Abstract:
A radio over fiber (ROF) link apparatus capable of a stable TDD wireless service for a time division duplexing (TDD) baseband signal includes a central access platform (CAP) for receiving various kinds of data including the TDD baseband signal from upper layers, multiplexing the data, electro-optically converting the multiplexed data, and transmitting the converted data as downstream data through an optical fiber, and opto-electrically converting upstream data received through the optical fiber, demultiplexing the converted upstream data, and transmitting the demultiplexed upstream data to the respective upper layers, and a remote access unit (RAU) for receiving the downstream data through the optical fiber, opto-electrically converting the received downstream data to the multiplexed data, demultiplexing the multiplexed data, performing a wireless access process of the demultiplexed data, and transmitting the wireless access processed data to a wireless local area network (WLAN) service terminal through an antenna.

Description:
CLAIM OF PRIORITY 
   This application claims the benefit of the earlier filing date, pursuant to 35 U.S.C. §119 to that patent application entitled “ROF Link Apparatus Capable of Stable TDD Wireless Service,” filed in the Korean Intellectual Property Office on Sep. 2, 2005 and assigned Serial No. 2005-81878, the contents of which are incorporated herein by reference. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates generally to a radio over fiber (ROF) link apparatus, and in particular, to an ROF link apparatus for transmitting a time division duplexing (TDD) wireless communication service without modulating a radio frequency (RF) band. 
   2. Description of the Related Art 
   Accompanying a variety, and a rapid increase, of information communication services, optical communication technology and wireless communication technology are being combined, thus increasing the necessity of a high-speed multimedia communication service. 
   Thus, research interests are concentrating on optical-wireless communication technology in which an ultra-high radio frequency is interlocked with a high-speed optical communication network to provide various kinds of bulk multimedia information communication services. By combining wired communication technology and wireless communication technology into an integrated technology of optical communication technology and wireless communication technology, a radio over fiber (ROF) technology is being vigorously studied. 
   Since an ROF system has many advantages, such as broadband channel capacity, low price, low power, and easy installation, operation, and management, the ROF technology provides appropriate solutions for high-speed wireless multimedia services for in-door applications such as airport terminals, shopping centers, and large-sized offices and out-door applications such as tunnels, narrow streets, and highways. 
     FIG. 1  is a block diagram of a conventional ROF link apparatus for a TDD wireless local area network (WLAN) service. 
   Referring to  FIG. 1 , the conventional ROF link apparatus includes a central station  100  and a base station  200 . The central station  100  receives data from an upper layer, converts the received data to an RF signal for wireless communication, electro-optically converts the RF signal to an optical signal and transmits the optical signal to the base station  200  through an optical fiber. The central station  100  further receives upstream data generated in an RF manner from the base station  200  through the optical fiber, opto-electrically converts the received upstream data to an RF signal, converts the RF signal to baseband data, and transmits the baseband data to the upper layer. The base station  200  similarly, receives downstream data from the central station  100  through the optical fiber, opto-electrically converts the downstream data to an RF signal, and transmits the RF signal to a WLAN service terminal  300  through an antenna, and further receives upstream data from the WLAN service terminal  300 , opto-electrically converts the upstream data to an optical signal, and transmits the optical signal to the central station  100  through the optical fiber. The central station  100  operates in TDD wireless communication protocol. 
   In the TDD wireless communication, the same frequency band is time divided and used for transmitting upstream data and downstream data. That is, an assigned frequency band is used to transmit downstream data in a specific time and used to transmit upstream data after the downstream data is transmitted. Thus, as the same frequency band is generally used to transmit upstream and downstream data, a TDD wireless system has better frequency usage efficiency than a conventional frequency division duplexing (FDD) wireless system. However, since a technique of processing data by dividing a short time period is required, the TDD wireless system has a relatively complex system configuration. Recently, TDD wireless systems are used in wireless services such as WLAN and mobile Internet. 
   When data is processed in the TDD wireless communication, as illustrated in  FIG. 1 , TDD data of a baseband is modulated to TDD data of an RF band using WLAN access points (APs)  101  and  102  operating in a TDD method included in the central station  100 . 
   The central station  100  also includes an RF coupler/divider  103  for performing coupling and dividing operations to process an RF input and an RF output to and from the WLAN APs  101  and  102 , and an opto-electrical converter  105  and an opto-electrical converter  104  for transmitting data through the optical fiber. 
   Each of the WLAN APs  101  and  102  includes an Ethernet switching unit for connecting with the upper layer, a baseband processing unit for converting baseband data input through the Ethernet switching unit to RF data, and an RF transceiver module for transmitting the converted RF data to the RF coupler/divider  103 . Although only a downstream operation of the WLAN APs  101  and  102  has been described, and as the upstream operation is opposite to the downstream operation, the upstream operation need be not discussed in detail herein. However, it would be well within the knowledge of those skilled in the art to understand the upstream operation based on the discussion of the downstream operation discussed herein. 
   The base station  200  includes an opto-electrical converter  106  for converting an optical signal received from the central station  100  to an electrical signal, an electro-optical converter  107  for converting an electrical signal to an optical signal and transmitting the converted optical signal to the central station  100 , and an RF amplifier  108  for amplifying downstream data (RF signal) converted to the electrical signal using the opto-electrical converter  106  to output through the antenna, and amplifying a weak RF signal received through the antenna to transmit to the central station  100  through the electro-optical converter  107 . 
   As described above, the ROF link apparatus has a structure in which the central station  100  and the base station  200  are connected through the optical fiber, i.e., an optical relay structure of a general wireless communication system. However, since the TDD method is applied to the ROF link apparatus, the WLAN APs  101  and  102  are disposed in the central station  100 . 
   Operations of the WLAN APs  101  and  102  will now be described. During a specific transmission time, an RF signal output from the WLAN AP  101  is modulated to an optical signal by the electro-optical converter  104  and transmitted to the base station  200  through the optical fiber. The transmitted optical signal is converted to an RF signal by the opto-electrical converter  106 , amplified by the RF amplifier  108 , and propagated through the antenna. 
   The operation described above is performed during the transmission time by the ROF link apparatus using the TDD method. 
   During a reception time, a weak upstream RF signal input through the antenna is low-noise amplified by a low noise amplifier (LNA: not shown) of the base station  200 , amplified to an RF signal having a constant level by the RF amplifier  108 , converted to an optical signal by the electro-optical converter  107 , and transmitted to the central station  100  through the optical fiber. The transmitted optical signal is converted to an RF signal by the opto-electrical converter  105 , input to the WLAN AP  102  through the RF coupler/divider  103 , and processed by the WLAN AP  102 . 
   As described above, when the WLAN APs  101  and  102  are disposed in the central station  100  for the TDD wireless communication of the ROF link apparatus, and a delay according to the length of the optical fiber may occur in the time division processing of the TDD method, throughput of service data may be reduced according to the length of the optical fiber, or the TDD system may not operate at all because of the amount of an optical signal loss. 
   When a signal is transmitted through a single mode optical fiber, a propagation delay time is around 5 μs/km is introduced in a typical environment. In a TDD WLAN system, a subsequent data frame can be transmitted only if an acknowledgement message, indicating that the other party has received a data frame without an error, is received within tens μs after an AP transmits the data frame. Thus, if an optical signal propagation delay time is too longer because the length of an optical fiber is longer than a predetermined distance, the possibility the acknowledgement message is received within a pre-defined time is high; thereby disabling a normal operation. 
   In the TDD WLAN system, based on the structure of the ROF link apparatus, a serviceable range is limited due to the optical signal propagation delay time and not an optical fiber link propagation loss. Thus, extension of the serviceable range is not sufficiently utilized, which is an advantage of ROF systems. 
   SUMMARY OF THE INVENTION 
   An object of the present invention is to substantially solve at least the above problems and/or disadvantages. Accordingly, an object of the present invention is to provide an ROF link apparatus capable of a stable TDD wireless service by disposing an AP for conversion to an RF signal in a base station to extend a serviceable range of the ROF link apparatus supporting TDD communication. 
   According to one aspect of the present invention, there is provided an ROF link apparatus capable of a stable TDD wireless service for a TDD baseband signal, the ROF link apparatus comprising a central access platform (CAP) for receiving various kinds of data including the TDD baseband signal from upper layers, multiplexing the data, electro-optically converting the multiplexed data, and transmitting the converted data as downstream data through an optical fiber, and opto-electrically converting upstream data received through the optical fiber, demultiplexing the converted upstream data, and transmitting the demultiplexed upstream data to the respective upper layers and a remote access unit (RAU) for receiving the downstream data through the optical fiber, opto-electrically converting the received downstream data to the multiplexed data, demultiplexing the multiplexed data, performing a wireless access process of the demultiplexed data, and transmitting the wireless access processed data to a WLAN service terminal through an antenna. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawing in which: 
       FIG. 1  is a block diagram of a conventional ROF link apparatus for a TDD WLAN service; 
       FIG. 2  is a block diagram of an ROF link apparatus capable of a TDD wireless service according to a first preferred embodiment of the present invention; 
       FIG. 3  is a diagram for explaining a frequency characteristic for multiplexing/demultiplexing in the ROF link apparatus illustrated in  FIG. 2 ; 
       FIG. 4  is a block diagram of an ROF link apparatus capable of a TDD wireless service according to a second preferred embodiment of the present invention; and 
       FIG. 5  is a diagram for explaining a frequency characteristic for multiplexing/demultiplexing in the ROF link apparatus illustrated in  FIG. 4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Preferred embodiments of the present invention will be described herein below with reference to the accompanying drawings. For the purposes of clarity and simplicity, well-known functions or constructions are not described in detail as they would obscure the invention in unnecessary detail. 
     FIG. 2  is a block diagram of an ROF link apparatus capable of a TDD wireless service according to a first preferred embodiment of the present invention. 
   In the illustrated embodiment, one TDD wireless service and two non-TDD RF services are supported. Although different kinds of services can be changed according to a particular situation, the configuration in the illustrated embodiment is related to the TDD wireless service. 
   Referring to  FIG. 2 , the ROF link apparatus includes a central access platform (CAP)  21  and a remote access unit (RAU)  22 . The CAP  21  receives various kinds of data from upper layers, multiplexes the received data, electro-optically converts the multiplexed data, and transmits the converted data to the RAU  22  through an optical fiber. Similarly, the CAP  21  receives upstream data from the RAU  22  through the optical fiber, opto-electrically converts the received upstream data, demultiplexes the converted upstream data, and transmits the demultiplexed upstream data to the respective upper layers. The RAU  22  receives downstream data from the CAP  21  through the optical fiber, opto-electrically converts the received downstream multiplexed data, demultiplexes the multiplexed downstream data, processes the demultiplexed downstream data in a wireless access method, and transmits the downstream data processed in the wireless access method to a WLAN service terminal through an antenna. The RAU  22  further receives upstream data from the WLAN service terminal, processes the received upstream data in the wireless access method, electro-optically converts the upstream data processed in the wireless access method, and transmits the converted upstream data to the CAP  21  through the optical fiber. 
   The current embodiment illustrated in  FIG. 2  shows a downlink system structure in which the CAP  21  multiplexes a TDD baseband signal and two RF signals, electro-optically converts the signals to an optical signal, and transmits the converted optical signal to the RAU  22  including an AP. Although the downlink system structure is described in  FIG. 2 , and as an uplink system structure processes data in a direction opposite that of the downlink system structure, the uplink system structure would be easily understood by those skilled in the art, and thus its description is herein. 
   In more detail, the CAP  21  includes a TDD processing unit  201  for receiving a TDD baseband electrical signal from an upper layer and downstream processing the received TDD baseband electrical signal, first and second RF processing units  202 - 1  and  202 - 2  for receiving RF signals from upper layers and downstream processing the received RF signals, a multiplexer  203  for multiplexing signals output from the TDD processing unit  201  and the first and second RF processing units  202 - 1  and  202 - 2  to a single electrical signal, and an electro-optical converter  204  for converting the electrical signals multiplexed by the multiplexer  203  to an optical signal. 
   The RAU  22  includes an opto-electrical converter  205  for converting the optical signal received through the optical fiber to an electrical signal, a demultiplexer  206  for demultiplexing the electrical signals multiplexed by the multiplexer  203  of the CAP  21 , first and second RF amplifiers  208 - 1  and  208 - 2  for amplifying RF signals among the demultiplexed signals, a TDD AP  207  for processing a TDD baseband electrical signal among the demultiplexed signals to an RF signal, and a coupler  209  for coupling the RF signals output from the TDD AP  207  and the first and second RF amplifiers  208 - 1  and  208 - 2  to a single signal and outputting the single signal through the antenna. 
   In one aspect of the invention for processing a TDD WLAN signal, the TDD baseband electrical signal in the CAP  21  is a 100 Base-TX Ethernet (100 Mb/s) signal. To convert the 100 Base-TX Ethernet signal to an optical signal using the electro-optical converter  204 , a media conversion process from the 100 Base-TX signal to a 100 Base-FX signal is necessary. The media conversion process is performed by the TDD processing unit  201 . 
   Accordingly, the RAU  22  also needs a process for converting the 100 Base-FX signal to opto-electrically converted by the opto-electrical converter  205  passing through the demultiplexer  206  to a 100 Base-TX signal. This process is performed by the TDD AP  207  in the illustrated embodiment of the invention. To do this, a media converter may be added to a front end of the TDD AP  207  to consider the TDD AP  207  as a simple AP. In  FIG. 2 , this media conversion function is performed by the TDD AP  207 , but also may be performed separately (not shown). 
   When a baseband signal is directly modulated to an optical signal, an extinction ratio can be an important element for determining a transmission characteristic. For a 100 Base-TX Ethernet system, it can be considered that a data transmission capability is not degraded with an extinction ratio of 1 to 2 dB. Thus, when a baseband signal and RF signals are multiplexed and simultaneously optical modulated, rigid correlation setting between an optical modulation index (OMI) of an electro-optical converter for the RF signals and an extinction ratio of the baseband signal does not have to be significantly considered. 
     FIG. 3  is a diagram for explaining a frequency characteristic for multiplexing/demultiplexing in the ROF link apparatus illustrated in  FIG. 2 . 
   Referring to  FIG. 3 , according to the frequency characteristic for multiplexing/demultiplexing in the ROF link apparatus illustrated in  FIG. 2 , a baseband signal  31  and RF signals  32  and  33  are multiplexed/demultiplexed based on respective frequencies. 
     FIG. 4  is a block diagram of an ROF link apparatus capable of a TDD wireless service according to a second preferred embodiment of the present invention. 
   Unlike the ROF link apparatus illustrated in  FIG. 2 , the ROF link apparatus illustrated in  FIG. 4  modulates a baseband signal to an intermediate frequency (IF) signal and transmits the IF signal to an RAU  42  in a sub-carrier multiplexing (SCM) method without transmitting the baseband signal to the RAU  42 . Thus, an IF modulator  403  for IF modulating a signal output from a TDD processing unit  401  is further included in a CAP  41 , an IF demodulator  408  for IF demodulating a demultiplexed IF signal is further included in the RAU  42 , and the other operations and configurations are the same as those illustrated in  FIG. 2 . 
   A configuration of the ROF link apparatus illustrated in  FIG. 4  will now be described. The CAP  41  includes the TDD processing unit  401  for receiving a TDD baseband electrical signal from an upper layer and downstream processing the received TDD baseband electrical signal, the IF modulator  403  for modulating a baseband signal output from the TDD processing unit  401  to an IF signal, first and second RF processing units  402 - 1  and  402 - 2  for receiving RF signals from upper layers and downstream processing the received RF signals, a multiplexer  404  for multiplexing signals output from the IF modulator  403  and the first and second RF processing units  402 - 1  and  402 - 2  to a single electrical signal, and an electro-optical converter  405  for converting the multiplexed electrical signal to an optical signal. 
   The RAU  42  includes an opto-electrical converter  406  for converting the optical signal received through an optical fiber to an electrical signal, a demultiplexer  407  for demultiplexing the electrical signals multiplexed by the multiplexer  404  of the CAP  41 , first and second RF amplifiers  410 - 1  and  410 - 2  for amplifying RF signals among the demultiplexed signals, the IF demodulator  408  for demodulating an IF signal among the demultiplexed signals to TDD baseband data, a TDD AP  409  for processing the TDD baseband data received from the IF demodulator  408  to an RF signal, and a coupler  411  for coupling the RF signals output from the TDD AP  409  and the first and second RF amplifiers  410 - 1  and  410 - 2  to a single signal and outputting the single signal through an antenna. 
   For a TDD WLAN signal, the baseband signal in the CAP  41  is a 100 Base-TX Ethernet (100 Mb/s) signal. To convert the 100 Base-TX Ethernet signal to an optical signal using the electro-optical converter  405 , a media conversion process from the 100 Base-TX signal to a 100 Base-FX signal is necessary. The media conversion process is performed by the TDD processing unit  401 . 
   Accordingly, the RAU  42  also needs a process of converting the 100 Base-FX signal opto-electrically converted by the opto-electrical converter  406  and passing through the demultiplexer  407  to the 100 Base-TX signal. This process is performed by the TDD AP  409  in the instant embodiment. To do this, a media converter may be added to a front end of the TDD AP  409  to consider the TDD AP  409  as a simple AP. In  FIG. 4 , this media conversion function is performed by the TDD AP  409 . 
   When a baseband signal is directly modulated to an optical signal, an extinction ratio can be an important element for determining a transmission characteristic. For a 100 Base-TX Ethernet system, it can be considered that a data transmission capability is not degraded with an extinction ratio of 1 to 2 dB. Thus, when a baseband signal and RF signals are multiplexed and simultaneously optical modulated, rigid correlation setting between an optical modulation index (OMI) of an electro-optical converter for the RF signals and an extinction ratio of the baseband signal does not have to be significantly considered. 
     FIG. 5  is a diagram for explaining a frequency characteristic for multiplexing/demultiplexing in the ROF link apparatus illustrated in  FIG. 4 . 
   Referring to  FIG. 5 , according to the frequency characteristic for multiplexing/demultiplexing in the ROF link apparatus illustrated in  FIG. 4 , a baseband signal  51  and RF signals  52  and  53  are multiplexed/demultiplexed based on respective frequencies. 
   As described above, according to the embodiments of the present invention, by disposing an AP in an RAU of an ROF link apparatus capable of a TDD wireless service, a normal TDD wireless service can be provided with a native advantage of the ROF link apparatus, i.e., the extension of a serviceable range. 
   While the invention has been shown and described with reference to a certain preferred embodiment 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.