Abstract:
A repeating apparatus and method using wireless optical transmission is disclosed. The repeating apparatus includes a donor device for transmitting two identical copies of an optical signal by receiving a RF signal from a base station and electro-optic converting the RF signal to an optical signal, and for transmitting a RF signal by receiving two identical copies of the optical signal and optic-electro converting the optical signal to a RF signal; and a coverage device for transmitting a RF signal to a mobile communication terminal by receiving two identical copies of the optical signal from the donor device and optic-electro converting the two identical copies of the optical signal to the RF signal, and transmitting two optical signals to the donor device by receiving the RF signal from the mobile communication terminal and elector-optic converting the RF signal to the optical signal.

Description:
FIELD OF THE INVENTION 
   The present invention relates to an apparatus and method for repeating signal by using wireless optical transmission; and, more particularly, to an apparatus and method for repeating signal by using wireless optical transmission for simultaneously transmitting the two identical copies of a wireless optical signal through two different routes, thereby minimizing the error rate and securing a stable environment for data transmission. 
   DESCRIPTION OF RELATED ART 
   Hereinafter, a mobile communication system will be quoted as a working example of the preferred embodiments of the present invention. 
     FIG. 1  is a perspective view of a wireless communication system. 
   Referring to  FIG. 1 , the mobile communication system includes a mobile communication terminal  10 , a base station  11 , a base station controller  12 , a switching center  13 , a location register  15  and a gateway switching center  14 . 
   The base station  11  provides a wireless connection between the mobile communication terminal  10  and the mobile communication network. The base station controller  12  controls and manages the base station  11 . The switching center  13  establishes a call connection to the mobile communication terminal  10 . The location register  15  keeps track of the location of the mobile communication terminal  10  so as to make mobile communication services more accessible. The gateway switching center  14  provides a gateway to external public switched telephone networks (PSTNs) and other mobile communication service providers. 
   For the mobile communication network, the base station  11  is the most important factor in determining its economic efficiency and service quality. Regarding the construction of the mobile communication network, the economic efficiency always comes first in determining where to locate the base stations  10  and thus, there must be shadow region occurred such as a huge building, hills, undergrounds and mountains. The shadow region is hereinafter referred to as an area in which the transmission of radio waves is blocked by objects like buildings, mountains and suchlike. The reasons for the above-mentioned service inaccessibility come in a wide variety of forms. Nevertheless, these shortcomings in the mobile communication network have been easily overcome by the repeater. 
     FIGS. 2A to 2E  are diagrams showing conventional repeaters implemented in the mobile communication network. 
   Referring to  FIG. 2A and 2E , the conventional repeaters includes a repeater  21 , a cable optical repeater  22  using an optical cable, a microwave repeater  23  using microwaves, a miniature base station  24  and a wireless optical repeater  25  using a wireless optical signal. 
   Firstly, referring to  FIG. 2A , a repeater  21  is a general type repeater. The repeater  21  amplifies a radio frequency (RF) signal transmitted in mobile communication bandwidth from a base station  11  and transmits the RF signal to the shadow region. The repeater  21  is cost-effective as well as easy to install and operate. On the other hand, the one major drawback of the repeater  21  is that a radio wave signal between transmitting and receiving antennas could be coupled together and it causes oscillations. For the above-mentioned reason, the RF signals of the two antennas need to be kept separate. For keeping the RF signals separate, antennas need to be installed with a vertical clearance and the gain of the repeater must be limited. Therefore, the intensity of transmissible radio waves becomes limited and the mobile communication service coverage area would be limited too. 
   Referring to  FIG. 2B , the cable optical repeater  22  converts a RF signal into an optical signal and transmits the optical signal through an optical cable installed between the base station  11  and the shadow area. The cable optical repeater  22  securely transports information across a long distance such as over 10 km without increased level of noise and signal distortion. On the other hand, the potential pitfall of using the cable optical repeater  22  and an optical cable is that this optical transmission technology can not be embraced prior to the provision of relevant infrastructure. In addition to the above, installing, hiring and fixing an optical cable are extremely costly in most cases. 
   Referring to  FIG. 2C , the microwave repeater  23  transforms a RF signal into a microwave, typically relating to frequency signals ranging from 8 GHz to 30 GHz, by using a microwave repeater device installed between the base station and a cell. The microwave repeater  23  is easy to install as well as inexpensive to maintain. On the other hand, the one major drawback of using the microwave repeater  23  is that the use of frequencies within microwave bandwidth brings up the legal and security issues such as paying fees for the usage, dealing with local authority and wire-tapping. 
   Referring to  FIG. 2D , the miniature base station  24  is a miniaturized version of the base station  11 , but otherwise the installation of such station  24  is problematic as well as costly. 
   Referring to  FIG. 2E , the wireless optical repeater  25  includes a donor device  26  and a coverage device  25 . The donor device  26  transforms a wireless signal received from the base station  11  into an optical signal. The coverage device  25  transforms the optical signal transmitted from the donor device  26  back into a wireless signal for further transmission. 
   Unlike in the repeater  21 , there exists no need in the wireless optical repeater  25  for installing antennas with a vertical clearance or entailing limited mobile communication service coverage due to a prescribed limit on the intensity of transmissible radio waves. As another advantage of the wireless optical repeater  25 , the cost-effectiveness thereof, in conjunction with the user-friendliness thereof, makes such repeaters favored. In the wireless optical repeater  25 , there are not any legal and security issues as such in relation to the use of frequencies within microwave bandwidth, as is usually the case in the microwave repeater  23 . Hereinafter, the configuration and operation of the wireless optical repeater  25  are explained in details. 
     FIG. 3  is a diagram showing a conventional donor device in a repeater. 
   Referring to  FIG. 3 , the donor device of the repeater using conventional wireless optical transmission includes a donor antenna  30 , a duplexer  31 , a forward process unit  32 , a transmitter telescope  34 , a receiver telescope  35  and a backward process unit  33 . 
   The donor antenna  30  transmits a RF signal to the base station  11 , and vice versa. The duplexer  31  passes a RF signal transmitted from the base station  11  to a forward process unit  32  via the donor antenna  30 . On the contrary, the duplexer  31  passes a RF signal transmitted from a backward process unit  33  on to the base station  11  via the donor antenna  30 . The forward process unit  32  transforms the RF signal transmitted from the duplexer  31  into the optical signal. The transmitter telescope  34  sends out an optical signal on receipt of the optical signal from the forward process unit  32 . The receiver telescope  35  receives the optical signal transmitted from a coverage device. The backward process unit  33  transforms the optical signal transmitted from the receiver telescope  35  into a RF signal. 
   The forward process unit  32  includes a low noise amplifier  321 , a RF filter  322 , an electro-optic (E/O) converter  323  and an optical amplifier  324 . The operation of the forward process unit  32  is performed as follows. A RF signal received from the donor antenna  30  through the duplexer  31  is transmitted to the low noise amplifier  321 . Low noise amplification at the low noise amplifier  321  is performed first. Secondly, the RF filter  322  filters the low noise amplified RF signal. Thirdly, the E/O converter  323  converts the low noise amplified RF signal to an optical signal. The optical amplifier  324  amplifies the optical signal. Lastly, the amplified optical signal is transmitted by the transmitter telescope  34 . 
   The backward process unit  33  includes an optic-electro (O/E) converter  334 , a low noise amplifier  333 , a RF filter  332 , and a power amplifier  324 . The operation of the backward process unit  33  is described as follows. An optical signal received from the receiver telescope  35  transmitted to the optic-electro (O/E) converter. The optic-electro (O/E) converter  334  converts the optical signal to RF signals. Secondly, the low noise amplifier  333  amplifies low noise in the RF signals. The RF filter  332  filters the low noise amplified RF signal. Power is amplified at the power amplifier  333  and the amplified RF signal is transmitted from a donor antenna  30  via a duplexer  31  to the base station  11 . 
     FIG. 4  is a perspective view of a coverage device used in a repeater using wireless optical transmission. 
   Referring to  FIG. 4 , the coverage device has the same configuration as the donor device does, but otherwise the coverage device follows the same process as the donor device does but in a reverse order. 
   The coverage device includes a receiver telescope  44 , a forward process unit  42 , a coverage antenna  40 , a duplexer  41 , a backward process unit  43  and a transmitter telescope  35 . 
   The receiver telescope  44  receives an optical signal transmitted from the donor device. The forward process unit  42  transforms the optical signal transmitted from the receiver telescope  44  into a RF signal. The coverage antenna  40  transmits a RF signal to the mobile communication terminal  10 , and vice versa. The duplexer  41  transmits the RF signal transmitted from the forward process unit  42  via the coverage antenna  40 . On the contrary, the duplexer  41  passes the RF signal transmitted from the mobile communication terminal  10  on to the backward process unit  43  via the coverage antenna  40 . The backward process unit  43  transforms the RF signal transmitted from the duplexer  41  into an optical signal. The transmitter telescope  45  sends out an optical signal on receipt of the optical signal from the forward process unit  32 . 
   The forward process unit  42  includes an optic-electro (O/E) converter  421 , a low noise amplifier  422 , a RF filter  423  and a power amplifier  424 . 
   The backward process unit  43  includes a low noise amplifier  434 , a RF filter  433 , an electro-optic (E/O) converter  432  and an optical amplifier  431 . The operation of the backward process unit  43  is that of the donor device in reverse. 
   It is often the case that the wireless optical signal transmission is climate-dependent and vulnerable to a malfunction caused by an external force. In other words, the successful transmission of the wireless optical signal is contingent on the weather conditions. Here, if data is to be transmitted through an optical cable at the rate of 1 Gbps, that is, 1 bit is roughly equivalent to 1 ns, an instant blockage in the optical cable will do a significant damage to the transmission of data as a whole, resulting in data being corrupted. This is the one major drawback of the wireless optical signal transmission. 
   SUMMARY OF THE INVENTION 
   It is, therefore, an object of the present invention to provide an apparatus and method for repeating signal by using wireless optical transmission for simultaneously transmitting the two identical copies of a wireless optical signal through two different routes, thereby minimizing the error rate and securing a stable environment for data transmission. 
   In accordance with an aspect of the present invention, there is provided an apparatus for repeating signal by using wireless optical transmission, including: a donor device for transmitting two identical copies of an optical signal through two different routes by receiving a RF signal from a base station and electro-optic converting the RF signal to an optical signal, and for transmitting a RF signal of the base station by receiving two identical copies of the optical signal through two different routes and optic-electro converting the optical signal to a RF signal; and a coverage device for transmitting a RF signal to a mobile communication terminal by receiving two identical copies of the optical signal through two different routes from the donor device and optic-electro converting the two identical copies of the optical signal to the RF signal, and transmitting two identical copies of the optical signal through two different routes to the donor device by receiving the RF signal from the mobile communication terminal and elector-optic converting the RF signal to the optical signal. 
   In accordance with another aspect of the present invention, there is provided a method for repeating signal by using wireless optical transmission, the method including the steps of: a) at a donor device, receiving a RF signal from a base station, electro-optic converting the RF signal into an optical signal and transmitting two identical copies of the optical signal through two different routes; b) at a coverage device, receiving two identical copies of the optical signal through two different routes from the donor device, optic-electro converting the optical signal into the RF signal and transmitting the RF signal to a mobile communication terminal; c) at the coverage device, receiving the RF signal from the mobile communication terminal, electro-optic converting the RF signal into an optical signal and transmitting two identical copies of the optical signal through two different routes; and d) at the donor device, receiving a two identical copies of the optical signal through two different routes from the coverage device, optic-electro converting the optical signal into a RF signal and transmitting the RF signal to the base station. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and features of the present invention will become apparent from the following description of the preferred embodiments given in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a perspective view of a wireless communication system; 
       FIGS. 2A and 2E  are a perspective view illustrating various conventional repeaters of wireless communication system; 
       FIG. 3  is a perspective view illustrating the configuration of a conventional donor device used in a repeater using conventional wireless optical transmission; 
       FIG. 4  is a perspective view of a conventional coverage device used in a repeater using wireless optical transmission; 
       FIG. 5  is a perspective view illustrating the configuration of a donor device used in a repeater using wireless optical transmission in accordance with a preferred embodiment of the present invention; and 
       FIG. 6  is a perspective view illustrating the configuration of a coverage device used in a repeater using wireless optical transmission in accordance with a preferred embodiment of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Other objects and aspects of the invention will become apparent from the following description of the embodiments with reference to the accompanying drawings, which is set forth hereinafter. 
     FIG. 5  is a perspective view illustrating the configuration of a donor device used in a repeater using wireless optical transmission in accordance with a preferred embodiment of the present invention. 
   Referring to  FIG. 5 , the donor device includes a donor antenna  50 , a duplexer  51 , a forward process unit  52 , a transmitter telescope  55 , an optical circulator  54 , a transceiver telescope  56 , a receiver telescope  57  and a backward process unit  53 . 
   The donor antenna  50  transmits a RF signal to the base station  11 , and vice versa. The duplexer  51  passes the RF signal transmitted from the base station  11  to a forward process unit  52  via the donor antenna  50 . On the contrary, the duplexer  51  passes a RF signal transmitted from a backward process unit  53  on to the base station  11  via the donor antenna  50 . The forward process unit  52  transforms the RF signal transmitted from the duplexer  51  into an optical signal. The transmitter telescope  55  transmits the optical signal from the forward process unit  52 . The optical circulator  54  receives the optical signal from the forward process unit  52  and transmits to the transceiver telescope  56 . Also, the optical circulator  54  receives the optical signal from the transceiver telescope  56  and transmits the optical signal to the backward process unit  53 . The transceiver telescope  56  transmits the optical signal from the optical circulator  54  and receives an optical signal from a coverage device. The receiver telescope  57  receives the optical signal transmitted from the coverage device. The backward process unit  53  transforms the optical signal transmitted from the receiver telescope  57  or the optical circulator  54  into a RF signal which is, in turn, to be delivered to the duplexer  51 . 
   The forward process unit  52  includes a low noise amplifier  521 , a RF filter  522 , an electro-optic (E/O) converter  523  and an optical amplifier  524 . The low noise amplifier  521  reduces noise on a RF signal transmitted from the duplexer  51 , thereby amplifying the RF signal. The RF filter  522  filters the RF signal transmitted from the low noise amplifier  521 . The E/O converter transforms the RF signal transmitted from the RF filter  522  into an optical signal using a laser diode (LD). The optical amplifier  524  amplifies the optical signal transmitted from the E/O converter. Here, the wavelength of the signal in the LD must be equal to the wavelength of the amplified signal coming out of the optical amplifier  524 . A direct modulation method using the laser diode (LD) modulates a signal multiplexed by an electric pulse string to an optical signal by inputting the multiplexed signal to a driving unit of the diode. The modulated optical signal acts according to the response characteristic of the LD wherein an optical pulse is transmitted via the turning on and off of the LD following a relevant bit string. However, an indirect modulation method may be used in the present invention instead of using the direct modulation method. In the indirect modulation method, the laser diode is always turned on and the signal is modulated by using external modulator. 
   The backward process unit  53  includes an optic-electro (O/E) converter  534 , a low noise amplifier  533 , a RF filter  532 , and an optical amplifier  531 . The O/E converter  534  converts an optical signal transmitted from the receiver telescope  57  or the optical circulator  54  into a RF signal. The low noise amplifier  533  reduces noise on the RF signal transmitted from the O/E converter  534 , thereby amplifying the signal. The RF filter  532  filters the RF signal transmitted from the low noise amplifier  533 . The power amplifier  531  amplifies the RF signal transmitted from the RF filter  532 . Here, the optical signal being transmitted from the coverage device to the donor device is attenuated for various reasons. Accordingly, if the optical signal is not significantly attenuated, it is not necessary to amplify the optical signal to be converted to the RF signal. Therefore, it is possible that the optical signal may be directly converted to the RF signal without amplifying the optical signal. However, if the optical signal is significantly attenuated, the optical signal needs to be amplified. 
   Furthermore, the making use of the low noise amplifier  533  and the RF filter  532  is contingent on the intensity of a converted RF signal coming out of the optic-electro (O/E) converter  534 . 
   On the other hand, each of the two identical copies of an optical signal processed at the forward process unit  52  is transmitted simultaneously to its corresponding destination, optical circulator  54  and transmitter telescope  55 . The optical circulator  54  transmits an optical signal to the coverage device via the transceiver telescope  56 . The above-mentioned safety mechanism is put in place to secure a stable environment for data transmission. In the event of an interruption to one of two routes, thereby doing damage to a signal therein, a signal transmitted via the other route can be used to restore the damaged signal. Accordingly, the reliability of data transmission is secured when each of the two output signals is checked through to determine if one signal is the exact match of the other. 
   The above mentioned operations are same for the donor device of the backward process unit  53 . Specifically, two identical copies of an optical signal is transmitted to the backward process unit  53  via two different routes, namely one via the optical circulator  54  and the other via the receiver telescope  57 . Most often, the backward process units  53  takes an optical signal transmitted from the optical circulator  54  and then transforms the signal into a RF signal. On the contrary, in the event of an interruption to the signal transmission, an optical signal transmitted from the receiver telescope  57  is used instead. On the other hand, both of the two optical signals are transformed into a RF signal. Then, each of the two RF signals is checked through to determine if one signal is the exact match of the other to make certain the data transmission is reliable. 
   Following on from the above, as regards the above-mentioned selection process performed at the O/E converter  534  of the backward process unit  53  by a controller (not shown) coupled to the O/E converter  534 , wherein the controller may be implemented by using a computer. The controller controls the O/E converter  534  to choose between the two available optical signals and to pass the select signal to the low noise amplifier  533 . Accordingly, the way in which each of the two signals is checked through to determine if one signal is the exact match of the other is as follows. Firstly, the O/E converter  534  transforms both of the two optical signals into a RF signal which is, in turn, to be fed into the controller connected to the O/E converter  534 . Secondly, the controller controls the O/E converter  534  to filter out those signals in which the two RF signals do not match. Thirdly, a select signal is transferred to the low noise amplifier  533 . The same operations are implemented for the selection process performed at the O/E converter  621  of the forward process unit  62  as illustrated below in  FIG. 6 . 
     FIG. 6  is a perspective view illustrating a coverage device used in a repeater using wireless optical transmission in accordance with a preferred embodiment of the present invention. 
   Referring to  FIG. 6 , the coverage device has the same configuration as a donor device does, but otherwise the coverage device follows the same process as the donor device does but in a reverse order. 
   The coverage device used in the repeater using wireless optical transmission includes a receiver telescope  65 , a transceiver telescope  66 , an optical circulator  64 , a forward process unit  62 , a coverage antenna  60 , a duplexer  61 , a backward process unit  63  and a transmitter telescope  67 . 
   The receiver telescope  65  receives an optical signal transmitted from the donor device. The transceiver telescope  66  passes the optical signal from the optical circulator  64  to the donor device. In addition, the transceiver telescope  66  receives an optical signal transmitted from the donor device. The optical circulator  64  causes an optical signal to branch toward the forward process unit  62  on receipt of the signal from a transceiver telescope  66 . In addition, the optical circulator  64  causes an optical signal to branch toward a transceiver telescope  66  on receipt of the signal from the backward process unit  63 . The forward process unit  62  transforms an optical signal transmitted from the optical circulator  64  or the receiver telescope  65  into a RF signal. The coverage antenna  60  transmits a RF signal to the mobile communication terminal  10 , and vice versa. The duplexer  61  transmits a RF signal transmitted from a forward process unit  62  via the coverage antenna  60 . On the contrary, the duplexer  61  passes a RF signal transmitted from the mobile communication terminal  10  on to the backward process unit  63  via the coverage antenna  60 . The backward process unit  63  transforms the RF signal transmitted from the duplexer  61  into an optical signal. The transmitter telescope  67  transmits the optical signal on receipt of the optical signal from the backward process unit  52 . 
   The forward process unit  62  includes an optic-electro (O/E) converter  621 , a low noise amplifier  622 , a RF filter  623  and a power amplifier  624 . The backward process unit  63  includes a low noise amplifier  634 , a RF filter  633 , an electro-optic (E/O) converter  632 , and an optical amplifier  631 . The operation of the coverage device is that of the donor device in a reverse order. 
   On the other hand, the use of the optical amplifier, the low noise amplifier and the RF filter is decided based on the weather conditions as well as the type of a place in which a repeater is located. In other words, the use of a high power optical amplifier is a must for a long distance communication. In a harsh climate, there exists a need for the increase in the intensity of an optical signal. 
   The transceiver telescope in the above mentioned preferred embodiment of the present invention can be implemented by using a receiver and a transmitter separately in another preferred embodiment. If the transceiver telescope is implemented by using the independent receiver and transmitter, the optical circulator is not required. 
   The effect of the present invention as recited in the above is briefly summarized herein as follows. As is usually the case in a typical wireless optical repeater using wireless optical communication, the wireless optical signal transmission is climate-dependent and vulnerable to a malfunction caused by an external force. Unlike the typical wireless optical repeater, a repeater using wireless optical transmission in accordance with the preferred embodiments of the present invention simultaneously sends out the two identical copies of a wireless optical signal via two different routes, thereby minimizing the error rate and securing a stable environment for data transmission. 
   While the present invention has been described with respect to certain preferred embodiments, it will be apparent to those skilled in the art that various changes and modifications may be made without departing from the scope of the invention as defined in the following claims.