An Evolved Universal Terrestrial Radio Access Network (E-UTRAN) system will hereinafter be described with reference to FIG. 1.
FIG. 1 is a conceptual diagram illustrating an E-UTRAN structure. The E-UTRAN has evolved from a legacy UTRAN, and basic standardization thereof is now being conducted by the 3rd Generation Partnership Project (3GPP). The E-UMTS system may also be called a Long Term Evolution (LTE) system.
The E-UTRAN includes one or more “eNode B(s)” or “eNB(s)”. The eNBs are connected through an X2 interface. Each eNB is connected to a User Equipment (UE) through a radio interface and is connected to an Evolved Packet Core (EPC) through an S1 interface.
The EPC may include a Mobility Management Entity (MME), a Serving-Gateway (S-GW), and a Packet Data Network-Gateway (PDN-GW). The MME may include UE access information or UE capability information, and this information is generally adapted to manage UE mobility. The S-GW is a gateway in which the E-UTRAN is located at an end point, and the PDN-GW is a gateway in which a Packet Data Network (PDN) is located at an end point.
Radio interface protocol layers between the UE and the network are classified into a first layer (L1), a second layer (L2), and a third layer (L3) on the basis of three lower layers of an Open System Interconnection (OSI) reference model well known in the field of communication systems. The first layer (L1) provides an information transfer service using a physical channel. A radio resource control (RRC) layer located at the third layer (L3) controls radio resources between the UE and the network. For this operation, the RRC layer exchanges RRC messages between the UE and the network.
FIG. 2 illustrates a control plane of a radio interface protocol between a User Equipment (UE) and a UMTS Terrestrial Radio Access Network (UTRAN) according to the 3GPP wireless access network standard. FIG. 3 illustrates a user plane (U-Plane) of a radio interface protocol between a User Equipment (UE) and an E-UTRAN according to the 3GPP wireless access network standard.
A radio interface protocol includes a physical layer, a data link layer, and a network layer horizontally. Vertically, the radio interface protocol includes a user plane for transmitting data information and a control plane for transmitting a control signal (i.e., a signaling message). The protocol layers shown in FIG. 2 may be classified into a first layer (L1), a second layer (L2), and a third layer (L3) on the basis of the three lower layers of an Open System Interconnection (OSI) reference model well known in the field of communication systems. The UE and the E-UTRAN include a pair of such radio protocol layers, and are used to transmit data via an air interface.
A physical layer serving as the first layer (L1) transmits an information transfer service to an upper layer over a physical channel. The physical layer is connected to a Medium Access Control (MAC) layer serving as an upper layer over a transport channel. Data is transferred from the MAC layer to the physical layer or the transport channel, or is also transferred from the physical layer to the MAC layer. In addition, data is transferred between different physical layers over the physical channel. In other words, data is transferred from a transmitting physical layer to a receiving physical layer over the physical channel. The above-mentioned physical channel is modulated according to an orthogonal frequency division multiplexing (OFDM) scheme, so that the physical channel uses time and frequency information as radio resources.
The MAC layer of the second layer (L2) transmits services to a Radio Link Control (RLC) layer serving as an upper layer over a logical channel. The RLC layer of the second layer (L2) supports reliable data transmission.
The RLC layer function may be implemented as a functional block contained in the MAC layer. In this case, the RLC layer may not be present. A Packet Data Convergence Protocol (PDCP) layer of the second layer (L2) performs a header compression function to reduce the size of an IP packet header having relatively large and unnecessary control information in order to efficiently transmit IP packets such as IPv4 or IPv6 packets in a radio interval with a narrow bandwidth.
The radio resource control (RRC) layer located at the lowest of the third layer (L3) is defined on a control plane only. In association with configuration, re-configuration, and release of radio bearers (RBs), the RRC layer controls the logical channel, the transport channel, and the physical channels. In this case, the above radio bearer (RB) is provided from the second layer (L2) to perform data communication between the UE and the UTRAN. If an RRC connection is located between the RRC layer of the UE and the RRC layer of the radio network, the UE stays in an RRC connected (RRC_CONNECTED) state. Otherwise, the UE stays in an RRC idle (RRC_IDEL) state.
There are a plurality of downlink transport channels for transmitting data from the network to the UE, for example, a broadcast channel (BCH) for transmitting system information, a paging channel (PCH) for transmitting paging messages, and a downlink shared channel (DL-SCH) for transmitting user traffic or control messages. Traffic or control messages of a downlink multicast or a broadcast service (Multimedia Broadcast/Multicast Service: MBMS) may be transmitted over a downlink multicast channel (MCH). In the meantime, there are a plurality of uplink transport channels for transmitting data from the UE to the network, for example, a random access channel (RACH) for transmitting initial control messages, and an uplink shared channel for transmitting user traffic or control messages.
A plurality of logical channels are located above the transport channel, and are mapped to the transport channel. For example, the logical channels may be a broadcast channel (BCCH), a paging control channel (PCCH), a common control channel (CCCH), a multicast control channel (MCCH), and a multicast traffic channel (MTCH).
A physical channel includes a plurality of subframes on the time axis and a plurality of subcarriers on the frequency axis. Here, one subframe includes a plurality of symbols on the time axis. One subframe includes a plurality of resource blocks and one resource block includes a plurality of symbols and a plurality of subcarriers. Each subframe can use specific subcarriers of a specific symbol (e.g., a first symbol) of the subframe for a Physical Downlink Control Channel (PDCCH) (i.e., an L1/L2 control channel). Each subframe has 0.5 ms. A Transmission Time Interval (TTI), which is a time unit during which data is transmitted, is 1 ms.
A detailed description will hereinafter be given of the RRC state and the RRC connection method, a UE's RRC state and its RRC connection method. The RRC state indicates whether a UE's RRC layer is logically connected to an E-UTRAN's RRC layer. If it is determined that the UE's RRC layer is logically connected to the E-UTRAN's RRC layer, this state is called an RRC connected (RRC_CONNECTED) state. If the UE's RRC layer is not logically connected to the E-UTRAN's RRC layer, this state is called an RRC idle (RRC_IDLE) state. A UE in the RRC connected (RRC_CONNECTED) state has an RRC connection, such that the E-UTRAN can recognize the presence of the corresponding UE in units of a cell. As a result, the UE can be effectively controlled. Otherwise, a UE in an RRC idle (RRC_IDLE) state cannot be recognized by the E-UTRAN, but is controlled by a core network (CN) in units of a tracking area larger than the cell. In other words, only the presence or absence of the above RRC-connected UE is recognized in units of a large region. If the RRC-connected UE desires to receive a general mobile communication service such as a voice or data service, the UE must enter the RRC connection state. Associated detailed description will hereinafter be described in detail.
If a user initially powers on his or her UE, the UE searches for an appropriate cell, and remains in an RRC_IDLE state in the searched cell. The UE in the RRC_IDLE state establishes an RRC connection in association with the E-UTRAN's RRC layer through an RRC connection procedure when it needs to establish the RRC connection, such that it is shifted to the RRC_CONNECTED state. The UE under the RRC_IDLE state must establish the RRC connection for a variety of reasons. For example, if uplink data transmission is needed when placing a call, or if a paging message is received from the E-UTRAN such that a response message to the paging message must be transmitted, the UE under the RRC_IDLE state needs to connect the RRC connection.
A Non-Access Stratum (NAS) layer located above the RRC layer performs functions such as session management and mobility management.
In order to manage UE mobility, an EPS Mobility Management-REGISTERED (EMM-REGISTERED) state and an EMM-DEREGISTERED state are defined in the NAS layer. The EMM-REGISTERED state and the EMM-DEREGISTERED state are applied to a UE and a Mobility Management Entity (MME). The user equipment (UE) is initially in the EMM-DEREGISTERED state, and carries out an ‘Initial Attach’ procedure to access a network, thereby being registered in the corresponding network. If this ‘Attach’ procedure has been successfully carried out, the UE and the MME enter the EMM-REGISTERED state.
In order to manage a signaling connection between the UE and the EPC, an EPS Connection Management (ECM)-IDLE state and an ECM-CONNECTED state are defined. The above-mentioned states are applied to the UE and the MME. The UE in the ECM-IDLE state is in the ECM-CONNECTED state when establishing an RRC connection with an E-UTRAN. If the MME in the ECM-IDLE state makes an S1 connection with the E-UTRAN, it enters the ECM-CONNECTED state. If the UE is in the ECM-IDLE state, the E-UTRAN has no context information of the UE. Therefore, the UE in the ECM-IDLE state carries out a UE-based mobility procedure (e.g., cell selection or cell reselection) without receiving a command from the network. Otherwise, if the UE is in the ECM-CONNECTED state, UE mobility is managed by the network. If the UE is in the ECM-IDLE state and the UE's location recognized by the network changes to another UE location, the UE performs a Tracking Area Update procedure, such that it informs the network of the UE's location.
System information will hereinafter be described in detail. The system information includes requisite information that must be recognized by the UE that desires to access a base station (BS). Accordingly, the UE must receive all the system information before accessing the BS, and must always include the latest system information. In addition, the system information must be recognized by all UEs contained in one cell, such that the BS periodically transmits the system information.
The system information is classified into a master information block (MIB), a scheduling block (SB), a system information block (SIB), etc. The MIB includes physical configuration information (e.g., a bandwidth) of the corresponding cell. The SB includes transmission information such as a transmission period of each SIB. The SIB is an aggregate (or a set) of mutually-associated system information. For example, a certain SIB includes only information of a neighbor cell, and a certain SIB includes only information of an uplink radio channel used in the UE.
A base station (BS) transmits a paging message so as to inform the UE of the presence or absence of changed system information. In this case, the paging message includes a system information change indicator. The UE receives the paging message. If the received paging message includes a system information change indicator, the UE receives system information through a logical channel (e.g., a broadcast channel (BCCH)).
A relay will hereinafter be described in detail. The relay means a technology for mediating data between the UE and the BS. In more detail, if the UE is distant from the BS in the LTE system, since the UE has difficulty in easily communicating with the BS, the relay technology for solving this difficulty has been newly introduced to the LTE-A system. In order to perform such a relay function, a new network node called a relay node (RN) has been introduced between the UE and the BS, and the BS managing this relay node is referred to as a donor BS (also called a Donor eNodeB or DeNB). In addition, an interface between one RN that is newly generated by the RN and a donor BS is defined as a ‘Un interface’, such that this interface can be distinguished from a ‘Uu interface’ acting as an interface between a UE and a network node.
If the relay node (RB) provides a service to the UE through the Uu interface, the relay node (RN) can receive downlink (DL) data from the donor BS only through a Multimedia Broadcast Multicast Service Single Frequency Network (MBSFN) subframe so as to avoid interference. In this case, according to the conventional art, the donor BS can transmit system information of the donor BS to the relay node (RN) using the following two schemes. In accordance with a first one of the two schemes, the donor BS transmits system information for a relay node to one relay node (RN) through a dedicated RRC signaling. In accordance with a second scheme, the donor BS transmits system information for a relay node to the MBSFN subframe such that the system information can be utilized for a plurality of relay nodes (RNs). The first scheme has a disadvantage in that the donor BS has to transmit system information to each relay node (RN) when several relay nodes (RNs) are connected to the donor BS (donor eNB). The second scheme has a disadvantage in that the donor BS has to broadcast system information and system information for a relay node in different ways, such that duplicated system information is unavoidably broadcast.