Abstract:
A communication system and method for supporting direct communication between a femto base station and a macro base station are provided. The communication system includes at least one macro base station which allocates a resource to one or more user equipments. The communication system also includes at least one femto base station which operates in a base station mode for serving terminals located within a service area of the at least one femto base station and operates in a user equipment mode for communicating with the at least one macro base station using the resource allocated by the at least one macro base station.

Description:
CLAIMS OF PRIORITY 
       [0001]    This application claims priority under 35 U.S.C. §119(a) to an application entitled “COMMUNICATION SYSTEM AND METHOD FOR SUPPORTING DIRECT COMMUNICATION BETWEEN FEMTO CELL AND MACROCELL” filed in the Korean Intellectual Property Office on Apr. 25, 2008 and assigned Serial No. 10-2008-0038694, the contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates generally to a communication system and method, and more particularly, to a communication system and method for supporting direct communication between a femto base station and a macro base station. 
         [0004]    2. Description of the Related Art 
         [0005]    A femtocell is an indoor cellular base station characterized by a small size and a low power capacity for use in a residential or a small business environment. The term “femtocell” is used to describe a more evolved cell, although it may also be referred to as a picocell. The femtocell is a small base station which connects to a broadband router and bridges the conventional 2 nd  Generation (2G) and 3rd Generation (3G) voice and data traffic to a mobile operator&#39;s backbone network using a residential DSL. 
         [0006]    3 rd  Generation Partnership Project Long Term Evolution (3GPP LTE) describes a base station part that includes an evolved Node B (eNB) and a home eNB. The eNB is responsible for managing an ordinary macrocell, and the home eNB is responsible for managing a femtocell. Within a mobile communication system composed of eNBs and home eNBs, the eNBs are connected to the core network by means of an S1 interface. More particularly, the eNBs (not including the home eNBs) are interconnected with each other by means of an X2 interface. The home eNBs are connected to the core network through an Internet Service Provider (ISP) network, which differentiates the home eNB from the conventional base station. This is advantageous in view of wired and wireless integration service, but also results in problems with respect to radio resource management. 
         [0007]    One significant problem is that there is no communication interface specified between a home eNB and an eNB (or another home eNB) like the X2 interface between the eNBs. Accordingly, in order for a home eNB to communicate with another home eNB or an eNB, it must connect to the core network. The absence of a direct communication interface between a home eNB and an eNB (or another home eNB) is likely to cause problems especially when a delay sensitive communication is required between them. Accordingly, there is a need to develop a communication mechanism between the home eNBs and between a home eNB and an eNB. 
       SUMMARY OF THE INVENTION 
       [0008]    The present invention has been made to address at least the above problems and/or disadvantages and to provide at least the advantages described below. Accordingly, an aspect of the present invention provides a communication method and system for supporting direct communication between a femtocell base station and a macrocell base station. 
         [0009]    According to one aspect of the present invention, a mobile communication system includes at least one macro base station which allocates a resource to user equipments. The mobile communication system also includes at least one femto base station which operates in a base station mode for serving terminals located within a service area of the at least one femto base station and operates in a user equipment mode for communicating with the at least one macro base station using the resource allocated by the at least one macro base station. 
         [0010]    The at least one femto base station may transition from the base station mode to the user equipment mode and may maintain the user equipment mode for a duration of the resource allocated by the at least one macro base station. The at least one macro base station may allocate a persistent resource to the at least one femto base station. 
         [0011]    The at least one femto base station may have a first period and a second period, shorter than the first period, and may enter the user equipment mode at a start of the first period and when the at least one femto base station has data to transmit at a start of the second period. 
         [0012]    The at least one femto base station may enter the user equipment mode when a predetermined event occurs and may operate in the user equipment mode for a duration of the resource allocated by the at least one macro base station. The at least one macro base station may allocate a non-persistent resource to the at least one femto base station. 
         [0013]    The at least one femto base station may exchange control data for a handover with the at least one macro base station in the user equipment mode. 
         [0014]    According to another aspect of the present invention, a communication method of a femto base station in a communication system including at least one macro base station and at least one femto base station is provided. A base station mode is operated in for serving at least one user equipment located in a service area of the femto base station. A user equipment mode is operated in for communicating with the at least one macro base station using resource allocated by the at least one macro base station. 
         [0015]    Operating in the user equipment mode may include transitioning from the base station mode to the user equipment mode. 
         [0016]    Transitioning from the base station mode to the user equipment mode may occur periodically. 
         [0017]    Operating in user equipment mode may include transitioning, when a predetermined event occurs in the base station mode, from the base station mode to the user equipment mode. Operating in the user equipment mode may further include determining, when a start of a first period is not detected, whether a start of a second period, which is shorter than the first period is detected, and transitioning, when at start of the second period is detected and transmission data exists, from the base station mode to the user equipment mode. 
         [0018]    When the femto base station transitions from the base station mode to the user equipment mode, the macro base station may allocate a persistent resource. 
         [0019]    Operating in the user equipment mode may include exchanging control data required for a handover with the at least one macro base station. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0020]    The above and other aspects, features and advantages of the present invention will be more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
           [0021]      FIGS. 1A and 1B  are schematic diagrams illustrating a communication system according to an embodiment of the present invention; 
           [0022]      FIGS. 2A to 2C  are block diagrams illustrating communication systems according to embodiments of the present invention; 
           [0023]      FIG. 3  is a timing diagram illustrating a periodic mode transition of a femto-BS of the communication system according to an embodiment of the present invention; 
           [0024]      FIG. 4  is a flowchart illustrating an operation mode transition procedure of a femto-BS according to an embodiment of the present invention; 
           [0025]      FIG. 5  is a timing diagram illustrating an event-driven mode transition of a femto-BS of the communication system according to an embodiment of the present invention; 
           [0026]      FIG. 6  is a flowchart illustrating an operation mode transition procedure of a femto-BS according to another embodiment of the present invention; and 
           [0027]      FIG. 7  is a signaling diagram illustrating a handover process between a femto-BS and a macro-BS in a communication system according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       [0028]    Preferred embodiments of the present invention are described in detail with reference to the accompanying drawings. The same or similar components are designated by the same or similar reference numerals although they are illustrated in different drawings. Detailed descriptions of constructions or processes known in the art may be omitted to avoid obscuring the subject matter of the present invention. 
         [0029]      FIGS. 1A and 1B  are schematic diagrams illustrating a communication system according to an embodiment of the present invention. 
         [0030]    Referring to  FIGS. 1A and 1B , the communication system includes a macrocell  1000  and a femtocell  2000 . 
         [0031]    The macrocell  1000  is a cell of an ordinary cellular communication network. A macro Base Station (macro-BS)  100  manages the macrocell  1000 . The femtocell  2000  is smaller in coverage than the macro cell  1000 . For example, the femtocell  2000  may have coverage for a house at most, or a room of the house. The femtocell  2000  can be deployed for covering indoor and outdoor shadow area, and provides multiple users with high quality data services. A femto Base Station (femto-BS)  200  manages the femtocell  2000 . The femto-BS  200  can cover a shadow area of the macro-BS  100 , or provide the service to a specific User Equipment (UE). In an embodiment of the present invention, the femto-BS  200  provides the service to UEs located in its radio coverage and communicates with the macro-BS  100  directly. For this purpose, the femto-BS  200  operates in two operation modes: a BS mode for serving the UEs located in its coverage, and a UE mode for communicating, as a UE, with the macro-BS  100 . 
         [0032]    The femto-BS  200  operates in the BS mode during a “predetermined time period” and operates in the UE mode in a fraction of the “predetermined time period.” The predetermined time period can be negotiated between the macro-BS  100  and the femto-BS  200 . The predetermined time period also can be determined by the femto-BS  200 . The fraction of the predetermined time period can be a time duration for the femto-BS  200  to exchange data with the macro-BS  100 . 
         [0033]      FIG. 1A  shows a situation in which the femto-BS  200  operates in the BS mode. The macro-BS  100  serves the UE located in the macrocell  1000 , and the femto-BS  200  serves the UE  300  located in the femtocell  2000 . 
         [0034]    The femtocell  2000  is defined by the radio coverage of the femto-BS  200 , and the femto-BS  200  is located within the macrocell  1000  defined by the radio coverage of the macro-BS  100 , such that the femtocell  2000  is overlapped with the macrocell  1000 . 
         [0035]    In the BS mode, the femto-BS  200  is connected to the core network via an ISP network to relay data between the UE  300  and another UE. 
         [0036]      FIG. 1B  shows a situation in which the femto-BS  200  operates in the UE mode. In this situation, the femto-BS  200  acts as a UE served by the macro BS  100 . In order to be served by the macro-BS  100 , the femto-BS  200  transitions to the UE mode periodically or when a specific event occurs. For example, when the UE  300  associated with the macro-BS  100  enters the femtocell area and requests handover to the femto-BS  200 , the femto-BS  200  transitions from the BS mode to the UE mode. 
         [0037]    In order to communicate with the macro-BS  100 , the femto-BS  200  must be in the macrocell  1000  of the macro-BS  100 . In this case, the femto-BS  200  operates in the UE mode and is served by the macro-BS  100  within the macrocell  1000 . 
         [0038]    Before transitioning to the UE mode, the femto-BS  200  causes the UE served by the femto-BS  200  to enter a sleep mode. After transitioning to the UE mode, the femto-BS is allocated a resource for communication with the macro-BS  100 . Since the femto-BS  200  can communicate with the macro-BS  100  directly, it is possible to solve problems caused by transmission delay, particularly when a seamless message exchange is required as in a handover process. 
         [0039]    Since the femto-BS  200  acts as the UE in its UE mode, it consumes transmission power (Tx power) like an ordinary UE. Accordingly, it is advantageous to ignore interferences to other indoor wireless devices and neighbor cells in transmission and reception power. 
         [0040]      FIGS. 2A to 2C  are block diagrams illustrating communication systems according to embodiments of the present invention. 
         [0041]    Referring to  FIG. 2A , a mobile communication system includes a core network  400  (or  500 ), a macro-BS  100  connected to the core network  400  (or  500 ), a femto-BS  200  connected to the core network via an ISP network  600 , another network  700  connected to the core network  400  (or  500 ), and UEs  300  that are connected to the macro-BS  100 , the femto-BS  200 , and the other network  700 . The core network  400  (or  500 ) is an operator&#39;s network structured as shown in  FIGS. 2   b  and  2   c.    
         [0042]    In  FIG. 2B , the core network  400  includes a Mobility Management Entity (MME)  410 , a Serving Gateway (S-GW)  420 , a Packet Data Network Gateway (P-GW)  430 , a Serving General Packet Radio Service (GPRS) Support Node (SGSN)  440 , a Radio Network Controller (RNC)  450 , and a Secure Gateway (Secure GW)  460 . Here, the MME  410  is an entity for managing the UE&#39;s mobility, and S-GW  420  is a gateway connecting to the macro-BS  100  by means of an S1 interface. The MME  410  and S-GW  420  are integrally called an access Gateway (aGW). The P-GW  430  is a gateway connected to a Packet Data Network (PDN), and the core network  400  connects the other network  700  by means of the P-GW  430 . The secure GW  460  provides an Internet Protocol (IP)-based Iub interface for the interconnection of the femto-BS  200  and the RNC  450 . The RNC  450  is an entity that manages the radio resources of the femto-BS (NodeB)  200 , and the SGSN  440  is an entity that supports the GPRS. 
         [0043]    In  FIG. 2C , the core network  500  includes an MME  510 , an S-GW  520 , a P-GW  530 , and an Unlicensed Mobile Access Network/General Access Network (UMAN/GAN) controller  540 . The UMAN controller is an entity that controls the Unlicensed Mobile Access (UMA), and the GAN controller is an entity that controls legacy network access including UMA. The UMAN/GAN controller  540  is configured with the functions of the secure GW  460  of  FIG. 2   b . The MME  510 , S-GW  520 , and P-GW  530  are identical with those of  FIG. 2   b  in function. 
         [0044]    The macro-BS  100  is connected to the MME  410  ( 510 ) and S-GW  420  ( 520 ) by means of an S1 interface, and the femto-BS  200  is connected to the core network  400  ( 500 ) via the ISP network  600 . The femto-BS  200  is connected to the core network  400  via the ISP network  600  and the secure GW  460  in  FIG. 2   b , whereas the femto-BS  200  is connected to the core network  500  via the ISP network  600  and the UMAN/GAN controller  540 . 
         [0045]    The macro-BS  100  can be an eNB. 
         [0046]    In an embodiment of the present invention, the macro-BS  100  allocates resources to the femto-BS  200  such that the femto-BS  200  can communicate with the macro-BS  100  in the UE mode. The resources include uplink and downlink resources. The macro-BS  100  performs frame-level scheduling fairly on the radio frame composed of the uplink and downlink. For example, the macro-BS  100  performs scheduling with a radio frame of 10 ms divided into 20 frames. The macro-BS  100  continues allocation of resources enough for processing the traffic in the service of which traffic is predictable. This kind of resource is called a persistent resource. When the femto-BS  200  requests periodic frame allocation, the macro-BS  100  allocates the persistent resource. 
         [0047]    As for the service of traffic that is unpredictable, the macro-BS  100  allocates the resource temporarily. This kind of resource is called a non-persistent resource. When the femto-BS  200  requests the macro-BS  100  to allocate resources for a specific event, the macro-BS  100  allocates the non-persistent resource. 
         [0048]    The numbers of frames for allocating persistent and non-persistent resources to the femto-BS  200  can be predetermined values. In an embodiment of the present invention, the resource allocation is described under the assumption of 3 frames for the persistent resource and 4 frames for the non-persistent resources. The persistent and non-persistent resource frames include uplink and downlink frames, respectively. 
         [0049]    As aforementioned, the femto-BS  200  transitions between the UE mode and the BS mode alternately. In the case of transition from the UE mode to the BS mode, the femto-BS  200  stores the synchronization with the UE, which is being served by the femto-BS  200  in the BS mode. If it is the first attempt to access the macro-BS  100 , the femto-BS  200  is required to perform a synchronization process with the macro-BS  100 . 
         [0050]    The femto-BS  200 , operating in the UE mode, requests the macro-BS  100  to allocate transmission frames. The femto-BS  200  can communicate directly with the macro-BS for the allocated frames. Particularly, the femto-BS  200  performs a transition from the BS mode to the UE mode periodically or when a specific event occurs. 
         [0051]    In order to transition to the UE mode periodically, the femto-BS  200  is provided with a timer. When the timer expires, the femto-BS  200  transitions to the UE mode and requests the macro-BS  100  to allocate a transmission resource. Also, the femto-BS  200  can be configured to transition to the UE mode and request resource allocation when a specific event occurs. The transmission resource includes uplink and downlink resources. 
         [0052]    The periodic mode transition and event-driven mode transition are described separately. 
         [0053]      FIG. 3  is a timing diagram illustrating a periodic mode transition of a femto-BS of the communication system according to an embodiment of the present invention. 
         [0054]    In an embodiment of the present invention, the femto-BS  200  operates either in the BS mode or in the UE mode. The femto-BS  200  communicates with the UE in the BS mode and with the macro-BS  100  in the UE mode. As aforementioned, the femto-BS  200  is allocated the uplink and downlink resources from the macro-BS  100  for communication with the macro BS  100 . Accordingly, the femto-BS  200  sends the macro-BS  100  a resource allocation request periodically. As aforementioned, upon receipt of the periodic resource allocation request, the macro-BS  100  allocates the persistent resource. In an embodiment of the present invention, the femto-BS  200  sends the macro-BS  100  the resource allocation request every long period (first period) and, when there is data to transmit at a start of every short period (second period). The short period is shorter than the long period in time. Preferably, a short period value is set to a divisor of a long period value. 
         [0055]    In an embodiment of the present invention, the macro-BS  100  performs scheduling the uplink and downlink resources fairly with the radio frame divided into 20 frames. In  FIG. 3 , the long period (first period) is composed of 20 frames and the short period (second period) is composed of 10 frames. In order to simplify the explanation, the 20 frames are called 1 st  to 20 th  frames. 
         [0056]    The femto-BS  200  is provided with a first timer for counting the long period (first period) and a second timer for counting the short period (second period). When the first timer has expired, the femto-BS  200  enters the UE mode and requests the macro-BS  100  for a resource. 
         [0057]    When the second timer has expired, the femto-BS  200  enters the UE mode and requests the macro-BS  100  for a transmission resource. When there is no data to transmit at the expiry of the second timer, the femto-BS  200  stays in the BS mode without transmission of the resource allocation request. In this manner, the femto-BS  200  can transmit data during the short period only when there is data to transmit at the expiry of the second timer, thereby reducing waste of radio resources. 
         [0058]    Referring to  FIG. 3 , the first period is composed of 20 frames starting with the 1 st  frame, and the second period is composed of contiguous 10 frames starting with the 10 th  frame. 
         [0059]    The femto-BS  200  operates in the UE mode for the 1 st  to 4 th  frames and operates in the BS mode for the 5 th  to 8 th  frames. 
         [0060]    That is, the femto-BS  200  enters the UE mode and requests the macro BS  100  for a resource at the 1 st  frame. As aforementioned, the femto-BS  200  maintains its UE mode for 3 frames with the persistent resource. Consequentially, the femto-BS  200  stays in the UE mode for the 4 frames (the 1 st  to 4 th  frames). 
         [0061]    The femto-BS  200  checks whether there is data to transmit at the 11 th  frame and, if any, transitions to the UE mode; otherwise the femto-BS  200  maintains the BS mode.  FIG. 3  is depicted under the assumption that there is no data to transmit. If there is data to transmit, the femto-BS  200  operates in the UE mode for the 11 th  to 14 th  frames. 
         [0062]    As shown in  FIG. 3 , the femto-BS  200 , operating in the UE mode, requests the macro-BS  100  for the transmission resource at the 1 st  frame. The femto-BS  200  can use the 2 nd  to 4 th  frames for the uplink and downlink. That is, the femto-BS  200  can communicate with the macro-BS  100  in the UE mode. The femto-BS  200  operating in the BS mode can communicate with the UE for the 5 th  to 20 th  frames. 
         [0063]      FIG. 4  is a flowchart illustrating an operation mode transition procedure of a femto-BS according to an embodiment of the present invention. 
         [0064]    It is assumed that the femto-BS  200  stores the synchronizations with the macro-BS  100  and the UE  300 , and the macro-BS  100  allocates the persistent resource for 3 frames. 
         [0065]    Referring to  FIG. 4 , the femto-BS  200  first operates in the BS mode in step S 401 . In the BS mode, the femto-BS communicates with the UE  300 . 
         [0066]    While operating in the BS mode, the femto-BS  200  monitors to detect the start of the first period in step S 403 . If the start of the first period is detected, the femto BS  200  transitions to the UE mode in step S 409 . That is, the femto-BS  200  determines whether the first timer has expired and, if expired, enters the UE mode. In the UE mode, the femto-BS  200  requests the macro-BS  100  for a resource and the macro-BS  100  responds by allocating the persistent resource. As described above, the persistent resource is allocated for 3 frames. As in the case of  FIG. 3 , the femto-BS  200  requests the macro-BS  100  for a resource at the 1 st  frame and the macro-BS  100  responds by allocating the persistent resource for the 2 nd  to 4 th  frames such that the femto-BS  200  can communicate with the macro-BS  100  for the 2 nd  to 4 th  frames. 
         [0067]    If the start of the first period is not detected yet, the femto-BS  200  determines whether the start of the second period is detected in step S 405 . The start of the second period is determined by detecting the expiry of the second timer. If the start of the second period is detected, the femto-BS  200  determines whether there is data to transmit in step S 407 ; otherwise, the process goes to step S 401 . 
         [0068]    If there is data to transmit at step S 407 , the femto-BS  200  enters the UE mode in step S 409 . 
         [0069]    Upon entering the UE mode, the femto-BS  200  requests the macro-BS  100  for the persistent resource and communicates with the macro-BS  100  directly for the predetermined number of frames. Referring to  FIG. 3 , when there is data to transmit, the femto-BS  200  requests the macro-BS  100  for a resource at the 11 th  frame and communicates with the macro-BS  100  for the 12 th  to 14 th  frames. 
         [0070]    Otherwise, if there is no data to transmit at step S 407 , the process goes to step S 401 .  FIG. 3  shows the situation in which the femto-BS  200  has no data to transmit. 
         [0071]    As described above, the femto-BS  200  operates in the BS mode for a predetermined time duration for serving the UE located in its service area and operates in the UE mode during a fraction of the predetermined time for communicating with the macro-BS  100  serving the femto-BS  200  as a UE. 
         [0072]    The transition between the BS and UE modes repeats as long as the femto-BS  200  does not break down or power off. Step S 411  is provided in consideration of a problematic situation. When an unexpected problem, such as power-off of the femto-BS  200 , occurs at step S 411 , the mode transition procedure ends. When no problem occurs, the femto-BS  200  transitions between the BS and UE modes, repeating steps S 401  to S 411 . 
         [0073]    As described above, the femto-BS  200  enters the UE mode periodically for communicating with the macro-BS  100 . In an embodiment of the present invention, the femto-BS  200  can be configured to enter the UE mode when a specific event occurs. The event-driven mode transition procedure is described hereinafter in detail. 
         [0074]      FIG. 5  is a timing diagram illustrating an event-driven mode transition of a femto-BS of the communication system according to an embodiment of the present invention. 
         [0075]    In an embodiment of the present invention, the femto-BS  200  operates either in the BS mode or in the UE mode. The femto-BS  200  communicates with the UE in the BS mode and with the macro-BS  100  in the UE mode. The femto-BS  200  requests the macro-BS  100  for resource allocation when a predetermined event occurs. Unlike the periodic request in the periodic mode transition, the femto-BS  200  requests a resource when it detects a specific event, and the macro-BS  100  responds by allocating the non-persistent resource to the femto-BS  200 . As aforementioned, the non-persistent resource is allocated in a unit of 4 frames. 
         [0076]    The predetermined event is an event requiring a resource for communication between the femto-BS  200  and the macro-BS  100 . For example, the event can be a handover. In an embodiment of the present invention, when handover occurs, the femto-BS  200  enters the UE mode for exchanging control data related to the handover with the macro-BS  100 . 
         [0077]    When a specific event occurs, the femto-BS  200  enters the UE mode and requests the macro-BS  100  for resource. After the macro-BS  100  allocates the persistent resource to the femto-BS  200 , the femto-BS  200  communicates with the macro BS  100  for a preset number of frames. In  FIG. 5 , the persistent resource is 4 frames. 
         [0078]      FIG. 5  is depicted under the assumption that the specific event occurs at the 6 th  frame and the 19 th  frame. Since 4 frames are allocated as the non-persistent resource, the femto-BS  200  operates in the UE mode for the 6 th  to 10 th  frames and the 19 th  to 23 rd  frames, and operates in the BS mode for the 1 st  to 5 th  frames, the 11 th  to 18 th  frames, and the 24 th  and 25 th  frames. 
         [0079]    The event-driven mode transition is described only with the event occurring at the 6 th  frame. If the event is detected at the 6 th  frame, the femto-BS  200  enters the UE mode and requests the macro-BS  100  for a resource. The femto-BS  200 , allocated a resource from the macro-BS  100 , stays in the UE mode for the 7 th  to 10 th  frames as the non-persistent resource. That is, the femto-BS  200  operates in the UE mode for the 6 th  to 10 th  frames. Also, if the event is detected at the 19 th  frame, the femto-BS  200  enters the UE mode and requests the macro-BS  100  for a resource. The femto-BS  200  operates in the UE mode for the 20 th  to 23 rd  frames. 
         [0080]    The femto-BS  200 , operating in the UE mode, requests that the macro-BS  100  allocate an uplink resource for the 6 th  to 10 th  frames. The femto-BS  200  can be allocated the uplink and downlink resources for the 7 th  to 11 th  and 20 th  to 23 rd  frames. In the BS mode, the femto-BS  200  communicates with the UE  300  for the 1 st  to 5 th  frames, the 11 th  to 18 th  frames, and the 24 th  and 25 th  frames. 
         [0081]      FIG. 6  is a flowchart illustrating an operation mode transition procedure of a femto-BS according to another embodiment of the present invention. 
         [0082]      FIG. 6  is depicted under the assumption that the femto-BS  200  stores the synchronizations with the macro-BS  100  and the UE  300 , and the macro-BS  100  allocates the non-persistent resource to the femto-BS  200  for 4 frames. 
         [0083]    Referring to  FIG. 6 , the femto-BS  200  first operates in the BS mode in step S 601 . In the BS mode, the femto-BS  200  communicates with the UE  300  located in its service area. 
         [0084]    While operating in the BS mode, the femto-BS  200  monitors to detect a mode transition trigger event in step S 603 . The mode transition trigger event may be generation of data to transmit to the macro-BS  100  directly. The mode transition trigger event can be a handover event which requires exchange of control data with the macro-BS  100 . 
         [0085]    If a mode transition trigger event is detected, the femto-BS  200  transitions to the UE mode in step S 605 . The femto-BS  200  operating in the UE mode requests the macro-BS  100  for a resource and the macro-BS  100  responds by allocating the non-persistent resource to the femto-BS  200  such that the femto-BS  200  communicates with the macro-BS  100  for the preset number of frames with the non-persistent resource. As assumed above, the preset number of frames is 4. Referring to  FIG. 5 , the femto-BS  200  requests the macro-BS  100  for a resource at the 6 th  frame and communicates directly with the macro-BS  100  for the 7 th  to 10 th  frames allocated by the macro-BS  100 . 
         [0086]    If no mode transition trigger event is detected, the process goes to step S 601 . 
         [0087]    As described above, the femto-BS  200  operates in the BS mode for predetermined time duration and operates in the UE mode for a fraction of the predetermined time duration. The mode transition between the BS and UE modes continues as long as the femto-BS  200  does not break down or power off. Step S 607  is provided in consideration of a problematic situation. When an unexpected problem, such as power-off of the femto-BS  200 , occurs at step S 607 , the mode transition procedure ends; otherwise, the process goes to step S 601 . That is, the femto-BS  200  transitions between the BS and UE modes, repeating steps S 601  to S 607  as long as any problematic event does not occur. 
         [0088]    As described above, the femto-BS  200  operating in the BS mode transitions to the UE mode periodically or when a mode transition trigger event occurs. In the UE mode, the femto-BS  200  can communicate with the macro-BS  100  directly. The direct communication between the femto-BS  200  and the macro-BS  100  is advantageous to exchange control messages, especially in the handover process. A handover process between the femto-BS  200  and the macro-BS  100  is described hereinafter in detail. 
         [0089]      FIG. 7  is a signaling diagram illustrating a handover process between a femto-BS and a macro-BS in a communication system according to an embodiment of the present invention.  FIG. 7  shows a situation in which a UE  300  performs a handover from the femto-BS  200  to the macro-BS  100 . Although not depicted, the handover from the macro-BS  100  to the femto-BS  200  can be performed in similar manner. 
         [0090]    Referring to  FIG. 7 , the femto-BS  200  sends a Measurement Request message to the UE  300  in step S 701 . Upon receipt of the Measurement Request message, the UE  300  measures the neighbor cells and frequency bands according to the cause of the Measurement Request message and, when the measurement satisfies a predetermined condition, sends a Measurement Report message to the femto-BS  200  in step S 703 . Upon receipt of the Measurement Report message, the femto-BS  200  makes a handover (HO) decision with reference to the Measurement Report message in step S 705 . Next, the femto-BS  200  sends a HO request message to the macro-BS  100  in step S 707 . Upon receipt of the HO Request message, the macro-BS  100  performs an Admission control process in step S 709  and sends an HO Request Acknowledgement (ACK) message to the femto-BS  200  in step S 711 . 
         [0091]    Upon receipt of the HO Request ACK message, the femto-BS  200  sends an HO Command message to the UE  300  in step S 713 . 
         [0092]    Upon receipt of the HO Command message, the UE  300  reconfigures its radio environment in step S 715 . While reconfiguring the radio environment, the UE  300  is detached from the old cell (femto-BS) and synchronizes with the new cell (macro-BS). That is, the UE accesses the macro-BS  100  at step S 715 . 
         [0093]    Meanwhile, after sending the HO command message, the femto-BS  200  delivers buffered and in-transit data to the target eNB (macro-BS) in step S 717 . Next, the femto-BS  200  sends a Sequence Number (SN) Status Transfer message to the macro-BS  100  in step S 719 . 
         [0094]    The UE  300  sends an HO confirm message to the macro-BS  100  in step S 721 . After receiving the SN Status Transfer message from the femto-BS  200  and the HO Confirm message from the UE  300 , the macro-BS  100  sends a Path Switch Request message indicating the cell switch of the UE  300  to the MME  510  in step S 723 . Upon receipt of the Path Switch Request message, the MME  510  sends a User Plane Update Request message to the S-GW  530  in step S 725 . Upon receipt of the User Plane Update Request message, the S-GW  530  switches the DL path in step S 727 . At this time, the S-GW  530  switches the DL path to the macro-BS  100 . Next, the S-GW  530  sends a User Plane Update Response message to the MME  510  in response to the User Plane Update Request message in step S 729 . 
         [0095]    Upon receipt of the User Plane Update Response message, the MME  520  sends a Path Switch Request ACK message to the macro-BS  100  in response to the Path Switch Request message in step S 731 . Upon receipt of the Path Switch Request ACK message, the macro-BS  100  sends a Release Resource message to the femto-BS  200  in step S 733 . 
         [0096]    Upon receipt of the Release Resource message, the femto-BS  200  flushes its DL buffer and continues delivering in-transit data in step S 735 . 
         [0097]    Next, the femto-BS  200  forwards the DL data destined to the UE  300  to the macro-BS  100  in step S 737 . 
         [0098]    After completing the DL data forwarding, the femto-BS  200  releases the resources allocated to the UE  300  in step S 739 . 
         [0099]    Steps S 701 , S 703 , and S 713  are performed when the femto-BS  200  operates in the BS mode, and steps S 707 , S 711 , S 719 , S 733 , and S 737  are performed when the femto-BS  200  operates in the UE mode. 
         [0100]    Particularly, the HO Request message at step S 703  and the HO Request ACK message at step S 711  should be exchanged promptly for facilitating the handover process. The SN Status Transfer message is sent at step S 719  for informing the macro-BS  100  of the buffered data to transmit to the UE  300  such that the buffered data is forwarded to the macro-BS  100  at step S 739 . When this handover occurs while the UE  300  receives a real time service, any delay of these message flows causes service breakage. 
         [0101]    If the direct communication between the femto-BS  200  and the macro BS  100  is not supported, the femto-BS  200  should establish a communication channel with the macro-BS  100  via the ISP network  600 , the GAN controller  540 , and the S-GW  520  for exchanging the control data with the macro-BS  100 , resulting in handover processing delay. In embodiments of the present invention, since the femto-BS  200  can communicate with the macro-BS  100  directly in the UE mode, it is possible to solve the problems caused by the control data exchange delay. 
         [0102]    As described above, the communication system comprising the femto-BS and the macro-BS provides a direct radio interface between the femto-BS and macro-BS, thereby securing latency-free communication. 
         [0103]    Also, the communication of the present invention allows a femto-BS to communicate with a macro base station directly in the UE mode so as to avoid transmission delay of control data, resulting in improvement of network performance. 
         [0104]    While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and detail may be made therein without departing from the spirit and scope of the present invention as defined by the appended claims.