Abstract:
A signal transmitting apparatus for an optical base station is disclosed. According to the invention, a base station outputs a digital IQ signal to an optical connecting unit. The optical connecting unit processes the digital IQ signal digitally, and transmits the digital signal over an optical network to a remotely located optical base station. The remote station digitally processes the signal before converting to an RF signal for transmission. The invention advantageously decreases signal loss and noise associated with analog processing in the optical connecting units and remote stations in the related art. System reliability is also improved.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a base station for communications, and more particularly, to a base station using optical communications. 
     2. Background of the Related Art 
       FIG. 1  is a conceptual view of a general optical base station. As shown therein, a mobile communication base station  10  transmits a radio signal with sufficient power to reach to every terminal within a service area. However, in view of the characteristics of the radio wave, shadow areas may exist, causing a failure to communicate with a terminal. Shadow areas may be the result of a skyscraper, a rear side of hill areas, or an underground living area. In addition, if a new skyscraper is built, a new shadow area is generated. 
     In order to provide the shadow area with a communication service, a communication service provider should establish a new base station or operate a small-sized remote base station  100  using the same frequency, data signal, and control signal as those of a pertinent base station. In most cases, in order to effectively operate within a limited frequency spectrum, the remote station  100  is installed. The remote base station  100  is also called an optical base station. 
       FIG. 2  is a schematic view showing the construction of a signal transmitting apparatus for an optical base station in accordance with the related art. As shown in  FIG. 2 , the related art signal transmitting apparatus for an optical base station includes a base station  10  controlling a plurality of optical base stations  100  or transmitting and receiving a radio frequency (RF) signal to and from the plurality of optical base stations  100  through an optical connecting unit  70 . The optical connecting unit  70  converts the RF signal outputted from the base station  10  into an optical signal and outputs the optical signal through an optical cable  90  to the plurality of optical base stations  100 . Additionally, optical connecting unit  70  converts optical signals received through the optical cable  90  into an RF signal and transmits the RF signal to the base station  10 . The remote station  100  converts the optical signal received through the optical cable  90  from the optical connecting unit  70  into a high power RF signal used in a mobile communication system and transmits the high power RF signal to an antenna. The remote station  100  down/up-converts the RF signal received by the antenna, converts the converted signal into an optical signal, and outputs to the optical signal optical cable  90 . 
       FIG. 4  is a view showing the construction of the signal transmission apparatus for an optical base station of  FIG. 2 . Referring to  FIG. 4 , the optical connecting unit  70  includes a divider  20  receiving the RF signal from the base station IO and dividing it to a plurality of base station connecting units  80 . Optical connecting unit  70  also includes a plurality of base station connecting units  80  adjusting a voltage level of a signal outputted from the divider  20 , converting the adjusted signal into an optical signal, and transmitting the optical signal through the optical cable  90  to the plurality of optical base stations  100 . Each of the base station connecting units  80  also convert the optical signal received through the optical cable  90  to an RF signal, adjust a voltage level of the converted signal, and output it. Optical connecting unit  70  also includes a combiner  60  combining the output signals of each base station connecting unit  80  and outputting it through a single path to base station  10 . 
     The base station connecting unit  80  includes a transmission signal level controller  30  adjusting a voltage level of the signal outputted from the divider  20 . Base station connecting unit  80  also includes an optical transceiver  40  converting the output signal of the transmission signal level controller  30  into an optical signal and outputting the optical signal to the optical cable  90 , or converting the optical signal received through the optical cable  90  into an RF signal. Base station connecting unit  80  also includes a reception signal level controller  50  adjusting a voltage level of the signal outputted from the optical transceiver  40  and outputting it to the combiner  60 . 
     The optical connecting unit  70  includes a plurality of base station connecting units  80  equal to the number of remote stations  100 . 
     The remote station  100  includes an optical transceiver  120  converting an optical signal received through the optical cable  90  into an RF signal, or converting the RF signal outputted from an down/up converter  170  into an optical signal and outputting the optical signal to the optical cable  90 . Remote station  100  also includes an up-converter  130  up-converting the output signal of the optical transceiver  120  into an RF signal used in a mobile communication system and a high power amplifier (HPA)  140  amplifying the RF signal outputted from the up-converter  130  into a high power signal. Remote station  100  also includes a duplexer  150  filtering the output signal of the HPA  140  and outputting it to an antenna  110 , or outputting a signal collected by the antenna  110  to a low noise amplifier (LNA)  160 , which amplifies the output signal of the duplexer  150 . An down/up converter  170  is also provided for down-converting the output signal of the LNA  160 , SAW-filtering the down-converted signal, and up-converting it. The operation of the down/up converter  170  is a process for reducing an influence of a different mobile communication service signal. 
     In a transmission process of the above-described related art signal transmitting apparatus, the RF signal outputted from the base station  10  is divided to multiple optical base stations  100  by the divider  20  and outputted to each remote station  100  connects to base station connecting unit  80 . The transmission signal level controller  30  of each base station connecting unit  80  adjusts the output signal of the divider  20  to a suitable voltage level and then applies it to the optical transceiver  40 . The optical transceiver  40  converts the applied signal into an optical signal and outputs the optical signal to the optical cable  90  connected to the plurality of remote stations  100 . 
     Upon receiving the optical signal through the optical cable  90 , the optical transceiver  120  of the remote station  100  converts the optical signal into a RF signal. The signal outputted from the optical transceiver  120  passes to the up-converter  130 , the high power amplifier  140  and the duplexer  150 , and then is transmitted through the antenna  110  to a corresponding terminal. 
     The operation of the reception signal transmitting apparatus for an optical base station will now be described in detail with reference to  FIG. 4 . The RF signal outputted from the base station  10  is transmitted to the divider  20  of the optical connecting unit  70 , and the divider  20  divides the RF signal into a plurality of base station connecting units  80 . The transmission signal level controller  30  of each base station connecting unit  80  adjusts the divided RF signal to a suitable level and transmits it to the optical transceiver  40 . The optical transceiver  40  converts the received RF signal into an optical signal and transmits the optical signal through the optical cable  90  to a pertinent remote station  100 . 
     The optical transceiver  120  of each remote station  100  converts the optical signal received through the optical cable  90  into an RF signal and outputs the RF signal to the up-converter  130 . The up-converter  130  up-converts the received signal into an RF signal for use in a mobile communication system and outputs it to the HPA  140 . The HPA  140  amplifies the inputted signal to a high power signal and transmits the high power signal through the duplexer  150  to the antenna  110 . 
     A signal collected by the antenna  110  is applied to the LNA  160  by the duplexer  150 , and the LNA  160  amplifies the applied signal and transmits it to the down/up converter  170 . The down/up converter  170  down-converts the inputted signal, then up-converts the down-converted signal by performing a SAW-filtering thereon, and outputs a resulting signal to the optical transceiver  120 . The optical transceiver  120  converts the output signal of the down/up converter  170  into an optical signal and transmits the optical signal through the optical cable  90  to the base station connecting unit  80  of the optical connecting unit  70 . The optical connecting unit  70  includes one base station connecting unit  80  for each remote station  100 . 
     The optical transceiver  40  of the base station connecting unit  80  restores the optical signal received through the optical cable  90  to an RF signal. Reception signal level controller  50  adjusts the restored RF signal to a suitable voltage level and outputs it to the combiner  60 . The combiner  60  combines the output signals of each base station connecting unit  80  and transmits it to the base station  10 . 
       FIG. 3  is a drawing illustrating a signal transmitting apparatus for an optical base station adopting an E2DM method in accordance with a different related art. As shown therein, when an RF signal outputted from the base station  10  is transmitted to the optical connecting unit  75 , a transmission signal level controller  35  of the optical connecting unit  75  adjusts a voltage level of the received RF signal and outputs it to an optical transceiver  180 . Then, the optical transceiver  180  converts the inputted RF signal into an optical signal and applies the optical signal to an optical distribution unit  190 . The optical distribution unit  190  distributes the applied RF signal to a plurality of remote stations  105  through the optical cable  90 . 
     The optical transceiver  200  of the remote station  105  converts the distributed optical signal into an RF signal and outputs the RF signal to a transmission signal level controller  230 , and the transmission signal level controller  230  adjusts the inputted signal to a suitable voltage level and applies it to an HPA  140 . The signal applied to the HPA  140  is amplified to a high power signal and transmitted through a duplexer  150  and an antenna  112  to a terminal. 
     Meanwhile, a radio signal of the terminal is collected by the antennas  112  and  114 , and the collected signal is transmitted to the base station  10  though two paths performing a receiving process. The antennas  112  and  114  are diversity antennas for preventing a fading phenomenon of a received signal, and are separately disposed to maintain an optimum distance there between in consideration of a wave length. The RF signal collected by the antennas  112  and  114  is outputted to each LNA  160  and  160 ′ by duplexers  150  and  150 ′, amplified by the LNAs  160  and  160 ′, and transmitted to notch filters  210  and  210 ′. The notch filters  210  and  210 ′ removes a noise of a certain band from the received signal and outputs the signal without a noise to reception signal level controllers  220  and  220 ′. The reception signal level controllers  220  and  220 ′ adjust a voltage level of the inputted signal and apply it to the optical transceiver  200 . The optical transceiver  200  converts the applied signals of the two paths into an optical signal and outputs it to the optical cable  90 . 
     The signal outputted to the optical cable  90  is transmitted to the optical distributor  190  of the optical connecting unit  75 . The optical distributor  190  combines the optical signals outputted from the plurality of remote stations  105  and applies the combined signal to the optical transceiver  180 . The optical transceiver  180  converts the applied optical signal into an RF signal and divides it into two paths for outputting. The signal outputted from the optical transceiver  180  is inputted into the reception signal level controllers  55  and  55 ′, adjusted to have a suitable voltage level, and transmitted to the base station  10 . 
     The above-described related art signal transmitting apparatus for a remote station can be used with a 3 wave division multiplexing (3WDM) method. Unlike the signal transmitting apparatus for an optical base station of the related art in which the optical connecting unit  70  includes the plurality of base station connecting units  80 , the 3WDM method operates an optical connecting unit  75  with only one optical distribution unit  190  and uses a diversity antenna. 
     As noted, in the above two embodiments, the optical connecting unit  70  and the remote stations  100  and  105  are based on an analog interface for processing the RF signal. 
     The related art signal transmitting apparatus for an optical base station has many problems and disadvantages. For example, in the apparatus of  FIGS. 2 and 4 , as the RF analog signal is converted into an optical signal and is passed through the optical transceivers  40  and  120 , the reception performance is degraded and the level of a noise signal is heightened. In order to solve the problem, the gain characteristic of the LNA  160  can be improved, but in view of the input limitation characteristic of the optical transceivers  120  and  40 , any improvement will be marginal. Likewise, the apparatus of  FIG. 3  solves the problem of fading by receiving a signal of a terminal in the diversity method, but it fails to solve the problem that the reception signal is degraded as a consequence of analog signal processing. 
     The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background. 
     SUMMARY OF THE INVENTION 
     An object of the invention is to solve at least the above problems and/or disadvantages and to provide at least the advantages described hereinafter. 
     Another object of the present invention is to provide a digital signal interface-based signal transmitting apparatus and method for an optical base station. 
     Another object of the invention is to reduce the level of a noise signal in an optical communication system. 
     In order to achieve at least the above objects in while or in part, and in accordance with the purposes of the invention, as embodied and broadly described, there is provided a communications system, including a base station configured to output a first digital in phase and quadrature phase (I/Q) signal, an optical connecting unit configured to convert the first digital I/Q signal into an optical signal and output the converted optical signal through an optical cable, and an optical base station coupled to receive the optical signal through the optical cable and configured to convert the optical signal into a second digital I/Q signal, and convert the second digital I/Q signal into a first RF signal for transmission. 
     To further achieve at least these advantages in whole or in parts, and in accordance with the purposes of the invention, there is provided a signal transmitting method for a communications system, including converting a first digital I/Q signal outputted from a base station into an optical signal, transmitting the optical signal through an optical cable to an optical base station, converting the optical signal received through the optical cable into a second digital I/Q signal, converting the second digital I/Q signal into a RF signal, and transmitting the RF signal through an antenna. 
     To further achieve at least these advantages in whole or in parts, and in accordance with the purposes of the invention, there is provided a signal receiving method for a communications system, including receiving an RF signal through an antenna of a first station, converting the received RF signal to a first digital electronic signal, converting the first digital electronic signal to a digital optical signal, transmitting the digital optical signal over an optical link to an optical connecting unit, converting the digital optical signal to a second digital electronic signal in the optical coupling unit, and providing the second digital electronic signal from the optical coupling unit to a second station. 
     To further achieve at least these advantages in whole or in parts, and in accordance with the purposes of the invention, there is provided a communications system, including means for converting a first digital electronic signal outputted from a first station into a first digital optical signal, means for transmitting the first digital optical signal to a second station, means for converting the first digital optical signal to a second digital electronic signal, means for converting the second digital electronic signal to a first RF signal, and means for transmitting the first RF signal. 
     To further achieve at least these advantages in whole or in parts, and in accordance with the purposes of the invention, there is provided a signal transmitting method in a communication system, including converting a digital I/Q signal to an optical signal in an optical connecting unit, transferring the optical signal over an optical cable to a remote station, and converting the optical signal into an RF signal for transmission. 
     To further achieve at least these advantages in whole or in parts, and in accordance with the purposes of the invention, there is provided a communication system, including an optical connecting unit, configured to receive a first digital I/Q signal and convert the first digital I/Q signal into a first digital optical signal, and a remote base station, coupled to receive the first digital optical signal and configured to convert the first digital optical signal to a first analog RF signal for transmission. 
     To further achieve at least these advantages in whole or in parts, and in accordance with the purposes of the invention, there is provided a communication system, including an optical connection unit, configured to convert a first digital I/Q signal to a first optical signal and to convert a second optical signal to a second digital I/Q signal, and a remote base station, coupled to receive the first optical signal, and configured to convert the first optical signal to a third digital I/Q signal, convert the third digital I/Q signal to a first RF signal, transmit the first RF signal, receive a second RF signal, convert the second RF signal to a fourth digital I/Q signal, and convert the fourth digital I/Q signal to the second optical signal. 
     Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein: 
         FIG. 1  is a drawing illustrating a concept of a general optical base station; 
         FIG. 2  is a drawing illustrating a schematic construction of a signal transmitting apparatus for an optical base station in accordance with the related art; 
         FIG. 3  is a drawing illustrating the construction of the signal transmitting apparatus for an optical base station adopting a 3 wave division multiplexing (3WDM) method in accordance with the related art; 
         FIG. 4  is a drawing illustrating a detailed construction of signal transmitting apparatus for an optical base station of  FIG. 2  in accordance with the related art; 
         FIG. 5  is a drawing illustrating a signal transmitting apparatus for an optical base station in accordance with a preferred embodiment of the present invention; and 
         FIG. 6  is a drawing illustrating in detail a signal transmitting apparatus for an optical base station in accordance with the preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 5  is a drawing illustrating a signal transmitting apparatus for an optical base station in accordance with a preferred embodiment of the present invention, and  FIG. 6  is a drawing illustrating additional details of a signal transmitting apparatus for the optical base station of  FIG. 5 . 
     As shown in  FIGS. 5 and 6 , a signal transmitting apparatus for an optical base station of the present invention preferably includes a base station  11  to output a digital I/Q signal and an optical connecting unit  78  to convert the digital in-phase/quadrature phase (I/Q) signal outputted from the base station  10  into an optical signal and outputting the optical signal through an optical cable  90 . The system preferably may also include a remote station  108  converting the optical signal received through the optical cable  90  into a digital I/Q signal, converting the converted digital I/Q signal into a high power RF signal, and transmitting the high power RF signal to an antenna  116 . 
     The remote station  108  preferably includes an optical transceiver  125  converting the optical signal received through the optical cable  90  into a digital I/Q signal, a multiplexer/demultiplexer unit  310  demultiplexing the digital I/Q signal outputted from the optical transceiver  125 , and an up-converter unit  235  digital/analog-converting the output signal of the multiplexer/demultiplexer unit  310 , up-converting it, band-pass filtering it, and outputting an RF signal. Remote station  108  also preferably includes an HPA  140  high-power amplifying the output signal of the up-converter unit  235  and outputting it, a duplexer  150  filtering the output signal of the HPA  140  and outputting it to an antenna  116 , a clock unit  320  providing an synchronous signal to the multiplexer/demultiplexer unit  310 , and a reference clock unit  330  providing a synchronous signal of the clock unit  320  to the up-converter unit  235  and a plurality of down-converter units  225  and  225 ′. 
     The remote station  108  preferably includes a plurality of duplexers  150  and  150 ′ respectively removing a noise component of a signal collected by a plurality of antennas  116  and  118 , a plurality of LNAs  160  and  160 ′ amplifying the RF signals outputted from the plurality of duplexers  150  and  150 ′, and a plurality of down-converter units  225  and  225 ′ band-pass filtering the RF signals outputted from the plurality of LNAs  160  and  160 ′, down-converting them, analog/digital converting them, and applying them to the multiplexer/demultiplexer unit  310 . 
     The optical connecting unit  78  preferably includes a multiplexer/demultiplexer unit  300  multiplexing the digital I/Q signal outputted from the base station  11  and outputting it an optical transceiver  45  converting the output signal of the multiplexer/demultiplexer unit  300  into an optical signal and transmitting the optical signal through the optical cable  90  to the plurality of remote stations  108  and a clock unit  320 ′ providing a synchronous signal to the multiplexer/demultiplexer unit  300 . 
     The operation of the signal transmitting apparatus for an optical base station according the preferred embodiment will now be described with reference to the  FIG. 6 . Unlike the related art signal transmitting apparatus for an optical base station which is based on an RF-signal (i.e. an analog signal) interface, the signal transmitting apparatus for an optical base station of the present invention is based on an I/Q signal (a digital signal) interface. 
     The base station  11  of the present invention preferably outputs a plurality of digital channel signals. That is, signals transmitted between the base station  11  and the optical connecting unit  78  are digital I/Q signals. Digital I/Q signals outputted from a plurality of channel cards  12  of the base station  11  are preferably transmitted to the multiplexer/demultiplexer unit  300  of the optical connecting unit  78 . The multiplexer/demultiplexer unit  300  preferably converts the plurality of received digital I/Q signals from parallel to serial, multiplexes them, and outputs a digital serial signal. Then, the multiplexer/demultiplexer unit  300  preferably transmits the digital serial signal to the optical transceiver  45 . 
     The optical transceiver  45  preferably converts the received digital serial signal to an optical signal. The converted optical signal is preferably transmitted through the optical cable  90  to the plurality of remote stations  108 . The optical transceiver  125  of the remote station  108  preferably converts the optical signal received through the optical cable  90  into a digital serial signal, an electric signal, and outputs the converted signal to the multiplexer/demultiplexer unit  310 . 
     The multiplexer/demultiplexer unit  310  preferably converts the received digital serial signal from serial to parallel, demultiplexer it and outputs it to the up-converter unit  235 . The multiplexer/demultiplexer unit  310  then preferably performs demultiplexing in synchronization with a clock signal of the clock unit  320 . 
     The multiplexer/demultiplexer unit  310  preferably converts the received digital serial signal from serial to parallel, demultiplexes it and outputs it to the up-converter unit  235 . The multiplexer/demultiplexer unit  310  then preferably performs demultiplexing in synchronization with a clock signal of the clock unit  320 . 
     The HPA  140  preferably amplifies the inputted RF signal to a high power signal. The amplified RF signal is preferably transmitted to the duplexer  150  and transmitted through the antenna  116 . 
     Since the remote station  108  preferably employs a diversity receiving method, it preferably includes a dual purpose antenna  116  for transmission and reception, and a reception dedicated antenna  118 . The two antennas  116  and  118  preferably are isolated to maintain an optimum distance there between in consideration of a wave length. The first antenna  116  and the second antenna  118  respectively preferably apply the received RF signal to the duplexers  150  and  150 ′. The RF signals applied to the duplexers  150  and  150 ′ are preferably amplified by the LNAs  160  and  160 ′ and outputted to the down-converter units  225  and  225 ′. 
     The down-converter units  225  and  225 ′ preferably band-pass filter the inputted RF signal, down-convert, analog/digital convert and generate a digital I/Q signal. Then the down-converter units  225  and  225 ′ preferably output the generated digital I/Q signal to the multiplexer/demultiplexer unit  310 . The down-converter units  225  and  225 ′ preferably perform a down-converting operation in synchronization with the clock signal outputted from the reference clock unit  330 . The reference clock unit  330  preferably receives the clock signal from the clock unit  320 . 
     The multiplexer/demultiplexer unit  310  preferably converts the inputted digital I/Q signals of the two paths from parallel to serial, multiplexes it and outputs a digital serial signal. Then, the multiplexer/demultiplexer unit  310  preferably transmits the outputted digital serial signal to the optical transceiver  125 . 
     The optical transceiver  125  preferably converts the received digital serial signal into an optical signal. The converted optical signal is preferably transmitted through the optical cable  90  to the optical connecting unit  78 . 
     The optical transceiver  45  of the optical connecting unit  78  preferably restores the optical signal received through the optical cable  90  to a digital serial signal, an electric signal, and outputs the restored signal to the multiplexer/demultiplexer unit  300 . 
     The multiplexer/demultiplexer  300  preferably converts the inputted I/Q signal from serial to parallel, demultiplexer it and outputs a digital I/Q signal. Then, the multiplexer/demultiplexer unit  300  preferably performs the demultiplexing in synchronization with the clock signal of the clock unit  320 . The clock signal has been generated in the base station, and the clock units  320  and  320 ′ receive through the optical transceiver. The outputted digital I/Q signal is preferably transmitted to the base station  11 . 
     The preferred embodiment of the invention has many advantages compared to the related art signal transmitting apparatus. For example, optical base station of the present invention can avoid signal degradation in analog stages of the base station, since the digital I/Q signal of the base station  10  is directly transmitted to the optical connecting unit  78 . Also, the digital interface-based apparatus of the present invention improves the noise figure by virtue of a noise floor which occurs in the analog interface-based apparatus of the related art. 
     In addition, since the up-converter unit  235  and the plurality of down-converter units  225  and  225 ′ are preferably components of remote station  108  in the present invention, the base station  11  can be constructed with only a channel bank. 
     Moreover, since the optical signal transmitted between the base station  11  and the remote station  108  is a digital I/Q signal, the performance degradation due to the optical transceivers  45  and  125  can be considerably reduced. Also, since the digital signal can be easily restored in the event of an error, the reliability of the system can be improved. 
     The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structure described herein as performing the recited function and not only structural equivalents but also equivalent structures.