WO2016022239A1

WO2016022239A1 - Inter-radio access technology (irat) cell reselection

Info

Publication number: WO2016022239A1
Application number: PCT/US2015/039311
Authority: WO
Inventors: Ming Yang; Tom Chin
Original assignee: Qualcomm Incorporated
Priority date: 2014-08-05
Filing date: 2015-07-07
Publication date: 2016-02-11

Abstract

In a method and process of wireless communication, when a first candidate target cell is no longer viable for cell selection, a UE instead camps on a second candidate cell, without performing a cell evaluation of the second candidate cell.

Description

INTER-RADIO ACCESS TECHNOLOGY (IRAT) CELL RESELECTION

BACKGROUND

Field

[0001] The present disclosure relates generally to wireless communication systems, and more particularly, to cell reselection in a wireless network.

Background

[0002] Wireless communication networks are widely deployed to provide various communication services such as telephony, video, data, messaging, broadcasts, and so on. Such networks, which are usually multiple access networks, support

communications for multiple users by sharing the available network resources. One example of such a network is the universal terrestrial radio access network (UTRAN). The UTRAN is the radio access network (RAN) defined as a part of the universal mobile telecommunications system (UMTS), a third generation (3G) mobile phone technology supported by the 3rd Generation Partnership Project (3GPP). The UMTS, which is the successor to global system for mobile communications (GSM)

technologies, currently supports various air interface standards, such as wideband-code division multiple access (W-CDMA), time division-code division multiple access (TD- CDMA), and time division-synchronous code division multiple access (TD-SCDMA). For example, China is pursuing TD-SCDMA as the underlying air interface in the UTRAN architecture with its existing GSM infrastructure as the core network. The UMTS also supports enhanced 3G data communications protocols, such as high speed packet access (HSPA), which provides higher data transfer speeds and capacity to associated UMTS networks. HSPA is a collection of two mobile telephony protocols, high speed downlink packet access (HSDPA) and high speed uplink packet access (HSUPA), which extends and improves the performance of existing wideband protocols.

[0003] As the demand for mobile broadband access continues to increase, research and development continue to advance the UMTS technologies not only to meet the growing demand for mobile broadband access, but to advance and enhance the user experience with mobile communications. SUMMARY

[0004] According to one aspect of the present disclosure, a method for wireless communication includes performing a first search and measurement to identify a first candidate cell for cell reselection. A cell evaluation of the first candidate cell is performed when the first candidate cell meets cell reselection conditions and when a reselection timer is started. A second candidate cell is identified during a second search and measurement after the cell reselection timer expires for the first candidate cell and a UE tunes to a target frequency and/or target radio access technology (RAT) of the first candidate cell. The method further includes camping on the second candidate cell, without performing a cell evaluation of the second candidate cell, when the first candidate cell signal quality is below a first threshold, and a difference between a signal quality of the first and second candidates is above a second threshold.

[0005] According to another aspect of the present disclosure, an apparatus for wireless communication includes means for performing a first search and measurement to identify a first candidate cell for cell reselection. The apparatus may also include means for performing a cell evaluation of the first candidate cell when the first candidate cell meets cell reselection conditions and a reselection timer is started. The apparatus may also include means for identifying a second candidate cell during a second search and measurement after the cell reselection timer expires for the first candidate cell and a UE tunes to a target frequency and/or target radio access technology (RAT) of the first candidate cell. The apparatus further includes means for camping on the second candidate cell, without performing a cell evaluation of the second candidate cell, when the first candidate cell signal quality is below a first threshold, and a difference between a signal quality of the first and second candidates is above a second threshold.

[0006] Another aspect discloses a computer program product for wireless communications in a wireless network having a non-transitory computer-readable medium. The computer readable medium has non-transitory program code recorded thereon which, when executed by the processor(s), causes the processor(s) to perform a first search and measurement to identify a first candidate cell for cell reselection. The program code also causes the processor(s) to perform a cell evaluation of the first candidate cell when the first candidate cell meets cell reselection conditions and a reselection timer is started. The program code also causes the processor(s) to identify a second candidate cell during a second search and measurement after the cell reselection timer expires for the first candidate cell and a UE tunes to a target frequency and/or target radio access technology (RAT) of the first candidate cell. The program code further causes the processor(s) to camp on the second candidate cell, without performing a cell evaluation of the second candidate cell, when the first candidate cell signal quality is below a first threshold, and a difference between a signal quality of the first and second candidates is above a second threshold.

[0007] Another aspect discloses an apparatus for wireless communication and includes a memory and at least one processor coupled to the memory. The processor(s) is configured to perform a first search and measurement to identify a first candidate cell for cell reselection. The processor(s) is also configured to perform a cell evaluation of the first candidate cell when the first candidate cell meets cell reselection conditions and a reselection timer is started. The processor(s) is also configured to identify a second candidate cell during a second search and measurement after the cell reselection timer expires for the first candidate cell and a UE tunes to a target frequency and/or target radio access technology (RAT) of the first candidate cell. The processor(s) is further configured to camp on the second candidate cell, without performing a cell evaluation of the second candidate cell, when the first candidate cell signal quality is below a first threshold, and a difference between a signal quality of the first and second candidates is above a second threshold.

[0008] This has outlined, rather broadly, the features and technical advantages of the present disclosure in order that the detailed description that follows may be better understood. Additional features and advantages of the disclosure will be described below. It should be appreciated by those skilled in the art that this disclosure may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present disclosure. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the teachings of the disclosure as set forth in the appended claims. The novel features, which are believed to be characteristic of the disclosure, both as to its organization and method of operation, together with further objects and advantages, will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present disclosure.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The features, nature, and advantages of the present disclosure will become more apparent from the detailed description set forth below when taken in conjunction with the drawings in which like reference characters identify correspondingly throughout.

[0010] FIGURE 1 is a block diagram conceptually illustrating an example of a telecommunications system.

[0011] FIGURE 2 is a block diagram conceptually illustrating an example of a frame structure in a telecommunications system.

[0012] FIGURE 3 is a block diagram conceptually illustrating an example of a node B in communication with a UE in a telecommunications system.

[0013] FIGURE 4 illustrates overlapping network coverage areas between two different radio access technologies.

[0014] FIGURE 5 is a block diagram illustrating a method for performing cell reselection, by a mobile device.

[0015] FIGURE 6 is a diagram illustrating an example of a hardware implementation for a mobile device employing a processing system to perform cell reselection.

DETAILED DESCRIPTION

[0016] The detailed description set forth below, in connection with the appended drawings, is intended as a description of various configurations and is not intended to represent the only configurations in which the concepts described herein may be practiced. The detailed description includes specific details for the purpose of providing a thorough understanding of the various concepts. However, it will be apparent to those skilled in the art that these concepts may be practiced without these specific details. In some instances, well-known structures and components are shown in block diagram form in order to avoid obscuring such concepts.

[0017] Turning now to FIGURE 1, a block diagram is shown illustrating an example of a telecommunications system 100. The various concepts presented throughout this disclosure may be implemented across a broad variety of telecommunication systems, network architectures, and communication standards. By way of example and without limitation, the aspects of the present disclosure illustrated in FIGURE 1 are presented with reference to a UMTS system employing a TD-SCDMA standard. In this example, the UMTS system includes a radio access network (RAN) 102 (e.g., UTRAN) that provides various wireless services including telephony, video, data, messaging, broadcasts, and/or other services. The RAN 102 may be divided into a number of radio network subsystems (RNSs) such as an RNS 107, each controlled by a radio network controller (RNC) such as an RNC 106. For clarity, only the RNC 106 and the RNS 107 are shown; however, the RAN 102 may include any number of RNCs and RNSs in addition to the RNC 106 and RNS 107. The RNC 106 is an apparatus responsible for, among other things, assigning, reconfiguring and releasing radio resources within the RNS 107. The RNC 106 may be interconnected to other RNCs (not shown) in the RAN 102 through various types of interfaces such as a direct physical connection, a virtual network, or the like, using any suitable transport network.

[0018] The geographic region covered by the RNS 107 may be divided into a number of cells, with a radio transceiver apparatus serving each cell. A radio transceiver apparatus is commonly referred to as a node B in UMTS applications, but may also be referred to by those skilled in the art as a base station (BS), a base transceiver station (BTS), a radio base station, a radio transceiver, a transceiver function, a basic service set (BSS), an extended service set (ESS), an access point (AP), or some other suitable terminology. For clarity, two node Bs 108 are shown; however, the RNS 107 may include any number of wireless node Bs. The node Bs 108 provide wireless access points to a core network 104 for any number of mobile apparatuses. Examples of a mobile apparatus include a cellular phone, a smart phone, a session initiation protocol (SIP) phone, a laptop, a notebook, a netbook, a smartbook, a personal digital assistant (PDA), a satellite radio, a global positioning system (GPS) device, a multimedia device, a video device, a digital audio player (e.g., MP3 player), a camera, a game console, or any other similar functioning device. The mobile apparatus is commonly referred to as user equipment (UE) in UMTS applications, but may also be referred to by those skilled in the art as a mobile station (MS), a subscriber station, a mobile unit, a subscriber unit, a wireless unit, a remote unit, a mobile device, a wireless device, a wireless

communications device, a remote device, a mobile subscriber station, an access terminal (AT), a mobile terminal, a wireless terminal, a remote terminal, a handset, a terminal, a user agent, a mobile client, a client, or some other suitable terminology. For illustrative purposes, three UEs 110 are shown in communication with the node Bs 108. The downlink (DL), also called the forward link, refers to the communication link from a node B to a UE, and the uplink (UL), also called the reverse link, refers to the communication link from a UE to a node B.

[0019] The core network 104, as shown, includes a GSM core network. However, as those skilled in the art will recognize, the various concepts presented throughout this disclosure may be implemented in a RAN, or other suitable access network, to provide UEs with access to types of core networks other than GSM networks.

[0020] In this example, the core network 104 supports circuit-switched services with a mobile switching center (MSC) 112 and a gateway MSC (GMSC) 114. One or more RNCs, such as the RNC 106, may be connected to the MSC 112. The MSC 112 is an apparatus that controls call setup, call routing, and UE mobility functions. The MSC 112 also includes a visitor location register (VLR) (not shown) that contains subscriber- related information for the duration that a UE is in the coverage area of the MSC 112. The GMSC 114 provides a gateway through the MSC 112 for the UE to access a circuit- switched network 116. The GMSC 114 includes a home location register (HLR) (not shown) containing subscriber data, such as the data reflecting the details of the services to which a particular user has subscribed. The HLR is also associated with an authentication center (AuC) that contains subscriber-specific authentication data. When a call is received for a particular UE, the GMSC 1 14 queries the HLR to determine the UE's location and forwards the call to the particular MSC serving that location.

[0021] The core network 104 also supports packet-data services with a serving GPRS support node (SGSN) 118 and a gateway GPRS support node (GGSN) 120. GPRS, which stands for General Packet Radio Service, is designed to provide packet-data services at speeds higher than those available with standard GSM circuit-switched data services. The GGSN 120 provides a connection for the RAN 102 to a packet-based network 122. The packet-based network 122 may be the Internet, a private data network, or some other suitable packet-based network. The primary function of the GGSN 120 is to provide the UEs 110 with packet-based network connectivity. Data packets are transferred between the GGSN 120 and the UEs 110 through the SGSN 118, which performs primarily the same functions in the packet-based domain as the MSC 112 performs in the circuit- switched domain.

[0022] The UMTS air interface is a spread spectrum Direct- Sequence Code Division Multiple Access (DS-CDMA) system. The spread spectrum DS-CDMA spreads user data over a much wider bandwidth through multiplication by a sequence of

pseudorandom bits called "chips." The TD-SCDMA standard is based on such direct sequence spread spectrum technology and additionally calls for a time division duplexing (TDD), rather than a frequency division duplexing (FDD) as used in many FDD mode UMTS/W-CDMA systems. TDD uses the same carrier frequency for both the uplink (UL) and downlink (DL) between a node B 108 and a UE 110, but divides uplink and downlink transmissions into different time slots in the carrier.

[0023] FIGURE 2 shows a frame structure 200 for a TD-SCDMA carrier. TD- SCDMA is based on time division and code division to allow multiple UEs to share the same radio bandwidth on a particular frequency channel. The bandwidth of each frequency channel in a TD-SCDMA system ordinarily is 1.6 MHz. The TD-SCDMA carrier ordinarily operates at 1.28 Mega-chips-per-second (Mcps). A TD-SCDMA frame 202 that is 10 ms in length is illustrated. The frame 202 has two 5 ms subframes 204, and each of the subframes 204 includes seven time slots, TS0 through TS6.

[0024] The downlink and uplink transmissions share the same bandwidth in different time slots. The first time slot, TS0, is usually allocated for downlink communication, while the second time slot, TS1, is usually allocated for uplink communication. The remaining time slots, TS2 through TS6, may be used for either uplink or downlink, which allows for greater flexibility during times of higher data transmission times in either the uplink or downlink directions. A downlink pilot time slot (DwPTS) 206 and an uplink pilot time slot (UpPTS) 210 (also known as the uplink pilot channel (UpPCH)) are located between TSO and TS1 separated by a gap known as a guard period (GP) 208. The particular allocation of timeslots TS2 through TS6 to uplink or downlink illustrated in FIGURE 2 is an example, and different frames 202 and sub frames 204 may have different uplink/downlink allocations.

[0025] In each time slot, there are multiple code channels. Each time slot, TS0-TS6, may allow data transmission multiplexed on a maximum of sixteen code channels. Data transmission on a code channel includes two data portions 212 (each with a length of three-hundred-fifty-two chips) separated by a midamble 214 (with a length of one- hundred- forty- four chips) and followed by a guard period (GP) 216 (with a length of sixteen chips). The midamble 214 may be used for features, such as channel estimation, while the guard period 216 may be used to avoid inter-burst interference. Also transmitted in the data portion is some Layer 1 control information, including synchronization shift (SS) bits 218. Synchronization shift bits 218 only appear in the second part of the data portion. The synchronization shift bits 218 immediately following the midamble 214 can indicate three cases: decrease shift, increase shift, or do nothing in the upload transmit timing. The positions of the synchronization shift bits 218 are not generally used during uplink communications.

[0026] FIGURE 3 is a block diagram of a node B 310 in communication with a UE 350 in a RAN 300, where the RAN 300 may be the RAN 102 in FIGURE 1, the node B 310 may be the node B 108 in FIGURE 1 , and the UE 350 may be the UE 110 in FIGURE 1. In the downlink communication, a transmit processor 320 may receive data from a data source 312 and control signals from a controller/processor 340. The transmit processor 320 provides various signal processing functions for the data and control signals, as well as reference signals (e.g., pilot signals). For example, the transmit processor 320 may provide cyclic redundancy check (CRC) codes for error detection, coding and interleaving to facilitate forward error correction (FEC), mapping to signal constellations based on various modulation schemes (e.g., binary phase-shift keying (BPSK), quadrature phase-shift keying (QPSK), M-phase-shift keying (M-PSK), M-quadrature amplitude modulation (M-QAM), and the like), spreading with orthogonal variable spreading factors (OVSF), and multiplying with scrambling codes to produce a series of symbols. Channel estimates from a channel processor 344 may be used by a controller/processor 340 to determine the coding, modulation, spreading, and/or scrambling schemes for the transmit processor 320. These channel estimates may be derived from a reference signal transmitted by the UE 350 or from feedback contained in the midamble 214 (FIGURE 2) from the UE 350. The symbols generated by the transmit processor 320 are provided to a transmit frame processor 330 to create a frame structure such as frame structure 200. The transmit frame processor 330 creates this frame structure by multiplexing the symbols with a midamble 214 (FIGURE 2) from the controller/processor 340, resulting in a series of frames. The frames are then provided to a transmitter 332, which provides various signal conditioning functions including amplifying, filtering, and modulating the frames onto a carrier for downlink transmission over the wireless medium through smart antennas 334. The smart antennas 334 may be implemented with beam steering bidirectional adaptive antenna arrays or other similar beam technologies.

[0027] At the UE 350, a receiver 354 receives the downlink transmission through an antenna 352 and processes the transmission to recover the information modulated onto the carrier. The information recovered by the receiver 354 is provided to a receive frame processor 360, which parses each frame, and provides the midamble 214

(FIGURE 2) to a channel processor 394 and the data, control, and reference signals to a receive processor 370. The receive processor 370 then performs the inverse of the processing performed by the transmit processor 320 in the node B 310. More specifically, the receive processor 370 descrambles and despreads the symbols, and then determines the most likely signal constellation points transmitted by the node B 310 based on the modulation scheme. These soft decisions may be based on channel estimates computed by the channel processor 394. The soft decisions are then decoded and deinterleaved to recover the data, control, and reference signals. The CRC codes are then checked to determine whether the frames were successfully decoded. The data carried by the successfully decoded frames will then be provided to a data sink 372, which represents applications running in the UE 350 and/or various user interfaces (e.g., display). Control signals carried by successfully decoded frames will be provided to a controller/processor 390. When frames are unsuccessfully decoded by the receive processor 370, the controller/processor 390 may also use an acknowledgement (ACK) and/or negative acknowledgement (NACK) protocol to support retransmission requests for those frames. [0028] In the uplink, data from a data source 378 and control signals from the controller/processor 390 are provided to a transmit processor 380. The data source 378 may represent applications running in the UE 350 and various user interfaces (e.g., keyboard). Similar to the functionality described in connection with the downlink transmission by the node B 310, the transmit processor 380 provides various signal processing functions including CRC codes, coding and interleaving to facilitate FEC, mapping to signal constellations, spreading with OVSFs, and scrambling to produce a series of symbols. Channel estimates, derived by the channel processor 394 from a reference signal transmitted by the node B 310 or from feedback contained in the midamble 214 transmitted by the node B 310, may be used to select the appropriate coding, modulation, spreading, and/or scrambling schemes. The symbols produced by the transmit processor 380 will be provided to a transmit frame processor 382 to create a frame structure such as frame structure 200. The transmit frame processor 382 creates this frame structure by multiplexing the symbols with a midamble 214 from the controller/processor 390, resulting in a series of frames. The frames are then provided to a transmitter 356, which provides various signal conditioning functions including amplification, filtering, and modulating the frames onto a carrier for uplink transmission over the wireless medium through the antenna 352.

[0029] The uplink transmission is processed at the node B 310 in a manner similar to that described in connection with the receiver function at the UE 350. A receiver 335 receives the uplink transmission through the antenna 334 and processes the transmission to recover the information modulated onto the carrier. The information recovered by the receiver 335 is provided to a receive frame processor 336, which parses each frame, and provides the midamble 214 (FIGURE 2) to the channel processor 344 and the data, control, and reference signals to a receive processor 338. The receive processor 338 performs the inverse of the processing performed by the transmit processor 380 in the UE 350. The data and control signals carried by the successfully decoded frames may then be provided to a data sink 339 and the controller/processor, respectively. If some of the frames were unsuccessfully decoded by the receive processor, the

controller/processor 340 may also use an acknowledgement (ACK) and/or negative acknowledgement (NACK) protocol to support retransmission requests for those frames. [0030] The controller/processors 340 and 390 may be used to direct the operation at the node B 310 and the UE 350, respectively. For example, the controller/processors 340 and 390 may provide various functions including timing, peripheral interfaces, voltage regulation, power management, and other control functions. The computer- readable media of memories 342 and 392 may store data and software for the node B 310 and the UE 350, respectively. For example, a non-transitory portion of the memory 392 of the UE 350 may store a reselection module 391 which, when executed by the controller/processor 390, configures the UE 350 to perform cell reselection.

[0031] Some networks, such as a newer or newly deployed network, may cover only a portion of a geographical area. Another network, such as an older more established network, may better cover the area, including remaining portions of the geographical area. FIGURE 4 illustrates coverage of an established network utilizing a first type of radio access technology (RAT-1), such as a TD-SCDMA network, and also illustrates a newly deployed network utilizing a second type of radio access technology (RAT-2), such as a Long Term Evolution (LTE) network.

[0032] The geographical area 400 may include RAT-1 cells 402 and RAT-2 cells 404. This arrangement of overlapping cells of different RATs may apply to various combinations of RAT technologies, and in some cases, the cells of more than two RATs may be overlaid in a same geographic area (e.g., overlapping GSM, TD-SCDMA, and/or LTE cells in a same geographic location). A user equipment (UE) 350 may move from one cell, such as a RAT-1 cell 402, to another cell, such as a RAT-2 cell 404. The movement of the UE 350 may specify a handover or a cell reselection.

[0033] The handover or cell reselection may be performed when the UE 350 moves from a coverage area of a first RAT to the coverage area of a second RAT, or vice versa. A handover or cell reselection may also be performed, among other reasons, when there is a coverage hole or lack of coverage in one network or when there is traffic balancing between a first RAT and the second RAT networks. As part of that handover or cell reselection process, while in a connected mode with a first system (e.g., TD- SCDMA) a UE 350 may be specified to perform a measurement of a neighboring cell (such as an LTE cell). For example, the UE 350 may measure the neighbor cells of a second network for signal strength, frequency channel, and base station identity code (BSIC). The UE 350 may then connect to the strongest cell of the second network. Such measurement may be referred to as inter radio access technology (IRAT) measurement.

[0034] The UE 350 may send a serving cell a measurement report indicating results of the IRAT measurement performed by the UE. The serving cell may then trigger a handover of the UE 350 to a new cell in the other RAT based on the measurement report. The measurement may include a serving cell signal strength, such as a received signal code power (RSCP) for a pilot channel (e.g., primary common control physical channel (PCCPCH)). The signal strength/quality is compared to a serving system threshold. The serving system threshold can be indicated to the UE 350 through dedicated radio resource control (RRC) signaling from the network. The measurement may also include a neighbor cell received signal strength indicator (RSSI). The neighbor cell signal strength can be compared with a neighbor system threshold. Before handover or cell reselection, in addition to the measurement processes, the base station IDs (e.g., BSICs) are confirmed and re-confirmed.

IRAT Cell Reselection

[0035] When a conventional UE is camped on a first RAT, such as TD-SCDMA, and is in idle mode, CELL PCH (cell paging channel listening) mode, and/or URA PCH (UTRAN registration area listening) mode, the UE may select and monitor the paging indicator channel (PICH) and paging channel (PCH) of the serving cell, monitor various system information, perform measurements for serving and neighboring cells, execute a cell reselection evaluation process, and reselect to a neighbor cell if that cell meets reselection trigger conditions. The cell reselection process includes an evaluation phase and an acquisition phase. During the evaluation phase the UE locates an LTE cell with a signal quality above a cell reselection threshold and starts a reselection timer.

Expiration of the timer triggers cell reselection. The UE then performs cell acquisition and collects system information for the selected LTE cell.

[0036] In LTE deployment, when the UE camps on the TD-SCDMA cell, the LTE neighbor frequencies are transmitted via broadcast messages from the TD-SCDMA network. The frequencies may be transmitted with or without cell IDs. The UE may only be informed of the LTE frequencies and not which cell(s) is on the particular frequency. The UE then performs a search to determine which cell is on a particular frequency.

[0037] When IRAT LTE neighbor measurement conditions are met, the UE performs IRAT LTE neighbor measurements. Examples of conditions for performing

measurements include, but are not limited to: a) the LTE neighbor frequencies are configured as higher priority than the TD-SCDMA serving cell; and b) the LTE neighbor frequencies are configured as lower priority than the TD-SCDMA serving cell while the TD-SCDMA serving cell signal quality is below a threshold indicated by the TD-SCDMA network. Those skilled in the art will appreciate the term signal quality is non-limiting and may include any type of signal metric, such as, but not limited to the strength and/or the quality of a signal.

[0038] During the period when IRAT measurements are performed, if the IRAT cell reselection conditions are met during the reselection time period (i.e., Treselection), the UE begins cell reselection to the target LTE cell detected during the IRAT

measurements. The UE then begins acquisition on the LTE frequency and attempts to camp on the target LTE cell after collecting the broadcast system information (e.g., system information blocks (SIBs)). Examples of the reselection conditions, include, but are not limited to: a), when the LTE neighbor frequencies are configured as higher priority than the TD-SCDMA serving cell, and the detected LTE cell signal quality is above a threshold defined by the TD-SCDMA network; and/or, b) when the LTE neighbor frequencies are configured as lower priority than the TD-SCDMA serving cell, and the TD-SCDMA serving cell signal quality is below a threshold and the LTE neighbor detected cell signal quality is above a threshold.

[0039] During the acquisition phase, the target LTE cell may no longer be the best cell for acquisition. For example, sudden radio frequency variation and high mobility may impact the viability of the target LTE cell. In some instances, IRAT reselection may fail when the target LTE cell is no longer available for acquisition. Additionally, reselection may fail when the UE cannot collect all of the required MIBs/SIBs on the target LTE cell. Further, reselection may also fail when the target LTE cell no longer meets the minimum signal quality requirement. [0040] Even if the UE successfully reselects to the target cell, altered circumstances may result in a rapidly degrading connection to the target cell, such that the UE may begin immediately searching for and evaluating neighbor cells for reselection after camping on the target cell. This re-initiation of the cell reselection process wastes battery power.

[0041] Aspects of the present disclosure are directed to a cell reselection procedure that includes consideration of the target cell and a candidate "best" cell. The best cell for cell reselection may be a second cell, different from the target cell. During the reselection procedure, if the signal quality of the target cell becomes too weak (or is lost altogether), the UE finds a second candidate target cell. Instead of reselection to the first target cell, the UE reselects to the second target cell.

[0042] In one aspect, during the evaluation phase of cell reselection, the UE searches for and identifies a first target cell on a particular frequency meeting cell reselection criteria. The UE then evaluates the first target cell and starts a reselection timer. When the reselection timer expires, the UE performs a second search and identifies a second target cell as a candidate for cell reselection. If the first target cell is no longer viable for cell reselection, then the UE camps on the second target cell instead of the first target cell.

[0043] In particular, after the reselection timer expires for the first target cell, the UE tunes to the first target frequency and/or target radio access technology of the first cell, and then performs a second search and measurement procedure to identify a second target cell. Those skilled in the art will appreciate the UE can identify multiple additional target cells.

[0044] The UE then evaluates the signal quality of the first target cell and the difference in signal quality between the first target cell and the second target cell. In one aspect, the UE will go directly to the second target cell when the signal quality of the second target cell is much stronger than the signal quality of the first target cell. In this scenario, the UE camps on the second target cell without performing a cell evaluation of the second target cell. In another aspect, if the signal quality of the first and second target cells is almost the same, then the UE will camp on the first target cell. Additionally, if the signal quality of the first target cell is below a threshold value, the UE will camp on the second target cell. In another aspect, if the UE cannot detect the signal of the first target cell and/or cannot collect system information (e.g., SIBs) from the first target cell, then the UE will camp on the second target cell.

[0045] In one aspect, when the signal quality of the first target cell is above a threshold value, the UE camps on the first target cell. The threshold may be a UE derived threshold that is based on receiver performance, i.e., whether the UE was able to reliably access service.

[0046] Alternately, in another aspect, the UE camps on the first target cell when the signal quality of the first target cell is below a first threshold and the difference in signal quality of the first and second target cells is below a second threshold. The threshold values may be predetermined values known by the UE.

[0047] In yet another aspect, the UE aborts camping on the first target cell when the signal quality of the first target cell is below a first threshold and the difference in signal quality of the first and second target cells is above a second threshold. In this aspect, the UE camps on the best cell. For example, the threshold may be based on a time duration between the last IRAT measurement and LTE acquisition, as well as an LTE intra- frequency cell reselection threshold. In one example, the first target cell is no longer detected after the reselection timer expires.

[0048] The first and second target cells may be on the same frequency. In another aspect, the first and second target cells are on different frequencies. Additionally, the first and second target cells may be of the same of different RATs.

[0049] According the present disclosure, IRAT cell reselection failure can be avoided. Thus, the likelihood of missing paging is reduced.

[0050] FIGURE 5 illustrates a cell reselection procedure 500. At block 502, the UE performs a first search and measurement to identify a first candidate cell for cell reselection. When the first candidate cell meets cell reselection conditions and a reselection timer is started, the UE performs a cell evaluation of the first candidate cell at block 504. After the cell reselection timer expires for the first candidate cell and the UE tunes to a target frequency and/or target radio access technology (RAT) of the first candidate cell, the UE identifies a second candidate cell during a second search and measurement at block 506. In block 508, the UE camps on the second candidate cell, without performing a cell evaluation of the second candidate cell, when the first candidate cell signal quality is below a first threshold, and a difference between a signal quality of the first and second candidates is above a third threshold.

[0051] FIGURE 6 is a diagram illustrating an example of a hardware implementation for an apparatus 600 employing a processing system 614. The processing system 614 may be implemented with a bus architecture, represented generally by the bus 624. The bus 624 may include any number of interconnecting buses and bridges depending on the specific application of the processing system 614 and the overall design constraints. The bus 624 links together various circuits including one or more processors and/or hardware modules, represented by the processor 622 the modules 602, 604, and 606 and the non-transitory computer-readable medium 626. The bus 624 may also link various other circuits such as timing sources, peripherals, voltage regulators, and power management circuits, which are well known in the art, and therefore, will not be described any further.

[0052] The apparatus includes a processing system 614 coupled to a transceiver 630. The transceiver 630 is coupled to one or more antennas 620. The transceiver 630 enables communicating with various other apparatus over a transmission medium. The processing system 614 includes a processor 622 coupled to a non-transitory computer- readable medium 626. The processor 622 is responsible for general processing, including the execution of software stored on the computer-readable medium 626. The software, when executed by the processor 622, causes the processing system 614 to perform the various functions described for any particular apparatus. The computer- readable medium 626 may also be used for storing data that is manipulated by the processor 622 when executing software.

[0053] The processing system 614 includes a search and measurement module 602 for performing search and measurement procedures. The processing system 614 includes a cell evaluation module 604 for performing cell evaluations. The processing system 614 includes a camping module for camping on a second candidate cell without performing a cell evaluation of the second candidate cell. The modules may be software modules running in the processor 622, resident/stored in the computer readable medium 626, one or more hardware modules coupled to the processor 622, or some combination thereof. The processing system 614 may be a component of the UE 350 and may include the memory 392, and/or the controller/processor 390.

[0054] In one configuration, an apparatus such as a UE is configured for wireless communication including means for performing a first search and measurement. In one aspect, the performing means may be the antennas 352, the receiver 354, the channel processor 394, the receive frame processor 360, the receive processor 370, the transmitter 356, the transmit frame processor 382, the transmit processor 380, the controller/processor 390, the memory 392, cell reselection module 391, search and measurement module 602 and/or the processing system 614 configured to perform the performing means.

[0055] The UE is also configured for wireless communication including means for performing cell evaluation. In one aspect, the performing cell evaluation means may be the antennas 352, the receiver 354, the channel processor 394, the receive frame processor 360, the receive processor 370, the transmitter 356, the transmit frame processor 382, the transmit processor 380, the controller/processor 390, the memory 392, cell reselection module 391, cell evaluation module 604, and/or the processing system 614 configured to perform the performing means.

[0056] The UE is also configured for wireless communication including means for identifying. In one aspect, the identifying means may be the antennas 352, the receiver 354, the channel processor 394, the receive frame processor 360, the receive processor 370, the transmitter 356, the transmit frame processor 382, the transmit processor 380, the controller/processor 390, the memory 392, cell reselection module 391, search and measurement module 602 and/or the processing system 614 configured to perform the identifying means.

[0057] The UE B is also configured to include means for camping. In one aspect, the camping means may be the antennas 352, the receiver 354, the channel processor 394, the receive frame processor 360, the receive processor 370, the transmitter 356, the transmit frame processor 382, the transmit processor 380, the controller/processor 390, the memory 392, cell reselection module 391, camping module 606, and/or the processing system 614 configured to perform the camping means. In one configuration, the means functions correspond to the aforementioned structures. In another aspect, the aforementioned means may be a module or any apparatus configured to perform the functions recited by the aforementioned means.

[0058] Several aspects of a telecommunications system has been presented with reference to TD-SCDMA, and LTE. However, as those skilled in the art will readily appreciate, various aspects described throughout this disclosure may be extended to other inter-RAT measurements and handovers, including IRAT operations in other wireless telecommunication systems, network architectures, and communication standards. Although discussed in the context of the first RAT (RAT-1) being a third- generation protocol and the second RAT (RAT-2) being a fourth-generation protocol, the RATs can be any protocol and any generation of technology (including a same generation), so long as the first RAT and the second RAT (and third RAT, etc.) are different protocols. By way of example, various aspects may be extended to other UMTS systems such as W-CDMA, high speed downlink packet access (HSDPA), high speed uplink packet access (HSUPA), high speed packet access plus (HSPA+) and TD- CDMA. Aspects may also be extended to various systems employing long term evolution (LTE) (in FDD, TDD, or both modes), LTE-Advanced (LTE-A) (in FDD, TDD, or both modes), CDMA2000, evolution-data optimized (EV-DO), ultra mobile broadband (UMB), IEEE 802.11 (Wi-Fi), IEEE 802.16 (WiMAX), IEEE 802.20, ultra- wideband (UWB), Bluetooth, and/or other suitable systems. The actual

telecommunication standard, network architecture, and/or communication standard employed will depend on the specific application and the overall design constraints imposed on the system.

[0059] Several processors have been described in connection with various apparatuses and methods. These processors may be implemented using electronic hardware, computer software, or any combination thereof. Whether such processors are implemented as hardware or software will depend upon the particular application and overall design constraints imposed on the system. By way of example, a processor, any portion of a processor, or any combination of processors presented in this disclosure may be implemented with a microprocessor, microcontroller, digital signal processor (DSP), a field-programmable gate array (FPGA), a programmable logic device (PLD), a state machine, gated logic, discrete hardware circuits, and other suitable processing components configured to perform the various functions described throughout this disclosure. The functionality of a processor, any portion of a processor, or any combination of processors presented in this disclosure may be implemented with software being executed by a microprocessor, microcontroller, DSP, or other suitable platform.

[0060] Software shall be construed broadly to mean instructions, instruction sets, code, code segments, program code, programs, subprograms, software modules, applications, software applications, software packages, routines, subroutines, objects, executables, threads of execution, procedures, functions, etc., whether referred to as software, firmware, middleware, microcode, hardware description language, or otherwise. The software may reside on a non-transitory computer-readable medium. A computer- readable medium may include, by way of example, memory such as a magnetic storage device (e.g., hard disk, floppy disk, magnetic strip), an optical disk (e.g., compact disc (CD), digital versatile disc (DVD)), a smart card, a flash memory device (e.g., card, stick, key drive), random access memory (RAM), read only memory (ROM), programmable ROM (PROM), erasable PROM (EPROM), electrically erasable PROM (EEPROM), a register, or a removable disk. Although memory 392 is shown separate from the various processors, the memory or portions of the memory may be internal to the processors (e.g., cache or register).

[0061] Computer-readable media may be embodied in a computer-program product. By way of example, a computer-program product may include a computer-readable medium in packaging materials. Those skilled in the art will recognize how best to implement the described functionality presented throughout this disclosure depending on the particular application and the overall design constraints imposed on the overall system.

[0062] It is to be understood that the specific order or hierarchy of steps in the methods disclosed is an illustration of exemplary processes. Based upon design preferences, it is understood that the specific order or hierarchy of steps in the methods may be rearranged. The accompanying method claims present elements of the various steps in a sample order, and are not meant to be limited to the specific order or hierarchy presented unless specifically recited therein. [0063] It is also to be understood that the term "signal quality" is non-limiting. Signal quality is intended to cover any type of signal metric such as received signal code power (RSCP), reference signal received power (RSRP), reference signal received quality (RSRQ), received signal strength indicator (RSSI), signal to noise ratio (SNR), signal to interference plus noise ratio (SINR), etc.

[0064] The previous description is provided to enable any person skilled in the art to practice the various aspects described herein. Various modifications to these aspects will be readily apparent to those skilled in the art, and the generic principles defined herein may be applied to other aspects. Thus, the claims are not intended to be limited to the aspects shown herein, but is to be accorded the full scope consistent with the language of the claims, wherein reference to an element in the singular is not intended to mean "one and only one" unless specifically so stated, but rather "one or more." Unless specifically stated otherwise, the term "some" refers to one or more. A phrase referring to "at least one of a list of items refers to any combination of those items, including single members. As an example, "at least one of: a, b, or c" is intended to cover: a; b; c; a and b; a and c; b and c; and a, b and c. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. § 112, sixth paragraph, unless the element is expressly recited using the phrase "means for" or, in the case of a method claim, the element is recited using the phrase "step for."

Claims

CLAIMS WHAT IS CLAIMED IS:

1. A method of wireless communication, comprising:

performing a first search and measurement to identify a first candidate cell for cell reselection;

performing a cell evaluation of the first candidate cell when the first candidate cell meets cell reselection conditions and a reselection timer is started; identifying a second candidate cell during a second search and measurement after the reselection timer expires for the first candidate cell and a UE tunes to a target frequency and/or target radio access technology (RAT) of the first candidate cell; and

camping on the second candidate cell, without performing a cell evaluation of the second candidate cell, when the first candidate cell signal quality is below a first threshold, and a difference between a signal quality of the first and second candidates is above a second threshold.

2. The method of claim 1, further comprising camping on the first candidate cell when the first candidate cell signal quality is above the first threshold.

3. The method of claim 1, further comprising camping on the first candidate cell when the first candidate cell signal quality is below the first threshold and the difference between the signal quality of the first and second candidates is below the second threshold.

4. The method of claim 1, in which the first candidate cell and the second candidate cell are on a same frequency and/or a same RAT.

5. The method of claim 1, further comprising:

camping on the second candidate cell, without performing the cell evaluation of the second candidate cell, when the first candidate cell is not detected after the reselection timer expires.

6. An apparatus for wireless communication, comprising:

means for performing a first search and measurement to identify a first candidate cell for cell reselection;

means for performing a cell evaluation of the first candidate cell when the first candidate cell meets cell reselection conditions and a reselection timer is started;

means for identifying a second candidate cell during a second search and measurement after the reselection timer expires for the first candidate cell and a UE tunes to a target frequency and/or target radio access technology (RAT) of the first candidate cell; and

means for camping on the second candidate cell, without performing a cell evaluation of the second candidate cell, when the first candidate cell signal quality is below a first threshold, and a difference between a signal quality of the first and second candidates is above a second threshold.

7. The apparatus of claim 6, further comprising means for camping on the first candidate cell when the first candidate cell signal quality is above the first threshold.

8. The apparatus of claim 6, further comprising means for camping on the first candidate cell when the first candidate cell signal quality is below the first threshold and the difference between the signal quality of the first and second candidates is below the second threshold.

9. The apparatus of claim 6, in which the first candidate cell and the second

candidate cell are on a same frequency and/or a same RAT.

10. The apparatus of claim 6, further comprising:

means for camping on the second candidate cell, without performing the cell evaluation of the second candidate cell, when the first candidate cell is not detected after the reselection timer expires.

11. An apparatus for wireless communication, comprising:

a memory; and

at least one processor coupled to the memory and configured:

to perform a first search and measurement to identify a first candidate cell for cell reselection;

to perform a cell evaluation of the first candidate cell when the first candidate cell meets cell reselection conditions and a reselection timer is started; to identify a second candidate cell during a second search and

measurement after the reselection timer expires for the first candidate cell and a UE tunes to a target frequency and/or target radio access technology (RAT) of the first candidate cell; and

to camp on the second candidate cell, without performing a cell evaluation of the second candidate cell, when the first candidate cell signal quality is below a first threshold, and a difference between a signal quality of the first and second candidates is above a second threshold.

12. The apparatus of claim 11, in which the at least one processor is further

configured to camp on the first candidate cell when the first candidate cell signal quality is above the first threshold.

13. The apparatus of claim 11, in which the at least one processor is further

configured to camp on the first candidate cell when the first candidate cell signal quality is below the first threshold and the difference between the signal quality of the first and second candidates is below the second threshold.

14. The apparatus of claim 11, in which the first candidate cell and the second

candidate cell are on a same frequency and/or a same RAT.

15. The apparatus of claim 11, in which the at least one processor is further

configured to camp on the second candidate cell, without performing the cell evaluation of the second candidate cell, when the first candidate cell is not detected after the reselection timer expires.

16. A computer program product for wireless communication, comprising:

a non-transitory computer-readable medium having program code recorded thereon, the program code comprising:

program code to perform a first search and measurement to identify a first candidate cell for cell reselection;

program code to perform a cell evaluation of the first candidate cell when the first candidate cell meets cell reselection conditions and a reselection timer is started;

program code to identify a second candidate cell during a second search and measurement after the reselection timer expires for the first candidate cell and a UE tunes to a target frequency and/or target radio access technology (RAT) of the first candidate cell; and

program code to camp on the second candidate cell, without performing a cell evaluation of the second candidate cell, when the first candidate cell signal quality is below a first threshold, and a difference between a signal quality of the first and second candidates is above a second threshold.

17. The computer program product of claim 16, further comprising program code to camp on the first candidate cell when the first candidate cell signal quality is above the first threshold.

18. The computer program product of claim 16, further comprising program code to camp on the first candidate cell when the first candidate cell signal quality is below the first threshold and the difference between the signal quality of the first and second candidates is below the second threshold.

19. The computer program product of claim 16 in which the first candidate cell and the second candidate cell are on a same frequency and/or a same RAT.

20. The computer program product of claim 16, further comprising program code to camp on the second candidate cell, without performing the cell evaluation of the second candidate cell, when the first candidate cell is not detected after the reselection timer expires.