Abstract:
A DOCSIS cable modem service can be extended by providing wireless links that connect users that are beyond the physical reach of the system. This may require that the downstream data are transferred over a wireless link to a remote subscriber radio frequency (RF) unit connected to a cable modem that provides the downstream data to the subscriber. Similarly, upstream data are sent from the subscriber cable modem over the wireless link to the wireless hub transceiver, where such data are inserted back to the distribution coax cable. This insertion causes the injection of noise into the DOCSIS cable modem system. Connecting a plurality of such devices can cause noise beyond the system limitations. By using a burst detect system, the RF receiver portion of the device is connected to the DOCSIS cable only when injecting data upstream, thereby reducing the overall noise injection.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application claims priority from U.S. provisional patent application Ser. No. 60/525,365, filed on Nov. 25, 2003, which application is incorporated herein in its entirety by this reference thereto. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Technical Field  
         [0003]     The invention relates generally to noise reduction in DOCSIS cable modem services more specifically, to the invention related to the reduction of noise injection for upstream data transmission from a wireless link to a DOCSIS cable modem service system.  
         [0004]     2. Discussion of the Prior Art  
         [0005]     The delivery of data using a cable television (CATV) system has become common in residential areas where CATV is commonly available. The data are delivered both downstream and upstream using available channels and/or frequencies. Because providers of CATV expect to deliver additional services, such as data for Internet connectivity, there is an interest to serve as large as possible number of clients. However, it is not always possible to provide a direct connection to each location.  
         [0006]     In U.S. patent application Ser. No. 10/463,483, titled “AN APPARATUS AND METHOD FOR EXTENDING DOCSIS CABLE MODEM SERVICE OVER WIRELESS LINKS” (hereinafter “the 483 Application”), assigned to a common assignee of this application, which is hereby incorporated by reference for all that it contains, a wireless connectivity to a CATV data delivery system is shown.  
         [0007]      FIG. 1  illustrates a modified CATV distribution system  100  that uses wireless communication to deliver data to shopping mall  120 . A splitter unit (SU)  310  is connected to the distribution coax cable  115  which, in turn, couples to subscribers  110 , in some cases via cable modems  117 , delivers downstream data to a wireless hub transceiver (WHT)  300 . The WHT  300  provides the SU  310  with upstream data. The WHT  300  uses an antenna  230  to communicate with an antenna  240 , as explained in more detail below. A receiving unit comprised of an antenna  240  and a subscriber radio frequency unit (SRFU)  242  is described in detail in U.S. patent application Ser. No. 10/282,533, titled SYSTEM AND METHOD FOR WIRELESS CABLE DATA TRANSMISSION assigned to a common assignee of this application and hereby incorporated by reference for all that it contains. The SRFU  242  is further connected to a cable modem, thereby enabling a subscriber in the mall  120  to receive data communication through extension of the Data Over Cable Service Interface Specifications (DOCSIS) cable modem service over a wireless link. A more detailed description of an exemplary SRFU  242  is provided below. A person skilled in the art may easily modify such a receiving unit to support a  64  quadrature amplitude modulation (QAM). The SU  310  provides upstream and downstream connectivity to the WHT  300 . In one embodiment, the SU  310  further provides the AC power required for the operation of the WHT  300 .  
         [0008]     The connection of a WHT  300  unit naturally causes the injection of upstream noise into the CATV system. The noise levels allowed on a CATV system, i.e. a signal-to-noise ratio (SNR), is on the order of 30 deciBels (dB). A WHT  300  unit would require a signal to have a challenging 35 dB SNR or higher to be transparent to the CATV system. In a system requiring the placement of a plurality of WHTs  300 , the noise injection in the system would be beyond that allowed by CATV system specifications. Prior art solutions favor the use of switching units on and off or, in other words, connecting and disconnecting the units. This, however, creates spectral spreading because high speed switching transients occur, rendering them at least as problematic. The injection of an unacceptable level of noise by a receiver happens whenever a user attempts to optimize the wireless link and therefore impairs the signal-to-noise ratio of the cable upstream systems. A person skilled-in-the-art would realize that connecting more wireless extension hubs would make the noise situation even worse.  
         [0009]     It would be therefore advantageous to provide an apparatus and a method that allows such a CATV system to operate in the presence of a plurality of wireless devices connected to the system, and particularly to a plurality of WHT  300  units connected to that system.  
       SUMMARY OF THE INVENTION  
       [0010]     A DOCSIS cable modem service can be extended by providing wireless links that connect users that are beyond the physical reach of the system. This may require that the downstream data are transferred over a wireless link to a remote subscriber radio frequency (RF) unit which is connected to a cable modem that provides the downstream data to the subscriber. Similarly, upstream data are sent from the subscriber cable modem over the wireless link to the wireless hub transceiver, where such data are inserted back to the distribution coax cable. This insertion causes the injection of noise into the DOCSIS cable modem system. Connecting a plurality of such devices can cause noise beyond the system limitations. By using a burst detect system the RF receiver portion of the device is connected to the DOCSIS cable only when injecting data upstream thereby reducing the overall noise injection. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]      FIG. 1  is a schematic diagram showing distribution of CATV coax cables in a residential area with a wireless extension;  
         [0012]      FIG. 2  is a schematic block diagram showing a wireless hub transceiver connected to a distribution coax cable in a CATV system;  
         [0013]      FIG. 3  is a schematic block diagram showing a variable gain upstream receiver;  
         [0014]      FIG. 4  is a flowchart showing the steps for the control of noise injection into a CATV system;  
         [0015]      FIG. 5  is a schematic block diagram showing circuit designed in accordance with the invention; and  
         [0016]      FIG. 6  is a schematic block diagram showing a circuit having a noise-floor sampler in accordance with the invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0017]     Reference is now made to  FIG. 2  which is a detailed block diagram of a wireless hub transceiver (WHIT)  300  this is connected through a splitter unit (SU)  310  to a distribution coax cable  115 . The WHT  300  comprises a splitter  320 , an embedded cable modem controller  340 , a downstream channel unit  350 , an automatic gain control  360 , a programmable gain  310 , an upstream channel unit  380 , an up-converter transmitter unit  390 , and a down-converter receiver unit  395 . Both the up-converter and down-converter are connected to an antenna  230 . A DC power unit  330  is optionally connected to a splitter  320  if it is possible to provide AC power from the distribution coax cable  115 .  
         [0018]     Operation of the WHT  300  is performed under the control of embedded cable modem controller  340 . Various control signals are delivered to components of the WHT  300 . The upstream channel unit is controlled for both the center&#39;s Data Over Cable Service Interface Specifications (DOCSIS) upstream frequency (f us ) for wireless operation, as well as for the upstream DOCSIS bandwidth (bw us ) which is selective at doubling steps starting from 200 Hz up to 3.2 MHZ, or 6.4 MHz in the case of DOCSIS 2.0. DOCSIS carriers support frequencies of 50-860 MHz for downstream communication and 5-48 MHz for upstream communication. A programmable gain unit (PGU)  370  is connected between the down-converter receiver unit  395  and the upstream channel unit  380 . The gain is controlled by means of an embedded cable modem controller  395  by providing the upstream gain (gus) parameter. A detailed description of this operation is provided in the &#39;483 application.  
         [0019]     The programmable gain unit  370  is a source of noise that is injected back into the cable modem cables, thus reducing the signal-to-noise ratio (SNR) required for proper operation of the system. Moreover, connecting a plurality of WHT  300  units to distribution coax cable  115  results in levels of noise that are unacceptable. The inventors have noted that the data provided using DOCSIS are in bursts and, hence, an upstream receiver sends data only when such a burst occurs. At all other times the system is affected by the noise generated by PGU  370 . Further, notably, is the fact that DOCSIS allows bursts of data from a single transmitter at a time (TDMA), such that channel use is effective. Therefore, the inventors have discovered that it would be advantageous to detect the presence of a data burst and enable the PGU  370  to provide data to the CATV system only upon presence of such data.  
         [0020]      FIG. 3  is a schematic block diagram of a variable gain amplifier  370  of an upstream receiver. An input signal is received at a low pass filter (LPF)  371  and then fed to both a logarithmic amplifier  372  and to a variable gain amplifier  375 . The output of the logarithmic amplifier  372  is a logarithmic function of the input signal. Therefore, low signals are essentially amplified more than large signals. The output of the logarithmic amplifier  372  is fed to a positive peak detector  373  that is capable of detecting a peak in the signal. This peak is compared by a comparator  374  to a reference value and, if it exceeds that value, it switches the gain of a linear variable gain amplifier (LVGA)  376 . The LVGA  376  receives the signal from the variable gain amplifier  375  that amplifies the input signal to the desired level of operation. The LVGA  375  changes its gain quickly based on a control, theoretically in the range of minus infinity dB to infinity dB, or for practical purposes from a few minus tens of negative dB to a few tens of positive dB. For example, an acceptable range is −70 dB to 14 dB. When the burst detection path, i.e. the logarithmic amplifier  372 , the positive peak detector  373 , and comparator  374 , provides an indication that a data burst is in progress, the gain of the LVGA  376  is switched from the maximum negative dB value to the maximum positive dB value. This means that only a very small, if any, amount of noise is added when there is no data burst to be transferred because the level of amplification is small. However, when there is a data burst, the amplification is increased as may be required to provide a signal per the specifications for the subject CATV cable system. The resultant signal is transferred through a low pass filter (LPF)  377  to ensure that no high frequency signals get through to the CATV cable system.  
         [0021]     Reference is now made to  FIG. 4  which is an exemplary and non-limiting flow chart  400  of a method for the purpose of reducing noise injection during bursts of upstream data into a CATV cable system. In step S 410  an input signal is received. Specifically, this is an upstream data stream received from a receiver, such as the WHT  300 . In step S 420  a channel bandpass filter (CBPF) is applied on the incoming signal, allowing only a signal in the channel to trigger the burst detect. In step S 430 , a burst detection is performed, i.e. it detects whether a data burst actually exists. As explained above, under the DOCSIS specification a receiver receives data in burst of transmission and is on-line only during these burst periods, and otherwise should not interfere with the ability of other transmitters to send data bursts. In step S 440 , it is checked whether a data burst was actually received. If the answer is positive, then execution continues with step S 450  where the gain, for example the gain of the LVGA  376 , is adjusted to provide the maximum allowable amplification to the signal. Otherwise, if the answer is negative, the execution continues with step S 460  where the gain, for example the gain of the LVGA  376 , is adjusted to provide the minimum allowable amplification to the signal. By using one of these two gain settings, it is possible to provide the signal to a CATV cable system with the minimum noise level, thereby maintaining the required SNR of the system. In step S 470 , a low pass filter is applied to the resultant signal to avoid transmission of any high frequencies that may have been added to the signal during the processing.  
         [0022]     Referring now to  FIG. 5 , an exemplary and non-limiting schematic of a circuit  500  comprised in accordance with the disclosed invention is shown. For clarity purposes, certain components of the circuit are boxed to indicate their functionality in respect to the block diagram of  FIG. 3 , discussed above. The low pass filters  501  and  507  correspond to LPF  371  and LPF  377 , respectively. Both are comprised of passive components, such as resistors, capacitors, and inductors. The logarithmic amplifier  502  corresponds to the logarithmic amplifier  372 , and is comprised of a standard logarithmic amplifier, such as the Analog Devices&#39; AD8307 component. The peak detector  503  corresponds to the peak detector  373 , and comprises an operational amplifier, diodes, and a passive integrator in the form of, for example, a 1,000 picoFarad capacitor. This comparator  504  appears in the schematic of the circuit in two parts, i.e.  504 -A and  504 -B, which together correspond to the comparator  374 . Specifically the circuit  504 -B shapes the signal as it goes through the linear variable gain amplifier (LVGA)  506 , in a fashion that is compatible with the burst width of a DOCSIS upstream signal. The variable gain amplifier  505  corresponds to the variable gain amplifier  375 , and may be implemented using an amplifier, such as Maxim&#39;s MAX3514 CATV upstream amplifier. The LVGA  506  corresponds to the LVGA  376 , and may be implemented through the use of National Semiconductor&#39;s LMH6503 variable gain amplifier.  
         [0023]     In another embodiment of the invention it may be desirable to compensate for environmental changes, such as temperature, and its effect on system gain. A person skilled in the art would note that noise levels in a system, especially in a system exposed to the elements, is at least affected by the ambient temperature. As a result, the system noise levels may increase or decrease. For this purpose an analog-to-digital converter (ADC) is used as a noise floor sampler to sample the noise floor levels over a period of time, for example every 10 milliseconds, which corresponds to a rate of 100 samples per second. The samples are averaged over a fixed number of samples, such as  64 , and the average is used as the current base noise level of the system. Such a sampling occurs while there is no other signal in the system, i.e. in between bursts. After the digital processing, the result is then sent to a digital-to-analog converter (DAC) such as TLV5637, shown as U26 of block  504 -A, and then applied to the comparator  374 .  
         [0024]     Reference is now made to  FIG. 6 , which is an exemplary and non-limiting schematic diagram  600  of a circuit having a noise floor sampler  610 . The noise-floor sampler  610  samples the noise-floor while no burst is present. The output of noise-floor  610  is connected to the comparator  374  in lieu of the reference voltage discussed in connection with  FIG. 3  above. This allows the comparator to adjust itself in response to changes in the noise-floor that result from the environment in which the circuit is placed, for example, in response to changes in temperature.  
         [0025]     The values in the text and figures are exemplary only and are not meant to limit the invention. Although the invention has been described herein with reference to certain preferred embodiments, one skilled in the art will readily appreciate that other applications may be substituted for those set forth herein without departing from the spirit and scope of the present invention. Accordingly, the invention should only be limited by the claims included below.