Walk-through checkout station

Systems and methods for implementing a radio frequency identifier (RFID) system are provided. The methods include transmitting a radio frequency (RF) signal, by an RFID interrogator with multiple antennas. The methods include receiving a superimposed received signal. The superimposed received signal includes replies from a first RFID tag and a second RFID tag that are overlapping in time. The methods also include separating the replies from the first RFID tag and second RFID tag though spatial processing of the superimposed received signal.

BACKGROUND

Technical Field

The present invention relates to radio-frequency identification (RFID) and more particularly to detection of RFID tags.

Description of the Related Art

RFID tags come in a variety of configurations, sizes, read ranges, memory amounts, etc. Even though the use of RFID tags is not yet widespread, some stores are leveraging RFIDs in their daily operations and some governments have embarked on initiatives to deploy RFID based check-out in the next few years. The use of RFID tags for theft prevention has already been in place for almost a decade now, where expensive goods or small items that can be hidden or misplaced easily are tagged and RFID readers (placed at entrance/exit doors) alert the retailer if an item leaves the store without being already paid for.

SUMMARY

According to an aspect of the present invention, a method is provided for implementing a radio frequency identifier (RFID) system. The method includes transmitting, by an RFID interrogator with multiple antennas, an RF signal. The method includes receiving a superimposed received signal. The superimposed received signal includes replies from a first RFID tag and a second RFID tag that are overlapping in time. The method also includes separating the replies from the first RFID tag and second RFID tag though spatial processing of the superimposed received signal.

According to another aspect of the present invention, a method is provided for implementing a radio frequency identifier (RFID) system. The method includes perturbing a transmitted wave. The wave is transmitted by an RF interrogator. The method includes generating a quasi-static process for a stationary period. The stationary period is at least a time required to excite an RFID tag, receive a message from the RF interrogator and finish the reply by the RFID tag. The method further includes generating different average energy at the RFID tag in different stationary period within a given reading cycle by the RF interrogator. The given reading cycle includes multiple stationary periods.

According to another aspect of the present invention, a system is provided for implementing a radio frequency identifier (RFID) system. The system includes a processor device operatively coupled to a memory device, the processor device being configured to transmit a radio frequency (RF) signal, by an RFID interrogator with multiple antennas. The processor device also receives a superimposed received signal. The superimposed received signal includes replies from a first RFID tag and a second RFID tag that are overlapping in time. The processor device also separates the replies from the first RFID tag and second RFID tag though spatial processing of the superimposed received signal.

According to another aspect of the present invention, a system is provided for implementing a radio frequency identifier (RFID) system. The system includes a processor device operatively coupled to a memory device, the processor device being configured to perturb a transmitted wave. The wave is transmitted by an RF interrogator. The processor device generates a quasi-static process for a stationary period. The stationary period is at least a time required to excite an RFID tag, receive a message from the RF interrogator and finish the reply by the RFID tag. The processor device further generates different average energy at the RFID tag in different stationary period within a given reading cycle by the RF interrogator. The given reading cycle includes multiple stationary periods.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In accordance with embodiments of the present invention, systems and methods are provided to/for implementing a walk-through checkout station that includes a radio frequency identifier (RFID) interrogator with multiple antennas. The RFID interrogator can excite multiple RFID tags by transmitting signals from the interrogator, receiving replies from RFID tags that are overlapping in time and separating the replies from RFID tags though spatial processing of the superimposed received signal.

In one embodiment, the transmitted wave is perturbed by the RFID interrogator to generate a quasi-static process where the stationary period is at least the time required to excite an RFID tag. Messages are then received by the RFID tag from the interrogator and a reply is sent by the same RFID tag, generating different average energy at the RFID tag in different stationary period within a given reading cycle by RFID interrogator. The reading cycle includes multiple stationary periods.

Referring now in detail to the figures in which like numerals represent the same or similar elements and initially toFIG. 1, a block diagram illustrating a walk-through checkout station system100is provided, in accordance with example embodiments.

As shown,FIG. 1illustrates a walk-through checkout station system100in which a checkout process can be completed using a walk-through checkout station102(for example, lane, etc.) based on RFID technology in accordance with example embodiments described herein. Walk-through checkout station102includes a scanning area120, and a pay station150. The scanning area120includes (or is connected to, wired or wirelessly) an RFID detection system125and can include cameras130positioned to perform facial recognition through facial matching. A person (illustrated as persons105-1and105-2) can take a basket (illustrated as baskets110-1and110-2) into the scanning area120, with items (that include RFID tags) to be purchased being listed on a display135after RFID detection by RFID detection system125. In some embodiments, the scanning area120can include a shielded returns section in which items are removed from the area and from the total (for example, by a conveyor belt, not shown). The person can then proceed to the pay station150.

In example embodiments described herein, the RFID detection system125can read the correct RFID tag (not individually shown inFIG. 1) corresponding to an item in the basket110in presence of other RFID tags which may include RFID tags that are not to be read as the price tag for an item and are used for different purposes, such as theft prevention, inventory, etc.). RFID detection system125can read the RFID tag without the need to locate the RFID tag, or correctly place it, for example horizontally or flat on the surface. RFID detection system125can read multiple tags at a time or in a very short interval. RFID detection system125can also trace an item (based on an RFID tag), for example a particular shirt from a set of shirts that have the same size, fabric, color, pattern, etc., to its time of checkout and corresponding customer (for example, basket110). RFID detection system125can minimize the possibility of error (for example, in skipping an item, etc.) at the time of scanning.

Walk-through checkout station system100can include one or more areas defined as tunnels (or lanes), where the presence of an RFID tag can be distinguished in any of these areas. As described herein, a tunnel refers to a three-dimensional (3D) volume. A tunnel can be separated from the rest of the 3D volume by a boundary where the boundary is a connected surface (for example, a physical boundary). Part of the boundary may be marked by actual objects such as wall, doors, etc., where some other parts may remain open and considered to be a virtual boundary.

Walk-through checkout station system100can include tunnels designed to use multiple antennas and a combination of the received signal from those antennas. In example embodiments, walk-through checkout station system100can accommodate the use of RF absorbers, and RF reflectors (not shown inFIG. 1) to facilitate the shaping of such tunnels. RFID detection system125can use a combination of received signal from different antennas along with their characteristics such as received signal strength indictor (RSSI), phase, doppler, etc., can be used to infer possible position of the RFID tags. RFID detection system125can observe such readings of the RFID tag by different antennas over time (e.g., number of readings, time between readings, etc.) to improve the efficiency of the classifying process.

Referring now toFIG. 2, a top view of an enhanced multi-lane walk-through checkout station is provided, in accordance with example embodiments.

As shown inFIG. 2, an enhanced multi-lane walk-through checkout station200(top view) includes antennas205deployed at the sides of each scanning area120(shown as120-1to120-3for three adjacent walk-through checkout station systems100, by way of example). The walk-through checkout station systems100can include inside RF antenna (210) placements, doors215, outside RF antenna (220) placements and lane curvature230to enhance the inside coverage and attenuate the RF signals that escape the scanning area120(for example, a checkout area). In example embodiments, customers (not shown inFIG. 2) can walk through a scanning area120of a particular walk-through checkout station systems100and the RFID tags240(shown, by way of example as240-1to240-n) are read and charged into an account associated with the customer.

As shown inFIG. 2, multiple RFID tags240can be positioned in a checkout tunnel250that includes RF antennas210of an RFID reader associated with RFID detection system125. In some embodiments RFID detection system125can include one or more radio frequency (RF) reflectors and/or RF absorbers (not individually shown). The RFID detection system125implemented with RFID tags240bring advantages into automation of the checkout process as well as the store management. Since RFID tags240are read wirelessly, there is no need to locate and present the RFID tag240in a specific way to the RFID reader (or interrogator) antenna. Further, multiple RFID tags240) can be scanned at (for example, substantially) the same time by having them in the scanning area120at once. The RFID detection system125reduces the possibility of human error by limiting reading of RFID tags240to those in the scanning area120and reading directly without a person presenting the RFID tag240to a reader as is the case in barcode systems. Hence, it is not possible to present a different (wrong) RFID tag240, or not to scan the RFID tag240, or scan an RFID tag240twice or more in error. RFID tags240can mark each item with a unique ID (as opposed to a common ID used for the same item type in barcodes). Hence, it is easier to know exactly which item is sold and adjust the price of a similar item differently than others, for example, for a distressed or open-box merchandise.

The example embodiments provide a system in which one or more areas are defined as (checkout) tunnels250(formed in the scanning area120) and distinguish the presence of an RFID tag240in these areas. The checkout tunnels250refer to a defined 3D volume (that can be defined as a portion of a particular open space or three-dimensional volume, as opposed to a closed underground space). A checkout tunnel250is separated from the rest of the three-dimensional volume by its boundary (for example, where the boundary is a connected surface). Part (or all) of the boundary may be marked by actual objects such as wall, doors, etc. where some other part (or, in some embodiments, all) may remain open and considered to be a virtual boundary.

A session can be defined as a four-dimensional (4D) section marked by a limited time interval in a particular tunnel250. RFID detection system125identifies RFID tags240accurately that are in different sessions. In other words, RFID detection system125identifies the presence of an RFID tag140within a tunnel250in a particular time interval. Although multiple sessions may overlap over some time interval (e.g., two customers being in adjacent tunnels (for example, within scanning area120-1and120-2holding items) RFID detection system125can correctly assign each of the RFID tags240to each session. RFID detection system125handles possible interference from different antennas in a way that these sessions are separable.

FIG. 2shows a walk-through checkout system that consists of three adjacent lanes (for example, three walk-through checkout station systems100). According to example embodiments, the length of each lane can be approximately 2 meters and the width, and the heights can be both approximately 1 meter. In the illustrated example, there are three antennas210at each side per lane (or scanning area120) that account for a total of 6 antennas per lane (for example, antennas210-1to210-6in scanning area120-1). In other embodiments there can be any number (greater than one) of antennas.

RFID detection system125performs separation of each tunnel within a three-dimensional space in addition to implementing multi-session detection. RFID detection system125can implement processes to identify each RFID tag240that is placed within the session (scanning area120) and nothing else. RFID detection system125handles possible tags in vicinity of the tunnel250(for example, RFID tag240-4with respect to scanning area120-1(and the checkout tunnel250formed within)) such that tags outside of the tunnel250are considered out of the session. RFID detection system125can also logically and/or physically separate two different session that are running back to back on the same tunnel250(and consequently produce meaningful results).

Example embodiments accommodate the use of RF absorbers, and RF reflectors (not individually shown inFIG. 2) to facilitate the shaping of such tunnels250. According to example embodiments, the received signal from different antennas210can be combined along with their characteristics such as received signal strength indicator (RSSI), phase, doppler, etc. and used to infer possible position of the RFID tag240. Additionally, RFID detection system125can observe such readings of the RFID tags240by different antennas210over time to improve the efficiency.

The walk-through checkout station systems100is designed to account for the possible availability of wandering RFID tags240(such as240-4in the instance of scanning area120-1) around the checkout area and make sure that those RFID tags240can be identified as outside tags. In general, if the RFID tags240are not read at all these tags are considered outside tags. However, the RFID tags240that are in close proximity of the tunnel250have a chance to be read by one or some of the antennas210. RFID detection system125identifies these tags as outside tags through a smart detection process. In addition, the physical structure and design of the walk-through checkout station systems100also enhances such detection (and differentiation).

RFID detection system125is designed with a high read efficiency and coverage of the antenna system to guarantee that RFID tags240that are in the checkout area are read. The time that it takes for the system to identify the tags that are inside highly depends on the number of antennas210, the antenna placement, the process that controls the transmission of the RF signals through multiple antennas210, the physical structure of the checkout lanes (for example, lane curvature230), and the placement of the reflectors and absorbers within the checkout structure.

In example embodiments, RFID detection system125can correctly separate and identify the RFID tags240in each lane of multiple adjacent lanes (for example, scanning areas120-1to120-3). RFID detection system125can correctly separate and identify the tags in sessions in different lanes including in instances in which sessions start and end at different time. RFID detection system125identifies the RFID tags240while accounting for the presence of another customer (and hence RFID tags240for that customer that do not belong to the session) in an adjacent lane.

Further, the structure of the walk-through checkout station systems100is designed to significantly improve the time that it takes to finish the readings in each session. The lane curvature230enhances the walk-through checkout station systems100in multiple ways. The lane curvature230works in a similar manner to an optical lens (as described in the field of optics) by reflecting back the signals within the scanning area120. The two curved walls (lane curvature230) in both sides can reflect most of the RF signals transmitted from each antenna210several times and generate a full and rich coverage within the checkout lane. Additionally, the same lane curvature230causes less RF signal to be able to escape the checkout lane even when the doors215are open or if no door215is used. This compensates (or accounts for) different customers entering or exiting different lanes and instances in which some outside RFID tags240may be read by the RFID detection system125when a door215is open.

Referring now toFIG. 3, a high-level system for implementing a walk-through gate is illustratively depicted in accordance with an embodiment of the present invention.

Exemplary computer system (e.g., a server or a network device) for implementing a walk-through gate with signal separation is shown in accordance with an embodiment of the present invention. The computer system300includes at least one processing device (CPU)305operatively coupled to other components via a system bus302. A cache306, a Read Only Memory (ROM)308, a Random-Access Memory (RAM)210, an input/output (I/O) adapter320, a network adapter390, a user interface adapter350, an RFID detection system125, and a display adapter360, can be operatively coupled to the system bus302.

A first storage device322and a second storage device329can be operatively coupled to system bus302by the I/O adapter320. The storage devices322and329can be any of a disk storage device (e.g., a magnetic or optical disk storage device), a solid-state magnetic device, and so forth. The storage devices322and329can be the same type of storage device or different types of storage devices. Either or both of the storage devices322and329can be configured to operate as a data store or database to store various logs of RF signal data372(e.g., signal measurements from various portions of the walk-through checkout station systems100). The RFID detection system125can include software and/or hardware as described herein below.

A transceiver395can be operatively coupled to system bus302by network adapter390. A display device362is operatively coupled to system bus302by display adapter360. RFID (reader, or interrogator) data372can be operatively coupled to system bus302directly or indirectly, for example via RFID detection system125. The RFID detection system125can be configured to receive RF signal data372.

A first user input device352and a second user input device359can be operatively coupled to system bus302by user interface adapter350. The user input devices352and359can be any of a sensor, a keyboard, a mouse, a keypad, a joystick, an image capture device, a motion sensing device, a power measurement device, a microphone, a device incorporating the functionality of at least two of the preceding devices, and so forth. Of course, other types of input devices can also be used in accordance with the present invention. The user input devices352and359can be the same type of user input device or different types of user input devices. The user input devices352and359can be used to input and output information to and from system300.

Other embodiments of the present invention can optionally include further processing units including a graphics processing unit (“GPU”), a mother board, or alternatively/additionally another storage medium, an operating system, one or more application software, as well as including one or more communication interfaces (e.g., RS232, Ethernet, Wi-Fi, Bluetooth, USB). Useful examples of computing devices optionally included in or integrable with embodiments of the present invention include, but are not limited to, personal computers, smart phones, laptops, mobile computing devices, tablet PCs, and servers. In accordance with embodiments of the present invention, an event record log source can be a computer storage medium.

Of course, the computer system300can also include other elements (not shown), as readily contemplated by one of skill in the art, as well as omit certain elements. For example, various other input devices and/or output devices can be included in computer system300, depending upon the particular implementation of the same, as readily understood by one of ordinary skill in the art. For example, various types of wireless and/or wired input and/or output devices can be used. Moreover, additional processors, controllers, memories, and so forth, in various configurations can also be utilized as readily appreciated by one of ordinary skill in the art. These and other variations of the computer system300are readily contemplated by one of ordinary skill in the art given the teachings of the present invention provided herein.

It should be understood that multiple computing devices can be operably linked to form a computer network in a manner as to distribute and share one or more resources, such as clustered computing devices and server banks/farms. In embodiments of the present invention each of the aforementioned elements (e.g., device, medium, source, or module) can be directly or indirectly communicably connected (e.g., via a wireless a wired electronic connection) to at least one other element of the system. As described in more detail below, some embodiments of the present invention can be wholly contained within a single computing device. Other embodiments, however, can encompass a plurality of interconnected or networked devices and resources.

Referring now toFIG. 4, a block diagram of a high-level RFID detection system125is illustratively depicted, in accordance with example embodiments of the present invention.

As shown inFIG. 4, RFID detection system125includes RFID readings410, received from RFID tags240within a tunnel250(as described with respect toFIG. 2) and from other lanes and outside the tunnels250. RFID detection system125also includes an RFID classifier420, that classifies RFID tags240as belonging to a particular tunnel250, other tunnels250or outside of the tunnels250, and an RFID session manager, that manages time windows. The ensuing discussion can be further understood with reference toFIG. 2, by way of example.

The coverage and RFID readings410of the entire volume inside a checkout lane (scanning area120) is partially determined by physical design of the system. In some instances, the structure can generate RFID readings410from unwanted RFID tags240. The probability of (and sensitivity to) unwanted RFID readings410increases when power increases or other techniques are performed to generate a full covered area inside the checkout lane. RFID classifier420can eliminate (for example, some of) these unwanted readings from outside RFID tags240and/or tags that belong to an adjacent tunnel (250) or lane.

In general, the reading of an RFID tag240at any particular location and given orientation is a probabilistic phenomenon. Hence, the process of interrogating an RFID tag240can be analyzed as a communication channel where transmission from different antennas (including antennas210in the tunnels250) are interpreted as transmission of different symbols as an input to a channel and the response from the tag is considered a sequence of binary output; ‘1’ for positive response and ‘0’ for no response. The probability of 0 and 1 in the output would be a function of the transmitted symbol, i.e., the transmitted antenna and chosen power.

RFID classifier420can further model the channel as a mixture Gaussian distribution instead of discrete memoryless channel where the received RSSI for each transmitted symbol has a Gaussian distribution and depends on the transmitted symbols as well as the transmit power.

RFID classifier420can identify if the RFID tag240is in a particular location based on identifying which channel is more likely to generate an output sequence based on a known transmit codeword of the symbols. This can be done based on joint typicality between the transmit and receive sequence. Identifying the tag location can also be interpreted in relation to the problem of ‘identification via channel’ where RFID classifier420would determine if a particular codeword was transmitted rather than what codeword was transmitted.

In example embodiments, RFID classifier420determines the location of the RFID tag240in a manner that approximates the joint typical decoding in a suboptimal form (for example, to speed up the decision-making process). Each reading of an RFID tag240from a particular antenna210would receive a positive or negative metric. A given RFID tag's240metric can be calculated for each lane as well as the outside region. The metric is dependent on which lane (for example, tunnel250) the metric is being computed for, and also depends on the antenna port number as well as other reading parameters such as RSSI, etc. The metric is then positively or negatively combined for all the readings within a time window.

RFID session manager430can determine the time window to have a minimum duration to ensure that RFID classifier420can have enough confidence that every RFID tag240inside the checkout region is read at least one time. RFID classifier420then uses a sequential detection process. The process continues until every RFID tag240that has been read during the session by one of the inside antennas210within the tunnel250are decided. In this instance, a decision means that RFID classifier420has identified the RFID tags240to be inside this lane, in another lane or in an outside area.

At each time slot and after updating a metric, RFID classifier420checks if any particular RFID tag240within the list of a lane can be claimed by this lane, another lane or outside area. In some embodiments, RFID detection system125can receive RFID tag240signal measurements (RFID readings410) from outside lanes in a multilane system. The claim process is based on the computed metrics and comparison with a threshold. Note that in some corner cases, two lanes may potentially be able to claim a tag in a given time slot. In such situation, the process can pick the lane which has the largest difference with its corresponding threshold.

According to example embodiments, RFID session manager430may implement a maximum delay for the time window where the decision for a particular lane has to be made. Since the sequential detection may not reach a decision by the end of the maximum delay time, at this time RFID classifier420uses a truncated sequential detection process where a single threshold is used to make a final decision. Regardless of the truncated sequential detection process, this final threshold can be implemented towards the optimization of the precision performance.

This means that RFID classifier420may value more a decision that is right about not assigning an item from outside the checkout lane to this lane. However, these decision criteria may perform poorly in recall performance. RFID classifier420may also drop some of the RFID readings410from the RFID tags240that are inside the checkout lane because RFID classifier420is not sure enough about those RFID tags240(for example, a confidence level is below a predetermined threshold).

Referring now toFIG. 5, a scenario500for reading RFID tags using multiple antennas is illustratively depicted in accordance with an embodiment of the present invention.

As shown inFIG. 5, multiple scanning areas120can be fabricated from panels, including from a flexible material, such as plywood, that will not break while bent to create the curved areas. Multi antenna systems (for example, including multiple antennas210) can be used to detect RFID tags240(for example, in retail systems), however, the appearance of very low energy points (for example, NULL points, as further described herein below with respect toFIG. 6) within the fields of antennas can stymie the effectiveness of multi antenna configurations. The scanning areas can include RF reflectors520and RF absorbers530.

According to example embodiments, the multiple antennas210can be used in designing RFID interrogators510or RFID systems that use such RFID interrogators510. Multiple antennas210can be used in an instance in which an RFID interrogator510has multiple ports that are working in time division duplex (TDD) mode or in frequency division duplex (FDD) mode and each port is potentially connected to a different antenna210. In some cases, a single port can be broken down into multiple sub-ports that operate in TDD mode as well. In other instances, the multiple antennas210can be used as phased array antennas to steer the physical beam in particular direction(s) or scan a 3D volume by using multiple beams consecutively in time.

According to example embodiments, RFID interrogators510can be implemented for simultaneous multi-tag reading in a single frequency or multiple frequencies. RFID interrogators510can thereby mitigate or account for the collision problem in the areas that there are many RFID tags240around. By reducing the effects of collisions, RFID interrogators510can increase the read rates of the RFID tags240and also addresses (or eliminates) the problem of “deserted tags” which happens when some RFID tags240can never be heard or have a chance to reply due to the interaction of multiple RFID tags240in their proximity.

The multiple antennas210can be deployed in order to generate different directions at different points since the orientation of the RFID tags240in many applications is an unknown variable (for example, dependent on the placement of the RFID tag240by the customer). The lower the projection of the incident vector from the RFID antennas210to the plane of the RFID tag240, the better the reading, which means that the orthogonal incident vector that has zero projection is the ideal case, and the parallel incident vector to the plane of the tag is the least desired.

Referring now toFIG. 6, a scenario600of null points due to using multiple antennas is illustratively depicted in accordance with an embodiment of the present invention.

As shown inFIG. 6, low energy points (named NULL points610) can be generated (by multiple antennas, for example, antennas210-1to210-n) within the field of an RFID interrogator510(for example, that includes multiple antennas210with a splitter630) especially in the near field and hence make the RFID tags240unreadable when placed in such NULL points610. The orientation of the RFID tags240, the use of multiple antennas210to increase the coverage, and the use of multiple antennas210combined with precoding for simultaneous readings of multiple tags are leading factors in generation of NULL points610.

According to example embodiments, RFID interrogator510can implement multi-tag reading for simultaneous reading of multiple RFID tags240. RFID interrogator510can only read one tag at a time. If multiple RFID tags240are interrogated the simultaneous or even partial overlapping response from multiple RFID tags240would result in a collision which makes the multiple RFID tags240unreadable. To this end, a random timer can be used by the excited RFID tags240to determine when to respond. This alleviates the problem to an extent if the number of simultaneously excited tags is low. However, for areas with large tag population the overlapping response from multiple RFID tags240would be unavoidable which can result in a severe loss in reading performance.

RFID interrogator510can exploit multiple antennas to separate responses from the tags that are reasonably far apart in a signal domain. The spatial separation of the tags usually makes then separable in the signal domains as well, however such spatial separation is not always necessary. The signal received from an RFID tag240by multiple antennas210(for example210-1to210-n) can be combined in a way that the response from the RFID tags240are separated in the signal domain. For example, a unitary processing of the received signals, such as described below, provides a framework that can be extrapolated to more complex cases.

Let U be a N×N unitary matrix used to process the vector of the received signals y by N antennas. The processed vector {tilde over (y)}=Uy consists of two orthogonal views of the received signal y. If the channel from the RFID tag240to the receive antennas210at the RFID interrogator510is modeled as H then the received vector y is given by y=Hx+z where x is the vector of the transmitted signal vector from the tags, and z is additive noise. If the channel H itself has two orthogonal columns then using U=HH, where HHis the conjugate transpose of H decomposes the received signal into two orthogonal components.

In some instances, the columns of the channel matrix H are not necessarily orthogonal but could be close to orthogonal. Nonetheless, even in the presence of much more pronounced correlation between the channels of different tags, RFID interrogator510can improve the interference of other tags from a given tag by using a preprocessing where the matrix U is not unitary matrix. The correlation between the channels of a given RFID tag240to that of different tags in a multiple antenna receiver is closely related to the projections of the corresponding columns of the channel matrix. RFID interrogator510can use the multiple antennas210for the purpose of simultaneous multi-tag reading.

The reading performance of the RFID tags240can be considerably affected by the orientations of the RFID tags240. RFID tags240can include a chip that processes the received signal and by changing the impedance of its port, which is connected to the antenna, modulates the backscattered signal. RFID tags240can also include an antenna (for example, a planar antenna that consists of one or multiple loops). The antenna of the RFID tag240can be designed to have particular impedance. The antenna of the RFID tag240absorbs power in order to power up the chip and also to reflect the backscatter signal.

In different orientations of RFID tags240, the power that is absorbed by the antenna as well as the power of the backscattered signal at the receiver considerably varies. This can cause some RFID tags240to be unreadable even when they are in close proximity to the antennas210(for example, about a meter away). The power absorbed by the tag antenna may be maximized when the incident wave is normal (or orthogonal) to the plane of the tag antenna. The direction of the electric field of the radiated wave with respect to the antenna at the incident point generates current in the conductive surface of the antenna.

The example systems described herein can vary the direction of the electric field of the transmitted wave with respect to time at the points in the space where RFID interrogator510reads the RFID tags240. In the far field, RFID interrogator510can use circular polarization or perturbation to read the RFID tags240. RFID interrogator510can implement perturbation to generate signals that fluctuates at the receiving antenna (of the RFID tags240) in such a way that the fluctuating signals induces energy in the antenna more homogenously and irrespective of its orientation.

RFID interrogator510can implement perturbation to provide higher energy in (almost) all directions to the RFID tags240. This can be interpreted as having higher induced energy at a time fraction that is enough to power up the tag and gather its replies for almost all orientation of the RFID tag240when it is placed at a given point.

Therefore, the difference between the time scale for perturbation and the time scale in which the particular symbol is transmitted to or received from the tag is an essential factor. RFID interrogator510can implement the perturbation time scale to be at least in the order of the (largest) transmission packet to an RFID tag240, which is equivalent to the transmission time required by an RFID interrogator510to make an RFID tag240wake up, process, and send a reply for an inquiry.

Perturbation can be viewed as a quasi-stationary process where its time scale cannot be too short or too long. If the time scale is too short the perturbation is not effective as it will not cause the receiving antenna (for example, at the RFID tag240) to gather enough energy to read, and if the time scale is too long the effective receive average power at the receiving antenna would not change since the received wave only varies with the modulated data. However, by controlling the time scale of the perturbation, RFID interrogator510can control the average power induced at the receiving antenna.

For example if the time required for exciting the RFID tag240, transmitting the data to the tag, and receiving the backscattered data from the RFID tag240is Ttagseconds, RFID interrogator510can implement the time scale of perturbation to be in the same order, e.g., kTtagfor some natural number k, say k=2. On the other hand, if the time that the RFID tag240is supposed to be read is Tinterrogationthis time scale is usually orders of magnitude larger, say Tinterrogation˜100Ttagwhich means that the perturbation process generates several different wave patterns in Tinterrogation. Since, one expect the reading process to be completed in Tinterrogationeffectively the max energy that can be induced to the RFID tag240in any of the perturbation cycles is considered as the reading power for the RFID tag240.

According to example embodiments, the systems can satisfy the conditions based on mechanical movements of the antennas or an RF reflecting body in proximity of the antennas or in any paths (not necessarily direct path or line-of-sight) between an RFID interrogator510and the RFID tag240. Even slight shaking of the antenna (at the interrogator or the tag) can cause the RFID tag240to be readable due to the fact that such mechanical movement even at high speed is usually considered a quasi-stationary process in comparison to the time scale of a symbol transmission in the form of an electrical signal. On the other hand, RFID interrogator510and the RFID tag240can make such movement=fast enough to span many changes during Tinterrogationthat could be in the order of a second or tenth of a second. The design of perturbation in electrical signal domain has to be such that it satisfies the aforementioned condition.

To enhance the coverage area of reading RFID tags240, RFID interrogator510uses multiple antennas210. The multiple antennas210can form an array such as a phased array antenna and connect to a single port or may be used to connect to different ports. A port for an RFID interrogator510can be defined as an antenna connection which is polled in a combination of time division duplex (TDD) and frequency division duplex (FDD) fashion. This combination could mean that the ports are polled in TDD mode and, within the time allocated to an individual port, multiple frequencies may be used in succession. RFID interrogator510can also use a combination of ports simultaneously active but in different frequencies.

When multiple antennas210are active in the same frequency band, regardless of having them connected to a single antenna port (e.g., as a phased array antennas) or to multiple antenna ports, their interference pattern would have some NULL point610. A NULL point610is a volume where the RFID tag240is not readable. This does not mean that the sum of the interfering signals from different antennas210or reflectors520are necessarily zero. This only means that the induced power into RFID tag240is below the threshold to activate it or its reply is weak such that it cannot be decoded as it may have received by the RFID interrogator's510antenna. Note that the definition of the NULL point610can be sensitive to the type of RFID tag240used or the orientation of the RFID tag240.

The NULL point610may in fact arise even if only a single port is active. The combination of the same band transmission from multiple antennas210as well as the reflections of the signal from the surrounding environment together constitute the formation of a NULL point610. Such NULL points610in the near field of the antennas210are more prominent specifically when the antennas210are placed far apart, (e.g., to cover a particular volume such as a cube, one may place multiple antennas210at multiple sides of the cube looking toward inside of the cube). Therefore, by adding antennas to the system to increase coverage, the system can cause unwanted NULL points in the same volume at the points that already had coverage.

RFID interrogator510can implement perturbation to combat low energy points in the field of an RFID interrogator510. Perturbation of the reading field of a reader (for example, RFID interrogator510) at a receiving antenna (e.g., the antenna of a tag) can be defined as a changing, in a quasi-stationary fashion, the directions, phase, or amplitude of the travelling waves that affect the receiving antennas to receive powers higher than a threshold in a stationary period of the perturbation process within a given multiples of such stationary periods. The effective reading power of the perturbation in a given location can be defined as the maximum of the received power at the receiving antennas over a stationary period among the overall time spent for reading the RFID tag240.

Note that the efficiency of a given perturbation method at an individual point can be defined as the ratio of the reading power of the perturbation over the induced power at the receiving antennas without perturbation. The overall efficiency may then be defined as the minimum of the efficiency over all points in the reading region. If the perturbation scheme consist of at least one stationary time period where no signal modification is performed, then the efficiency is always a positive number that is greater than one, which means that such perturbation can only make the reading easier by possibly generating larger average induced power over a stationary period.

RFID interrogator510can implement mechanical perturbation of antennas by shaking the antennas. By controlling the shaking of a physical antenna such as patch antenna that is usually used in RFID systems, RFID interrogator510can keep almost the same antenna patterns for the main antenna lobe for example, for a 60 degree antenna) while such small shaking of the antennas generates varying phases at each given point. This is because even though the shaking pattern (for example, for a shaking of about 1 to 3 degrees on a given axis that lies on the surface of a patch antenna) does not affect the antenna pattern by much, but the phases of the signal at different directions can be quite different. Moreover, such mechanical shaking may generate different reflections off the surrounding reflecting materials (for example reflector520).

RFID interrogator510can generate perturbation by changing the interference pattern of the signals based on the movement of a reflecting body such as a simple reflector plane (for example, reflector520) that rotates along an axis. The reflector520would not change the phase or gain of the signals that are transmitted from the antenna; however, the reflector520generates a varying (both gain and phase) reflecting signal or modifies the gain and phase of other indirect paths from antennas to the RFID tag240. Hence, this approach can also modify the average signals that is induced in the receiving antenna of the tag over a quasi-stationary period where a packet is transmitted to the tag from the interrogator.

This approach is especially effective where such reflecting body moves closer to the since a small movement of the antenna can generate bigger changes to the interfering signals at the receiving antenna of the RFID tag240. Moreover, the most effective form of movement is when the axis of the movement is normal (orthogonal) to the ground. In other words, the least effect of the perturbation along a particular axis is in the direction of the axis itself. Hence having a direction that is orthogonal or almost orthogonal to the ground makes such perturbation the most effective in the azimuth direction.

The system can combine (1) mechanical perturbation of the antenna, (2) mechanical perturbation of a reflecting body, and (3) electrical perturbation of antenna feeds, to generate better measurements from the RFID tags240. With respect to electrical perturbation of antenna feeds, for the sake of clarity of analysis and presentation, the following discussion assumes that the placement of the antennas and their orientation is fixed. The electromagnetic wave at each point may be represented by its electrical field and direction of transmission.

At a given point in the space, referred to herein as v, the electrical field as a result of transmission from antenna Ailocated at uiand pointing in direction ai may be represented as E(v−ui, ai). The electric field as a result of transmission from multiple antennas210can then be determined as the superposition of all such electric fields for all antennas210, say i∈S where S is the set of indices of all antennas. Note that there might be reflections from different points denoted by indices j∈T where T is the set of all such points also contribute to the field at location v. Hence, RFID interrogator510determines:
E(v)=Σi∈SE(v−ui, ai)+Σj∈TE(v−uj, bj)   Eqn. 1

where uj, j∈T is the location of reflection point j and bjis the direction of the reflecting wave.

Note that there might be several different reflection direction bjfor a given reflection point due to the fact that multiple different beam may arrive at different directions and reflect back in different direction from the same point. Note further that the number of point in the set T is not necessarily finite, and hence the summation could mean an integration for such cases. RFID interrogator510can use an approximation by considering only finite points (even though T might not be finite).

Consider an instance in which a single stream is transmitted through multiple antennas210and also consider the case that reflection has negligible effect that can be ignored. The electric field E(v−ui, ai) may be written as wixH(v−ui, ai) , where wiis the complex number that includes the gain and phase of transmitted signal from antenna i. This gain includes the antenna gain as well as the processing gain in the RF chain of this antenna.

The transmitted signal is represented by x and the channel from the antenna to the point v is represented by H(v−ui, ai) that explicitly depends on the direction of antenna and the relative position of the receiving point v with respect to the position of the antenna ui. For example if a patch antenna is used the gain of H(v−ui, ai) is higher when v−ui, is parallel to aiand as its angle increases the gain decreases. The phase of H(v−ui, ai) also depends on the angle between v−ui, and ai.

In instances in which the vector of H(v−ui, ai) for all i is represented as a column vector h and the vector of wifor all i as a column vector w the received signal at location v is given by y=hTwx where hTrepresents the transpose of a matrix (or vector) h.

The perturbation can be performed on w. One such perturbation could be represented by

(w~)=ww+δ⁢(w+δ)
where δ is a vector which has random or pseudo-random entries. To limit the effect of perturbation RFID interrogator510can force δ to have smaller norm than η∥w∥ for some threshold η. For example, RFID interrogator510can use a particular case that the vector δ is picked at random but always has a norm that is equal to η∥w∥. In this case,

(w~)=11+η⁢(w+η⁢wj⁢δ⁢δ).
In some cases, RFID interrogator510can pick (for example, select, receive, etc.) δ from a given codebook or set instead of being generated at random.

In instances where multiple streams are transmitted at the same times, the transmitted signal is represented as Σk=1Nwkxkwhere N represents the number of the streams, xkrepresent the transmitted signal for the kth stream and wkrepresents the vector of the gains for all antennas i, i∈S. Note that wkrepresents a column vector while wirepresents the entry of one of such vector which is the gain (and also including the phase through complex number notation) of antenna i.

where W is a |S|×N matrix composed of column vectors wkfor all streams k=1, . . . , N and x is the vector of the transmitted signals for all the streams. The perturbation works on each stream separately. These perturbations can be represented as

where v is a |S|×N matrix composed of column vectors δkfor all streams k=1, . . . , N, and δkis the perturbation vector for the stream k. Here diag(.) represents an operator that generates a square matrix with the diagonal elements noted in the argument of the function.

RFID interrogator510can change the gain of each stream xkwithin the limit of not violating the maximum transmitted power out of an antenna aperture set forth by the respected operating region. Such gain adjustment may be represented by multiplying a matrix G=diag(g1, . . . , gNfrom the right to {tilde over (W)}|, we have

Each antenna in the aforementioned formulation may be a directional antenna, or a multi element antenna such as phased array antenna. The dependency of H(v−ui, ai) on aiincorporates the effect of such individual directional antennas into the respective channel seen by this antenna.

The readings can also be improved based on perturbation of the receiving antenna. The same process of the mechanical or electrical perturbation in the transmitter side as discussed above may be applied in the receiving side as well. However, if the receiving side is not equipped with multiple antenna or is a passive device such as a passive RFID tag240, then the electrical perturbation of the antenna feeds may not be possible. However, the mechanical perturbation of the receiving antenna can still be performed. In instances where the NULL points610are sensitive to the orientation of the receiving antennas210, perturbation can be used to change the orientation of the RFID tag240in favor of a more energy absorbing orientation.

In some instances, increased movement of the receiving antennas can take the RFID tag240out of the NULL point610and effectively combat the low energy absorption in a NULL point position. The type of movement selected is dependent on the type of application that is being implemented. In many applications, the NULL point610is preferably addressed without moving the receiving antenna, e.g., the position and/or orientation of the RFID tags240. For example, it may not be desirable to ask customers or checkout attendant to move the items in order to make them readable.

RFID interrogator510can implement multi-tag reading in RFID systems. Consider multi element antennas. Assume N streams are transmitted by the RFID interrogator510using a precoding technique. Even though streams are precoded with different weights and hence they are spatially distinguishable, RFID interrogator510can use different frequencies in transmission of at least two different streams.

Such spatial separation makes the RFID tags240easier to respond in different frequencies and in the uplink, i.e., when the RF signal is received by the RFID interrogator510from multiple RFID tags240, the RF signal is easier to detect among the signals of multiple RFID tags240. The received signal model at a given point v is yj=hjTWx where hjis the channel of the tag j located at vj, W is a |S|×N matrix composed of column vectors wk for all streams k=1, . . . , N and x is the vector of the transmitted signals for all the streams.

The precoder W can be designed such that the received signal at different tags of interest has a one to one correspondence to different streams while the effect of interference from undesired streams on a tag is minimized. If RFID interrogator510selects a subset of users that have orthogonal or close to orthogonal channels hjthen the precoder W may consist of the conjugate transpose of the row vector hjT, that is a column vector (hjT)Hwhere (.)Hrepresent the conjugate transpose of (.).

In other cases where the column of W generated by gathering the conjugate transpose of the RFID tag's240channels does not have orthogonal column, RFID interrogator510can use a pseudo-inverse of W or regularized inverse of W (that is related to minimum mean squared error (MMSE) detection) instead of W itself. The use of zero-forcing precoding or regularized zero-forcing precoding can effectively generate different channels where multiple tags may be read at different channels simultaneously. Once different RFID tags240are excited and reply simultaneously, or with overlapping packets, RFID interrogator510can distinguish between the replies from different RFID tags240at the RFID interrogator510. RFID interrogator510can apply same process of multiuser detection to separate the replies from different RFID tags240.

RFID interrogator510can use the reciprocal of the same zero-forcing or regularized zero-forcing process that was done in the downlink to separate the replies from different tags from the superimposed signal in the uplink (since there is no channel estimation information available). RFID interrogator510thereby uses the reciprocity of the channels in the downlink and uplink in the time domain and hence does not need to perform any channel estimation.

According to example embodiments, RFID interrogator510can use a beamforming vector for transmission of a stream to an RFID tag240can perform the perturbation by modifying the beamforming vector by adding a random vector to the beamforming vector. RFID tag240can also perform perturbation by modifying the beamforming vector by adding a vector from a group of vectors to the beamforming vector.

FIG. 7is a flow diagram illustrating a system/method700for implementing a walk-through checkout station that includes an RFID interrogator with multiple antennas, in accordance with the present invention.

At block710, the RFID interrogator510transmits by multiple antennas, an RF signal. The RF signal is transmitted within a scanning area through which multiple RFID tags240can be carried.

At block720, the RFID interrogator510receives a superimposed received signal. The superimposed received signal includes replies from a first RFID tag and a second RFID tag that are overlapping in time. RFID interrogator510may receive the superimposed signal (or portions of the signal) from stationary reflectors that shapes a reading region (within a scanning area).

At block730, the RFID interrogator510separates the replies from the first RFID tag and second RFID tag though spatial processing of the superimposed received signal.

FIG. 8is a flow diagram illustrating a system/method800for implementing a walk-through checkout station that includes an RFID interrogator with multiple antennas, in accordance with the present invention.

At block810, perturbing, by a processing device, a transmitted wave. The wave is transmitted by an RF interrogator. In an example embodiment, the perturbation is generated by movement of a reflecting body. In another example embodiment, the perturbation is generated by movement of a transmit antenna. In a further example embodiment, the perturbation is generated by electrical manipulation of a transmit antenna feed.

At block820, a quasi-static process is generated for a stationary period. The stationary period is at least a time required to excite at least one RFID tag, receive a message from the interrogator and finish the reply by the at least one RFID tag.

At block830, different average energy is generated at the RFID tags in different stationary periods within a given reading cycle by the RF interrogator. The given reading cycle includes multiple stationary periods.