NETWORK BACKGROUND OF THE INVENTION

Field of the Invention

[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. Discussion of the Related Art

[0002] First of all, 3GPP LTE (3 ^rd 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 LI (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 LI (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 an radio resource overhead for the RACH and enhance the efficiency of the random access.

SUMMARY OF THE INVENTION

[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: receiving system information comprising random access channel (RACH) configuration information from the network, wherein the RACH configuration information informs the UE of a first RACH configuration index among predetermined multiple RACH configuration indexes; selecting a second RACH configuration index based on a comparison between radio links with the macro eNB and with the RRHs; and transmitting a random access sequence based on the second RACH configuration index is proposed.

[0025] The second RACH configuration index may be for a RACH format having a shorter time length compared to a RACH format for the first RACH configuration index.

[0026] Preferably, selecting the second RACH configuration index may be performed, if a RACH format for the first RACH configuration index is not a RACH format having a shortest time length among all of predetermined RACH formats.

[0027] The first RACH configuration index may be for the macro eNB while the second RACH configuration index is for one of the RRHs.

[0028] Preferably, the first and the second RACH configuration indexes may inform the UE of a same subframe number.

[0029] Preferably, the comparison can be based on a first type pathloss between the macro eNB and the UE, and one or more second type pathloss between each of the RRHs and the UE. The second RACH configuration index may be selected, if one of the second type pathloss is less than the first type pathloss.

[0030] Preferably, a transmission power of the random access sequence can be adjusted according to the second RACH configuration index. The transmission power of the random access sequence can be adjusted using power related parameters for one of the RRHs corresponding to the second RACH configuration index. The power related parameters may comprise a power margin and an offset value for the one of the RRHs.

[0031] 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 radio frequency (RF) unit configured to receive system information comprising random access channel (RACH) configuration information from the network, wherein the RACH configuration information informs the UE of a first RACH configuration index among predetermined multiple RACH configuration indexes; and a processor coupled with the RF unit and configured to select a second RACH configuration index based on a comparison between radio links with the macro eNB and with the RRHs, wherein the processor controls the RF unit to transmit a random access sequence based on the second RACH configuration index is proposed.

[0032] The second RACH configuration index may be for a RACH format having a shorter time length compared to a RACH format for the first RACH configuration index.

[0033] The processor may be further configured to perform selecting the second

RACH configuration index, if a RACH format for the first RACH configuration index is not a RACH format having a shortest time length among all of predetermined RACH formats.

[0034] The first RACH configuration index may be for the macro eNB while the second RACH configuration index is for one of the RRHs.

[0035] Preferably, the first and the second RACH configuration indexes may inform the UE of a same subframe number.

[0036] Preferably, the processor may perform the comparison based on a first type pathloss between the macro eNB and the UE, and one or more second type pathloss between each of the RRHs and the UE. The processor may select the second RACH configuration index, if one of the second type pathloss is less than the first type pathloss.

[0037] Preferably, the processor may be further configured to adjust a transmission power of the random access sequence according to the second RACH configuration index. The processor may adjust the transmission power of the random access sequence using power related parameters for one of the RRHs corresponding to the second RACH configuration index. The power related parameters may comprise a power margin and an offset value for the one of the RRHs.

[0038] 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

[0039] 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:

[0040] FIG. 1 is a schematic diagram of E-UMTS network structure as an example of a mobile communication system;

[0041] 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;

[0042] FIGs. 4 and 5 are procedural diagrams illustrating Contention-based Random Access Procedure and Non-Contention-based Random Access Procedure;

[0043] Fig. 6 shows a random access preamble format; [0044] Fig. 7 shows RACH TTI allocation scheme;

[0045] Fig. 8 is for illustrating the enhanced random access scheme according to a preferred embodiment of the invention; and

[0046] Fig. 9 shows apparatuses for implementing the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0047] 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 3 GPP LTE.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] (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);

[0053] (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;

[0054] (3) A UE in RRC CONNECTED state, handing over from its current serving cell to a target cell;

[0055] (4) A transition from RRC JDLE state to RRC CONNECTED, for example for initial access or tracking area updates;

[0056] (5) Recovering from radio link failure.

[0057] 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.

[0058] 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. [0059] 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.

[0060] The two procedures are outlined in the following.

[0061] FIGs. 4 and 5 are procedural diagrams illustrating Contention-based Random Access Procedure and Non-Contention-based Random Access Procedure.

[0062] 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).

[0063] 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.

[0064] Regarding the above procedures, a preamble transmission (message 1) is more specifically explained.

[0065] Fig. 6 shows a random access preamble format.

[0066] The preamble for the RACH consists of a cyclic prefix of length ^CP and a sequence part of length ^SEQ · The length of the LTE RACH preamble sequence was designed to satisfy coverage requirement. High cell coverage requires a longer CP (cyclic prefix) and GT (guard time) in order to absorb the corresponding round-trip delay. LTE Rel8 defines 4 PRACH formats as following:

[0067] [Table 1]

[0068] Format 0: 1TTI (1 ms) burst for small-medium cells (up to ~ 14 km)

[0069] Format 1 : 2TTI (2 ms) burst for large cells (up to ~77 km)

[0070] Format 2: 2TTI (2 ms) burst for medium cells (up to ~29 km)

[0071] Format 3: 3TTI (3 ms) burst for very large cells (up to ~ 100 km)

[0072] Fig. 7 shows RACH TTI allocation scheme.

[0073] The number of TTIs (Transmission Time Intervals) for RACH preamble transmission is decided by eNB according to the cell coverage requirement. Due to round trip delay and delay spread of signal, the UEs at cell edge require more TTIs for timing detection than those close to the eNB. Therefore the LTE reference scheme for all UEs within the cell is to satisfy the cell border UEs as shown in the right side of Fig 7.

[0074] For this end, the network transmits/broadcasts system information comprising random access channel (RACH) configuration information to UEs within a cell. The RACH configuration information informs each of the UEs of a RACH configuration index among predetermined multiple RACH configuration indexes. Table 2 shows these multiple RACH configuration.

[0075] [Table 2]

[0076] Table 2 shows the predetermined mapping relationship among RACH configuration index, preamble format, system frame number, and subframe number. The preamble formats are as shown in table 1. System frame number and subframe number indicate the resources allocated to the random access within the cell.

[0077] However, more or less TTI would be required for timing detection regarding the UE radio condition and location, i.e. the UE path loss. Therefore, the RACH enhancement design is proposed here with the goal to reduce interference in a cell with the minimum required RACH overhead and also to save the UE's battery power.

[0078] The UEs are differentiated by using different access scheme. The main focus is put on the UEs' different random access requirement according to his radio conditions and path loss. According to a preferred embodiment of the invention, prior to perform the random access, a UE determines itself which random access scheme is better suited to his situation. For example when Macro eNB (TPO) signals the use of any PRACH formats from 1 to 3, the UE would estimate itself whether the use of other format (e.g. format 0) which can fit into a minimum TTI allocation is allowed based on his radio condition and pathloss.

[0079] This approach should receive significant attention, on heterogeneous networks scenarios consisting of a macro eNodeB and oremote radio heads (RRHs) as shown in figure 8.

[0080] Fig. 8 is for illustrating the enhanced random access scheme according to a preferred embodiment of the invention.

[0081] As shown in Fig. 8, the heterogeneous network can comprises a Macro eNB (as shown as a transmission point 0 (TP 0) in Fig. 8) and one or more RRHs (as shown as transmission points 1 and 2 (TP 1 and TP 2). Application of a RRH is effective in coordinating geographically separate cells with negligible coordination latency. RRH can be any of Femto base station, Relay Station, etc.

[0082] Besides the well understood setup in which macro eNB 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. According to an embodiment of the present invention, the UE would perform the random access with targeting its closest transmission point with minimal required access resources (e.g. RACH TTI allocation, RACH transmission power, preamble length). This would reduce interference in a cell and also save the UE's battery power.

[0083] Specifically, when UEs 0-2 receive system information comprising RACH configuration information from Macro eNB, and it informs the UEs of a first RACH configuration index among predetermined multiple RACH configuration indexes, each of the UEs does not just transmit a random access preamble sequence based on the first RACH configuration index. Rather, each of the UEs of the present embodiment compares radio links with the macro eNB and with the RRHs. The comparison can be based on a first type pathloss between the macro eNB and the UE, and one or more second type pathloss between each of the RRHs and the UE. When one of the second type pathloss is less than the first type pathloss, UE can select another RACH configuration index (second RACH configuration index) from Table 2. For example, UE 1 and UE 2 in Figure 8 may select second RACH configuration index rather than the first RACH configuration index, since the closest TP for these UEs is not a macro eNB. When UE 1 or 2 determines that the pathloss between TP 1 or 2 is less than the pathloss between the macro eNB, UE 1 or 2 may select second RACH configuration index from table 2.

[0084] Preferably, the second RACH configuration index is for a RACH format having a shorter time length compared to a RACH format for the first RACH configuration index. For example, if the system information informs UE 1 of the RACH configuration index of 32 in table 2, and if the UE 1 determines that pathloss between the TP 1 is the least significant, the UE 1 may select the RACH configuration index of 5 in table 2. As can be seen, the RACH configuration index 5 indicates preamble format 0 which takes 1 TTI and the RACH configuration index 32 indicates preamble format 2 which takes 2 TTIs.

[0085] For this purpose, it is preferable to configure such that selecting the second RACH configuration index is performed, if a RACH format for the first RACH configuration index is not a RACH format having a shortest time length among all of predetermined RACH formats. As shown in table 2, RACH configuration indexes 0 to 15 indicate the same preamble format 0. Thus, one embodiment of the present invention proposes to perform selecting the second RACH configuration index only when the received system information does not indicates RACH configuration indexes 0 to 15.

[0086] The selection would be triggered by the UE selection of preambles group A. Indeed Rel.8, contention based access splits RACH preambles into two groups (A and B). Their selection is used to indicate information on size of Msg3 and the requested resource blocks. 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. If the UE selects preambles group (A) and/or the PRACH config index >15 (note: the preamble format 0 is signaled by the eNB by the PRACH config index range from 0 to 15), the UE may trigger the enhanced access procedure by using different access scheme.

[0087] One embodiment of the present invention proposes to select the second RACH configuration index among the RACH configuration indexes for the same subframe number indicated by the first RACH configuration index. When the network (macro eNB) transmits/broadcasts system information indicating the first RACH configuration index mapped to a specific subframe number, it is probable that the network expect the UEs to transmit the random access preamble at that specific subframe. Therefore, as exemplified above, when UE 1 receives system information indicating the RACH configuration index of 32 in table 2, the UE 1 shall select the RACH configuration index mapped to the subframe number 7. In the above example, UE 1 selected the RACH configuration index of 5 in table 2 mapped to the subframe number 7.

[0088] For the similar rationale, it is preferable to select the second RACH configuration index among those for the same system frame number of the first RACH configuration index.

[0089] For another embodiment of the present invention, the UE may select the second RACH configuration index based on broadcast information from TPs that allow the connection to the closest transmission point. Rather than based on the comparison of the pathlosses, UE 1 can acquire information that the TP 1 is the closest TP and/or RACH overhead can be reduced by accessing TP 1 rather than macro eNB.

[0090] Based on the above embodiments of the invention, one preferred embodiment proposes adjusting a transmission power of the random access sequence according to the second RACH configuration index.

[0091] When the use of minimum TTI allocation is determined by the UE, there is a need to further adjust the preamble transmission power. A preamble transmission power PPRACH is determined as in Equation 1 .

[0092] [Equation 1 ]

PPRACH = min{ JCMAX _> PREAMBLE RECEIVED TARGET POWER + PL }_[dBm], [0093] Where: [0094] ^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)

[0095] 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.

[0096] [Equation 2]

PREAMBLE_RECEIVED_TARGET_POWER=preambleInitialReceivedTar getPower+ DELTA PREAMBLE

[0097] With :

[0098] preamblelnitialReceivedTargetPower signalled by the eNB

[0099] DELTA_PREAMBLE is the preamble format based power offset values specified for each preamble format as follows

[00100] Format 0: O dB

[00101] Format 1 : O dB

[00102] Format 2 : -3 dB

[00103] Format 3 : -3 dB

[00104] 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 TPl 's PL (i.e. TPl 's max. transmission power) and TPl 's preamblelnitialReceivedTargetPower. 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 [00105] [Equation 3]

PL = min{PL _eNB , PL _RRH o, PLRRHI , PLRRH3 ,· · · }_[dBm],

[00106] According to the present embodiment, preamblelnitialReceivedTargetPower can be the target received preamble power at the RRH. Additional adjustment to the DELTA PREAMBLE based on the power offset { offset ) values can be also included.

[00107] Based on the above explanation, apparatus for the present invention is explained.

[00108] Fig. 9 shows apparatuses for implementing the present invention.

[00109] As shown in Fig. 9, 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 1 1 , a memory 12, and a radio frequency (RF) unit 13. The processor 1 1 may be configured to implement proposed procedures and/or methods described in this document. The memory 12 is coupled with the processor 1 1 and stores a variety of information to operate the processor 1 1. The RF unit 13 is coupled with the processor 1 1 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.

[00110] 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.

[00111] The above-described enhanced random access technology and apparatus are explained mainly with reference to the example that they are applied to the 3 GPP 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.