Abstract:
An apparatus for and a method of transmitting analog return signals in a digital return path of a cable television system (CATV) is disclosed. In one embodiment, at a node of the CATV system, an analog CATV return signal is converted to a stream of digital samples at approximately 100 MHz. Signals outside of a desired frequency band are removed with a digital filter having predetermined filter coefficients. The resulting stream of digital samples is up-sampled to generate another stream of digital samples at a rate that is four times the center frequency of a predetermined frequency band. The resulting stream is then punctured to generate yet another stream with a data rate that is lower than 100 MHz. Zero samples are removed, and the remaining digital samples are serialized and converted to optical signals for transmission via an optical medium of the CATV return path. A reverse process at a hub or head end of the CATV return system restores the signals of the desired frequency band.

Description:
The present application claims priority to, under 35 U.S.C. 119(e), U.S. Provisional Patent Application 60/357,071, filed Feb. 12, 2002, which is incorporated herein by reference. 
    
    
     BRIEF DESCRIPTION OF THE INVENTION 
     The present invention relates generally to cable television systems (CATV). More specifically, the present invention pertains to a method and system for lowering the data rate of digital return path links for a CATV hybrid fiber coax system. 
     BACKGROUND OF THE INVENTION 
     Cable television systems (CATV) were initially deployed so that remotely located communities were allowed to place a receiver on a hilltop and to use coaxial cable and amplifiers to distribute received signals down to the town that otherwise had poor signal reception. These early systems brought the signal down from the antennas to a “head end” and then distributed the signals out from this point. Since the purpose was to distribute television channels throughout a community, the systems were designed to be one-way and did not have the capability to take information back from subscribers to the head end. 
     Over time, it was realized that the basic system infrastructure could be made to operate two-way with the addition of some new components. Two-way CATV was used for many years to carry back some locally generated video programming to the head end where it could be up-converted to a carrier frequency compatible with the normal television channels. 
     Definitions for CATV systems today call the normal broadcast direction from the head end to the subscribers the “forward path” and the direction from the subscribers back to the head end the “return path.” A good review of much of today&#39;s existing return path technology is contained in the book entitled  Return Systems for Hybrid Fiber Coax Cable TV Networks  by Donald Raskin and Dean Stoneback, hereby incorporated by reference as background information. 
     One innovation, which has become pervasive throughout the CATV industry over the past decade, is the introduction of fiber optics technology. Optical links have been used to break up the original tree and branch architecture of most CATV systems and to replace that with an architecture labeled Hybrid Fiber/Coax (HFC). In this approach, optical fibers connect the head end of the system to neighborhood nodes, and then coaxial cable is used to connect the neighborhood nodes to homes, businesses and the like in a small geographical area. 
       FIG. 1  shows the architecture of a HFC cable television system. Television programming and data from external sources are sent to the customers over the “forward path.” Television signals and data are sent from a head end  10  to multiple hubs  12  over optical link  11 . At each hub  12 , data is sent to multiple nodes  14  over optical links  13 . At each node  14 , the optical signals are converted to electrical signals and sent to customers over a coaxial cable  15 . In the United States, the frequency range of these signals is between 55 to 850 MHz. 
     Data or television programming from the customer to external destinations, also known as return signals or return data, are sent over the “return path.” From the customers to the nodes  14 , return signals are sent over the coaxial cables  15 . In the United States, the frequency range of the return signals is between 5 to 42 MHz. At the nodes  14 , the return signals are converted to optical signals and sent to the hub  12 . The hub combines signals from multiple nodes  14  and sends the combined signals to the head end  10 . 
       FIG. 2  is a block diagram of a digital return path  100  of a prior art HFC cable television system that uses conventional return path optical fiber links. As shown, analog return signals, which include signals generated by cable modems and set top boxes, are present on the coaxial cable  102  returning from the customer. The coaxial cable  102  is terminated at a node  14  where the analog return signals are converted to a digital representation by an A/D converter  112 . The digital signal is used to modulate a optical data transmitter  114  and the resulting optical signal is sent over an optical fiber  106  to an intermediate or head end hub  12 . At the hub  12 , the optical signal is detected by an optical receiver  122 , and the detected digital signal is used to drive a D/A converter  124  whose output is the recovered analog return signals. 
     The analog return signals present on the coaxial cable  102  are typically a collection of independent signals. In the United States, because the analog return signals are in the frequency range of 5 to 42 MHz, the sampling rate of the A/D converter is about 100 MHz, slightly more than twice the highest frequency in the band. A 10-bit A/D converter operating at a sampling rate of 100 MHz is typically used for digitizing the return signals. As a result, data will be output from the A/D converter  112  at a rate of about 1 Gbps. Further, the optical data transmitter  114  and the optical data receiver  122  must be capable of transmitting and receiving optical signals at a rate of 1 Gbps or higher. The high transmission data rate requires the use of expensive equipment, or short transmission distances, or both. Bandwidth limitations of the data transmission equipment also limits the number of analog return signals that can be aggregated for transmission on the same optical fiber. 
     Accordingly, there exists a need for a method of and system for lowering the data rate in the return path of a CATV system. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention is an apparatus for and a method of transmitting analog return signals in a digital return path of a cable television system (CATV). In this embodiment, at a node of the CATV system, an analog CATV return signal is converted to a stream of digital samples at approximately 100 MHz. Signals outside of a desired frequency band are removed with a digital filter having predetermined filter coefficients. The resulting stream of digital samples is up-sampled to generate another stream of digital samples at a rate that is four times the center frequency of a predetermined frequency band. The resulting stream is then punctured to generate yet another stream with a data rate that is lower than 100 MHz. Zero samples (i.e., sample having a value of zero) are removed, and the remaining digital samples are serialized and converted to optical signals for transmission via an optical medium of the CATV return path. In one particular embodiment, the transported data stream has a data rate that is less than half of the 100 MHz data rate. 
     In furtherance of the present embodiment, at a hub or head end of the CATV system, the optical signals are converted to electrical signals and deserialized to form a stream of digital samples. Zeros samples are reinserted, and the resulting stream is filtered by a digital filter that has the same filter coefficients as the filter in the node of the CATV system. The filtered stream of digital samples are then up-sampled to a rate of approximately 100 MHz. The up-sampled stream is converted by a digital-to-analog converter to restore the signals in the desired frequency band. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and aspects of the present invention will be more readily apparent from the following description and appended claims when taken in conjunction with the accompanying drawings, in which: 
         FIG. 1  shows the architecture of a cable television system; 
         FIG. 2  is a block diagram of a cable television (CATV) digital return path of the prior art; 
         FIG. 3  is a block diagram of a CATV return path according to one embodiment of the present invention; 
         FIG. 4  illustrates a relationship between spectral energy and frequency of signals carried by a conventional CATV digital return path and a desired frequency band that is carried by a CATV digital return path of  FIG. 3 ; 
         FIG. 5  illustrates an encoder that can be used in the CATV digital return path of  FIG. 3 ; 
         FIG. 6  illustrates a decoder that can be used in the CATV digital return path of  FIG. 3 ; 
         FIG. 7  illustrates samples of a 35.3 MHz sinusoidal waveform sampled at a 100 MHz rate; 
         FIG. 8  illustrates the coefficients of the bandpass interpolation filter of  FIG. 5  according to one embodiment of the present invention; 
         FIG. 9  illustrates the frequency response of the bandpass interpolation filter of  FIG. 5  according to one embodiment of the present invention; 
         FIG. 10  illustrates the output of the bandpass interpolation filter of  FIG. 5  when interpolated 48/34 times the input frequency or approximately 141.176 MHz; 
         FIG. 11  illustrates the result of puncturing the output of the bandpass interpolation filter  FIG. 5  according to one embodiment of the present invention; 
         FIG. 12  illustrates the result of interpolating the filter output of the bandpass filter of  FIG. 6  to the full rate of approximately 100 MHz according to one embodiment of the present invention; and 
         FIG. 13  illustrates the result of converting the output of the bandpass interpolation filter of  FIG. 6  and low-pass filtering the analog signal according to one embodiment of the present invention. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       FIG. 3  is a block diagram depicting a CATV return path  200  according to one embodiment of the present invention. At the CATV return path transmitter  210 , an A/D converter  112  receives an analog return signal from a co-axial cable  201  and generates a stream of data at a full sampling rate (e.g., 100 MHz). A signal encoder  213  encodes the output of the A/D converter  112  and generates another stream of data at a lower data rate. The low data rate output of the signal encoder  213  is provided to the optical data transmitter  114  for transmission to a hub  220  as optical signals. According to the present invention, the hub  220  can be an intermediate hub or a head end hub. 
     At the hub  220 , an optical data receiver  122  receives the optical signals from the transmitter  210  and converts the signals to a low data rate data stream that is a replica of the data stream generated by the signal encoder  213 . More specifically, the optical data receiver  122  preferably includes an optoelectronic receiver that receives the optical signals and converts the optical signals into a serial bit stream, and a deserializer for converting the serial bit stream into a stream of multiple-bit digital values (sometimes called samples). A signal decoder  223  receives and decodes the output of the optical data receiver  122  and generates a stream of data at a full sampling rate. The output of the decoder  223  is provided to the D/A converter  124  for conversion into analog signals. In this embodiment, the signal encoder  213  and signal decoder  223  enable digital data to be transmitted across the optical link at a lower rate than N*F bits per second (where N is the number of bits and F is the sampling frequency of the A/D converter  112 ). The entire spectrum of the analog return signal originally present on cable  201 , however, is not recreated at the output of the hub  220 . Only frequencies within a desired frequency band of the analog return signal are recovered at the hub  220 . 
     The analog return signal carried by the co-axial cable  201  is an analog signal with signal components from 5 to 42 MHz.  FIG. 4  illustrates the spectral density of the signal components of a typical analog return signal. In prior art CATV systems, most or all of the signal components from 5 to 42 MHz are communicated via the return path to the head end. A typical sampling rate of the analog return signal is 100 MHz, which is higher than twice the highest frequency transmitted in the return path. In some CATV systems, users of the CATV return path only use specific portions of the return path spectrum. Thus, in those systems, only those portions of the return path spectrum carrying useful information need be transmitted from the node  210  to the hub  220 . Other portions of the return path spectrum can be filtered out. In one particular embodiment as shown in  FIG. 4 , the desired signal is only in a portion of the return path spectrum approximately between 34 MHz and 40 MHz with a total bandwidth of approximately 6 MHz. When only a specific portion of the return path spectrum is transmitted, (e.g., the spectrum between 34 MHz and 40 MHz) the data rate of the optical link can be significantly reduced. 
     According to one embodiment of the present invention, logic for transmitting a signal that embodies a specific portion of the return path spectrum is implemented in the signal encoder  213 . One implementation of the signal encoder  213  is shown in  FIG. 5 . As shown, a stream of A/ID samples at the Full Rate of 100 MHz is first filtered by a digital FIR (Finite Impulse Response) band-pass interpolation filter  510  to form a band-limited data stream. The filter rate of the bandpass interpolation filter  510  is chosen as the least common multiple of the Full Rate and an integer multiple (e.g. four times) of Center Frequency. As used herein, Center Frequency refers to the frequency approximately at the center of the frequency band to be retained. For example, if the frequency band to be retained is the band between 32-38 MHz, the Center Frequency will be approximately 35 MHz. The Center Frequency is preferably less than one half the Full Rate. 
     In the present embodiment, A/D samples enter the filter at the Full Rate (e.g., 100 MHz), and samples are read from the multiple phase taps of band-pass interpolation filter  510  at a rate that is four times (and more generally an integer multiple of) the Center Frequency to form another stream of samples. If the Center Frequency is 35 MHz, then samples are read from the band-pass interpolation filter  510  at a rate of 140 MHz, and the filter rate will be 700 MHz. In the present embodiment, the data rate at which samples are read from the output phase taps of the bandpass interpolation filter  510  is set by an NCO (Numerically Controlled Oscillator)  512 . 
     With reference again to  FIG. 5 , the interpolated samples are then punctured at an odd integer rate by logic circuits  514 . That is, samples are punctured at a rate of Center Frequency*4/k; where k is an odd integer. The value of k can be chosen as any odd number as long as the resulting sampling rate is less than twice the desired bandwidth (i.e., of the desired signal band). For a ⅓ puncture rate, only every third sample is retained. The other 2 of 3 of the samples are replaced by zeros. The retained samples are the Transport Samples. In the present embodiment, only the Transport Samples are sent to the optical data transmitter  114 . The samples that are replaced by zeros (or, “punctured”) are not sent over the optical link  11  to the hub  12  or head end  10 . 
     Attention now turns to  FIG. 6 , which is a block diagram depicting an implementation of signal decoder  223  in accordance with an embodiment of the present invention. The signal decoder  223  is coupled to SERDES circuits of the optical data receiver  122  to receive the transport stream generated by node  210 . As described above, the transport stream consists of punctured samples. That is, certain samples were replaced with zeros and were not transported. Thus, in the present embodiment, the zero-insertion logic  624  of the signal decoder  223  reinserts the zero samples in the transport stream to generate a “depunctured” or “restored” stream. The “depunctured” stream is filtered by a bandpass interpolation filter  626 , and the output phase taps of the interpolation filter  626  are read (by a multiplexer or similar apparatus  628 ) at the Full Rate of 100 MHz to form an output data stream. The samples of the output data stream are then sent to the D/A converter  124  ( FIG. 3 ) and an analog low pass output filter  230 , which reconstruct the desired analog waveform. The low pass output filter preferably filters out signals significantly above the desired band of signals, so as to reduce or eliminate high frequency noise generated by the reconstruction of the desired signal from digital samples. For example, with a desired signal band of 34 to 40 MHz, the low pass output filter would preferably filter out signal above approximately 50 MHz. 
     Example Implementation 
     Attention now turns to an example implementation that illustrates the principles of an embodiment of the present invention. In this example, a 35.3 MHz sinusoidal waveform sampled at a 100 MHz rate is used as the input signal.  FIG. 7  shows the samples of the 35.3 MHz sinusoidal input signal sampled at a 100 MHz rate. 
     Further, in this example, the bandpass interpolation filter  510  of the signal encoder has thirty-four active taps with forty-eight phases.  FIG. 8  shows the coefficients of the bandpass interpolation filter  510  in this particular example. The frequency response of the filter  510  in this particular example is shown in  FIG. 9 . The bandpass interpolation filter  510  processes the input signal allowing only the desired signals to pass. In this example, the 35.3 MHz sinusoidal input signal falls within the range of desired signals that are allowed to pass. (35.3 MHz corresponds to 112.96 on the horizontal scale of  FIG. 9 , and thus falls near the center of the region have 0 dB in amplitude attenuation.) 
     In the present example the output of the filter  510  is interpolated 48/34 times the (100 MHz) input frequency or approximately 141.18 MHz (which is approximately four times the center frequency of 35.3 MHz (35.3 MHz*4=141.2 MHz)), resulting in the interpolated samples of  FIG. 10 . The interpolated samples are then punctured to ⅓ the sample rate of 141.18 MHz or 47.06 MHz.  FIG. 11  shows the samples after puncturing. The punctured samples are set to zero in the  FIG. 11 . Only the non-zero samples are transported to the receiver. Thus, the transport data rate is reduced from 100 MHz to approximately 47.06 MHz. 
     At the receiver, the zeros in the punctured data stream are reinserted. The resulting data stream is filtered in the bandpass interpolation filter  626 , which has the same filter coefficients as the bandpass interpolation filter  510 . The bandpass interpolation filter  626 , however, is used with forty-eight active taps and thirty-four phases. The filter output is computed at the full rate of 100 MHz resulting in the samples shown in  FIG. 12 . The resulting samples are similar to the input samples ( FIG. 7 ) with only the phase shift of the system components. 
     The output of the bandpass interpolation filter  626  is passed to the D/A converter  124  ( FIG. 3 ) and filtered by an analog low pass filter  230  ( FIG. 3 ), resulting in the output of  FIG. 13 . 
     Preferred embodiments of the present invention and best modes for carrying out the invention have thus been disclosed. While the present invention has been described with reference to a few specific embodiments, the description is illustrative of the invention and is not to be construed as limiting the invention. Various modifications may occur to those skilled in the art without departing from the true spirit and scope of the invention. It should also be noted that some embodiments of the present invention described above can be implemented by hardware logic (e.g., Field Programmable Gate Array(s)). In addition, a person skilled in the art would realize upon reading the present disclosure that portions of the present invention can be implemented as computer executable programs executable by a digital signal processor. Further, although the embodiments described above use finite impulse response (FIR) digital filters for rate conversion, a person skilled in the art would realize upon reading the present disclosure that other embodiments of the invention can use infinite impulse response digital filters and variable time update periods.