Introduction Design Measurements Discussions Summary Design and Evaluation of a Multi-Modulation Retrodirective RFID Tag Mohammad Alhassoun Michael Varner Prof. Gregory Durgin The Propagation Group Georgia Institute of Technology IEEE International Conference on RFID, 2018 The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary Motivation Problem Overview Next-generation IoT sensors should Operate at high frequencies (mm-waves) Have high gain (reasonable communication distance) Be orientational independent Consume minimal power The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary Motivation Possible Solutions Problems 1 High gain tags 2 Orientation-independent tags Solutions 1 Use antenna arrays 2 Use isotropic (or semi-isotropic) antennas From antenna theory, you cannot do both! The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary Motivation The Best Solution We cannot use active beamformers because power consumption Alternatively, we can use retrodirective arrays Retrodirective arrays are the best RF-based solution to compensate for 1 Narrow beamwidth of passive arrays 2 Short range of high-frequency tags The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary Retrodirective Arrays Retrodirective Arrays: Definition Retrodirective arrays send waves back to the direction of incidence. Ideally, no power loss and maximum gain (in optics, similar to corner Retrodirective Feed Network reflectors) Retrodirective arrays act as passive, adaptive beamformers The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary Retrodirective Arrays Retrodirective Arrays: Example d d d Van Atta arrays Connects each antenna pair by a transmission L 2 lines L 2 = L 1 + nλ m L 1 Problems with Van Atta array are: 1 Limited to OOK or at best BPSK 2 You cannot incorporate two-terminal devices (e.g., tunnel diodes) The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary Theory Proposal A rat-race coupler can be Ant.#1 a retrodirective feed Γ 1 network. The two port scattering matrix is Ant.#2 � � (Γ 1 + Γ 2 ) e − jπ = 1 (Γ 1 − Γ 2 ) � � S (Γ 1 + Γ 2 ) e − jπ 2 (Γ 1 − Γ 2 ) Γ 2 Conditions 1 | Γ 1 | = | Γ 2 | 2 ∠ Γ 2 = ∠ Γ 1 + π The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary Theory Examples of Terminations + j 0 . 5 + j 2 . 0 Case 1 1 Port 1 is open ⇒ Γ 1 = 1 Case 2 • @5.8 GHz Port 2 is short ⇒ Γ 2 = − 1 2 Port 1 is short ⇒ Γ 1 = − 1 Port 2 is open ⇒ Γ 2 = 1 0 . 2 0 . 5 1 . 0 2 . 0 5 . 0 Observation In both cases, the coupler is retrodirective; however, two @5.8 GHz • (opposite) locations on Smith − j 0 . 5 − j 2 . 0 Chart. The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary Theory More to say Switching between retrodirective terminations changes only the phase Switching between a retrodirective and non-retrodirective state implements OOK No restrictions on the type of terminations Now, it is time to test the RCS of the device The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary Campaign Set Up d = 1 . 22 m Tx Tag Metal Plate VNA Rx The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary Campaign Specifications Location : Rooftop of the building (open range) Power : 25 dBm (+6 dBi antenna gain) Frequency Span : (3 . 8 − 7 . 8) GHz ( 4 GHz BW) Angular Span : − 90 ◦ to 90 ◦ Target Height : 1 . 73 cm Post-Processing Technique : Time Gating Tag designs : 1 Retrodirective (BPSK and OOK) 2 Single-element (BPSK and OOK) The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary BPSK BPSK Configuration and Results Recall A rat-race coupler is retrodirective if | Γ 1 | = | Γ 2 | and ∠ Γ 2 = ∠ Γ 1 + π For retrodirective tag: State#1: Γ 1 = 1 , Γ 2 = − 1 0 State#2: Γ 1 = − 1 , Γ 2 = 1 − 15 ∆ σ (dBsm) For single antenna tag: − 30 State#1: Open circuit − 45 State#2: Short circuit Retrodirective Non-retrodirective − 60 The measured differential − 90 − 70 − 50 − 30 − 10 10 30 50 70 90 RCS angles (degrees) The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary BPSK BPSK: Global Performance We expect 6 dB increase in the differential RCS What if we look at the Retrodirective 0 Mean retrodirective global performance? Non-retrodirective Mean non-retrodirective ∆ σ (dBsm) − 4 Within the beamwidth of 6 . 1 dB the (patch) antenna, − 8 how much increase on − 12 average? − 50 − 30 − 10 10 30 50 angles (degrees) The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary BPSK BPSK: Constellations 0 . 8 OS Retrodirective distance = 0 . 53 0 . 6 SO Retrodirective Open single For retrodirective tag: 0 . 4 Short single Quadrature (Volts) distance = 0 . 97 State#1: Γ 1 = 1 , Γ 2 = − 1 0 . 2 State#2: Γ 1 = − 1 , Γ 2 = 1 • 0 For single-antenna tag: − 0 . 2 State#1: Open circuit − 0 . 4 State#2: Short circuit − 0 . 6 − 0 . 8 − 1 − 0 . 5 0 0 . 5 1 In-phase (Volts) The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary OOK OOK: Constellations 0 . 8 OS Retrodirective SS Retrodirective For retrodirective tag: Open single 0 . 6 Load single Quadrature (Volts) State#1: Γ 1 = 1 , Γ 2 = − 1 State#2: Γ 1 = − 1 , distance = 0 . 56 0 . 4 Γ 2 = − 1 0 . 2 1 For single-antenna tag: = e c n a t s i d State#1: Open circuit 0 State#2: 50 Ω Load 0 0 . 2 0 . 4 0 . 6 0 . 8 1 In-phase (Volts) The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary Retrodirectivity Ideality Facor (RIF) Retrodirectivity Ideality Facor (RIF): Why? We want to measure the performance of the a retrodirective feed network Phase is the most important quantity Phase of the feed network must be compared with an ideal retrodirective network The ideal feed network is that of Van Atta arrays, a simple TEM Transmission line Therefore, we introduced a new metric: The Retrodirectivity Ideality Facor (RIF) The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
Introduction Design Measurements Discussions Summary Retrodirectivity Ideality Facor (RIF) Retrodirectivity Ideality Facor (RIF): Definition Definition Maximum deviations between the samples of the measured phase and the samples of the interpolated equivalent linear phase. max { RIF j } , ∀ j = 1 , . . . , # of states N � (Φ i, 21 − ˆ Φ i, 21 ) 2 i =1 RIF = 1 + N � Φ 2 ˆ i, 21 i =1 The Propagation Group c � Mohammad Alhassoun; malhassoun@gatech.edu The Propagation Group @ Georgia Tech Design and Evaluation of a Multi-Modulation Retrodirective RFID Tags
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