Cable modem optimized for high-speed data transmission from the home to the cable head

A cable modem provides upstream data signals in a cable system on a return channel. The upstream data signals are provided in a 50-550 MHz frequency range and yet do not affect the picture quality associated with conventional cable television signals. The data is provided on vestigial sidebands associated with the cable television signals or during black periods associated with the cable television signals. The data can be modulated in accordance with quadrature amplitude modulation (QAM) techniques.

FIELD OF THE INVENTION 
The present invention relates to cable communication systems. More 
particularly, the present invention relates to high-speed transmission of 
data from a home or residence to a central office or cable-head end. 
BACKGROUND OF THE INVENTION 
Conventional cable systems can utilize a cable modem to transmit and to 
receive data across a coaxial cable or line. The cable couples a head end 
or central office to numerous pieces of video equipment disposed in a 
multitude of residences. 
Cable systems are generally utilized as one-way systems in which cable 
television signals are transmitted from the head end to the video 
equipment coupled to the cable. The video equipment can include video 
cassette recorders (VCRs), television units, or other devices. The cable 
television signals are transmitted in a frequency range from approximately 
50 megahertz (MHz) to 550 MHz. The video equipment receives the cable 
television signals and either provides images in accordance with the cable 
television signals or stores the cable television signals. 
Cable modems can be utilized with existing cable systems to communicate 
data bidirectionally between the home and the head end. The cable modem 
transmits data from the home to the head end in a frequency range between 
approximately 5-39/42 (e.g., on a return channel). Generally, the return 
channel can utilize a 5-39 MHz range (low split) or a 5-42 MHz range (mid 
split). The cable modem receives data from the head end in a frequency 
range between 550-750 MHz (e.g., the downstream channel). The high-speed, 
downstream channel is often utilized to provide digitized services, such 
as, digitized entertainment to the residence, or to provide other data 
communications to the residence. 
The return channel is typically utilized to transmit small amounts of data, 
such as, requests for download information, while the downstream channel 
is utilized to transmit large amounts of data, such as, data associated 
with video signals, large programs, documents, or other applications. For 
example, in most internet applications, larger amounts of data are 
requested by the residential user than by the internet source. Further, in 
pay-per-view movie applications, the return channel is utilized to 
transmit relatively small data files, including billing and addressing 
information, while the downstream channel is utilized to transmit the 
relatively large data file, including the movie. 
The conventional return channel is not adequate for all cable modem 
applications, especially applications in which larger amounts of data must 
be transmitted to the head end. Heretofore, the return channel associated 
with conventional cable modems can be susceptible to interference from a 
number of other radio frequency (RF) sources, including amateur radio 
units and household motors, which provide distortion in the 5-39/42 MHz 
range. Additionally, the bandwidth associated with the conventional return 
channel (e.g., 5-39/42 MHz) is somewhat limited, thereby restricting the 
amount of data which can be sent to the head end. 
Since the frequency range between 50-550 MHz is utilized by all television 
sets coupled to the cable, it cannot conventionally be used for return 
channel applications. Indeed, the reception of all television sets would 
be affected by any signal placed in the frequency band between 50-550 MHz. 
Accordingly, conventional cable modems must transmit and receive data 
outside of the 50-550 MHz range to protect the reception of the large 
number of customers who are already committed to this aspect of the cable 
system. Thus, data cannot normally be transmitted upstream in the 50-550 
MHz frequency range because of the multi-drop nature of the cable system. 
Thus, there is a need for a high-speed return channel for cable modems. 
Further still, there is a need for more efficient use of the frequency 
band between 50-550 MHz in a cable system. 
SUMMARY OF THE INVENTION 
The present invention relates to a transmitter for use with a cable system 
including a cable. The cable is coupled between the transmitter and a 
cable-head end. The transmitter includes a terminal and a data modulator 
coupled to the terminal. The data modulator provides modulated data 
signals in a frequency band between 50-550 MHz across the cable, wherein 
the modulated data signals are transmitted so as not to interfere with 
reception of cable television signals. 
The present invention further relates to a cable modem including a receiver 
means and a transmitter means. The receiver means receives analog 
television signals in a frequency range between 50-550 MHz. The 
transmitter means transmits data signals on a vestigial sideband of 
carrier waves between the 50-550 MHz frequency range. The transmission of 
the data signals does not adversely affect the picture quality associated 
with the analog television signals. 
The present invention still further relates to a transmitter for use in a 
cable system. The transmitter provides data signals between 50-550 MHz on 
a return channel to a cable-head end. The transmission of the data signals 
does not adversely affect the picture quality associated with analog 
television signals in the 50-550 MHz frequency range. 
According to one exemplary aspect of the present invention, a cable modem 
provides data signals outside of the conventional return channel frequency 
range between 5-39/42 MHz. The return channel utilizes a black period 
associated with cable television signals or a vestigial sideband 
associated with cable television signals to transmit data in the 50-550 
MHz frequency range. The return channel advantageously does not adversely 
affect the reception of cable television signals in the 50-550 MHz 
frequency range.

DETAILED DESCRIPTION OF PREFERRED EXEMPLARY EMBODIMENTS 
With reference to FIG. 1, a cable system 12 includes central office or a 
head end 14, a fiber optic cable 16, a fiber node 18, a coaxial cable 20, 
and a multitude of residences 22. Cable 20 is preferably an insulated 
coaxial cable of the type utilized by cable systems which provide cable 
television signals to residences 22. Fiber optic cable 16 couples head end 
14 to fiber node 18. Fiber node 18 is coupled to cable 20. 
Head end 14 can also be coupled to a satellite cable television antenna 28, 
an asynchronous transfer mode (ATM) network 30, a public switched 
telephone network (PSTN) 32, or other services. Head end 14 transmits and 
receives signals to antenna 28, network 30, and network 32. Head end 14 
preferably receives cable television signals from antenna 28 and provides 
cable television signals across fiber optic cable 16 to fiber node 18. 
Fiber node 18 provides the cable television signal across cable 20 to 
residences 22. The cable television signals are analog signals within a 
frequency range approximately between 50-550 MHz. 
Additionally, head end 14 can receive cable television signals from 
networks 30 or 32. Head end 14 can also receive data from network 30 and 
32 and provide the data across cables 16 and 20 to residences 22. The data 
can include digitized services, digital entertainment, pay-per-view 
movies, or other information. For example, ATM network 30 can be coupled 
to the internet or worldwide web and information can be transmitted to and 
from the internet or worldwide web via cable 20 and cable 16. 
With reference to FIGS. 1 and 2, a cable modem 50 can be utilized in system 
12 to communicate information across cable 20. Preferably, cable modem 50 
is provided within residences 22. Cable modem 50 includes a data input 52, 
a data output 54, a modulator 56, a demodulator 58, and a terminal 60. 
Terminal 60 is a terminal preferably coupled to cable 20. 
Cable modem 50 advantageously transmits data or digital information 
upstream (e.g., from residence 22 to head end 14) in the frequency range 
approximately between 50-550 MHz. In contrast to conventional modems which 
utilize a return channel in the frequency range between approximately 1-50 
MHz, cable modem 50 utilizes the same frequency range for the return 
channel that is used for the cable television signals provided from head 
end 14 to residences 22. Data or digital information (e.g., modulated data 
signals) received by modem 50 from head end 14 across cable 20 is provided 
through demodulator 58 to output 54. The received data is preferably 
provided in the 550-750 MHz frequency range (e.g., on the downstream 
channel). 
Cable modem 50 is advantageously arranged so modulated data signals 
transmitted within the 50-550 MHz frequency range do not interfere with 
the picture quality associated with the cable television signals. 
Preferably, cable modem 50 includes a modulator, such as, modulator 56, 
which is configured so that the picture quality associated with the cable 
television signals received by residences 22 is not affected by data 
transmitted upstream along cable 20. Preferably, modulator 56 is arranged 
as a vestigial sideband modulator. 
With reference to FIG. 3, an exemplary bandwidth spectrum of a cable 
television signal 80 is shown. Cable television signal 80 has a bandwidth 
of 6 MHz and is within the 50-550 MHz frequency range. The 6 MHz bandwidth 
represents a television channel. 
A picture carrier frequency 82 is provided approximately 1.25 MHz from a 
starting frequency 84 associated with the channel. Information indicative 
of the picture associated with cable television signal 80 is modulated 
using a single sideband technique, wherein the picture information is 
contained between picture carrier frequency 82 and a frequency 
approximately 5.75 MHz above starting frequency 84. At a frequency 86, 
sound information can also be provided. 
Cable television signal 80 is transmitted utilizing a single sideband 
technique, wherein only one sideband contributes to the recovery of the 
information encoded on the cable television signal. Modulator 56 (FIG. 2) 
preferably utilizes a vestigial sideband 92 associated with cable 
television signal 80. Vestigial sideband 92 is in a frequency range 
between 0.5-1.25 MHz above frequency 84. Vestigial sideband 92 preferably 
has a bandwidth of 0.75 MHz and does not interfere with other television 
channels since it is greater than frequency 84. Additionally, vestigial 
sideband 92 should not interfere with its own channel associated with 
signal 80 because it is below carrier frequency 82. 
Modulator 56 (FIG. 2) preferably utilizes quadrature amplitude modulation 
(QAM) to provide modulation in vestigial sideband 92. Alternatively, 
modulator 56 could utilize phase shift key (PSK) modulation, frequency 
shift key modulation (FSK) or other types of modulation. Therefore, modem 
50 (FIG. 2) is able to utilize the 50-550 MHz frequency range to transmit 
information upstream on cable 20 by utilizing vestigial sideband 92. 
Utilizing the vestigial sideband will not provide an adverse effect on 
picture quality associated with cable television signal 80. Modem 50 can 
use a vestigial sideband similar to sideband 92 on any channel between the 
50-550 MHz frequency range. With reference to FIG. 4, modem 50 includes a 
transmitter 100, a receiver 102, and a hybrid circuit 128. Transmitter 100 
includes a band pass filter 104 and modulator 56. Receiver 102 includes a 
band pass filter 106 and demodulator 58. Modulator 56 is a QAM modulator 
and includes a scrambler 110, a bit-to-symbol mapper 112, an in-phase 
pulse generator 114, a quadrature pulse generator 116, a base band 
transmit filter 118, a base band transmit filter 120, a mixer 122, a mixer 
124, and a summer 126. 
Data is provided at input 52 through self-synchronizing scrambler 110. The 
scrambled data is provided to bit-to-symbol mapper 112, which provides 
symbols to pulse generators 114 and 116. Pulse generators 114 and 116 
provide quadrature pulse signals through base band transmit filters 118 
and 120, respectively, to mixers 122 and 124. Mixers 122 and 124 provide 
up-converted signals to summer 126. Mixers 122 and 124 provide the 
up-converted signals in a vestigial sideband, such as, sideband 92. Summer 
126 provides the modulated data signals (e.g., the summed, up-converted 
signals) in a vestigial sideband associated with cable television signals 
provided on cable 20. 
The modulated data signals are provided through band pass filter 104 and 
then hybrid circuit 128 to cable 20. Signals received on cable 20 are 
provided through hybrid circuit 128 and band pass filter 106 to 
demodulator 58. 
Demodulator 58 includes a mixer 130, a mixer 132, an in-phase equalizer 
filter 134, a quadrature equalizer filter 136, a slicer 138, a 
symbol-to-bit map 140, and an unscrambler 142. The modulated data signals 
are provided to mixers 130 and 132 and are down-converted. The 
down-converted signals are provided through equalizers 134 and 136 to 
slicer 138. Slicer 138 provides symbols representative of the modulated 
signals to symbol-to-bit mapper 140. Symbol-to-bit mapper 140 provides bit 
signals to unscrambler 142, which provides unscrambled data at output 54. 
The modulated data signals provided to mixers 130 and 132 are in the 
550-750 MHz frequency range. 
With reference to FIG. 5, a cable modem 250 in accordance with another 
exemplary embodiment, is shown. Cable modem 250 is similar to modem 50. 
However, cable modem 250 includes a black region modulator 256 instead of 
a vestigial sideband modulator 56. Modem 250, like modem 50, provides 
modulated data signals to cable 56 in the 50-550 MHz frequency band 
without interfering with the picture quality associated with the cable 
television signals. 
Black region modulator 256 is coupled to a timing reference circuit 258, 
which is also coupled to terminal 60. Timing reference circuit 258 
determines the black signal time periods associated with channels on the 
cable television signals. Black signal time periods are time periods when 
no information is provided on the cable television signal to accommodate a 
retrace operation associated with the cathode ray tube (CRT). During a 
retrace orientation, the cable television signal turns off the electron 
beam associated with the CRT so the retrace operation cannot be seen by 
the viewer. 
The retrace operation can occur at the end of a horizontal trace or scan of 
the electron beam. At the end of the horizontal trace, the electron beam 
must fly back and start a next horizontal trace. A retrace operation can 
also occur when the scan reaches the bottom of the screen associated with 
the CRT, and the electron beam has to come back to the top of the screen. 
During these retrace time periods, the cable television signal is driven 
into a black region, which essentially turns off a transmission of the 
electron beam so the retrace operation cannot be seen on the screen. 
Black signal modulator 256 is optimized to operate during the retrace time 
period to provide modulated data signals. The modulated data signals do 
not affect the cable television signals because they are occurring during 
retrace or flyback times (e.g., black regions of the cable television 
signals). The cable television signals are over-driven during the black 
regions so that the electron beam is totally turned off. Although the 
signal is over-driven, data still can be modulated on top of the 
over-driven signal. The data can be modulated with QAM, FSK, PSK, or other 
techniques. The use of black periods to include data has been utilized in 
other non-cable modem applications by companies, such as, "Datacast", to 
transmit data from a central source to computers at a relatively high 
rate. 
With reference to FIG. 1, modems in head end 14 or fiber node 18 can 
utilize echo-canceling techniques to remove the transmitted cable 
television signals between 50-550 MHz frequency range to obtain the 
modulated information, whether it be on a vestigial sideband or in a black 
region of the cable television signals. Echo-canceling techniques used in 
order to transmit and to receive data on a single medium, are well-known 
in the art. 
It is understood that, while the detailed drawings and specific examples 
given describe preferred exemplary embodiments of the present invention, 
they are for the purpose of illustration only. The method and apparatus of 
the present invention is not limited to the precise details and conditions 
disclosed. For example, although particular modulation and demodulation 
circuitry is described, other types of modulation and demodulation 
techniques can be utilized. In addition, modem 50 can be configured to 
utilize both the black region and the vestigial sideband, as well as, 
transmit in the 5-39/42 MHz frequency range, thereby maximizing the amount 
of data which can be transferred. Further, single lines in the drawings 
can represent multiple conductors. Thus, changes may be made to the 
details disclosed, without departing from the spirit of the invention 
defined by the following claims.