Wen Tzu Chen

An Accurate Tag Estimate Method for Improving the Performance of an RFID Anticollision Algorithm Based on Dynamic Frame Length ALOHA

In a radio-frequency identification (RFID) system, the dynamic frame length ALOHA protocol is widely adopted to solve the anticollision problem. Analysis for the anticollision problem can be divided into two primary parts. The concern of the first part is how to precisely estimate the number of tags. The other part involves determination of dynamic frame length to achieve maximum throughput or channel usage efficiency. In this paper, we present an accurate method for estimating tag quantity. This method is based on the maximum a posteriori probability decision. We also derive the optimal frame length using radio channel efficiency. Simulation results indicate the tag estimate error of the proposed method is less than 4%. Use of our proposed tag estimate method together with optimal frame length can achieve close to the theoretical maximum throughput of the framed ALOHA algorithm.

An Efficient Anti-Collision Method for Tag Identification in a RFID System

Radio frequency identification (RFID) technology is becoming increasingly attractive because of its high storage capacity and reprogrammability. There is a challenge to be overcome when a reader needs to read a number of tags within the readers interrogation zone at the same time. In this paper, we present an anti-collision scheme in a RFID system. The scheme is based on the dynamic framed ALOHA protocol developed for radio networks. In our scheme, we propose two methods to estimate the number of tags. Simulation results indicate that the total number of time slots for reading all tags is about 4 times the number of tags that need to be read, including acknowledgement time slots. The main advantages of our scheme are the great performance of uplink throughput and its easy implementation for both readers and tags.

Computer simulation of the human-body effects on a circular-loop-wire antenna for radio-pager communications at 152, 280, and 400 MHz

This paper presents an extensive computer simulation of the influence of the human body on a circular-loop-wire antenna to simulate the pager antenna. The coupled integral equations (CIEs) approach and the method of moments (MoM) are employed for numerical simulation of this antenna-body-coupling problem. The magnetic frill source is used to model the antenna-feeding structure. A realistically shaped full-scale human-body model (1.7 m) is constructed. A small loop antenna (loop radius b=1.7 cm and wire radius a=0.072 cm) of x, y, and z orientation, in free space or proximate to the human body at the top pocket (chest position) or belt level (waist position), is considered. Numerical results of the antenna characteristics and body absorption at 152, 280, and 400 MHz are presented and discussed for radio-paging applications. At 280 MHz, it is found that the real part of the impedance increases about five to ten times, and, hence, the antenna ohmic-loss radiation efficiency increases from 17% (in free space) to 69%, 44.3%, and 58.4%, respectively, for the x-, y-, and z-oriented loops when proximate to the body. The radiation efficiencies, reduced by the body-absorption effect, are 5%, 61%, and 25% for the x-, y-, and z-oriented loops, respectively. For the y-oriented loop, which is found to be the most suitable for paging communications, the antenna efficiencies are almost the same at the two location levels for all frequencies considered. The computed antenna characteristics influenced by the human body; including the input impedance, antenna patterns, cross-polarization field level, radiation efficiencies, and maximum and minimum power gains, are important for the antenna/RF design and the link-budget consideration.

Numerical computation of human interaction with arbitrarily oriented superquadric loop antennas in personal communications

Loop antennas are widely used in many personal communication systems such as radio pagers. This paper presents results from an extensive numerical simulation of the human interaction with loop antennas. The loop antenna with a superquadric curve, which is able to model the circular, ellipse, square, and rectangular loop is used to model the rectangular loop antenna with rounded corners. The magnetic frill source is used to model the antenna feeding structure. A realistically shaped full-scale human-body model (1.7 m) is constructed. The coupled integral equations (CIE) approach, which consists a Pocklington-type integral equation (PIE) for the loop antenna and a volume electric field integral equation (VEFIE) for the body with mutual coupling terms, are developed to numerically study this electromagnetic (EM) coupling problem. The method of moments (MoM) is employed for numerical solution. Numerical results for the antenna located at the chest pocket and waist-belt levels of the human body with arbitrary loop orientations are presented at 280-MHz VHF paging band. The pagers internal rectangular loop antenna with rounded corners is modeled by a superquadric loop antenna. It is found that the real part of the impedance (radiation resistance) increases about five times and, hence, the antenna ohmic-loss radiation efficiency increases from 4% (in free-space) to 33, 17, and 26% for the x-, y-, and z-oriented loops when proximate to the body. The radiation efficiencies, reduced by the body absorption effect, are 13, 40, and 27% for the x-, y-, and z-oriented loops, respectively. For the y-oriented loop, which is found to be the most suitable for radio-paging communications, it has the highest value of E/sub /spl theta// average power gain (product of the directive gain and the ohmic-loss and body-absorption efficiencies) in the horizontal plane. The computed antenna characteristics influenced by the human body, including the input impedance, antenna patterns, cross-polarization field level, radiation efficiencies, and maximum/minimum and average power gains, are very useful for the antenna/RF design and the link budget consideration of the personal communication systems.

A Feasible and Easy-to-Implement Anticollision Algorithm for the EPCglobal UHF Class-1 Generation-2 RFID Protocol

Dynamic frame slotted Aloha (DFSA) has been widely adopted to solve the anticollision problem in a radio frequency identification (RFID) system. In a DFSA procedure, the interrogator needs to continuously estimate tag backlog and select a new frame length for identifying the backlog. Intuitively, the accuracy of the tag estimator will affect the read performance. Hence, a considerable amount of research effort has been invested to improve the accuracy of backlog estimation. The improvement in general comes at the expense of large computation load and may lead to a serious challenge if one needs to implement such a kind of estimators in a real RFID system. This paper analyzes the influence of estimation error on read performance. Based on the analysis, we propose a feasible and easy-to-implement anticollision algorithm. Our proposed algorithm can achieve a normalized throughput of 35% that is very close to the theoretical maximum 36.1% for an EPCglobal UHF Class-1 Generation-2 system. The easy-to-implement advantage of our algorithm comes at the expense of only 1% reduction in normalized throughput as compared with the case where maximum throughput can be obtained. The results obtained are useful in designing fast and efficient interrogators. Note to Practitioners - A fast and efficient anticollision algorithm is required in many applications such as warehouse and supply chain management. Since most RFID interrogators are built with a single-chip microprocessor, whose computation ability or memory is limited. From the implementation point of view, a feasible algorithm should consider the compromise between complexity and performance. In order to easily implement the anticollision algorithm in a real RFID system, the proposed algorithm is based on a simple but sufficiently accurate method rather than complex models. The results obtained can be useful for the industry to develop fast and efficient RFID interrogators.

Numerical computation of the EM coupling between a circular loop antenna and a full-scale human-body model

This paper presents numerical computation of the electro-magnetic (EM) coupling between circular loop antennas and a full-scale human-body model. The loop antenna can be x-, y-, or z-oriented. Coupled integral equations (CIEs) and the method of moments (MoM) are employed to numerically solve this antenna-body-coupling problem. Numerical results of the antenna radiation characteristics influenced by the human body and the body-absorption rate from 50 to 400 MHz are presented. The applications of this study include assessment of a radio-frequency (RF) dose from a loop antenna and the body effect on the performance of the loop antenna used in personal communication devices such as radio pagers.

A 2.4 GHz polarization-diversity planar printed antenna for WLAN and wireless communication systems

This paper presents design simulation, fabrication and measurement of a 2.4 GHz polarization-diversity printed dipole-antenna for wireless communication applications. Two orthogonal printed dipole antennas and each with a microstrip via-hole balun, for vertical and horizontal polarization, are combined and fabricated on a PCB substrate. PIN diodes are used to switch and select the desired antenna polarization. HFSS (high frequency structure simulator) 3-D simulation and measured results of antenna radiation characteristics, including input SWR, radiation pattern coverage, and polarization diversity, are presented and compared.

Performance Comparison of Binary Search Tree and Framed ALOHA Algorithms for RFID Anti-Collision

Binary search tree and framed ALOHA algorithms are commonly adopted to solve the anti-collision problem in RFID systems. In this letter, the read efficiency of these two anti-collision algorithms is compared through computer simulations. Simulation results indicate the framed ALOHA algorithm requires less total read time than the binary search tree algorithm. The initial frame length strongly affects the uplink throughput for the framed ALOHA algorithm.

Optimal Frame Length Analysis and an Efficient Anti-Collision Algorithm With Early Adjustment of Frame Length for RFID Systems

This paper proposes an efficient anti-collision algorithm to improve the read performance of the EPC global ultra-high frequency (UHF) Class-1 Generation-2 radio-frequency identification (RFID) standard. Optimal frame length is one of the most important parameters to be adjusted in RFID anti-collision algorithms. Hence, we first derive the optimal frame length by analyzing the maximum of normalized throughput and by taking into account the unequal time intervals of successful, collision, and empty slots. We find that the optimal frame length should be set to 1.89 times of the number of tags when the ratio between collision-slot duration and empty-slot duration is 5. The proposed anti-collision algorithm is based on the mechanism of early adjustment of frame length and examines the fitness of frame length only at an optimal time slot in each read round. The primary advantage of our algorithm is the ability to achieve a good compromise between computation complexity and throughput performance. The results show that the proposed method provides up to 400 tags/s read speed and can obtain 5%-10% time-saving efficiency, as compared with typical dynamic framed-slotted ALOHA.

An effective medium contention method to improve the performance of IEEE 802.11

In this paper, we propose an effective medium access mechanism to enhance performance of the IEEE 802.11 distributed coordination function (DCF). One of the primary issues of 802.11 is a contention-based medium access control (MAC) mechanism over a limited medium, which is shared by many mobile users. In the original 802.11 DCF, the binary exponential backoff algorithm with specific contention window size is employed to coordinate the competition for shared channel. Instead of binary exponential increase, we adopt linear increase for the contention window that is determined according to the competing number of nodes. We also assume that the access point can broadcast the number of mobile nodes to each station through management frames. An analytical model is developed for the throughput performance of the wireless medium. Using simulation results from the NS2 simulator, we show that our model can accurately predict the system saturation throughput, and can obtain better performance in terms of throughput, fairness, and packet drop.