Abstract
A novel idea is investigated for radio frequency identification (RFID) and tagging, based on a radio frequency analogy of the ubiquitous optical barcode. The proposed method, referred to as an RF barcode, is based upon microwave resonators of different frequencies and requires no integrated circuits. RF barcodes, like optical barcodes, are passive devices requiring neither batteries nor reader power. These low cost components can potentially be "printed" on the tagged items or the packaging using metal-loaded ink. Each tag consists of parallel strips of microstrip half wavelength dipoles with varying widths as well varying lengths, each with a specific resonant frequency. A collection of these resonant dipoles forms a multi-bit tag with a specific binary code. With n different resonant RF barcodes, 2n-1 different items can be tagged and identified. Like optical barcodes, RF barcodes are disposable and read-only devices since the barcode reader cannot alter the information they contain, hence no read-write capability as is found with chip-based RFID devices. This is often referred to as a write-once, read-many (WORM) tag type. It is shown that RF barcodes have longer read range compared to optical barcodes; a range of up to 10m is achievable using allowable transmit levels in the license-exempt Industrial Scientific and Medical (ISM) frequency bands. It is shown for the first time that a useable number of bits can be achieved by operating within multiple bands. Prototype tags are demonstrated in the 2.4 GHz and 5-6 GHz frequency bands. Techniques are presented for accurately estimating barcode centre frequency, quality factor and bandwidth in order to maximise the number of information bits in the tag. A number of RF barcode reader design strategies are presented based on monostatic and bistatic detection methods. Both of these methods can potentially employ pulsed or continuous-sweep excitation. The advantages and disadvantages of each excitation technique are discussed in detail and results presented based on bistatic continuous frequency-sweep excitation.