Method and apparatus for obtaining identifier of small cell in wireless communication system having hierarchical cell structure

A method and an apparatus for obtaining a small cell identifier in a wireless communication system having a hierarchical cell structure are provided. The method includes obtaining a small cell identifier of a terminal in a wireless communication system of a hierarchical cell structure, transmitting resource information allocated for a Random Access Channel (RACH) of a terminal to a small base station, receiving state information regarding the RACH of the terminal from the small base station, determining at least one small base station adjacent to the terminal using the state information regarding the RACH, and transmitting cell IDentifier (ID) information of the determined small base station to the terminal.

PRIORITY

This application claims the benefit under 35 U.S.C. §119(a) of a Korean patent application filed in the Korean Intellectual Property Office on Jan. 10, 2011 and assigned Serial No. 10-2011-0002145 and a Korean patent application filed in the Korean Intellectual Property Office on Feb. 15, 2011 and assigned Serial No. 10-2011-0013146, the entire disclosure of each of which is hereby incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless communication system having a hierarchical cell structure. More particularly, the present invention relates to an apparatus and method for improving system performance in a wireless communication system of a hierarchical cell structure.

2. Description of the Related Art

A cellular communication system provides a wireless communication system of a hierarchical cell structure where base stations having different cell coverage coexist for swift communication of a terminal. That is, the cellular communication system provides a communication system where a small cell such as a pico cell and a femto cell, and a macro cell coexist.

The wireless communication system of a hierarchical cell structure according to the related art applies the same wireless communication standard to a macro cell and a small cell, and provides various transmission techniques for providing data to a low speed user and a high speed user.

FIG. 1illustrates a wireless communication system of a hierarchical cell structure according to the related art.

As illustrated inFIG. 1, the wireless communication system of the hierarchical cell structure may reduce a service shadow region generated in the case where only a macro base station100is used by allowing the macro base station100and a plurality of small base stations110and112to coexist, may extend a cell coverage region supportable by the macro base station100, and may improve a data transmission rate that can be provided per area.

In the wireless communication system of the hierarchical cell structure according to the related art, a terminal receives a service from one of the macro base station100and a small base station110. That is, when a first terminal120that receives a service from the macro base station100moves into the coverage region of the small base station110, the first terminal120performs a handover procedure from the macro base station100to the small base station110to disconnect a connection with the macro base station100and try a connection to the small base station110. However, when the first terminal120is moving at a high speed, since the first terminal120passes through the coverage region of the small base station110within a very short period of time, the first terminal120performs a handover procedure again. This frequent and unnecessary handover procedure increases the overhead of a system and reduces the performance of the system.

SUMMARY OF THE INVENTION

Aspects of the present invention are to address at least the above-mentioned problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention is to provide a method for improving a system performance in a wireless communication system of a hierarchical cell structure.

Another aspect of the present invention is to provide a method and an apparatus for allowing a macro base station to support a low speed user and a high speed user, and allowing a small base station to support a low speed user in a wireless communication system of a hierarchical cell structure.

Still another aspect of the present invention is to provide a method and an apparatus for allowing a terminal to always maintain a connection with a macro base station and to perform an additional connection with a neighbor small base station depending on a position and speed in a wireless communication system of a hierarchical cell structure.

Yet another aspect of the present invention is to provide a method and an apparatus for obtaining synchronization with a small cell under a circumstance where a small base station does not transmit a preamble in a wireless communication system of a hierarchical cell structure.

Further another aspect of the present invention is to provide a method and an apparatus for allowing a small base station to obtain synchronization using a preamble of a macro base station in a wireless communication system of a hierarchical cell structure.

Still another aspect of the present invention is to provide a method and an apparatus for obtaining an identifier of a small cell under a circumstance where a small base station does not transmit a preamble in a wireless communication system of a hierarchical cell structure.

Yet another aspect of the present invention is to provide a method and an apparatus for allowing a small base station to receive a Random Access Channel (RACH) signal transmitted from a macro terminal to a macro base station to report the same to the macro base station, thereby allowing the macro base station to transmit an identifier of a small base station adjacent to the macro terminal to a terminal in a wireless communication system of a hierarchical cell structure.

Yet still another aspect of the present invention is to provide a method and an apparatus for allowing a macro terminal to receive a common pilot signal of a small base station using an identifier of a neighbor small base station received from a macro base station, and determine a neighbor small base station through this received pilot signal in a wireless communication system of a hierarchical cell structure.

In accordance with an aspect of the present invention, a method of a macro base station, for obtaining a small cell identifier of a terminal in a wireless communication system of a hierarchical cell structure is provided. The method includes transmitting resource information allocated for an RACH of a terminal to a small base station, receiving state information regarding the RACH of the terminal from the small base station, determining at least one small base station adjacent to a relevant terminal using the state information regarding the RACH, and transmitting cell IDentifier (ID) information of the determined small base station to the relevant terminal.

In accordance with another aspect of the present invention, a method of a small base station for obtaining a small cell identifier of a terminal in a wireless communication system of a hierarchical cell structure is provided. The method includes receiving resource information allocated for an RACH of a terminal from a macro base station, receiving a signal of an RACH transmitted by the terminal to the macro base station using the received resource information, measuring state information regarding the RACH using a signal of the received RACH, and transmitting the measured state information regarding the RACH to the macro terminal.

In accordance with still another aspect of the present invention, a method of a terminal, for obtaining an identifier of a small cell in a wireless communication system having a hierarchical cell structure is provided. The method includes transmitting an RACH signal to a macro base station, receiving cell ID information of at least one neighbor small base station from the macro base station, and receiving a common pilot signal from at least one small base station using the received cell ID information.

In accordance with yet another aspect of the present invention, an apparatus of a macro base station, for obtaining a small cell identifier of a terminal in a wireless communication system having a hierarchical cell structure is provided. The apparatus includes a transceiver for transmitting/receiving a signal to/from at least one of a terminal and a small base station, and a controller for controlling to transmit resource information allocated for an RACH of a terminal to a small base station, receive state information regarding the RACH of the terminal from the small base station, determine at least one small base station adjacent to a relevant terminal using the state information regarding the RACH, and transmit cell ID information of the determined small base station to the relevant terminal.

In accordance with another aspect of the present invention, an apparatus of a small base station, for obtaining a small cell identifier of a terminal in a wireless communication system of a hierarchical cell structure is provided. The apparatus includes a transceiver for transmitting/receiving a signal to/from at least one of a terminal and a macro base station, and a controller for controlling to receive resource information allocated for an RACH of a terminal from the macro base station, receive a signal of an RACH transmitted by the terminal to the macro base station using the received resource information, measure state information regarding the RACH using a signal of the received RACH, and transmit the measured state information regarding the RACH to the macro terminal.

In accordance with a further aspect of the present invention, an apparatus of a terminal, for obtaining an identifier of a small cell in a wireless communication system having a hierarchical cell structure is provided. The apparatus includes a transceiver for transmitting/receiving a signal to/from at least one of a macro base station and a small base station, and a controller for controlling to transmit an RACH signal to the macro base station, receive cell ID information of at least one neighbor small base station from the macro base station, and receive a common pilot signal from at least one small base station using the received cell ID information.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Exemplary embodiments of the present invention provide a method and an apparatus for obtaining an identifier of a small cell under a circumstance where a small base station does not transmit a preamble in order to improve a system performance in a wireless communication system of a hierarchical cell structure.

Exemplary embodiments of the present invention provide a method and an apparatus for allowing a macro base station to support both low speed users and high speed users and allowing a small base station to support only low speed users in order to prevent high speed user equipment from performing an unnecessary handover frequently. In addition, an exemplary embodiment of the present invention allows a terminal to always perform an initial access on a macro cell, and maintain a connection to the macro cell, so that a small cell does not transmit a preamble. As described above, additional data is transmitted during a section in which the small cell does not transmit a preamble, so that a transmission rate may improve.

FIG. 2is a view illustrating a system operating method in a wireless communication system of a hierarchical cell structure according to an exemplary embodiment of the present invention.

Referring toFIG. 2, a second terminal MS-2230is a User Equipment (UE) moving at a high speed. The second terminal MS-2230performs an initial access on a macro base station200to receive a service. At this point, since the second terminal230is a user at a high speed, the second terminal230does not perform a handover to a second small base station212but maintains a connection with the macro base station200even when the second terminal230moves to the region of the second small base station212.

In contrast, a first terminal220is a user moving at a low speed. The first terminal220performs an initial access on the macro base station200to receive a service. At this point, since the first terminal220is a user at a low speed, when the first terminal220moves into the region of the small base station210, the first terminal220maintains a connection with the macro base station200and performs an additional connection with the small base station210. That is, the first terminal220receives a high speed data service from the small base station210and maintains a connection with the macro base station200.

As described above, all terminals perform an initial access on the macro base station200to perform a connection, and always maintain a connection with the macro base station200. That is, during the initial access, all the terminals receive a preamble signal transmitted from the macro base station200to obtain synchronization, and may obtain a cell identifier (ID) (and/or a sector ID). Therefore, an exemplary embodiment of the present invention assumes that the small base station210does not transmit a preamble signal for obtaining initial synchronization and obtaining a cell ID (and/or a sector ID). Since the small base station210may transmit other data during a section for transmitting the preamble via this transmission structure, a data transmission rate may improve.

However, since the small base station does not transmit a preamble signal, a need exists for a method for allowing the UE to obtain synchronization with respect to the small base station210and obtain a cell ID (and/or a sector ID) with respect to the small base station210in preparation for a handover circumstance of the UE.

In the following, it is assumed that a terminal performs an initial access to a macro base station to connect thereto. At this point, a method for obtaining synchronization of a neighbor small base station and obtaining a cell ID (and/or a sector ID) is described.

FIG. 3Ais a view illustrating a reception point of a terminal with respect to a transmission signal of a macro base station and a small base station in a wireless communication system of a hierarchical cell structure according to an exemplary embodiment of the present invention.

Referring toFIG. 3A, assuming that a macro base station300and a small base station310simultaneously transmit a signal using a Global Positioning System (GPS) signal, since the macro base station300has a radius of hundreds of meters to several kilometers, and the small base station310has a radius of tens of meters, a time difference may occur in receiving a signal of the macro base station300and a signal of the small base station310depending on the position of a terminal. For example, a terminal302positioned at a distance away from the macro base station300and relatively close to the small base station310receives a signal transmitted from the small base station310, and the terminal302receives a signal transmitted from the macro base station300after a specific time330.

However, since the small base station, according to an exemplary embodiment of the present invention, does not transmit a preamble, the terminal cannot obtain synchronization with respect to the small base station separately. Therefore, a technique for removing a reception point difference300of the terminal needs to be suggested. Furthermore, when the small base station is installed in an indoor space, the small base station does not use a GPS in many cases. Therefore, a technique for synchronization inside a system without using the GPS needs to be proposed. Hereinafter, an exemplary technique for obtaining synchronization of the small base station that does not transmit a preamble is described.

FIG. 3Bis a view illustrating a method for obtaining synchronization in a wireless communication system of a hierarchical cell structure according to an exemplary embodiment of the present invention, andFIG. 3Cis a view illustrating a reception point of a terminal with respect to a transmission signal of a macro base station and a small base station in a wireless communication system of a hierarchical cell structure according to an exemplary embodiment of the present invention.

Referring toFIG. 3B, a small base station310receives a preamble signal322transmitted from a macro base station300to synchronize with the macro base station300in time and a frame, and transmits a common pilot signal324to a terminal302positioned inside the region of the small base station according to the matched synchronization. That is, as illustrated inFIG. 3C, the small base station310may receive a preamble signal322transmitted from the macro base station301to obtain a first time offset320for controlling a transmission point so that the terminal302may receive a signal of the macro base station300and a signal of the small base station310at a similar point, and the small base station310may transmit a signal to the terminal302according to the obtained first time offset320. Here, the first time offset320is a value corresponding to a distance between the small base station and the macro base station, and may be determined using the received preamble. Through this, the small base station310and the terminal302may be approximately synchronized with each other.

Of course, since the time/frame of the small base station310and the terminal302are not accurately synchronized, the terminal302may correct a time/frame synchronization error using a reference signal transmitted from the small base station310in order to obtain accurate time/frame synchronization between the small base station310and the terminal302.

That is, the terminal302that has performed an initial access on the macro base station301to obtain synchronization receives a cell ID326of the small base station from the macro base station300to receive a common pilot signal324for the small base station, thereby obtaining a second time offset332for the small base station to obtain accurate synchronization. At this point, all small base stations synchronize with the macro base station via the above-described method and transmit a signal to a terminal according to the matched synchronization, so that a signal which the terminal has received from a plurality of small base stations may be received at a point similar to a point of a signal received from the macro base station, particularly, within a Cyclic Prefix (CP). By doing so, interference avoidance techniques between small base stations may be applied. At this point, the second time offset332may change depending on a cell radius of a small base station.

Here, even when the terminal is simultaneously connected with the macro base station and the small base station, since it is not realistic that the macro base station and the small base station have different time synchronization, it is assumed that they have the same time and the same frame.

FIG. 4Ais a view illustrating a method for allowing a macro terminal to obtain a cell identifier of a small base station in a wireless communication system of a hierarchical cell structure according to an exemplary embodiment of the present invention.

Referring toFIG. 4A, first, a macro base station400shares resource information for a Random Access Channel (RACH)430of a terminal allocated by the macro base station with small base stations410,412,414positioned in a neighborhood. Here, the resource information includes a point at which the terminal transmits the RACH and sequence code information of the RACH used by the terminal. At this point, the macro base station400and each of the small base stations410,412,414transmit and receive a signal via a wired network.

The terminal420connected to the macro base station400periodically transmits an RACH signal432to the macro base station400to obtain synchronization and fine-control the obtained synchronization. At this point, the small base stations410and412may receive an RACH signal which the terminal420transmits to the macro base station400using resource information of an RACH obtained from the macro base station400. Of course, at this point, a third small base station414distant from the terminal420cannot receive an RACH signal of the terminal420.

The small base stations410and412that have received the RACH signal measure state information of the RACH, and reports the measured channel state information to the macro base station400.

Then, the macro base station400determines small base stations adjacent to the terminal420based on the state information of the RACH reported from the small base stations410and412, and transmits a neighbor base station list including a cell ID (and/or a sector ID) of neighbor small base stations.

Through this method, the terminal420may obtain cell ID (and/or sector ID) information434of neighbor small base stations, and perform a handover on the neighbor small base stations through this.

However, the neighbor base station list received from the macro base station400may include invalid cell ID (and/or sector ID) information434. Therefore, the terminal420needs to receive the neighbor base station list from the macro base station400, and then determine whether small base stations included in the list is a base station actually neighboring the terminal420. The description thereof is described below in detail with reference toFIG. 4B.

FIG. 4Bis a view illustrating a method for allowing a macro terminal to determine a neighbor small base station in a wireless communication system of a hierarchical cell structure according to an exemplary embodiment of the present invention.

Referring toFIG. 4B, the terminal420is positioned at a region adjacent to the first small base station410and the second small base station412compared to the macro base station400. Therefore, the first small base station410and the second small base station412may receive an RACH signal which the terminal420periodically transmits to the macro base station400, and report state information for the RACH to the macro base station400.

Then, the macro base station400determines a small base station adjacent to the terminal420according to a preset method based on the reported channel state information. At this point, since the first small base station410and the second small base station412are positioned relatively close to the terminal420compared to the macro base station400, a channel state reported by the first small base station410and a channel state reported by the second small base station412may be better than a channel state which the macro base station400has measured with respect to the terminal420. Accordingly, the macro base station400may determine the first small base station410and the second small base station412as small base stations adjacent to the terminal400, and transmit a neighbor base station list including a cell ID (and/or sector ID) to the terminal400.

However, since a small base station uses low power, the terminal420may be positioned at the coverage region of the first small cell base station410and may not be positioned at the coverage region of the second small cell base station412. Therefore, even though the terminal420cannot receive service from the second small base station412, the terminal420attempts handover to the second small base station412according to the neighbor base station list received from the macro base station400and the handover fails.

To address this problem, in an exemplary implementation, the terminal420receives a neighbor base station list from the macro base station400, receives a common pilot signal from respective small neighbor base stations included in the neighbor base station list, and determines whether downlink data can be received from each small base station, thereby allowing the terminal420to finally determine a neighbor small base station for itself.

A method and an apparatus for allowing a terminal to obtain synchronization and a cell identifier of a small base station, and perform a handover from a macro base station to the small base station are described in more detail with reference toFIGS. 3 and 4.

FIG. 5is a view illustrating a signal flow in which a macro terminal obtains synchronization with respect to a small base station and a cell identifier to perform a handover in a wireless communication system of a hierarchical cell structure according to an exemplary embodiment of the present invention. Here, though one small base station has been exemplarily described for convenience in description, the same method is applicable to a plurality of small base stations.

Referring toFIG. 5, first, a macro base station504transmits a preamble every preset period in step505. At this point, a small base station502suspends providing a service to terminals connected to the small base station and receives a preamble of the macro base station504during a preset listening interval509.

The small base station502obtains time/frame synchronization with the macro base station504using the received preamble in step507. That is, the small base station502determines a time offset that may occur due to a difference in a distance from the macro base station and determines a signal transmission point for a terminal existing inside a region of the small base station502according to the time offset. At this point, the time offset may be determined depending on a distance between the macro base station and the small base station, so that small base stations whose distance from the macro base station is different may have different time offsets. As described above, the small base station502synchronizes in time/frame with the macro base station504and then transmits a signal to a terminal500inside the region of itself, thereby achieving approximate time/frame synchronization between the small base station502and the terminal500.

Meanwhile, the macro base station504transmits resource information which the macro base station504allocated for an RACH of a terminal to a small base station502positioned in the neighborhood in step510. That is, the macro base station504transmits resource information including an RACH transmission point of the terminal and sequence code information of an RACH used by the terminal to the small base station502. At this point, the macro base station504may transmit resource information allocated for the RACH to the small base station502via a wired network connected with the small base station502, and may transmit the resource information allocated for the RACH to the small base station502via a wireless network depending on system settings and an operation method.

Meanwhile, the terminal200performs an initial access procedure on the macro base station504to connect with the macro base station504in step512, and periodically transmits an RACH signal to the macro base station504in order to obtain synchronization in step514. At this point, the macro base station504receives an RACH signal transmitted from the terminal500. According to an exemplary embodiment of the present invention, the small base station502receives an RACH signal transmitted from the terminal500. That is, the small base station502may receive an RACH signal which the terminal500transmits to the macro base station504using resource information received from the macro base station504in step510.

The small base station502that has received the RACH signal of the terminal500measures state information for the RACH and reports the measured channel state information to the macro base station504in step516. That is, the small base station502may measure signal reception strength or a Signal-to-Noise Ratio (SNR) for the RACH, and report the same to the macro base station504. Here, the signal reception strength or the SNR for the RACH measured by the small base station502may be an index representing a relative distance between the terminal500corresponding to the RACH and the small base station502. At this point, the small base station502may transmit the measured channel state information to the macro base station504via a wired network connected to the macro base station504, and may transmit the measured channel state information to the macro base station504via a wireless network depending on system settings and an operation method.

The macro base station504determines small base stations adjacent to the terminal500based on the channel state information reported from the small base station502in step518. Here, the macro base station504receives channel state information from each of a plurality of small base stations, compares the received channel state information of each small base station with a threshold, and may determine small base stations whose channel state information is equal to or greater than the threshold as small base stations adjacent to the terminal500. In addition, the macro base station504receives channel state information from each of the plurality of small base stations, measures state information for an RACH of the terminal500, compares the received channel state information of each small base station with the channel state information measured by the macro base station504, and may determine small base stations whose channel state information is equal to or greater than the channel state information measured by the macro base station504as small base stations adjacent to the terminal502. Here, a description is made based on the assumption that the small base station502is determined as a small base station adjacent to the terminal.

When the small base stations adjacent to the terminal500are determined, the macro base station generates a candidate neighbor base station list including cell IDs (and/or sector IDs) of the determined small base stations, and proceeds to step520to transmit the candidate neighbor base station list to the terminal500.

Meanwhile, the small base station that has reported the channel state information to the macro base station504in step514periodically transmits a common pilot signal in step522. Here, a common pilot signal transmission point of the small base station502may be determined based on a preamble reception point of the macro base station504.

The terminal500receives a candidate neighbor base station list from the macro base station in step520, and receives a common pilot signal of the small base station502included in the candidate neighbor base station list in step522. Here, the terminal500may receive a common pilot signal of respective small base stations using cell ID (and/or sector ID) information of respective small base stations included in the candidate neighbor base station list. At this point, since a common pilot signal transmitted by all base stations is specialized for a cell ID of a relevant base station and transmitted, the terminal500may receive cell IDs of neighbor small base stations from the macro base station504, and then receive a common pilot signal transmitted from the small base station502.

The terminal500that has received the common pilot signal of the small base station502measures reception strength or an SNR of the common pilot signal, and compares the measured value with a threshold to determine whether it can receive downlink data from the relevant small base station502in step524. Here, when the reception strength or the SNR of the common pilot signal received from a specific small base station is equal to or greater than the threshold, the terminal500determines that it can receive downlink data from the specific small base station. In contrast, when the reception strength or the SNR of the common pilot signal received from the specific small base station is less than the threshold, the terminal500may determine that it cannot receive downlink data from the specific small base station in step526.

The terminal500proceeds to step528to determine small base stations from which downlink data can be received as handover candidate neighbor base stations of the terminal500, and determine cell IDs (and/or sector IDs) of the determined handover candidate neighbor base stations from a handover candidate neighbor base station list received in step520.

Thereafter, the terminal500determines whether a circumstance needing a handover to a small base station determined as a neighbor base station of the terminal500occurs, and when it is determined to be the circumstance needing the handover, the terminal500transmits a handover request message for a neighbor small base station to the macro base station504in step530. Here, to determine whether the circumstance needing the handover occurs, conventional well known methods may be used.

Thereafter, the macro base station504that has received the handover request message transmits a handover request message to the neighbor small base station serving as a handover target in step532, and the small base station502transmits a handover response message representing that a handover is possible to the macro base station504in step534. Then, the macro base station transmits a handover command message commanding a handover to the neighbor small base station502to the terminal500in step536. The terminal proceeds to step538to precisely correct a time/frame synchronization error for the small base station based on a common pilot signal received from the small base station502to obtain accurate time/frame synchronization. Thereafter, the terminal500and the small base station502perform a handover according to the obtained time/frame synchronization. Of course, at this point, the terminal500may maintain a connection with the macro base station504.

In the above description, step505to step509are procedures for obtaining synchronization for a small base station, and are not essential elements of the present invention.

FIG. 6is a flowchart illustrating an operation of a macro base station in a wireless communication system of a hierarchical cell structure according to an exemplary embodiment of the present invention.

Referring toFIG. 6, the macro base station transmits resource information allocated for an RACH of a terminal to a small base station positioned in the neighborhood in step600. At this point, the macro base station may transmit resource information allocated for the RACH to the small base station via a wired network connected with the small base station, and may transmit the resource information allocated for the RACH to the small base station via a wireless network depending on system settings and an operation method.

The macro base station determines whether channel state information for the RACH of the terminal is received from the small base station in step602. When the channel state information for the RACH of the terminal is received from the small base station, the macro base station proceeds to step604and selects a neighbor small base station of the terminal corresponding to the RACH based on the channel state information for the RACH received from the small base station. Here, the macro base station compares channel state information for an RACH received from each of a plurality of small base stations with a preset threshold, or compares the channel state information with channel state information measured by the macro base station. The macro base station may determine a small base station adjacent to the relevant terminal depending on the comparison result. Here, the channel state information measured by the macro base station denotes signal reception strength or an SNR of the macro base station with respect to an RACH of the terminal measured by small base stations.

The macro base station transmits a candidate neighbor base station list including selected neighbor small base stations to the relevant terminal in step606.

The macro base station proceeds to step608and determines whether a handover request message to a neighbor small base station is received from the terminal. When the handover request message is not received, the macro base station returns to step602to re-perform subsequent steps.

In contrast, when the handover request message is received, the macro base station proceeds to step610and transmits a handover request message to a small base station serving as a handover target of the terminal, receives a handover response message from the small base station in step612, and transmits a handover command message to the relevant terminal in step614. Thereafter, the macro base station returns to step602and performs the subsequent steps.

FIG. 7is a flowchart illustrating an operation procedure of a macro terminal in a wireless communication system of a hierarchical cell structure according to an exemplary embodiment of the present invention.

Referring toFIG. 7, the terminal transmits an RACH signal to a macro base station every preset period in order to obtain synchronization and fine-control synchronization with the macro base station that is currently being connected in step700.

The terminal receives a candidate neighbor base station list representing neighbor small base stations from the macro base station in step702, proceeds to step704to receive a common pilot signal of relevant small base stations using cell IDs (and/or sector IDs) of small base stations included in the received candidate neighbor base station list.

The terminal measures reception strength or an SNR of the common pilot signal received from the small base stations in step706, and finally determines a small base station adjacent to the terminal using the measured reception strength or SNR of the common pilot signal in step708. That is, the terminal compares the measured reception strength or SNR of the common pilot signal with a preset threshold to determine whether it can receive downlink data from the relevant small base station depending on the comparison result, and determines small base stations from which downlink data can be received as small base stations adjacent to the terminal.

The terminal determines whether a circumstance needing a handover to one of the finally determined small base stations occurs in step710. When the handover is not needed, the terminal returns to step704. When the handover is needed, the terminal proceeds to step712to transmit a handover request message for a neighbor small base station to the macro base station, and receive a handover command message from the macro base station in step714.

The terminal proceeds to step716and precisely corrects a time/frame synchronization error based on a common pilot signal of a small base station received in step704to obtain accurate time/frame synchronization. The terminal performs a handover to the relevant small base station according to the obtained time/frame synchronization in step718. The algorithm according to an exemplary embodiment of the present invention ends. At this point, the terminal may continue to maintain a connection with the macro base station.

FIG. 8is a flowchart illustrating an operation procedure of a small base station in a wireless communication system of a hierarchical cell structure according to an exemplary embodiment of the present invention.

Referring toFIG. 8, the small base station determines whether a section is a listening interval set in advance for receiving a preamble of a macro base station in step800. The listening interval denotes an interval for temporally suspending service providing for terminals connected to the small base station, and receiving a preamble of the macro base station. The reception of the preamble may be performed only one time initially or repeatedly performed every preset period depending on a company or a designer.

When a section is the listening interval, the small base station receives a preamble signal from the macro base station in step802, and obtains time/frame synchronization for the macro base station using the received preamble in step804. Through this, the small base station may approximately match time/frame synchronization with terminals inside the small base station. Here, the small base station predicts a time for a transmission frame of the macro base station via the preamble signal, obtains a time offset value depending on a difference between a distance from the macro base station using a transmission time and a reception time of the preamble signal, and applies the time offset value to the predicted transmission frame time to determine a transmission time of the small base station.

The small base station receives resource information allocated for an RACH of the terminal by the macro base station from the macro base station in step806. At this point, the small base station may receive the resource information via a wired network or a wireless network.

The small base station receives a signal of an RACH which a neighbor terminal transmits to the macro base station using resource information for an RACH of a terminal allocated by the macro base station in step808, and proceeds to step810to measure channel state information for the RACH using the received signal, and transmit the measured channel state information to the macro base station in step812. That is, the small base station measures reception strength and an SNR of a received signal via the RACH and reports the same to the macro base station.

The small base station transmits a common pilot signal in accordance with time/frame synchronization obtained for the macro base station in step814. Here, a transmission time of the common pilot signal of the small base station is determined depending on a point of receiving a preamble from the macro base station as described in step802.

The small base station determines whether a handover request message is received from the macro base station in step816. When the handover request message is not received, the small base station returns to step808and performs the subsequent steps.

In contrast, when the handover request message is received, the small base station proceeds to step818to transmit a handover response message to the macro base station, perform a handover procedure for a relevant terminal in step820, and returns to step808and perform the subsequent steps.

FIG. 9is a block diagram illustrating a macro base station in a wireless communication system of a hierarchical cell structure according to an exemplary embodiment of the present invention.

Referring toFIG. 9, the macro base station includes a transceiver900, a controller910, and a storage unit920. The controller910includes a neighbor base station determining unit912.

The transceiver900transmits/receives a signal to/from a terminal inside a coverage under control of the controller910, and transmits/receives a signal to/from a small base station via a wired line or wirelessly.

The controller910controls and processes an overall operation of the macro base station, and controls and processes an operation for a handover of the terminal. More particularly, the controller910includes the neighbor base station determining unit912to control and process a function for determining small base stations adjacent to the terminal and transmitting information regarding the adjacent small base stations to the terminal in order to allow the terminal to perform a handover to the small base station.

The neighbor base station determining unit912controls a function for transmitting resource information allocated for the RACH of the terminal to a small base station positioned in the neighborhood. When channel state information for an RACH of a specific terminal received from a small base station is received, the neighbor base station determining unit912controls and processes a function for determining a small base station adjacent to the specific terminal based on the received channel state information, generating a candidate neighbor base station list including information of the determined small base stations, and transmitting the list to the specific terminal. More particularly, the neighbor base station determining unit912controls and processes a function for incorporating cell IDs (and/or sector IDs) of the determined small base stations into the candidate neighbor base station list and transmitting the same.

The storage unit920stores various programs for an overall operation of the macro base station, and data. More particularly, the storage unit920stores cell ID (and/or sector ID) information of neighbor small base stations.

FIG. 10is a block diagram illustrating a macro terminal in a wireless communication system of a hierarchical cell structure according to an exemplary embodiment of the present invention.

Referring toFIG. 10, the terminal includes a transceiver1000, a controller1010, and a storage unit1020. The controller1010includes a neighbor base station determining unit1012, a handover controller1014, and a synchronization obtain unit1016.

The transceiver1000performs a function for transmitting/receiving a signal to/from a base station under control of the controller1010. More particularly, the transceiver1000receives a candidate neighbor base station list including a cell ID of a neighbor small base station from a macro base station and transmits the list to the controller1010under control of the controller1010, and receives a common pilot signal from a small base station and provides the same to the controller1010.

The controller1010controls and processes an overall operation of the terminal, and controls and processes a function for transmitting a signal via an RACH in order to obtain synchronization with a macro base station and fine-control the obtained synchronization. More particularly, when the candidate neighbor base station list is received from the macro base station via the neighbor base station determining unit1012, the controller1010controls and processes a function for obtaining cell IDs (and/or sector IDs) of small base stations included in the candidate neighbor base station list, receiving a common pilot signal from relevant small base stations to measure reception strength or an SNR of the common pilot signal, and determining a small base station adjacent to the terminal. In addition, the controller1010controls and processes a function for determining whether to perform a handover to the determined small base station via the handover controller1014and performing a handover to the neighbor small base station.

Additionally, the controller1010includes the synchronization obtain unit1016to control and process a function for obtaining time/frame synchronization using a preamble received from the macro base station when initially accessing the macro base station, and controls and processes a function for obtaining time/frame synchronization for a small base station depending on the speed and the position of the terminal. That is, when receiving a cell ID for a neighbor small base station from the macro base station while the terminal moves in low speed, the synchronization obtain unit1016controls a function for receiving a common pilot signal transmitted from a small base station corresponding to the received cell ID, and controls and processes a function for obtaining time/frame synchronization for a relevant small base station based on the received common pilot signal. That is, the synchronization obtain unit1016determines a fine synchronization error using the common pilot signal based on the time/frame synchronization obtained for the macro base station, and controls and processes a function for obtaining time/frame synchronization for the small base station based on the determined fine synchronization error.

The storage unit1020stores various programs for an overall operation of the terminal and data. More particularly, the storage unit1020stores cell ID (and/or sector ID) information of neighbor small base stations determined in the neighbor base station determining unit1012.

FIG. 11is a block diagram illustrating a small base station in a wireless communication system of a hierarchical cell structure according to an exemplary embodiment of the present invention.

Referring toFIG. 11, the small base station includes a transceiver1100, a controller1110, and a storage unit1120. More particularly, the controller1110includes a channel information measure unit1112and a synchronization obtain unit1114.

The transceiver1100transmits/receives a signal to/from a terminal inside a coverage under control of the controller1110, and transmits/receives a signal to/from a macro base station via a wired line or wirelessly. In addition, during a listening interval set in advance, the transceiver1100receives a preamble of the macro base station to provide the same to the controller1110under control of the controller1110, and transmits a common pilot signal under control of the controller1110. The controller1110controls and processes an overall operation of the small base station, and controls and processes an operation for a handover of the terminal. More particularly, the controller1110includes the channel information measure unit1112to control and process a function for receiving a signal of an RACH which a neighbor terminal receiving a service from a macro base station transmits to the macro base station to measure channel state information, and transmitting the same to the macro base station. Here, the channel information measure unit1112may receive a signal of an RACH which the terminal transmits to the macro base station by receiving resource information allocated for the RACH in advance from the macro base station.

Additionally, the controller1110includes the synchronization obtain unit1114to perform a function for time/frame synchronization with the macro base station using a preamble of the macro base station. After the time/frame synchronization with the macro base station, the controller1110controls and processes a function for transmitting/receiving a signal according to the time/frame synchronization. More particularly, the controller1110controls and processes a function for transmitting a common pilot signal.

The storage unit1120stores various programs for an overall operation of the small base station and data. More particularly, the storage unit1120stores resource information for an RACH allocated by the macro base station.

According to an exemplary embodiment of the present invention, in a wireless communication system of a hierarchical cell structure, a macro base station supports high speed and low speed users, and a small base station supports a low speed user, so that frequent occurrence of a handover may be prevented. A small base station does not transmit a preamble to improve a data transmission rate of the small base station. In addition, a small base station obtains synchronization using a preamble of a macro base station, so that a terminal may receive a signal from small base stations within a Cyclic Prefix (CP) to avoid interference between small cells. In addition, in a wireless communication system of a hierarchical cell structure, a small base station receives a random access channel signal which a macro terminal transmits to a macro base station to report the random access channel signal to the macro base station, and the macro base station receives an identifier of a small base station adjacent to the macro terminal to a terminal, so that the macro terminal may obtain a cell ID of the small base station even under a circumstance where the small base station does not transmit a preamble.