System and method for controlling range of successful interrogation by RFID interrogation device

CROSS REFERENCE TO RELATED APPLICATIONS

0001The present application claims the benefit of and priority to U.S. Provisional Patent Application No. 60/547,495 filed on Feb. 24, 2004, the entire disclosure of which is hereby incorporated by reference in its entirety.

FIELD OF THE INVENTION

0002The present invention relates in general to interrogation of radio-frequency identification (RFID) transponders, and particularly to a method and system for interrogating ‘passive’ RFID transponders while controlling a range of successful interrogation.

BACKGROUND OF THE INVENTION

0003RFID technologies are widely used for automatic identification. A basic RFID system includes an RFID tag or transponder carrying identification data and an RFID interrogator or reader that reads and/or writes the identification data. An RFID tag typically includes a microchip for data storage and processing, and a coupling element, such as an antenna coil, for communication. Tags may be classified as active or passive. Active tags have built-in power sources while passive tags are powered by radio waves received from the reader and thus cannot initiate any communications.

0004An RFID reader operates by writing data into the tags or interrogating tags for their data through a radio-frequency (RF) interface. During interrogation, the reader forms and transmits RF waves, which are used by tags to generate response data according to information stored therein. The reader also detects reflected or backscattered signals from the tags at the same frequency, or, in the case of a chirped interrogation waveform, at a slightly different frequency. The reader typically detects the reflected or backscattered signal by mixing this signal with a local oscillator signal. This detection mechanism is known as homodyne architecture.

0005In many applications of RFID techniques, such as automated vehicle identification and/or fare collection, or automated inventory of trucks entering loading docks, it is desirable that a particular interrogating device identifies only RFID tags located in a specific physical region. For example, in a warehouse or facility with multiple adjacent loading docks each accepting one vehicle at a time, it is desirable that the interrogating device or devices associated with a dock detect only RFID tags within the vehicle parked at or passing through that dock and not those of its neighbors. Similarly, in the case of automated vehicle identification which controls a tollgate or other passage restriction, it is desirable that a given tag reader sense only tags on vehicles in its assigned lane and not those of its neighbors.

0006Prior art solutions to this problem include the construction of physical barriers between separate regions in an attempt to prevent propagation of RF signals between the regions. Such barriers are expensive and inconvenient, as they must be either strongly absorbing or reflecting, and sufficiently large relative to the wavelength of the RF signals in question to minimize diffractive bypass of the obstacle.

0007Another approach, disclosed in U.S. Pat. No. 6,107,910, is to use a high-rate pseudorandom sequence to phase-modulate the transmitted signal, and convolve received signals with the sequence. By appropriate choice of the autocorrelation properties of the sequence employed, a null in the correlation can be created at a particular propagation delay, and used to reject signals at a certain distance, such as an adjacent lane or dock. However, this scheme suffers from the added complexity of a high-rate modulation imposed on the transmitter, and inflexibility in the placement of the rejected region relative to the accepted region.

0008A further approach, discussed in U.S. Pat. No. 6,097,301, employs control of the transmitted power to interrogate only the nearest RFID tag. This technique, however, is only applicable in situations where one tag will normally be close to the interrogation device with all other tags being far away. In many other applications, more than one tag may be equally or near equally close to the interrogation device, so that it is not possible to interrogate a just a single tag by ramping the transmitter power until a single tag is detected.

0009What is needed, therefore, is a flexible means of controlling the physical area interrogated by a given interrogation unit, such that all tags within the wanted area can be successfully read, while few or no tags in other areas are inadvertently interrogated.

SUMMARY OF THE INVENTION

0010The present invention is directed to controlling a range of successful interrogation by an RFID reader so that tags located in a specific physical area are likely to be successfully interrogated by the reader while the chance of the reader reading tags in other locations are minimized. In one embodiment of the present invention, a plurality of delineation RFID tags with known unique identifying numbers are placed in an areas of interest or wanted region, and the reader is characterized to determine an optimal setting for at least one transmission parameter based on responses from the delineation RFID tags and a predetermined figure of merit.

0011More specifically, in some embodiments a system is provided for reading an RFID tag in a defined region while reducing the possibility of reading other RFID tags outside the region, comprising an RFID reader at a fixed location in or near the region, the RFID reader capable of adjusting at least one associated transmission parameter. A plurality of delineation RFID tags each having a known and unique identification are placed at specified locations in the region, and the at least one transmission parameter associated with the reader is set according to responses from the delineation RFID tags to interrogation signals from the RFID reader and a predetermined figure of merit.

0012In another aspect, a method for controlling a range of successful interrogation by an RFID reader associated with at least one adjustable transmission parameter, is provided comprising placing a plurality of delineation RFID tags at specified locations in a wanted region. For each of a plurality of trial parameter settings for the at least one transmission parameter, an attempt to interrogate the plurality of delineation RFID tags using the RFID reader is made and responses from the delineation RFID tags are recorded. An optimal transmission parameter setting is determined for the reader based on the a predetermined figure of merit calculated using the responses from the delineation RFID tags at each of the plurality of trial parameter settings and a pre-determined criteria, and at least one transmission parameter is set associated with the RFID reader according to the optimal transmission parameter setting.

BRIEF DESCRIPTION OF THE DRAWINGS

0013Other aspects and advantages of the present invention will become apparent upon reading the detailed description of the invention and the appended claims provided, below, and upon reference to the drawings, in which:

0014<figref idref="DRAWINGS">FIG. 1</figref> is a block diagram of a system for controlling a range of successful interrogation by an RFID reader according to one embodiment of the present invention;

0015<figref idref="DRAWINGS">FIG. 2</figref> is a block diagram of an RFID reader according to one embodiment of the present invention;

0016<figref idref="DRAWINGS">FIG. 3A-3F</figref> are diagrams illustrating variations of a range of successful interrogation by an RFID reader when transmitted power from the RFID reader is varied;

0017<figref idref="DRAWINGS">FIG. 4</figref> is a block diagram of a system for controlling a range of successful interrogation by an RFID reader according to an alternative embodiment of the present invention;

0018<figref idref="DRAWINGS">FIG. 5</figref> is a block diagram of a system for controlling a range of successful interrogation by an RFID reader according to yet another alternative embodiment of the present invention;

0019<figref idref="DRAWINGS">FIGS. 6A and 6B</figref> are flowcharts illustrating a method for controlling a range of successful interrogation by an RFID reader according to one embodiment of the present invention;

0020<figref idref="DRAWINGS">FIGS. 7-9</figref> are diagrams each illustrating an overlaps between a range of successful interrogation and a wanted range according to an embodiment of the present invention.

0021<figref idref="DRAWINGS">FIG. 10</figref> is a block diagram of a system for controlling a range of successful interrogation by an RFID reader wherein additional receiving units are used in addition to, or in place of, delineation tags to provide better overlap between the range of successful interrogation and the wanted region, according to yet another embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

0022<figref idref="DRAWINGS">FIG. 1</figref> illustrates a system <b>100</b> for controlling a range of successful interrogation by an RFID reader according to one embodiment of the present invention. As shown in <figref idref="DRAWINGS">FIG. 1</figref>, system <b>100</b> generally comprises an RFID reader <b>110</b> and a plurality of delineation RFID tags <b>120</b> placed at specified locations in a region of interest or wanted region <b>102</b>. The wanted region <b>102</b> represents a region for intentional RFID interrogation. For example, in the situation of a tollgate, the wanted region <b>102</b> can be an area in front of the tollgate for cars to pass through while their toll meter is being read by an RFID reader installed at the tollgate. The plurality of delineation RFID tags <b>120</b> includes RFID tags <b>120</b>-<b>1</b>, <b>120</b>-<b>2</b>, . . . , and <b>120</b>-<i>n</i>, where n is a positive integer greater than 1. Each delineation RFID tag <b>120</b> can be a conventional RFID tag having a known and unique identification number. The delineation RFID tags <b>120</b> can be placed along the boundary of wanted region <b>102</b>, as shown in <figref idref="DRAWINGS">FIG. 1</figref>. They may also be staged through out wanted region <b>102</b>. The delineation RFID tags <b>120</b> are used to characterize reader <b>110</b> so that reader <b>110</b> is likely to successfully read only those RFID tags that are located in or near wanted region <b>102</b>. In other words, the delineation RFID tags <b>120</b> are used to characterize reader <b>110</b> so that wanted region <b>102</b> overlaps substantially with the range of successful interrogation by reader <b>110</b>.

0023In addition to the delineation RFID tags, system <b>100</b> may also comprise a plurality of anti-delineation RFID tags <b>130</b> placed at specified locations in or around an excluded region <b>103</b>. Excluded region <b>103</b> represents a region an RFID tag located wherein should not be inadvertently read by reader <b>110</b>. In the example of reader <b>110</b> being installed at a tollgate, the excluded region <b>103</b> can be an area or areas in front of neighboring tollgate(s). As shown in <figref idref="DRAWINGS">FIG. 1</figref>, the plurality of anti-delineation RFID tags <b>130</b> include RFID tags <b>130</b>-<b>1</b>, <b>130</b>-<b>2</b>, . . . , and <b>130</b>-<i>m</i>, where m is a positive integer greater than 1. Each anti-delineation RFID tag <b>130</b> can be a conventional RFID tag having a known and unique identification number that are different from the identification number in any of the delineation RFID tags <b>120</b>. The anti-delineation RFID tags <b>130</b> can be placed along a side of the boundary of excluded region <b>103</b> that faces the wanted region <b>102</b>, as shown in <figref idref="DRAWINGS">FIG. 1</figref>. They may also be staged throughout excluded region <b>103</b>. The anti-delineation RFID tags <b>130</b> can be used to further characterize reader <b>110</b> so that reader <b>110</b> is unlikely to read RFID tags that are located in or near excluded region <b>103</b>.

0024Although, for ease of illustration, <figref idref="DRAWINGS">FIG. 1</figref> shows that regions <b>102</b> and <b>103</b> are constrained in two dimensions, the present invention also includes situations where regions <b>102</b> and/or <b>103</b> are bounded in three-dimensions.

0025Reader <b>110</b> can be a conventional RFID reader having at least one transmission parameter that can be adjusted to limit its range of successful interrogation. In one embodiment of the present invention, the at least one transmission parameter can be adjusted to control the transmitted power from the reader, or the angular distribution of the transmitted power, or both. The at least one transmission parameter may also include a selection of one of a plurality of antennas for transmitting the interrogation signal from the reader when the reader is associated with more than one antennas. As an example, <figref idref="DRAWINGS">FIG. 2</figref> illustrates an RFID reader <b>200</b> that can be used as reader <b>110</b> according to an embodiment of the present invention. As shown in <figref idref="DRAWINGS">FIG. 2</figref>, reader <b>200</b> includes a crystal oscillator <b>202</b> configured to generate a clock signal, and a frequency synthesizer <b>204</b> configured to generate a continuous wave (CW) signal referencing the clock signal. Reader <b>200</b> further includes a local oscillator (LO) buffer amplifier <b>206</b> coupled to synthesizer <b>204</b> and configured to amplify the CW signal. LO buffer amplifier <b>206</b> also protects the synthesizer from disturbances created from other parts of reader <b>200</b>.

0026Reader <b>200</b> further includes a transmit (TX) chain <b>210</b> configured to form and transmit a transmit signal for interrogating a tag, and a receive (RX) chain <b>230</b> configured to receive the reflected or backscattered RF signal from the tag, and to generate a plurality of output signals from the RF signal. Transmit chain <b>210</b> includes an output power control module <b>212</b>, a modulator <b>214</b>, a power detector <b>216</b> and an attenuation driver <b>218</b>. Receive chain <b>230</b> includes a splitter <b>232</b>, a 90° hybrid <b>234</b>, an I-branch <b>240</b>, a Q-branch <b>250</b>, an IRM path <b>236</b>, an FSK receiver <b>238</b>, a filter <b>272</b>, analog to digital (A/D) converters <b>274</b> and <b>276</b>, and an optional phase shifter <b>270</b>.

0027Reader <b>200</b> further includes a splitter <b>208</b> coupled between LO buffer amplifier <b>206</b> and transmit/receive chains <b>210</b> and <b>230</b> and configured to split the CW signal from LO buffer amplifier <b>206</b> into a TX CW signal for the Transmit chain and a RX LO signal for the Receive chain. When more than one antenna can be used by reader <b>200</b>, reader <b>200</b> may also include an antenna select module <b>222</b> configured to select one of a plurality of antenna <b>224</b> for broadcasting the transmit signal or receiving the RF signal from the tag. Reader <b>200</b> further includes a directional coupler <b>220</b> coupled between antenna select module <b>222</b> and transmit/receive chains <b>210</b> and <b>230</b>. Directional coupler <b>220</b> is configured to pass the transmit signal from the transmit chain <b>210</b> to at least one antenna through antenna select module <b>222</b> and to couple the RF signals received by the antenna to the receive chain <b>230</b>.

0028Reader <b>200</b> further includes a controller <b>264</b> configured to control the operation of various components of reader <b>200</b> by processing a plurality of input signals from the various components and producing a plurality of output signals that are used by respective ones of the components. A conventional commercially available controller, after being programmed according to an RFID standard, can be used as controller <b>264</b>.

0029In one embodiment of the present invention, a host computer system can be used to operate reader <b>200</b> and characterize reader <b>200</b> according to characterization methods discussed below. Communication between reader <b>200</b> and the host computer is facilitated by a PC interface <b>262</b> in reader <b>200</b>. <figref idref="DRAWINGS">FIG. 2B</figref> is a block diagram of a computer system <b>280</b> that can be used to operate and characterize reader <b>200</b>. As shown in FIG. <b>2</b>B, computer system <b>280</b> is a conventional computer system including a central processing unit (CPU) <b>282</b>, a memory unit <b>284</b>, a plurality of data import/output (I/O) ports <b>286</b>, a user interface <b>288</b>, and a display device <b>290</b>. CPU <b>282</b>, memory unit <b>284</b>, I/O ports <b>286</b>, user interface <b>288</b>, and display device <b>290</b> are interconnected via a bus <b>292</b>. Reader <b>200</b> can be coupled to host computer <b>200</b> through one of the <b>1</b>/O ports <b>286</b>. Memory <b>284</b> stores therein program instructions that when executed by CPU <b>282</b> causes host computer <b>280</b> to perform the characterization methods for characterizing reader <b>200</b>, as discussed below. Memory <b>284</b> may also include a database storing therein data associated with the characterization methods, as discussed below.

0030For the sake of clarity, parts of reader <b>200</b> that are either conventional or otherwise unrelated to the present invention are not discussed in detail. More detailed description of one embodiment of reader <b>200</b> can be found in co-pending U.S. patent application Attorney Docket Number <b>463438-372 </b>(33889/US/3) entitled MULTIPROTOCOL RFID READER, which disclosure is incorporated herein by reference in its entirety. Although reader <b>200</b> is used herein for illustration purposes, the invention is not limited to using reader <b>200</b> as the RFID reader <b>110</b> in system <b>100</b>. Any interrogation device capable of adjusting its range of successful interrogation can be used as reader <b>110</b>.

0031As discussed above, in reader <b>200</b>, a signal generated by synthesizer <b>204</b> is employed to simultaneously provide the TX CW signal for the transmit chain and the RX LO signal for the receive chain. The TX CW signal is used to form the interrogation signal to be transmitted to one or more passive RFID tags, while the RX LO signal is used as a local oscillator signal to achieve homodyne detection of the backscattered signals from the tags. Thus, the transmitted power of the reader, i.e., the power in the interrogation signal, can be independently adjusted without affecting the operation of the receive chain by the employment of an output power control device placed in the transmit chain. In one embodiment of the present invention, the output power control device comprises a conventional variable attenuator in output control module <b>212</b>.

0032<figref idref="DRAWINGS">FIGS. 3A-3F</figref> illustrate responses of a plurality of RFID tags staged in a rectangular array in front of an antenna for transmitting interrogation signals from an RFID reader while the output power of the RFID reader is ramped from 17 dBm to 27 dBm. The shaded area in each rectangle represents a range of successful interrogation for a particular transmitted power setting. In other words, the RFID tags located in the shaded area have successfully responded to the interrogation signal transmitted from the antenna. When the transmitted power from the RFID reader is ramped, the range of successful interrogation also increases to cover a larger area in the rectangle <b>300</b>. Since the RFID tags in the shaded areas are successfully interrogated by the RFID reader while the RFID tags outside the shaded areas are not, for each transmitted power setting, the RFID tags located at the edge of the shaded area are considered to be at or slightly above a threshold for successful interrogation by the RFID reader. Whether an RFID tag is at or slightly above the threshold for successful interrogation can be determined by either increasing the transmitted power by a small increment (e.g., 2 dBm) and check if the reader is able to read the RFID tag which the read could not read before the change or by decreasing the transmitted power by a small decrement (e.g., 2 dBm) and check if the reader is no longer able to read the RFID tag which the read could successfully read; before the change.

0033In addition to the transmitted power, the spatial distribution of the transmitted radiation from reader <b>110</b> can also be adjusted. In one embodiment of the present invention, as shown in <figref idref="DRAWINGS">FIG. 4</figref>, a plurality of differently oriented and/or separately located antennas, such as antenna <b>1</b> and antenna <b>2</b> in <figref idref="DRAWINGS">FIG. 3</figref>, are provided and reader <b>200</b> can be operated to adjust its coverage by switching among these antennas using the antenna select module or an external switch <b>112</b>.

0034In another embodiment, as shown in <figref idref="DRAWINGS">FIG. 5</figref>, reader <b>200</b> can be coupled to an adaptive phased array <b>500</b> comprising two or more antennas, such as antenna <b>1</b>, antenna <b>2</b>, and antenna <b>3</b>, that are held in adjustable phase and amplitude relationships with each other such that the transmitted signal is the sum of the radiated field from each antenna. The antennas in the phased array <b>500</b> may also be configured to transmit differently polarized signals to reduce interferences among them. In <figref idref="DRAWINGS">FIG. 5</figref>, each antenna in the phased array <b>500</b> is coupled to reader <b>200</b> through an adjustable attenuator and phase shifter <b>510</b> to allow phase and amplitude adjustment of the analog signal transmitted from each antenna. In one embodiment of the present invention, each adjustable attenuator and phase shifter <b>510</b> is coupled to host computer <b>280</b> via a different one of the I/O ports <b>286</b>. So, the adjustments can be done according to a pre-selected algorithm by host computer <b>280</b>, as discussed below. The directivity and orientation of each antenna in phased array <b>500</b> can also be adaptively varied under automated control. Furthermore, digitally-controlled radios could be employed in which phase and amplitude controls for the transmit signal from each antenna are accomplished by an analog quadrature modulator, enabling the connections to remote antennas from the reader <b>200</b> to be made using low-frequency baseband signals. The phase adjustments could be continuous, or they could be implemented using well-known fixed phase shifting networks, such that a finite selection of parameter settings for phased array <b>500</b> is available.

0035<figref idref="DRAWINGS">FIG. 6A</figref> illustrates a method <b>600</b> for controlling an area of successful interrogation by reader <b>110</b> according to one embodiment of the present invention. As shown in <figref idref="DRAWINGS">FIG. 6A</figref>, method <b>600</b> comprises step <b>610</b> in which the plurality of delineation RFID tags <b>120</b> are installed in the wanted region. The number of delineation tags <b>120</b> used can vary with the geometry of the wanted region and the precision desired. In one embodiment of the present invention, the number of delineation tags <b>120</b> is between 3 and 30. As tags are generally inexpensive, last indefinitely, and small in size and weight, they can be placed readily and the number of delineation tags employed has little impact on the cost or difficulty of installation. The number of delineation tags, however, can affect the length of a calibration step discussed below. Reader <b>110</b> can be informed of the unique identification number for each delineating tag so placed. The delineation tags may be staged along the boundary of the wanted region with equal distance from each other if the boundary is relatively smooth, or more tags are placed at or near vertices of the boundary. The delineation tags may also be placed uniformed throughout the wanted region or non-uniformly throughout the region with more tags occupying seemingly harder to reach areas in the wanted region.

0036Method <b>600</b> may also comprise an optional step <b>620</b> in which a plurality of anti-delineation tags are placed in or near the excluded region <b>103</b>. These anti-delineation tags could simply be the same tags being used as delineation tags for a neighboring reader. The presence of the anti-delineation tags provides an additional input to the optimization algorithm, allowing reader <b>110</b> to optimally balance coverage of the wanted area with rejection of the excluded area. Again, the number of anti-delineation tags <b>120</b> used can vary with the geometry of the wanted region and the precision desired. In one embodiment of the present invention, the number of delineation tags <b>120</b> is generally greater than the number of anti-delineation tags. The anti-delineation tags may be placed along a side of the boundary of the excluded region that faces the wanted region. The anti-delineation tag may also be staged throughout the excluded region either uniformly or with emphasis placed on seemingly easy to reach areas by reader <b>110</b>, or otherwise.

0037When the installation is completed, method <b>600</b> proceeds to perform a characterization method in step <b>630</b> in which reader <b>110</b> is activated and a calibration of its operation region or range of successful interrogation is performed to determine an optimal setting for the at least one transmission parameter associated with the reader. Afterwards, in step <b>640</b>, the at least one transmission parameter associated with the reader is set according to the optimal setting.

0038The actual manner of characterization of transmission parameters depends somewhat on the implementation chosen. In one embodiment of the present invention, as shown in <figref idref="DRAWINGS">FIG. 6B</figref>, the characterization method in step <b>630</b> comprises step <b>631</b> in which a plurality of trial parameter settings for the at least one transmission parameter are determined. In the situation where the at least one transmission parameter includes only the transmitted power from reader <b>110</b>, the plurality of trial parameter settings may comprise a plurality of discrete transmitted power settings running from a lowest possible transmitted power setting to a highest possible transmitted power setting, or vise versa. The step change from one transmitted power setting to a next transmitted power setting depends on the difference between the lowest possible transmitted power setting and the highest possible transmitted power setting, the size of the wanted area, the separation between the wanted area and the excluded are, or their combinations.

0039In the situation where the at one transmission parameter includes a selection of one of a plurality of antennas for transmitting the interrogation signal in addition to the transmitted power from the reader, the plurality of trial parameter settings may comprise a plurality of discrete transmitted power settings running from a lowest possible transmitted power setting to a highest possible transmitted power setting, or vise versa, for each antenna selection.

0040The phased array <b>500</b> provides more adjustable transmission parameters and thus more freedom to control the range of successful interrogation. In the situation of the phased array <b>500</b> being provided, the characterization step <b>630</b> may include a simple exhaustive search combined with power optimization to determine a parameter setting for optimized coverage. Thus, the plurality of parameter settings may include different combinations of possible values of the transmission parameters. The different combinations may be exhaustive or selective based on theoretical calculations and/or empirical data. In the more complex schemes in which continuous adjustment of phase and amplitude is possible, multivariate optimization schemes such as the method of steepest descents, Monte Carlo optimization, simplex optimization, or other optimization techniques may be used in stead of or in addition to the characterization method in step <b>630</b>.

0041Still referring to <figref idref="DRAWINGS">FIG. 6B</figref>, the characterization method in step <b>630</b> further comprises step <b>632</b> in which the at least one transmission parameter associated with the reader is set according to a first one of the plurality of trial parameter settings. In the simplest case of the transmitted power being the at least one transmission parameter, the first one of the plurality of trial parameter settings may simply be the lowest possible transmitted power setting. The characterization method in step <b>630</b> further comprises step <b>633</b> in which an attempt to interrogate the delineation tags and optionally the anti-delineation tags are made and step <b>634</b> in which a first number or percentage of successfully interrogated delineation tags and optionally a second number or percentage of successfully interrogated anti-delineation tags are recorded.

0042Depending on the number of delineation or anti-delineation tags involved, the first number or percentage of successfully interrogated delineation or the second number of anti-delineation tags may need to be statistically averaged to ensure characterization accuracy. Therefore, the characterization method in step <b>630</b> may repeat the attempt and record steps <b>923</b> and <b>924</b> multiple times and calculate in step <b>925</b> a statistical average of the first number or percentage and optionally the second number or percentage based on the responses of the delineation tags and anti-delineation tags in the repeated attempts. The characterization method in step <b>630</b> proceeds to step <b>626</b> to determine if another parameter setting need to be tried. The determination may be based on whether all of the trial parameter settings have been tried or the responses of the delineation RFID tags and optionally the anti-delineation RFID tags for the trial parameter settings that have been tried so far. For example, in the situation of the transmitted power being the at least one transmission parameter, there is no need for an exhaustive search, the transmitted power can be ramped from a low setting to a certain setting at which a predetermined figure of merit calculated based on the first number or percentage and optionally the second number or percentage meets a predetermined criteria. The figure of merit may simply be the first number or percentage and the predetermined criteria be that the first number or percentage equals to or exceeds a predetermined number or percentage; or, the figure of merit may be, a weighted difference between the first number or percentage and the second number or percentage and the predetermined criteria be that the weighted difference equals to or exceeds a first predetermined value, i.e.: <ul id="ul0001" list-style="none"><li id="ul0001-0001" num="0000"><ul id="ul0002" list-style="none"><li id="ul0002-0001" num="0043"> A*first-number-or-percentage-B*second-number-or-percentage>=C1 <br /> Where A is the weight on the first number and B is the weight on the second number, and C1 is the first predetermined value. A and B can be any positive value that are selected based on specific implementation. </li></ul></li></ul>

0044In stead of ramping the transmitted power up, the transmitted power may also be ramped down from a high setting to a certain setting at which a predetermined figure of merit calculated based on the first number or percentage and optionally the second number or percentage meets a predetermined criteria. The figure of merit may simply be the second number or percentage and the predetermined criteria be that the second number or percentage is equal to or less than a predetermined number or percentage; or, the figure of merit may be a weighted difference between the first number or percentage and the second number or percentage and the predetermined criteria be that the weighted difference is near a second predetermined value, i.e.: <ul id="ul0003" list-style="none"><li id="ul0003-0001" num="0000"><ul id="ul0004" list-style="none"><li id="ul0004-0001" num="0045"> A*first-number-or-percentage-B*second-number-or-percentage˜C2 <br /> Where C2 is the second pre-determined value. </li></ul></li></ul>

0046As shown in <figref idref="DRAWINGS">FIG. 7</figref>, by performing step <b>630</b>, an optimal transmitted power can be selected so that the range of successful interrogation by reader <b>110</b> substantially overlaps with the wanted region <b>102</b>.

0047In response to the determination that more parameter settings need to be tried, the characterization method in step <b>630</b> proceeds to step <b>927</b> in which the at least one transmission parameter is adjusted according to a next one of the plurality of trial parameter settings and steps <b>923</b> through <b>925</b> are repeated. Otherwise, the characterization method in step <b>630</b> proceeds to step <b>928</b> in which an optimal parameter setting is determined based on responses of the tags in the prior attempts to read the tags by the reader and a predetermined figure of merit as compared with a predetermined criteria as discussed above for each trial parameter setting. In the more complicated situations involving multiple antennas, as shown in <figref idref="DRAWINGS">FIG. 4</figref> and <figref idref="DRAWINGS">FIG. 5</figref>, it is likely that multiple parameter settings will result in figures of merit that satisfy the predetermined criteria. When this happens, power optimization is performed to select an optimal parameter setting that has the least amount of transmitted power among the multiple parameter settings.

0048For example, when the at least one parameter includes a selection of one of a plurality of antennas for transmitting the interrogation signal in addition to the transmitted power, it is likely that either antenna can be selected to result in a figure of merit satisfying the predetermined criteria. <figref idref="DRAWINGS">FIG. 8</figref> represents a situation in which antenna <b>2</b> is selected to transmit the transmit signal from reader <b>110</b> because less transmitted power is involved to result in a same or even better overlap between the range of successful interrogation <b>800</b> and the wanted area <b>102</b> than that shown in <figref idref="DRAWINGS">FIG. 7</figref>.

0049As shown in <figref idref="DRAWINGS">FIG. 9</figref>, the parameters associated with phased array <b>500</b> can be set at a particular setting to result in a range <b>900</b> of successful interrogation that overlaps with the wanted region <b>102</b> even more substantially than range <b>800</b> in <figref idref="DRAWINGS">FIG. 8</figref>. At this setting, all of the delineation RFID tags can be successfully identified while no anti-delineation RFID tag is read.

0050It must be noted that in order for an automated controller to arrive at an optimized arrangement of any such adaptive system, the number of input data must equal or exceed the number of degrees of freedom present in the adaptation. In this case the input data can be the probability of successfully reading each of the delineation tags. Transmitted power control constitutes one degree of adaptive freedom, requiring at least one delineation tag for adjustment. In the case where two or more transmitting antennas are also available, but only one antenna is in use at any given time, two degrees of freedom are available for adaptation and thus at least two delineating tags must be employed:; Similarly, for more complex implementations the number of delineating tags must be expanded to sufficiently constrain the optimization problem so as to enable the system to arrive at an optimum coverage solution.

0051It is well-known that propagation of radio signals in complex environments such as indoor areas or obstructed outdoor locations results in strong, essentially unpredictable, local variations in signal strength in time and space (commonly known as fading of the signal). Thus it is preferable to employ significantly more delineating tags than strictly required to equal the degrees of adaptive freedom, so that a statistically valid optimization procedure can be performed which will be relatively unaffected by such local fading or signal variations. Again, as noted above, the cost of acquisition and placement of RFID tags is modest, and with appropriate automation the labor involved in identifying the placed delineation tag to the interrogation device may also be readily minimized, so that the use of redundant delineation tags does not constitute a significant obstacle to the use of method <b>600</b>.

0052Additional receiving units may be used in addition to, or in place of, delineation tags to provide better overlap between the range of successful interrogation and the wanted region. In this case, the received signal strength at a given receiver can be communicated to the control unit to help in adjusting the coverage area; some calibration may be required to establish the correspondence between the received signal strength and the likelihood of tag detection in the corresponding location. As shown in <figref idref="DRAWINGS">FIG. 10</figref>, the receiving units <b>1010</b> could be interrogating devices themselves, and multiple interrogating devices are connected to a control unit such as the host computer <b>280</b> to form a data network. The multiple interrogating devices may use delineating tags as well as information collected from each other concerning their respective signal strengths, to jointly optimize the coverage of the wanted region and minimize coverage of the excluded region, by adjusting their output powers as well as optionally their relative phase or directivity.

0053The present invention has been described in terms of a number of embodiments, but this description is not meant to limit the scope of the invention. Numerous variations will be apparent to those with skill in the art, without departing from the spirit of the invention disclosed herein.