IEEE INFOCOM 2018-IEEE Conference on Computer Communications dtd Fig.5:Illustration of device translation in 2D.The reader translates from position R to R'. (a)Spinning with shaking (b)Relative spinning model Fig.6:Illustration of relative spinning.(a)Although the C.Limitations of Single-Tag Based Approach turntable translates a lot when it is spinning,the relative distance between two tags remain unchanged.(b)From the The refined spinning signal is a good indicator to describe perspective of T1,T2 appears to move around Ti in a circle. the spinning in quiet settings.However,such signal heavily Thus,the relative phase only depends on the spinning itself depends on d,i.e.the distance between the reader and turntable instead of the shaking induced translation. center,as suggested in Eqn.6.As aforementioned,it is hard to hold the distance in noisy industrial settings.Even tiny shaking of the reader or the turntable would introduce not change.We can get the two tags'phase values when the unpredictable distances.This is the reason why the prior reader is translated to position R'as follows: work [2]requires a mandatory assumption that both the RFID 4π reader and spinning source have no additional displacements 0h(④≈元(d+△d-ncos(2rf.t+血+a》mod2m except those induced by the spinning during the measurement. 4T 02(t)≈ Further,the final received phase is derived from a combination (d+△d0-r2cos(2xf.t+2+a》mod2x of multiple copies of RF signals due to multipath effect.The Then,we define the relative phase of the two tags (denoted as measured phase value usually far deviates from the expected A(t))by subtracting their phase values.Since (a-b)mod one.Fig.3(c)shows the spinning signal acquired from a same c=(a mod c-b mod c)mod c,A0(t)can be given by: spinning process as shown in Fig.3(b)but under a noisy △0(t)=(0(t)-02(t)mod2m environment.Clearly,it totally cannot represent the original spinning any more.Therefore,we need to develop a more (ra cos(2+a)-r1 cos(2+a))mod 2 robust spinning signal. (r2 cos(+a)-ri cos(+))cos(2) (r2 sin(2 +a)-ri sin(1+a))sin(2fst)]mod 2 IV.SENSING WITH DUAL TAGS rcos(2f+arctan 2)mod 2 a (7) We call the instability caused by either motion of devices where or changes of environment as system shaking.The approach which can tolerate the system shaking is called as anti-shaking a1 =T2 coS(62+a)-r1 cos(1+a) sensing.We attach dual tags on the same spinning object to a2 r2 sin(2 +a)-ri sin(o1 +a) achieve more robust sensing r=Vai+a It is easy to find that r is actually the separated distance of A.Rationale Behind Anti-Shaking Sensing two tags.Both the variables d and Ad(t)are removed by the subtraction,which means the relative phase at an arbitrary time Why could dual tags resist shaking?We begin to answer this is independent of either the initial position or device translation question from line-of-sight scenario(i.e.free-space scenario), as long as the reader's direction does not change.Eqn.7 fully where the signal from the reader arrives along one dominate considers the initial angles of both tags when t=0,which path,and then discuss it in a more complex scenario with allows to attach tags at arbitrary positions on the turntable as multipath effect later. long as they are driven by the same spinning.Interestingly,the Relative phase:For simplicity,we assume that both tags relative phase can be finally converted into a cosine function and the reader lie on a two dimensional plane (extension to with the same frequency as the spinning,like what we discuss 3D will be addressed later).We consider the dual tags Ti and with a single tag. T2 are attached on a turntable,as shown in Fig.5.The reader We can also understand the relative phase from another situates at direction a (i.e.the angle of arrival).When the intuitive perspective.Relative to the position of T,the second tags rotate an angle of 2mft at time t,we observe Ad(t)tag T2 simply appears to move around a circle,as illustrated in translation between RO and R'O due to the shaking of the Fig.6(a).Although the turntable translates due to the shaking, reader or the target.Notice that here we have a reasonable the relative distance between two tags remains unchanged.In assumption that the reader is at a far distance compared to other words,two tags perform relative motion driven by the the movement of devices,thus,the angle of arrival a does spinning instead of the shaking.In this way,we can simplifyO d + ∆d d 2⇡fst T1 T2 R Shaking 0 R ↵ 2⇡fst Fig. 5: Illustration of device translation in 2D. The reader translates from position R to R0 . C. Limitations of Single-Tag Based Approach The refined spinning signal is a good indicator to describe the spinning in quiet settings. However, such signal heavily depends on d, i.e. the distance between the reader and turntable center, as suggested in Eqn. 6. As aforementioned, it is hard to hold the distance in noisy industrial settings. Even tiny shaking of the reader or the turntable would introduce unpredictable distances. This is the reason why the prior work [2] requires a mandatory assumption that both the RFID reader and spinning source have no additional displacements except those induced by the spinning during the measurement. Further, the final received phase is derived from a combination of multiple copies of RF signals due to multipath effect. The measured phase value usually far deviates from the expected one. Fig. 3(c) shows the spinning signal acquired from a same spinning process as shown in Fig. 3(b) but under a noisy environment. Clearly, it totally cannot represent the original spinning any more. Therefore, we need to develop a more robust spinning signal. IV. SENSING WITH DUAL TAGS We call the instability caused by either motion of devices or changes of environment as system shaking. The approach which can tolerate the system shaking is called as anti-shaking sensing. We attach dual tags on the same spinning object to achieve more robust sensing. A. Rationale Behind Anti-Shaking Sensing Why could dual tags resist shaking? We begin to answer this question from line-of-sight scenario (i.e. free-space scenario), where the signal from the reader arrives along one dominate path, and then discuss it in a more complex scenario with multipath effect later. Relative phase: For simplicity, we assume that both tags and the reader lie on a two dimensional plane (extension to 3D will be addressed later). We consider the dual tags T1 and T2 are attached on a turntable, as shown in Fig. 5. The reader situates at direction ↵ (i.e. the angle of arrival). When the tags rotate an angle of 2⇡fst at time t, we observe ∆d(t) translation between RO and R0 O due to the shaking of the reader or the target. Notice that here we have a reasonable assumption that the reader is at a far distance compared to the movement of devices, thus, the angle of arrival ↵ does t1 t2 t3 Translations of turntable (a) Spinning with shaking t1 t2 t3 T1 T2 r ↵ r cos(2⇡fst + φ + ↵) (b) Relative spinning model Fig. 6: Illustration of relative spinning. (a) Although the turntable translates a lot when it is spinning, the relative distance between two tags remain unchanged. (b) From the perspective of T1, T2 appears to move around T1 in a circle. Thus, the relative phase only depends on the spinning itself instead of the shaking induced translation. not change. We can get the two tags’ phase values when the reader is translated to position R0 as follows: ✓1(t) ⇡ 4⇡ λ (d + ∆d(t) − r1 cos(2⇡fst + φ1 + ↵)) mod 2⇡ ✓2(t) ⇡ 4⇡ λ (d + ∆d(t) − r2 cos(2⇡fst + φ2 + ↵)) mod 2⇡ Then, we define the relative phase of the two tags (denoted as ∆✓(t)) by subtracting their phase values. Since (a − b) mod c = (a mod c − b mod c) mod c, ∆✓(t) can be given by: ∆✓(t)=(✓1(t) − ✓2(t)) mod 2⇡ ⇡ 4⇡ λ (r2 cos(2⇡fst + φ2 + ↵) − r1 cos(2⇡fst + φ1 + ↵)) mod 2⇡ = 4⇡ λ [(r2 cos(φ2 + ↵) − r1 cos(φ1 + ↵)) cos(2⇡fst) − (r2 sin(φ2 + ↵) − r1 sin(φ1 + ↵)) sin(2⇡fst)] mod 2⇡ = 4⇡ λ r cos(2⇡fst + arctan a2 a1 ) mod 2⇡ (7) where 8 < : a1 = r2 cos(φ2 + ↵) − r1 cos(φ1 + ↵) a2 = r2 sin(φ2 + ↵) − r1 sin(φ1 + ↵) r = pa2 1 + a2 2 It is easy to find that r is actually the separated distance of two tags. Both the variables d and ∆d(t) are removed by the subtraction, which means the relative phase at an arbitrary time is independent of either the initial position or device translation as long as the reader’s direction does not change. Eqn. 7 fully considers the initial angles of both tags when t = 0, which allows to attach tags at arbitrary positions on the turntable as long as they are driven by the same spinning. Interestingly, the relative phase can be finally converted into a cosine function with the same frequency as the spinning, like what we discuss with a single tag. We can also understand the relative phase from another intuitive perspective. Relative to the position of T1, the second tag T2 simply appears to move around a circle, as illustrated in Fig. 6(a). Although the turntable translates due to the shaking, the relative distance between two tags remains unchanged. In other words, two tags perform relative motion driven by the spinning instead of the shaking. In this way, we can simplify IEEE INFOCOM 2018 - IEEE Conference on Computer Communications