Mixed orthogonal frequency coded SAW RFID tags

Abstract

Orthogonal frequency coded (OFC) SAW radio-frequency identification (RFID) tags are currently being explored as a multi-sensor platform because of their passive spread-spectrum operation, low loss, and resilience in harsh environments. Ongoing research continues to search for robust device embodiments that increase the number of identifiable codes, in the presence of intersymbol interference, while maintaining reasonable device lengths. This paper presents a technique that shortens the SAW response length while preserving code diversity and bandwidth by utilizing a multi-track SAW configuration. These new devices allow the time response of multiple OFC chips to overlap and yield a mixed-frequency chip having the sum of the chip bandwidths but shorter overall time response. The theoretical development is presented and examples are discussed for these new mixed orthogonal frequency coded (MOFC) SAW devices. Experimental results for MOFC sensors, fabricated on YZ-LiNbO3, with a 7% fractional bandwidth and five chip frequencies in three cells, provide a good contrast to similar OFC designs. Experimental results are presented for the simultaneous operation of eight wireless temperature sensors--four OFC and four MOFC--in a 915-MHz wireless correlator receiver system, highlighting the ability of these devices to operate in the same system.

Wireless SAW Strain Sensor Using Orthogonal Frequency Coding

“…The sensor operates at a frequency of 915MHz and has a bandwidth of 68MHz. The chips are separated into tracks to improve the interference effects between chips of adjacent frequencies [16]. Each chip has a time length of 74ns with a total response length of approximately 400ns (1.15μs to 1.52μs).…”

Concrete temperature monitoring using passive wireless surface acoustic wave sensor system

“…The combination of the VNA and the software provides a synchronous correlator receiver. Orthogonal frequency coding (OFC) [25][26][27] is a scheme used to encode the identity of a SAW temperature sensor using both time and frequency domain diversity. The bandwidth and time diversity, using adaptive correlation techniques, provide advantages for pulse compression and increased processing gain [28,29], which increases accuracy and precision over other approaches.…”

An Impedance-Loaded Orthogonal Frequency-Coded SAW Sensor for Passive Wireless Sensor Networks

“…To expand the capacity of SAW sensors, an OFC SAW sensor based on the principle of orthogonal frequency-coded division is proposed in [28][29][30]. This sensor could expand the capacity in a limited bandwidth using code diversity.…”