The present invention relates generally to radio frequency identification (RFID) systems, and more specifically to a system and method of performing interference monitoring in an RFID system. 
In recent years, radio frequency identification (RFID) systems have been employed in an ever increasing range of applications.  For example, RFID systems have been used in supply chain management applications to identify and track merchandise throughout manufacture, warehouse storage, transportation, distribution, and retail sale.  RFID systems have also been used in security applications to identify and track personnel for controlling access to restricted areas of buildings and plant facilities, thereby prohibiting access to such areas by individuals without the required authorization.  Accordingly, RFID systems have been increasingly employed in diverse applications to facilitate the identification and tracking of merchandise, personnel, and other items and/or individuals that need to be reliably monitored and/or controlled within a particular environment. 
A conventional RFID system typically includes at least one RFID transponder or tag, at least one RFID reader, and at least one controller or host computer.  For example, in a manufacturing environment, RFID tags can be attached to selected items of manufacture or equipment, and at least one RFID reader can be deployed in the environment to interrogate the tags as the tagged items pass predefined points on the manufacturing floor.  In a typical mode of operation, the reader transmits a radio frequency (RF) signal in the direction of a tag, which responds to the transmitted RF signal with another RF signal containing information identifying the item to which the tag is attached, and possibly other data acquired during the manufacture of the item.  The tag may also include at least one integrated transducer or environmental sensor for providing data such as the temperature or humidity level of the ambient environment.  The reader receives the information and data transmitted by the tag, and provides the tag data to the host computer for subsequent processing.  In this typical operating mode, the reader can be configured as a peripheral connected to a serial port of the host computer. 
More recently, RFID readers have been developed that are capable of being connected via a communications network to enterprise computer resources running one or more RFID-enabled client software applications.  Such readers have been deployed in complex systems including many readers (e.g., greater than 10) connected via one or more communications networks to a number of host computers, which may be part of an enterprise network server.  Such host computers can run client applications for processing tag data to control access to building and plant facilities, the movement of personnel and property, the operation of lighting/heating/ventilation/air conditioning facilities, and/or other diverse functions. 
Whether implemented as computer peripherals or networked devices, conventional RFID readers generally collect data from RFID tags much like optical barcode readers collect data from barcode labels.  However, whereas an optical barcode reader typically requires a direct line of sight to a barcode label to read the data imprinted on the label, the RF signals employed by the typical RFID reader can penetrate through and/or diffract around objects obstructing an RFID tag from the RF field of view of the reader, thereby allowing the reader to access data from a tag that, for example, might be buried beneath one or more boxes of merchandise.  In addition, unlike the optical barcode reader, the conventional RFID reader can operate on and distinguish between multiple RFID tags within the field of the reader. 
In the conventional RFID system, each RFID tag typically includes a small antenna operatively connected to a microchip.  For example, in the UHF band, the tag antenna can be just several inches long and can be implemented with conductive ink or etched in thin metal foil on a substrate of the microchip.  Further, each tag can be an active tag powered by a durable power source such as an internal battery, or a passive tag powered by inductive coupling, receiving induced power from RF signals transmitted by an RFID reader.  For example, an RFID reader may transmit a continuous unmodulated RF signal (i.e., a continuous wave, CW) or carrier signal for a predetermined minimum period of time to power a passive tag.  The volume of space within which a reader can deliver adequate power to a passive tag is known as the power coupling zone of the reader.  The internal battery of active tags may be employed to power integrated environmental sensors, and to maintain data and state information dynamically in an embedded memory of the tag.  Because passive tags do not have a durable power source, they do not include active semiconductor circuitry and must therefore maintain data and state information statically within its embedded memory.  In addition, passive tags have an essentially unlimited life span, while the life span of active tags is typically limited by the lifetime of the internal battery, which in some implementations may be replaceable. 
In conventional RFID systems that employ passive tags, each RFID reader typically follows a predefined sequence or protocol to interrogate and retrieve data from one or more RFID tags within the RF field of the reader (also known as the interrogation zone of the reader).  It is noted that the interrogation zone of a reader is generally determined by the physical positioning and orientation of the reader relative to the tags, and the setting of various parameters (e.g., the transmit power) employed by the reader during the interrogation sequence.  In systems employing passive tags, the interrogation zone is typically defined by the power coupling zone.  For example, a typical interrogation sequence performed by an RFID reader includes transmitting a CW to one or more passive tags within the reader's interrogation zone to power the tags, and transmitting a message packet (e.g., a request or command) by modulating the carrier signal.  The passive tag then reads the message packet while tapping some of the energy of the CW to maintain its power.  The message packet typically identifies one or a subset of the tags within the interrogation zone as the designated target of the message packet, and provides a request or command that the designated tag is expected to perform.  After the passive tag reads the information carried by the modulated carrier signal, the tag appropriately modulates the CW, and reflects a portion of the modulated wave back to the reader by changing the reflection characteristics of its antenna via a technique known as backscatter modulation.  In the event the interrogation sequence is employed in a system including active tags, the target active tag generates and transmits an appropriate response to the reader. 
During the typical interrogation sequence described above, the reader is tuned to detect changes in the small signals reflected from the antennae of the passive tags, or to receive the responses generated and transmitted by the active tags.  In the event the reader detects changes in signal reflections or receives responses from more than one tag in response to a message packet, the reader refines the identification (e.g., the address) of the target of the message in an iterative manner until only one tag provides data or information in response to the request or command contained within the packet.  For example, the tag address may be an electronic product code (EPC).  This process of iterative refinement of the communication between an RFID reader and a selected one of a plurality of RFID tags within the reader's interrogation zone is known as singulation.  Conventional singulation algorithms typically employ techniques similar to binary tree searches or randomized transmission delay techniques. 
After the reader has confirmed the presence of and received data from the targeted tag, it may send another message packet to a next tag until all of the tags within its interrogation zone have been addressed.  It is noted that some conventional interrogation protocols allow the creation of alias addresses for tags so that the reader is not required to transmit the actual tag address, which may carry private information.  For example, a tag can indicate to the reader how its alias tag address is related to its actual tag address via the backscatter transmission.  Further, the relationship between the alias address and the actual address can change each time the reader addresses that tag.  The reader then typically sends the data provided by the tags to the host computer for subsequent processing. 
However, the conventional RFID systems described above have a number of drawbacks.  For example, in the event the system employs a single RFID reader, various factors such as (1) multi-path signal reflections from items, individuals, and/or other tags in the vicinity of a targeted tag, (2) dielectric loading of the tag antenna caused by the item or individual to which the tag is attached, and (3) shadowing of the RF signal transmitted by the reader caused by shielding and/or absorbing material near the tag, may prevent the single reader from successfully addressing and accessing data from each tag within its interrogation zone.  The ability of an RFID reader to address RFID tags that may be at least partially obscured within its interrogation zone is known as the penetration ability of the reader.  For an RFID system that includes a single RFID reader, the penetration ability of the reader is typically limited by the reader's maximum transmit power. 
In addition, the interrogation zone of an RFID reader often changes depending upon the RF signal propagation characteristics in the environment in which the reader is deployed.  It may therefore be virtually impossible to infer the actual interrogation zone of the reader directly from the reader's transmission and reception settings and the transmission/reception capabilities of the tags.  For example, a particular transmitter setting of an RFID reader may result in significantly different qualities of reception for two different RFID tags disposed in substantially the same location.  Moreover, the interrogation zone of an RFID reader often fails to match the space that needs to be monitored in the RFID application. 
In addition, the conventional RFID system employing a single RFID reader is generally incapable of determining the location and/or direction of a moving RFID tag with precision.  As described above, RFID systems have been used to identify and track merchandise, equipment, or personnel within a particular environment.  Further, the tags attached to these items or individuals may be mobile or stationary, i.e., moving or disposed at fixed locations in the environment.  For example, in the event an RFID tag is attached to a forklift truck at a warehouse storage facility, a single RFID reader may be unable to determine whether the forklift is leaving or entering the facility at it passes the reader, which may be disposed at a dock bay.  For this reason, a single RFID reader is often accompanied by one or more photoelectric (electric eye) detectors for determining the location and/or direction of movement of mobile tags.  It is noted that an RFID reader can also be mobile (e.g., the reader may be a hand-held device or mounted to a vehicle) or stationary (e.g., the reader may be attached to a door, a wall, shelving, scaffolding, etc.). 
Conventional RFID systems that employ multiple RFID readers also have drawbacks.  Such systems have included tens to hundreds of RFID readers and/or other input/output devices connected to a communications network controlled by tens to hundreds of host computers running specialized client applications.  Such systems are frequently employed in applications that require many readers to perform tag monitoring within a relatively large space, to provide adequate resolution for determining the physical locations of tags within the designated space, and/or to provide frequent or continuous operation as tagged items or individuals move briskly through the interrogation zones of the readers. 
However, such dense deployments of RFID readers are problematic due to RF interference and potentially conflicting channel assignments of the readers.  For example, the space within which the transmissions of a first reader may interfere with the reception of a second RFID reader operating on the same channel as the first reader can be greater than the interrogation zones of the respective readers.  Significant RF interference may also occur when readers operate on adjacent channels.  Even if multiple readers operate on channels sufficiently far apart to avoid reader-to-reader interference, reader-to-tag interference may still occur since RFID tags are broadband devices capable of receiving RF transmissions from more than one reader at a time, which can confuse the tag circuitry. 
In addition, when multiple RFID readers are deployed within a conventional RFID system, the system is typically incapable of successfully coordinating the operation of the readers.  Further, each of the multiple readers is limited in time and frequency usage to the transaction paradigms typically employed by conventional standalone readers.  For example, each reader may only have access to its own information, and may therefore be unable to react to events detected by the other readers within the system.  As a result, the multiple readers are generally incapable of managing the large amount of potentially redundant information collected by the readers from the same group of tags within the same environment over an extended period of time.  Further, such conventional systems are generally unable to discriminate between significant events and essentially meaningless movements of items and/or individuals within the RFID environment.  Moreover, such conventional systems are generally incapable of keeping track of the various locations and functions of the many readers deployed within the environment, and assuring that all of the readers within the system are properly maintained and operational. 
It would therefore be desirable to have an improved system and method of accessing RFID tag data within an RFID environment.  Such a system and method would have the capability of identifying and tracking tagged merchandise, personnel, and other items and/or individuals within an RFID environment with increased reliability.  It would also be desirable to have a system and method of monitoring and characterizing the various sources of interference within an RFID environment, and adjusting the operational characteristics of RFID readers within the system based on these interference characterizations, thereby assuring that all of the readers within the system operate properly.