenna RFID systems verify the modeling advantages of the proposed method over the conventional linear ones. Disadvantages: Although the related works show that the multi-antenna RFID technology is a promising approach to solve the problem of tag anti-collision, one key statistical property of collided signals has been systematically overlooked, that is, the real
valued tag waveform can be regarded as a rectilinear signal with the maximal degree of impropriety, when observed from a quadrature receiver. While for improper complex-valued signals, additional performance gain can be achieved when both the received signal and its complex conjugate are jointly processed. This scheme, often referred to as the widely linear (WL) processing, has found its successful usage in other applications, where improper signals appear due to their underlying generating physics. Modules: Ultra high frequency: THE radio frequency identification (RFID) technology has been successfully applied in lots of different areas such as inventory, assets tracking and personal identification in recent years. In a typical multi-tag ultra high frequency (UHF) RFID system, tag collision problem has a significantly adverse impact on the transmission efficiency. One common solution to this problem, adopted in various RFID standards, such as ISO 18000-6C , is the framed slotted aloha (FSA) algorithm. However, since FSA-based RFID readers can handle slots that contain only one tag signal, the maximum system throughput cannot exceed. Recently, there has been a growing interest in resolving tag collision problem at the physical layer. For single antenna RFID systems, the work in used clustering and voltagelevel-selection for collision detection and signal recovery, and in, a tag rate reduction was paid to recover tag signals without a knowledge of the channel state information. Zero - forcing: Readers which can achieve a maximum ratio combining of receive signals in terms of the signal-to-noise ratio (SNR) maximization was present. The digital multibeam forming technique was employed in to fast the tag identification procedure. Moreover, in, both the zero-forcing (ZF) and linear minimum-mean-square-error (LMMSE) multi-antenna receivers were proposed for multiple-tag identification and their performance were quantified in theory for FSA-based RFID systems. Although the related works show that the multi-antenna RFID technology is a promising approach to solve the problem of tag anti-collision, one key statistical
property of collided signals has been systematically overlooked, that is, the real valued tag waveform can be regarded as a rectilinear signal with the maximal degree of impropriety, when observed from a quadrature receiver. Widely linear: While for improper complex-valued signals, additional performance gain can be achieved when both the received signal and its complex conjugate are jointly processed. This scheme, often referred to as the widely linear (WL) processing, has found its successful usage in other applications, where improper signals appear due to their underlying generating physics. In this letter, we first consider the tag anticollision problem in the context of multi-antenna RFID systems as a multiuser detection model mathematically. Furthermore, a novel tag signal recovery method is proposed based on the widely linear minimum-mean-square-error (WLMMSE) criterion to use the full second order statistics of collided tag signals. Numerical simulations in the MATLAB programming environment and real-world experiments built upon multi-antenna RFID systems verify the modeling advantages of the proposed method over the conventional linear ones.