Abstract:
The present invention discloses a positioning system based on Received Signal Strength having an antenna device with a directional patch antenna with circular polarization, and an installation method thereof. By having a circular polarization, being directional and of a patch design, the directional patch antenna can mitigate signal strength discrepancies caused by devices located at different angles, and signal strength interference due to presence of multi-paths, and front-end antenna radiation pattern and input impedance caused by ambient environment of the antenna device and other material compositions. Beacons are preferably installed on the ceiling, which can improve the line-of-sight (LOS) opportunities between the positioning devices and tracked objects.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention generally relates to an antenna device and, more particularly, to an antenna device of a directional patch design with circular polarization configured for RF positioning system. 
       BACKGROUND OF THE INVENTION 
       [0002]    Conventional positioning devices are become more popular in usage for determining accurate position or location of objects or persons in indoor and outdoor environment. The positioning device is generally equipped with an antenna with RF signal receiving capability, that is typically an omni-directional linear polarization antenna. In typical conventional beacon positioning devices, planar inverted-F antenna (PIFA) antenna is used, which is typically of linear polarization, having an axial ratio (AR) greater than 10 dB, omni-directional (with antenna directivity less than 3 dBi), and not of patch design. Conventional Beacon/iBeacon positioning devices are typically placed adjacent or next to tracked object at relatively easy to reach installation locations, rather than being placed at any particular elevated height that is difficult to reach, and as a result, are thus vulnerable to object signal blocking and experiencing negative signal interference effects due to having people (acting as obstructions) walking around the tracked object. 
         [0003]    The antenna radiation pattern and the input impedances of the conventional antennas are negatively affected significantly by vicinity surroundings, especially from metal object. The backside of conventional RF positioning device is typically securely and affixed on to an object, which can be a displayed object or an exhibited object, a shelf corner end, walls, ceilings, etc. . . . . The radiation pattern and input impedance of typical conventional antennas are affected by the interference from the aforementioned object located at backside of the positioning device. The former will negatively affect the accuracy and stability of the usage of signal strength for distance determination, and the later will affect the radiation efficiency of the antenna. 
         [0004]    As a result, due to the fact that conventional positioning system typically rely upon RSSI to perform triangulation positioning or fingerprinting positioning, which are subjected to be influenced by position or location of transceiver, and by taking into consideration of the significant degree of environmental impact of RF signal blocking and interferences, often resulting in poor stability of the RF signal, which led to inaccurate and unstable positioning data. Thus there is a need for improvement on the antenna design for RF indoor positioning system. 
       SUMMARY OF THE INVENTION 
       [0005]    The present invention provides a RF positioning system based on signal strength (Received Signal Strength, RSS-based) having one or more RF positioning devices, each equipped with an antenna device that is of a directional patch design with circular polarization. 
         [0006]    The present invention provides the antenna device of the directional patch design with circular polarization with improved performance capability with respect to overall received signal strength, return loss or power loss value, input matching, shielding and positioning signal interference prevention from adjacent metal object located at close proximity to the antenna device. 
         [0007]    The present invention also provides an installation method for the RF positioning devices for the RF positioning system to enhance the positioning accuracy and stability of tracked objects. 
         [0008]    The antenna device for RF positioning system of embodiments of present invention can mitigate negative signal interferences such as: (a) signal strength discrepancies caused by devices located at different angles, (b) signal strength interference due to presence of multi-pathes, and front-end antenna radiation pattern and input impedance caused by ambient environment of the antenna device and other material compositions. 
         [0009]    According to an embodiment of present invention, a positioning device such as a Beacon is preferably installed on an elevated height, such as a ceiling or a wall at a height of 3 m to 4 m (as measured from the floor to the height of the beacon) which can greatly improve the line-of-sight (LOS) opportunities between the RF positioning device and any tracked object. Thus improved line-of-sight (LOS) is one significant contributing factor to the improvement of the antenna of present invention. Through the use of affixing or mounting of the positioning device on a higher elevation/height level in contrast with conventional positioning device which are typically being placed at lower elevation/height level or near tracked object, the RF positioning device can be effectively prevented from being (inadvertently) blocked by object and people, thereby improving upon reliability of the signal strength (RSSI) used as basis for positioning determining scheme. 
         [0010]    As a result, the antenna device and installation method thereof of the RF positioning system of present invention based upon signal strength positioning scheme can effectively improve the positioning accuracy and stability of the overall RF positioning system. 
         [0011]    According to embodiments of present invention, the antenna device for RF positioning system has circular polarization instead of linear polarization. 
         [0012]    According to embodiments of present invention, the antenna device for RF positioning system is a directional antenna, instead of an omni-directional antenna. 
         [0013]    According to embodiments of present invention, the antenna device for RF positioning system is a patch antenna. 
         [0014]    According to embodiments of present invention, the antenna device for RF positioning system has reduced sensitivity of the direction and angle of the tracked object or device. 
         [0015]    According to embodiments of present invention, the antenna device for RF positioning system has higher antenna directivity, and thus more resistance to multipath by reflection and diffraction, and also more resistance to interference, and having more significant signal strength drop off between the RF positioning device coverage area and non-coverage area, a positioning engine using RF signal strength (RSSI, received signal strength indicator) as the measuring principle or basis can effectively enhance the positioning accuracy. 
         [0016]    According to embodiments of present invention, the antenna device for RF positioning system being of a patch antenna pattern can be radiating toward the broadside direction only, so that a field strength difference between backlobe and broadside of the antenna may be higher than 10 dB or more. Thus the (patch) antenna device according to embodiments of present invention is capable of reducing rear/backlobe signal radiation so that the front/broadside radiation of the antenna pattern and impedance matching are not affected by the interference from the ambient environment at the rear side of the antenna device. 
         [0017]    According to embodiments of present invention, the antenna device for RF positioning system has a substrate (by forming a grounded substrate) grounded so as to effectively shield an object located at the back of the antenna device and thus preventing interference by the object located at the back of the antenna device. The object can be a metal object. 
         [0018]    The present invention provides improved positioning accuracy of tracked object by using more than one RF positioning devices, each equipped with the same antenna device that is of the directional patch design with circular polarization. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]    The present invention will become more readily apparent to those ordinarily skilled in the art after reviewing the following detailed description and accompanying drawings, in which: 
           [0020]      FIG. 1  shows a block diagram of a RF positioning system according to an embodiment of present invention. 
           [0021]      FIG. 2  shows a perspective view of an antenna device of the embodiment of present invention. 
           [0022]      FIGS. 3A and 3B  show side-by-side performance comparison between a conventional PIFA antenna device (of  FIG. 3A ) versus the antenna device of the embodiment of present invention (of  FIG. 3B ). 
           [0023]      FIG. 4  shows antenna radiation patterns of the conventional pifa antenna device and the antenna device of the embodiment plotted with respect to x-z plane. 
           [0024]      FIG. 5  shows experimental results obtained on signal strength data (RSSI) with respect to rotation angles of a position-tracked device for a first beacon and a second beacon (of the embodiment of present invention). 
           [0025]      FIG. 6  shows installation locations of a group of antenna devices in a space. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0026]    The present invention will now be described more specifically with reference to the following embodiments. It is to be noted that the following descriptions of the embodiments of this invention are presented herein for purpose of illustration and description only. It is not intended to be exhaustive or to be limited to the precise form disclosed. 
         [0027]    As shown in  FIG. 1 , a RF positioning system according to an embodiment of present invention is provided, in which the RF positioning system  10  can be an indoor or outdoor RF location estimation system with operating principle based on received signal strength (RSS) of various types of RF signals of a set frequency band. The RF positioning system  10  includes one or more wireless electronic devices  15  (whose locations thereof are being measured and tracked), one or more RF positioning devices  20 , such as beacons or ibeacons, and a management software system  25  for the RF positioning system  10 . The description details and/or commercial availability of the wireless electronic devices  15  and the management software system  25  for the RF positioning system  10  are available and found in various public literature or commercial positioning system products, and are thus omitted herein for the sake of brevity. Each RF positioning device  20  includes an antenna device  30  configured for receiving RF signals from the one or more wireless electronic devices  15  that are being tracked for location data. The wireless electronic device  15  is also referred to a tracked object herein, and can be a mobile phone, a smartphone, a wireless IP camera, a wireless tablet device, a laptop, and a wireless autonomous robot, but is not limited to these. 
         [0028]    The antenna device  30  of the embodiment of present invention is shown in  FIG. 2 . In the illustrated embodiment, the antenna device  30  for the RF positioning system  10  has a directional patch design with circular polarization. The antenna device  30  includes a patch antenna  35 , a substrate layer  40 , a ground plane  45 , and a probe feed  50 . The ground plane  45  is a grounded dielectric layer providing shielding so that the antenna device  30  would only be radiating in an upward direction. In the illustrated embodiment, the substrate layer  40  of the antenna device  30  has a height H of 5 mm, a length L of 25 mm and a width W of 25 mm; the patch antenna  35  can be substantially square with a length of 15 mm and a width of 15 mm, respectively. The substrate layer  40  can be made of a material of dielectric constant of 5-8. The resonant frequency selected for the antenna device can be, for example, 2.4 GHz, but is not limited thereof, and can be configured also to operate between frequency range from 2.1 GHz to 5.6 GHz. The feed point  52  of the probe feed  50  is coupled to the patch antenna  35  at substantially in a center area of the patch antenna  35  at one end of the probe feed  50 , while the other end of the probe feed  50  is coupled to a coaxial connector (not shown) to form a coaxial probe feed. 
         [0029]      FIGS. 3A and 3B  show (side-by-side) comparison between performances of a conventional PIFA antenna device (see  FIG. 3A ) and that of the antenna device  30  of the embodiment of present invention (see  FIG. 3B ), both different antennas are respectively configured at two different operating configurations or settings, in which one operating configuration/setting has a metal object disposed 2 cm at the backside of the antenna devices thereof, respectively, and the other operating configuration/setting has no metal object disposed on the backside of the antennas, based upon experiments conducted. The device structures and return loss data of a (conventional/traditional) pifa antenna in comparison with the antenna device  30  of the embodiment of present invention having the patch antenna  35  being a directional patch antenna with circular polarization of present invention are shown to be different in  FIGS. 3A and 3B . The antenna device  30  effectively radiates a return loss required to be greater than −6 dB (&gt;−6 dB). From further comparison of  FIGS. 3A and 3B , it can be seen that the conventional pifa antenna device has a return loss of just −2 dB to −3 dB due to the presence of the metal object located at backside thereof at a 2 cm gap distance between the pifa antenna device and the metal object. On the other hand, the antenna device  30  (directional patch antenna with circular polarization) of the embodiment of present invention has a return loss performance of above −10 dB, with or without the metal object located at the same gap of 2 cm at the backside thereof. Thus, it is seen that the antenna device  30  (directional patch antenna with circular polarization) of present invention has significant improvement in performances including return loss, input matching and prevention of positioning signal interference by external metal object at close proximity to the antenna device. 
         [0030]    As further illustration on the behavior differences of the conventional pifa antenna device with respect to the antenna device  30  of the embodiment of present invention, a plurality of antenna radiation pattern data of the conventional pifa antenna device and the antenna device  30  of the embodiment are respectively plotted and shown in  FIG. 4 , in which the antenna radiation pattern data are collected and plotted with respect to x-z plane. The 3 dB beamwidth for the antenna device  30  (directional patch antenna with circular polarization) is about 60 degrees, which is less than the 3 dB beamwidth for the conventional pifa antenna device of about 80 degrees. It is appreciated by one skilled in the art that the lower angular value at 3 dB beamwidth for the antenna device  30  represents the fact that directivity is more pronounced or more significant in comparison with the conventional pifa antenna device in  FIG. 4 . In addition, the field effect on the back of the antenna device  30  is seen to be effectively suppressed. 
         [0031]      FIG. 5  shows experimental results obtained on signal strength data (RSSI) with respect to rotation angles of position-tracked device for two beacons, which include a first beacon  20   a  configured with a conventional pifa antenna device, and a second beacon  20   b  which is the RF positioning device  20  configured with the antenna device  30 . The first and second beacons  20   a ,  20   b  are only different in the antenna devices used/adopted therein, while maintaining the same for the other components thereof. A smartphone  37  is rotated at various different rotation angles held underneath the respective beacons  20   a ,  20   b . In other words, the smarthphone  37  is rotated from 0 degree to 360 degrees (full circle, θ). The beacon  20   b  (configured with the antenna device  30 ) of the embodiment of present invention (equipped with the directional patch antenna with circular polarization, herein also referred to as a beacon embodiment) is compared to the beacon  20   a  (equipped with the conventional pifa antenna device, herein also referred to as a conventional beacon) are (both) placed on a ceiling surface located side by side with respect to one another at close proximity (i.e. 3 cm apart). During an experiment, antenna energy from both beacons  20   a  and  20   b  are configured to be radiating downward toward the smartphone  37 . The smartphone  37  is rotated around its center axis from 0 degree to 360 degrees at a distance of 1.5 meters (m) away from both beacons  20   a ,  20   b  (the embodiment beacon and the conventional beacon), respectively. At the same 1.5 meter distance to the beacons  20   a ,  20   b , the embodiment beacon  20   b  achieved a power loss at about 5 dB, but the conventional beacon  20   a  has a more significant power loss at above 15 dB. From the experimental data shown in  FIG. 5 , the conventional beacon  20   a  seems to be inadequate in signal strength at rotation angle from about 120 degrees to about 240 degrees. Meanwhile, the embodiment beacon  20   b  demonstrates sufficient signal strength at all rotations angles of 0 degrees to 360 degrees for the smartphone  37 . 
         [0032]    As shown in  FIG. 6 , further experiments are conducted using a group of 5 beacons, which have been installed in a 8 meter by 7 meter space. Each of the 5 beacons used in the conducted experiments are either all conventional beacons  20   a  or all embodiment beacons  20   b , disposed at same respective mounting locations. The first and second beacons  20   a ,  20   b  are only different with respect to the antenna devices configured therein (i.e. conventional beacon versus embodiment beacon), while maintaining the same specifications for the other components thereof. An installation method for the RF positioning devices  20  for the RF positioning system  10  to enhance the positioning accuracy and stability of tracked objects (or wireless electronic device  15 ), such as the smartphone  37  includes secure mounting of the 5 embodiment beacons  20   b  (which is same as the RF positioning devices  20 ) onto the 1.5 meter tall ceiling of the 8 meter by 7 meter space in the following manner: (a) mount one embodiment beacon  20   b  on the ceiling in the center of the 8 meter by 7 meter space, in which the center is determined by drawing two erasable diagonal lines at the intersecting point thereof; (b) mounting each of the remaining four embodiment beacons  20  on the ceiling along a diagonal line at about 1 meter away from each corner point P of the space. The patch antenna  35  of the antenna device  30  is facing downwardly toward the floor of the space. 
         [0033]    The same smartphone (not shown) is moved around in the 8 meter by 7 meter space underneath the respective beacons  20   a ,  20   b . During experiment, antenna energy from all 5 beacons, namely the conventional beacons  20   a  or the embodiment beacons  20   b  are configured to be radiating downward toward the smartphone  37 . The results from the experiments showed that the conventional pifa antenna beacons  20   a  (conventional beacons) together achieved average positioning accuracy of about 3 meters for the smartphone; whereas, the embodiment beacons  20   b  (equipped with the directional patch antenna with circular polarization) together achieved average positioning accuracy of within 1 meter for the smartphone. Based on the experiments conducted, the embodiment beacons  20   b  equipped with the antenna device  30  when operating in a group configuration, i.e. 5 embodiment beacons  20   b , are superior in performance in positioning accuracy of tracked object such as the smartphone than the conventional beacons  20   a  that are equipped with conventional pifa antenna device when also operating in the group configuration, i.e. 5 conventional beacons  20   a.    
         [0034]    Most conventional RF positioning systems based on received signal strength, operate under existing positioning infrastructure, such as for example, using a WiFi access point (AP). Under the premise of not having to modify or change existing hardware, typical conventional methods for positioning improvements are accomplished via performing of software upgrades to the positioning algorithm, which is usually performed by a professional software engineer. Meanwhile, because normally the software engineer is not an antenna expert in positioning systems, thus the software engineer would not be motivated or reasoned to attempt antenna redesign for seeking positioning accuracy improvement. Indeed, many software engineers may not realize the potential advantages of a directional patch antenna with circular polarization for use in RF positioning systems when used correctly in a properly-configured spatial arrangement of the positioning devices. As a result, embodiments of present invention provide a fully-tested antenna device that is effectively configured for use for many RF positioning devices, i.e. Beacons and iBeacons, with actual data obtained which show positioning accuracy improvements for tracking wireless electronic devices, such as a smartphone, while operating in various settings, thereby giving the knowledge and confidence to software engineers for usage adoption in their own proprietary RF positioning systems in the future. In addition, because designing, fine tuning and testing of the antenna device  30  required to be performed while operating under positioning trials (i.e., for example, while tracking smartphones inside a beacon laid-out room) in a coordinated side-by-side manner, so that each positioning improvement realized can be properly as assessed as whether belonging to antenna-device related (causes or factors) or positioning-algorithm (software) related. As a result, the degree of difficulty for accomplishing antenna redesign while conducting positioning trials is relatively high. In addition, the antenna design along with the necessary impedance matching for the circular polarization antenna device thereof adds to the overall complexity of the endeavor, while any added cost of the antenna device would also be another consideration to those skilled in the art. 
         [0035]    While the invention has been described in terms of what is presently considered to be the most practical and preferred embodiments, it is to be understood that the invention needs not be limited to the disclosed embodiment. On the contrary, it is intended to cover various modifications and similar arrangements included within the spirit and scope of the appended claims which are to be accorded with the broadest interpretation so as to encompass all such modifications and similar structures.