WO2002017636A1

WO2002017636A1 - Transceiver applications in hfc networks

Info

Publication number: WO2002017636A1
Application number: PCT/EP2001/009393
Authority: WO
Inventors: Marcel F. C. Schemmann; Ventakesh Mutalik; Arnold K. Jansen Van Doorn
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2000-08-24
Filing date: 2001-08-14
Publication date: 2002-02-28
Also published as: JP2004507940A; EP1314318A1

Abstract

An HFC node and network with overlay architecture uses a transceiver. The transceiver is connected to a downstream signal that can sustain relatively high levels of distortion. This will generally be the quadrature amplitude modulated (QAM) signal that carriers digital television signals. The transceiver is also connected to an upstream signal that comes from consumers' homes and that contains data to be sent to the CAble TeleVision (CATV) provider. A second forward receiver receives a downstream signal that has more stringent requirements for distortion. This will generally be the 'normal' analog television signal, called an amplitude-modulated, vestigial side band (AM-VSB) signal. The two downstream signals are overlaid by an overlay coupler and then sent to consumers' homes. In general, because the QAM signal received by the transceiver, contains relatively high levels of distortion, the transceiver is connected to a filter that reduces the distortion before the QAM signal is combined with the AM-VSB signal.

Description

Transceiver applications in HFC networks

This invention generally relates to Hybrid Fiber Coax (HFC) networks and more specifically relates to using transceivers in HFC networks.

Cable television providers (CATN) are continually trying to broaden the services they offer to the consumer. For instance, CATV providers are trying to add additional channels of television and allow the consumer to send information upstream, to the CATN provider. This upstream information can range from requests for movies to highspeed internet access. Additionally, CATN providers are also beginning to deploy digital television signals and even are offering cable phone services.

With all of this information streaming upstream (to the CATN provider) and downstream (to the consumer), CATV providers have had to upgrade their aging, one-way coaxial cable systems. The primary upgrade currently being used and installed is called a "Hybrid Fiber Coax" (HFC) network. This network allows very fast, high bandwidth access to and from consumers' homes.

An example of an HFC network is shown in Fig 3. HFC network 300 of Fig. 3 comprises a head end 310, a fiber optic link 290, and a node 220. Head end 310 receives electrical signals from a cable television provider, converts these signals to light, and transmits these signals through fiber optic link 290 to node 220. HFC node 220 then converts the light signals from fiber optic link 290 to electrical signals. The electrical signals are added and sent to consumers' homes through composite signal 183. Additionally, some current CATV HFC networks are bi-directional, allowing consumers to transmit data to the CATV provider. This data is usually internet information telephone signals or requests for movies, but could be any type of information. Node 220 accepts this electrical signal, converts it to light and couples this to the head end, through fiber optic link 290.

To facilitate these objectives, head end 310 comprises two forward transmitters 140, 150 that accept data from head end electrical signals 105, 106, respectively. The forward transmitters are electro-optic devices that use electrical signals 105, 106 to modulate light. These modulated light signals 141, 244 are coupled to fiber optic link 290. Any suitable medium may be used to couple the signal from head end 310 to node 220. Because of this, the signals themselves, and not the medium in which they are being transmitted, will be discussed herein. Generally, there will be one or more splitters that send the signal to other nodes. For instance, in Fig. 3, modulated light signal 141 is split by splitter 130 into a downstream light signal 131 that is coupled to node 220 and another light signal 132 that is coupled to another node (not shown). Similarly, modulated light signal 244 is split by splitter 133 into a downstream light signal 234 that is coupled to node 220 and another light signal 235 that is coupled to another node (not shown).

Downstream light signals 131, 234 enter node 220 and are coupled to forward receivers 121, 222, respectively. These receivers convert the light back to electrical signals, called downstream electrical signals 125 and 223. The two downstream electrical signals 125 and 223 are combined by coupler 180. Coupler 180 combines the two signals into a combined signal 183.

Downstream signals 105, 141, 131, and 125 are usually Amplitude Modulated, Vestigial Side Band (AM-VSB) signals, which carry the well-known analog television channels. AM-VSB signals usually exist in a frequency band lower than 550 MegaHertz (MHz). Downstream signals 106, 244, 234, and 223 are also generally amplitude modulated. However, the modulation used is a particular flavor of Quadrature Amplitude Modulation (QAM) that meets the digital television standard. This is the digital television signal that can contain a number of digital television channels, and even other information, if desired. The QAM signal generally exists in a frequency band above 550 megahertz. Because these two signals inhabit two different frequency bands, coupler 180 can add the two signals without overlap. This HFC system is commonly known as having "overlay architecture" because both QAM and AM-VSB are on the same cable.

Combined signal 183 is then coupled, usually through a coaxial cable, to a consumer's home (not shown). In general, combined signal 183 will be coupled to several houses through a trunk or other distribution system (not shown).

Similarly, data from several homes will be gathered and potentially frequency multiplexed, to become upstream electrical signal 124. Return transmitter 226 converts upstream electrical signal 124 to upstream light signal 237, which can be joined during its travel with other upstream light signals. For example, in Fig. 3, splitter 136 (used as a coupler in this example) combines upstream signals 238 and 237 into upstream light signal 247. Return receiver 160 in head end 310 converts the light to a head end upstream electrical signal 107.

While HFC network 300 is an exciting system in terms of its potential to bring a wide variety of services to the consumer, the network is also relatively expensive. The number of discrete elements that make up the network is quite large, especially when one considers the staggering numbers of these devices necessary to provide this type of high bandwidth access to a large portion of the population of the world. Even a relatively trivial cost reduction in the parts that make up the network will substantially reduce deployment costs.

Thus, while it is desired that the deployment and use of HFC networks occur quickly due to the variety of additional services and programming offered by these networks to consumers, the cost associated with these networks can be prohibitive. Without an improvement that reduces the cost of deploying these networks, CATV providers will delay installation of these networks until the underlying parts can be purchased. In addition, consumers will, in the end, pay for these increased costs.

According to the present invention, an HFC node and network with overlay architecture uses a transceiver. The transceiver is connected to a downstream signal that can sustain relatively high levels of distortion. This will generally be the quadrature amplitude modulated (QAM) signal that carriers digital television signals. The transceiver is also connected to an upstream signal that comes from consumers' homes and that contains data to be sent to the CAble Television .(CATV) provider. A second forward receiver receives a downstream signal that has more stringent requirements for distortion. This will generally be the "normal" television signal, called an amplitude-modulated, vestigial side band (AM- VSB) signal. The two downstream signals are combined by a coupler and then sent to consumers' homes.

In general, because the internal structure of transceivers causes reflections, cross-talk, and other distortions, the QAM signal, after being converted to an electrical signal, can contain relatively high levels of distortion. To reduce this distortion, the transceiver's electrical signal output is connected to a filter that reduces the distortion in particular frequency bands before the QAM signal is combined with the AM-VSB signal.

Using a transceiver is a significant cost savings, as transceivers contain both a receiver and a transmitter in one small package. Furthermore, there is less cabling costs, as transceivers send and receive on the same cable. This should reduce the overall cost of deploying and upgrading HFC networks. Additional cost savings can be attained by using another transceiver at the head end of the CATV provider.

The foregoing and other features and advantages of the invention will be apparent from the following more particular description of preferred embodiments of the invention, as illustrated in the accompanying drawings. The preferred embodiments of the present invention will hereinafter be described in conjunction with the appended drawings, where like designations denote like elements, and:

Figs. 1 and 2 are block diagrams of an HFC system with overlay architecture in accordance with preferred embodiments of the present invention; and Fig. 3 is a block diagram of a prior art HFC system.

A transceiver is a device that contains both a receiver and a transmitter. As such, it generally is cheaper than independent receivers and transmitters due to having one single case, smaller runs of cable, relatively easier fabrication, less parts, etc. Transceivers for mixed electrical and optical systems, however, typically have a much more complex internal structure than independent receivers and transceivers. In a mixed electrical and optical system, a transmitter is an electro-optic device, such as a laser diode or light-emitting-diode, that converts an electrical signal to a modulated light wave. A receiver in this mixed system is also an electo-optical device that then receives the light wave and converts the light wave to an electrical signal. Because of the complex internal structure of transceivers, they are seldom used in mixed electrical and optical systems, if such systems need analog-grade quality (such as those systems using Amplitude Modulated, Vestigial Side Band Modulation). The internal structure of transceivers causes reflections, cross-talk, and distortions, such that a system must be robust in order to support the anomalies caused by transceivers. Returning to Fig. 3 for a moment, signals 105, 141, 131, and 125 generally carry Amplitude-Modulated, Vestigial Side Band (AM-VSB) signals. These signals carry the well-known, analog television signals, and occupy frequency ranges below about 550 MHz. This type of signal poses stringent demands on reflections, cross talk, and distortions. Because the signal is amplitude modulated, these anomalies can cause significant signal degradation in manners known to those skilled in the art. The low distortion tolerance of the AM-VSB signals does not allow a transceiver to be used in place of forward receiver 121 (and return transmitter 226), as the distortions caused by the transceiver would cause discernable imperfections in the AM-VSB signal. This would allow consumers watching their televisions to "see" the distortions, as the distortions cause errors in the television signals.

Signals 106, 244, 234, and 223 carry Quadrature Amplitude Modulated (QAM) signals that conform to the digital television standard and that broadcast digital television signals and other information. In particular, the Multimedia Cable Network System Partners' (MCNS) Data Over Cable System Interface Specification (DOCSIS) is a standard for cable modems. This standard allows 64 and 128 QAM (International Telecommunication Union Annex B with variable interleaving ), a carrier spacing of 6 MHz, data rates of 27 or 36 megabits per second (Mbps), MPEG-2 (Motion Picture Experts Group) framing, Forward Error Correction (FEC) using Reed Solomon codes, and encryption using Data Encryption Standard (DES). QAM is more distortion tolerant than is AM-VSB. Previously, it was thought that replacing forward receiver 222 (and return transmitter 226) with a transceiver would cause too much distortion and severe degradation of the QAM signal.

Thus, node 220 of Fig. 3 uses separate forward receivers 121 and 222 and return transmitter 226. Because these receivers and transmitters do not have the more complex structure of a transceiver, less emphasis must be placed on reducing distortion in network 200. For instance, transmitters 140, 150, and 226 will usually transmit data that varies slightly about a frequency. Ideally, only amplitude modulation should occur in the light waveform emitted by the transmitters. However, some small amount of variation in frequency also occurs. This variation in frequency may be described as a small amount of frequency modulation called chirping.

When this less-than-ideal light signal reaches receivers 121, 222, and 160, the receivers can limit this effect if they operate linearly with respect to the frequency changes. Because the optics of independent receivers tends to be excellent, the receivers can and do minimize or eliminate the non-ideal frequency modulation.

For transceivers, which have a complex internal optical structure, the transceiver's response tends to be wavelength dependent and non-linear. When the transceiver converts the slightly frequency-modulated light to an electrical signal, the wavelength dependency causes distortions that degrade the signal. Thus, it was previously thought that using transceivers in HFC network 200 was not possible. However, in accordance with the current invention, it is possible to use a transceiver in an HFC network.

Upstream signal 124 also usually meets an MNCS DOCSYS specification. This specification currently states that upstream signal 124 will use Quadrature Phase Shift Key (QPSK) modulation or 16 QAM modulation, a carrier frequency that is variable between 200 KiloHertz (KHz) and 3.2 MHz, a data rate between 320 Kilobits per second (Kbps) and 10 Mbps, FEC that uses Reed Solomon codes, and DES encryption.

Turning now to Fig. 1, an HFC network configured in accordance with the current invention is shown. Some of the devices shown in Fig. 1 have already been described in reference to Fig. 3; as such, only minor introductions will be discussed here for those devices that have already been introduced. HFC network comprises a head end 110, a fiber optic link 190, and a node 120, which connects to consumer's homes through combined signal 183. Head end 110 comprises a forward transmitter 140 that converts electrical signal 105 into modulated light signal 141. This light signal travels through fiber optic link 190, potentially being diverted at splitter 130, to become light signal 131 and 132.

Light signal 131 is then coupled to forward receiver 121, which converts the modulated light to an electrical signal 125. Signals 105, 141, 131, 132 and 125 generally contain AM-VSB signals. These signals are usually contained within the 50 to 550 MHz range. HFC network 100 also comprises, at head end 110 of the CATV provider, a forward transmitter 150 and a return receiver 160. Downstream electrical signal 106 is coupled to forward transmitter 150 and is generally a QAM signal, which is usually contained within the 550 to 750 MHz frequency band. Upstream electrical signal 107 is coupled to return receiver 160. Forward transmitter 150 converts downstream electrical signal 106 to downstream light signal 156. Return receiver 160 is coupled to upstream light signal 157. Downstream light signal 156, which is generally a QAM signal, and upstream light signal 157 are coupled to and combined by coupler 158 to produce light signal 144. Light signal 144, thus, has both a downstream signal and an upstream signal.

Illustratively, these two signals operate on two different wavelengths. For instance, forward transmitter 150 could generate a modulated light signal that has a 1550 nanometer (nm) wavelength, while return receiver 160 could receive a light signal that has a 1310 nm wavelength. Although it is theoretically possible to transmit and receive at the same wavelength, it is easier to transmit and receive if the transmission and reception frequencies are different. Coupler 158 couples both light signals 156 and 157 onto light signal 144. Therefore, light signals 144, 135, and 147 comprise two portions: one portion that contains a downstream digital television signal, and one portion that contains an upstream signal from one or more consumers' homes.

Coupler 158 may be a device that couples both signals together, without modification. In general, however, the coupler will be a wavelength division multiplexer that multiplexes signals 156 and 157 onto (or from) signal 144. Thus, signal 156 originates in forward transmitter 150 and is coupled onto signal 144 by coupler 158. Meanwhile, the upstream signal portion on signal 144 is wavelength demultiplexed by coupler 158 and placed on signal 157. Light signal 144, in fiber optic link 190, may be split to add additional signals or to receive additional signals. In the example of Fig. 1, light signal 144 has a splitter 133 that generates two light signals 135 and 147. Light signal 135 would go to another node for another group of homes, for instance. Light signal 147 is coupled to transceiver 122.

Transceiver 122 receives the downstream signal portion of light signal 147 and converts this light signal to downstream electrical signal 123. The transceiver converts the downstream signal portion of light signal 147 by demodulating the light signal that is centered at a wavelength of about 1550 nm in this example. Transceiver 122 also converts upstream electrical signal 124 to a modulated light signal that is centered at about a wavelength of 1310 nm in this example. This modulated light signal is coupled onto light signal 147, which has a downstream light signal portion (from forward transmitter 150) and an upstream light signal portion (from the transmitter portion of transceiver 122).

Electrical signal 123 is coupled to filter 128, which filters signal 123 to create filtered signal 126. Filtered signal 126 is coupled to coupler 180. Coupler 180 combines the AM-VSB electrical signal 125 with the filtered QAM signal 126 into combined signal 183. Not shown in Fig. 1, but most likely existing, are any amplifiers used to increase the signal power of electrical or light signals. Other devices, such as trunks to connect multiple houses to electrical signal 224 or light signals 132, 135, and 138 are not shown. Fig. 1 is a simple example used to illustrate important features of the current invention, but the Fig. is by no means all-inclusive. Ideally (ignoring the distortion caused by transceiver 122), electrical AM-VSB signal 125 does not overlap electrical QAM signal 126. In this manner, the two signals can be added without regard to interference. The term "does not overlap" means that the electrical AM-VSB and QAM signals are assigned separate frequency ranges such that there will be minimal or no interference between the two signals. However, those skilled in the art will realize that it is very hard not to have some minor amount of overlap (as measured by the power of one signal at a particular frequency that is in the other signal's assigned frequency range). Thus, the term "does not overlap" encompasses some minor, non-error-causing amount of overlap between the two signals. Note also that, because of the transceiver's distortions, electrical QAM signal 123 will have relatively significant distortion power in the AM-VSB frequency range (this is explained in more detail below). Thus, there will be overlap between the two signals. However, filter 228 should reduce this amount of overlap to a manageable, and non-error- causing level (also to be explained below).

Electrical signal 124 comes from one or more homes and generally carries relatively low frequency information, such as requests for movies, voice, or internet requests or uploads. As discussed previously, transceiver 122 uses this electrical signal to modulate light signal 147 at a particular wavelength. Transceiver 122 has a response that is wavelength dependent because of the complex internal structure of the transceiver. The wavelength dependency means that minor fluctuations in wavelength, due to the transmitter's small frequency fluctuations, cause distortions. Furthermore, there is usually some cross-talk between the receiver and transmitter portions of the transceiver. Additionally, the complex optical structure of the transceiver tends to create more reflections than separate receivers would.

The primary distortion mechanism for transceivers in HFC applications tends to be the wavelength dependency of the transceivers. This wavelength dependency causes distortions that have orders, as is well known in the art. The second-order distortion tends to be a very strong distortion that falls outside the 550 to 750 MHz band. That the relatively strong second-order distortion falls outside the 550 to 750 MHz frequency band is a fortuitous consequence of this frequency band's being inside one octave. Were the frequency band broader, the second-order distortion may fall inside the frequency range. Thus, the width of the digital television (QAM) frequency band is such that distortions that have the highest power are outside the digital television frequency band. The third-order distortion also tends to fall outside the band, but also has components inside the 550 to 750 MHz frequency band. The third-order distortion is smaller than the second-order distortion, but still can be somewhat problematic. Because of their relatively low power, fourth and higher- ordered distortions cause few if any problems in the QAM signal, although they may cause errors in the analog television (AM-VSB) signal if allowed to be joined to the analog television signal without filtering.

Because electrical signal 123 (and its corresponding light signals 144, 147 and 156) carry QAM, the problems associated with the third-order distortions that fall within the 550 to 750 MHz frequency band are reduced. QAM has a higher immunity to this type of noise than does AM-VSB. Because the third-order distortions are generally of a low power and the QAM has a somewhat high noise immunity, the signal degradations caused by the distortions in the QAM signal do not significantly affect the QAM signal. This allows the transceiver to be used in HFC network 200, yet produce distortions that affect the QAM signal but not to the extent that there are significant symbol errors. Thus, transceiver 122 does cause distortions in electrical QAM signal 123, but the distortions are relatively minor. There are, however, powerful second-order and less powerful third-order (and fourth-order, etc.) distortions that are not within the 550 to 750 MHz frequency range. In particular, many of these distortions have power at frequencies below the 550 MHz range, which is in the AM-VSB frequency range. If electrical QAM signal 123 is joined to electrical AM-VSB signal 125 by coupler 280 without any action taken to reduce or eliminate these distortions, the distortions will cause significant errors on the portion of combined signal 183 that contains the AM-VSB signal.

As stated previously, the AM-VSB signal is much less resistant to noise than the QAM signal is. Another way of saying this is that the AM-VSB signal has more stringent noise requirements than does the QAM signal. Furthermore, the second order distortion that is in the AM-VSB frequency band can be a high-power distortion. Even QAM, with its relatively high resistance to noise, would likely be affected by the second-order distortion, if the second-order distortion were in the QAM frequency band. The second-order distortion, therefore, can create significant damage to the AM-VSB signal. To reduce or eliminate the orders of distortion that exist in the AM-VSB frequency band of electrical QAM signal 123, filter 128 is preferably a high-pass filter that cuts off frequencies below 550 MHz. A band-pass filter may also be used, but distortions above 750 MHz will usually not affect the AM-VSB signal and QAM receivers should ignore these distortions. Filter 128 is preferably designed to reduce the level of the second- and third-order distortions to a reasonable level that will not create unacceptable errors in the

AM-VSB signal. The better filter 228 is designed, the faster the roll-off will be and the lower the power of the second- and third-order distortions in AM-VSB frequency band of QAM signal 123 will be. However, better filters also tend to be more costly.

Usually, one of the second-order distortions is centered at about 150 MHz, while one of the third order distortions is centered at about 500 MHz. Both of these are in the AM-VSB frequency range. Furthermore, the second-order distortions may be spread from 0 to 300 MHz, depending on the system, while the third-order distortions may be spread from several hundred MHz below 500 MHz to several hundred MHz above 700 Mhz. The power in these distortions is transceiver and system (the fiber being used, the amount of reflections caused by splitters, etc.) dependent. Typically, however, the frequencies around which the second- and third-order distortions are centered generally are only marginally affected by the transceiver and system. Certain combinations of transceivers and system components/configuration may cause relatively high power third-order distortions. In these cases, the roll-off of filter 228 may become important, as the filter may not sufficiently decrease the power of the third-order noise low enough to not cause errors in the AM-VSB signal (when the QAM signal is added to the AM-VSB signal). The higher the power of the third order distortions, the greater the roll-off that filter 228 should have. In general, because the third-order distortions will be relatively low power, filter 228 does not have to meet stringent roll-off criteria.

Thus, if the distortions that are caused by transceiver 122 and that are outside the digital television signal's frequency range are filtered, digital television signal 123 can be added to analog television signal 125 without causing too many errors in analog television signal 125. Additionally, the distortions that are caused by transceiver 122 and that are within the digital television signal's frequency band are of orders that have relatively low power. This fact, combined with the relatively high resistance to noise of digital television signal 123, allows the digital television signal to remain virtually unscathed by the distortions caused by the transceiver that are within its frequency range.

Comparing Figs. 1 and 3, it can be seen that one less signal (which is usually a fiber optic cable) enters node 120, than entered node 220 (of Fig. 3) and the transmitter 226 and receiver 222 of Fig. 3 have been replaced by one transceiver 122 and filter 128. This allows a fairly significant cost reduction for HFC network 100, especially because transceivers and filters are generally much cheaper than high quality receivers and transmitters. Fig. 2 illustrates an embodiment that has an even greater potential for cost savings. In this embodiment of the current invention, HFC network 200 has a transceiver 270 that performs the functionality of forward transmitter 150 and return receiver 160 of Figs. 1 and 3. Transceiver 270 converts downstream electrical signal 106 to a downstream light signal that is coupled onto light signal 144. Furthermore, a portion of light signal 144 that corresponds to an upstream light signal is converted to an upstream electrical signal 107.

Again, the downstream and upstream light signals would likely be centered around different wavelengths to more easily be able to send and receive the signals.

When implementing an HFC network having two transceivers, a design engineer should know the current and future capacity of the network. In certain instances, using two transceivers may not be appropriate. Using separate transmitters and receivers at one or both of the ends of the HFC network generally allows for greater throughput. Additionally, in most HFC networks, the upstream portion may be changed independently from the downstream portion. If the HFC network is designed such that the two-transceiver network is at the limits of its capacity, then additional fiber links, head-ends, and nodes will have to be added. Thus, although using two transceivers can provide cost reduction, the short-term cost reduction may not outweigh the additional capacity that might have to be added in the future.

One potential preferred method for using the current invention in an HFC network is: providing a transceiver suitable for converting upstream electrical signal 124 into the upstream part of light signal 147 and converting the downstream part of light signal 147 to a downstream electrical signal 123; providing a forward receiver suitable for converting the second downstream light signal 131 to downstream electrical signal 125, the downstream electrical signal 125 having a frequency band; filtering the downstream electrical signal 123 by attenuating the downstream electrical signal 123 at frequencies that correspond to the frequency band (thus removing or reducing the distortions caused in the frequency band of downstream signal 123 by transceiver 122); and combining downstream electrical signal 125 and filtered electrical signal 126 into a combined signal 183.

Thus, by using a transceiver appropriately using the transceiver to receive QAM and not AM-VSB in an HFC network, and by filtering the distortions caused by the transceiver that have power in the AM-VSB frequency band, the current invention allows the use of transceivers in HFC networks. The use of a transceiver is a significant cost reduction by using separate transmitters and receivers. The relatively high noise resistance of QAM allows the transceiver to produce distortions in the QAM frequency range with little or no significant errors.

The current invention is especially applicable to any mixed optical and electrical network wherein there is one signal that is relatively noise-resistant and that occupies a frequency range less than an octave and another signal that has stringent noise requirements and that occupies a frequency range different than the one signal's frequency range.

While the invention has been particularly shown and described with reference to preferred embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention.

Claims

CLAIMS:

1. A Hybrid Fiber Coax (HFC) network comprising:

- a transceiver suitable for coupling to a first light signal, the transceiver converting a downstream portion of the first light signal to a first downstream electrical signal and converting a first upstream electrical signal to an upstream portion of the first light signal;

- a forward receiver suitable for coupling to a second light signal, the forward receiver converting the second light signal to a second downstream electrical signal, the second downstream electrical signal having a second frequency band;

- a filter coupled to the first downstream electrical signal and creating a filtered electrical signal, the filter attenuating the first downstream electrical signal at frequencies that correspond to the second frequency band; and

- an overlay coupler that combines the second downstream electrical signal and the filtered electrical signal into a combined signal.

2. The HFC network of claim 1 wherein second electrical signal comprises an amplitude modulated, vestigial side band signal.

3. The HFC network of claim 1 wherein first downstream electrical signal comprises a quadrature amplitude modulated signal.

4. The HFC network of claim 3 wherein the first downstream electrical signal is in a digital television format.

5. The HFC network of claim 1 wherein the first downstream electrical signal has a first frequency band, wherein the first frequency band does not overlap the second frequency band, wherein the filter attenuates the first downstream electrical signal at frequencies that correspond to the second frequency band to reduce distortions caused by the transceiver that have power at frequencies in the second frequency band.

6. The HFC network of claim 1 wherein the first frequency band has a width such that distortions caused by the transceiver that have the highest power are outside the first frequency band.

7. The HFC network of claim 1 wherein the transceiver is coupled to the first light signal and wherein the HFC network further comprises a second transceiver that is coupled to the first light signal, the second transceiver converting a third downstream electrical signal into the downstream portion of the first light signal and converting the upstream portion of the first light signal into a second upstream electrical signal.

8. The HFC network of claim 7 wherein the downstream portion of the first light signal is centered at a first frequency and the upstream portion of the first light signal is centered at a different second frequency.

9. A Hybrid Fiber Coax (HFC) network comprising:

- a transceiver for converting a downstream portion of a first light signal to a downstream electrical digital television signal, and converting an upstream electrical signal to an upstream portion of the first light signal;

- a forward receiver for converting a second light signal to an electrical analog television signal having a frequency band;

- a filter for filtering the electrical digital television signal to attenuate the digital television signal at frequencies that correspond to the frequency band; and

- an overlay coupler that combines the electrical analog television signal and the filtered digital television signal into a combined signal.

10. The HFC network of claim 9 wherein analog television signal comprises an amplitude modulated, vestigial side band signal.

11. The HFC network of claim 9 wherein digital television signal comprises a quadrature amplitude modulated signal.

12. The HFC network of claim 9 wherein the downstream portion of the first light signal that carries the digital television signal is centered at a first frequency, and the upstream portion of the first light signal is centered at a second frequency.

13. The HFC network of claim 12 wherein the HFC network further comprises a second transceiver for converting a second electrical digital television signal into the downstream portion of the first light signal that carries the digital television signal and converting the upstream portion of the first light signal into a second upstream electrical signal.

14. A method performed in a Hybrid Fiber Coax (HFC) network, the method comprising the steps of: - providing a transceiver suitable for coupling to a first light signal to a first downstream electrical signal and to a first upstream electrical signal;

- converting a downstream portion of first light signal to the first downstream electrical signal having a first frequency band;

- converting the upstream electrical signal to an upstream portion of the first light signal;

- providing a forward receiver suitable for coupling to a second downstream electrical signal and a second downstream light signal;

- converting the second downstream light signal to the second downstream electrical signal, the second downstream electrical signal having a second frequency band; - filtering the first downstream electrical signal by attenuating the first downstream electrical signal at frequencies that coπespond to the second frequency band; and

- combining the second downstream electrical signal and the filtered electrical signal into a combined signal.

15. The method of claim 14 wherein second downstream electrical signal includes an amplitude modulated, vestigial side band signal.

16. The method of claim 14 wherein first downstream electrical signal includes a quadrature amplitude modulated signal.

17. The method of claim 14 wherein the first downstream electrical signal is in a digital television format.

18. The method of claim 14 wherein the first downstream electrical signal has a first frequency band, wherein the first frequency band does not overlap the second frequency band, wherein the filter attenuates the first downstream electrical signal at frequencies that correspond to the second frequency band to reduce distortions caused by the transceiver that have power at frequencies in the second frequency band.

19. The method of claim 14 wherein the first frequency band has a width such that second-order distortions of the first downstream electrical signal caused by the transceiver are outside the first frequency band.

20. The method of claim 14 further comprising the steps of:

- providing a second transceiver suitable for coupling to the first light signal, to a second downstream electrical signal, and to a second upstream electrical signal;

- converting the second downstream electrical signal into the downstream portion of the first light signal; and

- converting the upstream portion of the first light signal into the second upstream electrical signal.