Communications handoff using an adaptive antenna

One implementation provides a novel station-to-station handoff using an adaptive or directional antenna. The directional antenna generates a plurality of directional search beams to search for pilot signals from new base stations. The search beams are scanned one beam at a time to obtain the relative direction of all new base stations detected. From the detected base stations, a new base station with the strongest signal strength is selected and compared to the currently used base station. If the new base station has stronger signal strength than the current base station, then a communication handoff from the current base station to the new base station is initiated. In one feature of the present invention, two directional beams are used to communicate with the current and new base stations. Once a communication link is established with the new base station, the directional beam to the current base station is terminated.

REFERENCE TO CO-PENDING APPLICATIONS FOR PATENT

The present Application for Patent is related to the following co-pending U.S. patent application Ser. No. 11/182,236, entitled “Antenna Array Pattern Distortion Mitigation”, filed concurrently herewith, assigned to the assignee hereof, and expressly incorporated by reference herein.

BACKGROUND

Various embodiments of the invention pertain to wireless communication systems. At least one embodiment of the invention pertains to a system and method of transferring communications from a first base station to a second base station from a mobile communication device using a directional antenna.

Wireless communication networks enable communication devices to transmit and/or receive information while on the move. These wireless communication networks may be communicatively coupled to other public or private networks to enable the transfer of information to and from the mobile communication devices. Such communication networks typically include a plurality of base stations which provide wireless communication links to mobile communication devices. The base stations may be stationary (e.g., fixed to the ground) or mobile (e.g., mounted on satellites, etc.) and positioned to provide wide area coverage as the mobile communication device travels across different coverage areas.

As a mobile communication device moves around, its communication link with a base station may degrade. In this situation, the communication device switches or connects with another base station for a better quality communication link while its first link is still active. This process of establishing a communication link with another base station is referred to as a “handoff”. The handoff process typically faces the problem of maintaining a reliable communication link with the wireless communication network while switching base stations. Soft handoffs and hard handoffs are two commonly used types of handoffs. A soft handoff is one where a new communication link with a new base station is established before the existing communication link is terminated. In a hard handoff, an existing communication link is typically terminated before a new communication link is established.

Some mobile communication devices may utilize a directional or adaptive antenna. Directional and/or adaptive antennas are typically used to direct signal transmissions in a desired direction. These types of antennas have many advantages over omni-directional antennas when used in modern communications systems. These advantages occur for both transmission and reception of information-bearing signals. During transmission, the directional concentration of radiated energy beam towards a receiver significantly increases the amount of received power per unit of transmitted power. This generally improves the throughput rate of the transmitter-to-receiver link and allows higher rates of information transfer. Furthermore, in interference-limited systems the concentration of power towards the intended receiver reduces the interference caused by the transmitter to the rest of the system, hence increasing its overall capacity. However, performing an efficient handoff using an adaptive or directional antenna has several challenges since directional beams make it difficult to switch between receivers (e.g., base stations) while maintaining a reliable link at the same time.

SUMMARY

A communication device is provided having a first antenna configured to operate as a directional antenna and a control circuit communicatively coupled to the first antenna. The control circuit is configured to (a) establish a first communication link with a first base station via the first antenna, (b) search for a second base station, and (c) determine if a communication handoff should be performed from the first base station to the second base station. The control circuit is also configured to (a) initiate a communications handoff from the first base station to the second base station, (b) transmit information over the first antenna to the first base station, and (c) receive information over the first antenna from the first base station. Additionally, the control circuit establishes a second communication link with the second base station and terminates the first communication link with the first base station. The control circuit may search for the second base station by periodically, continuously, or sporadically scanning for pilot signals from base stations and stores the directional and signal strength information of the base stations found during the search. Searching for the second base station may include configuring the first antenna to generate a plurality of directional search beams, and scan each directional search beam for pilot signals from base stations. The plurality of directional search beams may be scanned one beam at a time to obtain the relative direction of the base stations and/or span all directions of the first antenna to obtain the relative direction of the base stations. The plurality of directional search beams may corresponds to predefined discrete sectors of the first antenna. According to one implementation, searching for the second base station includes obtaining known locations of one or more base stations and configuring the first antenna to search for at least one of the one or more base stations based on the known locations.

In one implementation, a second antenna is communicatively coupled to the control circuit and configured to search for the second base station. During a communications handoff, a second communication link is established with the second base station via the second antenna. If a signal is received indicating that the second base station is ready to communicate over the second communication link, the control circuit is further configured to switch the second communication link to a directional beam on the first antenna directed to the second base station.

Determining whether a communication handoff should be performed from the first base station to the second base station may include determining the signal strength of the first base station, determining the signal strength of the second base station, and initiating the communications handoff from the first base station to the second base station if the signal strength of the second base station is greater than the signal strength of the first base station. In one example, the communications handoff from the first base station to the second base station is initiated if the signal strength of the first base station is below a threshold value. The communication device may simultaneously transmit to both the first base station and the second base station using different directional beams. The control circuit may be configured to reconfigure the first antenna to operate as an omni-directional antenna during the communications handoff, and reconfigure the first antenna to operate as a directional antenna once a second communication link is established with the second base station. In one implementation, the first base station and second base stations are part of an aeronautical communications network.

A hard handoff may be initiated from the first base station to the second base station which includes terminating the first communication link with the first base station, and then establishing a second communication link with the second base station. Establishing the second communication link may include setting up a physical layer call between the communication device and the second base station.

Another implementation provides a wireless communication device comprising first means for adaptive wireless communications and control means coupled to the first means for adaptive wireless communications. The control means may be configured to establish a first communication link with a first base station, search for a new base station, and determine if a communication handoff should be performed from the first base station to a second base station. If a communication handoff is warranted, then the control means initiates a communication handoff from the first base station to the second base station, establishes a second communication link with the second base station, and, if a signal is received indicating that the second base station is ready to communicate over the second communication link, transmits over the second communication link and terminates the first communication link. The wireless communication device may also include a second means for wireless communications communicatively coupled to the control means, wherein the second means for wireless communications is employed to search for the new base station. The wireless communication device may be further configured to reconfigure the first means for adaptive wireless communications to operate as an omni-directional antenna during the communication handoff and reconfigure the first means for adaptive wireless communications to operate as a directional antenna once a second communication link is established with the second base station.

Another feature provides a method for switching communications between base stations using an adaptive antenna, including the steps of (a) establishing a first communication link with a current base station over the adaptive antenna, (b) searching for a new base station, (c) determining if a communication handoff should be performed from the current base station to the new base station, (d) initiating a communications handoff from the current base station to the new base station, (e) establishing a second communication link with the new base station, and (f) maintaining the first communication link with the current base station until the new base station is ready to communicate over the second communication link. The method for switching communications between base stations may further include transmitting over the second communication link through the adaptive antenna and terminating the first communication link once the new base stations begins receiving on the second communication link. The first communication link and second communication link may both be performed by the adaptive antenna. Additionally, the adaptive antenna may be configured to generate a plurality of directional search beams and periodically, continuously, or sporadically scanning each directional search beam for pilot signals from base stations, wherein the search beams are scanned one beam at a time to obtain the relative direction of base stations detected. In another implementation, the first communication link may be established via a first directional beam of the adaptive antenna and the second communication may be established via a second directional beam of a second antenna. Moreover, the adaptive antenna may be reconfigured to operate as an omni-directional antenna during the communications handoff, and reconfigured to operate as a directional antenna once a second communication link is established with the second base station. Known locations of one or more base stations may be obtained and used to configure the adaptive antenna to search for at least one of the one or more base stations.

Another implementation of the invention provides a machine-readable medium comprising instructions executable by a processor for performing a communications handoff between base stations using an adaptive antenna, which when executed by a processor, causes the processor to perform operations comprising (a) establishing a first communication link with a current base station, (b) configuring the adaptive antenna to generate a plurality of directional search beams to search for a new base station, (c) periodically, continuously, or sporadically scanning each directional search beam for pilot signals from base stations, wherein the search beams are scanned one beam at a time to obtain the relative direction of the base stations detected, (d) and determining if a communication handoff should be performed from the current base station to the new base station. If a communication handoff is to be performed, the processor performs the operations comprising (a) simultaneously transmitting to both the current base station and the new base station using different directional beams, (b) establishing a second communication link with the new base station, and (c) maintaining the first communication link with the current base station until the new base station is ready to communicate over the second communication link. Determining if a communication handoff should be performed may include the steps of (a) determining a signal strength of a detected new base station, (b) comparing the signal strength of the new base station to a signal strength for the current base station, and (c) initiating the communication handoff from the current base station to the new base station if the signal strength of the new base station is greater than the signal strength of the current base station. In one example, the communication handoff is a hard handoff where a second communication link with the new base station is established after terminating the first communication link with the current base station. Establishing the second communication link may include setting up a physical layer call between the communication device and the new base station.

Another implementation of the invention provides a communication system comprising (a) a plurality of networked base stations including a first base station and a second base station, and (b) a mobile communication transceiver including an adaptive antenna to wirelessly couple the communication transceiver to a base station, a control circuit communicatively coupled to the adaptive antenna. The control circuit may be configured to (a) establish a first communication link with the first base station, (b) configure the adaptive antenna to generate a plurality of directional search beams to search for new base stations, and (c) periodically, continuously, or sporadically scan each directional search beam for pilot signals from other base stations, wherein the search beams are scanned one beam at a time to obtain the relative direction of base stations detected. Additionally, the communication system may be further configured to (a) determine whether a communication handoff from the first base station to the second base station is warranted, and (b) if such communication handoff is warranted, then simultaneously transmit to the first base station and the second base station. The communication system then establishes a second communication link with the second base station and terminates the first communication link with the first base station when the second base station is ready to communicate over the second communication link.

In one implementation, the communication system includes a base station controller communicatively coupled to the first and second base stations and configured to facilitate transferring a communication link from the first base station to the second base station. The base station controller may be configured to transmit a signal to the mobile communication transceiver via the first communication link indicating that the second base station is ready to communicate over the second communication link.

The mobile communication transceiver may determine whether a communication handoff from the first base station to the second base station is warranted, and if such communication handoff is warranted, then performs a hard handoff by pointing the adaptive antenna to the second base station, and establishing a second communication link with the second base station by setting up a physical layer call between the mobile communication transceiver and the second base station. The mobile communication transceiver may terminate the first communication link with the first base station before establishing the second communication link with the second base station.

In one example, the communication system includes a base station controller communicatively coupled to the first and second base stations and, wherein the base station controller is configured to recognize when the mobile communication transceiver has switched from the first base station to the second base station and route unacknowledged packets addressed to the mobile communication transceiver to the to the second base station. The base station controller may recognize when the mobile communication transceiver has switched from the first base station to the second base station by monitoring for unacknowledged packets addressed to the mobile communication transceiver to the to the first base station.

Yet another implementation of the invention provides a processor comprising (a) an input interface to communicate with an adaptive antenna and (b) a circuit. The circuit being configured to (a) establish a first communication link with a current base station, (b) direct the adaptive antenna to generate a plurality of directional search beams to search for a new base station, (c) periodically, continuously, or sporadically scan each directional search beam for pilot signals from base stations, wherein the search beams are scanned one beam at a time to obtain the relative direction of the base stations detected, and (d) determine if a communication handoff should be performed from the current base station to the new base station. Additionally, the circuit may also be configured to (a) simultaneously transmit to both the current base station and the new base station using different directional beams, (b) establish a second communication link with the new base station, and (c) maintain the first communication link with the current base station until the new base station is ready to communicate over the second communication link.

DETAILED DESCRIPTION

In the following description, specific details are given to provide a thorough understanding of the embodiments. However, it will be understood by one of ordinary skill in the art that the embodiments may be practiced without these specific detail. For example, circuits may be shown in block diagrams in order not to obscure the embodiments in unnecessary detail. In other instances, well-known circuits, structures and techniques may be shown in detail in order not to obscure the embodiments.

Moreover, a storage medium may represent one or more devices for storing data, including read-only memory (ROM), random access memory (RAM), magnetic disk storage mediums, optical storage mediums, flash memory devices and/or other machine readable mediums for storing information. The term “machine readable medium” includes, but is not limited to portable or fixed storage devices, optical storage devices, wireless channels and various other mediums capable of storing, containing or carrying instruction(s) and/or data.

One feature of the invention provides a way of determining whether a communication device should initiate a communications handoff from a first base station to a second base station. Generally, the communication device uses a directional antenna to communicate with its current base station and periodically, continuously, or sporadically scans for better base stations using a plurality of directional beams. The directional beams are scanned one beam at a time to obtain the relative direction and/or signal strength of the base stations detected.

Another feature of the invention provides a system and method for performing a communication handoff between different base stations utilizing a directional antenna. For example, communications from a communication device to a first base station are efficiently switched to a second base station, with minimal or no interruptions, where the communication device uses one or more directional antennas. Generally, two directional transceivers may be configured to generate two directional beams, one for the first base station and one for the second base station. Alternatively, an adaptive antenna is configured to operate as an omni-directional antenna during the handoff process and switched back to a directional antenna, with a directional beam, once a communication link is established with the new base station.

FIG. 1illustrates a wireless communication system having a communication device102that performs a base station search and handoff according to one implementation of the present invention. In one implementation of the invention, communication device102may be a transceiver having an adaptive or directional antenna. Wireless communication device102may include a processing unit or control circuit to manage communication functions and/or configure the directional antenna (via weight vectors, for instance) to form the desired directional beam(s). A storage device or memory device may be coupled to the processing unit or control unit to store antenna settings, base station location information, etc. In some embodiments of the invention, the adaptive or directional antenna may be configured to form one or more continuous or discrete directional beams and/or operate as an omni-directional antenna. According to various implementations of the invention, wireless communication device102may be a consumer device (e.g., cell phone, personal assistant, or computer) and/or a network device (e.g., wired/wireless gateway, relay, and/or modem).

Wireless communication device102may communicate with a first base station104, which may be part of a wireless network. For example, a plurality of base stations104,108,110, and112may be part of an aeronautical communications network that enable communication device102, which may be mounted on an aircraft, to receive and/or transmit information to other devices beyond the aircraft. A base station controller (BSC)114may be communicatively coupled to the plurality of base stations to manage communication setup and forwarding among the base stations.

Communication device102may include an adaptive or directional antenna that permits it to focus its transmitted beam106toward the first base station104. The use of an adaptive or directional antenna has the advantage of focusing the beam(s)106to a desired receiver (e.g., base station104), reducing the amount of power needed for transmission of a signal s1(t), and reducing unwanted interference to other devices.

As communication device102moves from a first location (e.g., Location A) to a second location (e.g., Location B), it may move farther away from the first base station104and closer to other base stations (e.g., base station108). This may occur, for example, when the aircraft on which the communication device is mounted is in flight. As its distance from first base station104increases, the communication link between communication device102and first base station104may degrade. Various features of the present invention provide efficient ways of switching or handing off communications to a second base station with minimal or no interruptions in service.

As communication device102moves around in the coverage area of the wireless network (e.g., base stations104,108,110, and112), it may come within range of other base stations which may provide a better communication link than its existing link with first base station104. Therefore, communication device102may have a mechanism to periodically, continuously, or sporadically monitor if a better base station exists than its current base station (i.e., base station104). Besides using signal strength related metrics, the communication device102may also use position location of itself and/or the base stations104,108,110, and112to determine if a handoff is warranted and/or to which base station the handoff should occur.

FIG. 2illustrates a wireless communication system having a communication device202that communicates with a current base station206while searching for a better base station according to one implementation of the present invention. In order for communication device202to expeditiously determine whether a better base station is available, communication device202may periodically, continuously, or sporadically search for all base stations available in every direction or sectors relative to communication device202. This may be done in a number of ways.

In the most general case, the communication device202may form a receiving directional beam204of a given beamwidth in any direction in azimuth and elevation. Initially, communication device202forms directional beams204covering all sectors (e.g., Sectors1-N) and chooses the sector/beam from which it receives the highest Signal to Interference plus Noise (SINR) ratio with which to communicate. Beams204may be formed by configuring the directional antenna to create beams204in any desired direction. Directional beams204(in Sectors1-N) are scanned one beam at a time to obtain the relative direction and/or signal strength of the base stations detected. That is, the transceiver's operation is divided into timeslots to search for better base stations and to transmit and/or receive with its current base station.

In practice, the communication device's directional antenna may not be able to form beams in any direction but rather in a few discrete directions. Therefore, the best base station to select may be defined from the best beam position or sector that provides the highest SINR. For example, communication device202may utilize a directional antenna having N fixed beam positions, where N is an integer greater than one. Communication device202may scan through each of the N fixed positions to determine whether other base stations are within communication distance.

As part of the wireless network communication system, base stations206,208,210, and212may periodically broadcast a beacon or pilot signal (e.g., Pilot A, Pilot B, Pilot C, and Pilot D) to notify others of their presence. Communication device202may keep track of which base stations it finds at each beam position or sector. The pilot signals may also be used to determine the signal strength or SINR at each sector scanned by communication device202. The SINR is measured across all sectors or beam positions and the sectors or beam positions are ranked based on the SINR values. If there is a new sector or beam whose SINR is better (e.g., higher) or within a threshold of the currently used beam (e.g., beam to base station104), then a handoff to a new base station is initiated.

A handoff may also be initiated if a better base station is identified in the same sector or beam being used to communicate with the current base station. For example, if a pilot signal belonging to a new base station is identified on the same beam as the currently used base station, then the beam or sector used remains the same and signaling is used to change base stations. Two base stations or pilot signals on a single beam may be distinguished based on their different frequencies, base station ID, pilot signal signature, or signaling with the base station controller.

There are several different ways in which communication device202may search for a better base station. In one approach, communication device202schedules certain time periods to carry out the search during which time it does not receive data from the network (e.g., from current base station206). During this time, communication device202switches through all possible receiving beam positions in its antenna or sectors (e.g., Sectors1-N) and measures SINR from all base station pilot beams (e.g. Pilot A, Pilot B, Pilot C, and Pilot D) visible to the antenna beams. The disadvantage of this approach is that, during the search time periods, communication device202stops receiving from and/or transmitting to the current base station (e.g. base station206) causing a delay in communications that may result in loss of data for delay-sensitive applications. However, if the search is completed quickly enough (e.g., a few seconds or less) or the applications (e.g., systems using communication device202to communicate to the network and beyond) can tolerate some delay, then this approach is quite feasible.

Another approach to searching for better base stations is for communication device202to use two receivers, where one receiver is used to periodically, continuously or sporadically search for new base stations and the other receiver is used to maintain communications with current base station206.

An alternative to communication device202searching for new base stations is to use its knowledge of its current location (e.g., geographical position of the aircraft) of communication device202and/or the known locations of the base stations in the network to determine the antenna sector or beam through which to communicate. For instance, communication device202may have or receive information about its current location as well as neighboring base stations and uses this information to create a directional beam toward a selected base station.

Once a new sector or directional beam has been determined to provide better performance than the currently used sector or beam to current base station206, communication device202initiates a handoff to new station208associated with the new sector or antenna beam. Where a new base station is found on the same sector or receiving beam as the current base station, then the handoff may be done within the same sector or beam.

Once a better base station has been identified, the difficulty is in achieving a handoff to the new base station using a directional antenna. During the handoff, the communication link with the current base station is maintained to provide uninterrupted service while the new communication link with the new base station is established. Both the current and new base stations should receive signals from the communication device during the handoff process. This is quite challenging to do using a directional antenna. In various implementations, the handoff may be carried out with either one or two transceivers.

FIG. 3illustrates a communication device302having two directional beams to achieve a communication handoff according to one implementation of the invention. In order to maintain a smooth transition between a currently used base station304and a new base station306this approach uses two transceivers (corresponding to beams308and310, respectively). In this approach, communication device302initiates handoff to new base station306using a second transceiver310while it continues transmitting and receiving from the current base station304using a first transceiver308. Communication device302points one beam310toward the sector to which it wants to handoff, and another beam308toward the current sector with which it has been communicating. This allows new base station306to acquire the signal from communication device302while communication device302continues transmitting and receiving data with its current base station304. Once communication device302has been informed that the new base station306has successfully acquired the signal from communication device302, the communication device302switches communications from current base station304to new base station306and stops transmitting to the old sector (e.g., towards base station304). For example, a first transceiver is used to form beam308to current base station304while a second transceiver may be used to form beam310to new base station306. Once communications are established with the new base station306using the second transceiver, the first transceiver may be used to search for new beams. Thus, the purpose of the first and second transceivers may alternate the search and communication functions. In another implementation of the invention, once communications are established with new base station306, new beam310may be transferred or switched to the first transceiver, thereby freeing the second transceiver for future searching of new base stations.

In another approach, communication device302may use just one transceiver to perform the communication handoff from a current base station to a new base station. In this implementation, there are at least two different methods of carrying out the handoff.

FIG. 4illustrates one method of implementing a communication handoff between base stations according to one implementation of the present invention. A communication device402switches its directional antenna beam404to an omni-directional or a wide beamwidth antenna beam406and starts the handoff process. This may be accomplished by using an adaptive antenna that can be configured to operate as either a directional antenna or omni-directional antenna or using two separate antennas, e.g., one directional antenna and one omni-directional antenna. In this scheme, since the omni-directional or wide beamwidth antenna on communication device402has lower gain and receives more interference than the directional antenna, the communication device's data rate is likely lower. Therefore, once the communication device402starts the handoff process it may lower its data rate on both the forward link (transmission from communication device402) and reverse link (reception at communication device402) in order to avoid increasing the frame error rate.

Once the communication device402receives notification from its current base station408that the handoff has been completed (e.g., the new base station412has acquired the signal from the communication device402), it may then switch to a directional antenna beam410which points a narrow beam toward new base station412to which the handoff was completed. Narrower beam410increases the antenna gain and reduces interference received from other sectors in the system to support higher rates.

FIG. 5is a flow diagram illustrating another approach to a single transceiver communication handoff between base stations according to one implementation of the present invention. In general, this handoff method uses a hard handoff and the communication system's upper layer error recovery mechanisms. Once the communication device has determined the best new base station and the best sector or beam on which to communicate502, it may stop transmitting on the current beam to the current base station504. The communication device then starts transmitting using the new antenna beam to the new base station or sector506by basically setting up a physical layer connection to the new sector. This may be done by going through the usual physical layer call set up procedures.

Once the communication device sets up the physical layer connection to the new base station, the Base Station Controller (BSC) entity (see114inFIG. 1), which controls the base stations for functions such as handoffs and also interfaces to the wireline core network, may transmit all unacknowledged packets to the communication device through the new base station508. The BSC generally maintains the state of the packets that have been correctly received and acknowledged by the communication device. The BSC recognizes that a communication device, having a unique identifier, is the same communication device that stopped transmitting on a different sector or beam. The BSC starts routing the unacknowledged packets to the new base station. In other words, when a communication device that already had a connection to the BSC via a current base station tries to set up a new connection to the BSC through a new base station, the BSC switches communications to the communication device to the new base station while stopping transmission to the current base station. In this scheme the physical layer is established anew but the handoff is in effect carried out at the upper layer through the BSC.

FIG. 6illustrates a method of performing base station searching and handoff according to one implementation of the invention. This method outlines the steps performed during the base station searching and handoff processes that have been described in more detail above. A first communication link is established and maintained between the communication device and a current base station602. Since the communication device moves, it may come within range of other base stations. The communication device periodically, continuously, or sporadically searches for better base stations604. The detected base stations are then ranked according to their signal strength606. If any of the detected base stations are better (e.g., has a greater signal strength) than the current base station608, then a communication handoff is initiated. Otherwise, the communication device continues to transmit and/or receive over the communication device and search for better base stations. If the communication handoff is initiated, a new base station, one having the greatest signal strength, is selected from the detected base stations. The communication device transmits its signals to both the current base station and the new base station610. These transmissions to two base stations may be performed in any of the ways previously described. This enables the new base station to identify the signal and setup a link with the communication device while the current base station continues to provide a communication link. Once the communication link is established with the new base station612, the link and transmissions to the current base station is terminated. The communication device then repeats the process of transmitting and receiving over a base station while searching for a better base station.

FIG. 7shows an example device700may comprise an antenna710and a control circuit720, configured to perform communication handoff from a first base station to a second base station as described above. The processing circuit720may comprise of an input interface and circuits used in performing the communication handoff. Device700may also comprise a storage medium730that may comprise instructions executable by processing circuit820for performing a communications handoff between bases stations using antenna710.

The various features of the invention described herein can be implemented in different systems without departing from the invention. For example, some implementations of the invention may be performed with a moving or static communication device and a plurality of mobile or static base stations. For example, the base stations may be satellites and the communication device may be on either a moving vehicle or a static location.