Abstract:
Disclosed is a remote access unit for transmitting and receiving upstream and downstream data in which channels having different transmission scheme are multiplexed, and an optical network for bi-directional wireless communication using same. The remote access unit includes an antenna for receiving the downstream data and wirelessly transmitting same and for receiving the upstream data and providing same to the remote access unit, a switch for outputting downstream time division channels of the downstream data to the antenna and for receiving upstream time division channels of the upstream data from the antenna, and a controller for controlling the switch in order to prevent the upstream and downstream time division channels from overlapping.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims the benefit of the earlier filing dates of those patent applications, each entitled “Remote Access Unit and Optical Network for Bidirectional Wireless Communication Using the Same” filed in the Korean Intellectual Property Office on Aug. 29, 2005 and Jul. 19, 2005, and assigned Serial Nos. 2005-79399 and 2005-65388, respectively, the contents of which are hereby incorporated by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to a technology of transmitting radio frequency for wireless communication, and more particularly to a remote access unit for transmitting and receiving channels having different transmission schemes in a bi-direction.  
         [0004]     2. Description of the Related Art  
         [0005]     In recent wireless communication, various kinds of wireless communication services are provided. These services are referred to as 2 G wireless communication service, 3 G wireless communication service, wireless local area network (LAN), wireless Internet, etc. Additional wireless communication services use a radio-over-fiber type optical network such as an optical communication network combined with a wireless transmission system. The radio-over-fiber type optical network converts radio signals into optical signals and then transmits the radio signals as a form of the optical signals and converts the optical signals into radio signals to transmit the radio signals wirelessly.  
         [0006]      FIG. 1A  illustrates a schematic block diagram showing a conventional remote access unit. Referring to  FIG. 1A , the conventional remote access unit  100  including a first amplifier  110  for amplifying downstream data in which time division channels (TDD) and frequency division channels (FDD) are multiplexed, a second amplifier  150  for amplifying upstream signals received wirelessly, an antenna  130  for receiving a upstream data wirelessly and transmitting the downstream data wirelessly, a circulator  120  for providing the downstream data to the antenna  130  and also providing the upstream data received from the antenna toward the second amplifier  150 , and a filter  140  located between the circulator  120  and the second amplifier  150 .  
         [0007]      FIG. 1B  illustrates an exemplary wave form of upstream data in which upstream time division channels, upstream frequency division channel and frequency division channels of the downstream data are multiplexed.  FIG. 1B  illustrates a state in which some of the downstream frequency division channels are removed from the upstream data by the filter  140 .  
         [0008]      FIG. 2  illustrates an exemplary schematic block diagram showing a structure of an optical network including the conventional remote access unit shown in  FIG. 1A . Referring to  FIG. 2 , the conventional optical network  200  includes a central station  210  and a remote access unit  220 . The central station  210  includes an electric-optical converter  211  for converting downstream data into downstream optical signals, and a photo-electric converter  212  for converting upstream optical signals into upstream data.  
         [0009]     The remote access unit  220  includes a photo-electric converter  221  for photo-electrically converting the downstream optical signals into downstream data, a first amplifier  222  for amplifying the downstream data, an electric-optical converter  226  for electric-optically converting the upstream data into upstream optical signals, and a second amplifier  225  for amplifying the upstream data. A circulator  223  is located between the first and second amplifiers  222  and  225 , which is connected to an antenna  224 .  
         [0010]     Therefore, the conventional remote access unit and the optical network including the same can input and output signals in which time division channels and frequency division channels are multiplexed, but cannot separate and process channels from one another, based on transmitting schemes. Thus, a part of downstream signals is mixed with upstream signals, thereby causing a deterioration of elements for processing upstream signals.  
       SUMMARY OF THE INVENTION  
       [0011]     Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art and provides additional advantages, by providing a remote access unit which can perform bi-directional wireless communication using upstream and downstream signals in which time division channels and frequency division channels are multiplexed, and which also can restrict a deterioration of elements caused by mixing the transmitted signals in opposite directions.  
         [0012]     According to an aspect of the present invention, there is provided a remote access unit for transmitting and receiving upstream and downstream data in which channels having different schemes are multiplexed, which comprises an antenna for transmitting and receiving the downstream data wirelessly and for inputting the upstream data wirelessly received into the remote access unit, a switch for outputting downstream time division channels of the downstream data to the antenna and for receiving upstream time division channels of the upstream data inputted through the antenna, and a controller for controlling the switch in order to prevent the upstream and downstream time division channels from overlapping. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0013]     The above features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0014]      FIGS. 1A-1C  illustrate a schematic block diagram showing the conventional remote access unit and multiplexed signal composition;  
         [0015]      FIG. 2  illustrates a schematic block diagram showing the structure of an optical network including the conventional remote access unit;  
         [0016]      FIG. 3  illustrates a schematic block diagram showing the structure of a remote access unit according to the first embodiment of the present invention;  
         [0017]      FIGS. 4A through 4C  present graphs illustrating a process of treating data in the remote access unit shown in  FIG. 3 ;  
         [0018]      FIG. 5  illustrates a schematic block diagram showing the structure of a remote access unit according to the second embodiment of the present invention;  
         [0019]      FIG. 6  illustrates a schematic block diagram showing the structure of an optical network for bi-directional communication according to the third embodiment of the present invention;  
         [0020]      FIG. 7  illustrates a schematic block diagram showing the structure of an optical network for bi-directional communication according to the fourth embodiment of the present invention;  
         [0021]      FIG. 8  illustrates a schematic block diagram showing the structure of an optical network for bi-directional communication according to the fifth embodiment of the present invention;  
         [0022]      FIGS. 9 and 10  presents graphs illustrating the operation of the optical network shown in  FIG. 8 ;  
         [0023]      FIG. 11  illustrates a schematic block diagram showing the structure of an optical network for bi-directional communication according to the sixth embodiment of the present invention;  
         [0024]      FIG. 12  illustrates a graph illustrating the operation of the optical network shown in  FIG. 11 ; and  
         [0025]      FIG. 13  is a schematic block diagram showing the structure of an optical network for bi-directional communication according to the seventh embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0026]     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. For the purposes of clarity and simplicity, a detailed description of known functions and configurations incorporated herein will be omitted to avoid making the subject matter of the present invention unclear.  
         [0027]      FIG. 3  illustrates a schematic block diagram showing the structure of a remote access unit according to the first embodiment of the present invention, and  FIGS. 4A through 4C  presents graphs illustrating a process of treating data in the remote access unit shown in  FIG. 3 . Referring to  FIG. 3  and  FIGS. 4A through 4C , the remote access unit  300  according to this embodiment of the present invention includes first and second duplexer  320  and  350 , a triplexer  330 , downstream and upstream amplifiers  310  and  360 , a switch  370 , a controller  301 , and an antenna  340 . The remote access unit  300  transmits downstream data, in which downstream time division channels and downstream frequency division channels are multiplexed, through the antenna  340  wirelessly, and receives wireless upstream data, in which upstream time division channels and upstream frequency channels are multiplexed.  
         [0028]     The first duplexer  320  divides the downstream data into downstream time division channels and downstream frequency division channels. As shown in  FIG. 4A , the down frequency division channels are outputted from port  7  to a input port  12  triplexer  330 , while the downstream time division channels are inputted into the triplexer  330  through the switch  370 , represented as proceeding from port  6  of duplexer  1  to input port  1  of switch  370  and from output port  2  of switch  370  to port  11  of triplexer  330 .  
         [0029]     As shown in  FIG. 4B , the second duplexer  350  combines the upstream frequency division channels directly inputted from the triplexer  330  (i.e. output port  14  of triplexer  330  to input port  10  of duplexer  350 ) with the upstream time division channels inputted through the switch  370  so as to output the resultant toward the first upstream amplifier  360 .  
         [0030]     The triplexer  330  is located between the first and second duplexers  320  and  350 . As shown in  FIG. 4C , the triplexer  330  divides the upstream data inputted from the antenna  340  into upstream time division channels and upstream frequency division channels, while combining the downstream time division channels and the downstream frequency division channels with the downstream data and outputting the downstream data toward the antenna  340 .  
         [0031]     The switch  370  is located between the first and second duplexers  320  and  350 , and is connected with the triplexer  330 . Switch  370  is connected to the triplexer  330  or the second duplexer  350  based on control signals of the controller  301 . Specifically, the switch  370  outputs the downstream time division channels to the triplexer  330  or outputs the upstream time division channels to the second duplexer  350 , depending on the control signals of the controller  301 .  
         [0032]     The downstream amplifier  310  amplifies the downstream data and inputs the amplified downstream data into the first duplexer  320 . The upstream amplifier  360  amplifies the upstream data inputted from the second duplexer  350  and outputs the amplified upstream data outside the remote access unit  300 . A high power amplifier may be used as the first downstream amplifier  310 , and a low noise amplifier may be used as the first upstream amplifier  360 .  
         [0033]     The switch  370  and the triplexer  330  have an excellent capability for dividing channels between ports as compared with an element such as a circulator. Further, the switch  370  and the triplexer  330  can remove downstream data reflected by the antenna that is introduced into the upstream data. Therefore, the present invention restrains the downstream data from being introduced into the upstream data, thereby preventing deterioration of elements for upstream link from occurring, and also preventing a data loss and a malfunction of the elements, which may be caused due to the deterioration of the elements.  
         [0034]      FIG. 5  illustrates a schematic block diagram showing the structure of a remote access unit according to the second embodiment of the present invention. Referring to  FIG. 5 , the remote access unit  400  according to this embodiment of the present invention includes first and second duplexers  410  and  460 , a triplexer  430 , a first downstream amplifier  420 , a first upstream amplifiers  450 , a second downstream amplifier  490 , a second upstream amplifier  470 , a switch  480 , a controller  401  controlling the switch  480  and an antenna  440  for transmitting downstream data wirelessly and inputting upstream data received wirelessly into the remote access unit  400 .  
         [0035]     The first duplexer  410  divides the downstream data into downstream time division channels and downstream frequency division channels, as previously discussed. Then, the first duplexer  410  outputs the downstream time division channels toward the second downstream amplifier  490 , while outputting the downstream frequency division channels toward the first downstream amplifier  420 . The first downstream amplifier  420  amplifies and outputs the downstream frequency division channels to the triplexer  430 , and the second downstream amplifier  490  amplifies and outputs the downstream time division channels to the switch  480 .  
         [0036]     The triplexer  430  combines the downstream time division channels inputted from the switch  480  and the downstream frequency division channels inputted from the first amplifier  420  with the downstream data, and then outputs the downstream data toward the antenna  440 . The triplexer  430  further receives the upstream data inputted through the antenna  440  and divides it into upstream time division channels and upstream frequency division channels. Furthermore, the triplexer  430  outputs the upstream time division channels to the second upstream amplifier  470  through the witch  480 , while outputting the upstream frequency division channels to the first upstream amplifier  450 . The first upstream amplifier  450  amplifies and outputs the upstream frequency division channels toward the second duplexer  460 , and the second downstream amplifier  470  amplified and outputs the upstream time division channels toward the second duplexer  460 .  
         [0037]     The switch  480  is connected to the triplexer  430  or the second upstream amplifier  470 , depending on control signals of the controller  401 . Specifically, the switch  480  outputs the downstream time division channels to the triplexer  430 , or outputs the upstream time division channels to the second duplexer  460  through the second upstream amplifier  470 .  
         [0038]     The second duplexer  460  combines the upstream time division channels inputted from the second upstream amplifier  470  and the upstream frequency division channels inputted from the first upstream amplifier  450  with the upstream data and then outputs the upstream data.  
         [0039]      FIG. 6  illustrates a schematic block diagram showing the structure of an optical network for bi-directional communication according to a third embodiment of the present invention. Referring to  FIG. 6 , the optical network  500  according to this embodiment of the present invention includes a central station  510 , a remote access unit  520 , and first and second optical cables  501  and  502  for linking the central station  510  and the remote access unit  520 .  
         [0040]     The first optical cable  501  transmits downstream optical signals toward the remote access unit  520 , and the second optical cable  502  transmits upstream optical signals toward the central station  510 .  
         [0041]     The central station  510  includes an electric-optical converter for electric-optically converting the downstream data into the downstream optical signals and outputting the downstream optical signals to the remote access unit  620 , and a photo-electric converter  512  for photo-electrically converting the upstream optical signals inputted from the remote access unit  520  into the upstream data. The downstream data includes multiplexed downstream frequency division channels and downstream time division channels, and control signals. The upstream data includes multiplexed upstream frequency division channels and upstream time division channels.  
         [0042]     The remote access unit  520  includes downstream and upstream amplifiers  525  and  524 , a downstream photo-electric converter  531 , an upstream electric-optical converter  532 , first and second duplexers  521  and  523 , a switch  526 , a triplexer  522 , an antenna  527  wirelessly transmitting the downstream data and receiving the upstream data, a controller  529 , and a demultiplexer  528 .  
         [0043]     The downstream photo-electric converter  531  is linked to the downstream electric-optical converter  511  of the central station by means of the first optical cable  501 . Photo-electric converter  531  converts the downstream optical signals into the downstream data and then outputs the downstream data toward the demultiplexer  528 . The upstream electric-optical converter  532  is linked to the upstream photo-electric converter  512  of the central station  510  by means of the second optical cable  502 . The electro-optical  532  converts the upstream data into the upstream optical signal and then output the optical signals to the central station  510 .  
         [0044]     The demultiplexer  528  divides the downstream data and control signals from each other, and then outputs the control signals to the controller  529  and the downstream data toward the downstream amplifier  525 .  
         [0045]     The first duplexer  521  divides the downstream data inputted from the downstream amplifier  525  into downstream time division channels and downstream frequency division channels, and then outputs the downstream time division channels to the switch  526  and outputs the downstream frequency division channels toward the triplexer  522 .  
         [0046]     The triplexer  522  divides the upstream data inputted through the antenna  527  into upstream time division channels and upstream frequency division channels, and then outputs the upstream time division channels to the second duplexer  523  through the switch  526  and directly outputs the upstream frequency division channels toward the second duplexer  523 . Furthermore, the triplexer  522  combines the downstream frequency division channels inputted from the first duplexer  521  and the downstream time division channels inputted from the switch  526  with downstream data, and then outputs the downstream data toward the antenna  527 .  
         [0047]     The second duplexer  523  combines the upstream time division channels and the upstream frequency division channels with the upstream data and then outputs the upstream data. The upstream amplifier  524  amplifies and outputs the upstream data toward the upstream electric-optical converter  532 .  
         [0048]     The switch  526  selectively connects the first duplexer  521  or the second duplexer  523  to the triplexer  522 , depending on control signals of the controller  529 .  
         [0049]      FIG. 7  illustrates a schematic block diagram showing the structure of an optical network for bi-directional communication according to a fourth embodiment of the present invention. Referring to  FIG. 7 , the optical network  600  according to this embodiment of the present invention includes a central station  610 , a remote access unit  620 , and first and second optical cables  601  and  602  for linking the central  610  and the remote access unit  620 .  
         [0050]     The central station  610  includes a downstream electric-optical converter  611  for electric-optically converting downstream data into downstream optical signals and outputting the downstream optical signals toward the remote access unit  620 , and an upstream photo-electric converter  612  for photo-electrically converting upstream optical signals received from the remote access unit  620  into upstream data.  
         [0051]     The remote access unit  610  includes first and second downstream amplifiers  624  and  625 , first and second upstream amplifiers  629  and  628 , a downstream photo-electric converter  631 , an upstream electric-optical converter  632 , first and second duplexers  621  and  623 , a switch  626 , a triplexer  622 , an antenna  627  transmitting downstream data and receiving upstream data, a controller  634 , and a demultiplexer  633 .  
         [0052]     The demultiplexer  633  separates control signals from the downstream data and then outputs the control signals to the controller  634 . The controller  634  controls the switch  626  depending on the control signals. The switch  626  selectively connects the second downstream amplifier  625  or the second upstream amplifier  628  to the triplexer  633  according to the control signals of the controller  634 .  
         [0053]     The first duplexer  621  divides the downstream data inputted through the demultiplexer  633  into downstream frequency division channels and downstream time division channels, which in turn outputs the downstream time division channels toward the second downstream amplifier  625  and the downstream frequency division channels to the first downstream amplifier  624 .  
         [0054]     The first downstream amplifier  624  is located between the first duplexer  621  and the triplexer  622 , and amplifies and outputs the downstream frequency division channels toward the triplexer  622 . The second downstream amplifier  625  is disposed between the first duplexer  621  and the switch  626 , which amplifies and outputs the downstream time division channels toward the switch  626 .  
         [0055]     The first upstream amplifier  629  is located between the triplexer  622  and the second duplexer  623 , and amplifies and outputs the upstream frequency division channels toward the second duplexer  623 . The second upstream amplifier  628  is disposed between the switch  626  and the second duplexer  623 , and amplifies and outputs the upstream time division channels to the second duplexer  623 .  
         [0056]     The second duplexer  623  combines the upstream time division channels and the upstream frequency division channels into the upstream data stream and then outputs the upstream data toward the upstream electric-optical converter  632 . The upstream electric-optical converter  632  converts the upstream data into upstream optical signals and outputs the upstream optical signals to the central station  610 .  
         [0057]      FIG. 8  illustrates a schematic block diagram showing the structure of an optical network for bi-directional communication according to a fifth embodiment of the present invention. Referring to  FIG. 8 , the optical network  700  according to this embodiment of the present invention includes a central station  710 , a remote access unit  720 , and first and second optical cables  701  and  702  for linking the central station  710  and the remote access unit  720 .  
         [0058]     The central station  710  includes an upstream photo-electric converter  712  for photo-electrically converting upstream optical signals into upstream data and then detecting the upstream data, and a downstream electric-optical converter  711  for electric-optically converting downstream data into downstream optical signals and transmitting the downstream optical signals.  
         [0059]     The remote access unit  720  includes an antenna  725 , a controller  725 , a demultiplexer  733 , a first triplexer  721 , a second triplexer  723 , a duplexer  722 , a switch  724 , downstream and upstream amplifiers  726  and  727 , a downstream photo-electric converter  731 , and an upstream electric-optical converter  732 .  
         [0060]     The downstream photo-electric converter  731  converts the downstream optical signals inputted through the first optical cable  701  into the downstream data.  
         [0061]     The demultiplexer  733  is located between the downstream amplifier  726  and the downstream photo-electric converter  731 , and divides or separates control signals from the downstream data inputted from the downstream photo-electric converter  731  and outputs the control signals to the controller  734 .  
         [0062]      FIGS. 9 and 10  represents graphs illustrating the operation of the optical network shown in  FIG. 8 , wherein  FIG. 9  shows frequency response characteristics of the first triplexer  721  and the second triplexers  723  and  FIG. 10  shows the frequency response characteristic of a duplexer  722 . In this illustrative example, S 12  represents the broadcasting channel, S 13  represents the downstream frequency division channels, S 14  represents the downstream time division channels, S 31  represents the upstream frequency division channels and S 41  represents the upstream time division channels.  
         [0063]     As shown in  FIG. 9 , the first triplexer  721  divides or separates the downstream data inputted from the downstream amplifier  726  into broadcasting channels, downstream time division channels, and downstream frequency division channels. The downstream time division channels are outputted to the switch  724 , and the broadcasting channels are outputted to the second triplexer  723 . Furthermore, the downstream frequency division channels are outputted to the duplexer  722 .  
         [0064]     As shown in  FIG. 9 , the second triplexer  723  divides or separates the upstream data received through the antenna  725  into upstream time division channels and upstream frequency division channels. The upstream time division channels are outputted toward the switch  724 . Also, the upstream frequency division channels are outputted toward the duplexer  722 . Furthermore, the second triplexer  723  combines the upstream time division channels inputted from the switch  724  and the downstream frequency division channels inputted from the duplexer  722  with the broadcasting channels inputted from the first triplexer  721  so as to obtain and output downstream data to the antenna  725 .  
         [0065]     As shown in  FIG. 10 , the duplexer  722  outputs the downstream frequency division channels inputted from the first triplexer  721  toward the second triplexer  723 , while outputting the upstream frequency division channels inputted from the second triplexer  723  to the upstream amplifier  727 .  
         [0066]     The downstream amplifier  726  amplifies and outputs the downstream data inputted from the demultiplexer  733 . Meanwhile, the upstream amplifier  727  amplifies and outputs the upstream time division channels inputted from the switch  724  and the upstream frequency division channels inputted from the duplexer  722  and then outputs them to the upstream electric-optical converter  732 .  
         [0067]     The upstream electric-optical converter  732  converts the upstream data into upstream optical signals which are transmitted through the second optical cable  702  to the central station  710 .  
         [0068]     The switch  724  outputs downstream time division channels divided from the downstream data toward the antenna  725  through the second triplexer  723 , while receiving the upstream time division channels divided by the second triplexer  723  from the upstream data inputted through the antenna. Then, the upstream time division channels are then inputted into the upstream amplifier  727  from the switch  724 .  
         [0069]     The controller  734  controls the switch  724  so that the downstream time division channels and the upstream time division channels do not overlap. Specifically, the switch  724  is controlled by the controller  734  to prevent the upstream and downstream time division channels from overlapping.  
         [0070]      FIG. 11  illustrates a schematic block diagram showing a structure of an optical network for bi-directional communication according to a sixth embodiment of the present invention. The optical network  600  according to this embodiment of the present invention includes a central station  810 , a remote access unit  820 , and first and second optical cables  801  and  802  for linking the central station  810  and the remote access unit  820 . The first optical cable  801  transmits downstream optical signals from the central station  810  to the remote access unit  820 , and the second optical cable  802  transmits upstream optical signals from the remote access unit  820  to the central station  810 .  
         [0071]     The central station  810  includes an upstream photo-electric converter  812  for photo-electrically converting upstream optical signals into upstream data and then detecting the upstream data, and a downstream electric-optical converter  811  for electric-optical converting downstream data into downstream optical signals.  
         [0072]     The remote access unit  820  includes an antenna  823 , an upstream electric-optical converter  832  for converting the upstream data into upstream optical signals, a downstream photo-electric converter  831  for converting the downstream optical signals into the downstream data, a switch  824  for outputting downstream time division channels, divided from the downstream data, to the antenna  823  and for receiving upstream time division channels, divided from the upstream data, inputted through the antenna  823 , a controller  834  for controlling the switch  824  to prevent the downstream time division channels and the upstream time division channels from overlapping, a demultiplexer  833 , upstream and downstream amplifiers  825  and  826 , a triplexer  821 , and a multiplexing division coupler  822 .  
         [0073]     The demultiplexer  833  is located between the downstream photo-electric converter  831  and the downstream amplifier  826 , and separates control signals from the downstream data and outputs the control signals to the controller  834 .  
         [0074]     The triplexer  821  divides the downstream data inputted from the downstream amplifier  826  into broadcasting channels, downstream time division channels, and downstream frequency division channels. Triplexer  821  outputs the downstream time division channels toward the switch, while the downstream frequency division channels and the broadcasting channels are inputted into the multiplexing division coupler  822  through corresponding pathways.  
         [0075]      FIG. 12  is a graph illustrating the operation of the optical network shown in  FIG. 11 , which shows the operation of the multiplexing division coupler. As shown in  FIG. 12 , the multiplexing division coupler  822  combines the broadcasting channels and the downstream frequency division channels, which are divided by the triplexer  821 , and the downstream time division channels inputted from the switch  824  with the downstream data so as to output the downstream data toward the antenna  823  and divides the upstream data received through the antenna  823  into upstream time division channels and upstream frequency division channels so as to output the upstream time division channels to the switch  824  and output the upstream frequency division channels toward the upstream amplifier  825 .  
         [0076]      FIG. 13  illustrates a schematic block diagram showing the structure of an optical network for bi-directional communication according to a seventh embodiment of the present invention. Referring to  FIG. 13 , the optical network  900  for the bi-directional wireless communication according to this embodiment of the present invention includes a central station  910  for photo-electric converting upstream optical signals into upstream data so as to detect the upstream data and for electric-optically converting downstream data into downstream optical signals so as to transmit the downstream optical signals, a remote access unit  920 , and first and second optical cables  901  and  902  for linking the remote access unit  920  and the central station  910 . The central station  910  includes a downstream electric-optical converter  911  and an upstream photo-electric converter  912 .  
         [0077]     The remote access unit  920  includes an antenna  935  for receiving upstream data and for transmitting downstream data, an upstream electric-optical converter  932  for converting the upstream data into upstream optical signals so as to output the upstream optical signals toward the second optical cable  902 , a downstream photo-electric converter  931  for converting the downstream optical signals inputted through the first optical cable  901  into downstream data, a switch  924 , a controller  934 , a demultiplexer  933 , first and second triplexers  921  and  923 , a duplexer  922 , first, second and third downstream amplifiers  925 ,  926  and  927 , and first and second upstream amplifiers  928  and  929 .  
         [0078]     The demultiplexer  933  divides control signals from the downstream data, which outputs the control signals to the controller and transmits the remaining downstream data toward the first triplexer  921 .  
         [0079]     The first triplexer  921  divides the downstream data inputted from the demultiplexer  933  into downstream time division channels, downstream frequency division channels, and broadcasting channels. The downstream time division channels, divided from the downstream data, are inputted into the switch  924 .  
         [0080]     The second triplexer  923  combines the downstream time division channels inputted from the switch  924 , the downstream frequency division channels divided by the first triplexer  921 , and the broadcasting channels with the downstream data and then outputs the downstream data toward the antenna  935 . The second triplexer  923  further divides the upstream data inputted from the antenna  935  into upstream frequency division channels and upstream time division channels and outputs the upstream time division channels to the switch  924 .  
         [0081]     The duplexer  922  outputs the downstream frequency division channels divided by the first triplexer  921  toward the second triplexer  923 , while receiving the upstream frequency division channels from second triplexer  923 .  
         [0082]     The first downstream amplifier  925  is located between the first triplexer  921  and the second triplexer  923 , and amplifies and outputs the broadcasting channels to the second triplexer  923 . The second downstream amplifier  926  is disposed between the first triplexer  921  and the duplexer  922 , and amplifies and outputs the downstream frequency division channels to the duplexer  922 . The third downstream amplifier  927  is located between the first triplexer  921  and the switch  924 , and amplifies and outputs the downstream time division channels to the switch  924 .  
         [0083]     The first upstream amplifier  928  is located between the switch  924  and the upstream electric-optical converter  932 , and amplifies and outputs the upstream time division channels inputted from the switch  924  toward the electric-optical converter  932 . The second upstream amplifies  929  is disposed between the duplexer  922  and the upstream electric-optical converter  932 ; and amplifies and outputs the upstream frequency division channels toward the upstream electric-optical converter  932 .  
         [0084]     The controller  934  controls the switch  924  depending on the control signals, so as to prevent the downstream time division channels and the upstream time division channels from overlapping.  
         [0085]     The remote access unit according to the present invention respectively amplifies time division channels and frequency division channels, and then combines or transmits the channels, thereby preventing nonlinear phenomenon of active elements from occurring due to a leakage of downstream signals and also limiting deterioration of upstream link members which may be caused by the leakage of the downstream signals. That is, the remote access unit according to the present invention can prevent a part of downstream data from being introduced into the upstream link, thereby minimizing nonlinear phenomenon which may occur as elements processing upstream data using downstream data operates in a benefit saturation region or below a threshold.  
         [0086]     While the invention has been shown and described with reference to certain preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.