Hybrid Reflectarray RFID Reader Development

1. Introduction

1.1 Motivation

The chipless tag RFID system is the new cheaper solution that emerges from the limitations of achieving cheap chipped RFID system. It is expected to replace barcode technology in 2020. That results in the need for new efficient RFID readers with high gain, high sensitivity, and large reading range. A Chipless tag can't generate a signal without the reader sending an excitation signal to the tag itself. Therefore, the reader acts as a Master and the tag as a Slave. The main drawback of the Chipless tag RFID system is its low reading range where the back scattered tag signal is subjected to the fourth power reduction in magnitude with the reading distance as shown in Fig.1.1.

Besides, they also suffer from the reader low sensitivity, multipath interference, and low efficient tag localization [1- 4]. So, Chipless tag RFID reader must generate a high gain direct beam over a wide band of frequencies to accommodate multiple bits, reduce the effects of multipath propagation, and enhance the reader sensitivity which in turn leads to increasing the reader reading range.

Fig.1.1 Chipless RFID communication system.

It has become obvious that new types of high-gain and high-bandwidth directive antennas that efficiently designed with arrays are needed. In this thesis, we are introducing the design of unified and hybrid reflectarray antennas for chipless RFID reader systems.

Reflectarray antennas can be considered as a new type of antennas. A reflectarray antenna is composed of an array of radiating unit cells that provide a focused pencil beam in a determined direction when excited by a feeder. That can be achieved by choosing a sufficient radiating element (unit cell). The circular rings are the best reflectarray unit cell choice for its high linearity phase response.

Reflectarray antennas combined the advantages of both phased antenna arrays and parabolic antennas. Compared to the phased array, it has the possibility of beam steering. On the other hand, it has a simple feeding mechanism as that of the parabolic antenna which decreases the design complexity and losses of phased array feeding network and avoids the manufacturing complexity of parabolic antennas [5].

Table1.1 Comparison between Reflectarray antennas with other reflectors antennas

Parameters	Reflectarray antenna	ParabolicReflector	PhasedArray
Gain	High	High	High
Bandwidth	Low	High	Moderate
Beam Steering	Electronic	Mechanical	Electronic
Cost	Low	Moderate	High
Design Complexity	Low	High	High
Structure	Planar,Lightweight	Bulky	Moderate planar

From Table1.1,These features make the reflectarray antennas an efficient choice for chipless RFID reader applications.

For chipless tag RFID reader applications, to accommodate multiple bits increasing the rate of data transfer and the capacity,there will be a demand of antennas with high directive gain and at the same time maintain a large bandwidth.

Such large bandwidths are not achieved at low frequencies.

At higher frequency range such as microwaves, it has wider bandwidth and hence it can be used for simultaneous transmission of data. Previous studies have shown that chipless RFID readers operation is possible at 5 GHz and 6 GHz[6]. In this thesis, we will focus on the operation at 6 GHz.

However, the main drawback of the Reflectarray antennas which is its narrow bandwidth behavior. Reflectarray bandwidth is limited mainly by two main factors. The first is the narrow band of the radiating elements and the second is the differential spatial phase delay resulting from the different paths from the feeder to each point on the wavefront of the radiated beam [7]. In this thesis, hybrid reflectarray design method has been introduced to enhance the reflectarray bandwidth to make it an ideal choice for chipless RFID reader applications.

1.2 Objectives

In this thesis, a hybrid reflectarray RFID reader is designed to achieve

1. RFID reader gain maximization
2. RFID reading range extension
3. RFID reader efficient tag localization
4. RFID efficient signal transmission through the system channel

Circular rings (unit cells) based reflectarray antenna are designed for chipless RFID reader applications. Several unified unit cell reflectarrays and hybrid unit cell reflectarrays will be designed and compared at 6 GHz for chipless RFID reader application. As the bandwidth of the reflectarray is limited by the bandwidth of its basic unit cell, different shapes for circular unit cells will be introduced.

1.3 Thesis Outlines

This thesis is organized into five chapters as follows:

Chapter 2:Presents a literature survey on automatic identification techniques, Radio frequency identification (RFID) tags, Chip RFID communication system, Tags types Chipless RFID system, and the limitations of chipless RFID reader reading range.

Chapter 3:Introduces the features of the reflectarray antenna, principles of operation, and the reflectarray antenna specifications.

Chapter 4:Shows the criteria used for the selection and analysis of the unit cells used to design reflectarrays. In addition, the proposed unit cells that will be used in the reflectarray designs in the thesis will be introduced.

The dimensions, reflection loss curves and the reflection phase curve of each unit cell are presented in this chapter. The methods used in the CST (Computer Simulation Technology) simulator for obtaining the reflection phase curves are also given. A comparison between the different unit cells is given at the end of this chapter.

Chapter 5:Presents the design of unified RAproposed center-fed reflectarrays based on the unit cells introduced in chapter 4. The selection of a reflectarray design mechanism is introduced in this chapter. A comparison between these reflectarrays considering the gain, bandwidth, and side lobe levelis introduced in this chapter.

Chapter 6:Introduces the proposed Hybrid reflectarrays design mechanism. A comparison between the hybrid reflectarrays in terms of gain, bandwidth, and SLL is introduced. A comparison between the unified reflectarrays and hybrid reflectarrays is introduced at the end of the chapter.

Chapter 7:Introduces a comparison between the previous proposed work in that field and this work. This chapter also summarizes the concluding remarks and the major contributions of the thesis in addition to some suggestions for future work to improve the performance of reflectarrays.