This work exploits ultra-low cost, commodity, radio frequency identification (RFID) tags as elements of a reconfigurable intelligent surface (RIS). Such batteryless tags are powered and controlled by a software-defined radio reader, with properly modified software, so that a source-destination link is assisted, operating at a different band. Signal model includes small-scale and large-scale fading, direct link, as well as specific parameters relevant to reflection (i.e., backscatter) radio, such as antenna structural mode and reflection efficiency, typically overlooked in the literature. An algorithm is offered that computes the optimal RIS configuration with complexity of$\mathcal {O}(M\log {M})$in number of elements$M$, instead of intractable exponential complexity of exhaustive search, while accommodating any number$K\geq 2$of loads. With the proposed algorithm, it is shown that performance gains reach a plateau for constant element spacing and increasing number of elements, suggesting that the weak, passive nature of backscattered links limits the performance gains, even with perfect channel estimation. Channel estimation with linear minimum mean squared error (LMMSE) estimator is shown to be effective, provided that there are sufficient number of pilot symbols. A concrete way is offered to design and prototype a wireless, batteryless, RF-powered, reconfigurable surface and a proof-of-concept is experimentally demonstrated.

This work exploits commodity, ultra-low cost, com-mercial radio frequency identification tags (RFID) as the elements of a reconfigurable surface. Such batteryless tags are powered and controlled by a software-defined (SDR) reader, with properly modified software, so that a source-destination link is assisted, operating at a different carrier frequency. In terms of theory, the optimal gain and corresponding best element configuration is offered, with tractable polynomial complexity (instead of exponential) in number of elements. In terms of practice, a concrete way to design and prototype a wireless, batteryless, RF-powered, reconfigurable surface is offered and a proof-of-concept is experimentally demonstrated. It is also found that even with perfect channel estimation, the weak nature of backscattered links limits the performance gains, even for large number of surface elements. Impact of channel estimation errors is also studied. Future extensions at various carrier frequencies could be directly accommodated, through simple modifications in the antenna and matching network of each RFID tag/surface element.

Commercial radio frequency identification (RFID) readers have to resolve collisions between tags without sacrificing throughput. This work proposes a Viterbi joint sequence detector and a 2-symbol joint tag information detector that can resolve collision between two tags in the physical layer. In sharp contrast to prior art, the proposed closed-form signal model takes into account the asynchrony level between the two collided tag responses, due to the low-cost hardware of commercial RFID tags that follow industry’s Gen2 protocol. The asynchrony is considered as the time offset between the beginnings of the two tags’ RN16 responses and is modeled through a derived shaping matrix that depends on the delayed tag information. Performance evaluation of the proposed detectors shows improved performance compared to prior art, under different operating regimes. It is also found that for different time offset values, bit error rate (BER) does not present a monotonic behavior. Finally, it is shown that clustering techniques on the filtered received signal should explicitly take into account the time offset, since the latter can modify the number of observed clusters. Asynchronous detection has been overlooked in classic digital communications; in the era of batteryless, ultra-low cost tags, more work is clearly needed.

Commercial radio frequency identification (RFID) readers have to resolve collisions between tags, without sacrificing throughput. This work proposes a Viterbijoint sequence detectoras well as a 2-symbol joint tag information detector that can resolve a collision between two tags in the physical layer. In sharp contrast to prior art, the proposed closed-form signal model takes into account the asynchrony level between the two collided tag responses, which is not uncommon with commercial, low-cost RFID tags that follow industry’s Gen2 protocol. The asynchrony is considered as the time offset$\tau $between the beginnings of the two tags’ responses and is modeled through a derivedshapingmatrix that depends on the delayed tag information. Performance evaluation of the proposed detectors with simulated data under Ricean fading, as well as experimental data with software-defined radio, reveals improved performance compared to prior art, under various operating regimes. It is also shown that for different values of the parameter$\tau $, BER does not present a monotonic behaviour. As a collateral dividend, it is found that clustering techniques on thefilteredreceived signal should explicitly take into account the time offset$\tau $, since the latter modifies the number of observed clusters.