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This paper presents a low-cost, low-complexity FMCW-based ultra-wideband (UWB) chipless time domain reflectometry (TDR) radio frequency identification (RFID) system, which is suitable for implementing higher order modulation schemes. Higher order modulation significantly increases the information content in chipless RFID. The applicability of phaseshift keying (PSK) and pulse-position modulation (PPM) to chipless TDR RFID leads us to introduce an 8-PSK and an 8-PSK-and-2-PPM combination. The overall data capacity is increased to 16 bits compared to low-order modulation schemes, for example, on-off keying, which achieve only 4 bits. The tag, which is based on a meandered transmission line, can be printed cheaply as it consists solely of micro-strip structures. We built an appropriate interrogation unit since the achievable storage density of a chipless RFID tag greatly depends on the reader architecture and performance as well. The reader features a bandwidth of 1.25 GHz at a center frequency of 7.825 GHz, which allows for maximal power to meet most countries' UWB regulations. The reader's interrogation signal is a frequency-modulated continuous-wave (FMCW) signal generated by a voltage-controlled oscillator (VCO) that is stabilized by a phase-locked loop (PLL). With this setup the interrogation-unit build is inexpensive. As the reader as well as the tag impact the information content, the complete RFID system needs to be investigated. In addition to a detailed description of both system parts, tests demonstrate the performance of the TDR RFID systems.

This paper introduces three novel passive phase modulators for entirely electromagnetic chipless time domain RFID tags. The modulators overcome limitations of earlier approaches and allow for a highly increased signal space coverage, and, therefore, the implementation of high order PSK or QAM schemes. All three modulators are studied in theory and practically implemented up to a 16-PSK on a reconfigurable prototype tag with 4 modulation stages, realizing a 16 bit information content. Besides showing the measurement results, the assets and drawbacks of the three approaches are carefully compared and the achieved information density is related with approaches from literature.

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