FIG. 1 illustrates the structure of a wireless access protocol responsible for data transmission in a radio link of a Universal Mobile Telecommunication System (UMTS) which is a third generation mobile communication system. Data link layer corresponding to the second layer (Layer 2: L2) of the Open System Interconnection (OSI) reference model includes a Medium Access Control (MAC) layer, a Radio Link Control (RLC) layer, a Packet Data Convergence Protocol (PDCP) layer, and a Broadcast/Multicast Control (BMC) layer. The physical layer corresponds to the first layer (Layer 1: L1). Information exchange between protocol layers is performed through virtual access points that are referred to as “Service Access Points (SAPs),” which are represented by ovals in FIG. 1. Data units communicated between layers are given different names. These data units are referred to as “Service Data Units (SDUs)” and basic units that protocols use for transmitting data are referred to as “Protocol Data Units (PDUs).” In the following description of the invention, reference to data delivered between layers in the wireless access protocol structure indicates data blocks in specific units such as SDUs or PDUs as described above.
The MAC layer is a layer responsible for mapping between logical and transport channels. The MAC layer selects an appropriate transport channel for transmitting data received from the RLC layer and adds required control information to a header of a MAC PDU. Special functions performed by the MAC layer include a radio resource management function and a measurement function. The radio resource management function is not performed solely by the MAC layer. Instead, the radio resource management function serves to set operations of the MAC layer based on various MAC parameters received from a Radio Resource Control (RRC), which is located above the MAC layer, to control data transmission. Examples of the radio resource management function include a function to change mapping relations between logical and transport channels or to multiplex and transmit data through a scheduling function. The measurement function serves to measure the amount of traffic of a terminal and to report the measurement to an upper layer. The upper layer can change the configuration (or setting) of the MAC layer based on the measurement information obtained by the MAC layer of the terminal, thereby efficiently managing radio (or wireless) resources.
The RLC layer is located above the MAC layer and supports reliable data transmission. The RLC layer segments and concatenates RLC Service Data Units (SDUs) received from the above layer in order to construct data having a size suitable for a radio link.
An RLC layer at the receiving side supports data recombination in order to restore original RLC SDUs from the received RLC PDUs. Each RLC entity can operate in a Transparent Mode (TM), an Unacknowledged Mode (UM), or an Acknowledged Mode (AM) according to processing and transmission methods of RLC SDUs. When the RLC entity operates in the TM, it transfers an RLC SDU received from an upper entity or layer to the MAC layer without adding any header information to the RLC SDU. When the RLC entity operates in the UM, it segments/concatenates RLC SDUs to construct RLC PDUs and adds header information including a sequence number to each RLC PDU. However, in the UM, the RLC entity does not support data retransmission. When the RLC entity operates in the AM, it can use the RLC SDU segmentation/concatenation function to construct RLC PDUs and can perform retransmission when packet transmission has failed. Various parameters and variables such as a transmission window, a reception window, a timer, and a counter are used for the retransmission function in the AM.
The PDCP layer is used only in packet exchange regions and can compress and transmit IP packet headers so as to increase the transmission efficiency of packet data in wireless channels. The PDCP layer also manages sequence numbers in order to prevent data loss during Serving RNC (SRNC) relocation.
The BMC layer broadcasts cell broadcast messages received from a core network to multiple users through a common channel.
The physical layer, which is the first layer, provides an information transfer service to an upper layer using a physical channel. The physical layer is connected to the Media Access Control (MAC) layer located above the physical layer through a transport channel. Data is transferred between the MAC layer and the physical layer through the transport channel. Data is transferred between different physical layers (specifically, physical layers of transmitting and receiving sides) through a physical channel.
A Radio Resource Control (RRC) layer, which is located at the bottom of the third layer, is defined only in the control plane and is responsible for controlling logical, transport, and physical channels in association with configuration, re-configuration, and release of Radio Bearers (RBs). RBs are services that the second layer provides for data communication between terminals and a network including a base station. The control plane is a hierarchy in which control information is transferred in the vertical structure of the wireless access protocol of FIG. 1 and the user plane is a hierarchy in which user data/information is transferred.
As shown in FIG. 1, an RLC PDU generated in the RLC layer is transferred to the MAC layer and is handled as a MAC SDU in the MAC layer. While a MAC SDU, which is an RLC PDU received from the RLC layer, undergoes various functions of the MAC layer, various header information required for data processing is added to the MAC SDU. The header information can be altered depending on mapping relations between logical and transport channels.
Logical channels provide transport passages required for data exchange between the MAC and the RLC layer. Each logical channel is classified into control and traffic channels according to the type of data transmitted therethrough. The control channel transmits data of the control plane and the traffic channel transmits user traffic data. A logical channel is a type of data stream carrying a specific type of information. Each logical channel is generally connected to one RLC entity. One or more logical channels of the same type can also be connected to an RLC entity. Transport channels provide passages for data communication between the physical and MAC layers. A data stream in a logical channel is embodied as a MAC PDU in the MAC layer.
FIG. 2 illustrates a method in which a terminal receives data in an E-UMTS system. In a communication system employing Orthogonal Frequency Division Multiplexing (OFDM) as an example multi-carrier system, communication is performed using one or more frequency blocks that are allocated every specific time interval. More specifically, the transmitting and receiving sides mostly communicate control signals and data, except specific control signals or data, through a common physical channel such as a Physical Downlink Shared CHannel (PDSCH) that uses a common transport channel such as a Downlink Shared CHannel (PDSCH). Here, the transmitting and receiving sides may correspond respectively to a base station and a terminal, and vice versa. In the description of the invention, the term “transmitting side” refers to a base station and the term “receiving side” refers to a terminal for ease of explanation.
To accomplish the above communication, before the receiving side receives data of a common physical channel, the receiving side needs to receive control information regarding a receiving side(s) to which the data of the common physical channel is to be transmitted and regarding how the receiving side is to receive and decode the common physical channel. In the OFDM system, a channel for control information is first allocated to a basic processing interval (for example, a subframe period in FIG. 2) in the physical layer and a channel for data is then allocated to a next time interval. A Physical Downlink Control CHannel (PDCCH) in FIG. 2 corresponds to the control information channel and a Physical Downlink Shared CHannel (PDSCH) subsequent to the PDCCH corresponds to the data channel.
For example, let us assume that the PDCCH in the first subframe in FIG. 2 is CRC-masked with terminal identification information such as a Radio Network Temporary Identify (RNTI) and is then transmitted from a base station in a specific cell while the transmitted PDCCH includes information regarding data that is being transmitted using specific transport format information (for example, information regarding a modulation and coding method and a transport block size) through a specific radio resource such as a specific carrier set. In this case, upon receiving a PDCCH, each of a plurality of terminals in the specific cell checks whether or not the received PDCCH belongs to the terminal using terminal identification information such as an RNTI that the network has allocated to the terminal. If the received PDCCH belongs to the terminal, the terminal reads the specific radio resource information and the specific transport format information included in the PDCCH and receives a PDSCH in the same subframe.
In order to transmit data to the base station, the terminal needs to be allocated a radio resource for uplink transmission. To accomplish this, the terminal needs to request that the base station allocate a radio resource for data transmission. In the OFDM system, one or more terminals can use a single radio resource. If two or more terminals simultaneously transmit signals in uplink using the same radio resource, the base station cannot analyze the signals transmitted from the terminals. Therefore, the base station needs to perform scheduling such that only one terminal is permitted to use the radio resource in one radio resource processing unit.
To perform scheduling as described above, the base station may allocate radio resources to a terminal using the following methods before or while a call is connected between the base station and the terminal.
In the first method, the base station allocates radio resources to the terminal without being aware of how much uplink data the terminal has. However, this method causes waste of resources since radio resources are allocated to the terminal even when no data is transmitted from the terminal as the terminal has no uplink data for transmission.
In the second method, the terminal transmits information regarding a buffer associated directly with the amount of uplink data to the base station to allow the base station to know how much uplink data the terminal has and the base station then allocates uplink radio resources to the terminal based on the information.
The following are the cases where the terminal needs to transmit information regarding its buffer to the base station in the second method.
The first case is where the terminal has suddenly received data from an upper layer above the terminal before the terminal has not transmitted and received any data. In this case, since the terminal needs to be allocated radio resources for initial transmission, the terminal needs to send buffer information to the base station in order to request that the base station allocate radio resources to the terminal.
The second case is where the terminal actively transmits or receives data to or from the base station. In this case, the terminal transmits information regarding the amount of data in the terminal in order to allow the base station to determine the time until which the base station has to further allocate radio resources or to determine whether the base station is to increase or decrease the amount of radio resources allocated to the terminal afterwards. If the terminal does not transmit information regarding the amount of data in this case, the base station may stop allocating radio resources to the terminal since the base station has no updated information of the buffer of the terminal even though the terminal has data for transmission or the base station may continue allocating more radio resources than necessary, thereby reducing system efficiency.