Abstract:
Disclosed is a remote antenna unit and a wavelength division multiplexing network. The remote antenna unit includes: an optical circulator having a plurality of ports; an electric field absorption modulator having a first input/output end and a second reflective end, the electric field absorption modulator configured to demodulate an optical signal into a first data signal, and modulate the demodulated optical signal according to a second electric data signal; and a filter disposed between the optical circulator and the electric field absorption modulator, for the filter configured to transmit an optical signal having a particular wavelength from among optical signals input from a port of the optical circulator toward the electric field absorption modulator and reflecting the remaining optical signals into the port of the optical circulator, and inputting the optical signal from the electric field absorption modulator into the port of the optical circulator.

Description:
CLAIM OF PRIORITY  
       [0001]     This application claims priority to an application entitled “Remote Antenna Unit and Wavelength Division Multiplexing Radio-Over-Fiber Network Using the Same” filed with the Korean Intellectual Property Office on Apr. 4, 2005 and assigned Serial No. 2005-0028147, the contents of which is incorporated herein by reference.  
       BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an optical network, and more particularly to a wavelength division multiplexing radio-over-fiber network that supports bidirectional wireless communication.  
         [0004]     2. Description of the Related Art  
         [0005]     A radio-over-fiber network transmits radio signals through optical fibers. In particular, an optical transmitter converts radio signals into optical signals and transmits the optical signals through the optical fibers. An optical receiver converts optical signals received through the optical fibers into radio signals. When wavelength division multiplexing is applied to such a radio-over-fiber network, radio signals for a plurality of remote antenna units can be transmitted through one optical fiber. Thus, such a system effectively uses the broadband width of the optical fiber.  
         [0006]     Moreover, the wavelength division multiplexing radio-over-fiber network allows complicated electric devices dispersed in a plurality of base stations to be concentrated in a central office. In turn, a small number of optical fibers can be used because of the wavelength division multiplexing scheme.  
         [0007]      FIG. 1  is a block diagram of a conventional wavelength division multiplexing radio-over-fiber network. The radio-over-fiber network  100  includes a base station  110 , and first, second, third, and fourth remote antenna units  140 - 1 ,  140 - 2 ,  140 - 3 , and  140 - 4  connected in series to the base station  110  through an optical fiber  170 .  
         [0008]     The base station  110  includes second, fourth, sixth, and eighth optical transmitters  120 - 2 ,  120 - 4 ,  120 - 6 , and  120 - 8 , and first, third, fifth, and seventh optical receivers  130 - 1 ,  130 - 3 ,  130 - 5 , and  130 - 7 . As the index i increases in an order of 1, 3, 5, 7, . . . , the i th  optical receiver  130 - i  and the (i+1) th  optical transmitter  130 -( i+ 1) are alternately arranged and connected to one another in series. The (i+1) th  optical transmitter  130 -( i+ 1) downwardly transmits (i+1) th  optical signal λ i+1  having (i+1) th  wavelength. The i th  optical receiver  130 - i  receives an i th  optical signal i th  having an i th  wavelength. Herein, the word “downstream” indicates a direction from the base station  110  to the remote antenna units  140 - 1 ,  140 - 2 ,  140 - 3 , and  140 - 4 , while the term “upstream” means the direction opposite to the downstream. The optical transmitters  120 - 2 ,  120 - 4 ,  120 - 6 , and  120 - 8  pass the inputted optical signal therethrough. The optical receivers  130 - 1 ,  130 - 3 ,  130 - 5 , and  130 - 7  receive the optical signal assigned to each optical receiver and pass the rest of the optical signals therethrough. For example, the first optical signal inputted into the base station  110  passes through the eighth, sixth, fourth, and second optical transmitters and the seventh, fifth, and third optical receivers alternately and in order, and then is received by the first optical receiver  130 - 1 . On the other hand, the second optical signal outputted from the second optical transmitter  120 - 2  passes through the third, fifth, and seventh optical receivers and the fourth, sixth, and eighth optical transmitters alternately and in order, and then is downstream transmitted.  
         [0009]     The first, second, third, and fourth remote antenna units  140 - 1 ,  140 - 2 ,  140 - 3 , and  140 - 4  have structures identical to one another. The j th  remote antenna unit  140 - j  includes a ( 2   j− 1) th  optical transmitter  120 -( 2   j− 1), a ( 2   j ) th  optical receiver  130 - 2   j , a j th  circulator  150 - j , and a j th  antenna  160 - j , wherein the index j is a natural number below four.  
         [0010]     The (2 j− 1) th  optical transmitter  120 -( 2   j− 1) upstream transmits the ( 2   j −1) th  optical signal λ (2j-1)  created by electric data signal which is inputted from the j th  circulator  150 - j.    
         [0011]     The ( 2   j ) th  optical receiver  130 - 2   j  converts the ( 2   j ) th  optical signal λ (2j)  passing through the ( 2   j− 1) th  optical transmitter  120 -( 2   j −1) into electric data signal, and then outputs the electric data signal.  
         [0012]     The j th  circulator  150 - j  has first, second, and third ports. The first port of the j th  circulator  150 - j  is connected to the ( 2   j ) th  optical receiver  130 - 2   j , the second port of the j th  circulator  150 - j  is connected to the j th  antenna  160 - j , and the third port is connected to the ( 2   j− 1) th  optical transmitter  120 -( 2   j −1). The j th  circulator  150 - j  outputs electric data signal inputted into the first port to the second port, and outputs the electric data signal inputted into the second port to the third port.  
         [0013]     The j th  antenna  160 - j  converts radio signals received through the air into electric data signals, and then outputs the electric data signals to the j th  circulator  150 - j . Moreover, the j th  antenna  160 - j  converts electric data signals inputted from the j th  circulator  150 - j  into radio signals, and then emits the radio signals to the air.  
         [0014]     The optical transmitters  120 - 1 ,  120 - 3 ,  120 - 5 , and  120 - 7  pass the optical signals respectively inputted into the optical transmitters. The optical receivers  130 - 2 ,  130 - 4 ,  130 - 6 , and  130 - 8  receive the optical signals assigned to each receiver and pass the rest of optical signals therethrough as they are. For example, the eighth optical signal λ 8  passes through the first, third, fifth, and seventh optical transmitters and the second, fourth, and sixth optical receivers alternately and in order, and then is received by the eighth optical receiver  130 - 8 . On the other hand, the seventh optical signal  7  outputted from the seventh optical signal  120 - 7  passes through the sixth, fourth, and second optical receivers and the fifth, third, and first optical transmitters alternately and in order, and then is transmitted upstream.  
         [0015]     The first to eighth optical transmitters  120 - 1 ,  120 - 2 , . . . , and  120 - 8  have identical structures. The first to eighth optical receivers  130 - 1 ,  130 - 2 , . . . , and  130 - 8  also have identical structures.  
         [0016]      FIG. 2  illustrates a first optical transmitter  120 - 1  and a second optical receiver  130 - 2  included in a first remote antenna unit  140 - 1  shown in  FIG. 1 .  
         [0017]     The first optical transmitter  120 - 1  includes a first housing  210 , a first ferrule  215 , first and second lenses  220  and  230 , a first filter  225 , and laser diode  235 .  
         [0018]     The first housing  210  has a cylindrical shape in which the upper and lower ends are open.  
         [0019]     The first ferrule  215  is inserted into an upper portion of the first housing  210 , and has a pair of holes into which a part of an upstream optical fiber  170  and an intermediate optical fiber  170   b  are inserted. The second, fourth, sixth, and eighth optical signals λ 2 , λ 4 , λ 6  and λ 8  are inputted into the first optical transmitter  120 - 1  through the upstream optical fiber  170   a . The first, third, fifth, and seventh optical signals λ 1 , λ 3 , λ 5 , and λ 7  are outputted outside the first optical transmitter  120 - 1 . Moreover, the third, fifth, and seventh optical signals λ 3 , λ 5 , and λ 7  are inputted into the first optical transmitter  120 - 1  through the intermediate optical fiber  170   b , while the second, fourth, sixth, and eighth optical signals λ 2 , λ 4 , λ 6 , and λ 8  are outputted outside the first optical transmitter  120 - 1 .  
         [0020]     The first filter  225  is disposed at an intermediate portion of the first housing  210 . The first filter  225  reflects the optical signals inputted from the upstream optical fiber or the intermediate optical fiber  170   a  or  170   b  toward the intermediate optical fiber or the upstream optical fiber  170   b  or  170   a . The first filter  225  also transmits the first optical signal inputted from the laser diode  235  toward the upstream optical fiber  170   a.    
         [0021]     The first lens  220  is interposed between the first ferrule  215  and the first filter  225 . The first lens  220  enables the first, third, fifth, and seventh optical signals to converge at an end of the upstream optical fiber  170   a.    
         [0022]     The laser diode  235  is disposed at a lower portion of the first housing  210 . The laser diode  235  converts the electric data signals inputted from the first circulator  150 - 1  into the first optical signals and then outputs the first optical signals.  
         [0023]     The second lens  230  is interposed between the laser diode  235  and the first filter  225 . The second lens  230  makes the first optical signals converge at an end of the upstream optical fiber  170   a.    
         [0024]     The second optical receiver  130 - 2  includes a second housing  250 , a second ferrule  255 , third and fourth lenses  260  and  270 , a second filter  265 , and a photodiode  275 .  
         [0025]     The second housing  250  has a cylindrical shape in which upper and lower ends are open.  
         [0026]     The second ferrule  255  is inserted into an upper portion of the second housing  250 . The second ferrule  255  has a pair of holes into which a part of a downstream optical fiber  170   c  and the intermediate optical fiber  170   b  are inserted. The second, fourth, sixth, and eighth optical signals are inputted into the second optical receiver  130 - 2  through the intermediate optical fiber  170   b , while the third, fifth, and seventh optical signals are outputted outside of the second optical receiver  130 - 2 . Moreover, the third, fifth, and seventh optical signals are inputted into the second optical receiver  130 - 2  through the downstream optical fiber  170   c , and the fourth, sixth, and eighth optical signals are outputted outside of the second optical receiver  130 - 2 .  
         [0027]     The second filter  265  is disposed at an intermediate portion of the second housing  250 . The second filter  265  transmits only the second optical signal among the optical signals inputted from the intermediate optical fiber  170   b  toward the photodiode  275 . The second filter  265  also reflects the remaining optical signals toward the downstream optical fiber  170   c . Furthermore, the second filter  265  reflects the optical signals inputted from the downstream optical fiber  170   c  toward the intermediate optical fiber  170   b.    
         [0028]     The third lens  260  is interposed between the second ferrule  255  and the second filter  265 . The third lens  260  enables the optical signals reflected by the second filter  265  to converge at an end of the corresponding optical fiber.  
         [0029]     The photodiode  275  converts the second optical signal inputted from the second filter  265  into electric data signal, and then outputs the electric data signal.  
         [0030]     The fourth lens  270  is interposed between the photodiode  275  and the second filter  265 . The fourth lens  270  enables the second optical signal to converge into a light receiving portion of the photodiode  275 .  
         [0031]     However, the conventional wavelength division multiplexing radio-over-fiber network  100  described above has a number of limitations.  
         [0032]     For example, since each remote antenna unit has an optical transmitter and an optical receiver, the identical structural devices are repeatedly applied to the remote antenna unit. Thus, the network is complicated and the cost of realizing the network is increased.  
         [0033]     Further, the upstream wavelength band and downstream wavelength band must be separate, thereby decreasing the efficient use of the wavelength band.  
       SUMMARY OF THE INVENTION  
       [0034]     Accordingly, the present invention has been made to reduce or overcome the above-mentioned problems occurring in the prior art and provides additional advantages, by providing a remote antenna unit that has a simple structure and a lower cost for realizing the remote antenna unit.  
         [0035]     Another object of the present invention is to provide a wavelength division multiplexing radio-over-fiber network which can increase the efficiency of the wavelength band using the remote antenna unit.  
         [0036]     According to the first embodiment of the present invention, a remote antenna unit is provided. The remote antenna unit includes an optical circulator having a plurality of ports; an electric field absorption modulator having a first input/output end and a second reflective end, the electric field absorption modulator configured to demodulate an optical signal into a first data signal and modulate the demodulated optical signal according to a second electric data signal; and a filter disposed between the optical circulator and the electric field absorption modulator, the filter configured to transmit an optical signal having a particular wavelength from among optical signals input from a port of the optical circulator toward the electric field absorption modulator and reflecting the remaining optical signals into the port of the optical circulator, and input the optical signal from the electric field absorption modulator into the port of the optical circulator.  
         [0037]     According to the another aspect of the present invention, there is provided a radio-over-fiber network which comprises: a base station including an optical transmission portion for transmitting downstream optical signals having different wavelengths, and an optical reception portion for receiving upstream optical signals having different wavelengths; and a plurality of remote antenna units connected sequentially to the base station through optical fibers so as to be in a loop structure, each remote antenna unit which includes: an optical circulator having a plurality of ports; an electric field absorption modulator having a first input/output end and a second reflective end, the electric field absorption modulator configured to demodulate an optical signal into a first data signal and modulate the demodulated optical signal according to a second electric data signal; and a filter disposed between the optical circulator and the electric field absorption modulator, the filter configured to transmit an optical signal having a particular wavelength from among optical signals input from a port of the optical circulator toward the electric field absorption modulator and reflecting the remaining optical signals into the port of the optical circulator, and input the optical signal from the electric field absorption modulator into the port of the optical circulator. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0038]     The present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which:  
         [0039]      FIG. 1  is a block diagram of a conventional wavelength division multiplexing radio-over-fiber network;  
         [0040]      FIG. 2  illustrates a first optical transmitter and a second optical receiver included in a first remote antenna unit shown in  FIG. 1 ;  
         [0041]      FIG. 3  is a block diagram of a wavelength division multiplexing radio-over-fiber network according to a preferred embodiment of the present invention;  
         [0042]      FIG. 4  illustrates an m remote antenna unit shown in  FIG. 3 ; and  
         [0043]      FIG. 5  illustrates an m electric field absorption modulator shown in  FIG. 4 . 
     
    
     DETAILED DESCRIPTION  
       [0044]     Hereinafter, 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.  
         [0045]      FIG. 3  is a block diagram of a wavelength division multiplexing radio-over-fiber network according to a preferred embodiment of the present invention. The radio-over-fiber network  300  includes a base station  310 , and first, second, third, . . . , and N th  remote antenna units  400 - 1 ,  400 - 2 ,  400 - 3 , . . . , and  400 -N connected to the base station  310  through an optical fiber  360  in order and in a loop structure.  
         [0046]     The base station  310  includes an optical transmission portion  312  having first, second, third, . . . , and N th  optical transmitters  320 - 1 ,  320 - 2 ,  320 - 3 , . . . ,  320 -N and a first wavelength division multiplexer  330 , and an optical reception portion  314  having first, second, third, . . . , and N th  optical receivers  350 - 1 ,  350 - 2 ,  350 - 3 , . . . , and  350 -N and a second wavelength division multiplexer  340 .  
         [0047]     The first, second, third, . . . , and N th  optical transmitters  320 - 1 ,  320 - 2 ,  320 - 3 , . . . , and  320 -N have identical or similar structures with one another, and outputs first, second, third, . . . , and N th  downstream optical signals λ 1 , λ 2 , λ 3 , . . . , and λ N  (indicated by a solid line) having different wavelengths. The m th  optical transmitter  320 - m  outputs m upstream optical signal λ m  having an m th  wavelength which is modulated by m electric data signal, wherein the m is a natural number below N. The first, second, third, . . . , and N th  wavelengths are spaced at a given distance from one another. The first, second, third, . . . N th  and N th  optical transmitters  320 - 1 ,  320 - 2 ,  320 - 3 , . . . , and  320 -N can respectively use a laser diode.  
         [0048]     The first wavelength multiplexer  330  is provided with first, second, third, . . . , and N th  demultiplexing ports and a multiplexing port. The first, second, third, . . . , and N th  demultiplexing ports are connected by point to point to the first, second, third, . . . , and N th  optical transmitters  320 - 1 ,  320 - 2 ,  320 - 3 , . . . , and  320 -N one by one. The multiplexing port is connected through the optical fiber  360  to the first remote antenna unit  400 - 1 . The first wavelength division multiplexer  330  performs the wavelength division multiplexing relative to the first, second, third, . . . , and N th  downstream optical signals inputted into the first, second, third, . . . , and N th  demultiplexing ports, and then outputs the multiplexed downstream optical signals to the multiplexing port. At this time, the m th  downstream optical signal is inputted into the m th  demultiplexing port of the first wavelength division multiplexer  330 . 1×N arranged waveguide gratings may be used as the first and second wavelength division multiplexers  330  and  340 .  
         [0049]     The second wavelength division multiplexer  340  has first, second, third, . . . , N th  demultiplexing ports and a multiplexing port. The first, second, third, . . . , and N th  demultiplexing ports are point-to-point connected to the first, second, third, . . . , and N th  optical receivers  350 - 1 ,  350 - 2 ,  350 - 3 , . . . , and  350 - n  one by one. The multiplexing port is connected to the N th  remote antenna unit  400 -N through the optical fiber  360 . The second wavelength division multiplexer  340  performs the wavelength division demultiplexing with relation to the first, second, third, . . . , and N th  upstream optical signals λ 1 , λ 2 , λ 3 , . . . , and λ N  (indicated by a dotted line), and then outputs the demultiplexed upstream optical signals toward the first, second, third, . . . , and N th  demultiplexing ports. At this time, the second wavelength division multiplexer  340  outputs the m th  upstream optical signal toward the m th  demultiplexing port.  
         [0050]     The first, second, third, . . . , and N th  optical receivers  350 - 1 ,  350 - 2 ,  350 - 3 , . . . , and  350 -N have identical or similar structures, which respectively convert the inputted upstream optical signals into electric data signals and then output the electric data signals. The m th  optical receiver  350 - m  converts the m th  upstream optical signal inputted therein into (m′) th  electric data signal, and then outputs the (m′) th  data signal. Photodiodes may be used as the first, second, third, . . . , and N th  optical receivers  350 - 1 ,  350 - 2 ,  350 - 3 , . . . , and  350 -N, respectively.  
         [0051]     The first, second, third, . . . , and N th  remote antenna units  400 - 1 ,  400 - 2 ,  400 - 3 , . . . , and  400 -N have an identical or similar structures, i.e. loop structure, and are connected to the base station  310 . The downstream transmission from the base station  310  and the upstream transmission to the base station  310  are performed along the optical fiber  360  in an identical direction. That is, the downstream optical signals and the upstream optical signals propagating in the optical fiber  360  are transmitted in an identical direction. The base station  310  transmits the first, second, third, . . . , and N th  downstream optical signals toward the first remote antenna unit  400 - 1  while receiving the first, second, third, . . . , and N th  upstream optical signals from the N th  remote antenna unit  400 -N. For example, the N th  downstream optical signal passes through the first, second, third, . . . , and (N−1) th  remote antenna units  400 - 1 ,  400 - 2 ,  400 - 3 , . . . , and  400 -(N−1) in order, and then is received by the N th  remote antenna unit  400 -N. The first upstream optical signal passes through the second, third, fourth, . . . , and N th  remote antenna units  400 - 2 ,  400 - 3 ,  400 - 4 , . . . , and  400 -N in order, and then is received by the base station  310 . Specifically, the m th  downstream optical signal passes through the m −1  pieces of the remote antenna units in order, and then is received by the m th  remote antenna unit  400 - m . The upstream optical signal outputted from the m th  remote antenna unit  400 - m  passes through the N-m pieces of the remote antenna units in order, and then is received by the base station  310 .  
         [0052]      FIG. 4  illustrates the m th  remote antenna unit  400 - m . The m th  remote antenna unit  400 - m  includes an m th  housing  410 - m , an m th  optical circulator  420 - m , an m th  filter  430 - m , an m th  electric field absorption modulator  440 - m , and an m th  antenna  450 - m . The m th  housing  410 - m  has a cylindrical structure in which upper and lower ends are closed, and has a pair of holes formed on a peripheral surface thereof into which an upstream optical fiber  360   a  and a downstream optical fiber  360   b  are partially inserted.  
         [0053]     The m th  optical circulator  420 - m  is disposed at an upper portion of the m th  housing  410 - m , which has the first, second, and third ports. The first port of the m th  optical circulator  420 - m  is connected to the upstream optical fiber  360   a , the second port is connected to an auxiliary optical fiber  425 , and the third port is connected to the downstream optical fiber  360   b . The m th  optical circulator  420 - m  is a device without dependence on the wavelength of the optical signal and outputs the optical signal inputted into the first port toward the second port and outputs the optical signal inputted into the second port toward the third port. The m th , (m+1) th , (m+2) th , . . . , and N th  downstream optical signals and the first, second, third, . . . , and (m−1) th  upstream optical signals are inputted into the first port of the m th  optical circulator  420 - m , and then are outputted toward the second port. Meanwhile, the (m+1) th , (m+2) th , (m+3) th , . . . , and N th  downstream optical signals and the first, second, third, . . . , and m th  upstream optical signals are inputted into the second port, and then are outputted toward the third port. At this time, if the index m is one, no upstream optical signal is inputted into the first port. If the index m is two, no downstream optical signal is inputted into the second port.  
         [0054]     The m th  filter  430 - m  is disposed at an intermediate portion of the m th  housing  410 - m . The m th  filter  430 - m  transmits the m th  downstream optical signal among the optical signals inputted from the auxiliary optical fiber  425  toward the m th  electric field absorption modulator  440 - m  and reflects the rest of optical signals toward the auxiliary optical fiber  425 . Moreover, the m th  filter  430 - m  transmits the m th  upstream optical signal inputted from the m th  electric field absorption modulator  440 - m  toward the auxiliary optical fiber  425 . A thin film filter, in which a plurality of thin films are deposited on a glass substrate, may be used as the m th  filter  430 - m.    
         [0055]     The m th  electric field absorption modulator  440 - m  is disposed at a lower portion of the m th  housing  410 - m . The m th  electric field absorption modulator  440 - m  demodulates the m th  downstream optical signal inputted from the m th  filter  430 - m  into the m th  electric data signal so as to output the demodulated electric data signal. Moreover, the m th  electric field absorption modulator  440 - m  outputs the m th  upstream optical signal, which is inputted from the m th  antenna  450 - m  and is modulated into the (m′) th  electric data signal, toward the m th  filter  430 - m.    
         [0056]      FIG. 5  illustrates the m th  electric field absorption modulator  440 - m  shown in  FIG. 4 . The m th  electric absorption modulator  440 - m  is a reflective type, which includes upper and lower clad layers  441 - m  and  443 - m , an active layer  442 - m , an electrode  445 - m , and a reflective layer  444 - m.    
         [0057]     The upper and lower clad layers  441 - m  and  443 - m  perform a locking function of the optical signal in the active layer  442 - m . The active layer  442 - m  is interposed between the upper and lower clad layers  441 - m  and  443 - m , into which the optical signal propagates. The reflective layer  444 - m  is stacked on a second side end of the m th  electric field absorption modulator  440 - m  and reflects the inputted optical signal again. The electrode  445 - m  is stacked on the upper clad layer  443 - m  and is used as an input/output passage of the electric data signal. The first side end of the m electric field absorption modulator  440 - m  becomes an input/output passage for the optical signal. The m th  electric field absorption modulator  440 - m  demodulates the m th  downstream optical signal, into the m th  electric data signal EM. The m th  downstream optical signal has an optical carrier frequency and sidebands arranged in periodic intervals on both sides of the downstream optical signal. The m th  electric data signal is carried on the sidebands. The m th  electric data signal, which is generated by the m th  downstream optical signal from the m th  electric field absorption modulator  440 - m , is outputted through the electrode  445 - m  outside. The m th  electric field absorption modulator modulates the optical carrier frequency using the (m′) th  electric data signal Em′ applied to the electrode and then outputs the modulated optical carrier frequency. At this time, it is preferred to set the frequency of the m electric data signal different from the frequency of the (m′) th  electric data signal.  
         [0058]     Referring to  FIG. 4  again, the m th  antenna  450 - m  converts the radio signal received through the air into the (m′) th  electric data signal, while converting the m th  electric data signal inputted from the m th  electric field absorption modulator  400 - m  into a radio signal so as to emit the radio signal to the air.  
         [0059]     As described above, the remote antenna unit according to the present invention is provided with the optical circulator, the filter, and the reflective electric field absorption modulator. Thus the remote antenna unit has a simple structure and the lower cost of realization.  
         [0060]     Furthermore, in the wavelength division multiplexing radio-over-network using the remote antenna unit, identical wavelengths are repeatedly used for the upstream and downstream transmissions, so as to improve the efficiency of the wavelength bands in comparison with the conventional radio-over-network.  
         [0061]     While the invention has been shown and described with reference to the 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.