WO2013039283A1 - Enhanced random access to a heterogeneous network - Google Patents

Abstract

The present document is directed to a method for a user equipment (UE) to perform a random access to a network comprising a macro eNB and one or more remote radio heads (RRHs), and apparatus for the same. The method comprises selecting a sequence group among a plurality of predetermined sequence groups considering (a) a size of data to be transmitted from the UE, (b) a pathloss between the macro eNB and the UE, and (c) a metric related with at least one among the RRHs; selecting a preamble sequence from the selected sequence group; and transmitting the selected preamble sequence to the network.

Description

ENHANCED RANDOM ACCESS TO A HETEROGENEOUS NETWORK

[0001] The present document is directed to an enhanced random access to a heterogeneous network. More specifically, the present document is directed to a method for a user equipment (UE) to perform a random access to a network comprising a macro eNB and one or more remote radio heads (RRHs), and apparatus for the same.

[0002] First of all, 3GPP LTE (3rd generation partnership project) long term evolution: hereinafter called ‘LTE’) communication system is schematically described as a mobile communication system to which the present invention is applicable.

[0003] FIG. 1 is a schematic diagram of E-UMTS network structure as an example of a mobile communication system.

[0004] Referring to FIG. 1, E-UMTS (evolved universal mobile telecommunications system) is the system having evolved from UMTS (universal mobile telecommunications system) and its basic standardization is ongoing by 3GPP. Generally, the E-UMTS can be called LTE system.

[0005] E-UMTS network can be mainly divided into E-UTRAN (evolved-UMTS terrestrial radio access network) 101 and CN 102 (core network). The E-UTRAN 101 consists of a user equipment (hereinafter abbreviated UE) 103, a base station (hereinafter named eNode B or eNB) 104, and an access gateway (hereinafter abbreviated AG) 105 located at an end point of the network to be externally connected to an external network. The AG 105 can be divided into one part responsible for user traffic processing and the other part for processing control traffic. In this case, the AG for new user traffic processing and the AG for processing control traffic can communicate with each other using a new interface.

[0006] At least one cell can exist at one eNode B. Between eNode Bs, an interface for user or control traffic transmission is usable. And, the CN 102 can consist of a node for user registrations of the AG 105 and other UE 103. Moreover, an interface for discriminating the E-UTRAN 101 and the CN 102 is available.

[0007] Layers of a radio interface protocol between a user equipment and a network can be divided into L1 (first layer), L2 (second layer) and L3 (third layer) based on three lower layers of the open system interconnection (OSI) reference model widely known in the field of communication systems. A physical layer belonging to the first layer provides an information transfer service using a physical channel. A radio resource control (hereinafter abbreviated RRC) located on the third layer plays a role in controlling radio resources between the user equipment and the network. For this, the RRC layers exchange RRC messages between the user equipment and the network. The RRC layers can be distributed to network nodes including the eNode B 104, the AG 105 and the like. Moreover, the RRC layer can be provided to the eNode B 104 or the AG 105 only.

[0008] FIG. 2 and FIG. 3 are diagrams for structures of a radio interface protocol between a user equipment and UTRAN based on the 3GPP radio access network specifications.

[0009] Referring to FIG. 2 and FIG. 3, a radio interface protocol horizontally consists of a physical layer, a data link layer and a network layer. And, the radio interface protocol vertically consists of a user plane for data information transfer and a control plane for control signal delivery (signaling). In particular, FIG. 2 shows the respective layers of the radio protocol control plane and FIG. 3 shows the respective layers of the radio protocol user plane. The radio protocol layers shown in FIG. 2 and FIG. 3 can be divided into L1 (first layer), L2 (second layer) and L3 (third layer) based on three lower layers of the open system interconnection (OSI) reference model widely known in the field of communication systems.

[0010] The respective layers of the radio protocol control plane shown in FIG. 2 and the respective layers of the radio protocol user plane shown in FIG. 3 are explained as follows.

[0011] First of all, a physical (PHY) layer of a first layer provides an upper layer with an information transfer service using a physical channel. The physical (PHY) layer is connected to a medium access control (MAC) layer on an upper layer via a transport channel. And, data is transported between the medium access control (MAC) layer and the physical (PHY) layer via the transport channel. In this case, the transport channel can be classified into a dedicated transport channel or a common transport channel according to whether a channel is shared or not. Moreover, data are transported via the physical channel between different physical layers, i.e., between a physical layer of a transmitting side and a physical layer of a receiving side.

[0012] Various layers exist in the second layer. First of all, a medium access control (hereinafter abbreviated ‘MAC’) layer plays a role in mapping various logical channels to various transport channels. And, the MAC layer also plays a role as logical channel multiplexing in mapping several logical channels to one transport channel. The MAC layer is connected to a radio link control (RLC) layer of an upper layer via a logical channel. And, the logical channel can be mainly categorized into a control channel for transferring information of a control plane and a traffic channel for transferring information of a user plane according to a type of the transferred information.

[0013] A radio link control (hereinafter abbreviated RLC) of the second layer performs segmentation and concatenation on data received from an upper layer to play a role in adjusting a size of the data to be suitable for a lower layer to transfer the data to a radio section. And, the RLC layer provides three kinds of RLC modes including a transparent mode (hereinafter abbreviated TM), an unacknowledged mode (hereinafter abbreviated UM) and an acknowledged mode (hereinafter abbreviated AM) to secure various kinds of QoS demanded by each radio bearer (hereinafter abbreviated RB). In particular, the AM RLC performs a retransmission function through automatic repeat and request (ARQ) for the reliable data transfer.

[0014] A packet data convergence protocol (hereinafter abbreviated PDCP) layer of the second layer performs a header compression function for reducing a size of an IP packet header containing relatively large and unnecessary control information to efficiently transmit such an IP packet as IPv4 and IPv6 in a radio section having a small bandwidth. This enables a header part of data to carry mandatory information only to play a role in increasing transmission efficiency of the radio section. Moreover, in the LTE system, the PDCP layer performs a security function as well. This consists of ciphering for preventing data interception conducted by a third party and integrity protection for preventing data manipulation conducted by a third party.

[0015] A radio resource control (hereinafter abbreviated RRC) layer located at a most upper part of a third layer is defined in the control plane only and is responsible for controlling a logical channel, a transport channel and physical channels in association with configuration, reconfiguration and release of radio bearers (hereinafter abbreviated RBs). In this case, the RB means a logical path provided by the first and second layers of the radio protocol for the data delivery between the user equipment and the UTRAN. Generally, configuring an RB means to stipulate characteristics of radio protocol layers and channels required for providing a specific service and also means to configure detailed parameters and operational methods thereof. The RB can be classified into a signaling RB (SRB) or a data RB DRB). The SRB is used as a path for sending an RRC message in a control plane (C-plane) and the DRB is used as a path for transferring user data in a user plane (U-plane).

[0016] As a downlink transport channel for transporting data to a user equipment from a network, there is a broadcast channel (BCH) for transmitting system information and a downlink shared channel (SCH) for transmitting a user traffic or a control message. Downlink multicast, traffic of a broadcast service or a control message can be transmitted on downlink SCH or a separate downlink MCH (multicast channel). Meanwhile, as an uplink transport channel for transmitting data to a network from a user equipment, there is a random access channel (RACH) for transmitting an initial control message or an uplink shared channel (SCH) for transmitting user traffic or a control message.

[0017] As a downlink physical channel for transmitting information transferred on a downlink transport channel to a radio section between a network and a user equipment, there is a physical broadcast channel for transferring information of BCH, a physical multicast channel (PMCH) for transmitting information of MCH, a physical downlink shared channel for transmitting information of PCH and downlink SCH or a physical downlink control (or called DL L1/L2 control channel) for transmitting control information provided by first and second layers.

[0018] As an uplink physical channel for transmitting information forwarded on an uplink transport channel to a radio section between a network and a user equipment, there is a physical uplink shared channel (PUSCH) for transmitting information of uplink SCH, a physical random access channel (PRACH) for transmitting RACH information or a physical uplink control channel (PUCCH) for transmitting such control information, which is provided by first and second layers, as HARQ ACK, HARQ NACK, scheduling request (SR), channel quality indicator (CQI) report and the like.

[0019] An LTE User Equipment (UE) can only be scheduled for uplink transmission if its uplink transmission timing is synchronized. The LTE Random Access CHannel (RACH) therefore plays a key role as an interface between non-synchronized UEs and the orthogonal transmission scheme of the LTE uplink radio access.

[0020] However, the random access procedure of the current LTE system does not consider a situation where the network is heterogeneous. That is, when we carefully consider the network comprises a macro eNB and one or more remote radio heads (RRHs), we can reduce a radio resource overhead for the RACH and enhance the efficiency of the random access.

[0021] Accordingly, the present invention is directed to a method for a user equipment (UE) to perform a random access to a network comprising a macro eNB and one or more remote radio heads (RRHs), and apparatus for the same.

[0022] An object of the present invention is to provide an enhanced random access scheme not only considering a radio link between a UE and a macro eNB, but also considering a radio link between a UE and RRHs within a cell.

[0023] Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objectives and other advantages of the invention may be realized and attained by the structure particularly pointed out in the written description and claims hereof as well as the appended drawings.

[0024] To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a method for a user equipment (UE) to perform a random access to a network comprising a macro eNB and one or more remote radio heads (RRHs), the method comprising: selecting a sequence group among a plurality of predetermined sequence groups considering (a) a size of data to be transmitted from the UE, (b) a pathloss between the macro eNB and the UE, and (c) a metric related with at least one among the RRHs; selecting a preamble sequence from the selected sequence group; and transmitting the selected preamble sequence to the network is proposed.

[0025] The random access to the network may be based on a contention based random access scheme.

[0026] The method may further comprises: receiving system information from the network before transmitting the selected preamble sequence, wherein the system information may comprise random access channel (RACH) configuration information and information for the metric.

[0027] The information for the metric may comprise a number of the RRHs within the network.

[0028] The plurality of predetermined sequence groups may comprise a sequence group A and a sequence group B, and each of the sequence group A and the sequence group B may comprise one or more sub groups. Transmission powers for each of the sub groups may be different from each other.

[0029] A number of the sub groups of the sequence group A and the sequence group B may correspond to a number of the RRHs.

[0030] The information for the metric may comprise a number of the sub groups.

[0031] The information for the metric may comprise information for threshold values for each of the sub groups.

[0032] The method may further comprises: receiving a random access response from the network after transmitting the selected preamble sequence, wherein the random access response comprises at least one of (a) a timing advanced command for the at least one among the RRHs and (b) a power adjustment parameter for the at least one among the RRHs.

[0033] The random access response may be received from the macro eNB or at least one among the RRHs

[0034] The method may further comprises: transmitting uplink data to the network, based on the at least one of (a) the timing advanced command and (b) the power adjustment parameter.

[0035] To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a method for a user equipment (UE) to perform a random access to a network comprising a macro eNB and one or more transmission points (TPs), the method comprising: selecting a sequence group among a plurality of predetermined sequence groups based on a first criteria related with a size of data to be transmitted by the UE and a pathloss between the UE and the macro eNB, wherein the plurality of predetermined sequence groups comprise a first and a second groups, wherein the second group comprises one or more sub groups, wherein if the first sequence group is not selected based on the first criteria, either one of the subgroups is selected based on a second criteria related with a comparison between a radio condition with the macro eNB and a radio condition with one or more of the RRHs; selecting a preamble sequence from the selected sequence group or from the selected subgroup; and transmitting the selected preamble sequence to the network is proposed.

[0036] To achieve these objects and other advantages and in accordance with the purpose of the invention, as embodied and broadly described herein, a user equipment (UE) performing a random access to a network comprising a macro eNB and one or more remote radio heads (RRHs), the UE comprising: a processor configured to select a sequence group among a plurality of predetermined sequence groups considering (a) a size of data to be transmitted from the UE, (b) a pathloss between the macro eNB and the UE, and (c) a metric related with at least one among the RRHs, and to select a preamble sequence from the selected sequence group; and a radio frequency (RF) unit coupled with the processor and configured to transmit the selected preamble sequence to the network is proposed.

[0037] It is to be understood that both the foregoing general description and the following detailed description of the present invention are exemplary and explanatory and are intended to provide further explanation of the invention as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

[0038] The accompanying drawings, which are included to provide a further understanding of the invention and are incorporated in and constitute a part of this application, illustrate embodiment(s) of the invention and together with the description serve to explain the principle of the invention. In the drawings:

[0039] FIG. 1 is a schematic diagram of E-UMTS network structure as an example of a mobile communication system;

[0040] FIG. 2 and FIG. 3 are diagrams for structures of a radio interface protocol between a user equipment and UTRAN based on the 3GPP radio access network specifications;

[0041] FIGs. 4 and 5 are procedural diagrams illustrating Contention-based Random Access Procedure and Non-Contention-based Random Access Procedure;

[0042] Fig. 6 shows the concept of the heterogeneous network for the present invention;

[0043] Fig. 7 is for explaining the overall process of embodiments of the present invention; and

[0044] Fig. 8 shows apparatuses for implementing the present invention.

[0045] Reference will now be made in detail to the preferred embodiments of the present invention, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts. In the following detailed description of the invention includes details to help the full understanding of the present invention. Yet, it is apparent to those skilled in the art that the present invention can be implemented without these details. For instance, although the following detailed description is made in detail on the assumption that a mobile communication system is the 3GPP LTE system, it is applicable to other prescribed mobile communication systems by excluding unique items of the 3GPP LTE.

[0046] Occasionally, the structures and devices known to the public are omitted to avoid conceptional vagueness of the present invention or can be illustrated as block diagrams centering on their core functions.

[0047] Besides, in the following description, assume that a terminal is a generic term of such a mobile or fixed user-end device as a user equipment (UE), a mobile station (MS) and the like. Moreover, assume that eNB is a generic name of such a random node of a network end, such as a base station, which communicates with a terminal, as a Node B, an eNnode B and the like.

[0048] As stated above, the present invention is directed to a method for a user equipment (UE) to perform a random access to a network comprising a macro eNB and one or more remote radio heads (RRHs), and apparatus for the same. For better understanding of the invention, a random access procedure of the LTE system is first explained as an example.

[0049] The LTE random access procedure comes in two forms, allowing access to be either contention-based (implying an inherent risk of collision) or contention-free. A UE initiates a contention-based random access procedure for all use-cases listed as following.

[0050] (1) A UE in RRC_CONNECTED state, but not uplink-synchronized, needing to send new uplink data or control information (e.g. an event-triggered measurement report);

[0051] (2) A UE in RRC_CONNECTED state, but not uplink-synchronized, needing to receive new downlink data, and therefore to transmit correspondingACK/NACK in the uplink;

[0052] (3) A UE in RRC_CONNECTED state, handing over from its current serving cell to a target cell;

[0053] (4) A transition from RRC_IDLE state to RRC_CONNECTED, for example for initial access or tracking area updates;

[0054] (5) Recovering from radio link failure.

[0055] In this procedure, a random access preamble signature is randomly chosen by the UE, with the result that it is possible for more than one UE simultaneously to transmit the same signature, leading to a need for a subsequent contention resolution process.

[0056] For the use-cases (2) (new downlink data) and (3) (handover) the eNodeB has the option of preventing contention occurring by allocating a dedicated signature to a UE, resulting in contention-free access. This is faster than contention-based access - a factor which is particularly important for the case of handover, which is time-critical.

[0057] Unlike in WCDMA, a fixed number (64) of preamble signatures is available in each LTE cell, and the operation of the two types of RACH procedure depends on a partitioning of these signatures between those for contention-based access and those reserved for allocation to specific UEs on a contention-free basis.

[0058] The two procedures are outlined in the following.

[0059] FIGs. 4 and 5 are procedural diagrams illustrating Contention-based Random Access Procedure and Non-Contention-based Random Access Procedure.

[0060] The contention-based procedure consists of four-steps as shown in Figure 4:

* Step 1: Preamble transmission (message 1);

* Step 2: Random access response (message 2);

* Step 3: Layer 2 / Layer 3 (L2/L3) message (message 3);

* Step 4: Contention resolution message (message 4).

[0061] The slightly unpredictable latency of the random access procedure can be circumvented for some use cases where low latency is required, such as handover and resumption of downlink traffic for a UE, by allocating a dedicated signature to the UE on a per-need basis. In this case the procedure is simplified as shown in Figure 5. The procedure terminates with the RAR.

[0062] Now, the heterogeneous network to which the present invention is applied is explained.

[0063] Fig. 6 shows the concept of the heterogeneous network for the present invention.

[0064] Recently CoMP deployment scenarios received significant attention, especially those focused on heterogeneous networks consisting of a macro eNodeB and remote radio heads (RRHs)as shown in fig. 6. Application of a RRH is effective in coordinating geographically separate cells with negligible coordination latency. Besides the well understood setup in which macro and RRHs are configured with different cell IDs, an alternative configuration was proposed in which all nodes share the same cell ID. In such a configuration both Macro cell and its RRH(s) can be referred as transmission points (TP) of one cell. Configuring the same cell ID at both macro eNodeB and RRHs makes these nodes appear as a single cell sharing the same radio resources, at least from the viewpoint of control and Rel-8/9 legacy transmissions.

[0065] CoMP performance gain is obtained when the macro eNode B dynamically mute specific time/frequency resources to the UEs connected to the RRHs. Depending on the UE location within the cell, it can be optimized for downlink transmission to be performed from the closest single TP or the closest set of TP(s). Thus, it is preferable that the network can determine the TP(s) from which the UE is experiencing better radio conditions.

[0066] Based on these understandings, the proposed enhanced random access procedure is as follows.

[0067] In most casual random access procedure, there are 2 predetermined sequence groups (sequence groups A and B). The UE may select one of the sequence group based on the size of data to be transmitted from the UE and radio condition between the UE and the macro eNB. In terms of the radio condition criteria on selection of the sequence groups, it can be simply represented such that when the UE has a good radio condition with the macro eNB, the UE may select sequence group A, otherwise select sequence group B.

[0068] However, one embodiment of the present invention proposes to further consider some metric related with a corresponding RRH among the RRHs within the cell. By further considering the metric related with the RRH at selecting the sequence group, the random access procedure can be enhanced to reduce the RACH overheadand etc. For example in Fig. 6, UE 1 may select sequence group B based on the original criteria, since it is located far from the macro eNB. However, as shown in Fig. 6, since UE1 is close to TP 1, it may access to the TP 1 using the sequence group A, rather than the macro eNB (TP 0) using sequence group B. The metric related with the RRH can be used for this determination. It may be related with a pathloss between the UE 1 and TPs, but it is not necessary to be limitted thereto. System information may inform the UEs with only the number of RRHs within the network, and the UEs may acquire detailed information from each of RRHs. Further, sequence groups can be further partitioned into multiple sub-groups to support each of the RRHs. That is, UE 1 may select a specific sub-group for TP 1 and UE 2 may select another specific sub-group for TP 2.

[0069] In another embodiment, only the sequence group B may be further partitioned into sub-groups. And, when the sequence group A is not selected by a first criteria not related with the RRHs, the second criteria related with the RRHs is used to select one of sub-groups of sequence group B.

[0070] After, selecting the sequence group, the UE may select a preamble sequence from the selected sequence group and transmit the selected preamble sequence to the network.

[0071] Fig. 7 is for explaining the overall process of embodiments of the present invention.

[0072] The random access procedure can be initiated by a PDCCH order or by the MAC sublayer itself. The random access procedure initiated by a PDCCH order can be referred to as non-contention based random access procedure as explaiend with regards to Fig. 5, and the random access procedure initiated by the MAC sublayer itself can be referred to as contention based random access procedure as explained with regards to Fig. 4. Fig. 7 assumed that the random access procedure is contention based.

[0073] Before the procedure can be initiated, the following information is assumed to be available:

[0074] - the groups of Random Access Preambles and the set of available Random Access Preambles in each group:

[0075] The above mentioned embodiment assumed that there are two random access sequence groups A and B. But, there can be a situation where preamble sequence group B does not exist. The preambles that are contained in Random Access Preambles group A and Random Access Preambles group B can be calculated from the parameters indicating the number of random access preambles (numberOfRA-Preambles) and the size of the random access preamble group A (sizeOfRA-PreamblesGroupA). For this end, as shown in Fig. 7, the network may transmit/broadcast system information to UEs, and the UEs may try to retrieve information on the system information (step 0). The system information may include the above mentioned parameters.

[0076] If sizeOfRA-PreamblesGroupA is equal to numberOfRA-Preambles then there is no Random Access Preambles group B. Then, the preambles in Random Access Preamble group A are the preambles 0 to sizeOfRA-PreamblesGroupA-1. But, if Random Access Preambles group B exists, the preambles in Random Access Preamble group B are the preambles sizeOfRA-PreamblesGroupA to numberOfRA-Preambles-1 from the set of 64 preambles.

[0077] If Random Access Preambles group B exists, the thresholds, messagePowerOffsetGroupB and messageSizeGroupA, the configured UE transmitted power, PCMAX, and the offset between the preamble and Msg3, deltaPreambleMsg3, that are required for selecting one of the two groups of Random Access Preambles. These can be acquired from the received system information.

[0078] According to one embodiment of the present invention, the system information may further include information for the metric related with at least one among the RRHs within the network. The information for the metrc may include the number of RRHs within the network.

[0079] And, each of or one of the above mentioned sequence groups A and B can be further partitioned into multiple sub groups to support the preferred embodiment of the present invention. The information of the system information for the metrc may include the number of each subgroups. The number of sub-groups may corresponds to the number of RRHs, but it may not be limited to that.

[0080] When we did not considered the heterogeneous network condition, and when we only consider the situation when Random Access Preambles group B exists, the selection of the sequence group can be based on the following cirteria.

[0081] - if the potential message size (data available for transmission plus MAC header and, where required, MAC control elements) is greater than a predetermined size and if the pathloss between the macro eNB and the UE is less than a predetermined value, then the Random Access Preambles group B may be selected. Otherwise, the Random Access Preambles group A may be selected.

[0082] This can be simplified (in terms of pathloss) such that when the UE with good radio condition with the macro eNB, the UE may select the sequence group A, otherwise the sequence group B.

[0083] In one embodiment of the present invention, the sequence group B is further partitioned into sub-groups. And, if the sequence group A is not selected, a secondary criteria related with the RRHs are considered. For example, when UE 1 of Fig. 6 determines that the sequence group A is not selected and it is closer to TP 1 than the macro eNB (TP0), UE 1 may select sub-group of sequence group B for TP 1. The metric related with pathloss for TPs can be used for this selection. For this end, the system information may inform the UE of RRH related information, and the UEs can use this information to determine the pathlosses with RRHs.

[0084] In another embodiment of the present invention, both of the sequence groups A and B are further partitioned into sub-groups. And, the UE may further consider the metric related with at least one among RRHs within the network. Based on this consideration, the UE may selects appropriate sub-group and select the preamble sequence within this selected sub-group.

[0085] After, selecting the sequence group, the UE may select a preamble sequence from the selected sequence group and transmit the selected preamble sequence to the network (step 1 of Fig. 7). Then, the network tries to detect preambles within a predetermined time period.

[0086] The UE may receive random access response message including timing advanced command and power adjustment parameters (step 2 of Fig. 7). According to one embodiment of the present invention, the random access response may further include at least one of (a) a timing advanced command for the at least one among the RRHs and (b) a power adjustment parameter for the at least one among the RRHs. This random access response message can be received from the macro eNB or from one or more of corresponding RRHs. Based on these information, the UE may transmit uplink data to the network (step 3 of Fig. 7). The network may indicate collision or assign resource as requested by the message 3 (step 4 of Fig. 7).

[0087] In the case of the initial synchronization, in addition to the detection of synchronization signals, the UE may proceed to decode the Physical Broadcast CHannel (PBCH), from which system information is obtained (e.g. RACH configuration information). The RACH configuration information informs the UE of the RACH format, allocated subframe for RACH, etc.

[0088] In the case of new cell identication is used, the UE does not need to decode the PBCH; it simply makes quality-level measurements based on the reference signals (RS) transmitted from the newly-detected cell and reports these to the serving cell. For RRH configuration with the same cell ID, it is still under discussion whether the reference signals (RS) is transmitted from all TPs or not.

[0089] Assume that the RS is not transmitted from RRHs and LTE Rel. 8 initial synchronization procedure is used. The decoded PBCH may inform the UE of RACH configuration information and the signature grouping rule. A set of signatures is allocated for each time/frequency random access resource for each cell. In Rel.8, these signatures are divided into three groups:

[0090] The signatures of one of the groups are assigned explicitly to be used for the non-contention (dedicated) based access.

[0091] The other two groups (A and B) are used for the contention based access, and their selection is used to indicate information on size of Msg3 and the requested resource blocks limited by eNB. The UE with good radio condition would choose one of preamble from group A while those experienced with bad radio conditions (usually those far from eNB) would choose one of preamble from group B.

[0092] According to one of the embodiments, preamble group B is further partitioned according different level of radio conditions associated to the RRHs (difference between Macro eNB and RRHs experienced radio conditions)

[0093] The UE may estimate the radio condition compared to the Macro eNB also as compared to the RRH. Then, the UE may select randomly and transmits the preamble from group with better channel conditions. The UE may use the following equation when comparing.

[0094] [Equation 1]

[0095] pathloss < f(Macro eNB/RRH transmission power, margins)

[0096] Then, the eNB/RRH transmits to the UE the timing alignment and power adjustment based on the group to which the received signature preamble belongs to.

[0097] Other approaches to enhance the random access to the heterogeneous network can be thought. One of them can be referred to as contention based RACH procedure followed by non-contention based RACH.

[0098] In this approach, when the eNB detects the preamble from group B, the step 5 following the step 4 of Fig. 7 is proposed, in which eNB can assign specific/dedicated RRH resources to the UE by triggering the non-contention RACH procedure.

[0099] Another approach can be referred to as the use of RRH specific/information in contention based RACH procedure. In this approach, the UE may select and transmit a contention based preamble based on Rel.8 procedure (i.e. only 2 group of preamble sequence, group A and B). The eNB detects the preamble, and if the detected preamble belongs to the group B, then eNB indicates in Msg2 additional information related to the RRH, otherwise the former Rel8 procedure continues as explained.

[00100] The additional information, in case of preamble group B, may be any information (e.g. identifiers, resources, etc..) related to the RRH. For example, it can be the RRH’s specific identifier that UE would be included or scrambled into the Msg3.

[00101] Based on the above embodiments of the invention, one preferred embodiment proposes adjusting a transmission power of the random access sequence according to the selected sequence group.

[00102] When the sequence group selection is performed as explained above, there is a need to further adjust the preamble transmission power. A preamble transmission power PPRACH is determined as in Equation 2.

[00103] [Equation 2]

PPRACH = min{P_CMAX, PREAMBLE_RECEIVED_TARGET_POWER + PL}_[dBm],

[00104] Where:

[00105] P_CMAX is the configured UE transmit power (usually it is the maximum UE power, but in some area like hospital area the network has possibility to limit the UE max. transmission power by signalling)

[00106] PL is the downlink pathloss estimate calculated in the UE. The PL is based on the measured received power at the UE and the max. Tx. power which is signalled by the eNB.

[00107] [Equation 3]

PREAMBLE_RECEIVED_TARGET_POWER=preambleInitialReceivedTargetPower+ DELTA_PREAMBLE

[00108] With :

[00109] preambleInitialReceivedTargetPower signalled by the eNB

[00110] DELTA_PREAMBLE is the preamble format based power offset values specified for each preamble format as follows

[00111] Format 0: 0 dB

[00112] Format 1 : 0 dB

[00113] Format 2 : -3 dB

[00114] Format 3: -3 dB

[00115] Above parameters should be further modified according to the present embodiment, as the UE would connect to the closest TP (e.g. UE 1 connected to the TP1), so it needs to adjust preamble transmission power based on the TP1’s PL (i.e. TP1’s max. transmission power) and TP1’s preambleInitialReceivedTargetPower. These two parameters can be either signaled by the eNB (TP0) or fixed in the specification (e.g.: the RRH max. Tx power). In this case the PL would be defined as

[00116] [Equation 4]

PL = min{PLeNB, PLRRH0, PLRRH1, PLRRH3,…}_[dBm],

[00117] According to the present embodiment, preambleInitialReceivedTargetPower can be the target received preamble power at the RRH. Additional adjustment to the DELTA_PREAMBLE based on the power offset (delta_offsrt) values can be also included.

[00118] Based on the above explanation, apparatus for the present invention is explained.

[00119] Fig. 8 shows apparatuses for implementing the present invention.

[00120] As shown in Fig. 8, a wireless communication system can include one or more TPs and one or more UE 20. In downlink, a transmitter may be a part of the TP 10, and a receiver may be a part of the UE 20. In uplink, a transmitter may be a part of the UE 20, and a receiver may be a part of the TP 10. A TP 10 may include a processor 11, a memory 12, and a radio frequency (RF) unit 13. The processor 11 may be configured to implement proposed procedures and/or methods described in this document. The memory 12 is coupled with the processor 11 and stores a variety of information to operate the processor 11. The RF unit 13 is coupled with the processor 11 and transmits and/or receives a radio signal. TP 10 can be a macro eNB or any one of RRHs of the above explained embodiments.

[00121] A UE 20 may include a processor 21, a memory 22, and a RF unit 23. The processor 21 may be configured to implement proposed procedures and/or methods described in this application. The memory 22 is coupled with the processor 21 and stores a variety of information to operate the processor 21. The RF unit 23 is coupled with the processor 21 and transmits and/or receives a radio signal. The TP 10 and/or the UE 20 may have single antenna or multiple antennas. When at least one of the TP 10 and the UE 20 has multiple antennas, the wireless communication system may be called as multiple input multiple output (MIMO) system.

[00122] The above-described enhanced random access technology and apparatus are explained mainly with reference to the example that they are applied to the 3GPP LTE system. However, they are applicable to various mobile communication systems, such as IEEE based system employing ranging procedure corresponding to the random access procedure of LTE.

Claims (15)

1. A method for a user equipment (UE) to perform a random access to a network comprising a macro eNB and one or more remote radio heads (RRHs), the method comprising:

   selecting a sequence group among a plurality of predetermined sequence groups considering (a) a size of data to be transmitted from the UE, (b) a pathloss between the macro eNB and the UE, and (c) a metric related with at least one among the RRHs;

   selecting a preamble sequence from the selected sequence group; and

   transmitting the selected preamble sequence to the network.
2. The method of claim 1, wherein the random access to the network is based on a contention based random access scheme.
3. The method of claim 1, further comprising:

   receiving system information from the network before transmitting the selected preamble sequence, wherein the system information comprises random access channel (RACH) configuration information and information for the metric.
4. The method of claim 3, wherein the information for the metric comprises a number of the RRHs within the network.
5. The method of claim 3, wherein the plurality of predetermined sequence groups comprises a sequence group A and a sequence group B, and

   wherein each of the sequence group A and the sequence group B comprises one or more sub groups.
6. The method of claim 5, wherein transmission powers for each of the sub groups are different from each other.
7. The method of claim 5, wherein a number of the sub groups of the sequence group A and the sequence group B corresponds to a number of the RRHs.
8. The method of claim 5, wherein the information for the metric comprises a number of the sub groups.
9. The method of claim 5, wherein the information for the metric comprises information for threshold values for each of the sub groups.
10. The method of claim 1, further comprising:

    receiving a random access response from the network after transmitting the selected preamble sequence,

    wherein the random access response comprises at least one of (a) a timing advanced command for the at least one among the RRHs and (b) a power adjustment parameter for the at least one among the RRHs.
11. The method of claim 10, wherein the random access response is received from the macro eNB.
12. The method of claim 10, wherein the random access response is received from the at least one among the RRHs.
13. The method of claim 10, further comprising:

    transmitting uplink data to the network, based on the at least one of (a) the timing advanced command and (b) the power adjustment parameter.
14. A method for a user equipment (UE) to perform a random access to a network comprising a macro eNB and one or more transmission points (TPs), the method comprising:

    selecting a sequence group among a plurality of predetermined sequence groups based on a first criteria related with a size of data to be transmitted by the UE and a pathloss between the UE and the macro eNB,

    wherein the plurality of predetermined sequence groups comprise a first and a second groups, wherein the second group comprises one or more sub groups,

    wherein if the first sequence group is not selected based on the first criteria, either one of the subgroups is selected based on a second criteria related with a comparison between a radio condition with the macro eNB and a radio condition with one or more of the RRHs;

    selecting a preamble sequence from the selected sequence group or from the selected subgroup; and

    transmitting the selected preamble sequence to the network.
15. A user equipment (UE) performing a random access to a network comprising a macro eNB and one or more remote radio heads (RRHs), the UE comprising:

    a processor configured to select a sequence group among a plurality of predetermined sequence groups considering (a) a size of data to be transmitted from the UE, (b) a pathloss between the macro eNB and the UE, and (c) a metric related with at least one among the RRHs, and to select a preamble sequence from the selected sequence group; and

    a radio frequency (RF) unit coupled with the processor and configured to transmit the selected preamble sequence to the network.

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