Abstract:
A Femto base station which is arranged to connect to a wireless communication network and to generate a first cell for providing a wireless communication service to at least one user equipment existing within a coverage area of the first cell, comprising a transceiver comprising a transmitter and a receiver for the wireless communication with the user equipment and a controller, wherein the controller determines whether the Femto base station has entered an energy-saving mode or not, the controller disables the operation of the transmitter when it is determined that the base station has entered the energy-saving mode, and when the receiver detects a data transmission from the user equipment to a macro base station after disabling the operation of the transmitter, the controller reconfigures the transmitter. The macro base station generates a second cell with a broader coverage than the first cell and including the first cell.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a Femto base station, a energy-saving coordination node in wireless communication network enabling an efficient switching of operational modes of the Femto base station between an energy-saving mode and a normal mode. 
       BACKGROUND 
       [0002]    Recently, a home/residential base station called a “Femto base station (Femto-BS)” is proposed as a solution to the problem regarding a limited power reception level of a user equipment (UE) such as a mobile phone especially in remote areas and in residences. The Femto-BS generates a service area called as a Femto cell providing in-home mobility with maximum user data rates without loading a macro cell provided by macro base stations (macro-BSs). 
         [0003]    A radius of the Femto cell is approximately  10  meters and the Femto-BS provides the indoor coverage of the macro-BSs within a residence. It also increases the capacity of the data transmission to and from the user equipment, when the entire residence is in the macro-BS&#39;s coverage. 
         [0004]    The Femto-BS deployed at home usually has an idle period, for example, at night where the user of the Femto-BS is sleeping. If the Femto-BS enters an energy-saving mode (ES mode) from a normal operation mode to turn off a Downlink (DL) transmission from the Femto-BS to the UE connecting to the Femto cell for such a long period, it may save electricity at home. After the Femto-BS enters the ES mode and turns off the DL transmission, the end-user must turn it on manually using a mechanical switch of the Femto-BS or a near field communication (NFC), because a UE generally requires the DL common channels (DLCCH) from the base station before it initiates a call or data transmission through that base station. In addition, the UE may request to update software and its configuration or to download contents as a background operation while the UE itself is not used by the user overnight. 
       SUMMARY 
       [0005]    Therefore, it is desirable to make it possible to switch from the ES mode to the normal mode without the user operation. 
         [0006]    According to a first aspect of the invention, there is provided a Femto base station which is arranged to connect a wireless communication network and to generate a first cell for providing a wireless communication service to at least one user equipment existing in a coverage of the first cell. The Femto-base station comprises a transceiver comprising a transmitter and a receiver for the wireless communication with the user equipment, and a controller. The controller may determine whether the Femto base station enters an energy-saving mode or not. When the controller determines that the Femto base station enters the energy-saving mode, the controller may disable the operation of the transmitter. When the receiver detects a data transmission from the user equipment to a macro base station after disabling the operation of the transmitter, the controller may reconfigure the transmitter. Wherein the macro base station may be arranged to generate a second cell having a broader coverage than the first cell and including the first cell, for providing the wireless communication service to the user equipment. 
         [0007]    According to a second aspect of the invention, there is provided an energy-saving coordination node for a wireless communication network, in which the energy-saving coordination node is arranged to support operations of a Femto base station which generates a first cell for providing a wireless communication service to at least one user equipment existing in a coverage of the first cell. The energy-saving coordination node comprises a receiver for receiving configuration information of a physical random access channel (PRACH) of a macro base station which generates a second cell having a broader coverage than the first cell and including the first cell, for providing the wireless communication service to the user equipment, and a transmitter for transmitting the configuration information to the Femto base station. 
         [0008]    According to a third aspect of the invention, there is provided an energy-saving coordination node for a wireless communication network, in which the energy-saving coordination node is arranged to support operations of a Femto base station which generates a first cell for providing a wireless communication service to at least one user equipment existing in a coverage of the first cell. The energy-saving coordination node comprise a first receiver for receiving from the Femto base station of a notice that the Femto base station enters an energy-saving mode where a wireless transmission function of the Femto base station is disabled, a second receiver for receiving location information of the user equipment served by a macro base station from the macro base station which generates a second cell having a broader coverage than the first cell and including the first cell, for providing the wireless communication service to the user equipment, a controller for determining whether or not the user equipment can be served by the first cell based on the received location information, and a transmitter for transmitting a wake-up instruction to the Femto base station, when the controller determines that the user equipment can be served by the first cell. 
         [0009]    According to a fourth aspect of the invention, there is provided a wireless communication system including a Femto base station, a macro base station and an energy-saving coordination node. In the system, the Femto base station may be arranged to generate a first cell for providing a wireless communication service to at least one user equipment existing in a coverage of the first cell. The Femto base station comprises a transceiver comprising a transmitter and a receiver for the wireless communication with the user equipment, a network interface for communicating with the energy-saving coordination node, and a controller. The controller may determine whether the Femto base station enters an energy-saving mode or not. When the controller determines that the Femto base station enters the energy-saving mode, the controller disables the operation of the transmitter in the transceiver. The network interface may receive configuration information of a physical random access channel (PRACH) of the macro base station. When the receiver which is reconfigured using the configuration information detects data transmission from the user equipment to the macro base station after disabling the operation of the transmitter, the controller may reconfigure the transmitter. In the system, the macro base station may be arranged to generate a second cell having a broader coverage than the first cell and including the first cell, for providing the wireless communication service to the user equipment and to transmit the configuration information to the energy-saving coordination node. In the system, the energy-saving coordination node comprises a receiver for receiving the configuration information from the macro base station, and a transmitter transmitting the configuration information to the Femto base station. 
         [0010]    According to a fifth aspect of the invention, there is provided a wireless communication system including a Femto base station, a macro base station and an energy-saving coordination node. In the system, the Femto base station may be arranged to generate a first cell for providing a wireless communication service to at least one user equipment existing in a coverage of the first cell. The Femto base station comprises a transceiver comprising a transmitter and a receiver for the wireless communication with the user equipment, a network interface for communicating with the energy-saving coordination node, and a controller. The controller may determine whether the Femto base station enters an energy-saving mode or not. When the controller determines that the Femto base station enters the energy-saving mode, the controller may disable the operation of the transmitter in the transceiver. The network interface may transmit the notice of entering of the energy-saving mode to the energy-saving coordination node. When the receiver receives a wake-up instruction from the energy-saving coordination node, the controller may reconfigure the transmitter. In the system, the macro base station is arranged to generate a second cell having a broader coverage than the first cell and including the first cell, for providing the wireless communication service to the user equipment and to transmit location information of the user equipment in the second cell to the energy-saving coordination node. In the system, the energy-saving coordination node comprises a first receiver for receiving from the Femto base station of the notice, a second receiver for receiving the location information, a controller for determining whether or not the user equipment in the second cell can be served by the first cell based on the received location information, and a transmitter for transmitting a wake-up instruction to the Femto base station, when the controller determines that the user equipment can be served by the first cell. 
         [0011]    According to a sixth aspect of the invention, there is provided a computer program which causes a computer to function as a Femto base station which generates a first cell for providing a wireless communication service to at least one user equipment existing in a coverage of the first cell. The Femto base station comprises a transceiver comprising a transmitter and a receiver for the wireless communication with the user equipment and a controller. The controller may determine whether the Femto base station enters an energy-saving mode or not. When the controller determines that the Femto base station enters the energy-saving mode, the controller may disable the operation of the transmitter. When the receiver detects a data transmission from the user equipment to a macro base station after disabling the operation of the transmitter, the controller may reconfigure the transmitter. Wherein the macro base station may be arranged to generate a second cell having a broader coverage than the first cell and including the first cell, for providing the wireless communication service to the user equipment. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  shows a wireless communications network according to an exemplary embodiment of the invention; 
           [0013]      FIG. 2  illustrates an exemplary functional block diagram of the Femto-BS  101  according to the embodiment of the present invention; 
           [0014]      FIG. 3  illustrates an exemplary functional block diagram of MME  131  according to the embodiment of the present invention; 
           [0015]      FIG. 4  shows the functional components to achieve the exemplary embodiment of the present invention; 
           [0016]      FIG. 5  shows an example of a sequence diagram illustrating a procedure according to the exemplary embodiment of the present invention, when the Femto-BS  101  enters the ES mode; 
           [0017]      FIG. 6  shows an example of a sequence diagram illustrating a procedure according to the exemplary embodiment of the present invention; 
           [0018]      FIG. 7  shows an exemplary structure of the PRACH preamble according to “3GPP TS 36.211 V.9.0.0”. 
           [0019]      FIG. 8  shows another example of a sequence diagram illustrating a procedure according to the exemplary embodiment of the present invention, when the Femto-BS  101  recovers from the ES mode. 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    Embodiments of the present invention will now be described with reference to the attached drawings. Each embodiment described below will be helpful in understanding a variety of concepts from the generic to the more specific. 
         [0021]    It should be noted that the technical scope of the present invention is defined by claims, and is not limited by each embodiment described below. In addition, not all combinations of the features described in the embodiments are always indispensable for the present invention. 
         [0022]      FIG. 1  shows a wireless communication network according to an exemplary embodiment of the invention. It is a network which is generally in accordance with the Long Term Evolution (LTE) proposals of the Third Generation Partnership Project (3GPP). For further implementation details, the contents of the 3GPP Technical Specification (TS) 23.401, available from http://www.3gpp.org/ are expressly incorporated herein by reference. 
         [0023]      FIG. 1  shows a macro-BS  100  which generates a macro cell  103  for providing a wireless communication service to a plurality of user equipments including a UE  102 B. The UE  102 B is located within the coverage of the macro cell  103 . A Femto-BS  101  is located in a residence  105  to provide the UE  102 A within the residence with the wireless communication service as well as the macro-BS  100 . The Femto cell  104  generated by the Femto-BS  101  is within the coverage of the macro cell  103 . 
         [0024]    The UE  102  is a user equipment for communicating with other UEs over the wireless communication link according to a wireless communication protocol such as ANSI-136, GSM (Global Standard for Mobile) communication, GPRS (General Packet Radio Service), EDGE (Enhanced Data Rates for GSM Evolution), Code Division Multiple Access (CDMA), Wideband Code Division Multiple Access (WCDMA), CDMA2000, Long Term Evolution (LTE) and UMTS (Universal Mobile Telecommunications System). The UE  102  may be a mobile phone or other portable communication device which can communicate over the mobile communications network. 
         [0025]    The macro-BS  100  connects to a system architecture evolution gateway (SAE GW)  120  and a mobility management entity (MME)  131 . The Femto-BS  101  connects to the SAE GW  120  and the MME  131  via a broadband access network  140  or a dedicated line. The SAE GW  120  includes a serving gateway (serving GW)  121  and a packet data network gateway (PDN GW)  122 . The MME can be function as an energy-saving coordination node according to the present invention. The MME connects to HSS  132 . The HSS is a database for managing subscriber information of the UE  102  and information regarding the Femto-BS  101 . Detailed structure of the core network corresponds to the Long Term Evolution (LTE) and System Architecture Evolution (SAE) proposals of the Third Generation Partnership Project (3GPP). Indeed, for further implementation details, the contents of the 3GPP Technical Specification (TS) 23.401 are available. 
         [0026]      FIG. 2  illustrates an exemplary functional block diagram of the Femto-BS  101  according to the embodiment of the present invention. The Femto-BS  101  includes a core network interface (I/F)  201 , a baseband (BB) transceiver  202 , a radio frequency (RF) transceiver  203 , a controller  204  and a synchronization unit  205 . The core network interface  201  is an interface for communicating with a core network of the wireless communication system including the SAE GW  120  and the MME  131 . The BB transceiver includes a baseband transmitter  202 A and a baseband receiver  202 B. The baseband transmitter  202 A executes processing including encoding of downlink data to be transmitted in RF frames and timing information, and provides the processing results with the RF transmitter  203 A. The baseband receiver  202 B receives uplink data from RF receiver  203 B and decodes them to provide the core network interface  201 . The baseband receiver  202 B checks whether or not the decoded uplink data is a physical random access channel (PRACH) preamble transmitted from the UE  102 A, if so, the baseband receiver informs to the controller  204  that the PRACH preamble is detected. 
         [0027]    The RF transmitter  203 A receives data from the baseband transmitter  202 A and executes processing including D/A conversion, frequency conversion and amplification. The RF transmitter  203 A wirelessly transmits the processing results to the UE  102 . The RF receiver  203 B receives uplink data from the UE  102  and executes processing including A/D conversion, frequency conversion and amplification. The RF receiver  203 B transmits the processing results to the baseband receiver  202 B. 
         [0028]    The core network I/F  201  receives downlink data from the SAE GW  120  or the MME  131 , executes protocol conversion to the downlink data and provides the conversion result to the baseband transmitter  202 A. The core network I/F  201  also receives the uplink data from the baseband receiver  202 B and transmits it to the SAE GW  120  or the MME  131  after the protocol conversion. When the core network I/F  201  receives a control signal for the base station, it provides the control signal to the controller  204  after the protocol conversion. The core network I/F  201  forwards a response to the control signal from the controller  204  to the core network. 
         [0029]    The controller  204  controls operations of the core network I/F  201 , the BB transceiver  202 , the RF transceiver  203  and the synchronization unit  205  according to the control signal received from the core network. According to the exemplary embodiment of the present invention, the controller  204  controls an operation mode of the Femto-BS  101  including the energy-saving mode (ES mode) and normal mode. In order to shift to the ES mode from the normal mode, the controller  204  instructs the baseband transmitter  202 A and the RF transmitter  203 A to stop their operation. During the ES mode, when the baseband receiver  202 B detects the PRACH preamble from the UE  102 A and informs the detection to the controller  204 , the controller  204  reconfigures the baseband transmitter  202 A and the RF transmitter  203  to restart their operation to shift to the normal mode from the ES mode. 
         [0030]    The synchronization unit  205  creates a clock signal as a reference signal for respective operations in the core network I/F  201 , the BB transceiver  202 , the RF transceiver  203  and the controller  204 . The synchronization unit synchronizes an internal clock of the Femto-BS  101  to an external clock, such as the one from the core network or GPS. The synchronization unit  205  includes a system frame number (SFN) generator  206 . The SFN generator  206  manages a timing for generating frames, used in communication with the UE  102 , to which the system frame numbers are respectively assigned. 
         [0031]      FIG. 3  illustrates an exemplary functional block diagram of the MME  131  according to the embodiment of the present invention. The MME  131  includes an S 1 -MME interface  301 , a controller  302  and a database  303 . The S 1 -MME interface  301  is an interface for communicating with the Femto-BS  101  via the broadband access network  140  and with the macro BS  100 . S 1 -MME may interface  301  with the Femto-BS  101  via a dedicated line. The controller  302  controls operation of the MME  131  and executes processing according to the embodiments of the present invention. For more detailed structure of the interfaces correspond to the LTE/SAE proposals of the Third Generation Partnership Project (3GPP). Indeed, for further implementation details, the contents of the 3GPP Technical Specification (TS) 23.401 are available. The Database 303 may store in part or whole of information stored in the HSS  132  for the embodiment of the present invention. 
         [0032]      FIG. 4  shows the functional components to achieve the exemplary embodiment of the present invention. An Energy-Saving (ES) Coordinator  401  communicates with multiple Energy-Saving (ES) Agents  402  to  404 . The ES Coordinator  401  uses a database  405  to manage information regarding the Femto-BS  101  which is in the cell range of any one of macro-BSs  100  and the status of respective Femto-BSs  101  being in the ES mode. Information used to identify the macro-BS  100  which covers the Femto-BS  101  may be provisioned by the operator or by the end user to the ES Coordinator  401  by measuring and/or calculating the propagation delay (PD). According to one embodiment of the present invention, it also manages the information relating to the PRACH configuration that needs to be shared between the macro-BS  100  and the Femto-BS  101 . The ES Agent  402  puts the hardware into the ES mode and wakes it up either autonomously or by the instruction from the ES Coordinator  401 . In the exemplary embodiment of the present invention, the system depicted in  FIG. 4  can be applied to the Long Term Evolution (LTE) network shown in  FIG. 1 . Accordingly, the ES Coordinator  401  is deployed at the MME  131  as the energy-saving coordination node and the database  405  is deployed at HSS  132 . The MME  131  talks with the ES Agents in the Femto-BS  101  over the S 1 -MME interface  301 . The ES Coordinator  401  may also be deployed at the macro-BS  100  and may communicate with the ES Agents over the X2 interface. 
         [0033]      FIG. 5  shows an example of a sequence diagram illustrating a procedure according to the exemplary embodiment of the present invention, when the Femto-BS  101  enters the ES mode. 
         [0034]    At S 501 , the macro-BS  100  transmits to the MME  131  as the ES Coordinator  401 , configuration information including the PRACH configuration information of the macro-BS  100  and the SFN timing information necessary for the Femto-BS  101  to calculate the PRACH receiver window timing. 
         [0035]    At S 502 , the Femto-BS  101  receives the configuration information from the MME  131 . The received configuration information includes following elements. 
         [0036]    Configuration of the PRACH in the macro-BS  100 . The macro-BS  100  generates the PRACH configuration at every restart and informs the MME  131 . The MME  131  as the ES Coordinator  401  updates the HSS  132  with the latest informed values. The PRACH configuration information includes index information of a preamble format of the PRACH according to the table 5.7.1-1 and PRACH configuration index according to the table 5.7.1-2. The tables are defined in “3GPP TS 36.211 V.9.0.0, 5.7 Physical random access channel, 5.7.1 Time and frequency structure”. The PRACH configuration information further includes information to identify a set of possible sequences from which the UE  102 A chooses contents of the PRACH preamble including RACH ROOT SEQUENCE which is a logical index to be used in generating a root Zadoff-Chu (ZC) sequence, N CS  configuration which is an index used to determine a shift amount N CS  in ZC sequence, High-speed-flag specifying whether to select the PRACH preamble within a limited group or not. The frequency information used in the macro-BS  100  may also be included in the configuration information. 
         [0037]    SFN timing information (T 0 ) when a system frame number (SFN) of a frame used in the macro-BS  100  becomes zero. The SFN timing information are used with the index information to set the PRACH receiving window by the controller  204  and the SFN generator  206  in order to receive the PRACH preamble transmitted from the UE  102 A to the macro-BS  100  during the ES mode in the Femto-BS  101 . 
         [0038]    Propagation delay (PD) between the macro-BS  100  and the Femto-BS  101 . As one way of provisioning the PD information to the MME  131 , the network operator may calculate the propagation delay based on the distance between the macro-BS  100  and Femto-BS  101  on a map according to the address information of the owner of the Femto-BS  101 . Another way may be to measure the propagation delay between the UE  102 A and the macro-BS  100  in advance when the UE  102 A is served by the Femto-BS  101 , and to use the measured propagation delay as the PD information, since the distance between the Femto-BS  101  and the UE  102 A can be ignored against the distance between the UE  102 A and the macro-BS  100 . At S 503 , the controller  204  of the Femto-BS  101  autonomously determines whether to enter the ES mode based on the monitored usage of itself or by an instruction from the core network. For example, it can go to the ES mode when there is no user using the Femto-BS  101  for a certain period, and the period may vary according to the time of the day, usage statistics learned by the Femto-BS  101  or provided from the network side, user related information like IMS presence and sensor data in the proximity of the Femto-BS  101 . The period may even be zero which means the Femto-BS  101  immediately goes to the ES mode once the last UE  102 A has released the radio bearer to and from the Femto-BS  101 . The ES Coordinator  401  may provide the information of the UE&#39;s active cell, location and so on which the Femto-BS  101  may take into account. The behavior may take the user&#39;s preference in a configuration file into account. 
         [0039]    At S 504 , if the controller  204  determines to go to the ES mode, it instructs the baseband transmitter  202 A and the RF transmitter  203 A to stop the operation in order to turn off the downlink transmission to the UE  102 A. 
         [0040]    At S 505 , the controller  204  reconfigures a PRACH receiver unit including the baseband receiver  202 B, the RF receiver  203 B and the SFN generator  206  using the configuration information downloaded from the MME  131  at S 502 . 
         [0041]    During the reconfiguration, the controller  204  reconfigures the SFN Generator  206  to restart using the time information (T 0 ) and the propagation delay (PD) between the macro-BS  100  and Femto-BS  101 . For example, in case that the SFN cycle includes  4096  frames, the size of a single frame is 10 ms, the T 0  is 12:34:00:000.567, the PD is 0.01 ms, and the current time is 12:44:00:000.890, the restart time (RT) of the SFN generator  206  may be calculated as follows: 
         [0000]      RT=12:34:00:000.567+4096*10 ms*N−0.01 ms
 
         [0042]    Herein, N is a minimum integer which makes the RT larger than 12:34:00:000.890. 
         [0043]    Next, the controller sets the PRACH receiving window by designating the system frame number and sub frame number based on the PRACH configuration index and the index of the preamble format received from the MME  131 . For example, if the PRACH configuration index is “15” and the index of the preamble format is “0”, according to the table 5.7.1-2 defined in “3GPP TS 36.211 V.9.0.0”, the system frame number is set to “even” and the sub frame number is set to “9”. The sub frame number is set as the number of a leading frame of the PRACH preamble and the length of the PRACH receiving window is set as 0.8 ms or 1.6 ms depending on the designated preamble format. In the LTE system, a single system frame includes 10 sub frames each having a frame length of 1 ms. The controller  204  sets the PRACH receiving window to the baseband receiver  202 B and the reception frequency used in the macro-BS  100  to the RF receiver  203 B. 
         [0044]    Here, the propagation delay (Pb) from the UE  102 A to the Femto-BS  101  can be regarded as zero as the distance between them is very short. Assuming that the light speed c=3*10̂ 8 m/s, the cell radius of the Femto cell  104  is 12 m and the UE  102 A is on the edge (i.e. distance (b)=12 m), the propagation delay Pb=12/(3*10̂8)=4*10̂ (−8)=0.04 us. On the other hand, the UL sampling rate in the LTE is fs=30.72 MHz, where the OFDM sub-carrier spacing is 15 kHz with FFT size 2048. Duration of one sample Ts=1/fs=˜0.03 us. Thus, if the Femto-BS  101  puts the receiving window earlier by the propagation delay (PD) between the macro-BS and the Femto-BS than the receiving window of the macro-BS  100  in the time domain, the Femto-BS  101  can receive the PRACH preambles sent from the UE  102 A to the macro-BS  100 . Here, the delay of the preamble from the start of the receiving window can be called as the Time-Difference, and then the Time-Difference will be the same between the macro-BS and the Femto-BS. 
         [0045]    At S 505 , the controller stores the original configuration of the PRACH receiver unit, including the original SFN timing information for the SFN generator  206 , in cases in which the Femto-BS  101  recovers from the ES mode. 
         [0046]    At S 506 , the UE  102 A connecting to the Femto cell  104  loses the downlink synchronization to the Femto-BS  101  and at S 507  because the downlink transmission has been paused at S 504 , the UE  102 A executes a cell search and attaches to the macro cell  103  of the macro-BS  100 . 
         [0047]      FIG. 6  shows an example of a sequence diagram illustrating a procedure according to the exemplary embodiment of the present invention, when the Femto-BS  101  recovers from the ES mode. When the UE  102 A within the coverage of the Femto cell  104  initiates a call or attempts to establish data connection, it carries out the random access procedure with the macro-BS  100 . The UE  102 A transmits a short signal called the PRACH preamble to initiate the procedure. The reconfigured PRACH receiver unit of the Femto-BS  101  tries to detect the PRACH preambles. 
         [0048]    At S 601 , the macro-BS  100  transmits downlink common channels to the UE  102 A, including, for example a physical channel of PBCH and/or PDSCH. When it is necessary for the UE  102 A to access to the macro-BS  100 , for example, when the UE  102 A initiates a call, the UE  102 A decodes the downlink common channels, obtains parameters needed for PRACH and transmits the PRACH preamble to the macro-BS  100  at S 602 . These steps corresponds to procedures defined in the respective 3GPP standards. 
         [0049]    At S 602 , since the PRACH preamble transmission is carried over the designated system frame and the sub frame according to the PRACH configuration of the macro-BS  100 , the Femto-BS  101  can detect it using the PRACH receiving window. The baseband receiver  202 B decodes the uplink data and checks whether or not the decoded uplink data is the PRACH preamble from the UE  102 A. The baseband receiver  202  converts the decoded uplink data taken in PRACH receiving window as the time-domain representation into the frequency-domain representation using an FFT. The output of the FFT, representing the received signal in the frequency domain, is multiplied with the complex-conjugate frequency-domain representation of the root ZC sequence and the result is fed through an IFFT. By observing the IFFT outputs, it is possible to detect which of the shifts of the 64 ZC root sequences has been applied to and its delay. Basically, a peak of the IFFT output in interval Ni corresponds to the Nith cyclically shifted sequence and the delay is given by the position of the peak within the interval. If the decoded uplink data is identified as the any one of the 64 ZC sequences, the baseband receiver  202 B informs that the PRACH preamble is detected to the controller  204 . In addition, the Femto-BS  101  checks if the PRACH preamble is sent from the UE  102 A within the Femto cell  104  by looking at the position of the detected peak in the PRACH receiving window. If it is determined that it has come from outside of the coverage of the Femto cell  104 , the Femto-BS  101  ignores the received PRACH preamble and remains in the ES mode. In order to determine whether the UE  102  is outside of the coverage of the Femto cell  104 , the Baseband receiver may check a time difference between the reception timing of the PRACH preamble and the start point of the PRACH receiving window. If the time difference is larger than a predetermined threshold, it may be determined that the UE  102  locates outside of the Femto cell  104 . Measurements defined in the 3GPP Technical Specification (TS) 36.214, “5.2.4 Timing advance” and “5.2.5 eNB Rx-Tx time difference” may be utilized to measure the time difference. When the Femto-BS  101  detects the PRACH preamble, the power information of the detected peak may be utilized together with the measured time difference of the PRACH preamble relative to the start of the PRACH receiving window to increase the determination accuracy. The UE calculates the initial power of the PRACH preamble by adding the downlink path loss estimate to a target power, so that the Macro-BS receives the preamble by the target power (PREAMBLE RECEIVED TARGET POWER), according to 3GPP TS 36.213, section 6.1. As the distance between the macro-BS  100  and the Femto-BS  101  is provided to the Femto-BS  101  by the ES Coordinator  401  in advance, the path-loss from the UE  102 A in the proximity of the Femto-BS  101  to the macro-BS  100  may be estimated by the Femto-BS  101 . By informing the PREAMBLE RECEIVED TARGET POWER at the macro-BS  100  to the Femto-BS  101 , the Femto-BS  101  may estimate the initial transmission power of the PRACH preamble at the UE  102 A. If the difference of the PRACH preamble power received by the Femto-BS  101  from the estimated UE&#39;s initial transmission power is larger than a predefined threshold, then the Femto-BS may regard the UE  102 A is outside of its cell range. 
         [0050]    In order to make the ES mode effective, when the Femto-BS  101  detects the PRACH preamble within a predetermined time period (for example, 5 minutes) after entering the ES mode, the Femto-BS  101  may ignore the detection and may not reconfigure the transmitters  202 A and  203 A. 
         [0051]    At S 603 , the macro BS  100  transmits L 1  ACK to the UE  102 A in response to the PRACH preamble. At S 604 , the Femto-BS  101  recovers from the ES mode. The controller  204  restores the original configuration in the PRACH receiver unit including reconfiguration of the baseband transmitter  202 A and the RF transmitter  203 A to restart to DL transmission. 
         [0052]    At S 605 , a bearer setup is executed between the macro-BS  100  and the UE  102 A. At S 606 , the Femto-BS  101  restarts the downlink transmission. In response to the DL transmission restart, when the UE  102 A detects the DL signal, at S 607 , the UE  102 A measures strength and quality of the DL signal to determine whether to execute a handover from the macro-BS  100  to the Femto-BS  101 . If the UE  102 A determines that the DL signal from the Femto-BS  101  is better than the one from the macro-BS  100 , it executes the handover at S 608 . 
         [0053]    It should be noted that the random access procedure using the PRACH preamble is executed not only when a user initiates a call but also when the UE  102 A is paged by the network. So the Femto-BS  101  at the ES mode does not need to monitor the paging channel to wake up at an incoming call attempt. 
         [0054]      FIG. 7  shows an exemplary structure of the PRACH preamble according to “3GPP TS 36.211 V.9.0.0”. The PRACH preamble  700  has a length of 1 ms, 2 ms or 3 ms according to the selected preamble format and consists of two parts including a Cyclic Prefix (CP) part  701  and a Sequence part  702 . The Sequence part  702  involves a root ZC sequence which is shifted using N CS  according to the table 5.7.2-2 defined in “3GPP TS 36.211 V.9.0.0, 5.7 Physical random access channel, 5.7.2 Preamble sequence generation”. Since further details of the generation method of the PRACH preamble are described also in “3GPP TS 36.211 V.9.0.0, 5.7 Physical random access channel, 5.7.2 Preamble sequence generation”, their explanation will be omitted in this embodiment. 
         [0055]    In the above, embodiment of the present invention is described for such a case where the present invention is applied to the LTE system. In this type of applications, the time and the frequency information regarding the PRACH preamble is broadcasted to the UE  102 . Such a UE  102  that wants to make a random access sends a preamble sequence within the specific time-frequency resource. The preamble sequence is selected at random from a set of 64 sequences that is broadcasted as well. 
         [0056]    The present invention may be applied to W-CDMA system. According to 3GPP TS 25.213 section 4.3.3, the PRACH preamble in the W-CDMA is made by 256 repetitions of 16 symbols of a signature, which comes to 4096 chips length. There are 16 signatures that a UE can choose from. The preamble is scrambled by a scrambling code. There are 8192 preamble scrambling codes which are grouped into 512 groups of 16 codes. One group is assigned to a cell and broadcasted to the UE  102  in the cell. The UE  102  selects one of the  16  codes in the group when it initiates the PRACH procedure. Thus, there are 256 combinations from 16 signatures and 16 scrambling codes for the UE  102  to choose from. The UE  102  can select one of the 12 RACH sub-channels and send PRACH preambles on the selected sub-channel. According to 3GPP TS 25.211 Section 7, the timing of a RACH sub-channel is defined by a constant offset from the AICH access slot in the downlink. The AICH access slot is aligned to the PCCPCH timing which is aligned to the SFN timing. Thus, the Femto-BS  101  needs to align its SFN timing to the one in the macro cell, taking the propagation delay between the macro cell  103  and the Femto cell  104 . Accordingly, the ES Coordinator  401  must provide the Femto-BS  101  with at least following parameters as the PRACH configuration of the macro-BS  100 : 
         [0057]    The frequency information and the SFN timing information of the macro cell; 
         [0058]    The propagation delay (PD) from the Femto cell  104  to the macro cell  103 ; 
         [0059]    The scrambling code group for the PRACH preamble in the macro cell. 
         [0060]    In the embodiment of the present invention, the trigger which releases the Femto-BS  101  from the ES mode is not limited to the received PRACH preamble. In another embodiment of the present invention, the Femto-BS  101  recovers from the ES mode in response to a wake-up instruction from the network. In this embodiment, the MME  131  as the ES Coordinator  401  detects the location of the UE  102 A trying to access the macro-BS  100  through the macro-BS  100  and wakes up the Femto-BS  101 , if the UE  102 A seems to be within the coverage of the Femto cell  104 . In addition to the location information, the identities of the UE  102 A may be utilized to determine which Femto-BS the UE  102 A may want to wake up. 
         [0061]    The information regarding which Femto-BS is in the cell range of which macro-BS needs to be provisioned to the database in advance, in the same way as the first embodiment. In addition, the information regarding which UE  102  has the right to use which Femto-BS may be provisioned in the database. 
         [0062]    In this embodiment, when the Femto-BS  101  enters the ES mode, the Femto-BS  101  does not have to obtain PRACH configuration information from the network and to reconfigure the PRACH receiver unit. The Femto-BS  101  just turns off the downlink transmission for the connected UE  102 A. The Femto-BS  101  notifies the ES Coordinator  401  of its transition into the ES mode. Possible conditions to go to the ES mode may be the same as ones described above in association with S 503  of  FIG. 5 . 
         [0063]    When the UE  102 A executes the random access procedure through the coupled macro-BS  100 , the ES Coordinator  401  looks up the Femto-BS  101  based on the location of the UE  102 A and optionally using the identity of the UE  102 A. If the looked-up Femto-BS  101  is in the ES mode, the network sends a signal to wake it up. The ES Coordinator  401  may wake other Femto-BSs up at the same time around the waked-up of the Femto-BS  101  to be prepared for hand-over between the Femto BSs. 
         [0064]      FIG. 8  shows another example of a sequence diagram illustrating a procedure according to the exemplary embodiment of the present invention, when the Femto-BS  101  recovers from the ES mode. 
         [0065]    At S 801 , the Femto-BS  101  enters the ES mode, based on the determination result by the controller  204  of the Femto-BS  101 . The controller  204  instructs the RF transmitter  203 A and optionally the baseband transmitter  202 A to stop the operation in order to turn off the downlink transmission for the UE  102 A. 
         [0066]    At S 802 , the Femto-BS  101  informs that the Femto-BS  101  has entered the ES mode to the MME  131  as the ES Coordinator  401 . At S 803 , the UE  102 A which needs to access to the macro-BS, transmits the PRACH preamble to the macro-BS  100 . In response to the PRACH preamble transmission, the macro BS  100  transmits L 1  ACK to the UE  102 A and a bearer setup is executed between the macro-BS  100  and the UE  102 A. 
         [0067]    At S 804 , the macro-BS  101  measures the position of the UE  102 A. The measurements may be executed using the GPS coordinate information received at the UE  102 A and transferred to the macro-BS  100  from the UE  102 A. The macro-BS may use a round trip time between the macro-BS  100  and the UE  102 A. 
         [0068]    At S 805 , the macro-BS  100  informs the location information and the identity of the UE  102 A to the MME  131 . In this embodiment, the identity of the UE may be MS-ISDN (Mobile Subscriber Integrated Services Digital Network Number), IMEI (International Mobile Equipment Identity), IMSI (International Mobile Subscriber Identity) or RNTI (Radio Network Temporary Identifier) but not limited to. At S 806 , MME  131  retrieves from the HSS  132  information regarding the Femto-BS  101  which is able to serve the UE  102 A, the location information and the identity of that UE  102 A have been informed from the macro-BS  100 . The HSS  132  stores a table for managing respective Femto-BSs. The table can manage accessibility to Femto-BSs from UEs  102  by associating respective Femto-BSs with identities of the UEs which are permitted to use the corresponding Femto-BS. For example, the Femto-BS  101  in  FIG. 1  is associated with the UE  102 A in the table, while the Femto-BS  101  is not associated with the UE  102 B. The table further manages the location information of respective Femto-BSs and the cell radius of its Femto cell. At S 806 , the MME  131  can retrieve from the table in HSS  132  information regarding the Femto-BS relating to the UE  102  informed by the macro-BS using the identity of the UE, since the identity of the UE is associated with the information of Femto-BS. In alternative embodiment the table in the HSS  132  may be stored in the database  303  in the MME  131 , or it may be stored in an independent database. 
         [0069]    At S 807 , MME  131  determines whether the UE  102 A informed from the macro-BS  100  can be served by the Femto-BS  101  based on the location information of the UE  102 , the location information of the Femto-BS  101  retrieved from the HSS  131  and the cell radius of the Femto cell. If the UE  102  is located within the coverage of the Femto-BS  101 , the MME  131  sends a waking-up instruction to the Femto-BS at S 808 . However, in order to make the ES mode effective, even if the condition is satisfied to send the wake-up instruction, the MME  131  may not send the wake-up instruction in case where a predetermined time period (for example, 5 minutes) has not passed since the reception of the notice from the Femto-BS  101  at S 802 . 
         [0070]    At S 809 , in response to the waking-up instruction from the MME  131 , the Femto-BS  101  recovers from the ES mode by the controller  204  reconfiguring the RF transmitter  203 A to restart the DL transmission. 
         [0071]    In the above, the GPS coordinates are used as the location information of the UE  102  accessing the macro-BS  100  at S 804 , however, accuracy of the location information based on the GPS coordinates may not be sufficient in case where the UE  102  resides under a roof. On the other hand, at S 806 , the identity information of the UE  102  is used as a criteria to determine whether the Femto-BS  101  should wake up from the ES mode. 
         [0072]    Therefore, the following further embodiment may be available based on the combination of the above described two embodiments. In this further embodiment, the Femto-BS  101  goes into the ES mode according to the S 501  to  5507  of  FIG. 5 , and the Femto-BS  101  notifies the MME  131  as the ES Coordinator of entering the ES mode. The PRACH preamble transmitted by the UE  102  (S 602 ) is detected by the Femto-BS  101  in the same way as described in association with  FIG. 6 . However, the Femto-BS  101  starts a timer before executing S 604 . At S 602 , the PRACH preamble is detected by the macro BS  100  as well, which is followed by the  5803  to  5807  of  FIG. 8 . After the MME  131  determines that the UE  102  sent the PRACH preamble is permitted to access the Femto-BS  101  at S 807 , the MME  131  transmits the wake-up instruction to the Femto-BS at S 808 . 
         [0073]    Then the Femto-BS  101  makes the final decision whether it recovers from the ES mode by taking the fact into account that the PRACH preamble was detected at the Femto-BS  101  as well. If the controller  204  of the Femto-BS  101  determines to wake up, then the steps S 604  and thereafter continue. If the controller  204  determines not to wake up, or if the timer has elapsed before receiving the wake-up instruction, the Femto-BS  101  remains in the ES mode. 
         [0074]    According to the above described embodiments, the present invention can decrease energy consumptions in the Femto-BS by turning off the DL transmission. This decrease interference between the Femto-BS and other BSs and/or other equipments in its proximity. The present invention does not require any extra operation by the end user to decrease the energy consumption of the Femto-BS, and does not affect background operations such as software and configuration updates and contents download to the UEs thanks to the autonomous recovery function from the ES mode according to each embodiment. 
         [0075]    The Femto-BS may implement any one of the above described embodiments and switch among them depending on the environment such as the number of UEs in the proximity and the radio condition where the Femto-BS is located as well as other conditions like whether it is for private usage where the authorized users are known or it is for public usage where unknown UE may access. It should be noted that the present invention is not limited to the home use and the present invention may be applied to such a Femto-BS that locates in a public space. 
         [0076]    While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.