Abstract:
A method for enabling AC coupling or DC coupling when receiving burst data signals comprises generating a hold-over pattern, wherein the hold-over pattern is a AC balanced pattern when an AC coupling is required and a low-logic value signal when a DC coupling is required; inputting the generated hold-over pattern to an AC coupling circuit, when no burst data signal is received; inputting only a received burst data signal to the AC coupling circuit, during the reception of such signal; and upon receiving of the entire burst data signal, generating a reset signal causing to input the generated holdover pattern to an AC coupling circuit.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 12/967,461, filed on Dec. 14, 2010, which claims the benefit of U.S. provisional application No. 61/297,058, filed on Jan. 21, 2010. The contents of U.S. provisional application No. 61/297,058 are herein incorporated by reference. 
    
    
     TECHNICAL FIELD 
     The present invention relates generally to receiving high-speed burst data signals in optical networks. 
     BACKGROUND OF THE INVENTION 
     Many communication networks that provide high bit-rate transport over a shared medium are characterized by a non-continuous or burst data transmission. A typical PON includes a plurality of optical network units (ONUs) connected to an optical line terminal (OLT) via a passive optical splitter. Traffic data transmission is performed over two optical wavelengths, one for the downstream direction and another for the upstream direction. Thus, downstream transmission from the OLT is broadcast to all ONUs, where each ONU filters its respective data according to, for example, pre-assigned labels. In the upstream direction, an ONU transmits data to the OLT during different time slots allocated by the OLT. Transmission from an ONU to the OLT is in the form of a burst. 
     An OLT includes an optical transceiver that receives burst data and transmits continuous data. A received burst data signal is preceded by a low logic value (‘0’) signal transmitted on the optical line. This is performed mainly to enable the recovery of the received signals and without losing any data bits, as required, for example, by the Gigabit PON (GPON) communication standard. 
       FIG. 1  shows a schematic diagram of an OLT  100  that includes an optical transceiver  110  and a medium access control (MAC) module  120 . A transmitter (not shown) of the optical transceiver  110  generates and transmits optical signals respective of the input data signals. The optical transceiver  110  also includes a burst mode receiver (not shown) that receives burst signals sent from the ONUs. The optical transceiver  110  generates electric digital signals respective of the received burst data signals. The MAC module  120  processes digital electric signals provided by the optical transceiver  110 . 
     The MAC module  120  is a logic component implemented in en integrated circuit (IC). The MAC module  120  and optical transceiver  110  operate at different direct current (DC) levels. Typically, the DC level of the MAC module  120  is significantly lower than the DC level of the optical transceiver  110 , in particular, when the size of the IC including the MAC module  110  is designed to support advanced semiconductor fabrication techniques. 
     The data burst signals output by the optical transceiver  110  are offset by a certain biased DC level, which is typically the operation voltage of the transceiver. However, as the DC level of the MAC module  120  is lower than that of the transceiver  110 , the burst signal may not be properly received at the MAC module  120 . 
     Therefore, it would be advantageous to provide a solution for interfacing between the MAC module and the optical transceiver in order to properly receive data burst signals. 
     SUMMARY OF THE INVENTION 
     Certain embodiments of the invention include a method for enabling an alternating current (AC) coupling of burst data signals received at an optical line terminal (OLT) operable in a passive optical network (PON). The method comprises generating a hold-over pattern, wherein the hold-over pattern is a AC balanced pattern; inputting the generated hold-over pattern to an AC coupling circuit when no burst data signal is received, wherein the AC coupling circuit interfaces between a medium access control (MAC) module and a burst optical receiver of the OLT; inputting only a received burst data signal to the AC coupling circuit during the reception of the burst signal; and upon receiving of the entire burst data signal generating a reset signal causing to input the generated hold-over pattern to the AC coupling circuit. 
     Certain embodiments of the invention further include an optical line terminal (OLT) operable in a passive optical network (PON) for at least alternating current (AC) coupling input burst data signals. The OLT comprises an optical transmitter including at least an optical receiver for receiving input burst signals from a plurality of optical network units (ONUs) of the PON; a medium access control (MAC) module for generating at least a hold-over pattern and a reset signal; an AC coupling circuit connected between the optical transmitter and the MAC module for unbiasing direct current (DC) levels of the received input burst data signals; and an AC coupling control circuit for feeding either the hold-over pattern or a received burst signal to the AC coupling circuit based, in part, on the an optical signal-detect signal and reset signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The subject matter that is regarded as the invention is particularly pointed out and distinctly claimed in the claims at the conclusion of the specification. The foregoing and other objects, features, and advantages of the invention will be apparent from the following detailed description taken in conjunction with the accompanying drawings. 
         FIG. 1  is a schematic diagram of an OLT. 
         FIG. 2  is a schematic diagram of an OLT including an AC coupling circuit. 
         FIG. 3  is an AC coupling control circuit constructed in accordance with an embodiment of the invention. 
         FIG. 4  illustrates the operation of the AC coupling control circuit. 
         FIG. 5  is a flowchart describing a method for enabling AC coupling or DC coupling when receiving burst data signals. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The embodiments disclosed by the invention are only examples of the many possible advantageous uses and implementations of the innovative teachings presented herein. In general, statements made in the specification of the present application do not necessarily limit any of the various claimed inventions. Moreover, some statements may apply to some inventive features but not to others. In general, unless otherwise indicated, singular elements may be in plural and vice versa with no loss of generality. In the drawings, like numerals refer to like parts through several views. 
       FIG. 2  shows a schematic diagram of an OLT  200  implemented in accordance with an embodiment of the invention. The OLT  200  includes an optical transmitter (TRX)  210 , a MAC module  220  and an alternating current (AC) coupling circuit  230  coupled between them. That is, the AC coupling circuit  230  is utilized to interface between the MAC module  220  and the optical transceiver  210  in order to remove biased DC levels of received burst data signals. The optical transceiver  210  includes a burst optical receiver and a continuous optical transmitter (both are not shown). 
     The burst signals are AC balanced within the burst and between bursts, the signal is typically not AC balanced. For example, signals between bursts may include a long string of identical bits. The AC coupling circuit  230  allows for unbiasing the DC level of received data signals, while maintaining the AC swing characteristics of the signals. However, utilizing AC coupling may result in losing data bits, in particular, when a long string of information is repeatedly included in a sequence of identical bits or when an input data signal has not been received for a relatively long period of time (between bursts). This is due to the time required to charge and discharge the capacitors of the AC coupling circuit  230 . This problem may be critical in high-speed communication standards, such as, but not limited to, Gigabit PON (GPON) and XGPON (or 10XPON), where the number of allowed identical bits within the received data is bounded. However, when data is transmitted a long string of identical consecutive bits may be generated and received at the burst receiver  210 . 
     In order to eliminate the problems associated with the AC coupling and to ensure that the entire burst signal will be correctly received at the MAC module  220 , a data pattern (hereinafter the “hold-over pattern”) is generated by the MAC module  220  and input to the AC coupling control circuit  240  when no burst signal is received. The hold-over pattern may have two different forms, which are determined according to the desired coupling. Specifically, to achieve an efficient AC coupling the hold-over pattern is an AC balanced pattern, for example, a sequence of alternating ‘0’ bit and ‘1’ bit or a 50% duty-cycle clock signal. In this configuration, the hold-over pattern is input to the circuit  230  only when no burst data signal is received. This is accomplished using an AC coupling control circuit  240  which is further depicted in  FIG. 3 . To configure the AC coupling circuit  230  to function as a DC coupling interface, the hold-over pattern is set to a low-logic value (‘0’) signal. 
       FIG. 3  shows a schematic diagram of the AC coupling control circuit  240  constructed in accordance with an embodiment of the invention. The control circuit  240  includes a flip-flop  310 , a multiplexer (MUX)  320 , and a differential amplifier  330  that outputs a differential signal based on an input received from the MUX  320 . The inputs of the MUX  320  are the received burst data signal (at input Q 0 ) and the hold-over pattern (at input Q 1 ), the selection of the input is based on the signal at the select input (S). Specifically, when the output (d) of the flip-flop  310  is set (i.e., ‘1’), the input Q 0  of the MUX is selected and when the output (d) is clear (i.e., ‘0’), the input Q 1  is selected. 
     The flip-flop  310  is set when a Signal-Detect signal is asserted by the optical transceiver  210 , usually when energy of an incoming signal is detected by the receiver of the transceiver  210 . That is, as long as the Signal-Detect signal is active, the received burst signal is sent to the AC coupling circuit  230 . The flip-flop  310  is cleared when an RX_RESET signal is asserted by the MAC module  220 . This signal is output when the complete burst signal has been received. In another embodiment, the MAC module  220  outputs the RX_RESET signal during a ranging process of the PON or when a received signal is determined to be faulty. Once the RX_RESET is asserted, the hold-over pattern is input to the AC coupling circuit  230 . The hold-over pattern is generated by the MAC module  220 . 
     The operation of the AC coupling control circuit  240  is further illustrated in  FIG. 4 . In the AC coupling mode of operation, the hold-over pattern is an AC balanced pattern that is continuously generated by the MAC module  220 . The optical incoming signal is the signal as received from an ONU at the input of the optical transceiver  210 . The burst signal is an output signal of a burst mode receiver in the transceiver  210 . The optical Signal-Detect is also an output of the receiver and it is typically active (i.e., at a ‘1’ value) for the duration of the presence of a training sequence (preamble) or a similar pattern in the receiver, or the presence of incoming optical energy above a predefined level. In accordance with an embodiment of the invention, the Signal-Detect can be held at its active value by the flip-flop  310  until released by the RX_RESET signal. 
     In the exemplary diagrams shown in  FIG. 4 , there is no alignment between the incoming optical signal and the optical Signal-Detect signal. The RX_RESET is as generated by the MAC module  220  and is essential for the realization of a proper AC coupling mechanism. The differential signal is the output of the differential amplifier  330 , and it is being input to the AC coupling circuit  230 . 
     During the time interval of T 0  to T 1 , the differential signal consists of the hold-over pattern as no incoming signal is received. At T 1 , the optical Signal-Detect is asserted. The optical Signal-Detect is being held at a high-logic value by the flip-flop  310  until T 3 . As a result, an output signal of the burst mode receiver is input to the differential amplifier  330  and the differential signal consists of the received burst signal for the duration between T 1  and T 2 . At T 3  the RX_RESET signal is asserted causing the MUX  320  to output the hold-over pattern to the differential amplifier  330 . Thus, from T 3  until a new burst signal is received the differential signal consists of the hold-over pattern. 
     It should be noted that the burst signals transmitted by ONUs are scrambled signals using a polynomial method, thus having AC balanced properties. The hold-over pattern is also AC balanced, thus the differential signal, consisting of the hold over and the Burst data, have a good AC balance properties. Therefore, it is possible to pass the differential signal through the AC coupling circuit  230  without altering received burst signals or losing information contained in the received burst signals. In addition, capacitors of the AC coupling circuit  230  are chosen to have capacity values that will ensure proper signal integrity regardless of the physical characteristics of the PON (e.g., max CID, control delays, etc.). 
     In accordance with an embodiment of the invention, the OLT  200  can be configured to allow DC coupling without redesigning the OLT  200 . This is performed by setting the MAC module  220  to generate a hold-over pattern that consists of only a low-logic value (‘0) signal. In this embodiment, the operation of the AC coupling control circuit  240  is as described above. 
       FIG. 5  shows an exemplary and non-limiting flowchart  500  describing the method for enabling AC coupling or DC coupling when receiving burst data signals. At S 510 , a hold-over pattern is generated. The method can be utilized to allow an OLT-transceiver that includes an AC coupling circuit to perform as either an AC coupling or DC coupling interface. When an AC coupling is utilized, the hold-over pattern is an AC balanced pattern. In a DC interface configuration, the hold-over pattern is a low-logic value signal or can simply be tied to GND and VCC of the MAC module. It should be noted that the decision whether to operate in an AC or DC coupling mode depends on the DC level of the MAC module with respect to the transceiver. For example, when the DC level of the MAC module is lower than that of the transceiver, then AC coupling mode is applied; otherwise, DC coupling may be utilized. 
     At S 620  it is checked if the optical Signal-Detect signal is active, and if so at S 530  a received burst signal is output, otherwise, at S 540  the hold-over pattern is output, and thereafter execution returns to S 520 . Both the received burst signal and the pattern are fed into an AC coupling circuit, depending on the state of the Signal-Detect. At S 550 , a check is made to determine if the RX_RESET signal is asserted, and if so execution continues with S 540 ; otherwise, at S 560  it is checked if the execution should be terminated, for example, when shutting down the OLT. If S 560  results with a negative answer execution returns to S 550 . 
     The principles of the invention described herein are particularly useful in OLTs operable in PON communication standard, including but not limited to, GPON and XGPON (or 10XPON) where data bits of received signals cannot be lost or modified. It should be appreciated by one of ordinary skill in the art that the principles of the invention can be utilized to design OLT with either an AC or DC coupling interface. 
     The foregoing detailed description has set forth a few of the many for that the invention can take. It is intended that the foregoing detailed description be understood as an illustration of selected forms that the invention can take and not as a limitation to the definition of the invention. It is only the claims, including all equivalents that are intended to define the scope of this invention. 
     Most preferably, the principles of the invention are implemented as any combination of hardware, firmware, and software. Moreover, the software is preferably implemented as an application program tangibly embodied on a program storage unit or computer readable medium. The application program may be uploaded to, and executed by, a machine comprising any suitable architecture. Preferably, the machine is implemented on a computer platform having hardware such as one or more central processing units (“CPUs”), a memory, and input/output interfaces. The computer platform may also include an operating system and microinstruction code. The various processes and functions described herein may be either part of the microinstruction code or part of the application program, or any combination thereof, which may be executed by a CPU, whether or not such computer or processor is explicitly shown. In addition, various other peripheral units may be connected to the computer platform such as an additional data storage unit and a printing unit. Furthermore, a non-transitory computer readable medium is any computer readable medium except for a transitory propagating signal.