Abstract:
A passive optical network is disclosed that enables burst mode operation without some of the disadvantages for doing so in the prior art. An embodiment of the present invention comprises a receiver that receives optical signals from transmitters of a plurality of optical network units. For each transmission from an optical network unit, the receiver provides an output signal based on a comparison of the optical signal and a reference voltage that is specific to that optical network unit. A digital-to-analog converter generates the reference voltage in a data rate-independent manner based on information provided to it from the control plane.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to telecommunications in general, and, more particularly, to optical network equipment. 
       BACKGROUND OF THE INVENTION 
       [0002]    Passive optical networks (PONs) that operate in burst mode are attractive for providing fiber-to-the-premises because, among other reasons, they enable significant equipment sharing between multiple users. Total cost of implementing a fiber-to-the-premises network, therefore, can be reduced. 
         [0003]    A burst mode PON typically comprises several Optical Network Units (ONUs) connected to a central office through a shared optical fiber link. These ONUs communicate with the central office using a time-division-multiplexing scheme, wherein the central office allocates a transmission period to one ONU at a time to enable that ONU to transmit upstream data to the central office. In burst mode an ONU transmits its data without waiting for input from another device or waiting for an internal process to terminate before continuing the transfer of data. Since this type of transmission requires the use of transmission bandwidth for only the time while a transmitter is active, the remainder of the transmission bandwidth is available to other transmitters. As a result, a single fiber link can be shared by multiple ONUs. 
         [0004]    As the rate at which data is transferred through the network (i.e., the transmission data rate) increases, burst mode operation becomes increasingly difficult. Specifically, as data rates exceed 10 Gbit per second, however, the number of active components in the electronic circuitry required to support such operation becomes increasingly complex, expensive, and power hungry. 
         [0005]    One area wherein increased circuit complexity has become particularly problematic is the central office receiver that receives the upstream optical signals from the multiple ONUs. The challenges associated with receiving data at high data rates is exacerbated by the fact that the ONUs connected to the receiver are typically at different distances from the central office. Optical signals propagating through an optical fiber from the different ONUs, therefore, are subject to different losses of signal power due to the physical attributes of the fiber, different bends in the fiber, varied loss at fiber connectors within the span, and the like. In addition, the optical power launched by the transmitters of each ONU can vary significantly based on such factors as the age of the transmitter, efficiency of its optical coupling to the fiber, and differences in the electronics that drive the transmitters. As a result, the range of optical power in the optical signals received by the receiver can vary significantly. The receiver, however, must be able to generate an output signal based on all of these optical signals without incurring significant errors. 
       SUMMARY OF THE INVENTION 
       [0006]    The present invention enables burst mode operation of a passive optical network without some of the costs and disadvantages for doing so in the prior art. Embodiments of the present invention are particularly well-suited for use in optical telecommunications and data communications applications. 
         [0007]    Embodiments of the present invention, like the prior art, comprise an optical receiver that is capable of receiving time-division-multiplexed optical signals from a plurality of ONUs. In order to increase the receiver&#39;s immunity to noise and other deleterious effects, the receiver includes a comparator that generates an output signal based on a data stream received from an ONU and a generated reference voltage specific to that ONU. 
         [0008]    In the prior art, the reference voltage for each ONU is generated directly from the incoming data stream. As the data rate of the received data increases, however, the circuitry necessary to generate a reference voltage from the incoming data stream can become quite complex and expensive to implement. In addition, power consumption of such circuitry increases with its complexity. As a result, generation of a reference voltage directly from an incoming data stream becomes increasingly difficult—particularly for data rates that exceed 10 Gigabits per second. 
         [0009]    Unlike the prior art, embodiments of the present invention generate a reference voltage based on a power level set-point for the transmitting ONU. Power level set-points for each ONU are established during a dedicated ONU training period and stored in a database that comprises the look-up table. This database is accessible by the control plane of the passive optical network. A controller, typically located at the central office, reads a value based on the optical power set-point for the ONU and passes a digital bit pattern based on this value to a digital-to-analog converter. Embodiments of the present invention, therefore, operate across both the control layer and the physical layer of the network. The digital-to-analog converter receives the digital bit pattern and generates the reference voltage specific to this ONU. As a result, the generated reference voltage is correlated with the incoming data stream but does not have to be generated directly from it. This obviates the need for complex circuitry to generate the reference voltage. Further, the present invention enables the generation of the reference voltage in a data rate independent manner. 
         [0010]    In some embodiments, the present invention comprises a central office interconnected with a plurality of ONUs, wherein the central office comprises an optical receiver that receives optical signals from each the ONUs. The receiver comprises a reference voltage generator that includes a digital-to-analog converter. When the receiver receives an optical signal from an ONU, a digital bit pattern is provided to the digital-to-analog converter by the control plane of the network. The digital bit pattern is based on a power level set-point specific to that ONU. In some embodiments, the digital bit pattern is provided by a controller located at the central office. In some embodiments, the digital bit pattern is the power level set-point and is stored in a look-up table that is accessed by the controller. 
         [0011]    An embodiment of the present invention comprises: a first transmitter, wherein the first transmitter is characterized by a first power level set-point; a controller comprising a database that includes a first value, wherein the first value is based on the first power level set-point, and wherein the controller provides a first digital bit pattern that is based on the first value; and a receiver comprising an amplifier and a voltage generator, wherein the voltage generator receives the first digital bit pattern from the controller and generates a first reference voltage based on the first digital bit pattern; wherein the amplifier receives a first electrical signal and the first reference voltage, and wherein the first electrical signal is based on a first optical signal provided by the first transmitter, and wherein the amplifier generates a first output signal based on the first electrical signal and the first reference voltage. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  depicts a schematic diagram of a portion of a telecommunications network in accordance with the illustrative embodiment of the present invention. 
           [0013]      FIG. 2  depicts a schematic drawing of receiver  112  in accordance with the illustrative embodiment of the present invention. 
           [0014]      FIG. 3  depicts a method for receiving an optical signal in accordance with the illustrative embodiment of the present invention. 
           [0015]      FIG. 4  depicts a voltage signal based on an optical signal received at a burst-mode receiver in accordance with the illustrative embodiment of the present invention. 
           [0016]      FIG. 5  depicts a method for establishing a value for a power level set-point for an ONU in accordance with the illustrative embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0017]      FIG. 1  depicts a schematic diagram of a portion of a telecommunications network in accordance with the illustrative embodiment of the present invention. Network  100  comprises central office  102  and optical network units  106 - 1  and  106 - 2 . 
         [0018]    Network  100  operates in a burst mode transmission mode, wherein several ONUs share central office equipment through time-division multiplexing. In typical operation, central office  102  schedules a transmission period for an ONU. During this transmission period, the enabled ONU transmits its upstream data as a burst transmission. In some embodiments, all ONUs transmit their respective optical signals at substantially the same wavelength. In some embodiments, ONUs transmit optical signals at different wavelengths and the receiver is wavelength tunable to enable it to receive these multiple wavelengths. 
         [0019]    Central office  102  is a switching station that provides an interface between the long-haul portion of a telecommunications network and local subscribers, such as homes and businesses. A typical central office will serve tens of thousands of local subscribers. Central office  102  comprises transmitter  108 , receiver  112 , controller  114  and database  118 . Central office  102  communicates with each ONU through transmitter  108  and receiver  112 . 
         [0020]    Transmitter  108  is a conventional source for providing optical signal  104 , which comprises time-division multiplexed optical signals  104 - 1  and  104 - 2 . Central office  102  sends downstream data and control information, such as scheduling commands, to ONUs  106  via transmitter  108 . It will be clear to those skilled in the art how to make and use transmitter  108 . 
         [0021]    Receiver  112  is an optical receiver for receiving upstream data and signals from ONUs  106 . Receiver  112  operates in burst-mode, wherein ONUs  106 - 1  and  106 - 2  share the receiver and wherein each ONU transmits for only a transmission period that is scheduled by controller  114 . Receiver  112  is described in more detail below and with respect to  FIG. 2 . 
         [0022]    ONU  106 - 1  is located at a first subscriber location and provides conversion of downstream optical signals  104 - 1  into electrical signals that are usable by premises equipment. In similar fashion, ONU  106 - 2  is located at a second subscriber location and provides conversion of downstream optical signals  104 - 2  into electrical signals that are usable by premises equipment. ONU  106 - 1  and  106 - 2  (referred to collectively as ONUs  106 ) also convert electrical signals generated by their associated premises equipment into upstream optical signals  110 - 1  and  110 - 2 , respectively. In some embodiments, each of ONUs  106  comprises a receiver and transmitter that is dedicated to a single customer. In some embodiments, an ONU may serve more than one customer. 
         [0023]    Typically, each ONU interconnected with central office  102  is a different distance from the central office. This leads to a different amount of total signal attenuation along the length of their respective optical fibers. In addition, the source at each ONU may launch a different amount of optical power into its respective optical fiber, for example, due to the transmitter&#39;s age or quality of fiber coupling. As a result, the optical power level of transmissions received from different ONUs can vary significantly by the time they reach the central office. Receiver  112  must accommodate these different power levels without incurring significant errors. 
         [0024]    Controller  114  is a processing system that, among other functions, monitors network operation, communicates with other central offices, responds to protocol requests, and interfaces with each ONU to which it is connected. As will be discussed in more detail below and with respect to  FIGS. 2 and 3 , controller  114  also accesses database  118  and provides information pertaining to ONU power levels to receiver  112 . 
         [0025]    Database  118  is a database that comprises information about each ONU to which central office  102  is connected. Database  118  includes a look-up table that includes Value 1  and Value 2 , which are values corresponding to power level set-points for ONUs  106 - 1  and  106 - 2 , respectively. 
         [0026]      FIG. 2  depicts a schematic drawing of receiver  112  in accordance with the illustrative embodiment of the present invention. Receiver  112  comprises photodetector  202 , trans-impedance amplifier  206 , differential amplifier  210 , and digital-to-analog converter  214 . Receiver  112  converts optical signals  110 - 1  and  110 - 2  into output voltage signal  218 . 
         [0027]    In order to provide improved noise immunity, receiver  112  includes a comparator that provides an output voltage based on a comparison of electrical signal  208  (which is based on the intensity of one of optical signals  110 ) with a reference voltage (i.e., reference voltage  216 ). As a result, it is important that the reference voltage have a suitable magnitude. 
         [0028]    In the prior art, a reference voltage for a burst-mode receiver is typically generated based on the signal power of the incoming data. As the transmission rate of communications networks has increased, however, the complexity of the circuitry necessary to provide a suitable reference voltage has become a limiting factor. It is an aspect of the present invention that, since controller  114  schedules the transmission periods for each ONU, it is also capable of providing information that enables the generation of a suitable reference voltage for optical signals generated by each ONU. 
         [0029]      FIG. 3  depicts a method for receiving an optical signal in accordance with the illustrative embodiment of the present invention. In order to more clearly describe the present invention, method  300  is described herein with continuing reference to  FIGS. 1 and 2  and reference to  FIG. 4 . 
         [0030]    Method  300  begins with operation  301 , wherein controller  114  reserves transmission period  402  for ONU  106 - 1 . As a consequence of controller  114  scheduling this transmission period, controller  114  is aware of the fact that ONU  106 - 1  is the source optical signal  110 - 1 . As a result, controller  114  is enabled to select information, such as a power level set-point, from database  118  that pertains specifically to ONU  106 - 1 . 
         [0031]    At operation  302 , receiver  112  receives optical signal  110 - 1 . Optical signal  110 - 1  is detected by photodetector  202 , which generates photocurrent  204 . The instantaneous magnitude of photocurrent  204  is based on the instantaneous intensity of optical signal  110 - 1 . Photodetector  202  is a conventional photodetector suitable for detecting light included in optical signal  110 - 1 . It will be clear to one skilled in the art how to make and use photodetector  202 . 
         [0032]    At operation  303 , trans-impedance amplifier  206  converts photocurrent  204  into voltage signal  208 . In some embodiments, additional stages of amplification are included in receiver  112 . It will be clear to one skilled in the art how to make and use trans-impedance amplifier  206 . 
         [0033]      FIG. 4  depicts a voltage signal based on an optical signal received at a burst-mode receiver in accordance with the illustrative embodiment of the present invention. 
         [0034]    Voltage signal  208  is based on optical signal  110 . Optical signal  110  comprises optical signals  110 - 1  and  110 - 2 , which are received in transmission periods  402  and  404 , respectively (as scheduled by controller  114 ). During transmission period  402 , voltage signal  208  has a dynamic range that extends from substantially zero intensity to peak voltage  406 . Similarly, during transmission period  404 , voltage signal  208  has a dynamic range that extends from substantially zero intensity to peak voltage  410 . A desirable reference voltage has a magnitude between 40% and 50% of the peak voltage, and is most preferably 50% of the peak voltage. As a result, the preferable reference voltage is reference voltage value  408  during transmission period  402  and reference voltage value  412  during transmission period  404 . 
         [0035]    At operation  304 , controller  114  accesses database  118  to retrieve Value 1 . Value 1  is a digital bit pattern that corresponds to a power level set-point for ONU  106 - 1 . Value 1  is pre-determined at operation  301 . In some embodiments, Value 1  is another value that is used to generate a digital bit pattern that corresponds to a power level set-point for ONU  106 - 1 . Controller  114  passes Value 1  to receiver  112  as digital signal  212  on signal line  116 - 2 . 
         [0036]    At operation  305 , Value 1  is passed to digital-to-analog converter  214  (hereinafter referred to as DAC  214 ) as digital signal  212 . DAC  214  is a conventional digital-to-analog converter that has suitable output voltage range, resolution, and response time for providing reference voltage  216  at the data rate at which receiver  112  operates. It will be clear to one skilled in the art, after reading this specification, how to specify and use DAC  214 . DAC  214  receives digital signal  212  and converts it into an analog voltage (i.e., reference voltage  216 ) having a magnitude substantially equal to reference voltage value  408 . 
         [0037]    At operation  306 , differential amplifier  210  compares electrical signal  208  and reference voltage  216  and provides output signal  218  on signal line  116 - 1 . Differential amplifier  210  is a conventional differential amplifier suitable for the data rate at which receiver  112  operates. In some embodiments, output signal  218  represents a digital “high” signal when the magnitude of electrical signal  208  is greater than reference voltage value  408  and a digital “low” signal when the magnitude of electrical signal  208  is less than reference voltage value  408 . In some embodiments, output signal  218  represents a digital “low” signal when the magnitude of electrical signal  208  is greater than reference voltage value  408  and a digital “high” signal when the magnitude of electrical signal  208  is less than reference voltage value  408 . 
         [0038]    During transmission period  404 , method  300  is repeated for ONU  106 - 2 . Although in the illustrative embodiment, network  100  comprises only two ONUs, it will be clear to one of ordinary skill in the art, after reading this specification, how to make and use alternative embodiments of the present invention wherein network  100  comprises any number of ONUs. 
         [0039]      FIG. 5  depicts a method for establishing a value for a power level set-point for an ONU in accordance with the illustrative embodiment of the present invention. Method  500  is described herein using an example of establishing a power level set-point for ONU  106 - 1 . It will be clear to one skilled in the art, after reading this specification, how to apply method  500  to establish a value for a power level set-point for any ONU. It should be noted that method  500  needs to be run only once for each ONU, as long as the peak power level of the transmitter of the ONU remains substantially constant. In some instances, however, the peak power level of the transmitter might change due to aging, damage, perturbation of the optical fiber plant, and the like. In such a case, method  500  can be run again to reset the power level set-point as appropriate. 
         [0040]    Method  500  begins with operation  501 , wherein controller  114  assigns a testing timeslot to ONU  106 - 1 . 
         [0041]    At operation  502 , controller  114  transmits control signal  104 - 2  to ONU  106 - 1  via transmitter  108 . Control signal  104 - 2  informs ONU  106 - 1  of a training pattern to be transmitted back to controller  114 . ONU  106 - 1  embeds this training pattern in optical signal  110 - 1 , which is transmitted to controller  114  during the testing timeslot. 
         [0042]    At operation  503 , controller  114  provides a first test digital bit pattern to DAC  214  as digital signal  212  on signal line  116 - 2 . The first test digital bit pattern is converted into a first test reference voltage by DAC  214 . 
         [0043]    At operation  504 , controller  114  tests for receipt of the training pattern on output signal  218 , using the first test reference voltage as the reference voltage. If the training pattern is not successfully received, operations  503  and  504  are repeated with a different test digital bit pattern. 
         [0044]    At operation  505 , the training pattern is successfully detected. 
         [0045]    At operation  506 , the test digital bit pattern used to successfully detect the training pattern is stored as Value 1 . 
         [0046]    It is to be understood that the disclosure teaches just one example of the illustrative embodiment and that many variations of the invention can easily be devised by those skilled in the art after reading this disclosure and that the scope of the present invention is to be determined by the following claims.