This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE INFOCOM 2010 proceedings This paper was presented as part of the main Technical Program at IEEE INFOCOM 2010. arbitration.The ALOHA protocol was first developed for ran- and the tag's antenna gain is Gt.The wave length is A.Assume dom access in packet radio networks.To improve efficiency for the transmitter power from the reader is P measured in watts. RFID systems,the slotted ALOHA is developed [1][2].In [9]. By considering power flux density,the power received at the Schoute analyzes the performance of the dynamic frame-sized tag is P According to the radar principle. ALOHA.In [10]Floerkemeier presents a Bayesian strategy the amount of energy reflected by an object is dependent on to dynamically determine frame size.In [11],Vogt uses the the reflective area of the object.This area is referred to as Markov process to model the read process and suggests a set of the radar cross section (RCS).We use o to denote the RCS. dynamic frame sizes in the read process.An estimate function then the reflected power from the tag towards the reader is is also proposed to estimate the tag quantity.In [6].Lee et al. PThe power density yielded at the receiver of adopt Vogt's tag estimation method and derive the dynamic the reader is given byP.According to the frame size for achieving maximum channel efficiency.They (4Tr4 above analysis,we have the path loss as follows: claim that if the frame size approximately equals the number of tags plus one,the maximum channel efficiency can be P-P- (1) obtained.In [12].Murali et al.provide very fast and reliable estimation mechanisms for tag quantity in a more practical According to Eq.(1),we define the free space path loss over approach.To address the multiple-reading problem,Qian et al. distance r as PL(r)=()2.Apparently the above free propose the Lottery Frame scheme [13],a replicate-insensitive space propagation model is deterministic.However,this model estimation protocol.In [14].Bo et al.consider the problem rarely describes the actual propagation situation accurately. of identifying popular categories of RFID tags out of a large In realistic settings the propagation is probabilistic,because collection of tags.In [15],a fast missing tag detection protocol orientations of tags affect the backscatter efficiency,and phe- for RFID based inventory control applications was provided. nomenons such as absorption and multi-path fading further The unreliability of the physical layer in RFID systems make the attenuation not accurately predicted. has a crucial impact on tag reading.To show the effect of We denote the down-link communication from the reader errors on the C1G2 protocol,Mitsugi et al.[16]present to a tag as the forward channel,and denote the up-link simulation results that bit errors significantly degrade the communication from a tag to the reader as the reverse channel. C1G2 performance.In [3].Buettner et al.examine the per- For successful reading of a passive tag with the backscatter formance of the C1G2 RFID system in a realistic setting. scheme,there are two thresholds to meet the physical require- They identify factors that degrade overall performance and ments.The first is the tag power(sensitivity)threshold,Ps.It reliability with a focus on the physical layer.They find that is the minimum received power necessary to turn on an RFID physical layer considerations have a significant impact on chip.The second is the reader sensitivity threshold,Ps.It is reader performance,and that this is exacerbated by a lack of the minimum level of the tag signal that the reader can detect integration between physical and MAC layers.In [5],Aroor and resolve.Thus it must satisfy P2>Ps for the tag to be et al.identify the state of the technical capability of passive powered up and resolve the received signal,and also P>Prs UHF RFID systems using a simple,empirical,experimental for the reader to detect and resolve the received signal. approach.They examine various read distances for free space, near metal and near water situations.In [4].Ramakrishnan B.Tag inventory and access et al.describe the first comprehensive benchmark suite for The MAC protocol for the C1G2 system is based on Slotted passive UHF RFID tags.In [17]Ren et al.performed extensive ALOHA,where each frame has a number of slots and each experiments on an industrial conveyor belt to determine the active tag will reply in a randomly selected slot per frame. effects of mobility on RFID reader performance.In [18] When a reader(interrogator)wishes to read a set of tags,it first Jeffery et al.conduct experiments in realistic settings and find powers up and transmits a continuous wave (CW)to energize that within each reader's detection range,there are two distinct the tags.It then initiates a series of frames,varying the number regions:major detection region and minor detection region. of slots in each frame to best accommodate the number of III.PRELIMINARY tags.After all tags are read,the reader powers down.We refer to an individual frame as a Ouery Round,and the series of A.Far-Field Propagation and Backscatter Principle Query Rounds between power down periods as a Ouery Cycle. Class-1 Generation-2(C1G2)RFID systems [19]as defined For each Ouery Round,the reader can optionally transmit a by EPCglobal are based on UHF frequencies.They use far- Select command which limits the number of active tags by field communication and the physical property of backscatter- providing a bit mask.Then a Query command is transmitted ing power [20].Far-field communication uses the electric radio which contains the uplink frequency and data encoding,the O waves,where the reader sends a continuous base signal that is parameter determining the number of slots for the following reflected back by the tag's antenna.A backscatter tag operates frame,and a Target parameter.When a tag receives a Query by modulating the electronics connected to the antenna so as command,it chooses a random number in the range(0,20-1), to control the reflection of electromagnetic energy.Suppose a where O is in the range (0,15),and the value is stored in the reader and a tag are separated by a distance of r in the free slot counter of the tag.If a tag stores a 0 in its slot counter,it space propagation scenario.The reader's antenna gain is Gr will immediately backscatter a 16 bit random number,denoted Authorized licensed use limited to:Nanjing University.Downloaded on July 11,2010 at 07:37:18 UTC from IEEE Xplore.Restrictions apply.arbitration. The ALOHA protocol was first developed for ran￾dom access in packet radio networks. To improve efficiency for RFID systems, the slotted ALOHA is developed [1][2]. In [9], Schoute analyzes the performance of the dynamic frame-sized ALOHA. In [10] Floerkemeier presents a Bayesian strategy to dynamically determine frame size. In [11], Vogt uses the Markov process to model the read process and suggests a set of dynamic frame sizes in the read process. An estimate function is also proposed to estimate the tag quantity. In [6], Lee et al. adopt Vogt’s tag estimation method and derive the dynamic frame size for achieving maximum channel efficiency. They claim that if the frame size approximately equals the number of tags plus one, the maximum channel efficiency can be obtained. In [12], Murali et al. provide very fast and reliable estimation mechanisms for tag quantity in a more practical approach. To address the multiple-reading problem, Qian et al. propose the Lottery Frame scheme [13], a replicate-insensitive estimation protocol. In [14], Bo et al. consider the problem of identifying popular categories of RFID tags out of a large collection of tags. In [15], a fast missing tag detection protocol for RFID based inventory control applications was provided. The unreliability of the physical layer in RFID systems has a crucial impact on tag reading. To show the effect of errors on the C1G2 protocol, Mitsugi et al. [16] present simulation results that bit errors significantly degrade the C1G2 performance. In [3], Buettner et al. examine the per￾formance of the C1G2 RFID system in a realistic setting. They identify factors that degrade overall performance and reliability with a focus on the physical layer. They find that physical layer considerations have a significant impact on reader performance, and that this is exacerbated by a lack of integration between physical and MAC layers. In [5], Aroor et al. identify the state of the technical capability of passive UHF RFID systems using a simple, empirical, experimental approach. They examine various read distances for free space, near metal and near water situations. In [4], Ramakrishnan et al. describe the first comprehensive benchmark suite for passive UHF RFID tags. In [17] Ren et al. performed extensive experiments on an industrial conveyor belt to determine the effects of mobility on RFID reader performance. In [18] Jeffery et al. conduct experiments in realistic settings and find that within each reader’s detection range, there are two distinct regions: major detection region and minor detection region. III. PRELIMINARY A. Far-Field Propagation and Backscatter Principle Class-1 Generation-2 (C1G2) RFID systems [19] as defined by EPCglobal are based on UHF frequencies. They use far- field communication and the physical property of backscatter￾ing power [20]. Far-field communication uses the electric radio waves, where the reader sends a continuous base signal that is reflected back by the tag’s antenna. A backscatter tag operates by modulating the electronics connected to the antenna so as to control the reflection of electromagnetic energy. Suppose a reader and a tag are separated by a distance of r in the free space propagation scenario. The reader’s antenna gain is Gr and the tag’s antenna gain is Gt. The wave length is λ. Assume the transmitter power from the reader is P1 measured in watts. By considering power flux density, the power received at the tag is P2 = P1·Gr·Gt·λ2 (4π)2r2 . According to the radar principle, the amount of energy reflected by an object is dependent on the reflective area of the object. This area is referred to as the radar cross section (RCS). We use σ to denote the RCS, then the reflected power from the tag towards the reader is P3 = P1·Gr·σ (4π)2r2 . The power density yielded at the receiver of the reader is given by P4 = P1·G2 r·Gt·λ2·σ (4π)4r4 . According to the above analysis, we have the path loss as follows: P1 P2 = P3 P4 = 1 GrGt ( 4πr λ ) 2. (1) According to Eq.(1), we define the free space path loss over distance r as P Lf s(r)=( 4πr λ )2. Apparently the above free space propagation model is deterministic. However, this model rarely describes the actual propagation situation accurately. In realistic settings the propagation is probabilistic, because orientations of tags affect the backscatter efficiency, and phe￾nomenons such as absorption and multi-path fading further make the attenuation not accurately predicted. We denote the down-link communication from the reader to a tag as the forward channel, and denote the up-link communication from a tag to the reader as the reverse channel. For successful reading of a passive tag with the backscatter scheme, there are two thresholds to meet the physical require￾ments. The first is the tag power (sensitivity) threshold, Pts. It is the minimum received power necessary to turn on an RFID chip. The second is the reader sensitivity threshold, Prs. It is the minimum level of the tag signal that the reader can detect and resolve. Thus it must satisfy P2 > Pts for the tag to be powered up and resolve the received signal, and also P4 > Prs for the reader to detect and resolve the received signal. B. Tag inventory and access The MAC protocol for the C1G2 system is based on Slotted ALOHA, where each frame has a number of slots and each active tag will reply in a randomly selected slot per frame. When a reader (interrogator) wishes to read a set of tags, it first powers up and transmits a continuous wave (CW) to energize the tags. It then initiates a series of frames, varying the number of slots in each frame to best accommodate the number of tags. After all tags are read, the reader powers down. We refer to an individual frame as a Query Round, and the series of Query Rounds between power down periods as a Query Cycle. For each Query Round, the reader can optionally transmit a Select command which limits the number of active tags by providing a bit mask. Then a Query command is transmitted which contains the uplink frequency and data encoding, the Q parameter determining the number of slots for the following frame, and a Target parameter. When a tag receives a Query command, it chooses a random number in the range (0, 2Q−1), where Q is in the range (0,15), and the value is stored in the slot counter of the tag. If a tag stores a 0 in its slot counter, it will immediately backscatter a 16 bit random number, denoted This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE INFOCOM 2010 proceedings This paper was presented as part of the main Technical Program at IEEE INFOCOM 2010. Authorized licensed use limited to: Nanjing University. Downloaded on July 11,2010 at 07:37:18 UTC from IEEE Xplore. Restrictions apply