Abstract:
A method for controlling a handoff in an optical distributed network system using MIMO (multi input multi output) is disclosed. The method includes steps of classifying a channel into a first channel and a second channel, and setting an active cell, receiving and measuring a signal strength of a predetermined cell, and determining on/off of the predetermined cell, when the predetermined cell turns on, releasing the first channel of the active cell, switching a path to a first channel of the predetermined cell, and keeping the second channel of the active cell, receiving and measuring a signal strength of the active cell, and when the measured signal strength is a preset critical value or less, turning off the active cell and when the active cell turns off, releasing the second channel of the active cell, and switching the path to a second channel of the predetermined cell.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit of the earlier filing date, under 35 U.S.C. §119, to that patent application entitled “Method For Controlling Handoff In Optical Distributed Network System Using Multi Input Multi Output,” filed in the Korean Intellectual Property Office on Feb. 14, 2006 and assigned Serial No. 2006-14368, the contents of which are incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to a method for controlling a handoff in an optical distributed network system, and in particular, to a method for controlling a handoff in an optical distributed network system using multi input multi output (MIMO). 
         [0004]    2. Description of the Related Art 
         [0005]    Since the late 1970s the U.S. has developed a cellular mobile communication system. Korea is now beginning to provide a voice communication service by an advanced mobile phone service (AMPS) method that is based on an analogous 1 st  generation (1G) mobile communication system. In the middle 1990s, a code division multiple access (CDMA) system was commonly used as a 2 nd  generation (2G) mobile communication system. The CDMA system provides voice and low-rate data services. 
         [0006]    International mobile telecommunication-2000 (IMT-2000), is under development is a 3 rd  generation (3G) mobile communication system beginning from the late 1990s with the objection of improved radio multimedia service, global roaming, and high-rate data service. The 3 rd  generation mobile communication system has been developed to transmit data at a higher rate to accommodate the rapid increases in the amount of service data of the mobile communication system. 
         [0007]    As the 3 rd  generation mobile communication system begins to be commonly used, attention is being transferred to a beyond 3 rd  generation (B3G) or 4 th  generation (4G) mobile communication system. The B3G or 4 th  mobile communication systems are being standardized with an aim of creating an effective association and an integral service of a wire communication network and a wireless communication network. 
         [0008]    Accordingly, the wireless communication network is requires a technology for transmitting a large capacity data that comes close to the capacity of the wire communication network. For this, a mobile communication system using multi input multi output (MIMO) is presently under investigation. 
         [0009]    In general, the MIMO employs a multi transmission antenna and a multi reception antenna instead of one transmission antenna and one reception antenna, thereby improving an efficiency of data transmission. Basically, the MIMO greatly increases the data transmission efficiency by transmitting and receiving several signals using several antennas at the same time. Thus, it has an advantage of transmitting much more data than in the existing mobile communication system without increasing the required bandwidth. 
         [0010]    There is a great possibility in which a carrier frequency for transmitting data is set to a band higher than an existing frequency of 5 GHz. The prospect is that a cell radius would be gradually reduced to keep a high data rate and the same capacity as an existing capacity according to a free space propagation model. Thus, a distributed network system based on a picocell of about one hundred meter seems to be required. 
         [0011]    A conventional method for executing the distributed network system on a per-picocell basis uses an optical relay or multi hop technology. 
         [0012]    The multi hop technology, a technology recently proposed for constructing a picocell having many cellular systems, can widen a service boundary of a cell without installation of a separate wire line. However, the multi hop technology has a drawback in that frequency interference occurs and thus, is limited in constructing and managing the cell. 
         [0013]    However, the method using the optical relay has an advantage that it does not cause such a drawback and thus, is free from propagation interference in managing the picocell. Thus, in actuality, the distributed network system is using the optical relay. 
         [0014]      FIG. 1  illustrates a construction of the distributed network system using the optical relay. 
         [0015]    Referring to  FIG. 1 , the distributed network system includes a base station transceiver subsystem (BTS)  101 , a base station controller (BSC)  102 , a base station (BS)  103 , and a radio access unit (RAU)  104 . 
         [0016]    In a detailed description of the distributed network system using the optical relay, the base station transceiver subsystem  101  performs a function of radio access with a mobile terminal (MT) (not shown), and a function of wire and radio access between the mobile terminal and the base station controller  102 . 
         [0017]    The base station controller  102  is positioned between the base station  103  and a mobile services switching center (not shown), and manages and controls the base station transceiver subsystem  101  and the base station  103 . 
         [0018]    The base station  103  connects with the base station transceiver subsystem  101 . The base station  103  receives a signal from the mobile terminal provided within its managing picocell, over a wireless channel, and transmits the received signal to the mobile services switching center. Similarly, the base station  103  transmits a signal coming from the mobile services switching center, to the mobile terminal over the wireless channel. 
         [0019]    In general, in the distributed network system using the optical relay, a large area is divided into a small area that is called picocell, for the effective use of the wireless channel. The distributed network system performs a wireless communication with the mobile terminal through the base station  103  provided in each picocell. The picocell defines a wireless coverage area established by the base station  103  positioned in each picocell. Similarly, each of the other picocells defines a related wireless coverage area established by a corresponding base station  103  positioned among the associated picocell. 
         [0020]    The radio access units  104  connect with the base station transceiver subsystem  101 , and define the picocells around the corresponding base station  103 . The base station  103  and the mobile terminal perform the wireless communication with each other using the radio access units  104 . Moving in position in course of the wireless communication within the picocells, the mobile terminal measures a signal strength of each picocell, and clamps to the most relevant picocell. 
         [0021]    The most important issue of the distributed network system constructed by the many picocells is to process a handoff that is frequently implemented when the mobile terminal moves between the picocells so as to keep a state of uninterrupted communication. 
         [0022]    The handoff refers to changing a communication path to a cell to which movement is implemented, to keep a communication when the mobile terminal moves to another base station (or sector) out of an in-service base station (or sector) in the general mobile communication system. Unlike an analogous method supporting a hard handoff where an existing communication line is first cut and then is connected to a new base station, a CDMA method supports even a soft handoff where communication paths with two base stations (or sectors) are concurrently maintained. 
         [0023]      FIG. 2  illustrates an example of a conventional operation of processing the soft handoff in the mobile communication system. 
         [0024]      FIG. 2  conceptually exemplifies variations of intensities of signals received from a base station 1   202  and a neighbor base station 2   203  when a mobile terminal  201  moves from “a” point to “b” point within an overland (OL) area where a service boundary cell  1  of the base station 1   202  and a service boundary cell  2  of the neighbor base station 2   203  are overlapped. 
         [0025]    The operation of processing the soft handoff in the mobile communication system will be described with reference to  FIG. 2 . In this exemplary processing example, it is assumed that the mobile terminal  201  receiving a service in the service boundary cell  1  of the base station 1   202  moves to the service boundary cell  2  of the neighbor base station 2   203 . 
         [0026]    In a little more detailed description of the operation, the soft handoff is generated under the control of the base station controller  102  when the mobile terminal  201  is positioned in the area where the service boundary cell  1  of the base station 1   202  and the service boundary cell  2  of the base station 2   202  are overlapped. 
         [0027]    In other words, the mobile terminal  201  receiving the service from the base station 1   202  detects a signal strength at or exceeding a preset value (T_ADD), from the base station 2   203 . The detected signal strength indicates a handoff is necessary in the course of movement toward base station 2   203 . The mobile terminal  201  transmits the detected signal strength from base station 1   203  to the corresponding base station controller  102 . 
         [0028]    In response to this, the base station controller  102  checks whether or not the base station 2   203  is in an idle state. Checking in the idle state, the base station controller  102  allocates a channel between the base station 2   203  and the mobile terminal  201 . If the channel allocation to the base station 2   203  is implemented, the base station 1   202  and the base station 2   203  provide services to the mobile terminal  201  over the respective allocated channels. 
         [0029]    As the mobile terminal  201  continues to move toward base station 2   203 , at the signal strength received from the base station 1   202  reduces below a preset value (T_DROP). In this case, the mobile terminal  201  drops the in-service base station 1   202 , and receives the service only from the base station 2   203 . The mobile terminal  201  has a time margin of a predetermined time in which base station 1   202  is dropped. In other words, only when the time margin lapses after the signal strength is detected below the preset value (T_DROP) does the mobile terminal  201  drop the base station 1   202 . This prevents the handoff from being implemented when the signal intensity spontaneously falls. 
         [0030]    Pilot channel information for allowing the mobile terminal  201  to measure the signal strengths received from the base stations  202  and  203  includes an active set, a candidate set, a neighbor set, and a remaining set. The active set denotes a pilot of the base station (or the sector), such as a forward traffic channel allocated to the mobile terminal  201 . The candidate set denotes a pilot that is not currently the active set but is received by a sufficient intensity. The neighbor set denotes a pilot that does not currently exist at the active set or the candidate set but can become the candidate set. The remaining set denotes all pilots possible other than the above set in a current system. 
         [0031]    As shown in  FIG. 2 , the soft handoff is implemented in the handoff region by monitoring the signal strength at the mobile terminal  201  of the signal strengths from the base stations  202  and  203  using several pieces of pilot channel information, and, in a predetermined critical region, generating a message that the handoff is active, and concurrently transceiving the same data from both of the base stations  202  and  203 . 
         [0032]    However, the conventional soft handoff in the mobile communication system has a drawback in that it is difficult to overcome an essential latency time caused by protocol processing between the base station controller  102  and the mobile terminal  201 , in processing the handoff to be frequently implemented in a plurality of the picocells in the distributed network system for the B3C or 4 th  mobile communication system. 
       SUMMARY OF THE INVENTION 
       [0033]    The present invention provides a method for controlling a handoff in an optical distributed network system using multi-input multi-output frequently implemented between picocells for a B3G or 4 th  generation mobile communication system, and thus minimizing a latency time and making uniform data transmission possible. 
         [0034]    In one embodiment, there is provided a method for controlling a handoff in an optical distributed network system using MIMO (multi-input multi-output). The method includes steps of classifying a channel for transceiving data with a mobile terminal, into a first channel and a second channel, and setting an active cell, receiving a signal strength of a predetermined cell from the mobile terminal, measuring the received signal strength of the predetermined cell, and determining on/off status of the predetermined cell, when the predetermined cell is determined to be on, releasing the first channel of the active cell, switching a path to a first channel of the predetermined cell, and keeping the second channel of the active cell, receiving a signal strength signal strength of the active cell from the mobile terminal, measuring the received signal strength of the active cell, and when the measured signal strength of the active cell is at least or below a preset critical value, turning off the active cell, and when the active cell turns off, releasing the second channel of the active cell, and switching the path to a second channel of the predetermined cell. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0035]    The above features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings in which: 
           [0036]      FIG. 1  illustrates a construction of a conventional distributed network system using an optical relay; 
           [0037]      FIG. 2  illustrates an example of a conventional operation of processing a soft handoff in a mobile communication system; 
           [0038]      FIG. 3  illustrates a construction of an optical distributed network system using MIMO according to an exemplary embodiment of the present invention; 
           [0039]      FIG. 4  illustrates an internal construction of a micro cell of  FIG. 3 ; 
           [0040]      FIG. 5  illustrates a construction of a distributed network controller according to an exemplary embodiment of the present invention; 
           [0041]      FIG. 6  illustrates a construction of a radio access unit according to an exemplary embodiment of the present invention; 
           [0042]      FIG. 7  is a flowchart illustrating a handoff process of an optical distributed network system according to an exemplary embodiment of the present invention; 
           [0043]      FIG. 8  illustrates an internal construction of a macro cell by a unitary base station transceiver subsystem according to an exemplary embodiment of the present invention; 
           [0044]      FIG. 9  illustrates an example of a handoff process at a place where two micro cells are overlapped according to an exemplary embodiment of the present invention; 
           [0045]      FIG. 10  illustrates an example of an on/off operation of a picocell depending on a position movement of a mobile terminal of  FIG. 9 ; 
           [0046]      FIG. 11  illustrates an example of a handoff process at a place where three micro cells are overlapped according to an exemplary embodiment of the present invention; and 
           [0047]      FIG. 12  illustrates an example of an on/off operation of a picocell depending on a position movement of a mobile terminal of  FIG. 11 . 
       
    
    
     DETAILED DESCRIPTION 
       [0048]    A preferred embodiment of the present invention will now be described in detail with reference to the included drawings. In the drawings, the same or similar elements are denoted by the same reference numerals even though they are depicted in different drawings. In the following description, a detailed description of known functions and configurations incorporated herein has been omitted for conciseness. 
         [0049]      FIG. 3  illustrates a construction of an optical distributed network system using multi-input multi-output (MIMO) system according to an exemplary embodiment of the present invention. 
         [0050]    Referring to  FIG. 3 , the inventive optical distributed network system using the MIMO includes a mobile switching center (MSC)  301 ; a base station controller (BSC)  302 ; a base station transceiver subsystem  303  including a distributed network controller  303 A; and radio access units  304 , . . . ,  306 . 
         [0051]    The optical distributed network system using the MIMO will be described in more detail below. The mobile switching center  301 , a switching system widely used in a mobile communication system, performs the function of call connection depending on whether the call is incoming and outgoing. In a system aiming at a data communication in a 3 rd  generation partnership project 2 (3GPP2) camp, the mobile switching system is developing into a type in which it performs just only a switching function among various functions. This is called “MSCe”. The mobile switching center  301  refers to a device for performing the switching function for the call connection in an earlier 2 nd  or 3 rd  generation mobile communication system. Including the switching function, the mobile switching center  301  should be interpreted as the same meaning even though it is commonly used by a different name in a system developed in the future. 
         [0052]    The base station controller  302  controls a plurality of base stations  103  refers to a device for a connection of a data call including a voice signal between the mobile switching center  301  and the base station transceiver subsystem  303 . Thus, the base station controller  302  basically provides a communication path between the mobile switching center  301  and the base station transceiver subsystem  303 , and controls radio resource allocation and scheduling of the base station transceiver subsystem  303 . In general, these control functions are known to those having an ordinary knowledge in a wireless communication system art and thus, will not be described in more detail, herein. 
         [0053]    The base station transceiver subsystem  303  includes the distributed network controller  303 A according to the present invention, and includes a plurality of radio transceivers (not shown in  FIG. 3 ). Each of the radio transceivers, which are units for performing a voice or data communication with one wireless terminal, performs transmission/reception of data in a predetermined radio band set in the wireless communication system. The radio transceivers can include modems for modulating/demodulating and encoding/decoding transmission/reception data. In the present invention, one base station transceiver subsystem  303  connects at its lower level with the plurality of radio access units (RAU)  304  . . .  306 . 
         [0054]    The operation of the connections of the plurality of radio access units  304  . . .  306  will be described. The inventive wireless communication system uses a high frequency band. Using the high frequency band, the wireless communication system has a feature of strong straightness (direct-line) and weak diffraction when transmitting an electric wave. As shown in  FIG. 3 , the radio access units  304  . . .  306  have predetermined surrounding service boundaries. These service boundaries are called “picocells” in the present invention. One micro cell  320  consists of the illustrated picocells  314  . . .  316 . 
         [0055]    One macro cell  330  constitutes at least two micro cells  320 . Thus, the base station transceiver subsystem  303  can manage one macro cell  330  or manage at least one micro cell  320 . That the base station transceiver subsystem  303  can manage cells of sizes different from each other, such as the micro cell  320  or the macro cell  330 . This management determined on the number of mobile terminals located in a corresponding service boundary. 
         [0056]    Similarly, the micro cell  320  constitutes several picocells  314  . . .  316 . The number of the picocells  314  . . .  316  is determined based on the existing conditions, such as expected traffic circumstances and the number of users at a place where a system is to be installed. This is a fact obvious to those having an ordinary knowledge in the art. The MIMO, which is a key technology of a B3G or 4 th  generation mobile communication system, is embodied on a per-picocell basis. The mobile terminal (not shown) can communicate with the radio access unit of a service boundary where it is positioned, using the MIMO. 
         [0057]      FIG. 4  illustrates an internal construction of the micro cell  320  of  FIG. 3 . The radio access unit will be typically denoted by a reference numeral  304  below. 
         [0058]    Referring to  FIG. 4 , the base station transceiver subsystem  303  is positioned in the middle of the micro cell  320  to minimize the distance from each radio access unit  304 . However, it would be recognized that the base station may be positioned within any picocell. The present invention provides the distributed network controller  305  within the base station transceiver subsystem  303  to manage the plurality of radio access units  304 . 
         [0059]    As shown in  FIG. 4 , the base station transceiver subsystem  303  can also constitute one picocell. 
         [0060]    Accordingly, the base station transceiver subsystem  303  has to have the same construction as the radio access unit  304 . 
         [0061]    Connection between the base station transceiver subsystem  303  and the radio access units  304  constituting the respective picocells  314  . . .  316  uses an optical communication method. The present invention proposes that a communication between the base station transceiver subsystem  303  and the radio access unit  304  is based on a baseband optical communication. 
         [0062]    In addition, a signal strength of the mobile terminal in the picocells  314  . . .  316  is normalized in the radio access unit  304 , and is concurrently transmitted as an analog signal to the distributed network controller  305  over a separate optical channel. This is to remove a decoding and frame decapsulation time, thereby allowing for a faster handoff. A construction for using the optical communication method will be in detail described with reference to  FIGS. 5 and 6 . 
         [0063]    A communication process implemented in the inventive optical distributed network system described with reference to  FIGS. 3 and 4  will now be described below. 
         [0064]    The respective radio access units  304 , which are installed within the picocells  314  . . .  316  provided within the micro cell  320 , perform a communication between an uplink and a downlink within the unitary micro cell  320 , using the same frequency and the same channel, as follows. 
         [0065]    Typically, downlink data is broadcast transmitted while data transmitted in the uplink is uni-casted. Accordingly, the downlink data transmitted employs a method in which all terminals can receive the data and isolate only their own data. On the other hand, the uplink transmitted data employs a uni-casting method so that a specific mobile terminal can designate a transmission signal. This is to prevent a consumption of a frequency channel more than is needed. Designating the uplink data on a per-user basis can secure a user channel by a multiplexing method such as orthogonal frequency division multiplexing (OFDM). 
         [0066]    When the mobile terminal moves within the unitary micro cell  320 , a change between the picocells  314  . . .  316  can be generated. The mobile terminal can implement the handoff between the picocells  314  . . .  316  in such a soft handoff manner that a data swap between sectors or the picocells  314 , . . . ,  316  is performed by a rake receiver. The soft handoff refers to a soft handoff implemented between the sectors included in a specific picocell. In other words, it is distinguished from a soft handoff implemented between the picocells  314  . . .  316 . The handoff between the picocells  314  . . .  316  uses a normalized value of a signal strength of a signal received from the base station  103 , in the mobile terminal. The distributed network controller  305  controls the handoff between the picocells  314  . . .  316 , using the normalized value. 
         [0067]      FIG. 5  illustrates a construction of the distributed network controller according to an exemplary embodiment of the present invention. 
         [0068]    Referring to  FIG. 5 , the inventive distributed network controller  305  includes a data path switch  501 , a micro cell HD controller  502 , a picocell HD controller  503 , a picocell power meter  504 , modems  521 - 1 , . . . ,  521 - n , and optical transceiving units  522 - 1 , . . . ,  522 - n.    
         [0069]    The construction of the distributed network controller  305  will be described in a little more detail. The data path switch  501  switches to the picocell depending on whether the distributed network controller  305  transmits a signal received from an upper level, to any picocell. Here, the upper level refers to the base station controller  302  or its corresponding node for the call connection, and the picocell includes a cell where the base station transceiver subsystem  303  is positioned. 
         [0070]    The micro cell HD controller  502  detects whether the distributed network controller  305  transmits the reception signal from the upper level to any micro cell  320 , using a header, and controls the data path switch  501  to switch to the detected micro cell  320 . 
         [0071]    The picocell HD controller  503  detects whether the distributed network controller  305  transmits the signal to any picocell of the micro cell  320  detected by the micro cell HD controller  502 , using a header, and controls the data path switch  501  to switch to the detected picocell. 
         [0072]    The picocell power meter  504  receives the signal strengths, which are received from the radio access unit  304  of the picocell by the respective mobile terminals provided within the service boundary of the picocell  314  and uses the received signal strengths for power control and future scheduling. 
         [0073]    Depending on the method adopted by the B3G or 4G mobile communication system, the modem  521 - 1 , . . . ,  521 - n  modulates and encodes data to transmit, and demodulates and decodes a received signal. A modulation method can employ binary PSK (BPSK), quadrature phase shift keying (QPSK), 16-quadrature amplitude modulation (16 QAM), and 64-quadrature amplitude modulation (64-QAM) methods. A 16-QAM or more modulation method can be employed for higher data rate. Further, a modulation method having a higher order than the 64-QAM method can be employed. An encoding and decoding method can employ a convolution code method, a turbo code method, a quasi-complementary turbo code (QCTC) method, or a low density parity check (LDPC) encoding method. For the higher data rate, it will be most effective to employ the LDPC method, the turbo code method, or the quasi-complementary code method. 
         [0074]    The optical transceiving units  522 - 1  . . .  522 - n  refer to devices for performing an optical communication with the radio access unit  304 . The optical transceiving units  522 - 1  . . .  522 - n  receive electric signals from the modems  521 - 1 , . . . ,  521 - n , convert the received signals into optical signals, and transmit the converted signals. The optical transceiving units  522 - 1 , . . . ,  522 - n  receive optical signals from the radio access unit  304 , convert the received optical signals into electric signals, and output the converted signals to the modems  521 - 1 , . . . ,  521 - n . The radio access unit  304  will be described with reference to  FIG. 6 . 
         [0075]    As shown in  FIG. 5 , the distributed network controller  305  includes the data path switch  501  for enabling data swap between the micro cells  320 . The data path switch  501  can be constituted as a logical switch. 
         [0076]    The present invention proposes that the switch between the micro cells  320  to adapt the downlink data transmitted from the distributed network controller  305  and transmit the adapted data to a relevant picocell. Such a switching system refers to an input/output system corresponding to one picocell (or radio access unit  304 ). The switching system has a structure in which data to be inputted to, and outputted from, each picocell can be handled using data paths of the same number as the number of antennas of a MIMO-blast system to be installed in the radio access unit  304 . It can be used for a handoff function between the micro cells  320 . 
         [0077]      FIG. 6  illustrates a construction of the radio access unit according to an exemplary embodiment of the present invention. 
         [0078]    Referring to  FIG. 6 , an optical transceiving unit  601  has the same construction as the optical transceiving units  522 - 1  . . .  522 - n  of  FIG. 5  and thus, its detailed description will be omitted. 
         [0079]    An amplifier  602  amplifies a received signal into a processible-level signal, and outputs the amplified signal to a demultiplexer (DEMUX)  603 . The demultiplexer  603  divides data to transmit on a per-antenna basis in order to transmit the data by the MIMO, and outputs the divided data to encoders  604 - 1  . . .  604 - 3  associated with respective antennas.  FIG. 6  exemplifies a case where three multi-antennas are used by the MIMO. However, in actuality, two or more antennas may be used in a typical MIMO configuration. Accordingly, two, three, four or more antennas can be used depending on the MIMO used for the wireless communication system. 
         [0080]    The encoders encode each received data suitably according to channel circumstances, and then transmit the encoded data through the antennas. It should be noted that a construction for wireless processing between the encoders  604 - 1  . . .  604 - 3  and the antennas is known in the art and thus, its discussion is omitted with regard to  FIG. 6 . 
         [0081]    The antennas receive signals from the mobile terminals, respectively, and output the received signals to decoders  607 - 1  . . .  607 - 3 . A construction of wireless processing for converting a wireless signal into a baseband signal is known in the art and thus, is omitted. The respective decoders  607 - 1  . . .  607 - 3  decode the signals encoded and transmitted by the mobile terminal. The respective decoders  607 - 1  . . .  607 - 3  output the decoded signals to a multiplexer (MUX)  606  and a picocell power normalizer  605 . The signal inputted to the multiplexer  606  is a data signal. The signal inputted to the picocell power normalizer  605  is information on the signal strength, which is received from the base station  103 , measured, and fed-back by each mobile terminal. The signal strength measured by the mobile terminal is normalized in the picocell power normalizer  605  and is provided to the base station transceiving subsystem  303  as described in  FIG. 5 . The signal normalized in the picocell power normalizer  605  is transmitted using a baseband transmission method as described above. 
         [0082]    Even in the multiplexer  606 , transmission is identically performed using the baseband transmission method. The multiplexer converts the signals received from the respective decoders  607 - 1  . . .  607 - 3 , into one data stream. 
         [0083]    The MIMO used in the present invention can employ N (N is a natural number of 2 or more) number of antennas, and can employ a blast method. Accordingly, the distributed network controller  305  can have N number of data paths for one terminal. However, the signal strength from the mobile terminal is normalized for a distance, an optical attenuation, and each characteristic of the optical transceiving unit  601  so that a fast L 1  handoff can be implemented between the picocells  314  . . .  316 . This operation is possible when the mobile terminal initially registers with the wireless communication system, and is linked with a power control operation within the micro cell  320  so that a normalization value can be outputted. This signal is transmitted to the distributed network controller  305  of the base station transceiver subsystem  303  through an optical cable over a channel separate from the data communication. 
         [0084]      FIG. 7  is a flowchart illustrating a handoff process of the optical distributed network system according to an exemplary embodiment of the present invention. 
         [0085]    Referring to  FIG. 7 , the distributed network controller  305  classifies a channel for transceiving data with the mobile terminal, into a first channel and a second channel, and sets an active cell (S 701 ). The present invention illustrates the use of two antennas in the MIMO system. The switch of the distributed network controller  305  operates as a two-way switch. Whole data (D) is transmitted from the base station transceiver subsystem  303  to the mobile terminal through data paths of data  1  (D 1 ) and data  2  (D 2 ) by a MIMO-blast. 
         [0086]    When the whole data from the base station transceiver subsystem  303  to the mobile terminal is expressed as “D(D 1 +D 2 )”, the mobile terminal transceives the data in an “A” position over two channels (fn) and (fn+1). The channel is classified as the first channel and the second channel respectively. This classification is to define the rule for the allocation and release operations when data is switched to a multi path in the handoff process and the use of additional channels is considered to be within the scope of the invention. 
         [0087]    In an exemplary embodiment of the present invention, the picocells constituting the micro cell are constructed using the same channel, and the micro cells constituting one macro cell are constructed using the channels different from each other, respectively. 
         [0088]    As described above, the respective radio access units, which are installed within the picocells, perform the uplink and the downlink communication within the unitary micro cell  320 , using the same frequency and the same channel. 
         [0089]    Employed is a method in which the data transmitted to the downlink is broadcasted, and the data transmitted to the uplink is uni-casted. When the mobile terminal moves within a unitary micro cell  302 , a change between picocells  303  can be generated. The mobile terminal can implement the handoff in such a soft handoff manner that the data swap between the sectors or the picocells  303  is performed by the rake receiver. 
         [0090]    As the mobile terminal moves in position, the distributed network controller  305  receives a signal strength of a predetermined cell from the mobile terminal. The distributed network controller  305  measures the received signal strength of the predetermined cell, and determines on/off status of the predetermined cell. Turning on the predetermined cell (S 702 ), the distributed network controller  305  checks a channel of the predetermined cell. When it is determined that the predetermined cell has a greater signal strength than an active cell, the distributed network controller  305  releases the first channel of the active cell and switches the path to a first channel of the predetermined cell, and keeps the second channel of the active cell (S 703 ). 
         [0091]    When the distributed network controller  305  turns on the predetermined cell, the handoff between the micro cells is implemented. However, an actual operation is implemented by an operation between the picocells. This operation is implemented by transmitting the signal strength of the predetermined cell from the mobile terminal to the distributed network controller  305  and performing the on/off status determination. When the predetermined cell turns on, switching the data  1  (D 1 ) on a first channel of an adjacent neighbor cell and keeping a channel of the in-transmission data  2  (D 2 ) as it is. 
         [0092]    As the mobile terminal moves in position, the distributed network controller  305  receives the signal strength of the active cell from the mobile terminal (S 704 ). The distributed network controller  305  measures the received signal strength of the active cell. When the signal strength of the active cell is a preset critical value or less (S 705 ), the distributed network controller  305  turns off the active cell (S 706 ). 
         [0093]    The distributed network controller  305  releases the second channel of the active cell for transmitting the data  2  (D 2 ), and switches the path for transmitting the data  2  (D 2 ), to a second channel of the predetermined cell (S 707 ). 
         [0094]    In the handoff method of the optical distributed network system, the handoff process of the mobile terminal implemented in a position where two micro cells overlap and in a position where three micro cells overlap will be described in more detail with reference to  FIGS. 9 and 11 , respectively. 
         [0095]      FIG. 8  illustrates a construction of the macro cell by a unitary base station transceiver subsystem according to an exemplary embodiment of the present invention. 
         [0096]    Referring to  FIG. 8 , the unitary base station transceiver subsystem  303  connects at its lower level with the plurality of radio access units (not shown). The radio access units connecting with the unitary base station transceiver subsystem  303  have predetermined surrounding service boundaries. These service boundaries are referred to as “picocells” in the present invention. A micro cell  302  composed of a plurality of picocells  803 . A macro cell  801  is constituted collecting at least two micro cells  802 . Thus, the base station transceiver subsystem  303  can manage one macro cell  801  or manage one micro cell  802 . Furthermore, the base station transceiver subsystem  303  can manage cells of sizes different from each other such as the micro cell  802  or the macro cell  801  is decided depending on the number of mobile terminals provided in a corresponding service boundary. 
         [0097]    As shown in  FIG. 8 , the micro cell  802  is composed of several picocells  803 . The number of the picocells  803  should be cautiously decided considering an existing condition, such as expected traffic circumstances and the number of users at a place where a system is to be installed. This is a fact obvious to those having an ordinary knowledge in the art. The MIMO, which is a key technology of a B3G or 4 th  generation mobile communication system, is embodied on a per-picocell basis. The mobile terminal can communicate with the radio access unit of a service boundary where it is positioned, using the MIMO. 
         [0098]    As described above, connections between the base station transceiver subsystem  303  and the radio access units constituting each picocell  803  employ an optical communication method. The present invention proposes that a communication between the base station transceiver subsystem  303  and the radio access unit  304  be based on a baseband optical communication. 
         [0099]    The base station transceiver subsystem  303  is positioned in the middle of the micro cell  320  to minimize a distance from each radio access unit. The present invention provides the distributed network controller  305  within the base station transceiver subsystem  303  to manage the plurality of radio access units. 
         [0100]    As shown in  FIG. 8 , the unitary macro cell  801  consists of three micro cells  802 . Each micro cell  802  consists of seven picocells  803 . However, it is obvious that the numbers of the micro cells  802  and the picocells  803  constituting the macro cell  801  can be different depending on a state of the communication system. 
         [0101]    The picocells  803  constituting the micro cell  802  are constructed using the same channel and the micro cells  802  constituting one macro cell  801  are constructed using the channels different from each other, respectively. 
         [0102]    Thus, the picocells within each micro cell  802  are constructed using a set of the same channel ((f 1 , f 2 ), (f 3 , f 4 ), (f 5 , f 6 )), respectively, and the inter micro cells  802  are constructed using the channels different from each other, respectively. 
         [0103]    As described above, each of the radio access units installed within the picocells provided within the micro cell  802  performs a communication using an uplink and a downlink communication within the unitary micro cell  802 , using the same frequency and the same channel. 
         [0104]    Employed is a method in which downlink transmitted data is broadcasted and data transmitted to the uplink is uni-casted. In other words, the data transmitted to the downlink employs a method in which, by broadcasting data of each mobile terminal, all terminals can receive and hold only their own data. On the other hand, the uplink transmitted data employs a uni-casting method so that a specific mobile terminal can distinguish a transmission signal. This is to prevent a consumption of a frequency channel more than is needed. Distinguishing the uplink data on a per-user basis can secure a user channel by a multiplexing method such as orthogonal frequency division multiplexing (OFDM). 
         [0105]    When the mobile terminal moves within the unitary micro cell  802 , the mobile terminal can implement the handoff between the picocells in such a soft handoff manner that the data swap between the sectors or the picocells is performed by the rake receiver. 
         [0106]    If the mobile terminal is positioned in STATE 1 , only the data swap by the rake receiver is performed. This operation is normalized in the radio access unit, and is realized by the signal strength received by the distributed network controller  305  from the mobile terminal. If the mobile terminal is positioned in STATE 2  or STATE 3 , the handoff between the picocells with different channels set is implemented. 
         [0107]      FIG. 9  illustrates an example of a handoff process at a place where two micro cells overlap according to an exemplary embodiment of the present invention. 
         [0108]      FIG. 9  exemplifies that two antennas are used in the MIMO. Accordingly, the switch of the distributed network controller  305  operates as a two-way switch. The whole data (D) is transmitted from the base station transceiver subsystem  303  to the mobile terminal through the paths of the data  1  (D 1 ) and the data  2  (D 2 ) by the MIMO-blast. 
         [0109]    If the whole data transmitted from the base station transceiver subsystem  303  to the mobile terminal is expressed as “D(D 1 +D 2 )”, the mobile terminal transceives the data in an “A” position over two channels (fn) and (fn+1), respectively. The channel is classified as the first channel and the second channel, respectively. In the “A” position, the first channel of the D 1  is denoted by “f 1 ”, and the second channel of the D 2  is denoted by “f 2 ”. 
         [0110]    This classification is to define a rule for allocation and release operations when data is switched to a multi path in the handoff process. As described in  FIG. 8 , when the mobile terminal moves from the “A” position to a “B” position, a change between the picocells is generated within the same micro cell  802 . Thus, in the mobile terminal, only the data swap between the sectors or the picocells is performed by the rake receiver. 
         [0111]    This process is performed under the control of the radio access unit (not shown). The “B” position is in the same state as the “A” position. The mobile terminal transceives the data over the channels (f 1 ) and (f 2 ), using the same method as in the “A” position. 
         [0112]    If the mobile terminal moves from the “B” position to a “C” position, the handoff between the micro cells  802  is implemented. However, the actual operation is implemented by the operation between the picocells. This operation is implemented by transmitting a signal strength of a cell  2  from the mobile terminal to the distributed network controller  305  and performing the on/off state determination, and, at a time point when the cell  2  turns on, switching the data  1  (D 1 ) by a first channel (f 3 ) of channels (P 3 ) and (f 4 ) of an adjacent neighbor cell and keeping the in-transmission data  2  (D 2 ) by the second channel (f 2 ) of a cell  1  as it is. 
         [0113]    If the mobile terminal moves to a “D” position, the distributed network controller  305  turns off the cell  2 , releases the existing second channel (f 2 ) of the data  2  (D 2 ), and switches to a new second channel (f 4 ) of the cell  2 . 
         [0114]    Moving to an “E” position, the mobile terminal transceives the data over the channels (f 3 ) and (f 4 ) of the data  1  (D 1 ) and the data  2  (D 2 ) switched in the “D” position. In this case, the change between the picocells is generated within the same micro cell  802 . Thus, in the mobile terminal, only the data swap between the sectors or the picocells is performed by the rake receiver. 
         [0115]      FIG. 10  illustrates an example of an on/off operation of the picocell depending on a position movement of the mobile terminal of  FIG. 9 . 
         [0116]    Referring to  FIG. 10 , it is shown that the on/off operation of the picocell is performed depending on the position movement of the mobile terminal. As shown in  FIG. 10 , the cell  1  turns off when it enters the “C” position where the signal strength received from the mobile terminal is less than a critical value, and the cell  2  turns on from when it enters the “C” position where the signal strength received from the mobile terminal is at least the critical value. 
         [0117]    As described in  FIG. 9 , if the whole data transmitted from the base station transceiver subsystem  303  to the mobile terminal is expressed as “D(D 1 +D 2 )”, the mobile terminal transceives the data in the “A” position over the two channels (f 1 ) and (f 2 ), respectively. 
         [0118]    When the mobile terminal moves from the “A” position to the “B” position, the change between the picocells is generated within the same micro cell  802 . Thus, in the mobile terminal, only the data swap between the sectors or the picocells is performed by the rake receiver. The “B” position is in the same state as the “A” position. The mobile terminal transceives the data over the channels (f 1 ) and (f 2 ), using the same method as in the “A” position. 
         [0119]    When the mobile terminal moves from the “B” position to the “C” position, the distributed network controller  305  receives the signal strength of the cell  2  from the mobile terminal, and performs the on/off determination. If the cell  2  turns on, the hand off between the micro cells  802  is implemented. 
         [0120]    When the cell  2  turns on, the data D 1  is switched by the first channel (f 3 ) of the channels (f 3 ) and (f 4 ) of the adjacent neighbor cell, and the data D 2  in transmission is kept by the second channel (f 2 ) of the cell  1  as it is. 
         [0121]    When the mobile terminal moves to the “D” position, the distributed network controller  305  turns off the cell  2 , releases the existing second channel (f 2 ) of the data D 2 , and switches to the new second channel (f 4 ) of the cell  2 . 
         [0122]    Moving to the “E” position, the mobile terminal transceives the data over the channels (f 3 ) and (f 4 ) of the data D 1  and D 2  switched in the “D” position. In this case also, as described above, the change between the picocells is generated within the same micro cell  802 . Thus, in the mobile terminal, only the data swap between the sectors or the picocells is performed by the rake receiver. 
         [0123]    As the mobile terminal moves in position as above, the handoff is implemented at the place where two micro cells are overlapped. 
         [0124]      FIG. 11  illustrates an example of a handoff process at a place where three micro cells are overlap according to an exemplary embodiment of the present invention. 
         [0125]    In the handoff process at the place where three micro cells overlap, it is exemplified that two antennas are used in the MIMO system in the same manner as that of  FIG. 9 . If whole data transmitted from the base station transceiver subsystem  303  to the mobile terminal is expressed as “D(D 1 +D 2 )”, the mobile terminal transceives data in an “A” position over two channels (fn) and (fn+1), respectively. The channel is classified into a first channel and a second channel, respectively. A first channel and a second channel of a cell  1  are denoted by “f 1 ” and “f 2 ”, a first channel and a second channel of a cell  2  are denoted by “f 3 ” and “f 4 ”, and a first channel and a second channel of a cell  3  are denoted by “f 5 ” and “f 6 ”. 
         [0126]    The handoff process implemented as the mobile terminal moves from the “A” position to a “B” position is the same as the handoff process of  FIG. 9  at the place where two micro cells overlap and thus, its description will be omitted. 
         [0127]    If the mobile terminal moves from the “B” position to a “C” position, the distributed network controller  305  receives a signal strength of the cell  3  from the mobile terminal, and turns on the cell  3 . If so, in the “C” position, the D 1  transmitted over the first channel (f 3 ) of the cell  2  can be kept by the first channel (f 3 ) or can be set to the first channel (f 5 ) of the cell  3 . However, the signal strength received by the distributed network controller  305  from the mobile terminal can be switched to a channel of the channels (f 3 ) and (f 5 ) in the “C” position having better characteristics. 
         [0128]    As the cell  1 , the cell  2 , and the cell  3  overlap in the “C” position, the signal strength from the mobile terminal can be differentiated, and a movement direction of the mobile terminal among the neighbor picocells can be predicted. The movement direction of the mobile terminal can be obtained by calculating a movement direction of a cell where the mobile terminal moves, using the base station transceiver subsystem  305 . 
         [0129]    In an exemplary embodiment of the present invention, the signal strength received by the distributed network controller  305  from the mobile terminal in the “C” position is best in the cell  3 , Accordingly, the first channel (f 3 ) of the cell  2  is released, and the first channel (f 5 ) of the cell  3  is switched. At that time point, the existing second channel (f 2 ) of the cell  1  for transmitting the data  2  (D 2 ) is released, and, as the first channel (f 5 ) of the cell  3  is selected, the second channel (f 4 ) of its opposite cell  2  is switched. This is to keep a dual path for data transmission from two cells, in a handoff region. 
         [0130]    By the above operation, a signal strength profile in each cell can be naturally sequenced along the movement direction of the mobile terminal. 
         [0131]    In a “D” position, the mobile terminal keeps the final first channel and second channel depending on the movement direction in the “C” position. Last, in an “E” position, as the cell  2  turns off under the control of the distributed network controller  305 , the mobile terminal switches to the second channel (f 6 ) of the cell  3 . 
         [0132]      FIG. 12  illustrates an example of an on/off operation of the picocell depending on the position movement of the mobile terminal of  FIG. 11 . 
         [0133]      FIG. 12  shows the on/off operation of the picocell depending on the position movement of the mobile terminal. The cell  1  turns off when it enters the “D” position where the signal strength received from the mobile terminal is less than a critical value. 
         [0134]    The picocell  2  turns on starting from the “B” position where the signal strength received from the mobile terminal is at least the critical value. The picocell  3  turns on starting from the “C” position where the signal strength received from the mobile terminal is at least the critical value. When the mobile terminal moves from the “A” position to the “B” position, the distributed network controller  305  receives the signal strength of the cell  2 , and turns on the cell  2 . If so, the handoff between the micro cells  802  is implemented. When the cell  2  turns on, the data D 1  is switched to the first channel (f 3 ) of the channels (f 3 ) and (f 4 ) of an adjacent neighbor cell, and the data D 2  in transmission is kept by the second channel (t 2 ) of the cell  1  as it is. 
         [0135]    When the mobile terminal moves from the “B” position to the “C” position, the distributed network controller  305  receives the signal strength of the cell  3 , and turns on the cell  3 . If so, the distributed network controller  305  releases the first channel (f 3 ) of the cell  2  in the course of transmitting the data D 1 , and switches to the first channel (f 5 ) of the cell  3  where the signal strength from the mobile terminal is best. The existing second channel (f 2 ) of the cell  1  in the course of transmitting the data D 2  is released, and as the first channel (f 5 ) of the cell  3  is selected, the second channel (f 4 ) of its opposite cell  2  is switched. 
         [0136]    In the “D” position, the mobile terminal keeps the final first channel and second channel depending on the movement direction in the “C” position. Lastly, in the “E” position, as the cell  2  turns off under the control of the distributed network controller  305 , the second channel (f 6 ) of the cell  3  is switched. 
         [0137]    As the mobile terminal moves in position as above, the handoff is implemented at the place where three micro cells are overlapped. 
         [0138]    As described above, in the present invention, the handoff frequently implemented between the picocells is controlled in the optical distributed network system using the multi input multi output for the B3G or fourth generation mobile communication system, thereby minimizing a latency time and enabling uniform data transmission. 
         [0139]    The above-described methods according to the present invention can be realized in hardware or as software or computer code that can be stored in a recording medium such as a CD ROM, an RAM, a floppy disk, a hard disk, or a magneto-optical disk or downloaded over a network, so that the methods described herein can be rendered in such software using a general purpose computer, or a special processor or in programmable or dedicated hardware, such as an ASIC or FPGA. As would be understood in the art, the computer, the processor or the programmable hardware include memory components, e.g., RAM, ROM, Flash, etc. that may store or receive software or computer code that when accessed and executed by the computer, processor or hardware implement the processing methods described herein. 
         [0140]    While the invention has been shown and described with reference to a preferred embodiment thereof, it will be understood by those skilled in the art that various changes in form and details may be made without departing from the spirit and scope of the invention as defined by the appended claims.