Patent Grant
6992621
This disclosure relates in general to wireless communication and beam forming using passive beamformers and in particular, by way of example but not limitation, to improving at least one aspect of wireless communication by depopulating one or more ports of a passive beamformer and/or by increasing the order of a passive beamformer such as a Butler matrix.
In wireless communication, signals are sent from a transmitter to a receiver using electromagnetic waves that emanate from an antenna. These electromagnetic waves may be sent equally in all directions or focused in one or more desired directions. When the electromagnetic waves are focused in a desired direction, the pattern formed by the electromagnetic wave is termed a “beam” or “beam pattern.” Hence, the production and/or application of such electromagnetic beams are typically referred to as “beamforming.”
Beamforming may provide a number of benefits such as greater range and/or coverage per unit of transmitted power, improved resistance to interference, increased immunity to the deleterious effects of multipath transmission signals, and so forth. Beamforming can be achieved (i) using a finely tuned vector modulator to drive each antenna element to thereby arbitrarily form beam shapes, (ii) by implementing full adaptive beam forming, and (iii) by connecting a transmit/receive signal processor to each port of a Butler matrix.
A traditional Butler matrix is a passive device that forms beams of a pre-determined size and shape that emanate from an antenna array that is connected to the Butler matrix. The Butler matrix includes a first set of ports that connect to the antenna array and a second set of ports that connect to multiple transmit/receive signal processors. The first set of ports are denoted as antenna ports, and the second set of ports are denoted as transmit/receive ports. The number of ports in each of the first and second sets may be considered to determine the order of the Butler matrix. While not required, Butler matrices typically have an order that is a power of two, such as 4, 8, 16, 32, and so forth. In a conventional wireless communications environment, every port of the set of antenna ports of a Butler matrix is connected to an antenna element, and every port of the set of transmit/receive ports of a Butler matrix is connected to a signal processor.
By way of example, a Butler matrix may have an order of 16. In this case, there are 16 transmit/receive signal processors connected to the 16 transmit/receive ports of the Butler matrix, and there are 16 antenna elements connected to the 16 antenna ports of the Butler matrix. In operation, multiple individual beams of a fixed size and shape emanate from the antenna array. Signals transmitted in and received from each of the respective 16 beams map to a predetermined one of the 16 signal processors on the 16 transmit/receive ports of the Butler matrix. Thus, there is a one-to-one correspondence between (i) each beam formed by the combination of the Butler matrix and the antenna array and (ii) each signal processor that is connected to the Butler matrix.
Accordingly, there is a need for schemes and/or techniques for improving the variety and versatility of wireless communication and beamforming options.
Improving at least one aspect of wireless communication and beamforming is enabled by depopulating one or more ports of a passive beamformer such as a Butler matrix and/or by increasing the order thereof. In conjunction with such depopulation, one or more signal selection schemes may be employed to select a transmit/receive (TRX) port for wireless communication from among multiple TRX ports of a passive beamformer.
In an exemplary described access station implementation, an access station for wireless communications includes: a Butler matrix that has “M” antenna ports and “N” TRX ports; wherein at least a portion of the “M” antenna ports and/or at least a portion of the “N” TRX ports are depopulated.
In another exemplary described access station implementation, an access station for wireless communications includes: a Butler matrix that has multiple antenna ports and multiple TRX ports; a signal processor; and a signal selection device that is capable of coupling the signal processor to a subset of the multiple TRX ports responsive to a signal quality determination, the signal selection device adapted to switch the signal processor from a first TRX port of the subset of TRX ports to a second TRX port of the subset of TRX ports.
In yet another exemplary described access station implementation, an access station for wireless communications includes: a passive beamformer having multiple antenna ports and multiple TRX ports; and an antenna array having multiple antenna elements that are coupled to at least a portion of the multiple antenna ports of the passive beamformer, the multiple TRX ports numbering more than the multiple antenna elements; wherein signals that are applied to the multiple TRX ports of the passive beamformer are transceived on multiple communication beams that are formed jointly by the passive beamformer and the antenna array, and wherein the access station is adapted to have an aiming resolution for communication beams of the multiple communication beams that is finer than a width of a narrowest communication beam of the multiple communication beams.
In an exemplary described method implementation, a method for an access station includes the actions of: comparing a first signal quality from a first communication beam to a second signal quality from a second communication beam; if the first signal quality is greater than the second signal quality, then transceiving from a first TRX port of a Butler matrix; and if the second signal quality is greater than the first signal quality, then transceiving from a second TRX port of the Butler matrix.
Other method, system, apparatus, access station, Butler matrix, arrangement, etc. implementations are described herein.
The same numbers are used throughout the drawings to reference like and/or corresponding aspects, features, and components.
With respect to a Wi-Fi wireless communications system, access station 102 and/or remote clients 104 may operate in accordance with any IEEE 802.11 or similar standard. With respect to a cellular system, access station 102 and/or 11 remote clients 104 may operate in accordance with any analog or digital standard, including but not limited to those using time division/demand multiple access (TDMA), code division multiple access (CDMA), spread spectrum, some combination thereof, or any other such technology.
Access station 102 may be, for example, a nexus point, a trunking radio, a base station, a Wi-Fi switch, an access point, some combination and/or derivative thereof, and so forth. Remote clients 104 may be, for example, a hand-held device, a desktop or laptop computer, an expansion card or similar that is coupled to a desktop or laptop computer, a personal digital assistant (PDA), a car having a wireless communication device, a tablet or hand/palm-sized computer, a portable inventory-related scanning device, some combination thereof, and so forth. Remote clients 104 may operate in accordance with any standardized and/or specialized technology that is compatible with the operation of access station 102.
Access station 202 includes wireless I/O unit 206. Wireless I/O unit 206 includes an antenna array 208 that is implemented as two or more antennas, and optionally as a phased array of antennas. Wireless I/O unit 206 is capable of transmitting and/or receiving (i.e., transceiving) wireless communication(s) 106 via antenna array 208. These wireless communication(s) 106 are transmitted to and received from (i.e., transceived with respect to) remote client 104.
Exemplary Butler matrix 302 is a passive device that forms, in conjunction with antenna array 208, communication beams using signal combiners, signal splitters, and signal phase shifters. Butler matrix 302 includes a first side with multiple antenna ports (designated by “A”) and a second side with multiple transmit and/or receive signal processor ports (designated by “TRX”). The number of antenna ports and TRX ports indicate the order of the Butler matrix. Butler matrix 302 includes 16 antenna ports and 16 TRX ports. Thus, Butler matrix 302 has an order of 16.
Although Butler matrix 302 is so illustrated, antenna ports and TRX ports need not be distributed on separate, much less opposite, sides of a Butler matrix. Also, although not necessary, Butler matrices usually have an equal number of antenna ports and transmit and/or receive signal processor ports (or TRX ports). Furthermore, although Butler matrices are typically of an order that is a power of two (e.g., 2, 4, 8, 16, 32, 64 . . . 2n), they may alternatively be implemented with any number of ports.
The sixteen antenna ports of Butler matrix 302 are numbered from 0 to 15. Likewise, the sixteen TRX ports are numbered from 0 to 15. Antenna ports 0, 1 . . . 14, and 15 are coupled to and populated with sixteen antennas 208(0), 208(1). 208(14), and 208(15), respectively. Likewise, TRX ports 0, 1 . . . 14, and 15 are coupled to and populated with sixteen signal processors 304(0), 304(1) . . . 304(14), and 304(15), respectively. These signal processors are also directly or indirectly coupled to baseband processors 306 as indicated by the dashed lines. It should be noted that one or more active components (e.g., a power amplifier (PA), a low-noise amplifier (LNA), etc.) may also be coupled on the antenna port side of Butler matrix 302.
In an exemplary transmission operation, communication signals are provided from baseband processors 306 to the multiple transmit and/or receive signal processors (SP) 304. The multiple signal processors 304 forward the communication signals to the TRX ports 0, 1 . . . 14, and 15 of Butler matrix 302. After signal combination, signal splitting, and signal phase shifting, Butler matrix 302 outputs communication signals on the antenna ports 0, 1 . . . 14, and 15. Individual antennas 208 wirelessly transmit the communication signals, as altered by Butler matrix 302, from the antenna ports in predetermined beam patterns. The beam patterns are predetermined by the shape, orientation, constituency, etc. of antenna array 208 and by the alteration of the communication signals as “performed” by Butler matrix 302. In addition to transmissions, wireless signals such as wireless communications 106 (of
From the sixteen antennas 208(0) . . . 208(15), sixteen different communication beams 402(0) . . . 402(15) are formed as the wireless signals emanating from antennas 208 add and subtract from each other during electromagnetic propagation. Communication beams 402(1) . . . 402(15) spread out symmetrically from the central communication beam 402(0). The narrowest beam is the central beam 402(0), and the beams become wider as they spread outward from the center. For example, beam 402(15) is slightly wider than beam 402(0), and beam 402(5) is wider than beam 402(15). Also, beam 402(10) is wider still than beam 402(5).
The indices 0 . . . 15 for the sixteen different communication beams 402(0) . . . 402(15) may correspond to the indices 0 . . . 15 of the antenna ports of Butler matrix 302 as well as the indices 0 . . . 15 of the TRX ports thereof. However, no single communication beam 402(x) necessarily corresponds to a single antenna port x of Butler matrix 302 because each communication beam 402 is formed from the interplay of electromagnetic radiation with respect to multiple, including all, of the antennas of antenna array 208.
Due to real-world effects of the interactions between and among the wireless signals as they emanate from antenna array 208 (e.g., assuming a linear antenna array in a described implementation), communication beam 402(8) is degenerate such that its beam pattern is formed on both sides of antenna array 208. These real-world effects also account for the increasing widths of the other beams 402(1 . . . 7) and 402(15 . . . 9) as they spread outward from central beam 402(0).
The beam widths of the sixteen beams 0 . . . 15 are indicated in degrees within the ovals of
In a described implementation, all sixteen beams 0 . . . 15 are not utilized for wireless communications. Specifically, beams 7 and 9 are not used because they 8 are too wide and/or indiscriminate to be sufficiently beneficial. Furthermore, beam 8 is also ignored because its degenerate nature makes it even more difficult for it to be effectively utilized. These unused beams 7, 8, and 9 are indicated by dashed lines in
An access station 202 (of
It should be noted that beams 7, 8, and 9 need not be ignored and that the TRX ports 7, 8, and 9 of a Butler matrix 302 may be populated with signal processors (SP) 304 even if the beams 7, 8, and 9 are ignored. Also, if a Butler matrix 302 is of an order other than 16, then different communication beams and possibly a different total number of such communication beams may be ignored for efficiency and/or simplicity reasons when such different communication beams are too indiscriminate and/or too degenerate.
Specifically, signal processor 304(0) is coupled to TRX port 0 or 1, and signal processor 304(1) is coupled to TRX port 2 or 3. Similarly, signal processor 304(6) is coupled to TRX port 12 or 13, and signal processor 304(7) is coupled to TRX port 14 or 15. Each signal processor 304 is able to switch between being coupled to one of two TRX ports as specifically indicated by the dashed arrows at signal processor 304(0). This switching may be based, for example, on some quality measure. Exemplary approaches and methods for switching between TRX ports based on one or more quality measures are described further below with reference to
By way of example, signal processor 304(0) may transceive communication signals via TRX port 0 or TRX port 1 of Butler matrix 302. When coupled to TRX port 0, signal processor 304(0) “sees” (e.g., is able to transceive wireless communications via) a communication beam 0 that is formed by the combined action/configuration of Butler matrix 302 and antenna array 208. On the other hand, when coupled to TRX port 1, transceiver 304(0) sees a communication beam 1 that is formed by the combined action/configuration of Butler matrix 302 and antenna array 208. Other signal processors 304 may similarly see two different communication beams one beam at a time.
More specifically, for an implementation that is described also with reference to
A single signal processor 304 such as signal processor 304(0) is thus able to see two different antenna beam patterns, such as beams 402(0) and 402(1) (as shown in
In this described implementation, financial resources can thus be conserved by depopulating half of the TRX ports of a Butler matrix 302. This depopulation precipitates several effects. For example, in addition to switching overhead and/or delays, there is a concomitant reduction in simultaneous signal handling capability at access station 202 (of
Half of the sixteen antenna ports 0 . . . 15 of Butler matrix 302 are thus depopulated and the other half are populated. Specifically, antenna 208(0) is coupled to antenna port 0, and antenna 208(1) is coupled to antenna port 2. Similarly, antenna 208(6) is coupled to antenna port 12, and antenna 208(7) is coupled to antenna port 14. In other words, antennas 208(0 . . . 7) are coupled to antenna ports 0, 2, 4, 6, 8, 10, 12, and 14, respectively, of Butler matrix 302.
Assuming that other spatial parameters are maintained (e.g., that the distance between adjacent antenna elements of antenna array 208 are relatively unchanged), the width of each individual communication beam (not explicitly shown in
In other words, each of the sixteen different communication beams of a half-way populated Butler matrix 302 is approximately twice as wide as it would be if Butler matrix 302 were fully populated. For example, central communication beam 402(0) (of
Each of the sixteen signal processors of signal processors 304(0 . . . 15) may elect to effectively see half of one of these sixteen communication beams that are twice as wide as they would be if the sixteen antenna ports 0 . . . 15 of Butler matrix 302 were fully populated. More specifically, each signal processor 304 may actually transceive signals across the entire (e.g., 12° for a central beam) width of the communication beam. However, the beam steering resolution is finer than the beam width. In this case, the beam steering can occur in 6° increments while the beam width is at least 12°.
Hence, as desired and/or as detected from a signal quality perspective, signal processors 304 can elect to transceive over only the central half of each 12°-wide communication beam where the signal power is strongest. If the signal is being transceived to/from a point that is located outside this central portion of a communication beam, then a signal processor 304 (and/or a TRX port) that corresponds to an adjacent beam can assume transceiving responsibilities with respect to the central portion of the adjacent communication beam, especially if the signal quality of the resulting transceived signal is superior in the adjacent communication beam. In other words, the aiming resolution for the different communication beams as seen at the TRX ports of Butler matrix 302 of
Thus, each signal processor 304 that is connected to a different TRX port of Butler matrix 302 is associated with a different communication beam that is emanating from antennas 208(0 . . . 7). Although each such different communication beam is 12° wide, the respective peaks of the different communication beams may be directionally pointed every 6°. Analogous situations are described further below with particular reference to
In this described implementation, antenna array cost, size, and complexity can be reduced by depopulating half of the antenna ports of a Butler matrix 302. This depopulation precipitates several effects. For example, although the number of communication beams emanating from the antenna array remains constant, the width of each communication beam doubles and the overlap between communication beams increases. However, the beam steering capability of a related wireless I/O unit 206 maintains the same directionality resolution from the perspective of angular aiming precision for each signal processor 304. In other words, the number of pointing directions to which the communication beams can be aimed does not change.
Although the communication beams (not explicitly shown in
However, an individual signal processor 304 is not assigned to each TRX port full time. Instead, every two TRX ports share a single signal processor 304. Each signal processor 304 switches between being coupled (physically, operationally, and/or functionally) to one of two TRX ports as again indicated by the dashed lines at signal processor 304(0). This aspect of
The individual effects of depopulating the antenna ports and of depopulating the TRX ports of Butler matrix 302 are thus jointly experienced by the permutation of
Four antennas 208 are coupled to four different antenna ports, and four signal processors 304 are coupled to sixteen different TRX ports. Specifically, the four antennas 208(0), 208(1), 208(2), and 208(3) are coupled to the four antenna ports 3, 7, 11, and 15, respectively. Also, the four signal processors 304(0), 304(1), 304(2), and 304(3) are coupled to the sixteen TRX ports 0/1/2/3, 4/5/6/7, 8/9/10/11, and 12/13/14/15, respectively, taken four at time.
Each of the communication beams (not explicitly shown in
The sixteen TRX ports 0 . . . 15 are coupled to four different signal processors 304(0 . . . 3) such that only four of the sixteen TRX ports 0 . . . 15 are being used to transceive communication signals at any one moment. The particular TRX port of four possible TRX ports to which a given individual signal processor 304 is coupled is effectuated by a switching mechanism that is described further below with reference to
Thus, a wireless I/O unit 206 implementation may include a Butler matrix 302 that has been three-quarters depopulated with respect to either or both of the antenna ports and the TRX ports. It should be noted that other depopulation proportions besides one-half and three-quarters may alternatively be employed. Furthermore, such depopulation proportions need not be related to a power of two even though the complexity of such implementations that do deviate from a power of two consequently increases.
Specifically, the sixteen antennas 208(0) . . . 208(2) . . . 208(12) . . . 208(15) are coupled to sixteen antenna ports 0 . . . 4 . . . 24 . . . 30, respectively, of the thirty-two total antenna ports 0 . . . 31. Also, the sixteen signal processors 304(0). 304(2) . . . 304(14), and 304(15) are coupled to the thirty-two TRX ports 0/1 . . . 4/5 . . . 28/29, and 30/31, respectively, taken two at time.
With this permutation, supplanting a passive 16×16 Butler matrix 302 with a passive 32×32 Butler matrix 302 adds little to the cost of a wireless I/O unit 206 (of
Specifically, the steering resolution for antenna array 208 of
Other antenna array 208 and Butler matrix 302 configurations can alternatively be implemented. For example, a sixteen element antenna array 208 like that of
The various permutations of
As alluded to above individually, various Butler matrix port population configurations relate to various effects. Assume that a Butler matrix is fully populated at both its antenna ports and its TRX ports in an original configuration. For a first permutation, the TRX ports of the Butler matrix are depopulated, but the population of the antenna ports is unchanged. In this case, the cost of implementing such a permutation may be decreased by eliminating signal processors. Furthermore, the gain as well as the coverage and range may be maintained at a level comparable to that of the original, fully-populated state. There may be, however, a small performance penalty with respect to the number of remote clients that can be simultaneously serviced.
For a second permutation, the antenna ports of the Butler matrix are depopulated, but the population of the TRX ports is unchanged. In this case, the widths of the multiple communication beams are increased (e.g., doubled), but the signal processors can effectively steer each beam at an angular differential that is less than the beam widths. Thus, the same beam aiming resolution may be maintained because steering directionality is controllable at a resolution that is finer than the beam width.
In a third permutation, neither the antenna ports nor the TRX ports are depopulated, but the order of the Butler matrix is increased. The cost is approximately unchanged because Butler matrices are inexpensive relative to the remaining components of a wireless access station. Although the coverage area remains approximately the same, the gain and the range both increase. This increase can be approximately 40% when the order of a Butler matrix is doubled.
The M antennas 208(0), 208(1) . . . 208(M−1), which together form an antenna array 208, operate in combination with Butler matrix 302 to form multiple communication beams of a communication beam pattern 1106. In a described implementation and as illustrated, antenna array 208 and Butler matrix 302 jointly form N communication beams 1106(0), 1106(1) . . . 1106(N−1). Although not so illustrated, these N communication beams 1106(0 . . . N−1) may form an overall beam pattern identical, similar, and/or analogous to that of
Signal processor (SP) 304(0) is indirectly coupled to Butler matrix 302 by way of signal selection device 1102. Signal selection device 1102 selects the TRX port to which signal processor 304(0) should be coupled from among two or more TRX ports of Butler matrix 302. Signal selection device 1102 thus enables one or more signal processors 304 to implement or facilitate one or more kinds of signal selection schemes (e.g., such as those based on diversity) with respect to different communication beams 1106.
In the illustrated implementation, signal selection device 1102 selects from between TRX ports 0 and 1 of Butler matrix 302 for signal processor 304(0) as indicated by the dashed lines. This selection is made responsive to one or more communication signals from remote clients 104 (of
Signal quality determiner 1104 determines the signal quality of transceived signals as present at TRX port 0 and TRX port 1. This signal quality may include and/or relate to signal-to-noise ratio (SNR), interference level(s), multi-path variable(s) (e.g., a lowest delay spread), some combination thereof, and so forth. After signal quality determiner 1104 measures or otherwise determines at least one signal quality, signal selection device 1102 may analyze the determined signal quality in order to select the better (or best) TRX port.
In the illustrated implementation, signal selection device 1102 interprets the signal quality to select TRX port 0 or TRX port 1. For example, signal selection device 1102 may select the port having the better signal quality. This signal quality may reflect the better of two versions of a single signal from a single remote client 104, the better of two different signals from two different remote clients 104, the better communication beam 1106 (e.g., communication beam 1106(0) or 1106(1)) for transceiving a single signal from a single remote client 104, and so forth. Both of signal selection device 1102 and signal quality determiner 1104 may be comprised of hardware, software, firmware, some combination thereof, and so forth.
Flow diagram 1200 includes eight blocks 1202–1216 that may be implemented with any appropriate hardware, software, firmware, some combination thereof, and so forth and with any appropriate signal selection scheme. However, to improve clarity an exemplary implementation of the method of flow diagram 1200 is described with particular reference to
It should be noted (i) that the order in which the multiple blocks 1202–1216 are illustrated and/or described is not intended to be construed as a limitation and (ii) that the actions of any number of the described blocks, or portions thereof, can be combined or rearranged in any order to implement one or more methods for improving wireless communication and/or beamforming with Butler matrices.
At block 1202, a signal quality determiner is switched to a first TRX port of a Butler matrix. For example, signal quality determiner 1104 may be switched to TRX port 0 of Butler matrix 302 (of
At block 1206, the signal quality determiner is switched to a second TRX port of the Butler matrix. For example, signal quality determiner 1104 may be switched to TRX port 1 of Butler matrix 302. At block 1208, a signal quality from a second beam of the Butler matrix (in conjunction with the antenna array that is coupled thereto) is determined. For example, a second signal quality of a signal that is being transmitted or received within or proximate to communication beam 1106(1) is determined using signal quality determiner 1104. The determined first and second signal qualities may relate to the same signal with respect to the different communication beams 1106(1) and 1106(2), to different versions of the same signal, to different signals, and so forth.
At block 1210, the signal quality from the first beam of the Butler matrix is compared to the signal quality from the second beam of the Butler matrix. For example, signal selection device 1102 may compare the first signal quality that is related to communication beam 1106(0) to the second signal quality that is related to communication beam 1106(1). At block 1212, it is determined from the comparison whether the signal quality from the first beam of the Butler matrix is greater than the signal quality from the second beam of the Butler matrix. This determination may be accomplished, for example, by signal selection device 1102 determining a greater of two values for SNR, for interference level(s), for multi-path variable(s), some combination thereof, and so forth.
If the signal quality from the first beam of the Butler matrix is greater than the signal quality from the second beam of the Butler matrix (as determined at block 1212), then the first TRX port of the Butler matrix is selected for transceiving at block 1214. For example, signal selection device 1102 may couple signal processor 304(0) to TRX port 0 of Butler matrix 302. If, on the other hand, the signal quality from the first beam of the Butler matrix is not determined to be greater than the signal quality from the second beam of the Butler matrix, then the second TRX port of the Butler matrix is selected for transceiving at block 1216. For example, signal selection device 1102 may couple signal processor 304(0) to TRX port 1 of Butler matrix 302.
In a described implementation, the actions of the eight (8) blocks 1202–1216 are performed when at least one signal is present at one or more TRX ports. Any of many possible schemes may be implemented between the arrival of signals and/or for detecting a signal, as indicated by arrows 1218(A), 1218(B), and 1218(C). For example, a signal quality may be measured on each TRX port until a signal is detected. The signal quality for the detected signal is then determined on at least two TRX ports (and possibly over all TRX ports) to determine the better or best TRX port for receiving the signal. That better or best TRX port is then used for that signal until the transmission ceases, or until another signal quality measuring across multiple TRX ports is warranted (e.g., because of signal quality degradation, a timer expiration, etc.). The signal quality measuring/detecting may then continue and/or may also be continuing while the actions of flow diagram 1200 are occurring.
The implementations described hereinabove and illustrated in FIGS. 3 and 6–12 focus on a Butler matrix as an exemplary passive beamformer. However, other realizations for a passive beamformer may alternatively be used. For example, in addition to a Butler matrix, a passive beamformer may be implemented as a Rotman lens, a canonical beamformer, a lumped-element beamformer with static or variable inductors and capacitors, and so forth. For instance, a first Rotman lens with “x” TRX ports and “y” antenna ports can be substituted with a second Rotman lens with “x+w” (where w is positive) TRX ports to achieve a finer beam aiming resolution.
Although methods, systems, apparatuses, arrangements, schemes, approaches, and other implementations have been described in language specific to structural and functional features and/or flow diagrams, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features or flow diagrams described. Rather, the specific features and flow diagrams are disclosed as exemplary forms of implementing the claimed invention.
| Number | Name | Date | Kind |
|---|---|---|---|
| 3295134 | Lowe | Dec 1966 | A |
| 3836970 | Reitzig | Sep 1974 | A |
| 3858218 | Masak et al. | Dec 1974 | A |
| 4231040 | Walker | Oct 1980 | A |
| 4639732 | Acoraci et al. | Jan 1987 | A |
| 5115248 | Roederer | May 1992 | A |
| 5162804 | Uyeda | Nov 1992 | A |
| 5610617 | Gans et al. | Mar 1997 | A |
| 5666123 | Chrystie | Sep 1997 | A |
| 6104935 | Smith et al. | Aug 2000 | A |
| 6167036 | Beven | Dec 2000 | A |
| 6252544 | Hoffberg | Jun 2001 | B1 |
| 6340948 | Munoz-Garcia et al. | Jan 2002 | B1 |
| 6353410 | Powell | Mar 2002 | B1 |
| 6397082 | Searle | May 2002 | B1 |
| 6429812 | Hoffberg | Aug 2002 | B1 |
| 6470192 | Karlsson et al. | Oct 2002 | B1 |
| 20010016504 | Dam et al. | Aug 2001 | A1 |
| 20020163478 | Pleva et al. | Nov 2002 | A1 |
| Number | Date | Country | |
|---|---|---|---|
| 20040174299 A1 | Sep 2004 | US |
