This disclosure relates to wireless communications and, more particularly, to performing random access in carrier aggregation.
Long Term Evolution Advanced (LTE-A) is a mobile communication standard that is standardized by the 3rd Generation Partnership Project (3GPP) as a major enhancement of the 3GPP LTE standard. In LTE-A, carrier aggregation is introduced in order to support wider transmission bandwidth than LTE and potentially increase the peak data rate. Using carrier aggregation, multiple downlink/uplink component carriers may be aggregated, and radio resources may be allocated to user equipment (UE) based on the aggregation of carriers. In some instances, one of the multiple carriers may be designated as the primary cell (PCell). The PCell may provide system information and configure physical uplink control channel (PUCCH). The remaining carriers may be defined as the secondary cell (SCell). In some instances, a UE may be simultaneously served by both the PCell and the SCell.
The access device serving a PCell may be a primary access device, and the access device serving a SCell may be a secondary access device. LTE-A system may use a physical downlink control channel (PDCCH) to distribute data control information (DCI) messages amongst UEs. The PDCCH may include control channel element (CCE) candidates that are used to transmit DCI messages from an access device to UEs. The access device may select one or an aggregation of CCEs to transmit a DCI message to a UE. The UE may blind decode a subset of the PDCCH CCE candidates (or PDCCH candidates) when searching for a DCI message. In some instances, for each sub-frame, a UE may search both a common search space for PDCCH candidates transmitted to multiple UEs and a UE specific search space for PDCCH candidates to each UE.
a is a schematic representation of an example network deployment scenario;
b is a schematic representation of another example network deployment scenario;
Like reference symbols in the various drawings indicate like elements.
The present disclosure is directed to systems and methods that perform random access in carrier aggregation. In wireless communication systems, such as Long Term Evolution Advanced (LTE-A) systems, a user equipment (UE) may be served by multiple access devices including a primary access device and a secondary access device. The primary access device may serve a primary cell (PCell) of a first carrier, and the secondary access device may serve a secondary cell (SCell) of a second carrier. In some instances, the location of the primary access device and the location of the secondary access device may be different. Accordingly, the primary and secondary access devices' uplink configurations for the UE may also be different. In these cases, in addition to performing a regular random access procedure to obtain configuration information from the primary access device, the UE may also perform a random access procedure to obtain configuration information from the secondary access device. The configuration information may include a timing advance (TA) for uplink synchronization, and an uplink grant for uplink radio resource allocation.
In some implementations, a UE may transmit a random access preamble to a secondary access device to initiate a random access procedure. After receiving the random access preamble, the secondary access device may transmit a random access response that includes uplink configuration information to the UE. The random access response (RAR) may be scrambled using a random access radio network temporary identifier (RA-RNTI), where the RA-RNTI may be determined based on radio resources used to transmit the random access preamble. The RAR may be transmitted in the physical downlink shared channel (PDSCH) of the SCell. To help the UE locate the RAR, the secondary access device may encode data control information (DCI) associated with the RAR in a common search space of the physical downlink control channel (PDCCH) of the SCell. Therefore, the UE may identify DCI by performing blind decoding on PDCCH candidates in the common search space (CSS) of the PDCCH. The UE may then use the identified DCI to locate the RAR for the UE's uplink configuration information. The UE may also perform blind decoding on PDCCH candidates in a UE-specific search space (USS) of PDCCH. To avoid increasing the total number of blind decoding attempts, the UE may reduce the number of blind decoding attempts for the PDCCH candidates in the USS. In these implementations, the sum of the number of decoding attempts for the CSS and the reduced number of decoding attempts for USS may be equal to or less than the initial number of decoding attempts for the USS before the reduction.
In some implementations, instead of encoding the DCI as PDCCH candidates in the CSS to help the UE locate the RAR of the SCell, the secondary access device may encode the DCI as PDCCH candidates in the USS. Accordingly, the UE may perform blind decoding on the PDCCH candidates in the USS to identify the DCI associated with the RAR. In some implementations, a primary access device may encode the DCI associated with the RAR of the SCell. The primary access device may encode the DCI as PDCCH candidates in the CSS of the PCell. Accordingly, the UE may perform blind decoding on the PDCCH candidates in the CSS of the PCell to identify the DCI, and then, use the identified DCI to locate the RAR of the SCell.
As used herein, the term “UE” may refer to any mobile electronic device used by an end-user to communicate within a wireless communication system. UE may be referred to as mobile electronic device, user agent, user device, mobile station, subscriber station, or wireless terminal UE may be a cellular phone, personal data assistant (PDA), smartphone, laptop, tablet personal computer (PC), or other wireless communications device. Further, UEs may include pagers, portable computers, Session Initiation Protocol (SIP) phones, one or more processors within devices, or any other suitable processing devices capable of communicating information using a radio technology. The term UE may also refer to devices that have similar capabilities but that are not generally transportable, such as desktop computers, set-top boxes, or network nodes. The term “access device” may refer to any access network component, such as a base station, an LTE or LTE-A access device or eNode B (eNB), that may provide one or more UEs with access to other components.
a is a schematic representation of an example network deployment scenario 100a suitable for some of the various implementations of the disclosure. In the illustrated deployment scenario 100a, primary access devices 110a (e.g., eNBs) may be used to provide macro coverage, and secondary access devices 120a (e.g., remote radio heads, frequency selective repeaters) may be used to provide enhanced throughput at wireless hot spots. The primary access device 110a may serve a first carrier of a PCell 130a, and the secondary access device 120a may serve a second carrier of a SCell 140a. A UE 160a is located in the coverage area of both the PCell 130a and SCell 140a. Thus, the first carrier associated with the PCell 130a and the second carrier associated with the SCell 140a may be aggregated. In these instances, the UE 160a may be simultaneously served, using carrier aggregation, by both the primary access device 110a and the secondary access device 120a.
b is a schematic representation of another example network deployment scenario 100b suitable for some of the various implementations of the disclosure. In the illustrated deployment scenario 100b, primary access devices 110b (e.g., eNBs) may be used to provide macro coverage, and secondary access devices 120b (e.g., remote radio heads, frequency selective repeaters) may be used to extend the coverage for at least one of the carriers served by the primary access devices 110b. The primary access device 110b may serve both a PCell 130b (a first carrier) and a SCell 140b (a second carrier), and the secondary access device 120b may serve a cell extension 150b of the SCell 140b. The cell extension 150b may extend the coverage area of the SCell 140b. The UE 160b located in the cell extension 150b is in the coverage area of both the first carrier and the second carrier. Thus, the first carrier associated with the PCell 130a and the second carrier associated with the SCell 140a may be aggregated. In these instances, the UE 160b located in the cell extension 150b of the SCell 140b may be simultaneously served, using carrier aggregation, by both the primary access device 110b and the secondary access device 120b.
For the deployment scenarios 100a and 100b illustrated respectively in
The UE 160 that is suitable for some of the various implementations of the disclosure may include hardware components such as a processor, a machine-readable medium such as a memory (e.g., solid-state, optical, magnetic, etc.), a transceiver, and an antenna. The access device 110, 120 that are suitable for some of the various implementations of the disclosure may also include hardware components that are similar or complementary to the previously-described hardware components of the UE 160. That is, the access device 110, 120 may include a processor, a machine-readable medium such as a memory, a transceiver, and an antenna. The hardware components of the access device 110, 120 may have functions that are similar to, or different from the corresponding hardware components of the UE 160 as described above.
In order to maintain a constant total number of blind decodes performed in the SCell PDCCH during the RAR window 250, the UE may cancel blind decoding of any subsets of PDCCH candidates in the USS. Table 1 shows an example of blind decoding attempts for PDCCH candidates in the USS during the RAR window 250 as compared to normal operation outside of the RAR window 240, 260 with respect to aggregation levels.
In some implementations, the UE may stop monitoring PDCCH candidates configured by cell RNTI (C-RNTI) in the USS during the RAR window. In some implementations, the UE may monitor CCE subsets of a subset of the aggregation levels. For example, when the channel condition between the UE and a secondary access device is good enough to use the lower MCS level to achieve the same error probability, the CCE subset candidates may be encoded with a lower aggregation level (e.g. aggregation level 1, aggregation level 2). In these implementations, the UE may not monitor PDCCH candidates encoded with high aggregation levels (e.g., aggregation level 4, aggregation level 8). In some implementations, an access device may indicate a maximum aggregation level of CCEs during a SCell radio access channel (RACH) procedure to the UE. The UE may then decode PDCCH candidates with aggregation levels that are less than or equal to the indicated maximum aggregation level. For example, the access device may indicate to the UE that the maximum aggregation level of PDCCH candidates is 4. Based on receiving the indication, the UE may monitor PDCCH candidates with aggregation levels 1, 2 and 4.
In some implementations, more than one SCell may be activated for a UE during SCell RACH procedures. In these implementations, the secondary access device may indicate whether physical uplink shared channel (PUSCH) is scheduled during the SCell RACH procedures. When the uplink MIMO is configured, DCI format 4 is also scheduled to be monitored, and the number of blind decodes may increase to 44. Since the uplink PUSCH cannot be scheduled if uplink timing is not synchronized, the UE may cancel the scheduled monitoring of DCI format 4 before uplink timing synchronization. Therefore, the number of blind decodes may be maintained as 32 during the SCell RACH procedures. In some implementations, the UE may stop monitoring the CSS when a TA is acquired and/or when the RAR window 260 is expired.
The UE may not receive RAR until the RAR window has expired. After the RAR window is expired, the UE could re-transmit the random access preamble, or if the UE sends the random access preamble more than the allowed number of preamble transmissions, the UE may send the indication to the eNB with higher layer/MAC or physical layer signaling that the timing synchronization of the SCell has not been completed.
Since the RAR is sent in response to the random access preamble transmitted by a specific UE, the introduction of a new LCID value for identifying RAR may not affect the legacy UEs. If the UE identifies LCID value as 11011 (as defined in Table 2) in the MAC header 310, the remaining MAC payload may be interpreted by the UE as RAR MAC CE 320.
In some implementations, the UE may monitor PDCCH DCI format 1A configured by RA-RNTI in the USS instead of C-RNTI. Since the size of PDCCH 1A configured by RA-RNTI is the same as PDCCH 1A configured by C-RNTI in case of the separate scheduling, the number of blind decodes may not change. If the cross carrier scheduling is configured, the size of PDCCH 1A configured by RA-RNTI is different from the size configured by C-RNTI due to the carrier indicator field in PDCCH 1A configured by C-RNTI. In this case, in order to have the same size, the carrier indicator field may be included in PDCCH 1A configured by RA-RNTI and transmitted in the USS. The carrier indicator field in PDCCH 1A configured by RA-RNTI may be reserved as a certain value or used to support the cross carrier scheduling for PDCCH 1A configured by RA-RNTI. If the cross carrier scheduling is supported, RAR could be located in other serving cell than the corresponding SCell.
At block 720, the UE receives first PDCCH candidates of a CSS of the second carrier and second PDCCH candidates of a USS of the second carrier. At block 730, the UE performs a first blind decoding of the received first PDCCH candidates of the CSS of the second carrier. RA-RNTI is used in blind decoding of first PDCCH candidates. As mentioned with regard to
At block 750, the UE identifies an RAR based on the identified DCI. The RAR may be included in the PDSCH. The identified DCI may include information associated with the scheduling information of the RAR in the PDSCH. At block 760, the UE performs a second blind decoding of second PDCCH candidates of the USS of the second carrier. The number of blind decoding attempts for the first and second blind decodings may be maintained to be less than or equal to the configured total number (e.g., 32) of blind decoding attempts for decoding the USS in normal operations. The UE may reduce the blind decoding attempts during the RAR window based on any one of the implementations described in the illustration of
The RA-RNTI is generated based on indexes of PRACH time and frequency resources that are used to transmit the random access preamble. When the same PRACH frequency and time resources are allocated to both PCell and SCell, the DCI of the PCell and the DCI of the SCell may associate with the same RA-RNTI. In other words, PDCCH candidates associated with the same RA-RNTI may be received at the UE, if the UE transmit random access preamble(s) to the PCell and SCell using the same PRACH resources.
In some implementations, the primary access device (e.g., eNB) may allocate different PRACH time and frequency resources to a UE for PCell and SCell. As such, the UE may transmit the first random access preamble at block 1010 and the second random access preamble at block 1020 using different time and frequency resources. Accordingly, the corresponding RA-RNTIs associated with the PCell and the SCell may be different. The UE may then distinguish RARs from PCell and SCell based on the different RA-RNTIs.
In some implementations, RAPID may be configured to be different for the PCell and the SCell. For example, some of the random access preamble sequences (e.g., non-contention PRACH process) may be reserved exclusively for the SCell. In some instances, the UE may also transmit different random access preamble sequences on the PCell and the SCell RACH to avoid collision of RARs from the PCell and the SCell.
In some implementations, a new RA-RNTI may be reserved for the RAR of the SCell, instead of using the RA-RNTI calculated based on the PRACH resources used to transmit the random access preamble. An access device may signal the new RA-RNTI before a UE transmits the random access preamble to the secondary access device. For example, an RA-RNTI value may be included as a dedicated random access parameter for the SCell in order to distinguish the RARs transmitted in the PCell and the SCell.
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/US11/47438 | 8/11/2011 | WO | 00 | 9/19/2014 |