Abstract:
The present invention pertains to systems and methods for equalizing a digitally modulated input signal for transmission as an optical signal over an optical fiber. In detail, this equalization is accomplished prior to the signal&#39;s conversion to an optical signal, and prior to the signal being filtered by a vestigial sideband (VSB) filter. In particular, equalization is accomplished by giving weights to the taps of a tapped delay equalizer, wherein weights for respective taps are derived from the output signal after its conversion to a digital signal at the downstream end of the optical fiber.

Description:
FIELD OF THE INVENTION 
     The present invention pertains to systems and methods for equalizing an input signal to an optical vestigial sideband (VSB) filter. In particular, the present invention pertains to signal transmission systems wherein digitally modulated signals are to be filtered as optical signals by an optical VSB filter and then transmitted over an optical fiber. The present invention pertains particularly, but not exclusively, to systems and methods that incorporate a tapped delay equalizer which equalizes a digitally modulated input signal for transmission over an optical fiber, wherein tap weights for the tapped delay equalizer are derived from the output signal at the downstream end of the fiber optic. 
     BACKGROUND OF THE INVENTION 
     When an optical signal is modulated for transmission through an optical fiber on a carrier frequency f c  (f c =C/λ where C is a constant and λ is the optical wavelength), the modulated information signal will have two symmetric sidebands that are centered on the carrier frequency. In order to reduce fading due to fiber dispersion, and to conserve bandwidth in the transmission of such a signal, the optimal solution is to filter out one of the sidebands, either above or below f c . For various technical reasons, however, simultaneous preservation of one complete sideband and a complete removal of the other sideband is impossible. Nevertheless, although one of the sidebands may be partially suppressed, the complete preservation of the other sideband for transmission is highly desirable. 
     A partial solution for the difficulty mentioned above, is the use of a vestigial sideband (VSB) filter. As is well known in the pertinent art, a VSB filter is a band pass filter that effectively preserves one sideband while partially suppressing the other sideband. Just how much of the unwanted sideband can be actually suppressed, however, is a design consideration. As noted above, it is virtually impossible to suppress 100% of the unwanted sideband. The portion of the sideband which cannot be suppressed is then referred to as the vestigial sideband (VSB). 
     During a signal transmission it will happen that the VSB, which is transmitted with the unsuppressed sideband, will introduce impairments (distortions) into the transmitted signal. For signal integrity, these impairments need to be avoided, or at least minimized. For example, it is known that telecommunication signals can be adversely affected by group delays (i.e. time delays of amplitude envelopes), and phase delays (i.e. time delays of signal phase). Both of these types of delays result from interferences caused by the VSB. Also, and perhaps of greater concern, are Inter Symbol Interferences (ISI) that are introduced by the VSB during the demodulation of digital signals from an analog carrier signal. In any event, an optical information signal which is transmitted over an optical fiber will be somehow corrupted. 
     The primary object of VSB control is obviously to minimize impairments (distortions) in the received signal, while also preserving the integrity of the transmitted information signal as much as possible. With this objective in mind, closed loop feedback control technology has provided interesting possibilities. 
     In the context of signal telecommunications, an overview of closed loop control for a desired system output requires comparing the actual output of a system with the actual system input. In the case of a telecommunications system which seeks to preserve signal integrity, the desired system output will be the same as the system input (i.e. a signal transmission that results in a non-corrupted signal). When they are not the same, the signal has been corrupted during transmission. In this later case, a comparison of the actual output with the actual system input will generate an error signal. In a communications system, where it is known that the transmitted signal will be corrupted, the object is then to minimize the error signal. In essence, the question is what feedback will most effectively minimize the error signal. 
     An example of employing closed loop technology to control signal transmission using a VSB filter is provided by U.S. Pub. No. 2003/0058509 (hereinafter referred to as “Webb”). As disclosed in Webb, the control loop is used to adjust the wavelength of a laser that is providing the carrier frequency. Alternatively, Webb discloses the use of a wavelength control block to control the filter edge of the VSB filter. Unlike the disclosure of Webb, the present invention incorporates a tapped delay equalizer in the feedback loop which is established to reshape the input signal for the purpose of improving a VSB filter. 
     In light of the above, it is an object of the present invention to provide a system and method for equalizing a digitally modulated signal, for input as an optical signal to an optical VSB filter, for transmission of the optical signal over an optical fiber. Another object of the present invention is to provide a device which employs a tapped delay equalizer to equalize a digitally modulated signal for subsequent conversion and filtering as an optical signal by a VSB filter for transmission over an optical fiber. Still another object of the present invention is to provide a system for using a tapped delay equalizer, in combination with a VSB filter, to transmit optical signals over an optical fiber which is easy to manufacture, is simple to use and which is comparatively cost effective. 
     SUMMARY OF THE INVENTION 
     In accordance with the present invention, a system is provided for transmitting a digital information signal, as an optical information signal, over an optical fiber. After transmission over the optical fiber, the optical information signal is converted back into a digital information signal for further transmission. An important aspect of the present invention is the incorporation of a vestigial sideband (VSB) optical filter into the system which has been equalized to improve the VSB filter. For the present invention, this improvement is further improved by equalizing the digital information signal that is provided as the input to the system. For the present invention, the additional improvement afforded by equalization is provided by a tapped delay equalizer. In particular, this equalization compensates for signal impairments introduced by the VSB filter. 
     As envisioned for the present invention, the digital information signal can be a non-return to zero (NRZ) digital signal, a return to zero (RZ) digital signal, a quadrature amplitude modulation (QAM) signal, a duo-binary signal or any other suitable signal known in the pertinent art. Importantly, whatever digital information signal is used as the input signal, it will be characterized by a symbol rate, R s , and a time duration, T, for each symbol, with R s =1/T. 
     Structurally, the system includes a transmitter which receives a digital information signal as an electrical input, and it outputs an optical information signal for transmission over an optical fiber. To perform this function, the transmitter includes a driver chip, an electrical to optical (E/O) converter and an optical VSB filter, such as an optical thin film filter. 
     In this combination, the driver chip is provided for conditioning the electrical signal upstream of the E/O converter. To do this, the driver chip includes a tapped delay equalizer, an amplifier with gain and bias control, and a control circuitry for operating the driver chip. As indicated above, taps of the tapped delay equalizer are adjustable to alter the shape of the electrical signal (i.e. the digital information signal) that is input at the E/O converter. 
     In detail, the tapped delay equalizer which is positioned on the driver chip to receive the digital information signal as an input will have an n-number of taps. For the present invention, a time delay, d t , between adjacent taps can be engineered as desired for the particular driver chip. Accordingly, the chip needs to be configured with d t &lt;T. Moreover, although d t  may be the same between all adjacent taps (i.e. d t−1 =d t =d t+1 ), depending on the needs of the particular system, this may not necessarily be so (i.e. d t−1 ≠d t  and/or d t ≠d t+1 ). Further, using the n-number of taps, the adjustable equalizer will include an N-number of taps per symbol in the information signal. In general, the tapped delay equalizer is established with n-greater than N, and N greater than one (n&gt;N&gt;1). In this arrangement each tap is weighted, at least in part, based on the operational parameters of the VSB filter. As is well known by the skilled artisan, these operational parameters typically include a phase position of the VSB filter relative to the information signal, selectively measured amplitudes from the optical information signal, and group delays encountered between tap samples of the information signal. 
     Operationally, the tapped delay equalizer is employed to reshape the input digital information signal, to thereby compensate for impairments which are introduced into the optical information signal by the optical VSB filter. It is noteworthy that this signal reshaping can also account for variations in signal quality due to the length of the optical fiber. Further, in some implementations of the present invention, signal quality downstream of the optical fiber can be measured and the resulting data included for adjustments of the tapped delay equalizer. 
     On the driver chip, the amplifier with gain and bias control is connected to receive the shaped signal from the tapped delay equalizer. With this connection, the amplifier provides gain for the shaped signal, and it includes a biasing element to bias the shaped signal. The result here is an electrical digital information output signal from the driver chip which has a proper operating point. 
     Control circuitry, in addition to the tapped delay equalizer and the amplifier, is also provided on the driver chip. As indicated above, with the connection between the amplifier and the control circuitry, a suitable gain and a bias for the amplifier can be established. On the other hand, the connection between the control circuitry and the tapped delay equalizer allows the tap weights for individual taps of the tapped delay equalizer to be adjusted. The collective result of these corrective actions is a digital information signal that is ready for conversion to an optical information signal. 
     As indicated above, measurements of signal quality downstream of the optical fiber are to be used to adjust the equalizer. To do this, an analyzer is included in the system. Specifically, the analyzer is connected between the output of an optical to electrical (O/E) device at the downstream end of the fiber optic, and the driver/equalizer at the upstream end of the fiber optic. With this connection, the analyzer can be used to determine a transmission quality parameter such as a bit error rate (BER), along with other signal impairments mentioned above that have been introduced by the VSB optical filter and the optical fiber. 
     In more detail, the analyzer is connected between the O/E device and the tapped delay equalizer of the driver/equalizer to analyze samples of the digital information signal that is received downstream from the optical fiber. For this purpose the analyzer will include an oscilloscope that is connected into the analyzer to generate an eye diagram of the received digital information signal. Using the eye diagram, an n-number of values from the received digital information signal that are respectively based on operational parameters of the VSB filter are determined. These values are then used to create a control signal input to the tapped delay equalizer for respectively weighting each of the n taps of the tapped delay equalizer, to thereby minimize impairments introduced into the received information signal. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The novel features of this invention, as well as the invention itself, both as to its structure and its operation, will be best understood from the accompanying drawings, taken in conjunction with the accompanying description, in which similar reference characters refer to similar parts, and in which: 
         FIG. 1  is a schematic presentation of a communications link for transmitting a digitally modulated signal over an optical fiber in accordance with the present invention; 
         FIG. 2  is a schematic presentation of a driver/equalizer chip in accordance with the present invention, for use in the communications link shown in  FIG. 1 ; 
         FIG. 3A  is an eye diagram of a digital output signal, when a digitally modulated input signal has been transmitted as an optical signal over a fiber optic, when no optical VSB filter is used in signal transmission; 
         FIG. 3B  is an eye diagram of a digital output signal, when a digitally modulated input signal has been transmitted as an optical signal over a fiber optic, when a VSB filter has been used in signal transmission; and 
         FIG. 3C  is an eye diagram of a digital output signal, when a digitally modulated input signal has been transmitted as an optical signal over a fiber optic, when the digitally modulated input signal has been equalized and an optical VSB filter has been used in signal transmission. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring initially to  FIG. 1 , a system for transmitting optical signals in accordance with the present invention is shown, and is generally designated  10 . As shown, the system includes a transmitter  12  and a receiver  14  that are interconnected with each other by an optical fiber  16 .  FIG. 1  also shows that the system  10  includes an analyzer  18  which interconnects the receiver  14  with the transmitter  12 . 
     In overview, a digitally modulated information signal  20  is provided as input to the system  10  for transmission over the optical fiber  16  from the transmitter  12  to the receiver  14 . As envisioned for the present invention the digitally modulated information signal  20  will have a predetermined symbol rate, R s , and it will have characteristics and parameters that are well known in the art. 
     For the present invention, it is to be appreciated that the digitally modulated information signal  20  will experience several transformations as it passes through the system  10 . With this in mind, the general descriptor “information signal  20 ” is used in all references to the basic signal for all variations of the information signal  20 . In particular, these references include: 1) the original digitally modulated input information signal  20 ; 2) a digital (electrical) pre-transit equalized information signal  20   a;  3) an optical information signal  20  which is transmitted over the optical fiber  16 ; 4) a digital (electrical) post-transit information signal  20   b ; and 5) a digitally modulated output information signal  20 ′ which is received by a user of the system  10 . For reference purposes, these references for information signal  20  are all shown in  FIG. 1 . 
     As shown in  FIG. 1 , the transmitter  12  includes a data mapper  22  (optional) which may be provided to handle and format the input information signal  20  for data transfer purposes. Also included in the transmitter  12  is a driver/equalizer chip  24  which is provided to equalize the input information signal  20  for maximum transmission efficiency through the system  10 . In accordance with the present invention, once the input information signal  20  has been equalized, the resulting pre-transit equalized information signal  20   a  is converted into the optical information signal  20  by an Electrical/Optical (E/O) device  26 . 
     Still referring to  FIG. 1 , a vestigial sideband (VSB) filter  28  is provided with the transmitter  12  for filtering the optical information signal  20 . Once it is filtered, the optical information signal  20  is then passed to the optical fiber  16  for transmission over the optical fiber  16  to the receiver  14 . As is well known, the VSB filter  28  and the optical fiber  16  will introduce impairments to the optical information signal  20  during this transmission. In particular, the impairments will include phase delays in the optical information signal  20 , as well as group delays. 
     Upon receipt of the optical information signal  20  at the receiver  14 , an Optical/Electrical (O/E) device  30  is provided to convert the optical information signal  20  into a digital, post-transit information signal  20   b .  FIG. 1  shows that the post-transit information signal  20   b  passes through a data slicer  32  where the data in signal  20   b  can be appropriately narrowed. Also, a de-mapper  34  is provided, if necessary. 
     Still referring to  FIG. 1 , it will be seen that the post-transit information signal  20   b  can also be passed from the O/E device  30  to the analyzer  18 . As envisioned for the present invention, the analyzer  18  will typically include an oscilloscope which presents the post-transit information signal  20   b  as an eye diagram  36  (see  FIGS. 3A, 3B ). The post-transit information signal  20   b  is then passed back from the analyzer  18  to the driver/equalizer chip  24  of the transmitter  12 . 
       FIG. 2  shows that the driver/equalizer chip  24  includes an n-number of taps  38  which each have a respective delay d t . Importantly, individual delays d t  can be engineered for the driver/equalizer chip  24  as required for its particular application. Stated differently, d t  may be equal to d t−1 , or it may not. Also included in the driver/equalizer chip  24  are an n-number of amplifiers  40  which are respectively connected with the same n-numbered taps  38 . Importantly, for an operation of the present invention, there must be an N-number of taps  38  per symbol in the digitally modulated input information signal  20 , where N is greater than 1. Thus, n (total number of taps  38 ) must be equal to, or greater than 1. 
     As intended for the present invention, the analyzer  18  creates an eye diagram  36  which can be used to optimize a transmission of the optical information signal  20  over the optical fiber  16 . In particular, using the eye diagram  36  as a reference, an n-number of values are obtained from the post-transit information signal  20   b . The n-number of values which are obtained are then used by an equalizer control  42  in the driver/equalizer chip  24 . Specifically, the obtained values are used by the equalizer control  42  to establish amplitude control for the respectively numbered amplifiers  40 . 
       FIG. 2  also indicates that a summer  44  in the driver/equalizer chip  24  sums the outputs of the n-number of amplifiers  40 . Also, a bias/gain control  46  is provided which, together with the summer  44 , create the pre-transit equalized information signal  20   a.    
     In overview, the driver/equalizer chip  24  functions as a feedback control which operates to equalize the digitally modulated input information signal  20  for efficient transmission of the input information signal  20  from the transmitter  12  to the receiver  14 . Thus, at the receiver  14 , the post-transit information signal  20   b  is received as a VSB filtered output information signal  20 ′ having a substantially same information content as the input information signal  20 . 
       FIGS. 3A-C  are provided to respectively show typical eye diagrams  36   a - c  which are created by the analyzer  18 . As shown,  FIG. 3A  shows a post-transit information signal  20   b  which has not been filtered by a VSB filter  28 .  FIG. 3B  shows a signal  20   b  which has been filtered by a VSB filter  28 , but not equalized. And,  FIG. 3C  shows a signal  20   b  which has been both filtered and equalized. As is well known by the skilled artisan, the eye diagram  36   c  in  FIG. 3C  is preferable. 
     While the particular Adaptive Equalization for Vestigial Sideband (VSB) Transmissions as herein shown and disclosed in detail is fully capable of obtaining the objects and providing the advantages herein before stated, it is to be understood that it is merely illustrative of the presently preferred embodiments of the invention and that no limitations are intended to the details of construction or design herein shown other than as described in the appended claims.