IEEE INFOCOM 2018-IEEE Conference on Computer Communications 670 1612 120140 180 Mocioniless Linear Arc-shapod Fandom Ctjoct shaking 1120 Fig.12:Errors vs.trajectories Fig.13:Impact of spinning speed Fig.14:Impact of dual-tag distance 22.5ml 0.4 LOS .NLOS Tag type Enor (Hz) Fig.15:Impact of antenna distance Fig.16:Impact of diversity Fig.17:Impact of multipath shakes are performed up to a range of 30cm.Besides,in as long as their separation is fixed.We then set this separation view of the case that the spinning object shakes.we utilize to 3cm,5cm and 8cm respectively while keeping the same an orbital shaker to automatically shake the turntable along RPM and plot the recovered signals in Fig.14.We observe a restricted circular orbit with different speeds(see Fig.11). from this figure that although the three signals vary a lot in As a comparison study,we also consider the situation where pattern,their periods keep consistent (i.e.about 57ms).The both the antenna and turntable remain motionless.We compare averaged sensing accuracy is 0.10Hz,0.19Hz and 0.28Hz in the performance of Tagtwins against Tagbeat,which is not these three settings.In our experimentation,we choose the resistant to device translation.Fig.12 plots the sensing errors dual-tag distance as 5cm by default. in frequency.We find that both Tagtwins and Tagbeat achieve 3)Impact of Antenna Distance:Commercial RFID prod- high precision(around 0.2Hz)if the equipment does not move ucts can support a reading range of 6~7 meters in indoor during the experiment.However,if either the reader or the environment,so we change the distance between reader an- object observes some level of translation,even in a slight way, tenna and spinning object from 0.5m to 5m.Fig.15 shows the accuracy of Tagbeat will be affected severely,dropping to the accuracy with different distances.We have the following more than 7Hz.That is where our system wins out.In general, observations:(a)The performance achieves the best when the Tagtwins achieves a mean error of 0.27Hz in frequency with distance equals 1.5m.(b)When the antenna is too close to the the standard deviation of 0.53Hz,corresponding to 0.43ms tags,i.e.less than 0.5cm,the accuracy will drop.Recall that error in period,which is fairly good and can even rival those we have a premise in SIV that the antenna and turntable should of specialized tachometers. have a relatively large distance compared to their movement. and this premise will be broken if the antenna gets near C.Tuning Parameters the turntable (e.g.,distance is below two wavelengths,about We further discuss the following factors that may have an 64cm).Thus,shaking-induced translation can not be well influence on Tagtwins'performance handled by our relative signal.leading to more errors.(c)The 1)Impact of Spinning Speed:To check Tagtwins'effec-performance also decreases when the antenna is too far from tiveness under high frequency scenario,we tune the revolving the tags,i.e.more than 5m.This is understandable because speed of the turntable from 670 to 2,067 RPM with seven a larger distance will result in a lower reading rate,which levels.For each setting,we repeat the experiment for 50 times means fewer samples are collected.In summary,we suggest and Fig.13 depicts the averaged results.It can be seen that the a distance of 1m to 3m according to our empirical study. mean errors among various RPMs have little difference,from 4)Impact of Diversity:We experiment on four models of the minimum of 0.08Hz to the maximum of 0.42Hz.And the tags,namely“2×2”,“Square'”,“Squig"”and“Squiggle”to result is more accurate when the object spins at a low speed,study the influence of tag diversity.All these tag types have which is reasonable because more samples in one period can different antenna sizes and shapes as depicted in Fig.16.For be collected for recovering. each tag model,the result is averaged from 50 experiments 2)Impact of Dual-Tag Distance:As mentioned before,we with the same setting.We find that although the errors of have no requirement of the dual tags'geometric relationship all models maintain at a small value (less than 0.6Hz),thereMotionless Linear Arc−shaped Random Object shaking −2 0 2 4 6 8 10 12 14 Error (Hz) Tagtwins Tagbeat Fig. 12: Errors vs. trajectories 670 1049 1120 1340 1612 1832 2067 −0.4 −0.2 0 0.2 0.4 0.6 0.8 1 1.2 RPM Error (Hz) Fig. 13: Impact of spinning speed 0 20 40 60 80 100 120 140 160 180 200 −1.5 −1 −0.5 0 0.5 1 1.5 Time (ms) s(t) 3cm 5cm 8cm One period Fig. 14: Impact of dual-tag distance 0.5m 1.5m 3m 4m 5m 0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 Antenna distance Error (Hz) Fig. 15: Impact of antenna distance 2X2 Square Sguig Squiggle 0 0.2 0.4 0.6 0.8 1 1.2 Tag type Error (Hz) Size: 44.5mm×10.4mm Size: 94.8mm×8.1mm Size: 22.5mm×22.5mm Size: 44mm×46mm Fig. 16: Impact of diversity 0 1 2 3 4 5 6 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 Error (Hz) CDF LOS NLOS Fig. 17: Impact of multipath shakes are performed up to a range of 30cm. Besides, in view of the case that the spinning object shakes, we utilize an orbital shaker to automatically shake the turntable along a restricted circular orbit with different speeds (see Fig. 11). As a comparison study, we also consider the situation where both the antenna and turntable remain motionless. We compare the performance of Tagtwins against Tagbeat, which is not resistant to device translation. Fig. 12 plots the sensing errors in frequency. We find that both Tagtwins and Tagbeat achieve high precision (around 0.2Hz) if the equipment does not move during the experiment. However, if either the reader or the object observes some level of translation, even in a slight way, the accuracy of Tagbeat will be affected severely, dropping to more than 7Hz. That is where our system wins out. In general, Tagtwins achieves a mean error of 0.27Hz in frequency with the standard deviation of 0.53Hz, corresponding to 0.43ms error in period, which is fairly good and can even rival those of specialized tachometers. C. Tuning Parameters We further discuss the following factors that may have an influence on Tagtwins’ performance. 1) Impact of Spinning Speed: To check Tagtwins’ effec￾tiveness under high frequency scenario, we tune the revolving speed of the turntable from 670 to 2, 067 RPM with seven levels. For each setting, we repeat the experiment for 50 times and Fig. 13 depicts the averaged results. It can be seen that the mean errors among various RPMs have little difference, from the minimum of 0.08Hz to the maximum of 0.42Hz. And the result is more accurate when the object spins at a low speed, which is reasonable because more samples in one period can be collected for recovering. 2) Impact of Dual-Tag Distance: As mentioned before, we have no requirement of the dual tags’ geometric relationship as long as their separation is fixed. We then set this separation to 3cm, 5cm and 8cm respectively while keeping the same RPM and plot the recovered signals in Fig. 14. We observe from this figure that although the three signals vary a lot in pattern, their periods keep consistent (i.e. about 57ms). The averaged sensing accuracy is 0.10Hz, 0.19Hz and 0.28Hz in these three settings. In our experimentation, we choose the dual-tag distance as 5cm by default. 3) Impact of Antenna Distance: Commercial RFID prod￾ucts can support a reading range of 6 ⇠ 7 meters in indoor environment, so we change the distance between reader an￾tenna and spinning object from 0.5m to 5m. Fig. 15 shows the accuracy with different distances. We have the following observations: (a) The performance achieves the best when the distance equals 1.5m. (b) When the antenna is too close to the tags, i.e. less than 0.5cm, the accuracy will drop. Recall that we have a premise in §IV that the antenna and turntable should have a relatively large distance compared to their movement, and this premise will be broken if the antenna gets near the turntable (e.g., distance is below two wavelengths, about 64cm). Thus, shaking-induced translation can not be well handled by our relative signal, leading to more errors. (c) The performance also decreases when the antenna is too far from the tags, i.e. more than 5m. This is understandable because a larger distance will result in a lower reading rate, which means fewer samples are collected. In summary, we suggest a distance of 1m to 3m according to our empirical study. 4) Impact of Diversity: We experiment on four models of tags, namely “2 ⇥ 2”, “Square”, “Squig” and “Squiggle” to study the influence of tag diversity. All these tag types have different antenna sizes and shapes as depicted in Fig. 16. For each tag model, the result is averaged from 50 experiments with the same setting. We find that although the errors of all models maintain at a small value (less than 0.6Hz), there IEEE INFOCOM 2018 - IEEE Conference on Computer Communications