Abstract:
A device for and a method of decreasing the data rate of a digital return path link in a Cable Television Hybrid Fiber-Coax system (CATV system) is disclosed. At the node of the CATV system, the bandwidth of the a digital data stream representative of an analog return signal is limited to a desired frequency band. The bandwidth-limited data stream is then digitally re-sampled at a predetermined multiple of a center frequency of the frequency band. The re-sampled data stream is then separated into two data streams. Then, these separate data streams are digitally decimated to a lower data rate, interleaved and serialized for transmission to a head end of the CATV system. A reverse process reconstructs the original analog return signal&#39;s signal components within the desired frequency band at the head end.

Description:
The present application is a continuation of U.S. patent application Ser. No. 10/218,344, entitled BANDPASS COMPONENT DECIMATION AND TRANSMISSION OF DATA IN CABLE TELEVISION DIGITAL RETURN PATH, filed Aug. 12, 2002, which claims priority to, under 35 U.S.C. §119(e), U.S. Provisional Patent Application No. 60/355,023, filed Feb. 8, 2002. The foregoing applications are incorporated herein by reference in their entireties. 
    
    
     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 frequency range of 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.” Form the customer to the node, return signals are sent over the coaxial cable  15  in the frequency range of 5 to 42 MHz. At the node  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  24  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 hub  12 . At the intermediate 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. These recovered analog return signals are then combined in an analog fashion with analog return signals from other nodes. 
     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. Therefore, 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 results in more expensive equipment, or a lower transmission distance, or both. The high transmission data rate 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 transmitting data at a lower data rate on the return path of a Hybrid Fiber Coaxial CATV system. 
     SUMMARY OF THE INVENTION 
     An embodiment of the present invention is a device for and a method of decreasing the data rate of a digital return path link in a Hybrid Fiber-Coax Cable Television system (HFC-CATV system). In this embodiment, at the node of the CATV system, an analog return signal is digitized, and the bandwidth of the resulting digital data stream is limited to a desired frequency band. The bandwidth-limited data stream is re-sampled at a predetermined multiple of a center frequency of the frequency band. Then, the re-sampled data stream is separated into two data streams of in-phase and quadrature components at the re-sampling frequency. Thereafter, the data streams of in-phase and the quadrature components are digitally decimated to a lower data rate. Subsequently, the decimated data streams are interleaved and serialized for transmission to a head end via optical links. 
     A reverse process reconstructs the original return signal&#39;s bandwidth limited signal components at the head end of the CATV system. More specifically, at the head end of the CATV system, the data stream from the node is de-interleaved to form an in-phase data stream and a quadrature data stream. Then, the in-phase data stream and the quadrature data stream are digitally re-sampled and combined to form another data stream. This resulting data stream is bandpass filtered and re-sampled at a higher rate to form an output data stream, which is converted subsequently into analog form to recover an analog return signal. 
     In one embodiment, the decimated data stream has a data rate that is twice the bandwidth of the desired frequency band. If the bandwidth of the desired frequency band is low, low speed optical data transmitters and low speed optical data receivers can be used to transport the signals. Because low speed optical links are inexpensive, the overall cost of the CATV system is reduced. 
    
    
     
       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  depicts an example analog input waveform at 33 MHz; 
         FIG. 8  depicts samples of the example waveform of  FIG. 7  at a sampling rate of 100 MHz; 
         FIG. 9  depicts the filter coefficients of a 35.3 MHz bandpass filter; 
         FIG. 10  depicts the filter response of the 35.3 MHz bandpass filter having the filter coefficients of  FIG. 9 ; 
         FIG. 11  depicts samples of the example waveform of  FIG. 7  at a sampling rate of 141.176 MHz; 
         FIG. 12  depicts an in-phase component of the waveform of  FIG. 11 ; 
         FIG. 13  depicts a quadrature component of the waveform of  FIG. 11 ; 
         FIG. 14  illustrates the filter coefficients of an example 3 MHz lowpass interpolation filter; 
         FIG. 15  depicts the frequency response of a 3 MHz low pass interpolation filter having the filter coefficients of  FIG. 14 ; 
         FIG. 16  depicts a decimated in-phase data stream according to an embodiment of the invention; 
         FIG. 17  depicts a decimated quadrature data stream according to an embodiment of the invention; 
         FIG. 18  depicts a data stream generated by up-sampling the in-phase data stream of  FIG. 16 ; 
         FIG. 19  depicts a data stream generated by up-sampling the quadrature data stream of  FIG. 17 ; 
         FIG. 20  depicts a data stream generated by combining the up-sampled data streams of  FIGS. 18 and 19 ; 
         FIG. 21  depicts a data stream generated by resampling the combined data stream of  FIG. 20  at 100 mega-samples per second; and 
         FIG. 22  depicts an analog waveform generated using the data stream of  FIG. 21  and an analog lowpass filter. 
     
    
    
     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 corresponding to the one generated by the signal encoder  213 . 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 ). However, the entire spectrum of the analog return signal originally present on cable  201  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 in a predefined frequency range, such as 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, the logic for transmitting a signal that embodies a specific portion of the return path spectrum is implemented in the encoder  213 . One implementation of the encoder  213  is shown in  FIG. 5 . As shown, a stream of A/D samples at the Full Rate of 100 MHz is first filtered in a digital FIR (Finite Impulse Response) band-pass interpolation filter  510  to form a band-limited data stream. The filter rate of the band-pass interpolation filter  510  is chosen as a ratio of integers times the sample rate. As used herein, Center Frequency of a bandpass filter 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 of the bandpass filter will be approximately 35 MHz. The Center Frequency of the bandpass interpolation filter  510 , in one embodiment, is chosen to be 6/17 of the Full Rate (100 MHz), which is approximately 35.29 MHz. 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 a multiple (e.g., four times) of the Center Frequency of the bandpass filter  510  to form another stream of samples. In the present discussion, it is assumed that samples are read from the bass-pass interpolation filter  510  at a rate that is four times the Center Frequency. That is, in the present discussion, if the Center Frequency is 35.29 MHz, then samples are read from the band-pass interpolation filter  510  at a rate of 141.176 MHz. In the present embodiment, the data rate at which samples are read from the outputs of the bandpass interpolation filter  510  is set by an NCO (Numerically Controlled Oscillator)  512 . In other embodiments, the rate at which samples are read from the outputs of the bandpass interpolation filter  510  can be unequal to four times of the Center Frequency. 
     As an example, an analog input waveform of 33 MHz is shown in  FIG. 7 . When the 33 MHz analog waveform is sampled at 100 MHz, the resulting samples are shown in  FIG. 8 . In this example, the coefficients of the FIR filter  510  with its Center Frequency at 35.3 MHz are shown in  FIG. 9 , and the filter response of the FIR filter  510  is shown in  FIG. 10 . When the 33 MHz waveform is sampled by interpolation by the FIR filter  510  at 141.176 MHz, the samples that make up a band-limited data stream are obtained.  FIG. 11  depicts the band-limited data stream. 
     With reference again to  FIG. 5 , the band-limited data stream is provided to digital multipliers  514  where it is separated into two data streams, one of which carries in-phase components and the other of which carries quadrature components. The data stream carrying the in-phase components is referred to as the in-phase data stream. Likewise, the data stream carrying the quadrature components is referred to as the quadrature data stream. In the present embodiment, the separation is achieved by multiplying the band limited data stream by the cosine and sine waveforms whose frequency is the Center Frequency of the frequency band to be retained. The cosine and sine waveforms, in the present embodiment, are generated by a sin/cos generator  516  at a data rate of the band-limited data stream. In other words, the cosine and sine waveforms are generated at a rate of four times the Center Frequency. Thus, in the present embodiment, the cosine waveform will include a stream of +10−10+10−10 . . . , and the sine waveform will include a stream of 0+10−10+10−1 . . . . Digital multiplication of the band-limited data stream by the cosine waveform results in a stream of in-phase components, and digital multiplication of the band-limited data stream by the sine waveform results in a stream of “quadrature” components. As an example, the in-phase and quadrature waveforms are illustrated in  FIGS. 12 and 13 . Note that zeros are not output by the digital multipliers  514 . Thus, the data rate of the in-phase data stream  740  and that of the quadrature data stream  750  are approximately half of the data rate of the band-limited data stream  710 . 
     In the present embodiment, the Center Frequency used by sin/cos generator  516  is generated by a numerically controlled oscillator (NCO)  518 . In other embodiments, the cosine and sine waveforms are generated by a look up table in memory or by other computational means. 
     With reference again to  FIG. 5 , digital interpolation filters  520  up-sample the in-phase and quadrature data streams such that their outputs can be decimated accurately by a decimation filter  525  to a desired output rate. In one embodiment, the output rate is generated by an NCO  524 , and decimation is accomplished by only sampling the output of the interpolation filters  520  at the desired output data rate. In one embodiment, the desired output rate is at least twice the bandwidth of the desired frequency band. For example, if the bandwidth of the desired frequency band is 6 MHz, then the desired output rate is at least 12 MHz. 
     The FIR filter coefficients for an example implementation of one of the digital interpolation filters  520  are shown in  FIG. 14 . In this example, the digital interpolation filter  520  in 3 MHz lowpass interpolation filter. The frequency response of a 3 MHz lowpass interpolation filter is shown in  FIG. 15 . Further, in this example, the outputs of the digital interpolation filters  520  are decimated to a sample rate of 17.647 MHz. The decimated in-phase and quadrature data streams are illustrated in  FIGS. 16 and 17 . 
     With reference still to  FIG. 5 , the decimated data streams generated by the decimation filter  525  are then interleaved. The stream of interleaved samples is referred herein as the transport stream. The data rate of the transport stream, therefore, is the sum of the data rates of the decimated in-phase and quadrature streams determined by decimation filter  525 . Then, the transport stream is serialized by a SERDES circuit (not shown) and the resulting serial bit stream is used to drive the optical data transmitter  114  for generating optical signals for transmission to the hub  220 . 
     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 interleaved in-phase and quadrature components of the transmitted signal. At the signal decoder  223 , the transport samples are first deinterleaved by deinterleaving logic  612  to form two separate streams one of which is the decimated in-phase stream and the other is the decimated quadrature stream. Then, the in-phase stream and the quadrature stream are filtered by interpolation filters  614 . In one embodiment, the interpolation filters  614  are implemented in a similar fashion as interpolation filters  520  of the signal encoder  213 . In the present embodiment, the data rate at which samples are read from the outputs of the interpolation filters  614  is set by a NCO (Numerically Controlled Oscillator), which may be unequal to four times of the Center Frequency.  FIGS. 18 and 19  are the upsampled in-phase and quadrature data streams of the example 33 MHz waveform, which are nearly the same as the waveforms of  FIGS. 12 and 13 , differing only by computational errors. Here, the interpolation filters  614  up-sample the in-phase stream and the quadrature stream such that they have a data rate at four times the Center Frequency of the desired frequency band. In other embodiments, the interpolation filters  614  up-sample the in-phase stream and the quadrature stream to sample rates that are not equal to four times the Center Frequency. 
     With reference still to  FIG. 6 , the signal decoder  223  includes digital multipliers  618 ,  619  and sin/cos generator  620  for generating sine and cosine waveforms. As shown, the sin/cos generator  620  receives the Center Frequency from the NCO  622  and generates cosine and sine waveforms at the Center Frequency. Note that the cosine and sine waveforms, in the present embodiment, are generated at a data rate four times the Center Frequency. Thus, in the present embodiment, the cosine waveform will include a stream of +10−10+10−10 . . . , and the sine waveform will include a stream of 0+10−10+10−1 . . . . The in-phase stream is multiplied by the cosine waveform and the quadrature stream is multiplied by the sine waveform. Digital multiplication of the stream in phase by the cosine waveform results in a stream of values with alternating zeros, and digital multiplication of the quadrature stream by the sine waveform results in another stream of values with alternating zeros. 
     The outputs of the digital multipliers  618 ,  619  are added in by digital adder  624  to generate yet another data stream whose data rate is four times the Center Frequency. The upsampled and combined samples of the example 33 MHz waveform are shown in  FIG. 20 . The output of the digital adder  624  is processed by a bandpass interpolation filter  626 , which is constructed similarly to the bandpass interpolation filter  510 . The output of the bandpass interpolation filter  626  is decimated to an output data rate. In the present embodiment, the output data rate, which is defined by NCO  628 , is the Full Rate (e.g., 100 MHz). The 100 mega-sample per second resampled output of the bandpass filter is shown in  FIG. 21  for the example 33 MHz waveform. The digital samples output by the signal decoder  223  are sent to the D/A converter  124  to be converted to an analog signal. The analog signal thus recovered will have signal components within the desired frequency band. For the example 33 MHz waveform, the output of the D/A converter with an analog low pass filter is the recovered analog wave form of  FIG. 22 . 
     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. For instance, in another embodiment, the desired frequency band transmitted encompasses the full bandwidth of the input signal. That is, for a frequency band to be transmitted is 5 MHz to 42 MHz, and the Center Frequency is approximately 22.5 MHz. In other embodiments, the Center Frequency can be any frequency that is below one half of the frequency of the input data stream. 
     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)). However, a person skilled in the art would realize that portions of the present invention can be implemented as computer executable programs executable by a digital signal processor.