Patent Application
20080102834
1. Field of the Invention
This invention relates generally to communication systems, and, more particularly, to wireless communication systems.
2. Description of the Related Arts
The coverage area of a wireless communication system is typically divided into a number of cells, which may be grouped into one or more radio access networks. The coverage area of each cell in the wireless network is typically limited by the propagation loss of radio signals transmitted by base stations that provide coverage to the cell. Thus, the coverage area of each cell is determined by the location and the transmit power of the base station, as well as the topology of the cell and the location of any interfering objects. For example, the coverage area of a cell may be reduced if a building or a mountain is present near the base station. The boundaries of the cells are not rigidly defined and may vary with time. Thus, coverage areas may overlap such that multiple base stations may provide coverage to the overlapping regions, although the strength of the signal provided within the overlapping regions may be different for the different base stations.
Mobile units located in the coverage area of a cell may access the wireless communications system by establishing a wireless communication link with the base station associated to that cell. The wireless communication link is often referred to as the air interface. As discussed above, a mobile unit may be located in a region served by more than one base station. The mobile unit may then select the base station having the lowest propagation path loss (or highest channel quality) as it's serving base station. Roaming mobile units may travel through regions served by numerous base stations. Accordingly, a roaming mobile unit may handover from one base station to another as it enters and leaves cells served by different base stations. Stationary mobile units may also handover from one base station to another if the propagation path loss (or channel quality) associated with the base stations varies over time.
Handover typically refers to a category of procedures that may be used to support mobility for mobile units in a cellular wireless network. For example, handover techniques may be used to allocate different radio resources for the existing connection as the mobile unit is handed over from one cell to another cell. When performing handover from one cell to another cell this generally means that different radio resources need to be allocated for this connection. In Code Division Multiple Access (CDMA) networks like the Universal Mobile Telecommunication System (UMTS), the basic radio resources are carrier frequencies, transmit power, and spreading codes. Due to the wide bandwidth of the carriers in UMTS systems, most network operators have only two or three carriers available for the entire network and, hence, carrier frequencies are often re-used in every base station. Many mobiles then need to share the same frequency band. The signals transmitted between the cells and mobile units are therefore distinguished by their spreading code. Signal separation using the spreading codes is maintained when moving between cells. Soft and/or softer handover techniques that permit a mobile unit to communicate concurrently with multiple base stations are typically applied when the mobiles travel through the network using the same frequency in every cell. Due to the fact that frequencies are not changed, soft and/or softer handover is also known as intra-frequency handover in UMTS.
Mobile units may also be handed off between cells that operate on different frequencies. For example, the Third generation Partnership Project (3GPP) standards for application in UMTS networks specify an inter-frequency handover. As mentioned above, most operators only have a few carrier frequencies available for their networks. One carrier is typically used to provide continuous coverage and basic services and the additional carriers are made available when needed, e.g., to provide coverage to larger network areas or to selected hotspots. Mobile units may therefore need to change carrier frequencies, i.e. to perform an inter-frequency handover within the multi-carrier system. Inter-frequency handovers typically occur when the load conditions on the two carriers change, e.g., the cells on one carrier might become overloaded. Inter-frequency handovers may also be used when the coverage areas of additional carriers are limited, e.g., the second and/or third carrier might be only used in hotspot areas. For example, mobile units using a hotspot carrier may be handed over to other carriers if traveling towards the border of the hotspot.
In some cases, mobile units may even be handed off between base stations in different wireless communication systems. For example, the 3GPP standards also specify inter-system (or inter-Radio Access Technology) handover for application in UMTS networks. One example of a handover between different radio access technologies (RAT) would be handover between a UMTS network and a network that operates according to the Global System for Mobile communication (GSM) standard. Many network operators have deployed both a UMTS network and a GSM network. The GSM networks have been deployed longer than UMTS networks and therefore the GSM networks most often offer nearly continuous nationwide coverage. In contrast, UMTS networks have (and are expected to continue to have) numerous coverage holes as operators concentrate on offering the new UMTS services in areas of relatively high population density. Inter-RAT handover is therefore a very critical feature in mixed technology networks. For example, mobile units traveling outside the UMTS coverage area will be dropped if they do not perform a handover to the GSM access technology, which may also cause customer satisfaction to fall.
Conventional handover techniques, whether soft, inter-frequency, or inter-RAT, rely on measurements performed by the mobile unit and/or the network. For example, each mobile unit may perform measurements based on signals transmitted between the mobile units and base stations that are currently connected to the mobile unit. Mobile units may also measure characteristics of transmission signals in potential target cells. In a UMTS network, the measurements may be used to determine whether a handover should be performed. Mobile units can perform the measurements for intra-frequency (soft) handover on the current cell and neighbor cells continuously because the base stations all use the same carrier frequency. The signals for different mobile units can be easily distinguished from each other by their different spreading codes.
However, mobile units may need to be tuned to new frequency bands to perform the measurements for inter-frequency or inter-RAT measurements on potential target cells. In many cases artificial transmission gaps may be introduced in the continuous UMTS signals to allow the mobile unit to perform measurements on different frequency bands. For example, the Compressed Mode (CM) has been specified in the 3GPP standards. One basic CM method (named SF/2) allows transmitting the same amount of data over the primary carrier frequency in half of the time by reducing the spreading factor by half, which leaves transmission gaps during the unused time. The mobile unit may therefore tune one or more receivers to other frequency bands and perform the required measurements during the transmission gaps. The mobile units then switch back to the original carrier to continue information transmission.
Compressed mode operations have a number of drawbacks. For example, signals with reduced spreading factors are more susceptible to noise and interference, so the signal power is typically increased during the compressed mode period to maintain signal quality. As a result, the compressed mode, and especially the SF/2 method, can cause significant power variations that result in higher interference levels and network capacity losses. Accordingly, conventional networks attempt to minimize usage of the compressed mode.
One alternative to performing handover measurements in the compressed mode is blind handover. With blind handover, the mobile unit does not perform any measurements on the target cells and therefore the compressed mode is not used. Inter-frequency or inter-RAT handover are triggered only by the mobile unit's measurements performed on the currently used carrier frequency. For example, blind handover might be initiated when the quality of the signals on the current carrier drops below a minimum threshold. However, no measurements will be performed on the target cells (either the other RAT or a different UMTS carrier) and so the best target cell and the signal quality on the new carrier cannot be determined. Target cells for blind handover therefore need to be specified before the blind handover is initiated. For example, a network operator may define a target cell, carrier frequency, and/or RAT for every cell where blind handover is to be used. Usually the most appropriate target cell(s) will be identified during network planning using cell planning data. The mobile unit will then be handed over to the predetermined target cell when blind handover is triggered, e.g., due to bad signal quality on the current carrier.
The absence of target cell measurements in a blind handover increases the danger that handover might be directed towards an inappropriate target cell, e.g. if the selection process has not been carried out carefully enough. This would have a negative impact on the network performance and could also result in an increased handover failure or call drop rate. The use of fixed target cell definition may also be a drawback in many situations. First, the target cells for inter-RAT handover need to be specified in advance, which means that the network and/or cell planning for the different network technologies (e.g., UMTS and GSM) needs to be coordinated. For example, GSM target cell(s) must be identified in advance for every UMTS cell. In the case of inter-frequency handover, cell planning for the two frequency layers must be coordinated. Different networks and/or frequency layers are traditionally planned independently, and so the coordination effort might be substantial. Second, the best target cell(s) will not be dynamically updated during the operational phase. However, the network layout will change regularly during the operational phase. Thus, existing cells may not be available at all times and new base stations that provide service to new cells may be installed. It would therefore be necessary to update the target cells for blind handover each time the network layout changes. This is a very time consuming and costly procedure.
The present invention is directed to addressing the effects of one or more of the problems set forth above. The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an exhaustive overview of the invention. It is not intended to identify key or critical elements of the invention or to delineate the scope of the invention. Its sole purpose is to present some concepts in a simplified form as a prelude to the more detailed description that is discussed later.
In one embodiment of the present invention, methods are provided for selecting a target cell for a blind handover. The methods may include selecting at least one target cell for a blind handover of a first mobile unit based on at least one measurement performed by the first mobile unit or at least one second mobile unit for at least one measurement-based handover. The method may also include receiving information indicative of at least one target cell for a blind handover of a first mobile unit. The target cell is selected based on at least one measurement performed by the first mobile unit or at least one second mobile unit for at least one measurement-based handover.
The invention may be understood by reference to the following description taken in conjunction with the accompanying drawings, in which like reference numerals identify like elements, and in which:
While the invention is susceptible to various modifications and alternative forms, specific embodiments thereof have been shown by way of example in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific embodiments is not intended to limit the invention to the particular forms disclosed, but on the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.
Illustrative embodiments of the invention are described below. In the interest of clarity, not all features of an actual implementation are described in this specification. It will of course be appreciated that in the development of any such actual embodiment, numerous implementation-specific decisions should be made to achieve the developers' specific goals, such as compliance with system-related and business-related constraints, which will vary from one implementation to another. Moreover, it will be appreciated that such a development effort might be complex and time-consuming, but would nevertheless be a routine undertaking for those of ordinary skill in the art having the benefit of this disclosure.
Portions of the present invention and corresponding detailed description are presented in terms of software, or algorithms and symbolic representations of operations on data bits within a computer memory. These descriptions and representations are the ones by which those of ordinary skill in the art effectively convey the substance of their work to others of ordinary skill in the art. An algorithm, as the term is used here, and as it is used generally, is conceived to be a self-consistent sequence of steps leading to a desired result. The steps are those requiring physical manipulations of physical quantities. Usually, though not necessarily, these quantities take the form of optical, electrical, or magnetic signals capable of being stored, transferred, combined, compared, and otherwise manipulated. It has proven convenient at times, principally for reasons of common usage, to refer to these signals as bits, values, elements, symbols, characters, terms, numbers, or the like.
It should be borne in mind, however, that all of these and similar terms are to be associated with the appropriate physical quantities and are merely convenient labels applied to these quantities. Unless specifically stated otherwise, or as is apparent from the discussion, terms such as “processing” or “computing” or “calculating” or “determining” or “displaying” or the like, refer to the action and processes of a computer system, or similar electronic computing device, that manipulates and transforms data represented as physical, electronic quantities within the computer system's registers and memories into other data similarly represented as physical quantities within the computer system memories or registers or other such information storage, transmission or display devices.
Note also that the software implemented aspects of the invention are typically encoded on some form of program storage medium or implemented over some type of transmission medium. The program storage medium may be magnetic (e.g., a floppy disk or a hard drive) or optical (e.g., a compact disk read only memory, or “CD ROM”), and may be read only or random access. Similarly, the transmission medium may be twisted wire pairs, coaxial cable, optical fiber, or some other suitable transmission medium known to the art. The invention is not limited by these aspects of any given implementation.
The present invention will now be described with reference to the attached figures. Various structures, systems and devices are schematically depicted in the drawings for purposes of explanation only and so as to not obscure the present invention with details that are well known to those skilled in the art. Nevertheless, the attached drawings are included to describe and explain illustrative examples of the present invention. The words and phrases used herein should be understood and interpreted to have a meaning consistent with the understanding of those words and phrases by those skilled in the relevant art. No special definition of a term or phrase, i.e., a definition that is different from the ordinary and customary meaning as understood by those skilled in the art, is intended to be implied by consistent usage of the term or phrase herein. To the extent that a term or phrase is intended to have a special meaning, i.e., a meaning other than that understood by skilled artisans, such a special definition will be expressly set forth in the specification in a definitional manner that directly and unequivocally provides the special definition for the term or phrase.
The cells 105, 110 are separated into layers 115, 120. In one embodiment, the layers 115, 120 are distinguished based upon the frequency or frequencies that are used as carrier frequencies used for providing wireless connectivity to the associated cells 105, 110. For example, in a wireless communication system 100 that operates according to UMTS standards and/or protocols, wireless connectivity may be provided to the cells 105 using a first carrier frequency and to the cells 110 using a second carrier frequency. In one alternative embodiment, the layers 115, 120 are distinguished based upon the radio access technology used to provide wireless connectivity to the associated cells 105, 110. For example, wireless connectivity may be provided to the cells 105 according to UMTS standards and/or protocols and wireless connectivity may be provided to the cells 110 according to GSM standards and/or protocols.
The wireless communication system 100 provides wireless connectivity to one or more mobile units 125. Only one mobile unit 125 is shown in the illustrated embodiment, however, persons of ordinary skill in the art having benefit of the present disclosure should appreciate that any number of mobile units 125 may operate within the wireless communication system 100. Exemplary mobile units 125 may include, but are not limited to, devices such as mobile telephones, personal data assistants, smart phones, Global Positioning System devices, wireless network interface cards, desktop or laptop computers, and the like. The mobile unit 125 is capable of communicating with cells 105, 110 in either of the layers 115, 120. For example, the mobile unit 125 may be a UMTS-compatible device that is capable of communicating using any of the carrier frequencies provided by the wireless telecommunications system 100. For another example, the mobile unit 125 may include transceivers that are compatible with both UMTS and GSM standards, as well as any other radio access technology implemented by the wireless communication system 100.
The mobile unit 125 is initially associated with the cell 105(2) and therefore may access the wireless communication system 100 via the cell 105(2) and may receive information from the wireless communication system 100 via the cell 105(2). Techniques for transmitting information to the mobile unit 125 and/or receiving information from the mobile unit 125 over air interfaces established between the mobile unit 125 and the cell 105(2) are known to persons of ordinary skill in the art and an interest of clarity only those aspects of transmitting information over the air interface that are relevant to the present invention will be discussed further herein. The mobile unit 125 may then be handed over from the (serving) cell 105(2) to another (target) cell 105, 110. For example, a channel quality associated with the air interface between the mobile unit 125 and the cell 105(2) may degrade, which may result in the mobile unit requesting a handover. For another example, the mobile unit 125 may roam, as indicated by the arrow 130, resulting in the mobile unit 125 requesting a handover.
The mobile unit 125 may be handed over using either a blind handover algorithm or a measurement-based handover algorithm. As used herein and in accordance with usage in the art, the term “blind handover” will be understood to refer to a handover of the mobile unit 125 from a serving cell to a target cell that occurs without making measurements of characteristics of the newly established communication link (or the communication link that is about to be established) between the mobile unit 125 and the target cell. Blind handovers may also be referred to as inter-frequency handovers and/or inter-radio-access-technology handovers, depending on the frequencies and/or radio access technologies used by the cells 105, 110 involved in the blind handover. In contrast, the term “measurement-based handover” will be understood to refer to a handover of the mobile unit 125 from a serving cell to a target cell that is performed, at least in part, based on measurements of characteristics of the newly established communication link (or the communication link that is about to be established) between the mobile unit 125 and the target cell. Exemplary characteristics that may be measured during a measurement-based handover include, but not limited to, channel qualities, signal strengths, round-trip delays, and the like.
In the illustrated embodiment, the mobile unit 125 is handed over using a blind handover algorithm. For example, the mobile unit 125 may determine that the channel quality associated with the serving cell 105(2) has degraded and may therefore request a handover. The wireless communication system 100 may then select the target cell 110(2) based on measurements performed during previous measurement-based handovers and may initiate a blind handover of the mobile unit 125 from the serving cell 105(2) to the target cell 110(2). For another example, the mobile unit 125 may roam and determine that the signal strength and/or channel quality associated with the serving cell 105(2) has decreased. The mobile unit 125 may therefore request a handover. The wireless communication system 100 may select the target cell 110(3) based on measurements performed during previous measurement-based handovers and may initiate a blind handover of the mobile unit 125 from the serving cell 105(2) to the target cell 110(3).
In the illustrated embodiment, a mobile unit 215 is being (or is about to be) handed over from the base station 210(1) to the base station 210(2). The mobile unit 215 has established communication links over air interfaces 220 with corresponding base stations 210 and so the mobile unit 215 may be said to be in soft handover. However, persons of ordinary skill in the art having benefit of the present disclosure should appreciate that is not necessary for the mobile unit 215 to be in soft handover. In alternative embodiment, the mobile unit 215 may be in hard handover, softer handover, or other types of handovers. The base stations 210 shown in
The handover of the mobile unit 215 depicted in
The results of the measurements performed by the mobile unit during the measurement-based handover may be provided to the controller 205. In various alternative embodiments, the measurement results may be provided via either of the base stations 210 and in any form. The controller 205 may store information indicative of the results of the measurements in a memory unit 230, which may be an internal random access memory, a disk, a tape, or any other type of memory. Although the memory unit 230 is shown as an internal portion of the controller 205, persons of ordinary skill in the art having benefit of the present disclosure should appreciate that the present invention is not limited to internal memory units 230. In alternative embodiments, the information gathered during measurement-based handovers may be stored in external memory units, such as disks, tapes, memory elements mounted on remote computers, and the like. In some wireless communication systems 200, this information may be captured for performance analysis purposes that are not necessarily related to handovers.
The information indicative of the results of the measurements performed by the mobile units 215 in the past (i.e. in previous attempts) may be used to select target cells for blind handovers. In the illustrated embodiment, a processing unit 235 may access information associated with measurements performed during measurement-based handovers from the memory units 230 and use this information to select one or more target cells for a blind handover. The information used to select the target cells may be provided by the mobile unit that is requesting the blind handover, but this is not required for the practice of the present invention. In some embodiments, the processing unit 235 may also select target cells for blind handovers using information provided by other mobile units, which are not currently requesting a blind handover, but have previously performed measurements during a measurement-based handover.
In one embodiment, the gathered information may include statistics collected for measurement-based handover, which the processing unit 235 may process and/or evaluate offline. Analyzing the handover statistics and then defining appropriate relationships between source cells and target cells may therefore replace conventional cell planning techniques for determining target cells for blind handover. For example, blind handover target cells may be selected based on network measurements that indicate the state of the network at approximately the time the handover is requested. Target cell selection may therefore be performed dynamically, which may increase the accuracy and reliability of the selection algorithm as well as reducing the requirements for cell-planning and increasing the handover success rate.
The target cell statistics for measurement-based handover may also be evaluated and analyzed online, e.g. by the processing unit 235. For example, a list of the best target cells for blind handover from each source cell may be updated each time a measurement-based handover is performed. The list of target cells may be stored in the memory units 230 or at any other location. The processing unit 235 may access the list to determine the best target cell(s) when a blind handover is requested. Such an adaptive mechanism may easily adapt to any changes in the network layout and configuration. For example, an adaptive mechanism using online evaluation of the measurement results and handover statistics may automatically adapt to new constellations that are formed when cells become temporarily unavailable and/or new base stations with new cells are added to the network.
The handover algorithm may be selected (at 310) using any algorithm selection technique or method. In one embodiment, the handover algorithm may be selected (at 310) for each mobile unit depending on network conditions and performance. For example, the algorithm may be selected (at 310) when the need or request for a handover has been detected, e.g., the mobile units approaches a coverage border of the serving cell and therefore should be handed over to another carrier frequency or RAT associated with the target cell. The adaptive algorithm selection mechanism may dynamically choose between blind handover and measurement-based handover based on criteria that are related to the network structure, as well as network and handover performance. In a dynamic environment where the network configuration changes over time, the performance measures may also be continuously updated during network operation.
Typical performance measures and criteria proposed to be used for algorithm selection may include a handover failure rate, a network load, a number of target cells, and the like. For example, a measurement-based handover algorithm may be selected (at 310) if the failure rate for blind handover becomes too high. For another example, a blind handover algorithm may be selected (at 310) to avoid using the compressed mode if the network load becomes too high and a measurement-based handover algorithm may be selected (at 310) at least in part to update and improve the statistical data if the network load is low. For yet another example, a blind handover algorithm may be selected (at 310) if only few target cells were identified (e.g., 1 or 2) for the requested handover and a measurement-based handover algorithm may be selected (at 310) if too many (e.g., 3 or more) target cells are identified, possibly indicating insecure target cell information. In one embodiment, the handover algorithm selection mechanism may be individually adapted according to operator needs and desired functionality.
If the controller determines (at 315) that the blind handover was not selected, then a measurement-based handover of the mobile unit may be performed (at 320). For example, the mobile unit may initiate (at 320) a soft handover between the serving cell and one or more target cells. In one embodiment, the mobile unit may also perform measurements of various characteristics of the air interfaces involved in the measurement-based handover. Information indicative of these measurements may be stored and later used for various purposes, including target cell selection for a subsequent blind handover and algorithm selection.
If the controller determines (at 315) that a blind handover has been selected (at 310), the information indicative of one or more measurements performed during measurement-based handovers may be accessed (at 325). For example, the controller may access (at 325) information indicative of channel qualities, signal strengths, round-trip delays, and the like associated with the serving cell and/or the target cell(s). The access information may then be used to select (at 330) a target cell for the blind handover, as discussed above. A blind handover may then be performed (at 335) from the serving cell to the target cell.
In one embodiment, target cell selection (at 330) and algorithm selection (at 310) may be handled in a single generalized approach comparable to a neural network. Statistical measures used for the selection process may be enhanced by taking into account the likelihood that each mobile unit may be in soft handover many times, or at least may be able to measure other strong pilot signals, and therefore may be able to acquire information that may be used for both target cell selection and handover algorithm selection. For example, when a mobile unit determines that it should perform a handover (e.g., the mobile unit approaches the coverage border of the serving cell) some or all of the information collected by the mobile unit during previous handovers and some or all of the information collected from the network may be accessed (at 325) and used to select (at 310) the algorithm and select (at 330) the target cell (in case blind handover was selected).
The information used to select (at 310) the algorithm and/or select (at 330) the target cell may include a mobile unit measurement report containing information about the current network constellation (e.g., the measurement results of the strongest pilots). The selection information may also include the current network load in the source cells and the failure rates for blind handover in the corresponding network area. However, persons of ordinary skill in the art having benefit of the present disclosure should appreciate that other or additional information may also be applied to the decision mechanism, such that the outcome might be different than with the input data listed above. Handover may then be executed along the lines of the outcome of the decision mechanism. The results of this process (e.g. actual measurement results, the selected target cell, handover success or failure, etc.) may be fed back to refine and improve the statistical data for the next handover events. In this way the decision mechanism can learn about the network conditions and the appropriate handover algorithms. Each handover event may therefore improve the statistics such that the decision mechanism may select (at 310, 330) the handover algorithm and/or target cell(s) for current network conditions in an optimal way.
The particular embodiments disclosed above are illustrative only, as the invention may be modified and practiced in different but equivalent manners apparent to those skilled in the art having the benefit of the teachings herein. Furthermore, no limitations are intended to the details of construction or design herein shown, other than as described in the claims below. It is therefore evident that the particular embodiments disclosed above may be altered or modified and all such variations are considered within the scope and spirit of the invention. Accordingly, the protection sought herein is as set forth in the claims below.
This application is related to U.S. patent application Ser. No. ______, filed concurrently herewith.
