# Patent

## Title
Method for operating time alignment timer and communication device thereof

## Abstract

There is provided a method for operating time alignment timer (TAT). The method may comprise: operating a first TAT in a first medium access control (MAC) entity; operating a second TAT in a second MAC entity; and if the first TAT operated in the first MAC entity is expired, considering the second TAT operated in the second MAC entity as expired.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application is the National Phase of PCT/KR2014/000226 filed on Jan. 9, 2014, which claims priority under 35 U.S.C. 119(e) to U.S. Provisional Application No. 61/807,338 filed on Apr. 2, 2013, all of which are hereby expressly incorporated by reference into the present application.


TECHNICAL FIELD
The present invention relates to wireless communication, and more specifically, to a method for operating time alignment timer and a communication device thereof.
BACKGROUND ART
3rd generation partnership project (3GPP) long term evolution (LTE) is an improved version of a universal mobile telecommunication system (UMTS) and is introduced as the 3GPP release 8. The 3GPP LTE uses orthogonal frequency division multiple access (OFDMA) in a downlink, and uses single carrier-frequency division multiple access (SC-FDMA) in an uplink. The 3GPP LTE employs multiple input multiple output (MIMO) having up to four antennas. In recent years, there is an ongoing discussion on 3GPP LTE-advanced (LTE-A) that is an evolution of the 3GPP LTE.
Examples of techniques employed in the 3GPP LTE-A include carrier aggregation.
The carrier aggregation uses a plurality of component carriers. The component carrier is defined with a center frequency and a bandwidth. One downlink component carrier or a pair of an uplink component carrier and a downlink component carrier is mapped to one cell. When a user equipment receives a service by using a plurality of downlink component carriers, it can be said that the user equipment receives the service from a plurality of serving cells. That is, the plurality of serving cells provides a user equipment with various services.
In recent, there is a discussion for adopting small cells.
DISCLOSURE OF INVENTION
Technical Problem
In the related art as above explained, due to adoption of the small cells, it will be possible for the UE to have dual connectivities to both a conventional cell and a small cell. However, there is yet no concept and technique to realize dual connectivities.
Therefore, an object of the present invention is to provide solutions to realize dual connectivities.
Solution to Problem
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is provided a method for operating time alignment timer (TAT). The method may comprise: operating a first TAT in a first medium access control (MAC) entity; operating a second TAT in a second MAC entity; and if the first TAT operated in the first MAC entity is expired, considering the second TAT operated in the second MAC entity as expired.
The first MAC entity operating the first TAT may serve a signaling radio bearer (SRB). The second MAC entity operating the second TAT may do not serve SRB.
The method may further comprise connecting with a first base station via the first MAC entity; and connecting with a second base station via the second MAC entity;
The method may further comprise receiving configurations on the first TAT and the second TAT; and receiving time advance (TA) commands for the first TAT and the second TAT. Each TA command includes an identity of a corresponding MAC entity.
The operating of the first TAT may include: applying the TA command to the first MAC entity indicated by the identify in the TA command; and starting, by the first MAC entity indicated by the identify in the TA command, the first TAT.
The operating of the second TAT may include: applying the TA command to the second MAC entity indicated by the identify in the TA command; and starting, by the second MAC entity indicated by the identify in the TA command, the second TAT.
To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described herein, there is also provided a communication device configured for operating time alignment timer (TAT). The communication device may comprise: a radio frequency (RF) unit; and a processor connected with the RF unit thereby to control to: operate a first TAT in a first medium access control (MAC) entity; operate a second TAT in a second MAC entity; and if the first TAT operated in the first MAC entity is expired, consider the second TAT operated in the second MAC entity as expired.
Advantageous Effects of Invention
According to the present specification, the above-explained problem may be solved. Also, it is possible to minimize overhead of uplink signaling such as SRS which occurs during running time alignment timer (TAT) for other connectivity.



BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 shows a wireless communication system to which the present invention is applied.
FIG. 2 is a diagram showing a radio protocol architecture for a user plane.
FIG. 3 is a diagram showing a radio protocol architecture for a control plane.
FIG. 4 shows an example of a wideband system using carrier aggregation for 3GPP LTE-A.
FIG. 5 shows an example of a structure of DL layer 2 when carrier aggregation is used.
FIG. 6 shows an example of a structure of UL layer 2 when carrier aggregation is used.
FIG. 7 shows an example of transmitting a TAC.
FIG. 8 shows one exemplary concept of coexistence of a macro cell and small cells.
FIG. 9 shows one example of a first scenario of small cell deployment.
FIG. 10a shows one example of a second scenario of small cell deployment.
FIG. 10b shows another example of the second scenario of small cell deployment.
FIG. 11 shows one example of a third scenario of small cell deployment.
FIG. 12 shows a concept of dual connectivities
FIG. 13 shows radio protocols of eNodeBs for supporting dual connectivities.
FIG. 14 shows radio protocols of UE for supporting dual connectivities.
FIG. 15 shows one exemplary method according to one embodiment of the present disclosure.
FIG. 16 shows one exemplary method according to another embodiment of the present disclosure.
FIG. 17 is a block diagram showing a wireless communication system to implement an embodiment of the present invention.



MODE FOR THE INVENTION
Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. It will also be apparent to those skilled in the art that various modifications and variations can be made in the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.
Description will now be given in detail of a drain device and a refrigerator having the same according to an embodiment, with reference to the accompanying drawings.
The present invention will be described on the basis of a universal mobile telecommunication system (UMTS) and an evolved packet core (EPC). However, the present invention is not limited to such communication systems, and it may be also applicable to all kinds of communication systems and methods to which the technical spirit of the present invention is applied.
It should be noted that technological terms used herein are merely used to describe a specific embodiment, but not to limit the present invention. Also, unless particularly defined otherwise, technological terms used herein should be construed as a meaning that is generally understood by those having ordinary skill in the art to which the invention pertains, and should not be construed too broadly or too narrowly. Furthermore, if technological terms used herein are wrong terms unable to correctly express the spirit of the invention, then they should be replaced by technological terms that are properly understood by those skilled in the art. In addition, general terms used in this invention should be construed based on the definition of dictionary, or the context, and should not be construed too broadly or too narrowly.
Incidentally, unless clearly used otherwise, expressions in the singular number include a plural meaning. In this application, the terms “comprising” and “including” should not be construed to necessarily include all of the elements or steps disclosed herein, and should be construed not to include some of the elements or steps thereof, or should be construed to further include additional elements or steps.
The terms used herein including an ordinal number such as first, second, etc. can be used to describe various elements, but the elements should not be limited by those terms. The terms are used merely to distinguish an element from the other element. For example, a first element may be named to a second element, and similarly, a second element may be named to a first element.
In case where an element is “connected” or “linked” to the other element, it may be directly connected or linked to the other element, but another element may be existed therebetween. On the contrary, in case where an element is “directly connected” or “directly linked” to another element, it should be understood that any other element is not existed therebetween.
Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings, and the same or similar elements are designated with the same numeral references regardless of the numerals in the drawings and their redundant description will be omitted. In describing the present invention, moreover, the detailed description will be omitted when a specific description for publicly known technologies to which the invention pertains is judged to obscure the gist of the present invention. Also, it should be noted that the accompanying drawings are merely illustrated to easily explain the spirit of the invention, and therefore, they should not be construed to limit the spirit of the invention by the accompanying drawings. The spirit of the invention should be construed as being extended even to all changes, equivalents, and substitutes other than the accompanying drawings.
There is an exemplary UE (User Equipment) in accompanying drawings, however the UE may be referred to as terms such as a terminal, a mobile equipment (ME), a mobile station (MS), a user terminal (UT), a subscriber station (SS), a wireless device (WD), a handheld device (HD), an access terminal (AT), and etc. And, the UE may be implemented as a portable device such as a notebook, a mobile phone, a PDA, a smart phone, a multimedia device, etc, or as an unportable device such as a PC or a vehicle-mounted device.
FIG. 1 shows a wireless communication system to which the present invention is applied.
The wireless communication system may also be referred to as an evolved-UMTS terrestrial radio access network (E-UTRAN) or a long term evolution (LTE)/LTE-A system.
The E-UTRAN includes at least one base station (BS) 20 which provides a control plane and a user plane to a user equipment (UE) 10. The UE 10 may be fixed or mobile, and may be referred to as another terminology, such as a mobile station (MS), a user terminal (UT), a subscriber station (SS), a mobile terminal (MT), a wireless device, etc. The BS 20 is generally a fixed station that communicates with the UE 10 and may be referred to as another terminology, such as an evolved node-B (eNodeB), a base transceiver system (BTS), an access point, etc.
The BSs 20 are interconnected by means of an X2 interface. The BSs 20 are also connected by means of an S1 interface to an evolved packet core (EPC) 30, more specifically, to a mobility management entity (MME) through S1-MME and to a serving gateway (S-GW) through S1-U.
The EPC 30 includes an MME, an S-GW, and a packet data network-gateway (P-GW). The MME has access information of the UE or capability information of the UE, and such information is generally used for mobility management of the UE. The S-GW is a gateway having an E-UTRAN as an end point. The P-GW is a gateway having a PDN as an end point.
Layers of a radio interface protocol between the UE and the network can be classified into a first layer (L1), a second layer (L2), and a third layer (L3) based on the lower three layers of the open system interconnection (OSI) model that is well-known in the communication system. Among them, a physical (PHY) layer belonging to the first layer provides an information transfer service by using a physical channel, and a radio resource control (RRC) layer belonging to the third layer serves to control a radio resource between the UE and the network. For this, the RRC layer exchanges an RRC message between the UE and the BS.
FIG. 2 is a diagram showing a radio protocol architecture for a user plane. FIG. 3 is a diagram showing a radio protocol architecture for a control plane.
The user plane is a protocol stack for user data transmission. The control plane is a protocol stack for control signal transmission.
Referring to FIGS. 2 and 3, a PHY layer provides an upper layer with an information transfer service through a physical channel. The PHY layer is connected to a medium access control (MAC) layer which is an upper layer of the PHY layer through a transport channel. Data is transferred between the MAC layer and the PHY layer through the transport channel. The transport channel is classified according to how and with what characteristics data is transferred through a radio interface.
Between different PHY layers, i.e., a PHY layer of a transmitter and a PHY layer of a receiver, data is transferred through the physical channel. The physical channel may be modulated using an orthogonal frequency division multiplexing (OFDM) scheme, and may utilize time and frequency as a radio resource.
Functions of the MAC layer include mapping between a logical channel and a transport channel and multiplexing/de-multiplexing on a transport block provided to a physical channel over a transport channel of a MAC service data unit (SDU) belonging to the logical channel. The MAC layer provides a service to a radio link control (RLC) layer through the logical channel.
Functions of the RLC layer include RLC SDU concatenation, segmentation, and reassembly. To ensure a variety of quality of service (QoS) required by a radio bearer (RB), the RLC layer provides three operation modes, i.e., a transparent mode (TM), an unacknowledged mode (UM), and an acknowledged mode (AM). The AM RLC provides error correction by using an automatic repeat request (ARQ).
Functions of a packet data convergence protocol (PDCP) layer in the user plane include user data delivery, header compression, and ciphering. Functions of a PDCP layer in the control plane include control-plane data delivery and ciphering/integrity protection.
A radio resource control (RRC) layer is defined only in the control plane. The RRC layer serves to control the logical channel, the transport channel, and the physical channel in association with configuration, reconfiguration and release of radio bearers (RBs). An RB is a logical path provided by the first layer (i.e., the PHY layer) and the second layer (i.e., the MAC layer, the RLC layer, and the PDCP layer) for data delivery between the UE and the network.
The setup of the RB implies a process for specifying a radio protocol layer and channel properties to provide a particular service and for determining respective detailed parameters and operations. The RB can be classified into two types, i.e., a signaling RB (SRB) and a data RB (DRB). The SRB is used as a path for transmitting an RRC message in the control plane. The DRB is used as a path for transmitting user data in the user plane.
When an RRC connection is established between an RRC layer of the UE and an RRC layer of the network, the UE is in an RRC connected state (also may be referred as an RRC connected mode), and otherwise the UE is in an RRC idle state (also may be referred as an RRC idle mode).
Data is transmitted from the network to the UE through a downlink transport channel. Examples of the downlink transport channel include a broadcast channel (BCH) for transmitting system information and a downlink-shared channel (SCH) for transmitting user traffic or control messages. The user traffic of downlink multicast or broadcast services or the control messages can be transmitted on the downlink-SCH or an additional downlink multicast channel (MCH). Data is transmitted from the UE to the network through an uplink transport channel. Examples of the uplink transport channel include a random access channel (RACH) for transmitting an initial control message and an uplink SCH for transmitting user traffic or control messages.
Examples of logical channels belonging to a higher channel of the transport channel and mapped onto the transport channels include a broadcast channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), a multicast traffic channel (MTCH), etc.
The physical channel includes several OFDM symbols in a time domain and several subcarriers in a frequency domain. One subframe includes a plurality of OFDM symbols in the time domain. A resource block is a resource allocation unit, and includes a plurality of OFDM symbols and a plurality of subcarriers. Further, each subframe may use particular subcarriers of particular OFDM symbols (e.g., a first OFDM symbol) of a corresponding subframe for a physical downlink control channel (PDCCH), i.e., an L1/L2 control channel. A transmission time interval (TTI) is a unit time of subframe transmission.
Hereinafter, an RRC state of a UE and an RRC connection mechanism will be described.
The RRC state indicates whether an RRC layer of the UE is logically connected to an RRC layer of an E-UTRAN. If the two layers are connected to each other, it is called an RRC connected state, and if the two layers are not connected to each other, it is called an RRC idle state. When in the RRC connected state, the UE has an RRC connection and thus the E-UTRAN can recognize a presence of the UE in a cell unit. Accordingly, the UE can be effectively controlled. On the other hand, when in the RRC idle state, the UE cannot be recognized by the E-UTRAN, and is managed by a core network in a tracking area unit which is a unit of a wider area than a cell. That is, regarding the UE in the RRC idle state, only a presence or absence of the UE is recognized in a wide area unit. To get a typical mobile communication service such as voice or data, a transition to the RRC connected state is necessary.
When a user initially powers on the UE, the UE first searches for a proper cell and thereafter stays in the RRC idle state in the cell. Only when there is a need to establish an RRC connection, the UE staying in the RRC idle state establishes the RRC connection with the E-UTRAN through an RRC connection procedure and then transitions to the RRC connected state. Examples of a case where the UE in the RRC idle state needs to establish the RRC connection are various, such as a case where uplink data transmission is necessary due to telephony attempt of the user or the like or a case where a response message is transmitted in response to a paging message received from the E-UTRAN.
A non-access stratum (NAS) layer belongs to an upper layer of the RRC layer and serves to perform session management, mobility management, or the like.
Now, a radio link failure will be described.
A UE persistently performs measurement to maintain quality of a radio link with a serving cell from which the UE receives a service. The UE determines whether communication is impossible in a current situation due to deterioration of the quality of the radio link with the serving cell. If it is determined that the quality of the serving cell is so poor that communication is almost impossible, the UE determines the current situation as a radio link failure.
If the radio link failure is determined, the UE gives up maintaining communication with the current serving cell, selects a new cell through a cell selection (or cell reselection) procedure, and attempts RRC connection re-establishment to the new cell.
FIG. 4 shows an example of a wideband system using carrier aggregation for 3GPP LTE-A.
Referring to FIG. 4, each CC has a bandwidth of 20 MHz, which is a bandwidth of the 3GPP LTE. Up to 5 CCs may be aggregated, so maximum bandwidth of 100 MHz may be configured.
FIG. 5 shows an example of a structure of DL layer 2 when carrier aggregation is used. FIG. 6 shows an example of a structure of UL layer 2 when carrier aggregation is used.
The carrier aggregation may affect a MAC layer of the L2. For example, since the carrier aggregation uses a plurality of CCs, and each hybrid automatic repeat request (HARQ) entity manages each CC, the MAC layer of the 3GPP LTE-A using the carrier aggregation shall perform operations related to a plurality of HARQ entities. In addition, each HARQ entity processes a transport block independently. Therefore, when the carrier aggregation is used, a plurality of transport blocks may be transmitted or received at the same time through a plurality of CCs.
<Uplink Timing Synchronization>
Now, uplink timing synchronization will be described.
In LTE system based on an orthogonal frequency division multiplexing (OFDM) technique, there is a possibility that an interference to another user occurs in a process of performing communication between a UE and a BS. In order to minimize the interference, it is very important for the BS to manage uplink transmission timing of the UE.
More particularly, the terminal may exist in random area within a cell, and this implies that a data transmission time (i.e., traveling time of data from UE to base station) can be varied based on a location of the terminal. Namely, if the terminal is camped on edge of the cell, data transmission time of this specific terminal will be much longer than data transmission time of those terminals who camped on a center of the cell. In contrast, if the terminal is camped on the center of the cell, data transmission time of this specific terminal will be much shorter than data transmission time of those terminals who camped on the edge of the cell. The base station (eNB) must manage or handle all data or signals, which are transmitted by the terminals within the cell, in order to prevent the interferences between the terminals. Namely, the base station must adjust or manage a transmission timing of the terminals upon each terminal's condition, and such adjustment can be called as the timing alignment maintenance. One of the methods for maintaining the timing alignment is a random access procedure. Namely, during the random access procedure, the base station receives a random access preamble transmitted from the terminal, and the base station can calculate a time alignment (Sync) value using the received random access preamble, where the time alignment value is to adjust (i.e., faster or slower) a data transmission timing of the terminal. The calculated time alignment value can be notified to the terminal by a random access response, and the terminal can update the data transmission timing based on the calculated time alignment value. In other method, the base station may receive a sounding reference symbol (SRS) transmitted from the terminal periodically or randomly, the base station may calculate the time alignment (Sync) value based on the SRS, and the terminal may update the data transmission timing according to the calculated time alignment value.
As explained above, the base station (eNB) may measure a transmission timing of the terminal though a random access preamble or SRS, and may notify an adjustable timing value to the terminal. Here, the time alignment (Sync) value (i.e., the adjustable timing value) can be called as a time advance command (referred as ‘TAC’ hereafter). The TAC may be process in a MAC (Medium Access control) layer. Since the terminal does not camps on a fixed location, the transmission timing is frequently changed based on a terminal's moving location and/or a terminal's moving velocity. Concerning with this, if the terminal receives the time advance command (TAC) from the base station, the terminal expect that the time advance command is only valid for certain time duration. A time alignment timer (TAT) is used for indicating or representing the certain time duration. As such, the time alignment timer (TAT) is started when the terminal receives the TAC (time advance command) from the base station. The TAT value is transmitted to the terminal (UE) through a RRC (Radio Resource Control) signal such as system information (SI) or a radio bearer reconfiguration. Also, if the terminal receives a new TAC from the base station during an operation of the TAT, the TAT is restarted. Further, the terminal does not transmit any other uplink data or control signal (e.g., data on physical uplink shared channel (PUSCH), control signal on Physical uplink control channel (PUCCH) except for the random access preamble when the TAT is expired or not running. In more detail, if the TAT is expired, the terminal can perform the following operations: First, the terminal flush or empty a HARQ buffer for all serving cells. Second, the terminal informs a RRC layer to release a PUCCH/SRS resource for all serving cells. Also, the terminal releases a uplink resource and a downlink resource.
FIG. 7 shows an example of transmitting a TAC.
The section 6 of 3GPP (3rd Generation Partnership Project) TS (Technical Specification) 36.321 V8.5.0 (2009-03) can be incorporated herein by reference. (A) of FIG. 7 shows a TAC included in a random access response. (B) of FIG. 7 shows a TAC included in a MAC CE.
When the TAC is included in the random access response, this is case where new alignment is performed in a state where a UE is not time-aligned. Therefore, a precise regulation is required, and an 11-bit TAC is transmitted. When the TAC is included in the MAC CE, this is a case where it is used to extend time alignment in a situation where the UE is time-aligned, and thus a 6-bit TAC is transmitted.
However, since the UE does not always exist in a fixed location, transmission timing of the UE varies depending on a speed and location of the moving UE. By considering this, the UE assumes that time alignment is valid only during a specific time after receiving the TAC from a BS. A timer used for this is a time alignment timer (TAT).
The TAT is used to control how long the UE maintains the uplink time alignment.
Upon receiving the TAC from the BS, the UE applies the TAC and thereafter starts or restarts the TAT. The UE assumes that uplink time alignment with the BS is established only during the TAT is running.
A value of the TAT can be delivered by the BS to the UE through an RRC message such as system information or radio bearer reconfiguration.
<Small Cell>
Now, a concept of small cell will be described.
In the 3rd or 4th mobile communication system, an attempt to increase a cell capacity is continuously made in order to support a high-capacity service and a bidirectional service such as multimedia contents, streaming, and the like.
That is, as various large-capacity transmission technologies are required with development of communication and spread of multimedia technology, a method for increase a radio capacity includes a method of allocating more frequency resources, but there is a limit in allocating more frequency resources to a plurality of users with limited frequency resources.
An approach to use a high-frequency band and decrease a cell radius has been made in order to increase the cell capacity. When a small cell such as a pico cell or femto cell is adopted, a band higher than a frequency used in the existing cellular system may be used, and as a result, it is possible to transfer more information.
FIG. 8 shows one exemplary concept of coexistence of a macro cell and small cells.
As shown in FIG. 8, a cell of a conventional BS or eNodeB (200) may be called as a macro cell over small cells. Each small cell is operated by each small BS or eNodeB (300). When the conventional BS or eNodeB (200) may operate in use of a frequency F1, each small cell operates in use of a frequency F1 or F2. Small cells may be grouped in a cluster. It is noted that actual deployment of small cells are varied depending on operator's policy.
FIG. 9 shows one example of a first scenario of small cell deployment.
As shown in FIG. 9, the small cells may be deployed in the presence of an overlaid macro cell. That is, the small cells may be deployed in a coverage of the macro cell. In such deployment, the following may be considered.

    
    
        Co-channel deployment of the macro cell and small cells
        Outdoor small cell deployment
        Small cell cluster is considered
        The small cells are dense in cluster
        Details regarding the number/density of small cells per cluster, backhaul link for coordination among small cells and time synchronization among small cells may also be considered
        Both ideal backhaul and non-ideal backhaul may be also considered for the following interfaces: an interface between the small cells within the same cluster and an interface between a cluster of small cells and at least one macro eNodeB.
        Non-ideal backhaul is assumed for all other interfaces.
    
    


Here, the non-ideal backhaul means that there may be a delay up to 60 ms.
FIG. 10a shows one example of a second scenario of small cell deployment.
As shown in FIG. 10a, the small cells may be deployed outdoor. In such deployment, the following may be considered.

    
    
        The small cells are deployed in the presence of an overlaid macro network
        Separate frequency deployment of the macro cell and small cells
        Outdoor small cell deployment
        Small cell cluster is considered
        The small cells are dense in cluster
        Details regarding the number/density of small cells per cluster, backhaul link for coordination among small cells and time synchronization among small cells may also be considered.
        Both ideal backhaul and non-ideal backhaul may be also considered for the following interfaces: an interface between the small cells within the same cluster and an interface between a cluster of small cells and at least one macro eNB
        Non-ideal backhaul is assumed for all other interfaces
    
    


FIG. 10b shows another example of the second scenario of small cell deployment.
As shown in FIG. 10b, the small cells may be deployed indoor. In such deployment, the following may be considered.

    
    
        The small cells are deployed in the presence of an overlaid macro network
        Separate frequency deployment of the macro cell and small cells
        Indoor small cell deployment is considered
        Small cell cluster is considered
        The small cells are dense in cluster
        Details regarding the number/density of small cells per cluster, backhaul link for coordination among small cells and time synchronization among small cells may also be considered.
        A sparse scenario can be also considered such as the indoor hotspot scenario.
        Both ideal backhaul and non-ideal backhaul may be also considered for the following interfaces: an interface between the small cells within the same cluster and an interface between a cluster of small cells and at least one macro eNB
        Non-ideal backhaul is assumed for all other interfaces.
    
    


FIG. 11 shows one example of a third scenario of small cell deployment.
As shown in FIG. 11, the small cells may be deployed indoor. In such deployment, the following may be considered.

    
    
        Macro cell coverage is not present
        Indoor deployment scenario is considered
        Small cell cluster is considered
        The small cells are dense in cluster
        Details regarding the number/density of small cells per cluster, backhaul link for coordination among small cells and time synchronization among small cells may also be considered.
        A sparse scenario can be considered such as the indoor hotspot scenario.
        Both ideal backhaul and non-ideal backhaul may be also considered for the following interfaces: an interface between the small cells within the same cluster.
        Non-ideal backhaul is assumed for all other interfaces.
    
    


FIG. 12 shows a concept of dual connectivities
As illustrated in FIG. 12. the UE 100 has dual connectivities to both Macro cell and small cell. Here, the connectivity means the connection to eNodeB for data transfer. If the UE is served by both one macro cell and one small cell, it can be said that the UE has dual connectivities, i.e., one connectivity for the macro cell and another connectivity for the small cell. If the UE is served by small cells, it can be said that the UE has multiple connectivity.
The macro cell is served by CeNodeB (or CeNB) and the small cell or group of small cells is served by UeNodeB (or UeNB). The CeNodeB means Control plane eNodeB that is responsible for managing control plane specific operations, e.g., RRC connection control and mobility, e.g., transfer of control data on signaling radio bearers (SRBs). The UeNodeB means User plane eNodeB that is responsible for managing user plane specific operations, e.g., transfer of data on data radio bearers (DRBs).
The small cell of UeNodeB is responsible for transmitting best effort (BE) type traffic, while the macro cell of the CeNodeB is responsible for transmitting other types of traffic such as VoIP, streaming data, or signaling data.
It is noted that there is X3 interface between CeNodeB and UeNodeB that is similar to conventional X2 interface between eNodeBs.
FIG. 13 shows radio protocols of eNodeBs for supporting dual connectivities.
For dual or multiple connectivities, MAC functions of the UE 100 needs to be newly defined because from Layer 2 protocol point of view, RLC functions and configurations are bearer-specific while MAC functions and configurations are not.
To support dual or multiple connectivities, various protocol architectures are studied, and one of potential architectures is shown in FIG. 13. In this architecture, PDCP entity for UeNodeB is located in different network nodes, i.e. PDCP in CeNodeB.
As shown in FIG. 13, CeNodeB includes a PHY layer, a MAC layer, an RLC layer, a PDCH layer and an RRC layer while the UeNodeB includes a PHY layer, a MAC layer and an RLC layer. It is noted that the RRC layer and the PDCP layer exist only in the CeNodeB. In other words, there is the common RRC and PDCP layer and there is a set of RLC, MAC and PHY layers per connectivity. Accordingly, data on SRBs is signaled on CeNodeB and data on DRBs is signaled on either CeNodeB or UeNodeB according to the DRB configurations. That is, the CeNodeB can deliver data on DRBs in addition to control data on SRBs, while the UeNodeB can deliver data on only DRBs.
Here, the followings are considered:

    
    
        CeNodeB and UeNodeB can be different nodes.
        Transfer of data on SRBs is performed on CeNodeB.
        Transfer of data on DRBs is performed on either CeNodeB or UeNodeB. Whether path of data on DRBs is on CeNodeB or UeNodeB can be configured by the eNodeB, MME, or S-GW.
        There is X3 interface between CeNodeB and UeNodeB that is similar to conventional X2 interface between eNodeBs.
        Because RRC connection reconfiguration is managed in the CeNodeB, the CeNodeB sends information about DRB configurations to UeNodeB via X3 interface.
    
    


FIG. 14 shows radio protocols of UE for supporting dual connectivities.
As shown in FIG. 14, the UeNodeB is responsible for transmitting best effort (BE) DRB. The CeNodeB is responsible for transmitting SRB and DRB. As above explained, PDCP entity for UeNodeB is located in CeNodeB.
As shown in FIG. 14, on the UE 100 side, there are plural MAC entities for macro cell of CeNodeB and small cells of UeNodeB. In other word, the UE 100 setups each MAC entity for each connectivity. Accordingly, the UE 100 includes plural MAC entities for dual or multiple connectivities. Here, although FIG. 14 illustrates two PHY entities for dual connectivities, only one PHY entity may handle dual connectivities. For the connectivity to UeNodeB, the UE 100 may include the PDCP entity, the RLC entity and the MAC entity which handle BE-DRB. For connectivity to CeNodeB, the UE 100 may include plural RLC entities, plural PDCP entities which handle SRB and DRB.
Meanwhile, each of the CeNodeB and the UeNodeB owns a radio resource for itself and include a scheduler for scheduling the radio resource for itself. Here, each scheduler and each connectivity are 1-to-1 mapping. Therefore, each scheduler can manage an uplink (UL) timing of the UE per connectivity. And, each scheduler transmits configuration about uplink timing alignment per connectivity to the UE.
Therefore, uplink timing alignment related timer, e.g., time alignment timer (TAT) for each connectivity operates independently.
Here, the connectivity to the CeNodeB is important because it transfer SRB data such as handover command. However, there occurs a problem where other time alignment timers (TATs) for other connectivities are running while time alignment timer (TAT) for the connectivity to the CeNodeB is stopped.
Therefore, the present disclosure provides a solution that the time alignment timer (TAT) for the connectivity to the CeNodeB needs to supervise time alignment timer (TAT) for other connectivity to solve the problem.
For the solution, the present disclosure provides one example technique. According to the technique, if the time alignment timer (TAT) for connectivity for CeNodeB (or Macro eNodeB) expires, the UE considers other time alignment timers (TATs) for all connectivities as expired.
On the other hand, there may be another problem where data on any radio bearer can be delay or discarded when a connectivity relating to the radio bearer is released, deactivated or disconnected.
Therefore, the present disclosure provides another solution that a connectivity to CeNodeB is considered to a default connectivity, so that the UE can associate the radio bearer related to the removed or deactivated connectivity with the default connectivity.
FIG. 15 shows one exemplary method according to one embodiment of the present disclosure.
Referring to FIG. 15, it is illustrated how the time alignment timer (TAT) for the connectivity to the UeNodeB operates when the time alignment timer (TAT) for the connectivity to CeNodeB expires.
(1) In detail, the UE 100 may receive a configuration on dual connectivities to CeNodeB (or Macro eNodeB) 200 and UeNodeB (or small eNodeB) 300. The configuration may indicate that a first connectivity (connectivity 1) is for CeNodeB and a second connected (connectivity 2) is for UeNodeB. Then, the UE 100 may activate (or configure) each MAC entity for each connectivity.
(2) And, the UE 100 may receive configuration on a plurality of bearers. The configuration may indicate that a first radio bearer (radio bearer 1) is related to the first connectivity (connectivity 1) and a second radio bearer (radio bearer 2) is related to the second connectivity (connectivity 2). Then, the UE 100 may associate (or correlate) each MAC entity for each connectivity with each radio bearer.
(3) Also, the UE 100 may receive configuration on each uplink timing alignment related timer, e.g., time alignment timer (TAT). The configuration may indicate that a first time alignment timer (TAT 1) is for the first connectivity (connectivity 1) and a second time alignment timer (TAT 2) is for the second connectivity (connectivity 2).
(4) The UE 100 may receive time advance (TA) commands for the first and second connectivities. Each TA command may include an identity (ID) of a corresponding connectivity, a corresponding MAC entity, or a corresponding radio bearer.
(5) Then, The UE 100 starts TA timers for the first and second connectivity according to the TA commands. In detail, a first MAC entity for the first radio bearer related to the first connectivity starts the first TAT and a second MAC entity for the second radio bearer related to the second connectivity starts the second TAT. Here, to start each of the first and second TATs, the identity of the corresponding connectivity may be used. In particular, if the first MAC entity receives TA command including at least one of an identify of the first connectivity (connectivity 1), an identity of the first radio bearer and an identity of the first MAC entity, the first MAC entity starts the first TAT. Also, if the second MAC entity receives TA command including at least one of an identify of the second connectivity (connectivity 2), an identity of the second radio bearer and an identity of the second MAC entity, the second MAC entity starts the second TAT.
(6) Meanwhile, if at least one time alignment timer (TAT) is expired, the UE 100 considers all running time alignment timers (TATs) as expired. Especially, if the first TAT (TAT1) for the first connectivity for CeNodeB (or Macro eNodeB) is expired, the UE 100 considers the second TAT (TAT2) as expired.
In such a manner, the above-explained problem may be solved. Also, it is possible to minimize overhead of uplink signaling such as SRS which occurs during running time alignment timer (TAT) for other connectivity.
FIG. 16 shows one exemplary method according to another embodiment of the present disclosure.
Referring to FIG. 16, it is illustrated how the radio bearer is reconfigured to the connectivity upon removal or deactivation of connectivity.
(1) In detail, the UE 100 may receive a configuration on dual connectivities to CeNodeB (or Macro eNodeB) 200 and UeNodeB (or small eNodeB) 300. The configuration may indicate that a first connectivity (connectivity 1) is for CeNodeB and a second connected (connectivity 2) is for UeNodeB. Then, the UE 100 may activate (or configure) each MAC entity for each connectivity. Here, the configuration on the first connectivity (connectivity 1) to CeNodeB is considered as a default configuration.
(2) And, the UE 100 may receive configuration on a plurality of bearers. The configuration may indicate that a first radio bearer (radio bearer 1) is related to the first connectivity (connectivity 1) and a second radio bearer (radio bearer 2) is related to the second connectivity (connectivity 2). Then, the UE 100 may associate (or correlate) each MAC entity for each connectivity with each radio bearer.
(3) Afterward, the UE 100 is requested to remove or deactivate the second connectivity (connectivity 2). Or, the UE 100 removes or deactivates the second connectivity (connectivity 2) by itself according to the pre-defined conditions. For example, the UE 100 can be configured with connectivity timer for each connectivity. When the UE 100 is configured with a new connectivity, the UE starts the connectivity timer for the new connectivity. If the connectivity timer expires, the UE releases the connectivity.
(4) The UE 100 sets the second radio bearer (radio bearer 2) to the first connectivity (connectivity 1) which is defined as default connectivity.
(5) Then, the UE 100 transmits or removes data both on radio bearer 1 and 2 over the first connectivity (connectivity 1).
In such a manner, if at least one connectivity is removed or deactivated, the UE can associate a radio bearer related to the at least one removed or deactivated connectivity with another connectivity. Therefore, a delay or discard of data on the radio bearer can be minimized.
Hereinafter, other embodiments of the present disclosure will be explained
<Connectivity Grouping>
For realizing dual connectivity, from UE point of view, one MAC layer is needed for each eNodeB assuming that there is one connectivity per eNodeB. Because one eNodeB serves one or more cells and cells belonging to the same eNodeB can be handed in one MAC layer, the UE has one MAC layer per connectivity. For dual connectivity, it is assumed that the UE has at least one connectivity for macro cell(s) and one or more connectivity for small cells. For example, the UE is served by one macro cell and two small cells. Those small cells are served by different UeNodeBs. Then, the UE has 3 connectivity that requires 3 MAC layers.
The connectivity management can be done by CeNodeB, MME or S-GW. The following is included in the connectivity management.
Connectivity Identifier (Id)
The UE can be configured with connectivity Id for each connectivity by e.g., RRC messages. For example, the UE can be configured with connectivity Id 0 for CeNodeB, connectivity Id 1 for UeNodeB1, and connectivity Id 2 for UeNodeB2. The connectivity Id is generally used for identification of connectivity between the UE and eNodeB, e.g., when the connectivity is added, modified or removed.
Configuration Per Connectivity
With connectivity grouping, the common configuration for cells belonging to the same connectivity can be provided to the UE. For example, if the configurations are provided with the connectivity Id, the UE applies the configurations to the cells belonging to the connectivity indicated by the connectivity Id.
Default Configuration for Connectivity
Configurations for the connectivity for CeNodeB are considered as fault configuration. So, if the connectivity is removed, default configuration is applied to the configuration including radio bearer configured for the removed connectivity. For example, the UE is configured with radio bearers A and B and radio bearer A is configured for CeNodeB (connectivity 1) and radio bearer B is configured for UeNodeB (connectivity 2). If the connectivity 2 is removed, the UE considers the radio bearer B to be configured for connectivity 1.
Connectivity Timer
The UE can be configured with connectivity timer for each connectivity. When the UE is configured with a new connectivity, the UE starts the connectivity timer for the new connectivity. The UE re-starts the connectivity timer if the connectivity is modified. If the connectivity timer expires, the UE releases the connectivity.
Activation/Deactivation of the Connectivity
The eNodeB (e.g., CeNodeB) may order the UE to activate or deactivate one, some, all connectivity. When a new connectivity is added to the UE, the UE consider the connectivity to be deactivated. When the eNodeB asks the UE to activate the connectivity by PDCCH, MAC, RLC, PDCP, RRC signaling, the UE activates the connectivity. For the activated connectivity, the UE can use the data transfer on it. If the eNodeB asks the UE to deactivate the connectivity, then, the UE deactivates the connectivity. For the deactivated connectivity, the UE cannot use the data transfer on it.
<Buffer Status Reporting (BSR)>
Because the scheduler in each eNodeB schedules own radio resources, each scheduler needs to know the amount of data to schedule.
However, existing BSR mechanism only allows the UE to report the amount of data per logical cannel group (LCG) in one message to one eNodeB. It implies that the information about buffer status would need to be exchanged between the eNodeBs that are subject to dual connectivity. So, there would be a delay for the eNodeB to schedule.
Therefore, it is proposed that the BSR procedure is performed per connectivity. That is, radio bearers configured for a connectivity are considered for the BSR procedure for the connectivity. For example, it is assumed that the UE has 2 connectivity (connectivity 1 and 2) and 2 sets of radio bearers (set A and B). It is further assumed that set A is used for connectivity 1 and set B is used for connectivity 2. In this case, the BSR procedure for connectivity 1 is associated with the data on radio bearers in set A and the BSR procedure for connectivity 2 is associated with the data on radio bearers in set B. So,
If Data on Radio Bears in Set A Arrives,
The UE triggers the BSR for the connectivity 1. It means that the UE reports the BSR (i.e., BSR MAC CE) to the eNodeB which is subject to the connectivity 1. Also, if the UE does not have UL resources, then the UE triggers SR for the connectivity 1. It means that the UE sends SR on PUCCH or performs the Random Access procedure to/on the eNodeB which is subject to the connectivity 1. The BSR MAC CE includes information only about buffer status of radio bearers in set A.
If Data on Radio Bears in Set B Arrives,
The UE triggers the BSR for the connectivity 2. It means that the UE reports the BSR (i.e., BSR MAC CE) to the eNodeB which is subject to the connectivity 2. Also, if the UE does not have UL resources, then the UE triggers SR for the connectivity 2. It means that the UE sends SR on PUCCH or performs the Random Access procedure to/on the eNodeB which is subject to the connectivity 2. The BSR MAC CE includes information only about buffer status of radio bearers in set B.
Also, BSR configurations including periodicBSR-Timer, retxBSR-Timer and so on can be configured per connectivity. In addition to BSR configurations, those timers operate on each connectivity.
eNodeB may want to know total amount of UE's data (in UL). In this case, eNodeB can order the UE to report the total amount of data in UL. This order can be signaled by the PDCCH, MAC, RLC, PDCP, or RRC signaling. Also, eNodeB can configure the UE with periodic timer for reporting total amount of data in UL. The total amount of data can be indicated by amount of data per LCG, amount data per logical channel, amount of data per connectivity or etc.
Also, the UE can report the amount of data for connectivity if the connectivity is added, removed or modified. It means that the UE triggers the BSR when the connectivity is added, removed or modified. In those cases, the UE sends the BSR to eNodeBs for which the configured radio bears are changed. For example, the UE has two radio bears (A and B) for connectivity 1. If the UE is configured with a new connectivity 2 and radio bearer B is configured for connectivity 2, then the UE triggers the BSR for connectivity 2 and sends the BSR to the eNodeB which is subject to the connectivity 2, including the amount of data on radio bear B. Also, the UE triggers the BSR for connectivity 1 and sends it to the eNodeB which is subject to the connectivity 1, including the amount of data on radio bears on radio bear A.
If the connectivity is removed, the UE triggers the BSR and sends it to the CeNodeB (or other UeNodeBs) to indicate the amount of data for radio bears configured for the removed connectivity.
When the amount of data on radio bears configured for connectivity is indicated, the connectivity id can be indicated together to identify the connectivity. For example, when the UE report BSR for connectivity 1, then the UE also indicates connectivity id assigned for connectivity 1 along with the BSR.
<Logical Channel Prioritization (LCP)>
When the UE receives the UL grant from the eNodeB which is subject to certain connectivity, during the LCP procedure, the data on radio bearers configured and/or control information for the connectivity is only considered. For example, if the UE has 2 connectivity (A and B) and radio bearer “a” is configured for connectivity A and radio bearer “b” is configured for connectivity B, when the UE receives the UL grant from the eNodeB which is subject to the connectivity A, then the data on radio bearer “a” is considered for generating the MAC PDU by the received UL grant. I.e., in LCP procedure, the UL grant is only applicable to the data on radio bearers configured for connectivity for which the UL grant is assigned.
<Power Headroom Reporting (PHR)>
The PHR configurations per connectivity can be provided to the UE. Also, PHR related timers can operate per connectivity.
If the UE triggers PHR, it sends the PHR MAC CE. The PHR MAC CE includes the PH of cells belonging to the same connectivity.
When the connectivity is added, removed, or modified, the UE triggers the PHR for one, some, or all the configured connectivity.
When the UE reports the PHs for connectivity, the UE can indicate the connectivity Id.
<Maintenance of Uplink Timing Alignment>
Configuration about uplink timing alignment per connectivity can be provided to the UE. Uplink timing alignment related timer (e.g., timeAlignmentTimer) can operate per connectivity.
When the timeAlignmentTimer for connectivity for CeNodeB expires, the UE considers timeAlignmentTimer for all connectivity as expired.
When the Timing Advance Command is indicated, the connectivity Id is also indicated. Then, the UE applies the Timing Advance Command for the connectivity indicated by the connectivity Id and starts the timeAlignmentTimer for the connectivity indicated by the connectivity Id
<Random Access Procedure>
The Random Access procedure is also performed per connectivity. If the Random
Access procedure needs to be performed at the same time on 2 or more connectivity, the UE prioritizes the Random Access procedure on the connectivity of the CeNodeB over connectivity of the UeNodeBs.
The ways or methods to solve the problem of the related art according to the present disclosure, as described so far, can be implemented by hardware or software, or any combination thereof.
FIG. 17 is a block diagram showing a wireless communication system to implement an embodiment of the present invention.
An UE 100 includes a processor 101, memory 102, and a radio frequency (RF) unit 103. The memory 102 is connected to the processor 101 and configured to store various information used for the operations for the processor 101. The RF unit 103 is connected to the processor 101 and configured to send and/or receive a radio signal. The processor 101 implements the proposed functions, processed, and/or methods. In the described embodiments, the operation of the UE may be implemented by the processor 101.
The eNodeB (including CeNodeB and UeNodeB) 200/300 includes a processor 201/301, memory 202/302, and an RF unit 203/303. The memory 202/302 is connected to the processor 201/301 and configured to store various information used for the operations for the processor 201/301. The RF unit 203/303 is connected to the processor 201/301 and configured to send and/or receive a radio signal. The processor 201/301 implements the proposed functions, processed, and/or methods. In the described embodiments, the operation of the eNodeB may be implemented by the processor 201.
The processor may include Application-Specific Integrated Circuits (ASICs), other chipsets, logic circuits, and/or data processors. The memory may include Read-Only Memory (ROM), Random Access Memory (RAM), flash memory, memory cards, storage media and/or other storage devices. The RF unit may include a baseband circuit for processing a radio signal. When the above-described embodiment is implemented in software, the above-described scheme may be implemented using a module (process or function) which performs the above function. The module may be stored in the memory and executed by the processor. The memory may be disposed to the processor internally or externally and connected to the processor using a variety of well-known means.
In the above exemplary systems, although the methods have been described on the basis of the flowcharts using a series of the steps or blocks, the present invention is not limited to the sequence of the steps, and some of the steps may be performed at different sequences from the remaining steps or may be performed simultaneously with the remaining steps. Furthermore, those skilled in the art will understand that the steps shown in the flowcharts are not exclusive and may include other steps or one or more steps of the flowcharts may be deleted without affecting the scope of the present invention.

## Claims

1. A method of a communication device for operating a time alignment timer (TAT), the method comprising:
receiving configurations on a first TAT and a second TAT;
receiving a first time advance command (TAC) for the first TAT and a second TAC for the second TAT;
operating the first TAT in a first medium access control (MAC) entity;
operating the second TAT in a second MAC entity;
connecting with a first base station via the first MAC entity;
connecting with a second base station via the second MAC entity; and
if the first TAT operated in the first MAC entity is expired, considering the second TAT operated in the second MAC entity as expired, wherein the operating of the first TAT includes:
applying the first TAC to the first MAC entity; and
starting, by the first MAC entity, the first TAT, and

wherein the operating of the second TAT includes:
applying the second TAC to the second MAC entity; and
starting, by the second MAC entity, the second TAT.

2. The method of claim 1, wherein the first MAC entity operating the first TAT serves a signaling radio bearer (SRB).

3. The method of claim 1, wherein the second MAC entity operating the second TAT does not serve the SRB.

4. The method of claim 1, wherein each of the first and second TACs includes an identity of a corresponding MAC entity.

5. A communication device configured for operating a time alignment timer (TAT), the communication device comprising:
a radio frequency (RF) unit; and
a processor connected with the RF unit,
wherein the processor is configured to:
control the RF unit to receive configurations on a first TAT and a second TAT;
control the RF unit to receive a first time advance command (TAC) for the first TAT and a second TAC for the second TAT;
operate the first TAT in a first medium access control (MAC) entity;
operate the second TAT in a second MAC entity;
control the RF unit to connect with a first base station via the first MAC entity;
control the RF unit to connect with a second base station via the second MAC entity; and
if the first TAT operated in the first MAC entity is expired, consider the second TAT operated in the second MAC entity as expired,
wherein the operating of the first TAT includes:
applying the first TAC to the first MAC entity; and
starting, by the first MAC entity, the first TAT,

wherein the operating of the second TAT includes:
applying the second TAC to the second MAC entity; and

starting, by the second MAC entity, the second TAT.

6. The communication device of claim 5, wherein the first MAC entity operating the first TAT serves a signaling radio bearer (SRB).

7. The communication device of claim 5, wherein the second MAC entity operating the second TAT does not serve the SRB.