1. Field of the Invention
The present invention relates to a broadcast service and a unicast service, and more particularly to a method and a system for supporting simultaneous reception of a broadcast service and a unicast service.
2. Description of the Related Art
A Universal Mobile Telecommunication Service (UMTS) system is a 3rd Generation (3G) asynchronous mobile communication system, which uses wideband Code Division Multiple Access (CDMA) and is based on Global System for Mobile Communications (GSM) and General Packet Radio Services (GPRS), which are European mobile communication systems. In the 3rd Generation Partnership Project (3GPP), which is in charge of standardization of the UMTS, active discussion is being made about a Long Term Evolution (LTE) system as a next generation mobile communication system. The LTE is targeting commercialization by the year 2010 and the realization of high speed packet-based communication at a speed of about 100 Mbps. To this end, various schemes are being discussed, which include a scheme of reducing the number of nodes located in communication paths by simplifying the structure of a network, and a scheme of approaching a wireless protocol to a wireless channel as close as possible.
FIG. 1 illustrates an example of a structure of an evolved UMTS mobile communication system to which the present invention is applicable.
Referring to FIG. 1, an Evolved UMTS Radio Access Network (E-RAN) 110 has a simplified 2 node structure, which includes Evolved Node Bs (ENBs) 120, 122, 124, 126, and 128 and anchor nodes 130 and 132. A User Equipment (UE) 101 is connected to an Internet Protocol (IP) network 114 through the E-RAN 110. The ENBs 120 to 128 correspond to legacy Node Bs of the UMTS system and are connected to the UE 101 through a wireless channel. Differently from the legacy Node Bs, the ENBs 120 to 128 perform more complicated functions. In the LTE, all user traffics including the real-time service, such as Voice over IP (VoIP) using the Internet protocol, are provided through a shared channel. Therefore, the LTE requires an apparatus for collecting status information of UEs and performing scheduling by using the collected information. The ENBs 120 to 128 take charge of the scheduling. Usually, one ENB controls a plurality of cells. Further, the ENB performs Adaptive Modulation and Coding (AMC), which determines a modulation scheme and a channel coding rate in accordance with the channel status of a UE. As in the High Speed Uplink Packet Access (HSUPA; which is also called “Enhanced Dedicated Channel (E-DHC)”) and the High Speed Downlink Packet Access (HSDPA) of the UMTS, the Hybrid Automatic Repeat reQuest (HARQ) is performed between the ENBs 120 to 128 and the UE 101 in the LTE also. The HARQ process refers to a process of soft-combining previously-received data with retransmitted data without discarding the previously-received data, thereby improving the ratio of success in the reception. The HARQ process is used in order to improve the transmission efficiency in the high speed packet communication, such as the High Speed Downlink Packet Access (HSDPA) and the Enhanced Dedicated Channel (EDCH). In order to implement a maximum transmission speed of 100 Mbps, the LTE is expected to use Orthogonal Frequency Division Multiplexing (OFDM) in 20 MHz bandwidth as wireless connection technology. However, because the HARQ process alone cannot satisfy requirements for various Qualities of Service (QoSs), an outer Automatic Repeat reQuest (ARQ) in a higher layer may be performed between the UE 101 and the ENBs 120 to 128.
FIG. 2 illustrates an example of reception of a Multimedia Broadcast Multicast Service (MBMS) through an MBMS-dedicated frequency band in a 3GPP LTE system.
The LTE system, which is being discussed as a next generation mobile communication system of the 3GPP, employs MBMS-dedicated carriers for providing only an MBMS. Since only one MBMS is provided in an MBMS-dedicated carrier 210, uplink transmission is either impossible or extremely limited. Services requiring uplink transmission, such as a Routing Area Update (RAU), etc., can be provided through a unicast carrier cell 211 corresponding to existing communication service carriers. Most UEs use a single antenna, except for the antennas for Multiple Input Multiple Output (MIMO) or diversity. The antennas for MIMO or diversity can be also used for multi-carrier simultaneous reception. However, such use may increase hardware complexity and degrade reception capability. The present invention is based on an assumption that a UE can receive only a single carrier at each time. When a UE cannot perform simultaneous communication through two or more carriers, it performs a process by switching a Radio Frequency (RF) whenever necessary. For example, when an RAU is necessary during reception of an MBMS in an MBMS-dedicated carrier cell, the UE stops receiving the MBMS, switches to a unicast carrier cell, performs an RAU, switches back to the MBMS-dedicated carrier cell, and then restarts to receive the MBMS. Further, when the UE needs to perform a unicast service, such as reception or registration of a paging message, switching between carrier cells is necessary.
FIG. 3A is a signal flow diagram illustrating a process when a UE 301 should perform an RAU while receiving an MBMS.
Referring to FIG. 3A, in step 311, a UE 301 in a Radio Resource Control (RRC) idle mode receives an MBMS in an MBMS-dedicated carrier cell. Then, when an RAU procedure is triggered in step 312 while the UE 301 receives the MBMS, the UE 301 stops receiving the MBMS and switches to a unicast carrier cell in step 313. Then the UE 301 in the unicast carrier cell transmits an RAU message to the ENB 302 in step 314 and receives a response message to the RAU message from the ENB 302 in step 315. When the RAU procedure is finished, the UE 301 switches again to the MBMS-dedicated carrier cell in step 316 and restarts to receive the MBMS in step 317. Meanwhile, MBMS data loss 318 occurs in the interval for receiving the RAU procedure through the unicast carrier cell.
FIG. 3B is a signal flow diagram illustrating a process when a UE 301 should receive a unicast service while receiving an MBMS.
Referring to FIG. 3B, in step 321, a UE 301 in an RRC idle mode receives an MBMS in an MBMS-dedicated carrier cell. Then, when a unicast service is triggered in step 322 while the UE 301 receives the MBMS, the UE 301 stops receiving the MBMS and switches to a unicast carrier cell in step 323. Then the UE 301 performs a Radio Resource Control (RRC) connection and a Radio Bearer (RB) establishment in step 324, and then receives a response message in step 325. Therefore, MBMS data loss 327 occurs before the UE 301 returns to the MBMS-dedicated carrier cell. Thereafter, either an MBMS data loss 327 or a unicast data loss 328 occurs according to the cell the UE 301 is located in. That is, in step 326, MBMS data loss will occur if the UE 301 stays in the unicast carrier cell, while unicast data loss will occur if the UE 301 switches to the MBMS-dedicated carrier cell.