Patent Application
20140010555
The present invention relates generally to the field of communication networks, and, more particularly, to a method and apparatus for processing a received optical signal in a node of a PON or similar access network.
The following abbreviations are herewith defined, at least some of which are referred to within the following description of the state-of-the-art and the present invention.
CNR Carrier-to-Noise ratio
CSO Composite second order
SNR Signal-to-Noise ratio
A PON (passive optical network) can be used as an access network for providing various communication services to subscribers. These services may include, for example, Internet access, email, voice telephone, streaming audio and video, and television programming. Television programming may include broadcast and “cable” stations as well as access to movies for free viewing or purchase. Because a single cable—in the case of a PON, a fiber optic cable—must carry information representing all of the things to and from the subscriber, techniques have been developed for allowing all of this information traffic to traverse the fiber at the same time without interference.
The technique of video overlay involves sending video signals downstream through a PON using a different wavelength of light than is used for other information. The two (or more) wavelengths of light are then multiplexed for simultaneous transmission as needed to provide all of the information to the subscriber without undue delay. When the light carrying these signals reaches the subscriber the video overlay wavelength is separated again for separate processing. The device where this occurs is often called an ONU (optical network unit) and is located at the residence or business premises of the subscriber.
When the video overlay signal arrives at the ONU (or similar device) it is amplified to make it more useful to the subscriber equipment that will use or distribute it to other subscriber devices. As might be expected, this amplification is performed in a circuit that includes a number of electrical components after the optical video overlay signal is converted into an electrical signal. One or more of these components is in fact an amplifier. As the signal is being amplified, the output if the amplifier or amplifiers is influenced by the application of a bias voltage. If this bias voltage is static, that is constant and unchanging, it is set in advance.
The problem with setting a static bias voltage in advance is that the ONUs are often manufactured in large quantities before it is know who they will be sold to and where they will be installed. Different carriers have different requirements for acceptable input and output characteristics of the ONU. The use of a pre-set static bias voltage in the video overlay amplification circuit can mean in some cases that these requirements are not met, and additional work is needed to re-set the voltage. Of course, making a variety of ONUs is possible, but increases costs for the manufacturer.
In addition, it is difficult to in advance to know the environment the ONU will be used in. Some, for example, will be installed relatively-close to the CO (central office) where the optical signal originates while others are much further away. This can result in some ONUs receiving the optical signal at a much higher power level than others. This may be a problem, for example, if the static bias voltage is set to optimize amplifier distortion performance at relatively high optical input power to avoid common distortions such as CSO or CTB. If the ONU happens to be installed far from the CO, however, this setting may be accompanied by high thermal noise at the relatively-low optical input power and would impact receiver sensitivity performance.
Note that the techniques or schemes described herein as existing or possible are presented as background for the present invention, but no admission is made thereby that these techniques and schemes were heretofore commercialized or known to others besides the inventors.
Accordingly, there has been and still is a need to address the aforementioned shortcomings and other shortcomings associated with amplifying video overlay signals. These needs and other needs are addressed by the present invention.
The present invention is directed to a method and amplifier circuit for use in, for example, receiving a video overlay signal in a PON (passive optical network). In one embodiment, the present invention is amplifier circuit for receiving a video overlay signal and producing an amplified output for, for example, providing to a subscriber device. According to this aspect, the amplifier circuit includes a primary amplifier chain having at least one amplifier for amplifying the received signal and a bias voltage generator for generating a bias voltage for biasing the at least one amplifier in the primary amplifier chain based on at least one characteristic of the received signal. In this embodiment, the present invention may also include an ONU (for example, an ONT) in which the amplifier circuit is resident.
In most embodiments of this aspect of the present invention, the amplifier circuit also includes a light sensing device such as a photodiode for converting a received optical overlay signal and converting it into an electrical signal. In some embodiments, the primary amplifier chain includes one or more of a TIA (trans-impedance amplifier), an attenuator or attenuator circuit, and a post amplifier. In that case, the at least one amplifier may be the TIA or the post amplifier or both.
In this aspect the bias voltage generator may include a comparator for comparing a voltage signal representative of the overlay signal to a reference voltage. In some implementations the reference voltage is manually or automatically adjustable. A sensing circuit in communication with the light sensing device may be present for creating the voltage signal representative of the overlay signal. The bias voltage generator may further include a gain and level amplifier for amplifying the output of the comparator and a limiter circuit for limiting the output of the gain and level amplifier prior to applying the output as the bias voltage for the at least one amplifier of the primary amplifier chain.
In other embodiments the bias voltage generator comprises a microcontroller, the microcontroller for generating at least one bias voltage for biasing the at least one amplifier of the primary amplifier chain based on stored program instructions and the at least one characteristic of the received signal. In this embodiment, the bias voltage generator may also include a coupler for tapping the output of the primary amplifier circuit and providing a representation of the output to a measurement DSP, the measurement DSP for analyzing the output of the primary amplifier chain and providing the analysis as an input to the microcontroller. Signal characteristics to be analyzed by the measurement DSP may include one or more of MER, CNR, CTB, and CSO. In this case the bias voltage may also be based on the DSP analysis of the primary amplifier output analysis. In some embodiments, inputs to the microcontroller or the measurement DSP or both may be converted by an analog-to-digital converter, which may be a part of the microcontroller or one or more separate components. Likewise the voltage bias output of the microcontroller may be converted by a respective digital-to-analog converter, which again may be separate or integrally-formed with the microcontroller, prior to being applied to the at least one amplifier in the primary amplifier chain. In most embodiments a non-transitory memory device for storing program instructions is in communication with or integrally formed with the microcontroller. In some embodiments, the at least one bias voltage comprises a second bias voltage for biasing a second amplifier of the primary amplifier chain.
Additional aspects of the invention will be set forth, in part, in the detailed description, figures and any claims which follow, and in part will be derived from the detailed description, or can be learned by practice of the invention. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention as disclosed.
A more complete understanding of the present invention may be obtained by reference to the following detailed description when taken in conjunction with the accompanying drawings wherein:
The present invention provides an amplifier circuit for use in, for example, receiving the video overlay portion of a PON signal.
This exemplary PON 100 includes an OLT (optical line terminal) 110, which among other functions serves as a connection between a core network or networks (not shown). “Core network” is here being used in the general sense as a communication network through which subscribers may obtain downloaded content, communicate with others, access the Internet, and similar functions. The core network may itself include, for example, application servers and data storage devices for facilitating these functions or may interconnect with other networks, or both.
Downstream traffic from the core network is directed to one or more subscribers via the PON 100, and upstream traffic flows in the other direction, from subscribers toward the core network. Generally speaking, in this exemplary PON 100, the OLT 110 includes the apparatus (not shown in
For effecting downstream transmissions, the OLT includes at least one light source (not shown), which may for example be a laser or LED device. Light generated by the light source propagates along feeder fiber 115 until it is received in optical splitter/combiner 120 (sometimes referred to for convenience as simply an optical splitter).
In exemplary PON 100, optical splitter 120 distributes the light propagated downstream along feeder fiber 115 to a number of outputs. In
Optical splitter 120 is typically a passive device requiring no power but simply distributing light propagating in the downstream direction onto each one of its ports. In many PONs, therefore, the signals transmitted in the propagated light are identically passed to each of the access fibers. In the PON 100 of
An ONU may be associated with a single subscriber, as is, for example, an ONT (optical network terminal; not shown), and is often located on or near the subscriber's premises. In the example of
Note that in this exemplary PON 100, each of the ONUs receives the same downstream transmission but selects only that portion of the transmission stream addressed to it. Data not addressed to a particular ONU is simply discarded. Upstream traffic in exemplary PON propagates along the same path, originating in the ONUs 130a through 130n and transmitted in accordance with a time schedule established by OLT 110. The upstream traffic may use light of a different wavelength to avoid interference with downstream traffic, but the schedule is usually necessary so that ONU transmissions don't interfere with each other. A transmission from one of the ONUs 130a through 130n propagates along a respective one of the access fibers 125a through 125n to optical splitter combiner 120, where it is placed on feeder fiber 115 and eventually reaches a light detector (not shown) in OLT 110.
In many implementations, it is desirable to transmit video transmissions such as motion pictures or television programming to subscribers over the PON. One technique that has been developed for this purpose is often referred to as RF video overlay.
In the embodiment of
In the embodiment of
As should be apparent from the description above, the video overlay signal is in the usual case separately processed in the ONU 225. The arrangement for processing the video overlay signal will now be discussed in more detail.
In the embodiment of
As mentioned above, a bias voltage may be advantageously applied to the primary amplifier chain 345, often to the primary chain post amplifier 320 as shown in
For example, each ONU in a PON may be installed at a different distance from the central office compared with the other ONUs, and the difference in distance may for some units be quite large. In addition, carriers may specify broad input dynamic range requirements. This means that one setting of the amplifier bias voltage may result in uneven performance from one ONU to the next.
An improved video overlay amplifier circuit may be configured by introducing a dynamic bias voltage for the primary amplifier chain 345. In the embodiment of
In the embodiment of
In this manner a dynamic bias voltage is provided by bias voltage chain 350 to primary amplifier chain 345 of video overlay amplifier circuit 300. The dynamic bias voltage adjusts automatically as a function of (at least) the optical input received by the photodiode, within the parameters imposed by the design characteristics of the components of bias voltage chain 350, in this embodiment comparator 330, gain and level amplifier 335, and limiter circuit 340. Note that these parameters may vary from implementation to implementation.
Note that in the embodiment of
Other embodiments are possible, for example
In the embodiment of
In this embodiment, sensing circuit 425 creates a representation of the current produced by the photodiode 305 as a voltage signal, which can then be provided to microcontroller 430. Microcontroller 430 therefore is always aware of the strength of the input signal being received at photodiode 405. In this embodiment, it is also aware of the output at post amplifier 420. Coupler 435 provides the output to the subscriber equipment, but also permits an indication of the output to be provided to DSP (digital signal processor) 465 via an analog-to-digital (A/D) converter 460.
In this embodiment, DSP 465 performs selected measurements to determine the fidelity of the post amplifier output signal to desired parameters. The relevant parameters may include one or more of CNR (carrier-to-noise ratio), SNR (signal-to-noise ratio), MER (modulation error ratio), CSO (composite second order) performance, CTB (composite triple beat) performance, or other parameters as established by the network operator. The DSP 465 function may include, for example, FFT (fast fourier transform) spectral decomposition and filtering necessary to test specific parameters. Other functions may of course be employed.
In the embodiment of
Microcontroller 430 then determines the bias voltage that should be applied to the primary amplifier chain 445 and generates the necessary output. In this embodiment, a first bias voltage is applied to the TIA 410 via digital-to-analog (D/A) converter 475, and a second bias voltage is applied to post amplifier 420 via digital-analog converter 470
In the embodiment of
Note also that in other embodiments (not shown) only one of the bias voltages may be applied by the microcontroller 430. In yet other embodiments, either the first or second bias voltage may be static or a dynamic bias voltage may be applied by a bias voltage chain as shown in
Finally, note that while for illustration memory device 435, DSP 465, analog-to-digital converter 460, and digital-to-analog (D/A) converters 470 and 475 are shown as separate components from microcontroller 430, in other embodiments (not shown) any or all of these components may be integrally-formed with the microcontroller. And in accordance with the present invention, microcontroller 430 and DSP 465 may be implemented, separately or in a single unit, as a physical processor executing instructions stored as software in a non-transitory medium, or as a combination of executable software and hardware, or as hardware devices.
In this embodiment, the optical signal is then converted (step 515) into an electrical signal, for example by a photodiode. The resulting current signal is then converted (step 520) into a voltage signal, for example by a TIA, and typically at this stage amplified as well (not separately shown). The resulting electrical signal is attenuated (step 525) and provided to a post amplifier where it is amplified (step 530) before being provided a subscriber device (step 535). Note that as used herein, “subscriber device” includes such a device whether or not it is actually installed at a subscriber premises, and also any testing or sampling devices used to simulate the subscriber device or accept the signal of the post amplifier, for example during manufacture and testing.
In the embodiment of
In another embodiment, the bias voltage may be produced by a microcontroller that is provided with a representation of the input signal as in input. In this case, the microcontroller may also received measurements related to the output of the post amplifier as an input as well. In some embodiments, a bias voltage may be applied to the TIA as well as or instead of the post amplifier. If applied to both of course the bias voltages do not have to be identical. In any case, the process preferably continues as long as an input signal is being received.
Note that the sequence of operation illustrated in
Although multiple embodiments of the present invention have been illustrated in the accompanying Drawings and described in the foregoing Detailed Description, it should be understood that the present invention is not limited to the disclosed embodiments, but is capable of numerous rearrangements, modifications and substitutions without departing from the invention as set forth and defined by the following claims.
