3918 IEEE TRANSACTIONS ON COMMUNICATIONS, VOL. 62, NO. 11, NOVEMBER 2014 Detection of Multiple Tags Based on Impulsive Backscattered Signals Francesco Guidi, Member, IEEE, Nicolò Decarli, Member, IEEE, Stefania Bartoletti, Student Member, IEEE, Andrea Conti, Senior Member, IEEE, and Davide Dardari, Senior Member, IEEE Abstract—Passive and semipassive ultrawideband (UWB) radio-frequency identification (RFID) technology has been re- cently proposed to offer high-accuracy localization capabilities in next-generation RFID systems. This technology relies on the mod- ulation of backscattered signals, i.e., backscatter modulation, from multiple tags present in the environment. The detection of multiple tags based on backscattered signals is challenging in harsh envi- ronments with nonideal conditions such as clutter, near–far inter- ference effects, and clock drift. This paper analyzes the detection of multiple tags employing UWB backscatter modulation and pro- poses practical signaling, spreading codes, and detection schemes that are robust to nonideal conditions. A case study is presented to evaluate the performance of the proposed technique for the detec- tion of multiple tags based on impulsive backscattered signals. Index Terms—UWB backscatter modulation, RFID, multi-tag detection, near-far problem, aggregate interference. I. I NTRODUCTION I NTERNET of things paradigm envisages the physical world mapped into the Internet space. Within this context, a large number of novel applications, where objects will be responsive to the presence of people and other objects, is at the horizon [1]. Such a scenario calls for energy efficient wireless technologies able to provide both object identification and localization ca- pabilities. This is the concept of radio detection, identification, and localization (RaDIAL), where the radio-frequency identi- fication (RFID) of passive tags and their localization, resorting to real-time locating systems (RTLS), are offered with the same system. An RFID system is composed of a network of readers aiming to identify tags attached to objects and persons through wireless communications. In particular, the reader interrogates via a radio link the tags that answer by communicating both their Manuscript received February 7, 2014; revised July 17, 2014; accepted October 2, 2014. Date of publication October 15, 2014; date of current version November 18, 2014. This work was supported in part by the European Commission through the framework of the FP7 project SELECT under Grant 257544 and in part by the Italian Ministerial PRIN project GRETA under Grant 2010WHY5PR. This paper was presented in part at the IEEE Int. Conf. on Ultra-Wideband (ICUWB), Syracuse, NY, USA, Sep. 2012. The associate editor coordinating the review of this paper and approving it for publication was M. Buehrer. F. Guidi, N. Decarli, and D. Dardari are with the DEI, CNIT, University of Bologna, 47521 Cesena, Italy (e-mail: f.guidi@unibo.it; nicolo.decarli@ unibo.it; davide.dardari@unibo.it). S. Bartoletti and A. Conti are with the ENDIF, CNIT, University of Ferrara, 44100 Ferrara, Italy (e-mail: stefania.bartoletti@unife.it; a.conti@ieee.org). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TCOMM.2014.2363118 Fig. 1. Scenario with a reader interrogating several tags. identification and stored data [2]. Among all possible solutions, semi-passive tags are very promising for applications requiring extremely low power consumption, as the available energy from batteries or harvesters is used only for control logic operations. In fact, the tag-reader communication is based on backscatter modulation, which consists in changing the tag’s antenna load according to the data to be transmitted, therefore modifying how the antenna reflects back the interrogation signal [2]. Note that the backscatter modulation does not require the emission of new RF energy and hence it is usually classified as a passive communication scheme. Considering the convergence of RFID and high-definition RTLS toward the RaDIAL concept for enhancing the function- alities offered to the end user and enabling new potential wide market applications [3], the adoption of the ultra-wideband UWB technology [4], [5] is particularly appealing for its capa- bility of communication robustness and localization accuracy even in harsh propagation environments [6]–[11]. Fig. 1 shows an example scenario with a reader that inter- rogates semi-passive UWB tags located in the same area. To save energy, tags are normally in sleep mode and are woken-up through the transmission of a wake-up signal (e.g., an unmod- ulated ultra-high frequency (UHF) carrier) [12], [13]. Each tag reflects the incoming UWB interrogation signal by means of backscatter modulation according to its internal information and identification code. Signals backscattered by different tags can be distinguished through the adoption of different spreading 0090-6778 © 2014 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information.