Abstract:
The present invention discloses a system for full-duplex data transmission using polarization multiplexing comprises a central station having a first means for processing downlink signals and a second means for processing uplink signals, and a remote antenna unit connected to the central station via a transmission medium, having a third means for processing downlink signals and a fourth means for processing uplink signals, characterised in that the remote antenna unit is configured to receive downlink signals from the central station, and then to split a portion of the downlink signals to be used as uplink data transmission simultaneously with transmission of the downlink.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    The present application claims the priority benefit of Malaysian Patent Application No. PI 2016000131, filed Jan. 22, 2016, which is incorporated by reference in its entirety. 
       FIELD OF INVENTION 
       [0002]    The invention relates to a reusing optical carrier in uplink transmission for multiple services of radio-over-fiber communication using polarization multiplexing technique. 
       BACKGROUND OF THE INVENTION 
       [0003]    Radio-over-fiber (RoF) refers to a technology whereby light is modulated by a radio signal and transmitted over an optical fiber link to a remote antenna unit for facilitating wireless access. In a conventional ROF network, an optical transmitter converts radio signals into optical signals and transmits the optical signals through the optical fibers. At the other end of the optical fibers, an optical receiver is provided to convert the optical signals into radio signals. The drawbacks of the conventional remote antenna unit are as follows.
       1. Complexity of remote antenna unit (RAU): Complexity of RAU leads to increase in cost and power usage.   2. Capacity of optical fiber is not fully utilised: The conventional system does not utilise all bandwidth of the optical fiber as it uses separate fiber or different wavelength in uplink and downlink data transmission.   3. Different fiber cables for downlink and uplink transmission: in practise, separate fibers or different wavelength are used for downlink and uplink to avoid interference. However, this also leads to increase in cost and power usage.   4. Signal collision: When multiple services are transmitting in a single system, there are possibilities that the signals will collide or interfere with each other.       
 
         [0008]    Therefore, a need exists for the RAU to be designed with reduced components and less complexity due to multiple numbers of to be deployed at the residential or commercial area. The invention provides an economical and effective RAU system that implements a reuse carrier technique for full-duplex data transmission. 
       PRIOR ART 
       [0009]    EP 2485418 A1 discloses a polarization multiplexing system specifically on sending and receiving method of modulation and demodulation of polarized multiplexing signal. It claims particularly on the receiving apparatus for optical polarization division multiplexing so as to reduce cross talk in optical signals at a receiving, between x-polarization and y-polarization. This patent focuses on the mechanism to precisely de-multiplex the received polarized multiplexing signal. However, it is not related to full-duplex multiplexing system using a reused local multipoint distribution system (LMDS) carrier. 
         [0010]    U.S. Pat. No. 6,580,535 B1 discloses using two optical signals with same wavelength are polarization filtered at a sending end and multiplexed into orthogonal polarization orientations of a fiber. This polarization multiplexed signal is transmitted via optical data link (fiber) and to a receiving end having a splitter. However, this is only a half-duplex polarization multiplexing system and uses controller to control the polarization of the signals. It is not related to full-duplex multiplexing system using a reused local multipoint distribution system (LMDS) carrier. 
         [0011]    U.S. Pat. No. 8,032,025 B2 discloses a system for monitoring polarization detection unit that receives the multiplexed optical signal and measures a polarization state of light received at the optical splitter and a power level of light associated with the RF tone signal. The system further contains a feedback control unit in communication with the optical polarization of the light at the optical splitter to optimize a separation of the first and second data channels for optimal detection. However, this is only a half-duplex polarization multiplexing system. It is related to lull duplex multiplexing system using a reused local multipoint distribution system (LMDS) carrier. 
       SUMMARY OF INVENTION 
       [0012]    The invention provides a system for full-duplex data transmission using polarization multiplexing comprises a central station having a first means for processing downlink signals and a second means for processing uplink signals, and a remote antenna unit connected to the central station via a transmission medium, having a third means for processing downlink signals and a fourth means for processing uplink signals, characterised in that the remote antenna unit is configured to receive downlink signals from the central station, and then to split a portion of the downlink signals to be used as uplink data transmission simultaneously with transmission of the downlink. 
         [0013]    Preferably, the first means of the central station includes means for generating at least one wave carrier, means for splitting the generated wave carrier into at least two different polarization states, means for modulating at least one radio frequency signal into at least one of the polarization states, and means for multiplexing the at least two polarization states into the transmission medium. 
         [0014]    Preferably, the second means of the central station includes means for splitting multiplexed signals into at least two different polarization states, and means for converting the at least one of the signals into electrical signal. 
         [0015]    Preferably, the third means of the remote antenna unit includes means for splitting multiplexed signals into a first and second signals, the first signal is used for downlink transmission and the second signal is re-used for uplink transmission, means for splitting the first and second signals into two polarization states, and means for converting the first signals into electrical signals. 
         [0016]    Preferably, the fourth means of the remote antenna unit includes means for modulating at least one radio frequency signal into at least one of the polarization states of the second signal of the third means, and means for multiplexing the at least two polarization states of the second signal into the transmission medium. 
         [0017]    Preferably, the transmission medium is a fiber optic cable. 
         [0018]    Preferably, the means for generating at least one wave carrier is a Local Multipoint Distribution System Carrier Generator Module. 
         [0019]    Preferably, the means for splitting the generated wave carrier into at least two different polarization states is an optical polarization beam splitter. 
         [0020]    Preferably, the means for modulating at least one radio frequency signal into at least one of the polarization states is an optical modulator. 
         [0021]    Preferably, the means for multiplexing the at least two polarization states into the transmission medium is an optical polarization beam combiner. 
         [0022]    Preferably, the means for converting the optical wave signals into electrical signal is a photodetector. 
         [0023]    Preferably, the wavelength of the uplink and downlink are the same. 
         [0024]    Preferably, each signal is split into carrier with x-polarization and carrier with y-polarization. 
         [0025]    Preferably, the third means and the fourth means of the remote antenna unit are coupled with at least one antenna for wireless data transmission. 
         [0026]    In one embodiment of the invention, the system may further comprise means at both ends of the transmission medium for guiding the multiplexed signals to their designated path ways. The means for guiding the multiplexed signals to their designated path ways can be an optical circulator. 
         [0027]    In another embodiment of the invention, the system may further comprise means for amplifying the signals. The means for amplifying the signals can be optic amplifying module. 
         [0028]    One skilled in the art will readily appreciate that the invention is well adapted to carry out the objects and obtain the ends and advantages mentioned, as well as those inherent therein. The embodiments described herein are not intended as limitations on the scope of the invention. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0029]    For the purpose of facilitating an understanding of the invention, there is illustrated in the accompanying drawing the preferred embodiments from an inspection of which when considered in connection with the following description, the invention, its construction and operation and many of its advantages would be readily understood and appreciated. 
           [0030]      FIG. 1  illustrates a general full-duplex polarization multiplexing (Pol-Mux) system covering central station (CS) and remote antenna unit (RAU). 
           [0031]      FIG. 2  illustrates the detail of the full-duplex Pol-Mux system. 
           [0032]      FIG. 3  illustrates the optical domain and electrical domain representation of the x-SOP and y-SOP configuration for the downlink and uplink transmission. 
           [0033]      FIG. 4  illustrates the process for transmitting and receiving radio frequency (RF) signal in RF domain depending on the antenna arrangement. 
           [0034]      FIG. 5  is a flow chart diagram illustrating the downlink signal processing process of the CS. 
           [0035]      FIG. 6  is a flowchart diagram illustrating the downlink signal processing process of the RAU. 
           [0036]      FIG. 7  is a flow chart diagram illustrating the uplink signal processing process of the RAU. 
           [0037]      FIG. 8  is a flow chart diagram illustrating the uplink signal processing process of the CS. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    The invention will now be described in greater detail, by way of example, with reference to the drawings. 
         [0039]      FIG. 1  and  FIG. 2  show an exemplary embodiment of a full-duplex Pol-Mux system. The system comprises a CS  101 , a RAU  103  linked to the CS  101  via a transmission medium  102 , and a circulator  222 ,  223  at both end of the cable to guide the optical signal for downlink and uplink at their respective path. The transmission medium  102  can be fiber optic cable, glass conduit, waveguide, light frequency transmission line or even natural interface such as vacuum, liquid, or gas. Preferably, the transmission medium  102  is a single mode fiber. 
         [0040]    The CS  101  includes a downlink optical microwave signal processing unit  104  and an uplink optical microwave signal processing unit  105 . 
         [0041]    Preferably, the downlink optical microwave signal process unit  104  of CS  101  comprises:
       i. a local multipoint distribution system (LAMS) carrier generator module  201  for generating high frequency millimetre wave carrier;   ii. an optical polarization beam splitter (PBS)  202  for splitting the LMDS optical carrier into two different states of polarization (SOP), which is x-SOP and y-SOP, the x-SOP is LMDS optical carrier with x-polarization  003 , while y-SOP is LMDS optical carrier with y-polarization  002 ;   iii. two optical modulators  203 ,  204  for modulating RF signals from different sources with x-SOP and y-SOP optical carrier; the modulated optical signals A, f A  is RF signal A, f A    205  mixed with the y-SOP LMDS optical carrier; while the modulated optical signals B, f B  is RF signal B, f B    206  mixed with the x-SOP LMDS optical carrier; and   iv. an optical polarization beam combiner (PBC) for multiplexing the modulated optical signal A, f A  and the modulated optical signal B, f B  into a single fiber medium of the fiber optic cable  102 .       
 
         [0046]    Preferably, the uplink optical microwave signal processing unit  105  comprises:
       i. a PBS  224  for splitting the uplink signal from the RAU  103  into the two different states of polarization (SOP);   ii. two optic amplifying modules  225 ,  226  for amplifying the polarised uplink signals; and   iii. two photodetectors  227 ,  228  for converting the amplified and polarised uplink signals into electrical signals.       
 
         [0050]    The RAU  103  includes a downlink optical microwave signal processing unit  106  and an uplink optical microwave signal processing unit  107 . 
         [0051]    Preferably, the optical microwave signal processing unit  106  of RAU  103  includes an optical splitter  208  for splitting the multiplexed optical signal from the CS  101  into two-routes; the first route is for downlink and the second route is for uplink. The first route comprises:
       i. a PBS  209  for splitting the received signal into x-SOP signal and y-SOP signal;   ii. two photodetectors  211 ,  212  for converting the two signals into electrical signals;   iii. two RF amplifying modules  213 ,  214  for amplifying the respective signal in electrical domain; and   iv. two RF antennas  215 ,  216  used to convert the RF signals before transmitting wireless into the air.       
 
         [0056]    The second route comprises:
       i. a PBS  210  for splitting the received signal into x-SOP signal and y-SOP signal where y-SOP is up-converted of RP signal A, f A  at LMDS carrier and x-SOP is up-converted of RF signal B, f B  at LMDS carrier, the two signals is directed to the uplink optical microwave signal processing unit  107 .       
 
         [0058]    The uplink optical microwave signal processing unit comprises:
       ii. two antennas  229 ,  230 , for receiving the up-converted RF signal A, f A  and up-converted RF signal B, f B  from the air;   iii. two RF amplifying module  219 ,  220  for amplifying the up-converted RF signal A, F A  and up-converted RF signal B, f B ; a band pass filter at the desired band can be inserted between the antennas  229 ,  230 , and the amplifier  219 ,  220 , to filter out unwanted signal;   iv. two optical modulators  217 ,  218  to modulate the up-converted RF signals, f A  with up-converted of RF signal A, f A , and up-converted of RF signal B, f B  at LMDS carrier;   v. a PBC  221  to multiplex the modulated uplink at x-SOP, and modulate uplink at y-SOP into the fiber optic cable.       
 
         [0063]    The output of signal processed by each modules of the system is illustrated in  FIG. 3  as follows:
         001  is the LMDS 25 GHz carrier generated from the OCS;     002  is the LMDS optical carrier with y-polarization (y-SOP);     003  is the LMDS optical carrier with x-polarization (x-SOP);     004  is RF signal A, f A  mixed with y-SOP LMDS optical carrier;     005  is RF signal B, f B  mixed with x-SOP LMDS optical carrier;     006  is multiplexed optical signal, consist of RF signal A, f A  mixed with y-SOP LMDS optical carrier,  004  and RF signal B, f B  mixed with x-SOP LMDS optical carrier,  005 ;     007  is the downlink multiplexed optical signal  006  with reduced power splitted by optical splitter;     008  is the downlink multiplexed optical signal  006  with reduced power splitted by optical splitter  208  to be used as uplink reuse LMDS carrier;     009  is the up-converted of RF signal A, f A  at LMDS carrier at y-SOP;     010  is the up-converted of RF signal B, f B  at LMDS carrier at x-SOP;     011  is similar to  009  and to be used as uplink reuse LMDS carrier;     012  is similar to  010  and to be used as uplink reuse LMDS carrier;     013  is  009  after converted into electrical signal via the photodetector  211 ;     014  is  010  after converted into electrical signal via the photodetector  212 ;     015  is the received signal A, f A  with LMDS electrical carrier from antenna  229 ;     016  is the received signal B, f B  with LMDS electrical carrier from antenna  230 ;     017  is the  011  from the reuse downlink signal with RF signals A, f A  combined with received signal B, f B  with LMDS electrical carrier  016 , the black lines represent the reuse downlink signal with RF signals A, f A ,  011 ;     018  is the  012  from the reuse downlink signal and RF signals B, f B  combine with received signal A, f A  with LMDS electrical carrier  015  from the receiving antenna  230 ; grey lines represent received signal A, f A  with LMDS electrical carrier  015 , while black lines represent the reuse downlink signal and RF signals B, f B    012 ;     019  is the multiplexed optical signal for uplink consists of  017  and  018  by the PBC  221  into the fiber optic  102 ; grey lines represent the received signals from antenna while back lines represent the reuse downlink signals;     020  is the RF signal A, f A  and RF signal B, f B  mixed with LMDS electrical carrier in the air;       
 
         [0084]      FIG. 4  represents the way to transmit and to receive the RF signal in RF domain depending on the antenna arrangement; where; 
         [0085]      401  is an up-converted RF signal A, fA, at LMDS frequency band, before transmitting to the air by transmitter antenna  215 ;  401  can be represented by  013  of  FIG. 3 ; 
         [0086]      402  is an up-converted RF signal B, fB at LMDS frequency band, before transmitting to the air by transmitter antenna  216 ;  402  can be represented by  014  of  FIG. 3 ; 
         [0087]      403  is an up-converted RF signal A, fA and RF signal B, fB in the air transmission medium;  403  can be represented by  020  of  FIG. 3 ; 
         [0088]      404  is an up-converted RF signal A, fA, at LMDS frequency band at receiving antenna  229 ;  404  can be represented by  015  of  FIG. 3 ; 
         [0089]      405  is an up-converted RF signal B, fB at LMDS frequency band at receiving antenna  230 ;  405  can be represented by  016  of  FIG. 3 ; 
         [0090]      406  is an up-converted RF signal A, fA and RF signal B, fB at LMDS frequency band in the fiber upon uplink transmission as multiplexed optical signal for uplink;  406  can be represented by  019  of  FIG. 3 ; 
         [0091]      FIG. 5  illustrates the signal transmission in the downlink optical microwave signal processing unit  104  of CS  101 . In the first step, the LMDS optical carrier  001  is generated by using a technique called optical carrier suppression (OCS). In the second step, the LMDS optical carrier  001  from the OCS is split into two signals with different state of polarization, y-SOP  002  and x-SOP  003  by using the PBS  202 . In the third step, the RF signal A, f A    205  is mixed with y-SOP by optical modulator  203 , and the RF signal B, f B    206  is mixed with x-SOP by optical modulator  204 . In the fourth step, the RF signal A, f A  mixed with y-SOP LMDS optical carrier  004  and RF signal B, f B  mixed with x-SOP LMDS optical carrier  005  are combined or multiplexed into the single fiber optic cable medium but at different polarization axis by using PBC  207 , the multiplexed signals are transmitted to the RAU. 
         [0092]      FIG. 6  illustrates the signal transmission in the downlink optical microwave signal processing unit  106  of RAU  103 . Upon the multiplexed signal  006  reaches the RAU  103 , the optical splitter  208  split the optical signal into two different routes at any splitting ratio depending on the signal link budget. At the first route, the multiplexed optical signal is split back into their polarized signals x-SOP  010  and y-SOP  009  using PBS  209 . In the third step, these polarized signals are converted by their respective photodetectors  211 ,  212  into RF signal A, f A    013  and RF signal B, f B    014  which were carried by LADS carrier. In the fourth step, the RF signals  013 ,  014  are amplified respectively by the RF amplifying modules  213 ,  214 . In the final step, the amplified RF signals  013 ,  014  are transmitted through air (wirelessly) represented by  020  at respective frequency with no overlapping of spectrum via antennas  215 ,  216 . 
         [0093]      FIG. 7  illustrates the signal transmission in the uplink optical microwave signal processing unit  107  of RAU  103 . After the optical splitter  208  of the downlink processing unit  106  splits the received signal in two portions, one portion will be reused for the uplink carrier signal transmission. Preferably, the signal is split into two equal power signals  007 ,  008  by the optical splitter  208 . It should be noted that the signal can be divided into different ratio depending on the applications or the business packages provided by the service provider. The Signal  008  is split again by PBS  210 , into y-SOP and x-SOP signal where x-SOP is up-converted of RF signal B, f B  at LMDS carrier  012 , and y-SOP is up-converted of RF signal A, f A  at LMDS carrier  011  respectively. The RF signal A, f A    015  and the RF signal B, f B    016  are signals which received from the antenna  229 ,  230  and being amplified by RF amplifying modules  219 ,  220 . In one embodiment, the band pass filter at the desired band is inserted between antennas  229 ,  230  and amplifiers  219 ,  220  to filter out unwanted signal. The received RF signals A, f A    015 , is then modulated with the LMDS carrier at x-SOP containing RF signals B, f B    012 , and the received RF signal B, f B    016  is modulated with LMDS carrier at y-SOP  012  containing RF signals A, f A    011  by using optical modulators  217 ,  218 . Thus, for the uplink transmission, the reuse downlink signal RF signal B, f B  at LMDS carrier at x-SOP  012  with the received signal A, f A    015  with LMDS electrical carrier from antenna  229  is hereinafter referred to as modulated uplink at x-SOP  018 ; the reuse downlink signal RF signal A, f A  at LMDS carrier at y-SOP  011  with the received signal B, f B    016  with LMDS electrical carrier from antenna  230  is hereinafter referred to as modulated uplink at y-SOP  017 . The modulated uplink at y-SOP and modulated uplink at x-SOP  018  are then combined at the PBS  221  and multiplexed back into the fiber optic cable  102  via a circulator  222  as uplink transmission. The received signals from the antennas  229 ,  230  will not interfere with signals in the LMDS carrier due to different polarization and frequency allocation. 
         [0094]      FIG. 8  illustrates the signal transmission in the uplink optical microwave signal processing unit  105  of CS  101 . The multiplexed optical signal from the RAU  103  is split back into their polarised signals x-SOP and y-SOP using PBS  224 . These polarised signals are then amplified using optic amplifying modules  225 ,  226 . Finally, the amplified and polarised signals are converted to electrical signals by their respective photodetectors  227 ,  228 . 
         [0095]    The present disclosure includes as contained in the appended claims, as well as that of the foregoing description. Although this invention has been described in its preferred form with a degree of particularity, it is understood that the present disclosure of the preferred form has been made only by way of example and that numerous changes in the details of construction and the combination and arrangements of parts may be resorted to without departing from the scope of the invention.