Patent Publication Number: US-2013242892-A1

Title: Apparatuses and methods for handling random access procedures

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims priority of U.S. Provisional Application No. 61/611,758, filed on Mar. 16, 2012, the entirety of which is incorporated by reference herein. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention generally relates to the techniques for handling random access procedures, and more particularly, to apparatuses, systems, and methods for handling random access procedures in a multi-carrier system. 
     2. Description of the Related Art 
     In order to meet user demands for higher data rates in wireless communications, support for wider transmission bandwidths is required. A so-called Carrier Aggregation (CA) technique has been proposed to allow for the expansion of effective bandwidth delivered to a user terminal through concurrent utilization of radio resources across multiple carriers. The CA technique is most useful when it is difficult to derive a contiguous and extra wide bandwidth. With the CA technique, 2 or more frequency bands may be aggregated to form a larger transmission/receive bandwidth. Each of the aggregated frequency bands is generally referred to as a carrier frequency. 
     Specifically, each carrier frequency needs to be configured and activated before it may be used for data transmission or reception. The network side may first transmit a Radio Resource Control (RRC) message to configure the carrier frequencies for a User Equipment (UE) (or may be referred to as a Mobile Station (MS)), and then transmit a Medium Access Control (MAC) activation command to the UE to activate one of the carrier frequencies. After that, the UE may initiate a random access procedure on the activated carrier frequency for uplink timing alignment, requesting radio resources, or other purposes. However, if a UE is configured with more than one carrier frequency, there may be a situation where multiple random access procedures are requested to be performed on different carrier frequencies at the same time for the UE. However, in some radio access technologies, such as the Long Term Evolution (LTE) technology, only one random access procedure is allowed to be performed at a given time for a UE. As a result, confusion regarding which one of the random access procedures should be performed first may occur, which causes indeterminate behavior or even malfunction of the UE. 
     BRIEF SUMMARY OF THE INVENTION 
     The invention proposes solutions for the UE (referred to herein as mobile communication device) to solve the confusion regarding which random access procedure should be performed first among a plurality of requested random access procedures. 
     In one aspect of the invention, a mobile communication device comprising a wireless module and a controller module is provided for handling random access procedures in a multi-carrier system. The wireless module performs wireless transceiving to and from a cellular access network comprising a first SCell and a second SCell. The controller module performs operations of a Media Access Control (MAC) layer, wherein the operations comprises: receiving, during an ongoing random access procedure on the first SCell, a PDCCH order for initiating another random access procedure on the second SCell from the cellular access network via the wireless module, and not processing the PDCCH order. 
     In another aspect of the invention, a method for a mobile communication device to handle random access procedures in a MAC layer with a cellular access network comprising a first SCell and a second SCell is provided. The method comprises the steps of receiving, during an ongoing random access procedure on the first SCell, a PDCCH order for initiating another random access procedure on the second SCell from the cellular access network, and not processing the PDCCH order. 
     Other aspects and features of the invention will become apparent to those with ordinary skill in the art upon review of the following descriptions of specific embodiments of the mobile communication devices and methods for handling random access procedures. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The invention can be more fully understood by reading the subsequent detailed description and examples with references made to the accompanying drawings, wherein: 
         FIG. 1  is a block diagram illustrating a multi-carrier system according to an embodiment of the invention; 
         FIG. 2  is a message sequence chart illustrating the handling of random access procedures according to an embodiment of the invention; 
         FIG. 3  is a message sequence chart illustrating the handling of random access procedures according to another embodiment of the invention; and 
         FIG. 4  is a flow chart illustrating a method for handling random access procedures in the MAC layer according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The following description is of the best-contemplated mode of carrying out the invention. This description is made for the purpose of illustrating the general principles of the invention and should not be taken in a limiting sense. The scope of the invention is best determined by reference to the appended claims. 
       FIG. 1  is a block diagram illustrating a multi-carrier system according to an embodiment of the invention. In the multi-carrier system  100 , the mobile communication device  110  is wirelessly connected to the service network  120  via the air interface for obtaining wireless services, wherein both of the mobile communication device  110  and the service network  120  support CA technique and multiple carrier frequencies. The service network  120  comprises at least one cellular access network  121  and the core network  122 . In general, the cellular access network  121  is controlled by the core network  122  to provide the functionality of wireless transceiving, and the cellular access network  121  may comprise one or more cellular stations, such as base stations, Node-Bs, or evolved Node-B (eNB), depending on the radio access technology in use. Although not shown, the core network  122  may further enable interfacing with external networks, such as the Public Switched Telephone Network (PSTN), which is called the Circuit Switched (CS) domain functionality, or interfacing with the Internet Protocol (IP) based Network, such as the Internet, which is called the Packet Switched (PS) domain functionality. 
     The mobile communication device  110  comprises a wireless module  111  and a controller module  112 , wherein the wireless module  111  is configured to perform the functionality of wireless transceiving and the controller module  112  is configured to control the operation of the wireless module  111 . To further clarify, the wireless module  111  may be a Radio Frequency (RF) unit (not shown), and the controller module  112  may be a general-purpose processor or Micro-Control Unit (MCU) of a baseband unit (not shown). The baseband unit may contain multiple hardware devices to perform baseband signal processing, including analog to digital conversion (ADC)/digital to analog conversion (DAC), gain adjusting, modulation/demodulation, encoding/decoding, and so on. The RF unit may receive RF wireless signals, convert the received RF wireless signals to baseband signals, which are processed by the baseband unit, or receive baseband signals from the baseband unit and convert the received baseband signals to RF wireless signals, which are later transmitted. The RF unit may also contain multiple hardware devices to perform radio frequency conversion. For example, the RF unit may comprise a mixer to multiply the baseband signals with a carrier oscillated in the radio frequency of the wireless communications system, wherein the radio frequency may be 900 MHz, 2100 MHz, or 2.6 GHz utilized in LTE/LTE-Advanced technology, or others depending on the radio access technology in use. Although not shown, the mobile communication device  110  may further comprise other functional components, such as a display unit and/or keypad serving as the Man-Machine Interface (MMI), a storage unit storing the program codes of applications, or others. Similarly, the cellular stations of the cellular access network  121  may each comprise a wireless module and a controller module, wherein the wireless module is controlled by the controller module to perform the functionality of wireless transceiving. 
     Since each carrier frequency supported by the cellular access network  121  (i.e., the cellular station of the cellular access network  121 ) forms a separate cell with its own cell ID, the mobile communication device  110  may connect to one Primary Serving Cell (referred to herein as PCell) and up to 4 or more Secondary Serving Cells (referred to herein as SCells) depending on the radio access technology in use. The PCell is defined as the cell which is initially configured during connection establishment, while the SCells are cells which may be configured after connection establishment for providing additional radio resources. Note that, the random access procedures in the invention are performed on the SCells. 
     In one embodiment, the service network  120  may be an LTE network, the cellular access network  121  may be an Evolved Universal Terrestrial Radio Access Network (E-UTRAN) which comprises at least one eNB, and the core network  122  may be an Evolved Packet Core (EPC), while the mobile communication device  110  may be a UE or MS which utilizes the LTE technology. Please note that, in other embodiments, instead of the LTE technology, other radio access technologies which support the CA technique and multiple carrier frequencies, such as the LTE-Advanced technology, the Worldwide Interoperability for Microwave Access (WiMAX) technology, and others, may be utilized by the service network  120  and the mobile communication device  110 , and the invention is not limited thereto. 
     To further clarify, in the mobile communication device  110 , the controller module  112  controls the wireless module  111  to perform the random access procedures with the cellular station of the cellular access network  121 . Specifically, the controller module  112  is configured to perform the operations of the Media Access Control (MAC) layer for handling random access procedures. In other words, the methods for handling random access procedures proposed in the invention are applied in the MAC layer. Note that, the controller module  112  is not limited just to perform operations of the MAC layer, it may also be configured to perform operations of other layers of the used communication protocol, including the Non-Access Stratum (NAS), Radio Resource Control (RRC) layer, the Radio Link Control (RLC) layer, and Physical (PHY) layer, etc., for performing procedures other than the random access procedure. Detailed description of the procedures other than the random access procedure is omitted herein, since they are beyond the scope of the invention. 
       FIG. 2  is a message sequence chart illustrating the handling of random access procedures according to an embodiment of the invention. In this embodiment, the cellular access network  121  is an E-UTRAN supporting three or more carrier frequencies which form one PCell and two SCells of the same secondary Timing Advance Group (sTAG), and the mobile communication device  110  is a UE utilizing the LTE technology or LTE-Advanced technology. Particularly, the mobile communication device  110  is initially connected to the PCell and SCells of the cellular access network  121 , and receives a Preamble Assignment message from the cellular access network  121  on the PCell (step S 210 ). In this embodiment, the random access procedure on the SCell- 1  is a non-contention based random access procedure. Specifically, the cellular access network  121  may transmit an RRC message, such as an RRC Connection Reconfiguration message, to the mobile communications device  110  to configure the connection to the PCell and Scells, and the Preamble Assignment message is transmitted via the DL-SCH, which contains an assigned preamble for the mobile communication device  110 . Next, the mobile communication device  110  initiates a random access procedure on one SCell (denoted as SCell- 1 ) by transmitting a Random Access Preamble message via the P-RACH using the assigned preamble (step S 220 ). In one embodiment, the step S 220  may be performed in response to receiving a PDCCH order (not shown) which indicates that the mobile communication device  110  is to initiate the random access procedure on the SCell- 1 , or the step S 220  may be performed by the mobile communication device  110  spontaneously. 
     Before receiving a Random Access Response message on the SCell- 1 , the mobile communication device  110  receives a PDCCH order on the SCell- 2  from the cellular access network  121 , which indicates an initiation of another random access procedure on another SCell (denoted as SCell- 2 ) (step S 230 ). That is, the PDCCH order is received during the ongoing random access procedure on the SCell- 1 . The mobile communication device  110  does not process the received PDCCH order (step S 240 ), due to the ongoing random access procedure on the SCell- 1 . That is, the random access procedure on the SCell- 2  is not initiated in response to not processing the received PDCCH order, and the ongoing random access procedure on the SCell- 1  is not interrupted by the PDCCH order. In one embodiment, the mobile communication device  110  may discard the PDCCH order, so that the PDCCH order is not processed. 
     Thus, the ongoing random access procedure on the SCell- 1  continues. Subsequently, the mobile communication device  110  retransmits the Random Access Preamble message on the SCell- 1  when a guard timer for the random access procedure expires and no Random Access Response message is received from the cellular access network  121  (step S 250 ). In reply, the cellular access network  121  transmits a Random Access Response message on the SCell- 1  to the mobile communication device  110  (step S 260 ), and the procedure ends. Specifically, the Random Access Response message is transmitted via the DL-SCH, which contains timing advance information for uplink timing alignment of the mobile communication device  110 . 
       FIG. 3  is a message sequence chart illustrating the handling of random access procedures according to another embodiment of the invention. Similar to  FIG. 2 , the cellular access network  121  is an E-UTRAN supporting three or more carrier frequencies which form one PCell and two SCells of the same sTAG, and the mobile communication device  110  is a UE utilizing the LTE technology or LTE-Advanced technology. Particularly, the mobile communication device  110  is initially connected to the PCell and SCells of the cellular access network  121 . In one embodiment, the cellular access network  121  may transmit an RRC message, such as an RRC Connection Reconfiguration message, to the mobile communications device  110  to configure the connection to the PCell and Scells. To begin, the mobile communication device  110  initiates a random access procedure on one SCell (denoted as SCell- 1 ) by transmitting a Random Access Preamble message via the P-RACH (step S 310 ). In this embodiment, the random access procedure on the SCell- 1  is a contention based random access procedure, and the mobile communication device  110  selects one of a plurality of P-RACH configurations for the transmission of the Random Access Preamble message, which may be provided to the mobile communication device  110  prior to the initiation of the random access procedure. 
     Before receiving any response from the cellular access network  121  on the SCell- 1 , the mobile communication device  110  receives a PDCCH order on the SCell- 2  from the cellular access network  121 , which indicates an initiation of another random access procedure on another SCell (denoted as SCell- 2 ) (step S 320 ). That is, the PDCCH order is received during the ongoing random access procedure on the SCell- 1 . The mobile communication device  110  does not process the received PDCCH order (step S 330 ), due to the ongoing random access procedure on the SCell- 1 . That is, the random access procedure on the SCell- 2  is not initiated in response to not processing the received PDCCH order, and the ongoing random access procedure on the SCell- 1  is not interrupted by the PDCCH order. In one embodiment, the mobile communication device  110  may discard the PDCCH order, so that the PDCCH order is not processed. 
     Thus, the ongoing random access procedure on the SCell- 1  continues. Next, the mobile communication device  110  retransmits the Random Access Preamble message on the SCell- 1  when a guard timer for the random access procedure expires and no Random Access Response message is received from the cellular access network  121  (step S 340 ). In reply, the cellular access network  121  transmits a Random Access Response message on the SCell- 1  to the mobile communication device  110  (step S 350 ). Specifically, the Random Access Response message is transmitted via the DL-SCH, which contains timing advance information for uplink timing alignment, an uplink grant, and a temporary C-RNTI for the mobile communication device  110 . Subsequently, the mobile communication device  110  uses the uplink grant for schedule transmission to the cellular access network  121  along with the temporary C-RNTI (step S 360 ). In response to receiving the schedule transmission, the cellular access network  121  replies to the mobile communication device  110  with a Contention Resolution message via the DL-SCH using the temporary C-RNTI (step S 370 ), and the procedure ends. 
     Alternatively, for the step S 320 , the PDCCH order may be received before receiving a Contention Resolution message from the cellular access network  121 . In other words, the PDCCH order may be received after the reception of a Random Access Response message or after the schedule transmission, as long as a Contention Resolution message is not received yet. 
       FIG. 4  is a flow chart illustrating a method for handling random access procedures in the MAC layer according to an embodiment of the invention. The method may be applied to any mobile communication device supporting the CA technique and multiple carrier frequencies, such as a UE or MS utilizing the LTE/LTE-Advanced technology. Initially, the mobile communications device is wirelessly connected to a cellular access network comprising a first SCell and a second SCell of the same sTAG. To begin, the mobile communications device receives, during an ongoing random access procedure on the first SCell, a PDCCH order for initiating another random access procedure on the second SCell from the cellular access network (step S 410 ). In one embodiment, the PDCCH order is transmitted on the second SCell. 
     Subsequently, the mobile communications device does not process the PDCCH order (step S 420 ). In other words, the mobile communication device ignores or discards the received PDCCH order, so that the random access procedure on the second SCell is not initiated and the random access procedure on the first SCell is not interrupted by the received PDCCH order. Thus, the mobile communication device may unambiguously determine which random access procedure is to be performed when multiple random access procedures on different SCells are requested. 
     While the invention has been described by way of example and in terms of preferred embodiment, it is to be understood that the invention is not limited thereto. Those who are skilled in this technology can still make various alterations and modifications without departing from the scope and spirit of this invention. For example, the methods for handling random access procedures may also be applied for any evolutionary technology of the LTE/LTE-Advanced technology family or other radio access technologies which support CA technique and multiple carrier frequencies. Therefore, the scope of the invention shall be defined and protected by the following claims and their equivalents.