IEEE INFOCOM 2018-IEEE Conference on Computer Communications counters numerous challenges.First,the measured phase val- ues are discontinuous due to the operation of mod.Worsely, the phase may randomly jump radians because of the half- wave loss phenomenon [3].Second,1=2 =0 happens Turntable only when two tags are attached on a straight line which passes through the turntable center.Apparently,we do not Fig.2:Spinning sensing with dual-tags expect to attach tags under some scheduled rules,thus,the two because there are too many linear combinations.Many real formulas cannot be simply merged together like this.Third, scenarios are against such assumption.For instance,industrial all COTS tags are randomly and exclusively read in a time- sharing fashion to avoid signal collisions.It is impossible to operations can happen in unstable platforms (e.g.vehicles and ships),whose shaking would lead to dramatic translations of simultaneously acquire the two tags'phase values at a same readers and tags.It is also hard to let a worker stably hold a time point for the calculation of relative phase.To address these challenges,we firstly develop Relative Spinning Signal handheld reader for a long time measurement.Our empirical study suggests that even a 5cm noisy translation of the device (RSS).We analytically demonstrate that RSS is resilient to would make the spinning signal unrecoverable. surrounding noise even in the presence of multipath effect. Motivated by the above limitation,we present Tagtwins,a Importantly,the underlying sparsity assumption that compres- robust spinning sensing system that can work in noisy settings. sive reading [2]is built on still holds true.We then design Here noise means unpredictable shaking or translation of and implement Dual Compressive Reading (DCR)to recover RSS using COTS RFID devices,with no extra infrastructure devices (readers and/or spinning objects).Tagtwins addresses the challenge by attaching dual RFID tags on the spinning or pre-calibration efforts. surface,as shown in Fig.2,and develops a new formulation Contributions:Tagtwins enhances the RFID-enabled sys- tem that makes sense of mechanical rotation within sub-hertz of spinning signal that is shaking-resilient.We allow both the turntable and reader to be randomly and simultaneously shaken accuracy using dual tags'backscatter signals.It addresses when monitoring the spinning.Even if the shaking involves a practical problem of how to robustly sense spinning in unknown trajectories,we can accurately recover its spinning noisy settings.Second,we develop RSS to depict the shake- resilient sensing,and DCR to inspect high-frequency spinning. signal.To this end,we exploit the observation that the distance between two tags is fixed independent of how the turntable or Third,we implement and evaluate our prototype with extensive the reader shakes.Leveraging this,we develop the relative experiments,demonstrating the practicality and effectiveness spinning signal which is derived from the relative wireless of our design. channels of two tags,to depict the spinning that occurs in noisy II.OVERVIEW settings,without knowing any information on the absolute position or translation of the devices. Tagtwins is an RFID-based solution for inspecting me- To quickly grasp our basic idea,we give a simplified chanical spinning frequency of any objects.Although we explanation why our relative spinning signal can work.As present the system in the context of spinning in most of shown in Fig.2,the phase values of signals backscattered the time,Tagtwins'technique could be applied to any other from tag Ti and T2 are respectively given by modalities of periodic mechanical motion (like vibration or pendulum).Specifically,it decomposes the sensing problem 01()≈红(d-n1cos(2mft+1》mod2m into the following two cases: (1) Sensing with a single tag.We firstly consider a simplified 4π 02(t)≈ (d-ra cos(2f+))mod 2 case where a single tag is used to sense the spinning in quiet settings in Sec.III.Correspondingly,we develop the where A is the wavelength,d is the distance between the reader refined spinning signal with the RF phase values to address and turntable center,fs is the spinning frequency,ri,r2 are the discontinuity caused by either the modulus operation or distances of two tags to the turntable center,and 1.2 are the half-wave loss. the initial angles of two tags.The detailed geometric model is Sensing with dual tags.We then consider a general presented in Sec.III.Assuming1=2=0,we can obtain case where dual tags are used to defend against signal noises the relative phase by subtracting the above two equations: introduced by the devices or the surroundings in Sec.IV. △0)=4r (r2-r1)cos(2nfst)mod 2m (2) Correspondingly,we develop the relative spinning signal to enhance the system robustness. Clearly,the distance d is perfectly eliminated from the for- The next few sections elaborate on the above steps,provid- mula.This means that no mater how the reader or the turntable ing the technical details. moves,the relative phase is only dependent on fs.Meanwhile, A(t)also maintains the same frequency as the original. III.SENSING WITH A SINGLE TAG One might consider using the above relative phase as the In this section,we introduce RFID-based spinning sensing spinning signal.Unfortunately,performing it in practice en- with a single tag as well as its limitations.O T1 T2 Turntable Shaking Reader Fig. 2: Spinning sensing with dual-tags because there are too many linear combinations. Many real scenarios are against such assumption. For instance, industrial operations can happen in unstable platforms (e.g. vehicles and ships), whose shaking would lead to dramatic translations of readers and tags. It is also hard to let a worker stably hold a handheld reader for a long time measurement. Our empirical study suggests that even a 5cm noisy translation of the device would make the spinning signal unrecoverable. Motivated by the above limitation, we present Tagtwins, a robust spinning sensing system that can work in noisy settings. Here noise means unpredictable shaking or translation of devices (readers and/or spinning objects). Tagtwins addresses the challenge by attaching dual RFID tags on the spinning surface, as shown in Fig. 2, and develops a new formulation of spinning signal that is shaking-resilient. We allow both the turntable and reader to be randomly and simultaneously shaken when monitoring the spinning. Even if the shaking involves unknown trajectories, we can accurately recover its spinning signal. To this end, we exploit the observation that the distance between two tags is fixed independent of how the turntable or the reader shakes. Leveraging this, we develop the relative spinning signal which is derived from the relative wireless channels of two tags, to depict the spinning that occurs in noisy settings, without knowing any information on the absolute position or translation of the devices. To quickly grasp our basic idea, we give a simplified explanation why our relative spinning signal can work. As shown in Fig. 2, the phase values of signals backscattered from tag T1 and T2 are respectively given by ✓1(t) ⇡ 4⇡ λ (d − r1 cos(2⇡fst + φ1)) mod 2⇡ ✓2(t) ⇡ 4⇡ λ (d − r2 cos(2⇡fst + φ2)) mod 2⇡ (1) where λ is the wavelength, d is the distance between the reader and turntable center, fs is the spinning frequency, r1, r2 are distances of two tags to the turntable center, and φ1, φ2 are the initial angles of two tags. The detailed geometric model is presented in Sec. III. Assuming φ1 = φ2 = 0, we can obtain the relative phase by subtracting the above two equations: ∆✓(t) = 4⇡ λ (r2 − r1) cos(2⇡fst) mod 2⇡ (2) Clearly, the distance d is perfectly eliminated from the for￾mula. This means that no mater how the reader or the turntable moves, the relative phase is only dependent on fs. Meanwhile, ∆✓(t) also maintains the same frequency as the original. One might consider using the above relative phase as the spinning signal. Unfortunately, performing it in practice en￾counters numerous challenges. First, the measured phase val￾ues are discontinuous due to the operation of mod. Worsely, the phase may randomly jump ⇡ radians because of the half￾wave loss phenomenon [3]. Second, φ1 = φ2 = 0 happens only when two tags are attached on a straight line which passes through the turntable center. Apparently, we do not expect to attach tags under some scheduled rules, thus, the two formulas cannot be simply merged together like this. Third, all COTS tags are randomly and exclusively read in a time￾sharing fashion to avoid signal collisions. It is impossible to simultaneously acquire the two tags’ phase values at a same time point for the calculation of relative phase. To address these challenges, we firstly develop Relative Spinning Signal (RSS). We analytically demonstrate that RSS is resilient to surrounding noise even in the presence of multipath effect. Importantly, the underlying sparsity assumption that compres￾sive reading [2] is built on still holds true. We then design and implement Dual Compressive Reading (DCR) to recover RSS using COTS RFID devices, with no extra infrastructure or pre-calibration efforts. Contributions: Tagtwins enhances the RFID-enabled sys￾tem that makes sense of mechanical rotation within sub-hertz accuracy using dual tags’ backscatter signals. It addresses a practical problem of how to robustly sense spinning in noisy settings. Second, we develop RSS to depict the shake￾resilient sensing, and DCR to inspect high-frequency spinning. Third, we implement and evaluate our prototype with extensive experiments, demonstrating the practicality and effectiveness of our design. II. OVERVIEW Tagtwins is an RFID-based solution for inspecting me￾chanical spinning frequency of any objects. Although we present the system in the context of spinning in most of the time, Tagtwins’ technique could be applied to any other modalities of periodic mechanical motion (like vibration or pendulum). Specifically, it decomposes the sensing problem into the following two cases: • Sensing with a single tag. We firstly consider a simplified case where a single tag is used to sense the spinning in quiet settings in Sec. III. Correspondingly, we develop the refined spinning signal with the RF phase values to address the discontinuity caused by either the modulus operation or the half-wave loss. • Sensing with dual tags. We then consider a general case where dual tags are used to defend against signal noises introduced by the devices or the surroundings in Sec. IV. Correspondingly, we develop the relative spinning signal to enhance the system robustness. The next few sections elaborate on the above steps, provid￾ing the technical details. III. SENSING WITH A SINGLE TAG In this section, we introduce RFID-based spinning sensing with a single tag as well as its limitations. IEEE INFOCOM 2018 - IEEE Conference on Computer Communications