Abstract:
A method and apparatus for transmitting data to an Access Point (AP) and a Mobile Station (MS) in a Time Division Duplex (TDD) optical repeater. A main donor generates a control frame for controlling a remote, upon receipt of data from the AP, and transmits the control frame to the remote during a non-downlink transmission period. The remote analyzes the control frame received from the main donor, detects sync information about a downlink signal and an uplink signal, and remote control information from the analyzed control frame, performs a control operation according to the detected remote control information, and generates a status frame for notifying the main donor of the status of the remote upon receipt of data from the MS. The status frame is transmitted from the remote to the main donor during a non-uplink transmission period.

Description:
CLAIM OF PRIORITY 
     This application claims the benefit of priority under 35 U.S.C. §119(a) from a Korean Patent Application filed in the Korean Intellectual Property Office on Jan. 3, 2007 and assigned Serial No. 2007-537, the entire disclosure of which is hereby incorporated by reference in its entirety. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to optical communications and communications with one or more remote nodes. More particularly, the present invention relates to a method and system for controlling a remote node in a Time Division Duplex (TDD) optical repeater. 
     2. Description of the Related Art 
     The drastic development of computer, electronics, and communications technologies is a driving force behind the growth of a variety of wireless communication services being provided over a wireless network. The basic wireless communication service is wireless voice call service to mobile users. The wireless voice call service is typically provided irrespective of a time and location, and text service, which is supplementary to the wireless voice call service, has become increasing popular. Recently, wireless Internet service has emerged to provide wireless Internet access to mobile users. 
     Along with the development of the information and communications technology, 3 rd  Generation (3G) International Mobile Telecommunication-2000 (IMT-2000) standardized by the International Telecommunication Union (ITU) Radio communication sector (ITU-R), such as Code Division Multiple Access (CDMA), Evolution-Data Only (EV-DO), and Wideband CDMA (WCDMA), has been deployed. IMT-2000 is a mobile communication system that aims to provide a variety of application services by enabling direct roaming all over the world. The system permits an improved degree of service, thereby permitting personal mobility and service mobility, while ensuring communication quality near or at the level of wired phones, providing high-speed packet data service, and converging wired and wireless networks. Besides increasing voice quality and Wireless Application Protocol (WAP) service quality, IMT-2000 can provide various multimedia services like Audio On Demand (AOD) and Video On Demand (VOD) at higher rates than known by using conventional (i.e. legacy) mobile communication systems. 
     Moreover, legacy mobile communication systems have limitations in their effectiveness in providing ultra high-speed wireless Internet service because of high wireless Internet fees incurred by the high cost of installing Base Stations (BSs), as well as the limited content generally available for display by the relatively small-size screens of mobile terminals. Wireless Local Area Network (WLAN) is not feasible for providing public service due to propagation interference and narrow coverage. In this context, Wireless Broadband (WiBro) and 4 th  Generation (4G) wireless mobile communications have been proposed to provide the ultra high-speed wireless Internet service at a significantly lower price than known before, while ensuring portability and mobility. 
     Compared to CDMA and WCDMA, WiBro and 4G wireless mobile communications use a mobile Internet technology that adopts Time Division Duplexing (TDD) as a duplexing scheme and Orthogonal Frequency Division Multiplexing (OFDM) as a modulation scheme. 
     TDD is a bi-directional transmission scheme in which downlink transmission alternates with uplink transmission in time. TDD offers higher transmission efficiency than, for example, Frequency Division Duplex (FDD) that uses two different frequencies for the downlink and the uplink. TDD is also suitable for asymmetric or bursty application services through a dynamic time slot allocation. Orthogonal Frequency Division Multiple Access (OFDMA)/Time Division Multiple Access (TDMA) is a similar multiple access scheme to TDMA. which allocates all the subcarriers of a total frequency band to one user during a given time period, and then subsequently allocates them to another user during a next time period. OFDMA/TDMA advantageously increases the data rate per bandwidth and prevents multipath interference. 
     Typically, a mobile communication system divides a mobile communication service area into a plurality of cells and installs a BS at the center of each cell by introducing the concept of frequency reuse in order to expand the coverage area of a mobile communication network. The radiuses of the cells depend on the strength of signals and/or the amount of traffic in the cells. In other words, a cell radius is relatively smaller in a downtown area having a relatively large amount of traffic, whereas a cell radius is relatively larger in a suburban area with a relatively small amount of traffic, so that the traffic does not exceed the processing capacities of wireless BSs that provide mobile communication services to the cells. 
     Despite these efforts to provide better mobile communication services through the appropriate control of cell radiuses according to frequency reuse and the quantity of traffic therein, there still exists some limitations such as shadowing, in which there are areas where wireless signals cannot propagate, such as underground, the inside of buildings, tunnels, etc. in a downtown area. Installing a plurality of new wireless BSs to overcome shadowing in the shadowing area is neither cost-effective due to the costs of installing, maintaining and repairing such facilities in each of the shadowing areas, is generally unfavorable to cell design. As a solution to the problem of shadowing, mobile communication services can be provided in areas such as underground, the inside of buildings, tunnels, etc. using an optical repeater system in the shadowing area. The optical repeater system typically overcomes the shadowing by transmitting signals on a communication channel allocated to a mother BS from an optical repeater in an optical transmission scheme. 
     In particular, the use of an optical repeater is preferable to the 3G mobile communication system and the WiBro system, which have small cell radiuses because these systems use high frequencies and thus experience a large path loss, a small diffraction effect, and a large building transmission loss, compared to the 2 nd  Generation (2G) mobile communication system. 
     In order to relay a radio signal between a BS and a Mobile Station (MS), the optical repeater should distinguish a downlink signal from an uplink signal. In FDD, the optical repeater identifies the downlink signal and the uplink signal by means of a duplexer, whereas in TDD, it distinguishes the downlink signal from the uplink signal by use of a switch and selectively provides a path for each of the signals. For this purpose, the TDD optical repeater needs a control signal for accurately detecting the starting points of the downlink signal and the uplink signal, switching on/off the switch for the signals, and thus changing a signal path. The TDD optical repeater, which is typically located in or near the area where there is a shadowing problem, can receive the control signal from the BS by an optical cable. 
     Moreover, the TDD optical repeater should be equipped with a function for generating a switching control signal so as to control the switch by analyzing a transmission frame so that switching can occur between a downlink period and an uplink period. Due to signal transmission through the optical cable, the optical repeater may suffer from time delay during the transmission. Unless the switch control signal is compensated for the time delay of the optical cable, the switch control signal becomes inaccurate, making it difficult to accurately distinguish between the downlink signal and the uplink signal. 
     One solution to the time delay can be found, for example, in a Korean Patent Publication No. 2006-0010963 entitled “Method and System for Generating Switching Timing Signal for Separating Transmitting and Receiving Signal in Optical Repeater of Mobile Telecommunication Network Using TDD and OFDM Modulation”. 
       FIG. 1  is a block diagram of a conventional TDD optical repeater. Such repeaters can include a “donor” or “donor unit” and a “remote” or “coverage unit” which can bring wireless signals into a shadowing area such as a tunnel or inside a large building and distribute the signal where reception is needed. 
     Referring to  FIG. 1 , in TDD, a main donor  200  transmits a downlink signal to a remote  250  during a predetermined time period and the remote  250  transmits an uplink signal to the main donor  200  during a time period without any downlink signal. 
       FIG. 2  is a timing diagram illustrating the timings of transmitting the downlink signal and the uplink signal in the conventional TDD optical repeater. 
     Referring to  FIG. 2 , during an optical downlink signal transmission via an optical fiber, optical uplink signal transmission does not occur. In other words, the transmissions are mutually exclusive of each other. Because the downlink transmission and the uplink transmission are carried out in TDD, the optical downlink and uplink signals are both transmitted in the same manner, as opposed to, for example, a system where the downlink and the uplink transmissions are transmitted in a different manner. 
     Now referring back to  FIG. 1 , upon the generation of a downlink signal at a predetermined time in an Access Point (AP)  110 , the downlink signal is amplified in a Low Noise Amplifier (LNA)  205  and converted into an optical signal through electrooptic conversion in an Electro-Optic (E/O) converter  210 . The optical signal is transmitted to the remote  250  through a Wavelength Division Multiplexer (WDM)  215  via an optical fiber. For distinguishing an optical downlink signal from an optical uplink signal, wavelength division multiplexing is used. 
     Upon receipt of the optical downlink signal in the remote  250 , an Opto-Electric (O/E) converter  260  converts the optical downlink signal into an electrical signal after processing in a WDM  255 . A separator  265  separates the signal so that a portion goes to switching timing signal generator  290  and a portion to High Power Amplifier (HPA)  260 . A switch  275 , which receives the timing signal generator by generator  290  and the amplified signal from HPA  260  switches the electrical signal to an antenna according to a switching timing signal generated from a switching timing signal generator  290 . 
     Upon receipt of an uplink signal received through the antenna at a predetermined time, the switch  275  switches the uplink signal to an LNA  280  and an E/O converter  285  converts the electrical signal received from the LNA  280  into an optical signal. The optical signal is transmitted to the main donor  200  via the WDMs  255  and  215 . In the main donor  200 , an O/E converter  220  converts the optical uplink signal to an electrical signal and a High Power Amplifier (HPA)  225  amplifies the electrical signal and transmits the amplified signal to the AP  110 . 
     In the conventional TDD optical repeater such as shown in  FIG. 1 , no channel is allocated for controlling the on/off of the remote  250  or the amplifiers of the remote  250 , i.e. an HPA  270  and the LNA  280  and no channel is allocated to carry information about the status of the remote  250  to the main donor  200 . Therefore, the control of the remote  250  is unstable. 
     SUMMARY OF THE INVENTION 
     An aspect of exemplary embodiments of the present invention is to address at least some of the problems and/or disadvantages discussed above and to provide at least the advantages described herein below. Accordingly, an aspect of the present invention is to provide a method and system for facilitating control of a remote based on the characteristics of a TDD signal without using an additional optical transceiver in a TDD optical repeater by transmitting control information for the remote along with a downlink signal from a main donor to the remote, so that the remote extracts the control information, for control of the remote, and transmits status information about the status of the remote along with an uplink signal from the remote to the main donor. 
     In accordance with an aspect of the present invention, there is provided a method for transmitting data to an Access Point (AP) and a Mobile Station (MS) in a TDD optical repeater, in which a main donor generates a control frame for controlling a remote, upon receipt of data from the AP, and transmits the control frame to the remote during a non-downlink transmission period, and the remote analyzes the control frame received from the main donor, detects sync information about a downlink signal and an uplink signal and remote control information from the analyzed control frame, performs a control operation according to the detected remote control information, generates a status frame for notifying the main donor of the status of the remote, upon receipt of data from the MS, and transmits the status frame to the main donor during a non-uplink transmission period. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above features and advantages of certain exemplary embodiments of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  is a block diagram of a conventional TDD optical repeater; 
         FIG. 2  is a timing diagram illustrating the timings of transmitting a downlink signal and an uplink signal in the conventional TDD optical repeater; 
         FIG. 3  is a block diagram of a TDD optical repeater according to an exemplary embodiment of the present invention; 
         FIG. 4  is a timing diagram illustrating the timings of transmitting a downlink signal and an uplink signal in the TDD optical repeater according to an exemplary embodiment of the present invention; 
         FIG. 5  illustrates the format of a control frame by which a main donor controls a remote according to an exemplary embodiment of the present invention; 
         FIG. 6  illustrates the format of a status frame by which the remote notifies the main donor of its status according to an exemplary embodiment of the present invention; 
         FIG. 7  is a flowchart illustrating a method for controlling the remote with respect to a downlink signal in the TDD optical repeater according to an exemplary embodiment of the present invention; and 
         FIG. 8  is a flowchart illustrating a method for controlling the remote with respect to an uplink signal in the TDD optical repeater according to an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The matters defined in the description such as a detailed construction and elements are provided to assist in a comprehensive understanding of exemplary embodiments of the invention. Accordingly, those of ordinary skill in the art will recognize that various changes and modifications of the embodiments described herein can be made without departing from the scope and spirit of the invention. Also, descriptions of well-known functions and constructions may be omitted for clarity and conciseness to ensure appreciation of the invention by a person of ordinary skill in the art. Throughout the drawings, which are provided for illustration and do not limit the invention to the examples shown, the same drawing reference numerals will be understood to refer to the same elements, features and structures. 
       FIG. 3  is a block diagram of a TDD optical repeater according to an exemplary embodiment of the present invention and  FIG. 4  is a timing diagram illustrating the timings of transmitting a downlink signal and an uplink signal in the TDD optical repeater according to an exemplary embodiment of the present invention. 
     Referring to the example shown in  FIG. 3 , the TDD optical repeater may include a main donor  300  and at least one remote  350 . The main donor  300  includes a control frame generator  302 , an LNA  304 , a first signal combiner  305 , an E/O converter  310 , a WDM  315 , an O/E converter  320 , a first switch  322 , and an HPA  325 . 
     The remote  350  includes a WDM  355 , an O/E converter  360 , a signal separator  365 , an HPA  370 , a second switch  375 , an LNA  377 , a second signal combiner  380 , an E/O converter  385 , and a control signal generator  390 . The main donor  300  can be connected to a plurality of remotes  350  by optical fibers so as to expand the coverage area of the TDD optical repeater. 
     The main donor  300  typically communicates with the AP  110  by a transmission line T x  connected there between. Upon receipt of a downlink signal from the AP  110 , the main donor  300  converts the downlink signal into an optical signal by electro-optic conversion and transmits the optical signal to the remote  350  via an optical fiber. Upon receipt of an optical uplink signal from the remote  350 , the main donor  300  converts the optical signal to an electrical signal through opto-electric conversion and transmits the electrical signal to the AP  110  via the transmission line. 
     Upon receipt of an optical downlink signal from the main donor  300 , the remote  350  converts the optical signal into an electrical signal through opto-electric conversion and transmits the electrical signal to an MS  100 . Upon receipt of an uplink signal from the MS  100 , the remote  350  converts the uplink signal to an optical signal through electro-optic conversion and transmits the optical signal to the main donor  300  via the optical fiber. 
     Still referring to  FIG. 3 , for downlink transmissions and uplink transmissions, the above-described TDD optical repeater typically operates as follows. 
     In the main donor  300 , the LNA  304  low-noise-amplifies a downlink signal received from the AP  110 . 
     The control frame generator  302  generates a control frame with control information for controlling the remote  350 , as illustrated in  FIG. 4 , and transmits the control frame to the remote  350  during an uplink transmission following the downlink transmission, i.e. a non-downlink transmission period. The control frame includes information for controlling the HPA  370  and the LNA  377  to adjust the power of an uplink signal as well as a downlink signal from the remote  350 . The control frame generator  302  also generates a control frame according to a status frame with information about the status of the remote  350  received during a non-uplink transmission period after an uplink transmission from an MS. 
     The first signal combiner  305  combines the amplified downlink signal received from the LNA  305  with the control frame received from the control frame generator  302 . The E/O converter  310  converts the combined downlink signal received from the first signal combiner  310  into an optical signal. The WDM  315  transmits a plurality of optical signals received from the E/O converter  310  to the remote  350  by WDM. 
     Still referring to  FIG. 3 , each of the WDMs  315  and  355  divides a fiberoptic channel into a plurality of channels according to light wavelengths and uses them as a plurality of communication paths. For optical signal transmission, the WDM can act as a wavelength division multiplexer that transmits signals at different light wavelengths via a single optical fiber, and for optical signal reception, it can act as a wavelength division demultiplexer that demultiplexes a signal received via a single optical fiber into signals at different wavelengths. The E/O converters  310  and  385  can be configured, for example, using laser diodes and the O/E converters  320  and  360  can be configured, for example, using photo diodes. 
     In the remote  350 , the WDM  355  demultiplexes the received optical signal into a plurality of optical signals. The O/E converter  360  converts an optical signal into an electrical signal. 
     The signal separator  365  separates a control frame and a downlink signal from the electrical signal received from the O/E converter  360  and provides the control frame to the control signal generator  390  and the downlink signal to the HPA  370 . 
     The control signal generator  390  analyzes the control frame and controls the second switch  375 , the on/off of the remote  350 , the HPA  370 , and the LNA  377  according to the analysis result. 
     In addition, the control signal generator  390  typically generates a status frame with information about the status of the remote  350  for transmission to the main donor  300  during a downlink transmission following an uplink transmission from an MS, i.e. during a non-uplink transmission period. 
     The HPA  370  amplifies the electrically converted downlink signal to an effective predefined power level/range for transmitting the downlink signal over the air by an antenna according to HPA control information analyzed by the control signal generator  390 . The second switch  375  switches on/off according to time control information Δ t  analyzed by the control signal generator  390 , selectively establishes a downlink signal path, and radiates the amplified downlink signal received from the HPA  370  to an MS through an antenna. 
     When the remote  350  receives an uplink signal from the MS through the antenna according to remote-on/off information analyzed by the control signal generator  390 , the second switch  375  switches on/off according to time control information Δ t  analyzed by the control signal generator  390  and selectively establishes an uplink signal path. 
     The LNA  377  amplifies the uplink signal received from the second switch  375  according to LNA control information analyzed by the control signal generator  390 . 
     The second signal combiner  380  combines the amplified uplink signal received from the LNA  377  with a status frame generated from the control signal generator  390 . 
     The E/O converter  385  converts the combined uplink signal to an optical signal and the WDM  355  transmits the optical signal to the main donor  300  via an optical fiber. 
     In the main donor  300 , the WDM  315  demultiplexes the received optical signal into a plurality of optical signals. The O/E converter  320  converts an optical signal to an electrical signal. 
     The first switch  322  selectively establishes an uplink signal path to provide the uplink signal to the HPA  325 , or provides the status frame received during the non-uplink transmission period to the control frame generator  302 . 
     The HPA  325  amplifies the uplink signal to an effective power level for transmitting the uplink signal to the AP  110 , and transmits it to the AP  110 . 
       FIG. 5  illustrates the exemplary format of a control frame by which the main donor  300  typically controls the remote according to an exemplary embodiment of the present invention and  FIG. 6  illustrates the exemplary format of a status frame by which the remote  350  typically notifies the main donor  300  of its status according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 5 , the control frame for controlling the remote  350  includes Sync information for synchronizing between the main donor  300  and the remote  350  with respect to the a downlink signal and an uplink signal, time control information Δ t , remote-on/off information, HPA control information, and LNA control information. The control frame may further include other control information and has every time information required for TDD operation. 
     Referring to  FIG. 6 , the status frame indicating the status of the remote  350  includes Sync information for synchronizing between the main donor  300  and the remote  350  with respect to a downlink signal and an uplink signal, remote-on/off status information, HPA status information, and LNA status information. The control frame may further include other status information according to need. 
       FIG. 7  is a flowchart illustrating exemplary method steps for controlling the remote with respect to a downlink signal in the TDD optical repeater according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 7 , upon receipt of a downlink signal from the AP  110  at the main donor  300  of the TDD optical repeater, the control frame generator  302  generates a control frame for controlling the remote  350  in step S 700  and the main donor  300  transmits the control frame to the remote  350  during an uplink transmission following the downlink transmission from the AP  110 , i.e. during a non-downlink transmission period in step S 710 . 
     The control signal generator  390  of the remote  350  analyzes the control frame received from the main donor  300  in step S 720  and detects Sync information for synchronizing between the main donor  300  and the remote  350  with respect to a downlink signal and an uplink signal in step S 730 . Subsequently, the control signal generator  390  detects time in step S 740  for control information Δ t  for generating a switch control signal and then acts according to the type of control information detected. 
     In step S 750 , when detecting remote-on/off information from the control frame, the control signal generator  390  controls the on/off of the remote  350  according to the remote-on/off information. 
     In step S 760 , when detecting HPA control information, the control signal generator  390  controls the HPA  370  to amplify the downlink signal to an effective power level for radio transmission according to the HPA control information. In step S 770 , when detecting time control information Δ t  for controlling a switching timing, the control signal generator generates a switch control signal according to the time control information Δ t  and controls the second switch  375  to establish a downlink signal path according to the switch control signal in step S 780 . 
     The second switch  375  establishes the downlink signal path by switching on according to the switch control signal and only a downlink signal free of the control frame is transmitted through the antenna for transmission to the MS  100 . 
       FIG. 8  is a flowchart illustrating exemplary method steps for controlling the remote with respect to an uplink signal in the TDD optical repeater according to an exemplary embodiment of the present invention. 
     Referring to  FIG. 8 , the second switch  375  of remote  350  establishes an uplink signal path by switching on according to a switch control signal. Upon receipt of an uplink signal from an MS  100  through the antenna in step S 800 , the control signal generator  390  detects LNA control information from control information about the remote  350  in step S 810  and controls the LNA  377  to amplify an uplink signal component according to the LNA control information in step S 820 . 
     Meanwhile, at step S 830 , the control signal generator  390  generates a status frame indicating the status of the remote  350 , including Sync information for synchronizing between the main donor  300  and the remote  350  with respect to a downlink signal and an uplink signal, remote-on/off status information, HPA status information, and LNA status information, and transmits the status information to the main donor  300  during a non-uplink transmission period. 
     In step S 840 , the control frame generator  302  of the main donor  300  controls the first switch  322  to establish an uplink signal path, receives the status frame from the control signal generator  390 , and generates a control frame using the status frame. Then in step S 850 , the control signal generator  390  repeats the control procedure of the remote  350 . 
     In accordance with the present invention as described in the above examples, a remote can be easily controlled in a TDD optical repeater since control information for controlling the remote and status information about the remote are transmitted by the respective main donor  300  and remote  350  in time areas unused by the respective main donor and remote for TDD signal transmission. 
     While the invention has been shown and described with reference to certain exemplary embodiments of the present invention 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 of the present invention and the scope of the appended claims. For example, while the main donor  300  shows a transmission line and reception line between the AP  110  and the main donor  300 , it is within the spirit and scope of the invention that such transmission could be wireless.