# Patent

## Title
Forward handover under radio link failure

## Abstract

System(s) and method(s) are provided for forward handover under radio link failure. A configurable criterion for radio link failure (RLF) is established through a reference threshold for a channel quality metric; the threshold (CQITH) can be determined at least on a quality-of-service basis or a per-terminal basis. A low-overhead indication of RLF is conveyed as a NULL CQI when the channel quality metric is below threshold. Onset of RLF triggers preparation of forward handover (FHO) at a terminal within a predetermined timer, during which normal service can resume if link quality recovers above CQITH. Forward handover is pursued once timer elapses. Preparation at the terminal can be complemented by preparation implemented in advanced to RLF condition, in accordance to a disparate threshold applied to terminal's measurements. Preparation of FHO includes backhaul exchange of operational information like buffered data, and context.

## Background
CLAIM OF PRIORITY UNDER 35 U.S.C. §119
This Application for patent claims the benefit of U.S. Provisional Application Ser. No. 60/942,661 filed on Jun. 7, 2007, and entitled “A METHOD AND APPARATUS FOR FORWARD HANDOVER.” The entirety of this application is expressly incorporated herein by reference.


BACKGROUND
I. Field
The subject specification relates generally to wireless communications and, more particularly, to handover mechanism(s) for handover of a mobile terminal in a wireless communication system.
II. Background
Conventional technologies utilized for transmitting information within a mobile communication network (e.g., a cellular telephone network) include frequency, time and code division based techniques. In general, with frequency division based techniques calls are split based on a frequency access method, wherein respective calls are placed on a separate frequency. With time division based techniques, respective calls are assigned a certain portion of time on a designated frequency. With code division based techniques respective calls are associated with unique codes and spread over available frequencies. Respective technologies can accommodate multiple accesses by one or more users.
More particularly, frequency division based techniques typically separate the spectrum into distinct channels by splitting it into uniform chunks of bandwidth, for example, division of the frequency band allocated for wireless cellular telephone communication can be split into 30 channels, each of which can carry a voice conversation or, with digital service, carry digital data. Each channel can be assigned to only one user at a time. One commonly utilized variant is an orthogonal frequency division technique that effectively partitions the overall system bandwidth into multiple orthogonal subbands. These subbands are also referred to as tones, carriers, subcarriers, bins, and frequency channels. Each subband is associated with a subcarrier that can be modulated with data. With time division based techniques, a band is split time-wise into sequential time slices or time slots. Each user of a channel is provided with a time slice for transmitting and receiving information in a round-robin manner. For example, at any given time t, a user is provided access to the channel for a short burst. Then, access switches to another user who is provided with a short burst of time for transmitting and receiving information. The cycle of “taking turns” continues, and eventually each user is provided with multiple transmission and reception bursts.
Code division based techniques typically transmit data over a number of frequencies available at any time in a range. In general, data is digitized and spread over available bandwidth, wherein multiple users can be overlaid on the channel and respective users can be assigned a unique sequence code. Users can transmit in the same wide-band chunk of spectrum, wherein each user's signal is spread over the entire bandwidth by its respective unique spreading code. This technique can provide for sharing, wherein one or more users can concurrently transmit and receive. Such sharing can be achieved through spread spectrum digital modulation, wherein a user's stream of bits is encoded and spread across a very wide channel in a pseudo-random fashion. The receiver is designed to recognize the associated unique sequence code and undo the randomization in order to collect the bits for a particular user in a coherent manner.
A typical wireless communication network (e.g., employing frequency, time and code division techniques) includes one or more base stations that provide a coverage area and one or more mobile (e.g., wireless) terminals that can transmit and receive data within the coverage area. A typical base station can simultaneously transmit multiple data streams or flows for multicast or unicast services, and signaling. A mobile terminal within the coverage area of that base station can be interested in receiving one, more than one or all the data streams carried by the composite stream. Likewise, a mobile terminal can transmit data to the base station or another mobile terminal. Such communication between base station and mobile terminal or between mobile terminals can deteriorate due to radio link variations associated with interference inflicted by other terminals operating in a same or disparate coverage area, seasonal changes (e.g., weather fluctuation, time of day, . . . ), and so forth. Substantial degree of deterioration, or radio link failure, can lead to terminal handover to a disparate serving cell. Typically handover incurs a service disruption (e.g., a dropped call, a substantially slowed-down data transfer, and the like). There is therefore a need in the art for handover mechanism(s) that avoid service disruption without substantive overhead.
SUMMARY
The following presents a simplified summary in order to provide a basic understanding of some aspects of the disclosed embodiments. This summary is not an extensive overview and is intended to neither identify key or critical elements nor delineate the scope of such embodiments. Its purpose is to present some concepts of the described embodiments in a simplified form as a prelude to the more detailed description that is presented later.
The subject innovation provides system(s) and method(s) to implement forward handover under radio link failure. A configurable, unambiguous criterion for radio link failure (RLF) is established through a reference threshold for a channel quality metric. The threshold (CQITH) can be determined based at least in part on (a) quality of service (e.g., subscriber agreed service, traffic priority, time-frequency resources granted or available); (b) the capabilities of a served terminal (e.g., antenna configuration, multi-mode operation) that monitors radio link conditions, and the applications executed and mode of operation; or (c) network planning or configuration such as geographic deployment, frequency bands licensed for service, and so forth. A low-overhead (e.g., O(1 bit)) indication of RLF is conveyed as a NULL channel quality indicator when the channel quality metric is below the predetermined threshold. Onset of RLF triggers a timer for preparation of forward handover (FHO) during which service is retained and normal operation can resume if link quality recovers above CQITH. Forward handover is pursued once the timer expires. By controlling the magnitude of the time, latency associated with a handover procedure can be controlled. Preparation of FHO includes synchronization of a terminal that undergoes handover with a set of target cells, and exchange via a backhaul network backbone of operational information like system information, uplink conditions, buffered data, and context among a serving cell and the set of target cells.
In an aspect, the subject innovation discloses a method for facilitating handover in wireless system, the method comprising: establishing a radio link failure (RLF) criterion by configuring a threshold for a channel quality metric; receiving an indication of a radio link failure condition when the criterion is fulfilled; and preparing for handover.
In another aspect, a device that operates in a wireless environment is described, the device comprising: a processor configured to set a configurable threshold for a channel quality metric; to receive an indication of a radio link failure (RLF) condition when the channel quality metric is below the configurable threshold; to receive an indication of a radio link failure (RLF) condition when the channel quality metric is below threshold; and to prepare for handover under the RLF condition; and a memory coupled to the processor.
In yet another aspect, an apparatus that operates in a wireless environment is disclosed, the apparatus comprising: means for configuring a threshold for a channel quality metric; means for conveying the configured threshold for the channel quality metric, means for receiving an indication of a radio link failure (RLF) condition when the channel quality metric is below threshold; and means for preparing for handover under the RLF condition.
In a further yet aspect, the subject innovation discloses a computer program product comprising a computer-readable medium including: code for causing at least one computer to configure a threshold for a channel quality metric to determine a radio link failure (RLF) criterion; code for causing at least one computer to convey the configured threshold for the channel quality metric; code for causing at least one computer to receive an indication of a radio link failure condition when the channel quality metric is below threshold; and code for causing at least one computer to prepare for handover.
In an aspect, a method for forward handover under link failure in wireless system is disclosed, the method comprising: receiving a channel quality metric threshold, the threshold establishes a radio link failure criterion; conveying an indication of a RLF condition when the channel quality metric is below the threshold; preparing for forward handover; and pursuing forward handover.
In another aspect, an apparatus that operates in a wireless environment is described, the apparatus comprising: a processor configured to receive a channel quality metric threshold which determines a radio link failure criterion; to generate the channel quality metric; to convey a channel quality indicator (CQI) report that indicates a RLF condition when the channel quality metric is below the threshold; and to prepare for forward handover; and a memory coupled to the processor.
In yet another aspect, the subject innovation includes a wireless device comprising: means for receiving a radio link failure criterion, wherein the radio link failure criterion is determined by a threshold for a channel quality metric; means for conveying an indication of a RLF condition when the criterion is fulfilled; means for preparing for forward handover; and means for pursuing forward handover.
In a further aspect, the subject specification discloses a computer program product comprising a computer readable medium including: code for causing at least one computer to receive a channel quality metric threshold; code for causing at least one computer to convey an indication of a radio link failure condition when the channel quality metric is below the threshold; and code for causing at least one computer to pursue forward handover.
To the accomplishment of the foregoing and related ends, one or more embodiments comprise the features hereinafter fully described and particularly pointed out in the claims. The following description and the annexed drawings set forth in detail certain illustrative aspects and are indicative of but a few of the various ways in which the principles of the embodiments may be employed. Other advantages and novel features will become apparent from the following detailed description when considered in conjunction with the drawings and the disclosed embodiments are intended to include all such aspects and their equivalents.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 illustrates an example wireless multiple-access communication system in accordance with various aspects set forth herein.
FIG. 2 is a block diagram of an example system that facilitates a determination of radio link failure and forward handover in accordance with aspects described in the subject specification.
FIGS. 3A and 3B are two diagrams that depict, respectively, an example service disruption cycle and an example service recovery upon a radio link failure condition sets on.
FIGS. 4A and 4B illustrate two scenarios to determine a magnitude of a threshold CQITH that can be exploited to establish radio link failure based on channel performance metrics.
FIG. 5 is a diagram that illustrates example sources that determine radio link failure threshold.
FIG. 6 is a block diagram of an example system that facilitates handover under radio link failure in accordance with aspects disclosed herein.
FIG. 7 is a block diagram of an example embodiment of a transmitter system and a receiver system in MIMO operation that can exploit aspects described in the subject specification.
FIG. 8 is a block diagram of an example multiple-user MIMO system.
FIG. 9 is a flowchart of an example method for preparing for preparing for handover upon radio link failure according to aspects described in the subject specification.
FIG. 10 is a flowchart of an example method for establishing a radio link failure criterion according to aspects of the subject innovation described herein.
FIG. 11 presents a flowchart of an example method for preparing for forward handover under radio link failure according to aspects described herein.
FIG. 12 presents a flowchart of an example method for preparing for forward handover under radio link failure according to aspects set forth herein.
FIG. 13 illustrates a block diagram of an example system that enables establishing a criterion for radio link failure in accordance with aspects disclosed in the subject specification.
FIG. 14 illustrates a block diagram of an example system that enables utilization of an RLF indication to implement handover in accordance with aspect described in the subject specification.



DETAILED DESCRIPTION
Various embodiments are now described with reference to the drawings, wherein like reference numerals are used to refer to like elements throughout. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more embodiments. It may be evident, however, that such embodiment(s) may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more embodiments.
As used in this application, the terms “component,” “module,” “system,” and the like are intended to refer to a computer-related entity, either hardware, firmware, a combination of hardware and software, software, or software in execution. For example, a component may be, but is not limited to being, a process running on a processors a processor, an object, an executable, a thread of execution, a program, and/or a computer. By way of illustration, both an application running on a computing device and the computing device can be a component. One or more components can reside within a process and/or thread of execution and a component may be localized on one computer and/or distributed between two or more computers. In addition, these components can execute from various computer readable media having various data structures stored thereon. The components may communicate by way of local and/or remote processes such as in accordance with a signal having one or more data packets (e.g., data from one component interacting with another component in a local system, distributed system, and/or across a network such as the Internet with other systems by way of the signal).
Moreover, the term “or” is intended to mean an inclusive “or” rather than an exclusive “or.” That is, unless specified otherwise, or clear from context, “X employs A or B” is intended to mean any of the natural inclusive permutations. That is, if X employs A; X employs B; or X employs both A and B, then “X employs A or B” is satisfied under any of the foregoing instances. In addition, the articles “a” and “an” as used in this application and the appended claims should generally be construed to mean “one or more” unless specified otherwise or clear from context to be directed to a singular form.
Various embodiments are described herein in connection with a wireless terminal. A wireless terminal may refer to a device providing voice and/or data connectivity to a user. A wireless terminal may be connected to a computing device such as a laptop computer or desktop computer, or it may be a self contained device such as a personal digital assistant (PDA). A wireless terminal can also be called a system, a subscriber unit, a subscriber station, a mobile station, a mobile terminal, a mobile, a remote station, an access point, a remote terminal, an access terminal, a user terminal, a user device, a user equipment (UE), a cellular telephone, a personal communication service (PCS) telephone, a cordless telephone, a session initiation protocol (SIP) phone, a wireless local loop (WLL) station, a handheld device having wireless connection capability, or other processing device connected to a wireless modem, and the like.
Additionally, embodiments described in the subject specification related to a base station, which it may refer to a device in an access network that communicates over the air-interface, through one or more sectors, with wireless terminals, and with other base stations through wired or wireless backhaul network communication. The base station may act as a router between the wireless terminal and the rest of the access network, which may include an internet-protocol (IP) network, by converting received air-interface frames to IP packets. The base station can also coordinate management of attributes for the air interface through a scheduler which establishes communication priorities and resources. A base station may also be referred to as an access point, Node B, evolved Node B (eNodeB), home Node B, or some other terminology.
Referring now to the drawings, FIG. 1 is an illustration of a wireless multiple-access communication system 100 in accordance with various aspects. In one example, the wireless multiple-access communication system 100 includes multiple base stations 110 and multiple terminals 120. Further, one or more base stations 110 can communicate through a radio link 125 with one or more of the terminals 120. By way of non-limiting example, a base station 110 can be an access point, an eNode B, or another appropriate network entity. Each base station 110 provides communication coverage for a particular geographic area 102a-c. It should be appreciated that while the coverage areas are illustrated as substantially the same hexagonal cells, size and shape of coverage cells need not be neither substantially the same nor identical; size and shape can be determined by various factors such as geography, landscape, operator(s) spectrum boundaries, population (e.g., rural, urban, etc.), and so forth. As utilized herein, the term “cell” can refer to a base station 110 and/or its coverage area 102a-c depending on the context in which the term is used.
To improve system capacity, the coverage area 102a, 102b, or 102c corresponding to a base station 110 can be partitioned into multiple smaller areas (e.g., areas 104a, 104b, and 104c), or sectors. Each of the smaller areas, or sectors, 104a, 104b, and 104c can be served by a respective base transceiver subsystem (BTS, not shown. As utilized herein, the term “sector” can refer to a BTS and/or its coverage area depending on the context in which the term is used. In addition, in one example, sectors 104a, 104b, 104c in a cell 102a, 102b, 102c can be formed and served in accordance to beamforming configurations generated by groups of antennas (e.g., smart antennas; not shown) operated by a base station 110, where each group of antennas is responsible for communication with terminals 120 in a portion of the cell 102a, 102b, or 102c. It should be appreciated that sector generation through beamforming can be dynamic, in accordance with operation needs and communication (e.g., radio link) conditions, reflected through channel quality indicator reports. For example, a base station 110 serving cell 102a can have a first antenna group corresponding to sector 104a, a second antenna group corresponding to sector 104b, and a third antenna group corresponding to sector 104c. However, it should be appreciated that the various aspects disclosed herein can be used in a system having sectorized and/or unsectorized cells. Further, it should be appreciated that all suitable wireless communication networks having any number of sectorized and/or unsectorized cells are intended to fall within the scope of the hereto appended claims. For simplicity, the term “base station” as used herein can refer both to a station that serves a sector as well as a station that serves a cell. As further used herein, a “serving” access point is one with which a terminal has uplink traffic (data) transmissions, and a “target” (currently non-serving) access point is one with which a terminal may have downlink traffic and/or both downlink and uplink control transmissions but no uplink traffic. While the following description generally relates to a system in which each terminal communicates with one serving access point for simplicity, it should be appreciated that terminals can communicate with any number of serving access points.
In accordance with one aspect, terminals 120 can be dispersed throughout the system 100. Each terminal 120 can be stationary (e.g., a wireless router) or mobile (e.g., a cellular telephone). Mobile access terminals rely on handover mechanisms to switch from a serving access point to a disparate target cell to ensure mobility. In the subject innovation, mobility mechanism(s) can be triggered in accordance with radio link failure conditions configured by a network operator. By way of non-limiting example, a terminal 120 can refer to an access terminal (AT), a mobile station, user equipment, a subscriber station, a wireless device, a cellular telephone, a personal digital assistant (PDA), a wireless modem, a handheld device, and/or another appropriate device or network entity. Further, a terminal 120 can communicate with any number of base stations 110 or no base stations 110 at substantially any given moment.
In another example, the system 100 can utilize a centralized architecture by employing a system controller 130, or substantially any other network management component, that can be coupled to one or more base stations 110 and provide coordination and control for the base stations 110. In accordance with alternative aspects, system controller 130 can be a single network entity or a collection of network entities. Additionally, the system 100 can utilize a distributed architecture to allow the base stations 110 to communicate with each other as needed. Backhaul network backbone 135 can facilitate point-to-point communication between base stations employing such a distributed architecture. Backhaul link 135 can be a wire-based (e.g., T1/E1 lines or other links of the T-carrier/E-carrier protocol, digital subscriber lines) or fiber-optic-based. Communication over backhaul backbone 135 can be circuit-switched, packet-switched (e.g., based on internet protocol) or a combination thereof. In one example, system controller 130 can additionally contain one or more connections to multiple networks. These networks can include the Internet, other packet based networks, and/or circuit switched voice networks that can provide information to and/or from terminals 120 in communication with one or more base stations 110 in system 100. In another example, system controller 130 can include or be coupled with a scheduler (not shown) that can schedule transmissions to and/or from terminals 120. Alternatively, the scheduler can reside in each individual cell 102, each sector 104, or a combination thereof.
In an example, system 100 can utilize one or more multiple-access schemes, such as CDMA, TDMA, FDMA, OFDMA, Single-Carrier FDMA (SC-FDMA), and/or other suitable multiple-access schemes. TDMA utilizes time division multiplexing (TDM), wherein transmissions for different terminals 120 are orthogonalized by transmitting in different time intervals. FDMA utilizes frequency division multiplexing (FDM), wherein transmissions for different terminals 120 are orthogonalized by transmitting in different frequency subcarriers. In one example, TDMA and FDMA systems can also use code division multiplexing (CDM), wherein transmissions for multiple terminals can be orthogonalized using different orthogonal codes (e.g., Walsh codes, polyphase sequences, Kasami codes) even though they are sent in the same time interval or frequency sub-carrier. OFDMA utilizes Orthogonal Frequency Division Multiplexing (OFDM), and SC-FDMA utilizes Single-Carrier Frequency Division Multiplexing (SC-FDM). OFDM and SC-FDM can partition the system bandwidth into multiple orthogonal subcarriers (e.g., tones, bins, . . . ), each of which can be modulated with data. Typically, modulation symbols are sent in the frequency domain with OFDM and in the time domain with SC-FDM. Additionally and/or alternatively, the system bandwidth can be divided into one or more frequency carriers, each of which can contain one or more subcarriers. System 100 can also utilize a combination of multiple-access schemes, such as OFDMA and CDMA. While the power control techniques provided herein are generally described for an OFDMA system, it should be appreciated that the techniques described herein can similarly be applied to any wireless communication system.
In another example, base stations 110 and terminals 120 in system 100 can communicate data using one or more data channels and signaling using one or more control channels. Data channels utilized by system 100 can be assigned to active terminals 120 such that each data channel is used by only one terminal at any given time. Alternatively, data channels can be assigned to multiple terminals 120, which can be superimposed or orthogonally scheduled on a data channel. To conserve system resources, control channels utilized by system 100 can also be shared among multiple terminals 120 using, for example, code division multiplexing. In one example, data channels orthogonally multiplexed only in frequency and time (e.g., data channels not multiplexed using CDM) can be less susceptible to loss in orthogonality due to channel conditions and receiver imperfections than corresponding control channels.
FIG. 2 is a block diagram of an example system that facilitates a determination of radio link failure and forward handover in accordance with aspects described in the subject specification. In system 200, a serving eNode B 210 includes a handover component 215 that can facilitate handover to a target cell (not shown) once a radio link failure (RLF) condition has been identified. Handover component 215 can utilize processor 225 to execute handover algorithms 238 stored in memory 235. In an aspect, a RLF condition can be established by a network operator and stored within a set of handover algorithms 238. As an example, a RLF can be dictated by a set of channel quality metric thresholds {CQITH} 246. The set of {CQITH} 246 can be time-dependent (e.g., seasonal, time of day, adjusted in accordance to execution of an application, etc.) and/or based at least in part on the served user equipment 250, and specific performance metrics utilized to assess channel quality. Once a threshold channel quality indicator threshold CQITH 248 is assigned to UE 250, it can be conveyed thereto through downlink. It should be appreciated that optimal magnitude of CQITH arises from a tradeoff between mitigation of handover and retention of satisfactory link conditions to ensure satisfactory service. Intelligent component 218 can implement such an optimization.
Channel quality metrics that can be employed for establishing a radio link failure criterion based on channel quality threshold(s) {CQI} 246 include: reference signal received power (RSRP) indicates signal strength (ES) and reveals DL path loss, or attenuation, associated with attenuation of radiation conveyed in DL physical channels; reference signal strength indicator (RSSI), which typically conveys the effects of interference, indicating the strength of a signal relative to average background noise, e.g., carrier to interference ratio (C/I), or signal to noise ratio (SNR), the measured background noise can include intra-cell and inter-cell interference; reference signal over thermal noise (RSOT), typically conveyed as ES/N0. Derived metrics such as signal to noise and interference ratio (SINR) can also be employed to determine a performance metric threshold that establishes a radio link failure criterion. In addition, communication performance metrics like capacity or throughput can also be utilized to define a generalized radio link failure in a cell or sector.
An advantage of an unambiguously established RLF criterion based on channel quality and channel quality indicator reports is that service eNode B 210 receives DL CQI reports 290 periodically or aperiodically in accordance with CQI-report repetition factors, determined by CQI generator 255 for example, in substantially any technology utilized for wireless communication; thus, the RLF criterion can be regarded as universal. It should be noted that that such repetition factors for CQI=NULL 294 indication can be disparate from CQI-report repetition factors utilized by UE 250 within normal operation. Repetition factors can be adequately adjusted (typically upwards), e.g., via handover component 265, in order to ensure the integrity of RLF indication(s) since channel condition are in a failure condition. Another advantage is that multiple RLF criteria can be established through determination of disparate channel quality performance thresholds CQITH on a per-terminal basis, a per-flow basis, a per-QoS basis, and so on. Further yet, an advantage of utilization of CQI=NULL 294 as an indication of RLF is that it is a lightweight, low-overhead (O(1 bit)) mechanism.
User equipment 250 can receive CQITH 248 and store it in memory 285. In an aspect, as mentioned above, CQITH 248 can be utilized to establish a RLF condition. In an aspect, CQI generator 255, via processor 275, can determine a channel quality metric typically based on a set of DL pilot sequences and convey a DL CQI 290 report to service eNode B 210. When DL channel quality metric is below CQITH 248, a radio link failure condition arises, and UE 250 conveys a CQI=NULL 294 to indicate such RLF condition. Additionally, handover component 265 can initiate synchronization with disparate eNode Bs (e.g., target cells) in preparation for forward handover. Target cell search can proceed for a predetermined time interval which can be monitored by timer(s) 248 triggered by handover component 265 via processor 275 at the time a RLF condition has been indicated; in an aspect, the indication is conveyed by physical layer to upper layers in UE 250 (layer(s) not illustrated).
Additionally, an RLF indication (e.g., CQI=NULL 294) can trigger one or more RLF timer(s) 242; handover component 215 typically starts the RLF timer, which spans a time interval that is configurable by a network operator through handover component 215. Radio link failure timer(s) 242 can be configured to optimize mitigate jitter and latency during a communication session (e.g., voice or data transfer). Optimization can be effected through intelligent component 218.
Generally, DL channel conditions report(s) can be stored in memory 235 if necessary. Generation of broadband or narrowband downlink (DL) reference signals, which can be transmitted to user equipment 250 for generation of channel quality indications via CQI generator 255, can be implemented by handover component 215. Generation of DL channel conditions can effected periodically, the period for CQI reporting can be determined by traffic and load in a cell/sector covered by serving eNode B 210. In addition, the period for reporting can depend on reported DL channel conditions, in order to ensure accurate DL CQI 290 is received at serving eNode B 210. In addition, DL CQI 290 generation and reporting can be event triggered, e.g., an additional wireless device enters the coverage area of eNode B 210, an access terminal in a neighboring cell transmits a burst of data substantially increasing other sector interference, a data buffer size (e.g., residing in memory 285) increases and communication resources for UE 250, or a disparate UE, needs adjustment, and so on.
In an aspect, handover component 215 can rely upon an intelligent component 218 to autonomously identify (e.g., find, adapt, optimize) optimal values of CQITH 246, RLF timer(s) 242, or repetition factors for DL CQI report(s) 290 based at least in part on quality of service, access terminal(s) served, frequency planning, cell/sector capacity, or throughput, and battery utility, and so forth. To that and other ends related to adaptation or optimization in other portions of the subject description associated with additional functionalities of the subject innovation, the term “intelligence” refers to the ability to reason or draw conclusions about, e.g., infer, the current or future state of a system based on existing information about the system. Artificial intelligence can be employed to identify a specific context or action, or generate a probability distribution of specific states of a system without human intervention. Artificial intelligence relies on applying advanced mathematical algorithms—e.g., decision trees, neural networks, regression analysis, cluster analysis, genetic algorithms, and reinforced learning—to a set of available data (information) on the system.
In particular, to the accomplishment of the various automated aspects described above and other automated aspects relevant to the subject innovation described herein, an intelligent component (not shown) can employ one of numerous methodologies for learning from data and then drawing inferences from the models so constructed, e.g., Hidden Markov Models (HMMs) and related prototypical dependency models, more general probabilistic graphical models, such as Bayesian networks, e.g., created by structure search using a Bayesian model score or approximation, linear classifiers, such as support vector machines (SVMs), non-linear classifiers, such as methods referred to as “neural network” methodologies, fuzzy logic methodologies, and other approaches that perform data fusion, etc.
It is to be noted that processors 225 and 275 are configured to perform at least a portion of the functional actions, e.g., computations, necessary to implement the functionality of substantially any component in serving eNode B 210 and access terminal 250, respectively. Memories 235 and 285 can retain respective data structures, code instructions, algorithms, and the like, that can be employed by respective processors 216 and 236 when conferring base station 210 or UE 230 its functionality.
FIGS. 3A and 3B are two diagrams that depict, respectively, an example service disruption cycle (diagram 300) and an example service recovery (diagram 350) upon a radio link failure condition sets on. Service disruption. FIG. 3A presents an example time-dependent profile of a channel quality indicator CQI (e.g., SNR, SINR, C/I, inter-cell interference, intra-cell interference, . . . ) conveyed in reporting intervals Δt 328 (e.g., a set of transmission time intervals, or a set of sub-transmission time intervals): typically Δt 328 is dictated by the time-response of electronics associated with measurement components utilized to measure and process performance metric(s), e.g., RSOT, RSRP, etc., utilized to determine a CQI. It should be appreciated that Δt 328 can vary from reporting period to reporting period. In diagram 300, after reported values of CQI 324 during normal operation 332, a radio link failure condition 304 is attained and a CQI=NULL 312 indication is conveyed as a result of determined (e.g., estimated) CQI values below CQITH 320. In example diagram 300, RLF condition remains for a time interval τ1 316, after which forward handover (FHO) can be pursued 336. As sketched in block 308, it is to be noted that throughout time interval τ1 316, telecommunication service is retained—even though new traffic can be prevented from being scheduled—and forward handover can be prepared; e.g., an access terminal can pre-synchronize to, or conduct a coarse synchronization with, target eNode Bs. It is noted that time-region 308 in which FHO is prepared can span a time interval longer than τ1 316, with advanced preparation triggered in accordance to a threshold, different from CQITH 320, configured by a serving eNode B (e.g., eNode B 210) for UE measurements Service can be disrupted once FHO is pursued, and can typically be recovered once a mobile that pursues FHO attaches to a suitable target cell. It should be appreciated that timer τ1 316 can be configured on a per-terminal basis, per-sector basis, per-QoS basis, per-application basis, etc., and can be set to zero. Adjustment of the magnitude of time τ1 can facilitate control of the latency associated with a handover procedure.
Service recovery. As in diagram 300, FIG. 3B presents an example time-dependent profile of CQI conveyed in reporting intervals Δt 328. In diagram 350, radio link failure 354 onset is indicated by CQI=NULL 358, a conditions that spans a time interval τ2 362 after which radio link condition improves and normal operation resumes (e.g., recovery 366) as reflected by CQI above CQITH 320. It is to be noted that in a recovery 366 scenario, RLF time interval τ2 362 is shorter than configured time τ1 316—an upper bound timer in which service is retained and FHO can be prepared. In an aspect, recovery can entail newly scheduled packets, introduction of new radio bearers, etc. In view of possible operational scenarios in which service recovery occurs within time interval τ2 362, it should be appreciated that timer τ1 316 mitigates excessive FHO pursuits and ensuing target cell attachments. Such mitigation results in a reduction of operational complexity through a reduction of unwarranted (in view of possible service recovery) hops to target cells.
FIGS. 4A and 4B illustrate two scenarios to determine a magnitude of threshold CQITH to establish radio link failure based on channel performance metrics.
FIG. 4A is a diagram that illustrates spatial diversity of CQITH. In diagram 400, a service cell 405 is divided in three sectors 4101-4103, served by base station 415, each sector can be assigned a CQITH: CQITH(λ) 420λ with λ=1, 2, 3. Having spatial CQI threshold diversity provides at least the advantage of customization of threshold conditions according to specific characteristics of each sector. For example, sector 4101 can be a heavily populated are of a downtown and thus a mobile station can be submitted to substantial intra-sector interference, therefore a low CQITH(1) can be adequate to mitigate likely hopping of terminals into neighboring cell(s)/sector(s) as a result of likely poor radio conditions. In addition, sector 4102 can be a less populated area with generally high channel signal strength, thus an elevated CQITH(2) can be appropriate in order to ensure mobility and mitigate high load levels of the sector. Furthermore, sector 4103 can correspond to a region with a substantial number of roads and high vehicular traffic, in such scenario a low value of CQITH(3) can be appropriate since mobility is highly driven by the operation conditions (highly mobile terminals) and it may be desirable to mitigate handoff mobility associated with radio link failure. It should be appreciated that sectorizations are possible, with cells generally split into M sectors (M a positive integer) with M dependent on service cell.
FIG. 4B illustrates terminal diversity, or per-terminal configuration, of CQITH. In example diagram 450, coverage cell 455 serves mobile stations 46014603 through, respectively, radio links 4701-4703. Radio link failure conditions of such links can be established through respective channel performance metric thresholds: CQITH(κ) 475κ with κ=1, 2, 3. Each radio link failure threshold can be adjusted in accordance with user equipment capabilities—e.g., multi-mode operation, DL/UL maximum data rates, antenna configuration, video- and music-streaming capabilities, packet-switched operation—in order to optimize terminal performance.
FIG. 5 is a diagram 500 that illustrates example sources that determine radio link failure threshold. The sources comprise three groups which can facilitate determination of CQITH: (i) Quality of service 505. This group includes subscriber agreed service, traffic priority, and time-frequency resources granted. Subscriber agreed service can comprise guaranteed bit rate (GBR), average bit rate (ABR), and minimum bit rate (MINBR); block error rate (BLER), packet error rate (PER), bit error rate (BER); and peak data rate (PDR). Disparate subscribers can have access to disparate rate levels; for instance, enterprise subscribers can have access to hard QoS that ensures a specific GBR rather than a MINBR. Subscriber agreed quality of service can be determined on a per-flow basis, wherein disparate applications executed by a subscriber are serviced within different agreed rates. Granted time-frequency resources characterize a radio link; CQITH can be configured in accordance with operational system bandwidth, allocated subcarriers and/or resource blocks, allocated power, and so forth. In order to ensure agreed level of service, CQITH can be determined on a per subscriber-basis, or a per-flow basis.
(ii) Access terminal. Disparate application(s) executed in a mobile station can have disparate CQITH to ensure satisfactory execution and subscriber experience. In an aspect, radio link failure threshold (e.g., CQITH) can be configured in accordance with whether radio link resilience, or integrity, is critical to the application. For instance, voice communication, ecommerce, or wireless banking can depend critically on radio link failure, whereas application such as web browsing, or file transfer can tolerate substantive radio link failure without significant execution degradation. Additionally, disparate mobile stations can operate within different communication modes like single input-single output (SISO), single input-multiple output (SIMO), and single-user (SU) multiple-input-multiple output (MIMO) and multiple-user (MU) MIMO. Such modes of operation, and their afforded capacity, depend to substantially different extents on access to channel state information (e.g., DL CQI 290) at a served terminal. Consequently, a radio link failure threshold can be configured for each scheduled mode of operation of a terminal so as to ensure availability of channel quality indicators. Furthermore, user equipment generally possesses a specific set of technical capabilities like number of antennas, multi-mode (e.g., multiple system bandwidth operation; multiple telecommunication technology such as WCDMA, HSDPA; or telecommunication services like GPS) or single-mode chipsets, battery resources (e.g., long discharge characteristic time, solar-power assisted, . . . ), and so on, which can result in substantially different operation performance based at least in part on radio link threshold. For example, a multi-mode terminal that can operate with a substantive CQITH since it can synchronize to target eNode Bs serving disparate bandwidths and frequency bands.
(iii) Network planning. Radio link failure threshold (e.g., CQITH) can be configured in accordance with available (e.g., licensed) frequency bands and bandwidth for operation, and geographic deployment characteristics (e.g., urban, metropolitan, rural). It is to be noted that in LTE, bandwidth typically determines the number of frequency subcarriers available for communication.
FIG. 6 is a block diagram of an example system 600 that facilitates handover under radio link failure. Components of system 600 possess substantially the same functionality as in system 200. Serving eNode B 210 and can receive an indication of radio link failure, e.g., CQI=NULL 294, when channel quality metric(s) lead to a channel quality indicator below a threshold CQITH 248 (not shown in FIG. 6). Reports are generated by served user equipment 250 through a CQI generator 255, as discussed above. It should be appreciated that as the indication of radio link failure condition(s) can trigger handoff of user equipment 250, reports of downlink channel performance metrics DL CQI 290 are sufficient, as opposed to generation of both UL and DL CQI.
In an aspect, when a RLF condition indication (e.g., CQI=NULL 294) is received over the air-interface, serving eNode B 210 can trigger a RLF timer, which can span a time interval τ1 605, during which serving eNode B 210 can prepare a set of target cells 6401-640N (N a positive integer) for handover. In another aspect, preparation for handover can be initiated in advance to a RLF condition, advanced preparation can be triggered in accordance to a threshold ATH for measurements performed, and conveyed to serving Node B 210, by user equipment 250. Threshold for advanced preparation ATH is different from CQITH, even though it can be related thereto. It should be appreciated that ATH can also be a part of the set of thresholds 246 stored in memory 235 (FIG. 1). It is noted that substantially the same sources, as described hereinbefore, that can be exploited to determine RLF threshold, e.g., CQITH, can be utilized in substantially the same manner to determine a set of thresholds {ATH} for advanced handover preparation. Preparation can be supported through communication over backhaul network backbone 610, such communication benefits from being substantially insensitive to communication channel conditions, as the communication is point-to-point among base stations (e.g., 210 and 640J). Information exchanged among serving eNode B 210 and target eNode Bs 6401-640N in preparation for handover can include (i) system information 615, which can comprise location information (e.g., location area, routing area), cell identity, pilot sequences (e.g., root indices for Zadoff-Chu codes) utilized for synchronization, system bandwidth which in LTE, for example, can include multiple values ranging from 1.25 MH to 20 MHz; pilot sequences employed in DL and UL reference signals; system frame number; CQI channel directives that determine repetition factors of CQI reports and frequency, or periodicity, of determination of channel quality indications; cell DL timing, in particular in asynchronous systems; and UL timing offset with respect to DL communications, physical random access channel (PRACH) configuration (e.g., preamble size, or preamble content), or management frame(s) configuration; symbol and cyclic prefix timing; allocated power or power density, and so forth; (ii) buffered data 625, which can be scheduled or unscheduled traffic as an aspect of preparation for handover can include a cessation of data scheduling; and (iii) packet data protocol (PDP) context transfer 635 which can include logical and transport channels utilized by user equipment 250, operating radio bearers, radio resources control signaling, and so on; (iv) a set of UL channel condition metrics {UL CQI}1-N 638 between user equipment 250 and target cells 6401-640N, which can then be conveyed by serving eNode 210 to UE 250 to facilitate handover resolution, such metrics can be received by base station 210 periodically, or due to triggering events in a target cell, e.g., a specific cell/sector load level is reached, a specific intra-cell or inter-cell interference level is detected, new system information is scheduled, and so forth.
It should be appreciated that target cells are typically neighboring cells, wherein whether a cell/sector is a neighbor can be determined not only on spatial consideration but also on signal-strength consideration based on UL measurements conducted at target cells; for instance, such measurements can be triggered by handover component 265 when a RLF indication is generated by CQI generator 255.
To effect handover, handover component 265 can utilize received information associated with target cells 6401-640N to select a target eNode B, e.g., 640J, and pursue a forward handover (FHO) resolution 652 which generally comprise signaling, e.g., FHO request, FHO request acknowledge, and UL/DL resource grant(s), among the selected target cell and UE 250. Once FHO is resolved, data exchange 648 can take place, wherein the data includes buffered data 625 received over backhaul network link 610, X2 interface in LTE, in preparation for FHO. It should be noted that once FHO is resolved, synchronization among UE 250 and the target cell can be performed and system information associated with the target cell can be read. Handover algorithm(s) employed by UE 250, through handover component 265 and processor 275, to resolve forward handoff can reside in memory 285.
FIG. 7 is a block diagram 700 of an embodiment of a transmitter system 710 (such as eNode B 210) and a receiver system 750 (e.g. user equipment 250) in a multiple-input multiple-output (MIMO) system that can provide for cell (or sector) communication in a wireless environment in accordance with one or more aspects set forth herein. At the transmitter system 710, traffic data for a number of data streams can be provided from a data source 712 to transmit (TX) data processor 714. In an embodiment, each data stream is transmitted over a respective transmit antenna. TX data processor 714 formats, codes, and interleaves the traffic data for each data stream based on a particular coding scheme selected for that data stream to provide coded data. The coded data for each data stream may be multiplexed with pilot data using OFDM techniques. The pilot data is typically a known data pattern that is processed in a known manner and can be used at the receiver system to estimate the channel response. The multiplexed pilot and coded data for each data stream is then modulated (e.g., symbol mapped) based on a particular modulation scheme (e.g., binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), multiple phase-shift keying (M-PSK), or M-ary quadrature amplitude modulation (M-QAM), with M a positive integer) selected for that data stream to provide modulation symbols. The data rate, coding, and modulation for each data stream may be determined by instructions executed by processor 730, the instructions as well as the data can be stored in memory 732. Handover component 734 can execute, via processor 734, handover algorithms which can include determination of a radio link failure criterion, management of timers associated with RLF, or implementation of traffic scheduling changes (wherein a scheduler can reside within processor 770, for example) and exchange of system information with target cells in preparation for handover. Handover algorithms can be stored in memory 732.
The modulation symbols for all data streams are then provided to a TX MIMO processor 720, which may further process the modulation symbols (e.g., OFDM). TX MIMO processor 720 then provides NT modulation symbol streams to NT transceivers (TMTR/RCVR) 722A through 722T. In certain embodiments, TX MIMO processor 720 applies beamforming weights (or precoding) to the symbols of the data streams and to the antenna from which the symbol is being transmitted. Each transceiver 722 receives and processes a respective symbol stream to provide one or more analog signals, and further conditions (e.g., amplifies, filters, and upconverts) the analog signals to provide a modulated signal suitable for transmission over the MIMO channel. NT modulated signals from transceivers 722A through 722T are then transmitted from NT antennas 7241 through 724T, respectively. At receiver system 750, the transmitted modulated signals are received by NR antennas 7521 through 752R and the received signal from each antenna 752 is provided to a respective transceiver (RCVR/TMTR) 754A through 754R. Each transceiver 7541-754R conditions (e.g., filters, amplifies, and downconverts) a respective received signal, digitizes the conditioned signal to provide samples, and further processes the samples to provide a corresponding “received” symbol stream.
An RX data processor 760 then receives and processes the NR received symbol streams from NR transceivers 7541-754R based on a particular receiver processing technique to provide NT “detected” symbol streams. The RX data processor 760 then demodulates, deinterleaves, and decodes each detected symbol stream to recover the traffic data for the data stream. The processing by RX data processor 760 is complementary to that performed by TX MIMO processor 720 and TX data processor 714 at transmitter system 710. A processor 770 periodically determines which pre-coding matrix to use, such a matrix can be stored in memory 772. Processor 770 is configured to formulate uplink message(s) comprising a matrix index portion and a rank value portion. Memory 772 may store instructions that when executed by processor 770 result in formulation of the uplink message(s). Uplink message(s) can comprise various types of information regarding the communication link or received data stream(s)/flow(s), or a combination thereof. In particular, such information can comprise channel quality indicator report(s) (e.g., DL CQI 290). Handover component 734 can operate, at least partially, through processor 770. In particular, handover component 774 can generate an indication of radio link failure (e.g., CQI=NULL 358) through link (or channel) estimation and comparison with a RLF link failure criterion based on channel quality. Additionally, handover component 774 can execute, at least in part via processor 770, handover algorithms which can be stored in memory 772. The uplink message(s) is then processed by a TX data processor 738, which also receives traffic data for a number of data streams from a data source 736, modulated by a modulator 780, conditioned by transceiver 754A through 754R, and transmitted back to transmitter system 710.
At transmitter system 710, the modulated signals from receiver system 750 are received by antennas 7241-724T, conditioned by transceivers 722A-722T, demodulated by a demodulator 740, and processed by a RX data processor 742 to extract the reserve link message transmitted by the receiver system 750. Processor 730 then determines which pre-coding matrix to use for determining the beamforming weights and processes the extracted message.
A mobile station 230 can be scheduled to operate in SIMO, SU-MIMO, and MU-MIMO, depending at least in part on the channel quality indicators reported by said receiver. Next, communication in these modes of operation is described. It is noted that in SIMO mode a single antenna at the receiver (NR=1) is employed for communication; therefore, SIMO operation can be interpreted as a special case of SU-MIMO. Single-user MIMO mode of operation corresponds to the case in which a single receiver system 750 communicates with transmitter system 710, as previously illustrated FIG. 7 and according to the operation described in connection therewith. In such a system, the NT transmitters 7241-724T (also known as TX antennas) and NR receivers 7521-752R (also known as RX antennas) form a MIMO matrix channel (e.g., Rayleigh channel, or Gaussian channel, with slow or fast fading) for wireless communication. As mentioned above, the SU-MIMO channel is described by a NR×NT matrix of random complex numbers. The rank of the channel equals the algebraic rank of the NR×NT matrix, which in terms of space-time, or space-frequency coding, the rank equals the number NV≦min{NT, NR} of independent data streams (or layers) that can be sent over the SU-MIMO channel without inflicting inter-stream interference.
In one aspect, in SU-MIMO mode, transmitted/received symbols with OFDM, at tone ω, can be modeled by:

y(ω)=H(ω)c(ω)+n(ω).  (1)

Here, y(ω) is the received data stream and is a NR×1 vector, H(ω) is the channel response NR×NT matrix at tone ω (e.g., the Fourier transform of the time-dependent channel response matrix h), c(ω) is an NT×1 output symbol vector, and n(ω) is an NR×1 noise vector (e.g., additive white Gaussian noise). Precoding can convert a NV×1 layer vector to NT×1 precoding output vector. NV is the actual number of data streams (layers) transmitted by transmitter 710, and NV can be scheduled at the discretion of the transmitter (e.g., transmitter 710, eNode B 210, or access point 110) based at least in part on channel conditions (e.g., reported CQI, like DL CQI 290, according to a reporting approach established by a serving access point) and the rank reported in a scheduling request by a terminal (e.g., receiver 750). It should be appreciated that c(ω) is the result of at least one multiplexing scheme, and at least one precoding (or beamforming) scheme applied by the transmitter. Additionally, c(ω) can be convoluted with a power gain matrix, which determines the amount of power transmitter 710 allocates to transmit each data stream NV. It should be appreciated that such a power gain matrix can be a resource that is assigned to a terminal (e.g., access terminal 230, receiver 750, or UE 230) through a scheduler in the serving node in response, at least in part, to a reported CQI.

As mentioned above, according to an aspect, MU-MIMO operation of a set of terminals is within the scope of the subject innovation. Moreover, scheduled MU-MIMO terminals operate jointly with SU-MIMO terminals and SIMO terminals. FIG. 8 illustrates an example multiple-user MIMO system 800 in which three ATs 850P, 850U, and 850S, embodied in receivers substantially the same as receiver 850, communicate with transmitter 810, which embodies a Node B. It should be appreciated that operation of system 800 is representative of operation of substantially any group of wireless devices scheduled in MU-MIMO operation within a service cell by a centralized scheduler residing in a serving access point. As mentioned above, transmitter 710 has NT TX antennas 7241-724T, and each of the ATs has multiple RX antennas; namely, ATP has NP antennas 7521-752P, APU has NU antennas 7521-752U, and APS has NS antennas 752′-752S. Communication between terminals and the access point is effected through uplinks 815P, 815U, and 815S. Similarly, downlinks 810P, 810U, and 810S facilitate communication between Node B 710 and terminals ATP, ATU, and ATS, respectively. Additionally, communication between each terminal and base station is implemented in substantially the same manner, through substantially the same components, as illustrated in FIG. 7 and discussed in the description thereof.
Terminals can be located in substantially different locations (see FIG. 1) within the cell serviced by access point 710, therefore each user equipment 750p, 650U, and 650S has its own MIMO matrix channel hα and response matrix Hα (α=P, U, and S), with its own rank (or, equivalently, singular value decomposition), and its own associated channel quality indicator. Due to the plurality of users present in the cell serviced by the base station 710, intra-cell interference can be present and can affect CQI values reported by each of terminals 750P, 750U, and 750S.
Although illustrated with three terminals in FIG. 8, it should be appreciated that a MU-MIMO system can comprise substantially any number of terminals; each of such terminals indicated below with an index k. In accordance with various aspects, each of the access terminals 750P, 750U, and 750S can report CQI to Node B 710. Such terminals can report CQI from one or more antennas, utilizing a cyclic or parallel reporting approach. The frequency and the spectral characteristics, e.g., which subbands, of such reporting can be dictated by serving Node B 710. In addition, Node B 710 can dynamically re-schedule each of terminals 750P, 750U, and 750S in a disparate mode of operation, like SU-MIMO or SISO, and establish a disparate CQI reporting directive(s) for each of the terminals.
In one aspect, transmitted/received symbols with OFDM, at tone ω and for user k, can be modeled by:

yk(ω)=Hk(ω)ck(ω)+Hk(ω)Σ′cm(ω)+nk(ω).  (2)

Here, symbols have the same meaning as in Eq. (1). It should be appreciated that due to multi-user diversity, other-user interference in the signal received by user k is modeled with the second term in the left-hand side of Eq. (2). The prime (′) symbol indicates that transmitted symbol vector ck is excluded from the summation. The terms in the series represent reception by user k (through its channel response Hk) of symbols transmitted by a transmitter (e.g., eNode B 210) to the other users in the cell.

In view of the example systems presented and described above, methodologies for resolving handover based at least in part on uplink channel state information that may be implemented in accordance with the disclosed subject matter will be better appreciated with reference to the flowcharts of FIGS. 9-12. While, for purposes of simplicity of explanation, the methodologies are shown and described as a series of blocks, it is to be understood and appreciated that the claimed subject matter is not limited by the number or order of blocks, as some blocks may occur in different orders and/or concurrently with other blocks from what is depicted and described herein. Moreover, not all illustrated blocks may be required to implement the methodologies described hereinafter. It is to be appreciated that the functionality associated with the blocks may be implemented by software, hardware, a combination thereof or any other suitable means (e.g., device, system, process, component, . . . ). Additionally, it should be further appreciated that the methodologies disclosed hereinafter and throughout this specification are capable of being stored on an article of manufacture to facilitate transporting and transferring such methodologies to various devices. Those skilled in the art will understand and appreciate that a methodology may alternatively be represented as a series of interrelated states or events, such as in a state diagram. In addition, a methodology derived from a combination of at least portions of disparate methodologies may be represented as an interaction diagram or a call flow rather than through flow chart(s).
FIG. 9 is a flowchart of an example method 900 for preparing for preparing for handover upon radio link failure. In an aspect, example method 900 can be performed by a serving base station (e.g., eNode B 210). At act 910, a radio link failure criterion is established; e.g., a criterion is defined and conveyed to wireless network components (e.g., a mobile station). As an example, RLF criterion can be configured by a network operator via a network management component (e.g., mobility management entity, or a base station). The RLF criterion can be based on a channel signal strength metric, or channel quality metric, like SNR, SINR, C/I, RSOT, and so forth. At act 920 an indication of a radio link failure is received when the RLF criterion is fulfilled. In another aspect, such an indication is generally conveyed by an access terminal served by a base station that established the RLF criterion. At act 930, a radio link failure timer is triggered. The RLF timer can span a predetermined time interval allotted for conducting a handover. During the allotted time for the RLF time, service is retained and handover can be prepared, thus ensuring mobility without substantial service disruption. At act 940, handover is prepared. Such preparation can include scheduling changes (e.g., implemented by a serving base station), target cell system information communication, communication of context, and so on. As an example, a serving eNode B 210 can configure potential target eNBs with a mobile's context. Selection of potential target eNBs can be based on historic data and measurements or statically configured by the network. In an aspect, such preparation can complement handover preparation initiated in advance to RLF in order to ensure critical operational information is preconfigured prior to reaching a link failure condition. In an aspect, the extent to which advanced preparation occurs can be dictated by a threshold associated with message-based measurement reports conveyed by a served mobile terminal. At act 950, it is checked whether RLF timer has expired, for example by having elapsed a time interval greater that a predetermined upper bound (e.g., τ1 316). Upon timer expiration, forward handover is pursued at act 960, wherein pursuing handover can include synchronizing with a target cell, conveying a forward handover request and receiving a forward handover request acknowledgement. When RLF timer has failed to expire, it is checked at act 970 whether RLF condition(s) remains. In the affirmative case, normal service is resumed at act 980. It should be appreciated that normal service can include resuming scheduling traffic for a terminal that reported a radio link failure condition, or halting communication of buffering data to a set of target cells, and so forth. Moreover, in view of handover preparations, service can be resumed through a candidate target cell. Conversely, preparation(s) for handover proceeds at act 940.
FIG. 10 is a flowchart of an example method 1000 for establishing a radio link failure criterion. In an example, a network operator can configure the RLF criterion and utilize it as a part of handover algorithms to trigger both forward and backward handover. At act 1010, a channel quality metric threshold CQITH is configured. It is to be noted that multiple values of CQITH can be configured. Various metrics can be utilized, for instance: RSOT, RSSI, RSRP, SINR, and so forth. Such configuration can implemented on a per-terminal basis, a per-flow basis, or a per-subscriber basis. Additionally, threshold(s) CQITH can be time dependent. More particularly, CQITH can be determined based at least in part on (1) QoS specifications such as GBR, ABR, BER, PER, BLER, traffic handling priority which typically determines scheduling priorities—generally dictated by channel quality indicators in a served mobile station; (2) service sector or other spatial criteria; (3) user equipment capabilities or mode of operation; and (4) frequency planning such operational bandwidth, allocated frequency bands, frequency reuse, etc. In an aspect, configuration of CQITH can be autonomously and dynamically optimized to mitigate latency associated with handover while retaining satisfactory service (e.g., low jitter level for voice sessions), for example by intelligent component 218. At act 1020, a set of configured CQITH is retained. At act 1030, a configured CQITH is conveyed. In an aspect, threshold CQITH values are conveyed to an access terminal, which can employ them to indicate radio link failure and initiate FHO.
FIG. 11 presents a flowchart of an example method 1100 for preparing for forward handover under radio link failure. In an aspect, example method 1100 is carried out by a serving base station (e.g., eNode B 210) upon receiving an indication of radio link failure (e.g., an indication of CQI=NULL). It is to be noted that example method 110 can be performed in substantially any situation in which FHO has been signaled. At act 1110, traffic ceases to be scheduled. As an indication of handoff is received (e.g., radio link failure condition incurred), a serving eNode B can continue to transmit buffered data (e.g., packets), however, as service is expected to be handed over, scheduling of newly generated data is unwarranted. At act 1120 system information is received from a set of target cells. In an aspect, information is received through a backhaul network communication backbone (e.g., link 135; identified as X2 interface in LTE). System information can comprise for example, target cell system bandwidth; cell identity; pilot sequences (e.g., root indices for Zadoff-Chu codes) utilized for synchronization, UL sounding reference(s), and DL reference signals; physical random access channel (PRACH) configuration (e.g., preamble size, or preamble content), or management frame(s) configuration; symbol and cyclic prefix timing; allocated power or power density; etc. At act 1130, UL channel quality indicators from a set of target cells are received. At act 1140, buffered data for scheduled data flows of data streams is conveyed. Typically, the set of target cells that transmitted the system information receive the buffered data to ensure data integrity, which can be particularly critical for specific applications such as encrypted data transfer, or mission-critical applications like video-streaming in remote surgical or technical procedures. At act 1150, context is conveyed to the set of target cells. Context generally refers to service (e.g., Multimedia Broadcast Multimedia Service, security, billing,) context of a mobile station that prepares to implement FHO. At act 1160, uplink channel quality indicators received from a set of target cells are conveyed. In an aspect, such UL CQI can be employed by a mobile to determine an adequate target cell to attach to.
FIG. 12 presents a flowchart of an example method 1200 for preparing for forward handover under radio link failure. The example method 1200 is typically performed by a mobile station (e.g. user equipment 250), as it typically is the situation with FHO. At act 1210 a channel quality metric threshold CQITH is received. Various channel quality metrics can be utilized such as RSOT, RSSI, RSRP, SINR, and so forth. In an aspect, CQITH establishes a radio link failure condition, even under communication conditions in which service can be retained. It should be appreciated, in an aspect, that multiples values of CQITH can be received according to flows scheduled for the terminal receiving CQITH, the location of the terminal in a coverage cell or sector, the mode of operation of the terminal, and so forth. It is to be noted that CQITH can be time dependent. At act 1220, received value(s) of CQITH is retained. Threshold value(s) can be retained, for example in a memory connected to a handover component (e.g., handover component 265) in the mobile station that receives CQITH. At act 1230, an indication of radio link failure is conveyed. In an aspect, in LTE, such indication is communicated on the physical uplink control channel (PUCCH), while in control-less wireless technologies such as IEEE 802.11x, the indication can be conveyed on in-band management frames. Indication of radio link failure can be signaling of CQI=NULL when a channel quality metric associated with the metric that determines RLF is below CQITH. At act 1240, a preparation for forward handover is conducted. Preparation includes receiving system information of a set of target cells. The received system information facilitates preparing for forward handover, and it can comprise, for example, target cell system bandwidth, cell identity, pilot sequences (e.g., root indices for Zadoff-Chu codes) utilized for synchronization, symbol and cyclic prefix timing, allocated power or power density, etc. Processing of received system information can involve decoding of Q bits (Q a positive integer; with Q ̃O(10 bits) per target cell, for example). In addition, preparation for forward handover includes receiving uplink channel quality indicators for a set of target cells. At act 1250, forward handover (FHO) is pursued, which typically comprise handover signaling, e.g., FHO request, FHO request acknowledge, and reception of UL/DL resource grant(s) among a selected target cell and a mobile station (e.g., UE 250) that pursues FHO.
FIG. 13 illustrates a block diagram of an example system 1300 that enables establishment a criterion for radio link failure in accordance to aspects disclosed in the subject specification. System 1300 can reside, at least partially, within a base station (e.g., eNode B 210). System 1100 includes a logical grouping 1310 of electronic components that can act in conjunction. In an aspect, logical grouping 1310 includes an electronic component 1315 for configuring a threshold for a channel quality metric: an electronic component 1325 for conveying the configured threshold for the channel quality metric; and an electronic component 1335 for receiving an indication of a radio link failure (RLF) condition when the channel quality metric is below threshold. In addition, system 1300 can include electronic component 1345 for preparing for handover under the RLF condition.
System 1300 can also include a memory 1350 that retains instructions for executing functions associated with electrical components 1315, 1325, 1335, and 1345, as well as measured or computed data that may be generated during executing such functions. While shown as being external to memory 1180, it is to be understood that one or more of electronic components 1115, 1125, and 1135, 1145, 1155, 1165, and 1175 can exist within memory 1180.
FIG. 14 illustrates a block diagram of an example system 1400 that enables utilization of an RLF indication to implement handover. System 1400 can reside, at least partially, within a mobile (e.g., user equipment 250). System 1400 includes a logical grouping 1410 of electronic components that can act in conjunction. In an aspect, logical grouping 1410 includes an electronic component 1415 for receiving a radio link failure criterion, wherein the radio link failure criterion is determined by a threshold for a channel quality metric; an electronic component 1425 for conveying an indication of a RLF condition when the criterion is fulfilled; and an electronic component 1435 for preparing for forward handover. In addition, system 1400 can include electronic component 1445 for pursuing forward handover.
System 1400 can also include a memory 1450 that retains instructions for executing functions associated with electrical components 1415, 1425, 1435, and 1445, as well as measured or computed data that may be generated during executing such functions. While shown as being external to memory 1450, it is to be understood that one or more of electronic components 1415, 1425, 1435, and 1445, and can exist within memory 1450.
The techniques described herein may be used for various wireless communication networks such as Code Division Multiple Access (CDMA) networks, Time Division Multiple Access (TDMA) networks, Frequency Division Multiple Access (FDMA) networks, Orthogonal FDMA (OFDMA) networks, Single-Carrier FDMA (SC-FDMA) networks, etc. The terms “networks” and “systems” are often used interchangeably. A CDMA network may implement a radio technology such as Universal Terrestrial Radio Access (UTRA), cdma2000, etc. UTRA includes Wideband-CDMA (W-CDMA) and Low Chip Rate (LCR). cdma2000 covers IS-2000, IS-95 and IS-856 standards. A TDMA network may implement a radio technology such as Global System for Mobile Communications (GSM). An OFDMA network may implement a radio technology such as Evolved UTRA (E-UTRA), IEEE 802.11, IEEE 802.16, IEEE 802.20, Flash-OFDM®, etc. UTRA, E-UTRA, and GSM are part of Universal Mobile Telecommunication System (UMTS). Long Term Evolution (LTE) is an upcoming release of UMTS that uses E-UTRA. UTRA, E-UTRA, GSM, UMTS and LTE are described in documents from an organization named “3rd Generation Partnership Project” (3GPP). CDMA2000 is described in documents from an organization named “3rd Generation Partnership Project 2” (3GPP2).
For a software implementation, the techniques described herein may be implemented with modules (e.g., procedures, functions, and so on) that perform the functions described herein. The software codes may be stored in memory units, or memories, and executed by processors. A memory unit, or memory, may be implemented within the processor or external to the processor, in which case it can be communicatively coupled to the processor via various conventional means.
Various aspects or features described herein may be implemented as a method, apparatus, or article of manufacture employing standard programming and/or engineering techniques. The term “article of manufacture” as utilized herein is intended to encompass a computer program accessible from any computer-readable device, carrier, or media. For example, computer-readable media can include but are not limited to magnetic storage devices (e.g., hard disk, floppy disk, magnetic strips, etc.), optical disks (e.g., compact disk (CD), digital versatile disk (DVD), etc.), smart cards, and flash memory devices (e.g., EPROM, card, stick, key drive, etc.). Additionally, various storage media described herein can represent one or more devices and/or other machine-readable media for storing information. The term “machine-readable medium” can include, without being limited to include, wireless channels and various other media capable of storing, containing, and/or carrying instruction(s) and/or data.
As it employed herein, the term “processor” can refer to a classical architecture or a quantum computer. Classical architecture comprises, but is not limited to comprise, single-core processors; single-processors with software multithread execution capability; multi-core processors; multi-core processors with software multithread execution capability; multi-core processors with hardware multithread technology; parallel platforms; and parallel platforms with distributed shared memory. Additionally, a processor can refer to an integrated circuit, an application specific integrated circuit (ASIC), a digital signal processor (DSP), a field programmable gate array (FPGA), a programmable logic controller (PLC), a complex programmable logic device (CPLD), a discrete gate or transistor logic, discrete hardware components, or any combination thereof designed to perform the functions described herein. Quantum computer architecture may be based on qubits embodied in gated or self-assembled quantum dots, nuclear magnetic resonance platforms, superconducting Josephson junctions, etc. Processors can exploit nano-scale architectures such as, but not limited to, molecular and quantum-dot based transistors, switches and gates, in order to optimize space usage or enhance performance of user equipment. A processor may also be implemented as a combination of computing devices, e.g., a combination of a DSP and a microprocessor, a plurality of microprocessors, one or more microprocessors in conjunction with a DSP core, or any other such configuration.
Furthermore, in the subject specification and annexed drawings, the term “memory” refers to data stores, algorithm stores, and other information stores such as subscriber databases, billings databases, or specification/content stores. It will be appreciated that the memory components described herein can be either volatile memory or nonvolatile memory, or can include both volatile and nonvolatile memory. By way of illustration, and not limitation, nonvolatile memory can include read only memory (ROM), programmable ROM (PROM), electrically programmable ROM (EPROM), electrically erasable ROM (EEPROM), or flash memory. Volatile memory can include random access memory (RAM), which acts as external cache memory. By way of illustration and not limitation, RAM is available in many forms such as synchronous RAM (SRAM), dynamic RAM (DRAM), synchronous DRAM (SDRAM), double data rate SDRAM (DDR SDRAM), enhanced SDRAM (ESDRAM), Synchlink DRAM (SLDRAM), and direct Rambus RAM (DRRAM). Additionally, the disclosed memory components of systems and/or methods herein are intended to comprise, without being limited to, these and any other suitable types of memory.
What has been described above includes examples of one or more embodiments. It is, of course, not possible to describe every conceivable combination of components or methodologies for purposes of describing the aforementioned embodiments, but one of ordinary skill in the art may recognize that many further combinations and permutations of various embodiments are possible. Accordingly, the described embodiments are intended to embrace all such alterations, modifications and variations that fall within the spirit and scope of the appended claims. Furthermore, to the extent that the terms “includes,” “including,” “possess,” “possessing,” and the like are used in the subject specification, such terms are intended to be inclusive in a manner similar to the term “comprising” as “comprising” is interpreted when employed as a transitional word in a claim.

## Claims

1. A method for facilitating handover in wireless system, the method comprising:
establishing a radio link failure (RLF) criterion by configuring an autonomously optimized threshold for a channel quality metric;
receiving an indication of a radio link failure (RLF) condition when the RLF criterion is fulfilled, wherein the indication of the RLF condition includes a NULL channel quality indicator of RLF onset;
triggering a RLF timer upon receiving the indication, wherein the RLF timer is selected to mitigate excessive forward handover pursuits and ensuing target cell attachments; and
triggering preparation for handover in response to the NULL channel quality indicator of RLF onset.

2. The method of claim 1, wherein establishing the RLF criterion comprises conveying the threshold for the channel quality metric.

3. The method of claim 1, wherein configuring the threshold for the channel quality metric is based at least on one of a quality of service, a service sector, a user equipment, or a frequency plan.

4. The method of claim 3, wherein the quality of service includes at least one of a guaranteed bitrate, an average bitrate, a minimum bitrate, a block error rate, a packet error rate, a bit error rate, or a peak data rate.

5. The method of claim 3, wherein establishing the RLF criterion further comprises retaining a set of configured channel quality metric thresholds.

6. The method of claim 3, wherein configuring the threshold for the channel quality metric based on the user equipment comprises configuring based on one of a data flow, a mode of operation, or a user equipment capability.

7. The method of claim 6, wherein the channel quality metric includes one of a reference signal received power, a reference signal strength indicator, a reference signal over thermal noise, or a signal to noise and interference ratio.

8. The method of claim 1, wherein the preparation for handover comprises ceasing to schedule traffic.

9. The method of claim 8, wherein the preparation for handover further comprises receiving system information from a set of target cells.

10. The method of claim 9, wherein the preparation for handover further comprises receiving a set of uplink channel quality indicators from a set of target base stations.

11. The method of claim 10, wherein the preparation for handover further comprises conveying buffered data.

12. The method of claim 11, wherein the preparation for handover further comprises conveying context to the set of target cells.

13. The method of claim 12, wherein the preparation for handover further comprises conveying uplink channel quality indicators received from the set of target cells.

14. The method of claim 1, further comprising evaluating whether the RLF timer has expired and pursuing forward handover when the timer has expired.

15. The method of claim 14, when the RLF timer has failed to elapse, further comprising evaluating whether the RLF condition remains and resuming a normal operation for wireless communication when the RLF condition ceases to exist.

16. A device that operates in a wireless environment, the device comprising:
a processor configured
to set a configurable and autonomously optimized threshold for a channel quality metric;
to receive an indication of a radio link failure (RLF) condition when a channel quality metric is below the threshold, wherein the indication of the RLF condition includes a NULL channel quality indicator of RLF onset;
to trigger a RLF timer upon receiving the indication, and wherein the RLF timer is selected to mitigate excessive forward handover pursuits and ensuing target cell attachments; and
to trigger preparation for handover in response to the NULL channel quality indicator of RLF onset; and

a memory coupled to the processor.

17. The device of claim 16, wherein the processor is further configured to convey the threshold for the channel quality metric.

18. The device of claim 16, wherein the threshold for the channel quality metric is based at least on one of a quality of service, a service sector, a user equipment, or a frequency plan.

19. The device of claim 18, wherein the quality of service includes at least one of a guaranteed bitrate, an average bitrate, a minimum bitrate, a block error rate, a packet error rate, a bit error rate, or a peak data rate.

20. The device of claim 19, wherein a configuration of the threshold for the channel quality metric based on the user equipment comprises a configuration based on one of a data flow, a mode of operation, or a user equipment capability.

21. The device of claim 20, wherein the channel quality metric includes one of a reference signal received power, a reference signal strength indicator, a reference signal over thermal noise, or a signal to noise and interference ratio.

22. The device of claim 16, wherein to trigger preparation for handover in response to the NULL channel quality indicator of RLF onset further comprises to convey context to a set of target cells.

23. The device of claim 22, wherein to trigger preparation for handover in response to the NULL channel quality indicator of RLF onset further comprises to convey buffered data.

24. The device of claim 23, wherein to trigger preparation for handover in response to the NULL channel quality indicator of RLF onset further comprises to receive a set of uplink channel quality indicators from a set of target base stations.

25. The device of claim 24, wherein to trigger preparation for handover in response to the NULL channel quality indicator of RLF onset comprises to cease to schedule traffic.

26. The device of claim 25, wherein to trigger preparation for handover in response to the NULL channel quality indicator of RLF onset further comprises conveying uplink channel quality indicators received from the set of target cells.

27. The device of claim 16, wherein the processor is further configured to evaluate whether the RLF timer has expired and to disrupt a communication service when said RLF timer has expired.

28. The device of claim 27, when the RLF timer has failed to elapse, the processor further configured to evaluate whether the RLF condition remains and to resume a normal operation for wireless communication when the RLF condition ceases to exist.

29. The device of claim 28, wherein the memory stores a set of configured thresholds for a set of channel quality metrics.

30. An apparatus that operates in a wireless environment, the apparatus comprising:
means for configuring an autonomously optimized threshold for a channel quality metric;
means for conveying the threshold for the channel quality metric;
means for receiving an indication of a radio link failure (RLF) condition when a channel quality metric is below threshold, wherein the indication of the RLF condition includes a NULL channel quality indicator of RLF onset;
means for triggering a RLF timer upon receiving the indication, wherein the RLF timer is selected to mitigate excessive forward handover pursuits and ensuing target cell attachments; and
means for triggering preparation for handover in response to the NULL channel quality indicator of RLF onset.

31. A non-transitory computer-readable medium storing a computer program, wherein execution of the computer program is for:
configuring an autonomously optimized threshold for a channel quality metric to determine a radio link failure (RLF) criterion;
conveying the threshold for the channel quality metric;
receiving an indication of a radio link failure (RLF) condition when the channel quality metric is below threshold, wherein the indication of the RLF condition includes a NULL channel quality indicator of RLF onset;
triggering a RLF timer upon receiving the indication, wherein the RLF timer is selected to mitigate excessive forward handover pursuits and ensuing target cell attachments; and
triggering preparation for handover in response to the NULL channel quality indicator of RLF onset.

32. The non-transitory computer-readable medium of claim 31, wherein the threshold for the channel quality metric is based at least on one of a quality of service, a service sector, a user equipment, or a frequency plan.

33. A method for forward handover under link failure in a wireless system, the method comprising:
receiving a configurable channel quality metric threshold, the channel quality metric threshold being autonomously optimized and establishes a radio link failure (RLF) criterion;
conveying an indication of a RLF condition when a channel quality metric is below the threshold, wherein the indication of the RLF condition includes a NULL channel quality indicator of RLF onset, and reception of the indication triggers an RLF timer selected to mitigate excessive forward handover pursuits and ensuing target cell attachments;
preparing for forward handover triggered by the NULL channel quality indicator of RLF onset; and
pursuing forward handover.

34. The method of claim 33, wherein the configurable channel quality metric threshold is based at least on one of a quality of service, a service sector, a user equipment, or a frequency plan.

35. The method of claim 34, wherein the quality of service includes at least one of a guaranteed bitrate, an average bitrate, a minimum bitrate, a block error rate, a packet error rate, a bit error rate, or a peak data rate.

36. The method of claim 35, wherein a configuration of the channel quality metric threshold based on the user equipment comprises a configuration based on one of a data flow, a mode of operation, or a user equipment capability.

37. The method of claim 33, wherein preparing for forward handover includes receiving a set of uplink channel quality indicators from the set of target cells.

38. The method of claim 33, wherein pursuing forward handover comprises:
synchronizing with a target cell; and
conveying a forward handover request and receiving a forward handover request acknowledgement.

39. The method of claim 33, further comprising retaining the channel quality threshold.

40. An apparatus that operates in a wireless environment, the apparatus comprising:
a processor configured
to receive a channel quality metric threshold which determines a radio link failure (RLF) criterion and wherein the channel quality metric threshold is autonomously optimized;
to generate a channel quality metric;
to convey a channel quality indicator (CQI) report that indicates a RLF condition as a NULL channel quality indicator of RLF onset when the channel quality metric is below the channel quality metric threshold, wherein reception of the CQI report triggers an RLF timer selected to mitigate excessive forward handover pursuits and ensuing target cell attachments; and
to prepare for forward handover triggered by the NULL channel quality indicator of RLF onset; and

memory coupled to the processor.

41. The apparatus of claim 40, wherein the processor is further configured to pursue forward handover.

42. The apparatus of claim 41, wherein to pursue forward handover comprises
to synchronize with a target cell; and
to convey a forward handover request and receiving a forward handover request acknowledgement.

43. The apparatus of claim 42, wherein the channel quality metric includes one of a reference signal received power, a reference signal strength indicator, a reference signal over thermal noise, or a signal to noise and interference ratio.

44. The apparatus of claim 40, wherein the processor configured to prepare for forward handover is further configured to receive system information of a set of target cells; and to receive a set of uplink channel quality indicators from the set of target cells.

45. The apparatus of claim 44, wherein the memory stores the received channel quality metric threshold.

46. A wireless device comprising:
means for receiving a radio link failure (RLF) criterion, wherein the RLF criterion is determined by an autonomously optimized threshold for a channel quality metric;
means for conveying an indication of a RLF condition as a NULL channel quality indicator of RLF onset when the RLF criterion is fulfilled, wherein reception of the indication triggers an RLF timer selected to mitigate excessive forward handover pursuits and ensuing target cell attachments;
means for preparing for forward handover triggered by the NULL channel quality indicator of RLF onset; and
means for pursuing forward handover.

47. The wireless device of claim 46, wherein the RLF criterion is fulfilled when the channel quality metric is below the threshold.

48. The wireless device of claim 46, wherein the means for preparing for forward handover includes means for receiving a set of uplink channel quality indicators from the set of target cells.

49. The wireless device of claim 47, wherein the means for pursuing forward handover comprises:
means for synchronizing with a target cell; and
means for conveying a forward handover request and receiving a forward handover request acknowledgement.

50. A non-transitory computer readable medium storing a computer program, wherein execution of the computer program is for:
receiving a channel quality metric threshold, wherein the channel quality metric threshold is autonomously optimized;
conveying an indication of a radio link failure (RLF) condition as a NULL channel quality indicator of RLF onset when a channel quality metric is below the channel quality metric threshold, wherein reception of the indication triggers an RLF timer selected to mitigate excessive forward handover pursuits and ensuing target cell attachments; and
pursuing forward handover triggered by the NULL channel quality indicator of RLF onset.

51. The non-transitory computer readable medium of claim 50, wherein execution of the computer program is also for:
synchronizing with a target cell; and
conveying a forward handover request and receiving a forward handover request acknowledgement.

52. The non-transitory computer-readable medium of claim 51 wherein execution of the computer program is also for preparing for forward handover.

53. The non-transitory computer readable medium of claim 52, wherein execution of the computer program is also for:
receiving system information of a set of target cells; and
receiving a set of uplink channel quality indicators from the set of target cells.