Patent Publication Number: US-2013244654-A1

Title: Handoff determination in a heterogeneous network

Description:
CROSS REFERENCES TO RELATED APPLICATIONS 
     The present patent application is related to U.S. Utility application Ser. No. 13/231,379, entitled “Controlling and Enhancing Handoff between Wireless Access Points,” (Attorney Docket No. BP2572C2), filed Sep. 13, 2011. 
    
    
     TECHNICAL FIELD 
     The subject matter relates generally to communication systems; and, more particularly, to wireless cell or handoff determination for user equipment in a heterogeneous network. 
     DESCRIPTION OF RELATED ART 
     Traditionally, cellular networks were based on macro base stations. These macro base stations were transmitting at relatively high power and were intended to maximize coverage for user equipment (UE). 
     Through the evolution of the cellular technologies (for example, 2G to 2.5G, 2.5G to 3G, 3G to 4G, such as GSM, GPRS, UMTS, HSPA, LTE, et cetera), low power and more localized base stations concepts appear (for example, pico cells, femto cells). 
     Macro base stations were intended to universally service all user equipment; however, broadband and application usage caused conflicts and interference with the user equipment, such as through near-far interference. Heterogeneous networks sought to support a greater variety of traffic types, including varying data rates, cellular wireless, cellular data, local area network data, high speed packet access, and the like. However, problems result from the exponential increase in the number of base stations in a heterogeneous network cell, giving rise to frequent user device handoffs. The increase in handoff rate brought on quality of service degradation, loss of load control, and as a result, dropped connections to the user equipment. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a heterogeneous network in accordance with an embodiment of the invention. 
         FIG. 2  illustrates a heterogeneous network cell in accordance with an embodiment of the invention. 
         FIG. 3  is a block diagram illustrating user equipment in accordance with an embodiment of the invention. 
         FIG. 4  is a block diagram illustrating an access controller constructed in accordance with an embodiment of the invention. 
         FIG. 5  is a flow chart illustrating a method for coordinating user equipment handover in a heterogeneous network cell in accordance with an embodiment of the invention. 
         FIG. 6  is a flow chart illustrating a method for use in user equipment to facilitate handoff in a heterogeneous network cell in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an illustration of a heterogeneous network  100 . The heterogeneous network  100  includes a macro layer  102 , a small layer  104 , and a femto layer  106 . As should be appreciated, the layers are shown in a stacked relation; however, deployment is generally oriented along a plane defined by local terrain. 
     Each of the layers  102 ,  104  and  106  differs in base station density for a given cell area. For example, the macro layer  102  includes macro cell base stations  114  with towers  112 , which can provide a large wireless coverage area per each base station unit, and provide wireless service based upon Long Term Evolution (LTE) specifications, GSM specifications, High Speed Packet Access (HSPA), HSPA evolution (HSPA+), Global System for Mobile communications (GSM), et cetera. 
     The small layer  104  may have a moderate cell base station density as compared to a macro cell base station. As an example, six small cell base stations  116  per a macro sector (three sectors per macro cell) are not uncommon. A small cell base station  116  may have an effective service range of up to one kilometer and for up to two-hundred UE subscribers. 
     The femto layer  106  has the highest cell base station density as compared to a macro cell base station. For example, 180 access points, or femto cell base stations  126 , per macro sector (three sectors per macro cell) is not uncommon. The femto layer cells or access points include pico cell technologies, femto cell technologies, WiFi or IEEE 802.11 technologies, et cetera. A femto cell base station  126  has an effective range of about fifteen-to-fifty meters, and is generally used in residences and small enterprises in support of up to eight UE subscribers. A pico cell can provide service coverage indoors and outdoors for up to 16 UE subscribers. 
     Generally, the heterogeneous network  100  provides a solution to the conflicts and interference otherwise found in homogenous macro cell base station deployments. Also, as another consideration, because mobile data usage nearly doubles each year, deployment capacity in a macro cell based network cannot also increase at that rate. 
     Smaller cells of a heterogeneous network  100  supplement coverage in areas where macro cells service would be overkill in view of the overhead and complexity. The stacked layer effect of the heterogeneous network  100  serves to improve signaling integrity, uplink and downlink data rates, network capacity, and provide low latencies. 
     But the increase in the number of base stations and cells within a given area complicates the handoff process due to varying mobility rate of the user equipment (UE), such as the wireless phone  122  and portable computer  124 , and other user equipment, such as PDAs, tablet computers, et cetera, having wireless connection  118  capability with the base stations  114 ,  116  and  128 . In the example provided, a heterogeneous network handoff may in effect be a vertical handoff  110  along the layers  102 ,  104  and  106 , or a horizontal handoff  120  within a layer. 
     The handoff or handover consideration user equipment may require differing data rates based upon the application executing on the device; Quality of Service (QoS) requirements for different mobility content such as video playback from cloud networks, or for voice calls, whether circuit switched cellular or data packet based. 
     The added complexity leads to service degradation, loss of load control, and risk for dropped service, frustrating the UE service subscriber. 
     The user equipment handoff is further complicated by the movement of the device in a heterogeneous cell—that is, the duration that the UE may remain within a given service area in view of its movement before another handoff being required, as well as the availability of service resources. 
     Coordination of the handoff is coordinated by a network core  142 , which may include an access controller  144 . The access controller  144  may also be implemented as a form of base station controller. The access controller  144  receives status information from the base stations  114 ,  116  and  126  via communication paths  134 ,  136  and  138 , respectively. The communication paths may be coupled to a network  132  to facilitate the routing of the information to the access controller  144  and to other components of the heterogeneous network  100 . 
     Movement of the UE (e.g., wireless phone  122 , portable computer  124 ) is monitored by the access controller  144  such that handoff decisions to a cell base station of a cell layer  102 ,  104  or  106  may be initiated to provide service to the UE. Handoff of the UE between cell base stations of the heterogeneous network  100  is discussed in detail with respect to  FIGS. 2 to 6 . 
       FIG. 2  illustrates an embodiment of a heterogeneous network cell  202  showing the service area of the overlaying cell layers provided by a micro cell base station  114 , a small cell base station  116 , and femto base stations  126 . The interrelationship of the base station service areas are not shown to scale for simplicity purposes. 
     Each of the base stations of the heterogeneous network cell  202  includes a service area boundary. A handover of user equipment  204  refers to the process of transferring an ongoing session with the user equipment  204  from one cell base station to another. 
     Handoffs, or handovers, are generally based upon a parameter having a relatively short time factor, such as receive signal strength (RSS), bit error rate (BER), signal-to-interference plus noise ratio (SINR), et cetera. In the present example of  FIG. 2 , these parameters relating to signal quality for the user equipment with a serving cell, as provided by either of the macro cell base station  114 , the small cell base station  116 , and femto cell base stations  126 . 
     The parameter relating to signal quality, when coming within a given threshold, would trigger handoff procedures, which the access controller  114  coordinates among the network, and with the heterogeneous network cell  202  as provided by the present example. 
     Because frequent handovers between base stations  116  and  126  might be result by the UE  204  mobility, the velocity estimation  206  for the UE  204  is taken into consideration to prevent overly frequent handovers either horizontally between base stations of a layer or vertically in the overlaid layers  102 ,  104 , and  106  (see  FIG. 1 ). 
     Without taking velocity into consideration for the handover, the result is in excessive handoffs causing disruption in service continuity, loss in service quality, and likely dropped calls, particularly with cells in the layer based upon femto cell base stations  126 . 
     To facilitate smooth handoffs of the user equipment, the macro cell base station  114 , small cell base station  116 , and femto cell base stations  126  each provide a velocity estimation  206  for the user equipment served by the heterogeneous network cell  202 , such as user equipment  204 . As may be appreciated, a velocity estimation  206  may be provided by a single base station. Multiple velocity estimations  206  provided by multiple base stations to the network core  142  and access controller  144  may also be provided to mitigate erroneous velocity estimations (for example, a first femto cell base station  126  provides a velocity estimation of 30 mph, where other base stations in the cell  202  provide velocity estimations of 2 mph). 
     Additional parameters may be periodically provided by the base stations (or on request by the access controller  144 ) to further improve handover objectives such as sustaining service quality and continuity. Another parameter includes the nature of the application executed by the user equipment  204 , and the sustained data rate for the application. For example, the sustained data rate pertains to whether the application is a real-time application (for example VoIP session, circuit switch Voice, video streaming) or an application more tolerant of bursty, or inconsistent, data transmissions (for example, Internet browsing or file download). A further parameter is a data rate requested for suitable quality of service (QoS) for the user equipment  204 . 
     With these parameters provided by the base stations  114 ,  116  and  126 , the network core  142 , with access controller  144 , operates to aggregate and use these parameters in forming handoff decision, specifically to which base station the user equipment should next camp after the handoff. 
     Further, the parameters can be provided as an average weighting factor for handoff decisions to a macro cell base station  114 , a small cell base station  116 , or femto cell base station  126 . Specifically the average weighting factor taking into consideration the velocity estimation parameter, the QoS factor parameter, and the data rate parameter. 
     In operation, when the velocity estimation, either singular, aggregate, or averaged, exceeds a threshold, the handoff decision includes a cell base station of a cell layer that has a low base station density, such as a macro cell base station  114 . As an example, the velocity estimation parameter, or aggregated velocity estimation parameter, indicates high speed user equipment (e.g., 50 kmh) by exceeding a high speed threshold, has a data rate parameter with a low rate, and has an application parameter indicating a real time application executing by the user equipment. The handoff decision by the access controller is to a macro cell base station  114 , which would be a vertical handoff from either of the femto cell base stations  126  or the small cell base station  116 . 
     In another instance, when the velocity estimation exceeds or comes within another threshold indicating sufficiently static speed, the handoff decision includes a cell base station of a cell layer that has a highest base station density, such as the femto cell layer that includes femto cell base stations  126 . For another example, the velocity estimation parameter, or aggregated velocity estimation parameter, indicates a substantially static speed user equipment (e.g., 3 to 5 kmh, such as a pedestrian) that exceeds or comes within a static threshold, has a data rate parameter with a high data rate, and has an application parameter indicating a non-real-time application executing by the user equipment  204  (for example, Internet browsing). The handoff decision by the access controller  144  is to a femto cell base station  126 , which would be a vertical handoff from either of a macro cell base station  114  or a small cell base station  116 , or a horizontal handoff to another femto cell base station  126 . 
     In a further instance, when the velocity estimation  206  exceeds neither of the first threshold for high speed the second threshold for static speed, the handoff decision includes a cell base station of a cell layer that has a moderate base station density, such as the small cell base station  116 . For yet another example, the user equipment comes between a high velocity threshold and a substantially static velocity estimation threshold, sustain the wireless service, the handoff decision by the access controller  144  is to a small cell base station  116 , which would be a vertical handoff from either of a macro cell base station  114  or a femto cell base station  126  to the small cell base station  116 , or a horizontal handoff to another small cell base station  116  (not shown for clarity). 
     As shown by the example of  FIG. 2 , forming a handoff decision based upon a velocity estimation parameter, either singularly from a base station or aggregated from multiple base stations of the heterogeneous network cell  202 , minimizes the overall number and frequency of UE handovers in the system. Moreover, such consideration reduces the control load and call drops in the system, while enhancing the user experience, QoS and satisfaction. Also taking into consideration further parameters relating to the nature of the applications executed on the UE and the applicable QoS data rates serve to further refine the handoff decision in the context of the heterogeneous network cell  202 . 
     Furthermore, handoff of user equipment may be further enhanced by authentication to the components of the heterogeneous network structure. For example, when user equipment passes out of range of an access point, such as that provided by femto cell base stations  126  into a coverage area servicing cellular communications or even no communications, and then passes back into range provided by femto cell base stations  126 , the user oftentimes would be required to re-register before accessing the base station. 
     Once user equipment  204  is registered with a cell base station, such as a WiFi base station or femto cell base station  126 , information from the access session is passed to a shared UE authentication register  148  accessible by the cell base stations of the heterogeneous network cell  202  and network  100 . The shared register may also be local to a base station, as well as stored with the access controller  144  of the network core  142 . Shared registration and authentication of user equipment or mobile stations is discussed in detail in U.S. Utility application Ser. No. 13/231,379, entitled “Controlling and Enhancing Handoff between Wireless Access Points,” (Attorney Docket No. BP2572C2), filed Sep. 13, 2011, which is hereby incorporated herein by reference in its entirety and made part of the present U.S. Utility Patent Application for all purposes. 
       FIG. 3  is a block diagram illustrating of user equipment  204 . The user equipment  204 , such as wireless phone  122  or portable computer  124  (see  FIG. 1 ) supports standardized operations that are compatible with the teachings of the disclosure, with or without modification. In other embodiments, however, the user equipment  204  may support other operating standards. 
     The user equipment  204  includes an RF unit  302  implementing a physical layer in support of various protocol specifications deployed by a heterogeneous network  202  (such as that of IEEE 802.21, IMT-Advanced, et cetera), a processing module  304 , and a memory  306 . The RF unit  302  couples to an antenna  318  that may be located internal or external to the case of the user equipment  204 . The processing module  304  may be an Application Specific Integrated Circuit (ASIC) or another type of processor that is capable of operating the user equipment  204 . 
     The memory  306  may include both static and dynamic components, for example, dynamic RAM, static RAM, ROM, EEPROM, et cetera. In some embodiments, the memory  306  may be partially or fully contained upon an ASIC or other IC that also includes the processing module  304 . 
     A user interface  308  includes a display  310 , a keyboard  312 , a speaker/microphone  314 , and a data interface  316 , and may include other user interface components. The RF unit  302 , the processing module  304 , the memory  306 , and the user interface  308  couple via one or more communication buses/links. A battery  325  also couples to and powers the RF unit  302 , the processing module  304 , the memory  306 , and the user interface  308 . 
     Operational instructions  322  to facilitate handoff in a heterogeneous network are stored in memory  306 . The operational instructions are loaded to the processing module  304  for execution by the processing module  304 . The operational instructions  322  may be programmed into the user equipment  204  at the time of manufacture, during a service provisioning operation, such as an over-the-air service provisioning operation, or during a parameter updating operation. Upon execution, the operational instructions  322  cause the user equipment  204  to perform operations according to the present invention previously described with reference to  FIGS. 1 through 6 . 
     The structure of the user equipment  204  illustrated is only an example of one user equipment structure. Many other varied user equipment structures could be operated according to the teachings of the present disclosure. Upon execution of the operational instructions  322 , the user equipment  204  performs operations according to the present invention previously described herein in facilitating handoff in a heterogeneous network  100  (see  FIG. 1  and  FIG. 2 ). 
       FIG. 4  is a block diagram illustrating an access controller  144  of the network core  142  constructed according to an embodiment of the invention. The access controller  144  includes a processing module  402 , dynamic RAM  406 , static RAM  408 , 
     EPROM  410 , and at least one data storage device  412 , such as a hard drive, optical drive, tape drive, et cetera. These components (which may be contained on a peripheral processing card or module) intercouple via a local bus  436  and couples to a peripheral bus  438  (which may be a back plane) via an interface  416 . Peripheral cards couple to the peripheral bus  338 . The peripheral cards include a network infrastructure interface card  420 , which couples the access controller  144  to the heterogeneous network cell or cells  202 . Additional cards  318  may be provided with respect to coupling to other networks, or other functionality. 
     Structures and operational instructions  404  regarding coordinating user equipment handover in a heterogeneous network cell are stored in storage  412 . The operational instructions  404  are downloaded to the processing module  402  and/or the DRAM  406  for execution by the processing module  402 . While the operational instructions  404  are shown to reside within storage  412  within the access controller  144 , the operational instructions may also be loaded onto portable media such as magnetic media, optical media, or electronic media. Further, the operational instructions  404  may be electronically transmitted from one computer to another across a data communication path. 
     Upon execution of the operational instructions  404 , the access controller  144  performs operations according to the methods and processes described herein with reference to  FIGS. 1 through 6 . The structure of the access controller  144  illustrated is only one of many varied access controller structures that could be operated according to the descriptions contained herein. 
       FIG. 5  is a flow chart that illustrates a method  500  for coordinating user equipment handover in a heterogeneous network cell. The heterogeneous network cell includes a plurality of overlaying cell layers, each cell layer differing in base station density. At step  502 , the method receives a first parameter relating to signal quality for the user equipment with a serving cell of a cell layer of the plurality of overlaying cell layers. At step  504 , the method is receiving a second parameter based upon a velocity estimation of the user equipment within the heterogeneous network cell. The velocity estimation may be based on a variety estimation techniques, alone or in combination, such as Doppler phase estimation, round trip delay estimation, global positioning satellite (GPS) report, et cetera. Moreover, the velocity estimation may be an aggregate of estimations from the plurality of base stations of the heterogeneous network cell, as well as an averaged valuation to compensate for abnormal estimation results. 
     As depicted in  FIG. 5 , steps  502  and  504  periodically repeat in view of the changing environment caused by the mobility of the user equipment. Further, the reception of the signal quality and/or the velocity estimation for the user equipment may be in response to a request for the parameter data. Such data may be considered stale as based upon a timer variable or other such mechanism or routine. 
     At steps  506  and  508 , the method may further receive a third parameter relating to a quality of service (QoS) level for the user equipment, and receive a fourth parameter relating to an application type of the user equipment being real-time (such as VoIP communications, streaming audio and/or video) or non-real-time based (such as Internet browsing). 
     At step  510 , when the first parameter indicates a cell boundary, the method  500  proceeds to step  512 , forming a handoff decision to a cell base station of a cell layer of the plurality of overlaying cell layers based upon the second parameter relating to velocity estimation. Otherwise, the method returns to step  502 . 
     With respect to forming a handoff decision the method  500  considers that when the second parameter exceeds a first threshold, the handoff decision includes a cell base station of a cell layer that has a low base station density having a large coverage area, such as a macro layer. In another aspect, when the second parameter exceeds a second threshold indicative of substantially static or low velocity, the handoff decision includes a cell base station of a cell layer that has a highest base station density, such as a femto layer with comparatively numerous base station access points to service user equipment. In a third aspect, the handoff decision is based upon when the second parameter relating to the velocity estimation neither exceeds the first threshold and the second threshold, the handoff decision includes a cell base station of a cell layer that has a moderate base station density, such as the small layer. 
     At step  514 , the method continues by initiating a handoff to the cell base station based upon the handoff decision. 
       FIG. 6  is a flow chart that illustrates a method  600  for use in user equipment (UE) to facilitate handoff in a heterogeneous network cell. At step  602 , the method provides engaging a first cell base station of a heterogeneous network cell. The heterogeneous network cell includes a plurality of overlaying cell layers, each cell layer differing in base station density, wherein the first cell base station provides session support to the UE based upon a data rate and quality of service characteristics of a session. 
     The plurality of cell layers include a macro cell layer having a low base station density, a micro cell layer having a moderate base station density, and a femto cell layer having a highest base station density. 
     Step  604  continues by sensing movement of the UE within the heterogeneous network cell, and determining at step  606  a rate of movement of the user equipment. 
     The rate of movement of the UE may be based upon a Doppler phase estimation, round trip delay estimation, global positioning satellite (GPS) report, et cetera, either alone or in combination. 
     At step  608 , the method provides for transmitting the determined rate of movement. As depicted, steps  606  and  608  are periodically conducted to provide accurate estimation information regarding the user equipment. Moreover, the rate of movement may be conducted in response to a command and/or request to provide such determined velocity estimation. 
     The method then queries at step  610  whether a handoff command has been received. If not, the method returns to step  604 . If so, then the method continues at step  612 , engaging in a handoff to a second cell base station of the heterogeneous network cell. The second cell base station belonging to a cell layer of the heterogeneous network cell, the cell layer having base station density sufficient to support the determined rate of movement of the user equipment. 
     It is noted that the various modules described herein may be a single processing device or a plurality of processing devices. Such a processor or processing device may be a microprocessor, micro-controller, digital signal processor, microcomputer, central processing unit, field programmable gate array, programmable logic device, state machine, logic circuitry, analog circuitry, digital circuitry, and/or any device that manipulates signals (analog and/or digital) based on operational instructions. The operational instructions may be stored in a memory. The memory may be a single memory device or a plurality of memory devices. Such a memory device may be a read-only memory (ROM), random access memory (RAM), volatile memory, non-volatile memory, static memory, dynamic memory, flash memory, and/or any device that stores digital information. It is also noted that when the processing module implements one or more of its functions via a state machine, analog circuitry, digital circuitry, and/or logic circuitry, the memory storing the corresponding operational instructions is embedded with the circuitry comprising the state machine, analog circuitry, digital circuitry, and/or logic circuitry. In such an embodiment, a memory stores, and a processing module coupled thereto executes, operational instructions corresponding to at least some of the steps and/or functions illustrated and/or described herein. 
     The present invention has also been described above with the aid of method steps illustrating the performance of specified functions and relationships thereof. The boundaries and sequence of these functional building blocks and method steps have been arbitrarily defined herein for convenience of description. Alternate boundaries and sequences can be defined so long as the specified functions and relationships are appropriately performed. Any such alternate boundaries or sequences are thus within the scope and spirit of the claimed invention. 
     The present invention has been described above with the aid of functional building blocks illustrating the performance of certain significant functions. The boundaries of these functional building blocks have been arbitrarily defined for convenience of description. Alternate boundaries could be defined as long as the certain significant functions are appropriately performed. Similarly, flow diagram blocks may also have been arbitrarily defined herein to illustrate certain significant functionality. To the extent used, the flow diagram block boundaries and sequence could have been defined otherwise and still perform the certain significant functionality. Such alternate definitions of both functional building blocks and flow diagram blocks and sequences are thus within the scope and spirit of the claimed invention. 
     One of average skill in the art will also recognize that the functional building blocks, and other illustrative blocks, modules and components herein, can be implemented as illustrated or by discrete components, application specific integrated circuits, processors executing appropriate software and the like or any combination thereof. 
     Moreover, although described in detail for purposes of clarity and understanding by way of the aforementioned embodiments, the present invention is not limited to such embodiments. It will be obvious to one of average skill in the art that various changes and modifications may be practiced within the spirit and scope of the invention, as limited only by the scope of the appended claims.