What are the different encoding techniques used for chipless RFID tags?

Encoding refers to converting a message into a different transmission, storage, or processing form. Chipless RFID lacks specialized ASICs to lower costs and use time and frequency domain systems for encoding. Hybrid encoding combines time and frequency domain systems for custom encoding options.

A chipless RFID system is a radio frequency identification technology that does not use traditional silicon microchips in the tags. The chipless tags usually consist of a printed pattern of resonators or reflectors on a substrate material like paper, plastic, or fabric.

The chipless tags are inherently passive, requiring no battery or power source. As they also lack any microchip, these tags can be produced at a much lower cost than the traditional RFID tags, potentially below 1 cent per tag. They find applications suited for item-level tagging in retail, supply chain management, and other applications requiring low-cost tags.

Figure 1 illustrates how the reader and the tag in a chipless RFID system work together.

The information is encoded directly into the physical structure of the tag using time domain techniques, frequency domain techniques, and hybrid encoding.

Time domain encoding techniques

Time domain systems utilize the temporal characteristics of the reflected signal from the tag to encode information. A reader emits a specific electromagnetic signal towards the tag. This is followed by the tag reflecting the signal with characteristics of time delays or patterns. The reader then proceeds to capture and analyze the temporal characteristics of the reflected signal to decode the tag’s information.

Surface acoustic wave (SAW) tags use piezoelectric substrates to convert RF signals into acoustic waves, which are then reflected with specific time delays (Figure 2). Another more straightforward encoding method is the On-Off Keying method, where the reflection of the interrogation signal in a particular interval determines the logic states ‘0’ and ‘1’.

The pulse position modulation works in a unique way where the time interval is split into multiple windows of 2ⁿ, and the encoding is carried out by varying the pulse position in one of the windows. Phase modulation is another technique to increase the tag’s information density based on the phase modulation of the input pulse.

The main challenge in time-domain chipless RFID systems is ensuring robust tag detection in different environments, especially in noisy conditions.

Frequency domain encoding techniques

When operating in the frequency domain, the tag consists of multiple resonant elements. The reader transmits a wideband signal covering the tag’s operating frequencies. The signal is processed to analyze the backscattered signal’s amplitude and/or phase response (Figure 3).

A spectral signature is a frequency-based encoding technique in which each resonant element is tuned to a specific frequency within a predefined band. The frequency position coding technique helps encode the data by varying the resonant frequencies of the tag’s element.

Multi-resonant tags employ a grid of dipole antennas or other resonant structures tuned to different frequencies. The reader generates a frequency sweep signal and scans for signal dips or peaks corresponding to the resonances.

An advantage of the frequency domain encoding technique is that it allows higher data capacity than time domain approaches. As with the time domain encoding techniques, there is also no need for precise time synchronization.

Hybrid domain encoding techniques

Hybrid domain systems in chipless RFID combine multiple encoding techniques to improve performance and increase data capacity. There are many ways to implement them, but the main objective is to achieve a higher data capacity.

Time-frequency hybrid encoding combines the time and frequency domain characteristics to encode information. It utilizes the temporal and spectral properties of the tag’s response to increase data capacity. Phase-frequency encoding combines the phase and frequency information to increase the tag’s data capacity.

A higher bit density can be achieved when the tag elements’ spatial arrangement and spectral signatures are used to encode data, also known as spatial-spectral encoding. The amplitude and phase encoding can be clubbed with each other to work on the reflected signal at different frequencies to represent data. This approach enhances the encoding capacity.

Summary

The Chipless RFID system is meant to reduce costs compared to traditional chip-based RFID systems. Eliminating the ASICs reduces costs, but increasing the data storage capacity to compete with optical barcode technology is challenging.

Using various encoding techniques, chipless RFID technology can increase data storage capacity, improve read range, and reduce tag costs. The specific encoding technique is based on the required data capacity, tag size constraints, and the type of application requirements.

References

Time and Frequency Domains Analysis of Chipless RFID Back-Scattered Tag Reflection, IoT, MDPI
Chipless-RFID: A Review and Recent Developments, Sensors, MDPI
Chipless RFID tags and sensors: a review on time-domain techniques, Cambridge University Press
RFID Tags & Chipless RFID tags: What’s the Difference?, EnCstore.com
Chipless RFID, Wikipedia

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