Systems and methods for mitigating interference at an access node

Systems and methods are also described for mitigating interference at an access node. It may be determined, based on an interference metric for a first wireless device exceeding an interference criteria, that communication between the first wireless device and a cell of an access node is experiencing interference from a neighboring cell. A second wireless device receiving a beamformed transmission may be identified, wherein the beamformed transmission to the second wireless device is identified as an interference source for communication between the first wireless device and the cell of the access node. Transmissions to the first wireless device and the second wireless device may be scheduled such that the scheduled timings for transmissions to the first wireless device are different from the scheduling timings for transmissions to the second wireless device.

TECHNICAL BACKGROUND

Telecommunication systems, such as cellular networks or other wireless networks, provide wireless services to a plurality of wireless devices in a variety of conditions. For example, an access node may serve a group of wireless devices that experience strong signal conditions while other wireless devices experience poor signal conditions. In some cases, a system may leverage various techniques, such as beamforming, to better serve wireless devices with poor signal conditions. However, such techniques may interfere with other communication between the access node and wireless devices. Accordingly, a system that effectively detects interference caused by such techniques and, in some cases, effectively mitigates the interference can provide enhanced service to users of the system.

Overview

Systems and methods are described for detecting interference at an access node. A rate at which packets are unsuccessful received at a wireless device may be monitored, wherein the wireless device is in communication with a cell of an access node. The access node may retransmit one or more unsuccessfully received packets to the wireless device. A retransmission metric for retransmission attempts to the wireless device from the access node may be monitored. And it may be determined that communication between the cell of the access node and the wireless device is experiencing interference from a neighboring cell when the monitored rate and monitored retransmission metric meet the interference criteria.

Systems and methods are also described for mitigating interference at an access node. It may determined, based on an interference metric for a first wireless device exceeding an interference criteria, that communication between the first wireless device and a cell of an access node is experiencing interference from a neighboring cell. At least one neighboring cell in which one or more beamformed signals are transmitted is identified as a potential interference source. The identified neighboring cell may be instructed to terminate transmission of a beamformed signal to at least a second wireless device. It may then be determined whether the interference metric for the first wireless device continues to exceed the interference criteria after the termination of the beamformed signal. And the second wireless device may be identified as an interference source when the interference metric for the first wireless device does not continue to exceed the interference criteria.

Systems and methods are also described for mitigating interference at an access node. It may be determined, based on an interference metric for a first wireless device exceeding an interference criteria, that communication between the first wireless device and a cell of an access node is experiencing interference from a neighboring cell. A second wireless device receiving a beamformed transmission may be identified, wherein the beamformed transmission to the second wireless device is identified as an interference source for communication between the first wireless device and the cell of the access node. Transmissions to the first wireless device and the second wireless device may be scheduled such that the scheduled timings for transmissions to the first wireless device are different from the scheduling timings for transmissions to the second wireless device.

DETAILED DESCRIPTION

Systems and methods are described for detecting interference at an access node. For example, a wireless device in communication with an access node my experience interference. In an embodiment, a rate at which packets are unsuccessfully received at the wireless device and a retransmission metric (e.g., number of retransmission attempts for a packet that is eventually successfully received at the wireless device) may indicate a type of interference. For example, based on the rate and the retransmission metric for the wireless device, it may be detected that the wireless device is experiencing grating lobe interference caused by a beamformed signal transmitted from a neighboring cell (e.g., a neighboring cell at the access node).

Once such detection is performed, systems and methods may be leveraged to mitigate the experienced interference. In some embodiments, a wireless device may be identified as a source of the grating lobe interference because a beamformed signal transmitted from the neighboring cell of the access node to the identified wireless device is determined to be causing the grating lobe interference. For instance, this may be determined by instructing the access node to terminate transmission of the beamformed signal to the identified wireless devices and then monitoring the experienced interference. Where the experienced interference drops (e.g., drops below a threshold), the wireless device may be identified as a source for the grating lobe interference. In some examples, the access node may be instructed to refrain from transmitting a beamformed signal to the identified wireless device for a period of time.

In another embodiment, once it is detected that communication between the cell of the access node and the wireless device is experiencing grating lobe interference, and a wireless device has been identified as a source for that interference (e.g., based on a beamformed signal transmitted to the wireless device), one or more schedulers may be used to mitigate the interference. For example, one or more schedulers for the cells of the access node may be instructed to schedule transmissions to the two wireless devices at different times (e.g., separated by at least a delta time). In some instances, the delta time between scheduled transmissions will mitigate against the experienced interference.

FIG. 1illustrates an exemplary communication system100detect interference at an access node comprising wireless devices102and104, access node106, communication network108, and communication links110,112, and114. Other network elements may be present in the communication system100to facilitate communication but are omitted for clarity, such as controller nodes, base stations, base station controllers, gateways, mobile switching centers, dispatch application processors, and location registers such as a home location register or visitor location register. Furthermore, other network elements may be present to facilitate communication between access node106and communication network108which are omitted for clarity, including additional processing nodes, routers, gateways, and physical and/or wireless data links for carrying data among the various network elements.

Wireless devices102and104can be any device configured to communicate over communication system100using a wireless communication link. For example, wireless devices102and104can include a cell phone, a smart phone, a computing platform such as a laptop, palmtop, or a tablet, a personal digital assistant, or an internet access device, and combinations thereof. It is noted that while two wireless devices are illustrated inFIG. 1as being in communication with access node106, any number of wireless devices can be implemented.

Access node106is a network node capable of providing wireless communications to wireless devices102and104, and can be, for example, a base transceiver station, a radio base station, and an eNodeB device. Access node106may communicate with communication network108over communication link114. Although only access node106is illustrated inFIG. 1, wireless devices102and104(and other wireless device not depicted) can be in communication with a plurality of access nodes and/or small cells. The plurality of access nodes and/or small cells can be associated with different networks and can support different communication protocols and radio access technologies.

Communication links110,112, and114, can be wired or wireless communication links. Wired communication links can comprise, for example, twisted pair cable, coaxial cable or fiber optic cable, or combinations thereof. Wireless communication links can comprise a radio frequency, microwave, infrared, or other similar signal, and can use a suitable communication protocol, for example, GSM, CDMA, UMTS, HSPA, EV-DO, or 3GPP LTE, or combinations thereof. Other wireless protocols can also be used.

FIG. 2illustrates an exemplary diagram200of signal lobes for a beamformed signal according to an embodiment. For example diagram200shows one or more main lobes and one or more grating lobes. In an embodiment, a main lobe may comprise a signal transmitted from an access node toward a wireless device in order to enhanced signal conditions for the wireless device. The main lobe may be transmitted to the wireless device as a beamformed signal

In an embodiment, beamforming may be accomplished using a plurality of antennas at an access node that implements, for example, a multiple input multiple output (MIMO) protocol. The signals from each of the plurality of antennas may be controlled such that the net signal from the access node may be transmitted towards wireless device as a beamformed signal. In an embodiment, the beam may be formed by weighting the magnitude and/or phase of the signals transmitted by each individual antenna. For example, the signals may be weighted such that the emitted waveform from the antennas experiences constructive interference in the direction of the wireless device.

In some embodiments, one or more side lobes, or grating lobes, may be created as a result of the beamformed signal. For example, the weighting applied to the antennas of the access node may cause a plurality of additional lobes to also be transmitted from the access node along with the beamformed signal. As illustrated inFIG. 2, the side lobes or grating lobes may be of different shapes and sizes, and may be transmitted from the access node at a plurality of angles. In an embodiment, the size, shape, and angle that side lobes or grating lobes take may be based on the location for the wireless device receiving the beamformed transmission. This is because the antenna weighting used to achieve beamforming is based on the location of the wireless device.

FIG. 3illustrates a system300for detecting interference at an access node and mitigating the interference according to an embodiment. System300comprises wireless devices302and304, access node306, beamformed signals308and310, and grating lobe312. Wireless devices302and304may comprise devices similar to wireless device102. Access node306may comprise an access node similar to access node106.

In operation, access node306may establish communication with wireless devices302and304such that access node306provides the wireless devices access to a communication network (e.g., communication network108). In an embodiment, system300may use a plurality of carriers in order to provide wireless communication services. A plurality of carriers that comprise bandwidth for wireless communications (e.g., 1.25 GHz carrier, 1900 Mhz carrier, and 800 Mhz carrier, and the like) may include a plurality of channels (e.g., 5 Mhz channels, 10 Mhz channels, 15 Mhz channels, and the like) that may further be divided into subcarriers. In an embodiment, a frequency band may comprise a carrier, a channel, a subcarrier, a plurality of any of these, or any other suitable frequency band.

In an embodiment, wireless devices302and304may be located in different cells (or sectors) of access node306. For example, access node306may be segmented into a plurality of sectors each comprising an arch (e.g., 60°, 90°, 120°, and the like). Each cell may include its own set of wireless resources (e.g., frequency bands for providing wireless service) and, in some examples, its own scheduler for scheduling transmissions to wireless devices. In some embodiments, cells of access node306may reuse wireless resources in order to provide wireless services. For example, a wireless device in a first cell of access node306may be assigned to communicate using a first frequency band, and a wireless device in a second cell of access node308may similarly be assigned to communicate using the first frequency band. System300may leverage a transmission protocol that limits or mitigates against the interference caused by such resource reuse (e.g., the LTE protocol).

In an embodiment, system300may leverage beamforming to enhance the wireless services provided to wireless devices302and304. For example, one or more of wireless device302and304may experience poor channel conditions, and the wireless devices may therefore comprise a low channel quality (e.g., a channel quality indicator (CQI) below a threshold). The low channel quality may be based on a high interference level, distance from access node306, or other suitable factors. In an embodiment, access node306may perform beamforming such that a signal transmitted to wireless device302is adjusted based on the location of the wireless device. For example, beamformed signal308may be transmitted from access node306such that wireless device302may experience greater channel quality when communicating with access node306. In an embodiment, the beamformed signal308may comprise of signals transmitted over a frequency band assigned to wireless device302(e.g., assigned as the frequency band that access node306uses to communicate with wireless device302).

In an embodiment, beamformed signal310may similarly be transmitted from access node306such that wireless device304may experience greater channel quality when communicating with access node306. In other examples, access node306may communicate with wireless device304using a non-beamformed signal (e.g., default signal). In an embodiment, the beamformed signal308transmitted to wireless device302may cause one or more side lobes or grating lobes, as described herein. For example, grating lobe312may be transmitted from access node306as a result of beamformed signal308.

In the embodiment illustrated inFIG. 3, grating lobe312may interfere with communication between wireless device304and access node306. However, in some instances, the interference may go undetected based on the channel conditions reported by wireless device304. For example, wireless device304may receive a reference signal from access node306at a certain signal level (e.g., reference signal received power, RSRP). Based on the received signal level (RSRP), wireless device304may report channel conditions to access node306(e.g., reported CQI). The channel conditions may then be used by access node306to determine certain transmission parameters for wireless device304(e.g., modulation and coding scheme (MCS), priority, and the like). However, beamformed signal308and resultant grating lobe312transmitted from access node306may be transmitted over data carrying signals (e.g., resource blocks used for carrying user data), not over reference signals (e.g., resource blocks used for carrying reference signals or pilot signals). For instance, considering an LTE implementation, data carrying signals may be transmitted over a Physical Downlink Shard Channel (PDSCH), used for carrying user data to and from wireless devices. Reference signals may be transmitted across various channels, but may be limited to certain resource blocks within a frame or subframe (e.g., predetermined resource blocks according to a particular pattern).

Because of this, the received signal level for a reference signal at wireless device304from access node306may not experience the same interference as a received signal level for data carrying signals received at wireless device304from access node306. This mismatch may cause wireless device304to suffer from poor channel conditions due to interference that goes undetected. Accordingly, a system that effectively detects and, in some instances, mitigates such interference may provide enhanced wireless service to users of the system.

FIG. 4illustrates an exemplary method for detecting interference at an access node according to an embodiment. The method will be discussed with reference to the exemplary communication system300illustrated inFIG. 3, however, the method can be implemented with any suitable communication system.

Referring toFIG. 4, at step402, a rate at which packets are unsuccessful received at a wireless device may be monitored, wherein the wireless device is in communication with a cell of an access node. For example, a rate at which packets are unsuccessfully received at wireless device304may be monitored. The unsuccessfully received packets may be transmitted by access node306. In an embodiment, the rate may comprise a block error rate (BLER) for wireless device304.

At step404, the access node may retransmit one or more unsuccessfully received packets to the wireless device. For example, access node306may retransmit one or more unsuccessfully recited packets to wireless device304. For example, the retransmissions may be part of an automatic repeat request (ARQ) or hybrid automatic repeat request (HARD) protocol.

At step406, a retransmission metric for retransmission attempts to the wireless device from the access node may be monitored. For example, a retransmission metric for retransmission attempts from access node306to wireless device304may be monitored. In an embodiment, the monitored retransmission metric may comprise the number of retransmission attempts for a packet that is eventually successfully received at wireless device304.

At step408, the monitored rate and the monitored retransmission metric may be compared to an interference criteria. For example, the monitored rate and the monitored retransmission metric may each be compared to a threshold for the values.

At step410, it may be determined that communication between the cell of the access node and the wireless device is experiencing interference from a neighboring cell when the monitored rate and monitored retransmission metric meet the interference criteria. For example, it may be determined that communication between the cell of access node306and wireless device304is experiencing interference from a neighboring cell (e.g., neighboring cell of access node306). In an embodiment, it may be determined that the interference caused comprises grating lobe interference.

FIG. 5illustrates an exemplary method for mitigating interference at an access node according to an embodiment. The method will be discussed with reference to the exemplary communication system300illustrated inFIG. 3, however, the method can be implemented with any suitable communication system.

Referring toFIG. 5, at step502, it may be determined, based on an interference metric for a first wireless device exceeding an interference criteria, that communication between the first wireless device and a cell of an access node is experiencing interference from a neighboring cell. For example, wireless device304may be in communication with a cell of access node306. An interference metric for wireless device304may be monitored and compared to an interference criteria. In an embodiment, after the comparison it may be determined that the monitored interference metric exceeds the interference criteria. Based on the comparison, it may be determined that communication between wireless device304and the cell of access node306is experiencing interference from a neighboring cell (e.g., a neighboring cell of access node306).

At step504, at least one neighboring cell in which one or more beamformed signals are transmitted is identified as a potential interference source. For example, a neighboring cell of access node306transmits a beamformed signal to wireless device302. Accordingly, the neighboring cell may be identified as a potential interference source.

At step506, the identified neighboring cell may be instructed to terminate transmission of a beamformed signal to at least one second wireless device. For example, the neighboring cell of access node306may be instructed to terminate the beamformed signal to wireless device302.

At step508, it may then be determined whether the interference metric for the first wireless device continues to exceed the interference criteria after the termination of the beamformed signal. For example, the interference metric for wireless device304may be monitored after termination of the beamformed signal to wireless device302. The monitored interference metric may then be compared to the interference metric to determine whether monitored interference metric continues to exceed the interference criteria.

At step510, the second wireless device may be identified as an interference source when the interference metric for the first wireless device does not continue to exceed the interference criteria. For example, if, after termination of the beamformed signal to wireless device302, the monitored interference metric for wireless device304does not exceed the interference metric, it may be determined that the beamformed signal to wireless device302caused the interference for communication between wireless device304and access node306.

FIG. 6illustrates another exemplary method for mitigating interference at an access node according to an embodiment. The method will be discussed with reference to the exemplary communication system300illustrated inFIG. 3, however, the method can be implemented with any suitable communication system.

Referring toFIG. 6, at step602, it may be determined, based on an interference metric for a first wireless device exceeding an interference criteria, that communication between the first wireless device and a cell of an access node is experiencing interference from a neighboring cell. For example, wireless device304may be in communication with a cell of access node306. An interference metric for wireless device304may be monitored and compared to an interference criteria. In an embodiment, after the comparison it may be determined that the monitored interference metric exceeds the interference criteria. Based on the comparison, it may be determined that communication between wireless device304and the cell of access node306is experiencing interference from a neighboring cell (e.g., a neighboring cell of access node306).

At step604, a second wireless device receiving a beamformed transmission may be identified, wherein the beamformed transmission to the second wireless device is identified as an interference source for communication between the first wireless device and the cell of the access node. For example, wireless device302may be identified because the wireless device is receiving a beamformed signal (e.g., from access node306). The second wireless device may further be identified as an interference source for communication between wireless device304and the cell of access node306.

At step606, transmissions to the first wireless device and the second wireless device may be scheduled such that the scheduled timings for transmissions to the first wireless device are different from the scheduling timings for transmissions to the second wireless device. For example, transmissions to wireless device302and wireless device304(e.g., from access node306) may be scheduled such that the scheduled timings for transmissions to wireless device302are different from the scheduling timings for transmissions to wireless device304. For example, the scheduled timings may differ by at least one or more transmission time intervals (TTIs).

FIG. 7illustrates another exemplary communication system700to detect and mitigate interference at an access node. Communication system700may comprise wireless devices702and704, access node706, controller node708, gateway node710, communication network712, and communication links714,716,718,720,722, and724. Other network elements may be present in the communication system400to facilitate communication but are omitted for clarity, such as base stations, base station controllers, gateways, mobile switching centers, dispatch application processors, and location registers such as a home location register or visitor location register.

Wireless devices702and704can be any device configured to communicate over communication system700using a wireless communication link. For example, wireless devices702and704can include a cell phone, a smart phone, a computing platform such as a laptop, palmtop, or a tablet, a personal digital assistant, or an internet access device, and combinations thereof.

Access node706is a network node capable of providing wireless communications to wireless devices702and704, and can be, for example, a base transceiver station, a radio base station, or an eNodeB device. In an embodiment, access node706can comprise a serving access node for wireless device702and704. Access node706may communicate with controller node708over communication link718, and with gateway node710over communication link720.

Controller node708can be any network node configured to manage services within system700. Controller node708may provide other control and management functions for system700. The controller node708can be a single device having various functions or a plurality of devices having differing functions. For example, controller node708can include at least one of a multi-cell/multicast coordination entity (MCE), a mobility management entity (MME), a radio network controller (RNC), a mobile switching center (MSC), and a combination thereof.

Controller node708can comprise a processor and associated circuitry to execute or direct the execution of computer-readable instructions to obtain information. Controller node708can retrieve and execute software from storage, which can include a disk drive, a flash drive, memory circuitry, or some other memory device, and which can be local or remotely accessible. The software may comprise computer programs, firmware, or some other form of machine-readable instructions, and may include an operating system, utilities, drivers, network interfaces, applications, or some other type of software, including combinations thereof. Controller node708can receive instructions and other input at a user interface. Controller node708can comprise a processor and associated circuitry to execute or direct the execution of computer-readable instructions to obtain information.

Gateway node710is a network element which can comprise a processor and associated circuitry to execute or direct the execution of computer-readable instructions. Gateway node710may retrieve and execute software from storage, which can include a disk drive, flash drive, memory circuitry, or some other memory device, and which can be local or remotely accessible. The software comprises computer programs, firmware, or some other form of machine-readable instructions, and may include an operating system, utilities, drivers, network interfaces, applications, or some other type of software, including combinations thereof. In an embodiment, gateway node710can provide instructions to access node706related to channel selection in communications with wireless devices702and704. For example, gateway node710can comprise at least one of a serving gateway (SGW), a packet data network gateway (PDNGW), a cellular gateway (CGW), and a combination thereof.

Communication links714,716,718,720,722, and724can be wired or wireless communication links. Wired communication links can be, for example, twisted pair cable, coaxial cable or fiber optic cable, or combinations thereof. Wireless communication links can be a radio frequency, microwave, infrared, or other similar signal, and can use a suitable communication protocol, for example, Global System for Mobile telecommunications (GSM), Code Division Multiple Access (CDMA), or Long Term Evolution (LTE), or combinations thereof. Other wireless protocols can also be used.

Other network elements may be present in the communication system700to facilitate wireless communication but are omitted for clarity, such as base stations, base station controllers, gateways, mobile switching centers, dispatch application processors, and location registers such as a home location register or visitor location register. Furthermore, other network elements may be present to facilitate communication among access node706, controller node708, gateway node710, and communication network712which are omitted for clarity, including additional processing nodes, routers, gateways, and physical and/or wireless data links for carrying data among the various network elements. In an embodiment, any of controller node708, gateway node710, and one or more modules of access node706may perform all or parts of the methods ofFIGS. 4-6 and 9-11.

FIG. 8illustrates a system800for detecting and mitigating interference at an access node according to an embodiment. System800comprises wireless devices802,804, and806, access node808, beamformed signals810,812, and814, and grating lobe816. Wireless devices802,804, and806, may comprise devices similar to wireless device402. Access node808may comprise an access node similar to access node406.

In operation, access node808may establish communication with wireless devices802,804, and806such that access node808provides the wireless devices access to a communication network (e.g., communication network712). In an embodiment, system800may use a plurality of carriers in order to provide wireless communication services. A plurality of carriers that comprise bandwidth for wireless communications (e.g., 1.25 GHz carrier, 1900 Mhz carrier, and 800 Mhz carrier, and the like) may include a plurality of channels (e.g., 5 Mhz channels, 10 Mhz channels, 15 Mhz channels, and the like) that may further be divided into subcarriers. In an embodiment, a frequency band may comprise a carrier, a channel, a subcarrier, a plurality of any of these, or any other suitable frequency band.

In an embodiment, wireless devices806may be located in a different cell (or sector) of access node808than wireless device802and804. For example, access node808may be segmented into a plurality of sectors each comprising an arch (e.g., 60°, 90°, 120°, and the like). Each cell may include its own set of wireless resources (e.g., frequency bands for providing wireless service) and, in some examples, its own scheduler for scheduling transmissions to wireless devices. In some embodiments, cells of access node808may reuse wireless resources in order to provide wireless services. For example, a wireless device in a first cell of access node808may be assigned to communicate using a first frequency band, and a wireless device in a second cell of access node808may similarly be assigned to communicate using the first frequency band. System800may leverage a transmission protocol that limits or mitigates against the interference caused by such resource reuse (e.g., the LTE protocol).

In an embodiment, system800may leverage beamforming to enhance the wireless services provided to wireless devices802,804, and806. In an embodiment, access node808may perform beamforming such that a signal transmitted to wireless device802is adjusted based on the location of the wireless device. For example, beamformed signal810may be transmitted from access node808such that wireless device802may experience greater channel quality when communicating with access node808. In an embodiment, beamformed signal810may comprise of signals transmitted over a frequency band assigned to wireless device802(e.g., assigned as the frequency band that access node808uses to communicate with wireless device802).

In an embodiment, beamformed signal812may similarly be transmitted from access node808such that wireless device804may experience greater channel quality when communicating with access node808and beamformed signal814may similarly be transmitted from access node808such that wireless device806may experience greater channel quality when communicating with access node808. In other examples, access node808may communicate with wireless device806using a non-beamformed signal (e.g., default signal). In an embodiment, beamformed signal810transmitted to wireless device802and/or beamformed signal812transmitted to wireless device804may cause one or more side lobes or grating lobes, as described herein. For example, grating lobe816may be transmitted from access node808as a result of one or both of beamformed signals810and812.

In the embodiment illustrated inFIG. 8, grating lobe816may interfere with communication between wireless device806and access node808. However, in some instances, the interference may go undetected based on the channel conditions reported by wireless device806. For example, wireless device806may receive a reference signal from access node808at a certain signal level (e.g., reference signal received power, RSRP). Based on the received signal level (RSRP), wireless device806may report channel conditions to access node808(e.g., reported CQI). The channel conditions may then be used by access node808to determine certain transmission parameters for wireless device806(e.g., modulation and coding scheme (MCS), and the like). However, beamformed signals810,812, and814, and grating lobe816transmitted from access node808may be transmitted over data carrying signals (e.g., resource blocks used for carrying user data), not over reference signals (e.g., resource blocks used for carrying reference signals or pilot signals). For instance, considering an LTE implementation, data carrying signals may be transmitted over a Physical Downlink Shard Channel (PDSCH), used for carrying user data to and from wireless devices. Reference signals may be transmitted across various channels, but may be limited to certain resource blocks within a frame or subframe (e.g., predetermined resource blocks according to a particular pattern).

Because of this, the received signal level for a reference signal at wireless device806from access node808may not experience the same interference as a received signal level for data carrying signals received at wireless device806from access node808. This mismatch may cause wireless device806to suffer from poor channel conditions due to interference that goes undetected. Accordingly, a system that effectively detects such interference and, in some instances, mitigates the interference may provide enhanced wireless service to users of the system.

FIG. 9illustrates an exemplary method for detecting interference at an access node according to an embodiment. The method will be discussed with reference to the exemplary communication system800illustrated inFIG. 8, however, the method can be implemented with any suitable communication system.

Referring toFIG. 9, at step902, a rate at which packets are unsuccessful received at a wireless device may be monitored, wherein the wireless device is in communication with a cell of an access node. For example, a rate at which packets are unsuccessfully received at wireless device806may be monitored. The unsuccessfully received packets may be transmitted by access node808, where wireless device806is in communication with a cell of access node808. In an embodiment, the rate may comprise a block error rate (BLER) for wireless device806. In another embodiment, the rate may comprise a packet error rate (PER) for wireless device806.

At step904, the access node may retransmit one or more unsuccessfully received packets to the wireless device. For example, access node808may retransmit one or more unsuccessfully recited packets to wireless device806. For example, the retransmissions may be part of an automatic repeat request (ARQ) or hybrid automatic repeat request (HARQ) protocol. In an embodiment, access node808may implement a HARQ protocol such that a NACK message received from a wireless device indicates data (e.g., a packet) was not successfully received at the wireless device. Based on the HARQ protocol, access node808may retransmit the data (e.g., packet) associated with the NACK message so that it may be successfully received by the wireless device. In an embodiment, a NACK may be received from a wireless device based on a retransmitted packet from access node808, and thus access node808may retransmit the packet again according to the HARQ protocol. A packet may be retransmitted a number of times until a maximum HARQ retransmission threshold is reached (e.g., maximum of four retransmissions).

In an embodiment, the monitored rate may comprise a BLER for wireless device806, where BLER is based on an error rate per each transmission. Accordingly, errors that lead to HARQ retransmissions may each contribute to the BLER even when retransmissions eventually lead to successfully received data. In another embodiment, a monitored PER may discount an error when HARQ retransmissions succeed and the data associated with an unsuccessfully received packet is eventually received at wireless device806. In other words, BLER may account for each reception error even when the HARQ protocol corrects the reception error with retransmissions. PER may not account for errors that HARQ retransmissions eventually correct, and thus these errors may be masked. This difference may be based on the various Open Systems Interconnection (OSI) layers, or network stack layers, for the two rates. BLER is relative to the physical (PHY) layer of the OSI model while PER is relative to the network layer or data link (MAC) layer of the OSI model. For instance, for wireless links, often the data link (MAC) layer is leveraged to provide error free packets to the network layer, and thus HARQ retransmissions may correct errors at the data link (MAC) layer such that the error is not perceived at the network layer.

In an embodiment, one of BLER and PER may be monitored and further implemented (e.g., as a BLER or PER threshold) based on the interference experienced. In an embodiment, each of a consistent interference or an inconsistent interference may have a different affect on BLER than on PER, or analysis of each rate may detect different types of interference. Accordingly, detection embodiments may leverage one or both rates in order to provide robust interference detection.

At step906, a retransmission metric for retransmission attempts to the wireless device from the access node may be monitored. For example, a retransmission metric for retransmission attempts from access node808to wireless device806may be monitored.

In an embodiment, the monitored retransmission metric may comprise the number of retransmission attempts for a packet that is eventually successfully received at wireless device806. For example, based on a HARQ protocol, access node808may retransmit one or more packets to wireless device806when the wireless device transmits a NACK message back to the access node. In some instances, a retransmitted packet based on the HARQ protocol may also be unsuccessfully received at wireless device806, and thus access node808may retransmit the packet again. The retransmissions may continue until the packet is successfully received or the HARQ retransmission threshold is reached (e.g., maximum of four retransmissions).

In an embodiment, the monitored retransmission metric may comprise the average number of retransmission attempts from access node808for packets that are eventually successfully received at wireless device806. In this example, HARQ retransmissions may be analyzed such that the number of retransmission attempts for each packet that is eventually successfully received at wireless device806based on the HARQ protocol may be determined, and this number may then be divided by the number of packets that triggered the HARQ protocol to arrive at an average number of retransmissions. For instance, a first, second, and third packet may each trigger the HARQ protocol, where each packet is also eventually successfully received at wireless device806. The first packet may have taken 2 retransmissions, the second packet may have taken 3 retransmissions, and the third packet may have taken 4 retransmissions. Thus, the average number of retransmissions for these three packets may comprise (2+3+4)/3, or 3.

In another embodiment, the monitored retransmission metric may comprise the average number of retransmission attempts from access node808for packets that are eventually successfully received at wireless device806and for packets that are not eventually successfully received at wireless device806. For instance, the average number of retransmission attempts from access node808for a packet that is not successfully received at wireless device806after HARQ retransmissions will comprise the HARQ retransmission threshold.

At step908, the monitored rate and the monitored retransmission metric may be compared to an interference criteria. For example, the monitored rate and the monitored retransmission metric may each be compared to a threshold for the values.

In an embodiment, the monitored rate may comprise a BLER rate, and the interference criteria may include a BLER threshold. For instance, the BLER threshold may be the expected BLER without grating lobe interference (e.g., 10%, 15%, and the like). The BLER threshold may be theoretically derived or may be based on historical data.

In an embodiment, the monitored retransmission metric may comprise an average number of retransmission attempts for successfully received packets at wireless device806based on a HARQ protocol, and the interference criteria may include a number of retransmissions threshold. For instance, the number of retransmissions threshold may be the expected number of retransmissions without grating lobe interference (e.g., 1, 2, 3, and the like). The number of retransmissions threshold may be theoretically derived or may be based on historical data. In another embodiment, the monitored retransmission metric may comprise an average number of retransmission attempts for successfully received packets at wireless device806based on a HARQ protocol and unsuccessfully received packets at wireless device806based on the HARQ protocol, and the interference criteria may include a number of retransmissions threshold.

At step910, it may be determined that communication between the cell of the access node and the wireless device is experiencing interference from a neighboring cell when the monitored rate and monitored retransmission metric meet the interference criteria. For example, it may be determined that communication between the cell of access node808and wireless device806is experiencing interference from a neighboring cell (e.g., neighboring cell of access node808). In an embodiment, it may be determined that the interference caused comprises grating lobe interference.

For example, the monitored rate may comprise a BLER for wireless device806, the monitored retransmission metric may comprise an average number of retransmission attempts for successfully received packets at wireless device806based on a HARQ protocol, and the interference criteria may include a BLER threshold and a number of retransmissions threshold. In an embodiment, where the monitored BLER (meets or) exceeds the BLER threshold and the monitored average number of retransmission attempts for successfully received packets (meets or) exceeds a number of retransmissions threshold, it may be determined that communication between the cell of access node808and wireless device806is experiencing interference from a neighboring cell. In another embodiment, where the monitored BLER (meets or) exceeds the BLER threshold or the monitored average number of retransmission attempts for successfully received packets (meets or) exceeds a number of retransmissions threshold, it may be determined that communication between the cell of access node808and wireless device806is experiencing interference from a neighboring cell.

At step912, at least one neighboring cell may be instructed to adjust a transmission based on the determination that communication between the cell of the access node and the wireless device is experiencing interference from a neighboring cell. For example, at least one neighboring cell of access node808may be instructed to adjust a beamformed transmission to one or more wireless devices based on the determination that communication between the cell of access node808and wireless device806is experiencing interference from a neighboring cell (e.g., grating lobe interference). Various examples and embodiments that describe techniques for adjusting transmissions from a neighboring cell are further described herein.

FIG. 10illustrates an exemplary method for mitigating interference at an access node according to an embodiment. The method will be discussed with reference to the exemplary communication system800illustrated inFIG. 8, however, the method can be implemented with any suitable communication system.

Referring toFIG. 10, at step1002, it may be determined, based on an interference metric for a first wireless device exceeding an interference criteria, that communication between the first wireless device and a cell of an access node is experiencing interference from a neighboring cell. For example, wireless device806may be in communication with a cell of access node808. An interference metric for wireless device806may be monitored and compared to an interference criteria. In an embodiment, after the comparison it may be determined that the monitored interference metric (meets or) exceeds the interference criteria. Based on the comparison, it may be determined that communication between wireless device806and a cell of access node808is experiencing interference from a neighboring cell (e.g., a neighboring cell of access node808). For example, the interference may comprise grating lobe interference.

In an embodiment, the interference metric for wireless device806may comprise a packet error rate (PER). For example, the PER for wireless device806may be monitored while the wireless device is in communication with a cell of access node808. It may then be determined that the monitored packet error rate (meets or) exceeds an interference criteria, where the interference criteria may comprise a PER threshold. The PER threshold may be based on an expected PER when grating lobe interference is not experienced, and may be theoretically derived or may be based on historical data. In an embodiment, based on the monitored packet error rate exceeding the packet error rate threshold, it may be determined that communication between wireless device806and the cell of access node808is experiencing grating lobe interference from a beamformed signal transmitted by a neighboring cell (e.g., neighboring cell of access node808). Various other detection techniques for detecting that communication is experiencing grating lobe interference may also be implemented, as described herein. For instance, one or more of a BLER for wireless device806and a number of retransmission attempts from access node808to wireless device806may be monitored and compared to an interference criteria.

At step1004, at least one neighboring cell in which one or more beamformed signals are transmitted is identified as a potential interference source. For example, a neighboring cell of access node808transmits a beamformed signal to wireless device802and wireless device804. Accordingly, the neighboring cell is identified as a potential interference source. Other neighboring cells (e.g., of access node808) may similarly be identified when the neighboring cells transmit one or more beamformed signals to a wireless device.

At step1006, the identified neighboring cell may be instructed to terminate transmission of a beamformed signal to at least one second wireless device. For example, the neighboring cell of access node808may be instructed to terminate the beamformed signal to wireless device802. The instruction to terminate the beamformed signal may be valid for a predetermined period of time.

At step1008, it may then be determined whether the interference metric for the first wireless device continues to (meet or) exceed the interference criteria after the termination of the beamformed signal. For example, the interference metric for wireless device806may be monitored after termination of the beamformed signal to wireless device802. The monitored interference metric may then be compared to the interference criteria to determine whether monitored interference metric continues to (meet or) exceed the interference criteria.

In an embodiment, the monitored interference metric may comprise a PER for wireless device806, and it may be determined whether the monitored PER for wireless device806continues to meet or exceed a PER threshold after the beamformed signal to wireless device802has been terminated. In other embodiments, one or more of a BLER for wireless device806and a number of retransmission attempts from access node808to wireless device806may be monitored after the beamformed signal to wireless device802has been terminated, and the monitored values may be compared to an interference criteria. The method ofFIG. 10may progress from step1008to step1010when it is determined that interference metric for wireless device806(e.g., monitored PER after the termination of the beamformed signal to wireless device802) does not continue to (meet or) exceed the interference criteria

At step1010, the second wireless device may be identified as an interference source when the interference metric for the first wireless device does not continue to exceed the interference criteria. For example, if, after termination of the beamformed signal to wireless device802, the monitored interference metric for wireless device806does not (meet or) exceed the interference criteria, it may be determined that the beamformed signal to wireless device802caused the interference (e.g., grating lobe interference) for communication between wireless device806and the cell of access node808.

In an embodiment, the interference metric may comprise a PER for wireless device806. If, after termination of the beamformed signal to wireless device802, the monitored PER for wireless device806does not (meet or) exceed the PER threshold, it may be determined that the beamformed signal to wireless device802caused the interference for communication between wireless device806and access node808. For example, it may be determined that the beamformed signal to wireless device802from a neighboring cell of access node808caused grating lobe interference to the communication between wireless device806and the cell of access node808. Other embodiments where the interference metric comprises one or more of a BLER for wireless device806and a number of retransmission attempts from access node808to wireless device806may similarly be implemented.

At step1012, the neighboring cell of the access node may be instructed to refrain from transmitting a beamformed signal to the identified wireless device for a period of time. For example, the neighboring cell of access node808may be instructed to refrain from transmitting a beamformed signal to wireless device802(e.g., identified as an interference source) for a period of time, such as a predetermined period of time.

In other embodiments, the neighboring cell of access node808may be instructed to refrain from transmitting a beamformed signal to wireless device802for a period of time or until wireless device802moves to another location (e.g., a location at least a threshold distance away from the wireless device's location when it was identified as the interference source). Because grating lobe interference is caused by the weights assigned to antennas that transmit a beamformed signal, relocation of a wireless device may alter the circumstances that generate a grating lobe. In some embodiments, the neighboring cell of access node808may be instructed to refrain from transmitting a beamformed signal to wireless device802for a period of time and/or may be instructed to refrain from transmitting a beamformed signal to other wireless devices within a threshold distance of the location of wireless device802when it was identified as the interference source. A location for a wireless device may be determined based on a number of techniques known in the art, such as signal triangulation, a global positioning system, and the like.

In another example, the method ofFIG. 10may progress from step1008to step1014when it is determined that interference metric for wireless device806(e.g., monitored PER after the termination of the beamformed signal to wireless device802) continues to (meet or) exceed the interference criteria. At step1014, it may be whether the neighboring cell transmits a beamformed signal to a third wireless device. For example, if, after termination of the beamformed signal to wireless device802, the monitored interference metric for wireless device806continues to (meet or) exceed the interference metric, it may be determined whether the neighboring cell transmits a beamformed signal to a third wireless device. In an example, the neighboring cell of access node808may also transmit a beamformed signal to wireless device804. In addition, because termination of the beamformed signal to wireless device802did not cause the monitored interference metric for wireless device806to fall below the interference criteria (e.g., threshold), it may be determined that the beamformed signal transmitted to wireless device802is not causing the interference between wireless device806and the cell of access node808. Accordingly, wireless device802may be added to a cleared list of wireless devices that have been cleared as interference sources.

In an embodiment, the method ofFIG. 10may progress from step1014to step1006when it is determined that the neighboring cell transmits a beamformed signal to a third wireless device (e.g., wireless device804). At step1006, the neighboring cell may similarly be instructed to terminate the transmission of the beamformed signal to wireless device804. At step1008, it may then be determined whether the interference metric for wireless device806continues to (meet or) exceed the interference criteria after the termination of the beamformed signal to wireless device804. For example, the monitored interference metric may comprise a PER for wireless device806, and it may be determined whether the monitored PER for wireless device806continues to meet or exceed a PER threshold after the beamformed signal to wireless device804has been terminated. Other embodiments for these steps, as described with reference to wireless device802, may similarly be implemented.

The method ofFIG. 10may progress from step1008to step1010when it is determined that interference metric for the first wireless device (e.g., monitored PER after the termination of the beamformed signal to wireless device804) does not continue to (meet or) exceed the interference criteria. Similar to the descriptions herein for steps1010and step1012, wireless device804may be identified as the source for interference between the cell of access node808and wireless device806, and the neighboring cell may be instructed to refrain from transmitting a beamformed signal to wireless device804for a period of time. Other embodiments for these steps, as described with reference to wireless device802, may similarly be implemented.

In another example, the method ofFIG. 10may progress from step1008to step1014when it is determined that interference metric for wireless device806(e.g., monitored PER after the termination of the beamformed signal to wireless device804) continues to (meet or) exceed the interference criteria. In addition, because termination of the beamformed signal to wireless device804did not cause the monitored interference metric for wireless device806to fall below the interference criteria (e.g., threshold), it may be determined that the beamformed signal transmitted to wireless device804is not causing the interference between wireless device806and access node808. Accordingly, wireless device804may be added to a cleared list of wireless devices that have been cleared as interference sources.

In an embodiment, the method ofFIG. 10may progress from step1014to step1016when it is determined that the neighboring cell does not transmit a beamformed signal to a fourth wireless device. For example, wireless devices802and804may be included on the cleared list of wireless devices. It may be determined whether the neighboring cell transmits a beamformed signal to any wireless devices not included on the cleared list. In an embodiment, the method ofFIG. 10may progress from step1014to step1016when it is determined that the neighboring cell does not transmit a beamformed signal to any wireless devices no included on the cleared list.

At step1016, other sources of interference may be searched. For example, a second neighboring cell may be identified in which one or more beamformed signals are transmitted as a potential interference source. In an embodiment, the second neighboring cell may be a neighboring cell of access node808(and of the previously identified neighboring cell). Similar to the descriptions for the previously identified neighboring cell and wireless devices802and804, the wireless devices that receive a beamformed signal from the second neighboring cell may be cycled through to determine if one of the wireless devices is causing interference for communication between wireless device806and the cell of access node808.

For example, the second neighboring cell may be instructed to terminate transmission of a beamformed signal to at least a fifth wireless device that receives a beamformed signal from the second neighboring cell. It may be determined whether the interference metric for wireless device806continues to (meet or) exceed the interference criteria after termination of the beamformed signal to the fifth wireless device. The fifth wireless device may be identified as an interference source when the interference metric for wireless device806does not continue to (meet or) exceed the interference criteria after termination of the beamformed signal to the fifth wireless device. Similar to the descriptions herein, the second neighboring cell may be instructed to refrain from transmitting a beamformed signal to the fifth wireless device for a period of time when the fifth wireless device is identified as an interference source. Various other embodiments as described with reference to steps1012may also be implemented.

FIG. 11illustrates another exemplary method for mitigating interference at an access node according to an embodiment. The method will be discussed with reference to the exemplary communication system800illustrated inFIG. 8, however, the method can be implemented with any suitable communication system.

Referring toFIG. 11, at step1102, it may be determined, based on an interference metric for a first wireless device exceeding an interference criteria, that communication between the first wireless device and a cell of an access node is experiencing interference from a neighboring cell. For example, wireless device806may be in communication with a cell of access node808. An interference metric for wireless device806may be monitored and compared to an interference metric. In an embodiment, after the comparison it may be determined that the monitored interference metric (meets or) exceeds the interference criteria. Based on the comparison, it may be determined that communication between wireless device806and the cell of access node808is experiencing interference from a neighboring cell (e.g., a neighboring cell of access node808). For example, the interference may comprise grating lobe interference.

In an embodiment, the interference metric for wireless device806may comprise a packet error rate (PER). For example, the PER for wireless device806may be monitored while the wireless device is in communication with the cell of access node808. It may then be determined that the monitored packet error rate (meets or) exceeds an interference criteria, where the interference criteria may comprise a PER threshold. The PER threshold may be based on an expected PER when grating lobe interference is not experienced, and may be theoretically derived or may be based on historical data. In an embodiment, based on the monitored packet error rate exceeding the packet error rate threshold, it may be determined that communication between wireless device806and the cell of access node808is experiencing grating lobe interference from a beamformed signal transmitted by a neighboring cell (e.g., neighboring cell of access node808). Various other detection techniques for detecting that communication is experiencing grating lobe interference may also be implemented, as described herein. For instance, one or more of a BLER for wireless device806and a number of retransmission attempts from access node808to wireless device806may be monitored and compared to an interference criteria.

At step1104, a second wireless device receiving a beamformed transmission may be identified, wherein the beamformed transmission to the second wireless device is identified as an interference source for communication between the first wireless device and the cell of the access node. For example, wireless device802may be identified as an interference source because the wireless device is receiving a beamformed transmission (e.g., from a cell of access node808). In an embodiment, a wireless device that receives a beamformed signal based on a transmission from a neighboring cell of access node808may be identified as an interference source as further described with reference toFIG. 10. For example, the beamformed signal to wireless device802from access node808may be terminated, and it may be determined that the interference experienced by wireless device806and access node808changes based on the terminated beamformed signal to wireless device802. Various other techniques for identifying wireless device802as an interference source for communication between wireless device806and a cell of access node808may be implemented.

At step1106, transmissions to the first wireless device and the second wireless device may be scheduled such that the scheduled timings for transmissions to the first wireless device are different from the scheduling timings for transmissions to the second wireless device. For example, transmissions to wireless device802and wireless device806from access node808may be scheduled such that the scheduled timings for transmissions to wireless device802are different from the scheduling timings for transmissions to wireless device806. For example, the scheduled timings may differ by one or more transmission time intervals (TTIs).

In an embodiment, a cell of access node808may communicate with wireless device806while a neighboring cell of access node808may communicate with wireless device802. In some configurations, each cell of access node808will comprise a separate scheduler. For this example, the scheduler for the cell communicating with wireless device806may communicate with the scheduler for the neighboring cell communicating with wireless device802in order to accomplish the different scheduled timings for the two wireless devices. In other examples, a single scheduler may be used for multiple cells of access node808.

In an embodiment, the difference in scheduled timings may be based on a scheduling algorithm. For example, the schedulers for the cells of access node808may communicate such that wireless devices802and806are scheduled transmissions according to a round robin algorithm (e.g., one after the other).

In another embodiment, the schedulers may implement a delta time between the scheduled transmission times for wireless device802and wireless device806. For example, a first of wireless devices802and806may be scheduled a transmission time of T1. Here, the scheduler for the second of wireless devices802and806may schedule a transmission to the second wireless at a time that is at least a delta time ΔT different from T1. The delta time ΔT may comprise a predetermined period of time or a random period of time. In an embodiment, ΔT may comprise a predetermined or random variable (e.g., integer) multiplied by the duration for a TTI of system800. A TTI comprises a unit of time over which an uplink or downlink transmission may be scheduled (e.g., for an access node or a wireless device). Here, the TTI may be uniform for the multiple cells of access node808. The variable may be predetermined based on historical data or may be theoretically derived.

At step1108, the interference metric for the first wireless device may continue to be monitored after the scheduled transmissions at different times. For example, the interference metric for wireless device806may continue to be monitored after transmissions are sent to wireless devices802and806from access node808in accordance with the different scheduled timings for the wireless devices. In some embodiments, the different scheduled timings for transmission to wireless devices802and806will impact the interference experienced for communication between wireless device806and the cell of access node808at least because wireless device802was identified as an interference source for this communication.

At step1110, it may be determined whether the monitored interference metric for the first wireless device continues to exceed the interference criteria. For example, the monitored interference metric for wireless device806after the scheduled transmissions at different times may be compared to the interference criteria. In an embodiment, the interference metric for wireless device806may comprise a packet error rate (PER) and the interference criteria may be a PER threshold. Other examples of the interference metric and interference criteria may be implemented consistent with this disclosure. For instance, one or more of a BLER for wireless device806and a number of retransmission attempts from access node808to wireless device806may be monitored and compared to an interference criteria.

At step1112, a delta time period for scheduled timings for transmissions to the first and second wireless devices may be increased when it is determined that the monitored interference metric for the first wireless device continues to exceed the interference criteria. For example, a delta time period between scheduled timings for transmissions to wireless devices802and806may be increased when it is determined that the monitored interference metric for wireless device806continues to exceed the interference criteria. The determination that the monitored interference metric for wireless device806continues to exceed the interference criteria may indicate that communication between wireless device806and the cell of access node808continues to suffer from grating lobe interference.

As described herein, the delta time period may comprise a predetermined or random variable (e.g., integer) multiplied by the duration for a TTI of system800. To increase the delta time period, the variable may be increased (e.g., the integer value may be incremented) or the delta time period may be multiplied by a weight (e.g., 1.25, 1.5, 1.75, 2, and the like).

In an embodiment, based on the increased delta time period, communication between wireless device806and access node808should experience less interference. For example, a first of wireless devices802and806may be scheduled a transmission time of T1. Here, the scheduler for the second of wireless devices802and806may schedule a transmission to the second wireless at a time that is at least a delta time ΔT different from T1, where the delta time has been increased based on an incremented variable, multiplied weight, or any other suitable increase. In an embodiment, the larger delta time between scheduled transmissions for wireless device802and wireless device806will further reduce interference for communication between wireless device806and the cell of access node808caused by signals transmitted to wireless device802.

Although the methods described perform steps in a particular order for purposes of illustration, the methods discussed herein are not limited to any particular order or arrangement. One skilled in the art, using the disclosure provided herein, will appreciate that various steps of the methods can be omitted, rearranged, combined, and/or adapted in various ways.

FIG. 12illustrates an exemplary processing node1200in a communication system. Processing node1200comprises communication interface1202, user interface1204, and processing system1206in communication with communication interface1202and user interface1204. Processing node1200can be configured to determine a communication access node for a wireless device. Processing system1206includes storage1208, which can comprise a disk drive, flash drive, memory circuitry, or other memory device. Storage1208can store software1210which is used in the operation of the processing node1200. Storage1208may include a disk drive, flash drive, data storage circuitry, or some other memory apparatus. Software1210may include computer programs, firmware, or some other form of machine-readable instructions, including an operating system, utilities, drivers, network interfaces, applications, or some other type of software. Processing system1206may include a microprocessor and other circuitry to retrieve and execute software1210from storage1208. Processing node1200may further include other components such as a power management unit, a control interface unit, etc., which are omitted for clarity. Communication interface1202permits processing node1200to communicate with other network elements. User interface1204permits the configuration and control of the operation of processing node1200.

Examples of processing node1200include controller node708and gateway node710. Processing node1200can also be an adjunct or component of a network element, such as an element of access nodes106or706and the like. Processing node1200can also be another network element in a communication system. Further, the functionality of processing node1200can be distributed over two or more network elements of a communication system.

The above description and associated figures teach the best mode of the invention. The following claims specify the scope of the invention. Note that some aspects of the best mode may not fall within the scope of the invention as specified by the claims. Those skilled in the art will appreciate that the features described above can be combined in various ways to form multiple variations of the invention, and that various modifications may be made to the configuration and methodology of the exemplary embodiments disclosed herein without departing from the scope of the present teachings. Those skilled in the art also will appreciate that various features disclosed with respect to one exemplary embodiment herein may be used in combination with other exemplary embodiments with appropriate modifications, even if such combinations are not explicitly disclosed herein. As a result, the invention is not limited to the specific embodiments described above, but only by the following claims and their equivalents.