WO1998039920A1

WO1998039920A1 - Wireless upstream communications using frequency conversion

Info

Publication number: WO1998039920A1
Application number: PCT/US1998/003718
Authority: WO
Inventors: Paul Baran; Mark Laubach; Ali Raissinia
Original assignee: Com21, Inc.
Priority date: 1997-03-04
Filing date: 1998-02-25
Publication date: 1998-09-11
Also published as: AU6439898A

Abstract

A two-way cable TV network having a standard coax cable downstream path and in part an over-the-air upstream radio return path. A conventional head end unit is used for transmitting a downstream signal through a standard coaxial cable distribution network to a number of subscribers. For two-way communications a two-way cable modem at the subscriber's premises is used to accept downstream signals from the head end unit through the standard coaxial cable. However, on the upstream side, a transmitter appliqué coupled to the subscriber's two-way cable modem is used to up convert the signals generated by the two-way cable modem through superheterodyning techniques to a radio frequency which is suitable for over-the-air transmission. A transmitting antenna is coupled to the transmitter appliqué for transmitting the upstream signals over the air.

Description

WIRELESS UPSTREAM COMMUNICATIONS USING FREQUENCY CONVERSION

FIELD OF THE INVENTION The present invention relates to an over-the-air upstream path for data transmission on a cable TV system. In particular, the present invention relates to converting standard one-way cable systems into a two-way system by converting the same signal into different frequencies for wireless upstream communications.

BACKGROUND OF THE INVENTION

Currently, conventional coaxial cable TV systems can deliver tens of megabits per second data transmissions downstream (i.e. , from the headend unit to various subscribers) . Also presently feasible is the use of diplexing amplifiers and reverse amplifiers to transmit information upstream (i.e., from one or more subscribers back to the headend unit) on the common coaxial cable as well. However, to activate this reverse channel entails a major fixed cost of adding amplifiers and filters and balancing the upstream gain. If there many customers for the new two-way services, then the cable operator's investment in activating the two way capability can be justified. However, if there are but relatively few scattered upstream services customers, then the cable operator faces a significant fixed cost to activate the two way capability that cannot be fully amortized over the few customers. This is a "chicken and egg" type problem. On the one hand, the desired solution requires access before the market can be proven, but the access comes at the expense of an often unacceptable front end cost.

The use of a separate telephone line return for the upstream cable subscriber signals is well known and is used in a number of different cable modems. The idea of heterodyned frequency translation per se is also old art. And the use of frequency translation in cable systems is probably best developed in the RAD concept used to transmit PCS signals over a cable system. The RAD concept includes the use of inexpensive antennas and frequency shifters as a two way delivery mechanism to remote PCS devices. Over-the-air, two-way signals from cordless Personal Communications Service telephones, say at 1800 MHz from a number of remote antenna sites are frequency shifted and carried to and from one or more central sites. Today, only 10-15% of the cable plants have been converted to two-way operation. Although these alternative prior art systems offer some flexibility and enhanced functionalities, they nonetheless suffer from several disadvantages. Where telephone lines are used for upstream transmission, the upstream paths are seriously bandwidth limited. And with such approaches, a different protocol is needed in the upstream which requires implementation of a completely different set of hardware than being able to use the hardware for an eventually converted conventional 2-way cable system such as the Com21 head end system. Scalability and upgradeability are also potential problems with some prior art systems. In addition, there are significant software compatibility issues.

In light of such limitations of the prior art, there exists a need by the cable industry for an efficient, transparent, and cost-effective approach for converting standard one-way cable systems into two-way systems for a limited number of initial users, while later being readily convertible to conventional 2- way all cable or hybrid fiber optic cable (HFC) systems. The present invention provides a solution whereby a transparent radio return path is temporarily or permanently used for providing upstream communications, which allows cable operators to be able to selectively upgrade their systems on an as-needed basis. Costs are minimized by leveraging off the same frequency, scheduling, and modulation schemes to keep the conversion from the radio path signal to the signal on the cable as simple and straightforward as possible. And optionally at a later date, removing the radio path sections to use the conventional cable upstream path.

SUMMARY OF THE INVENTION

The present invention pertains to a method and apparatus for converting a traditional one-way cable system into a two-way system by utilizing an over-the air radio frequency (RF) return path to provide upstream communications. In the present invention, a transmitter applique is attached to a subscriber's conventional two-way cable modem that normally operates in a given modem frequency range generally at 5 to 42 MHz. This transmitter applique heterodynes the normal upstream cable modem signal to a frequency suitable for transmission over-the-air. At the receiving site, a matched reverse heterodyning of the received signal is performed via a receiver applique. This applique transposes the received signal back to the transmitting modem's original frequency and modulation format where it is received by the normal cable head end modem without any modifications. In the preferred embodiment, there is no change to the normal modulation and data formats whatsoever. Hence, the exact same receiving equipment and frequency channelization are preserved by making the use of the radio path essentially transparent to the overall cable system.

In one embodiment, the cable customers using this invention are those typically having a two-way cable modem that use a remote controlled noise blocking filter at the junction of the drop cable, to the cable tap at the feeder. This same signal, as used to open the noise blocking filter, can be used in the present invention to turn on and off the RF transmitter in the transmitter applique. The transmitter applique is a small unit containing the circuitry required to perform the superheterodyning of the original modem frequency to a radio frequency suitable for over-the-air transmission. The output of the transmitter in the applique is fed to a coax cable to convey the radio signal to a transmitter antenna. In one embodiment, the transmitter applique is mounted in close proximity to the two-way cable modem and thus is able to receive power and control signals from the two-way cable modem to minimize cost. In an alternative embodiment, the transmitter applique is interposed remotely between the cable modem and feeder tap, with the output of the applique's over-the-air signal connected to an antenna and thence received by one or more receiving antennas which are connected to the cable plant head end.

In the preferred embodiment, neither the frequencies nor the precise modulation used nor the formatting is modified to keep the conversion from the radio path signal to the signal on the cable as simple and straightforward as possible. While the purpose of using the radio path is to defer the expense of insertion of diplexing filters and reverse amplifiers throughout the entire cable system path until enough two way users are present to justify the common expenditure of the plant upgrade, there will be instances where the radio path alone will suffice forever and the full upstream cable conversion be indefinitely deferred.

BRIEF DESCRIPTION OF THE DRAWINGS

The operation of this invention can be best visualized by reference to the drawings.

Figure 1 shows a block diagram of a standard one-way cable TV network that has been upgraded into a two-way system by adding a radio return link.

Figure 2 is a circuit diagram of a transmitter applique unit and its connection to the two-way cable modem.

Figure 3 is a circuit diagram of the receiving site frequency downconverter.

Figure 4 is a circuit diagram of a receiver applique.

Figure 5 shows a block diagram of a standard one-way cable TV network that has been upgraded according to the present invention with a radio return link for enabling two-way communications .

Figure 6A shows a one-way line extender amplifier in a shielded case in a cable system. Figure 6B shows a plug-in diplexing filter module for two- way operation in a shielded case of a cable system.

Figure 6C shows an upstream amplifier module for two-way operation in a shielded case of a cable system.

Figure 7 shows that an upstream radio link can be added in lieu of the upstream amplifier module.

Figure 8 shows an exemplary transmitting antenna 801 as may be connected to a transmitter applique.

DETAILED DESCRIPTION OF THE INVENTION

A method and apparatus for converting a traditional one-way cable system into a two-way system by utilizing an over-the-air radio frequency (RF) return path to provide upstream communications is described. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of the present invention. It will be obvious, however, to one skilled in the art that the present invention may be practiced without these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to avoid obscuring the present invention.

Basically, starting with a two-way cable modem situated at a residence or office, a transmitter applique is attached that converts an upstream modem signal on a coaxial line (e.g., in the 5-42 MHz band) into a frequency range suitable for over-the- air radio transmission to the cable head end or to one or more intermediate points. It should be noted that the terminology, two-way cable modem, as used in this patent and interpreted in the claims, is not to be limited to simple conventional two way TV cable modems, but also includes all types and variations of subscriber terminal units (ΞTUs) , interactive TV set-top boxes, etc. Furthermore, it is contemplated that different frequencies ranges may be applied to the present invention than described herein. For instance, the selected radio channel can be one that is suitable for transmission without the necessity of separate FCC licensing, such as the frequency ranges 902-928 MHz. or 2400-2483.5 MHz or 5725-5850 MHz. At the cable head end, an inverse transformation is performed, shifting the received band back to the 5-42 MHz range where the received signal can be processed by a conventional head end controller. The up conversion and reciprocal down conversion process is performed by a conventional superheterodyning process, whereby a signal having one frequency is converted into a higher or lower frequency by mixing it with a locally generated signal and filtering out the undesired modulation products.

It should be noted that more cable plants are presently partially activated than totally activated. That is, reverse amplifiers and filters are activated on some trunks to support remote monitoring equipment, but not necessarily on the feeders connecting the subscribers. In such cases, it would not be necessary to bring the upstream signals back to the head end solely by radio. Rather, a partial path via upstream cable plus a partial path of over-the-air radio can be used, with the conversion back to the 5-42 MHz band accomplished ^"by heterodyning downconverters located on the activated return trunk. Thus, the upstream signals are then sent upstream to the cable modem at the cable head end partially via the cable and partially via radio. Again, it should be noted that since neither the frequency nor the precise modulation used is modified by the currently preferred embodiment of the present invention, the conversion from the radio path signal to the signal on the cable is implemented by a matched frequency translation. The transparency, multiplicity, and channelization aspects associated with such a wireless return path are described in detail in the patent application entitled, "CATV NETWORK AND CABLE MODEM SYSTEM HAVING A WIRELESS RETURN PATH," filed on , Serial No. . Figure 1 shows a block diagram of the present invention in its simplest form. A head end unit 101 of a cable TV (CATV) plant transmits data/TV signals downstream over standard coaxial cable 102 to a two-way cable modem 103 installed at a subscriber's location. The subscriber transmits data back upstream by using transmitter applique 104 to upconvert the signal generated by the cable modem 103 into a radio frequency suitable for over-the-air transmission via transmitting antenna 105. This radio signal is then received by antenna 106. A receiver applique 107 downconverts the received signal which is then sent to the head end unit 101 for further processing.

Referring now to Figure 2, a circuit diagram of a transmitter applique 104 and its interface with an associated cable modem 103 is shown. Essentially, transmitter applique 104 is comprised of circuitry required to perform the conversion from 5-42 MHz to a frequency suitable for radio transmissions. It can be a standalone unit or integrated as part of the cable modem 103. It should be noted that the present invention can be used with a variety of cable modems. One example is the P3 two- way cable modem designed and manufactured by Com21, Inc. of Milpitas, CA. Transmitter applique 201 accepts downstream as well as upstream communications. On the downstream side, a downstream signal in the 5-42 MHz range being transmitted over a standard cable system is accessed by tap 202 via feeder cable 203. This signal is then fed through drop cable 204 to applique 201, which passes it on as an input to the STU 205. On the upstream side, a directional coupler 206 is used to direct the 5-42 MHz upstream data signal from STU 205 to a mixer 207. The mixer 207 takes the 5-42 MHz data signal and mixes it with a higher frequency signal (e.g., 886 MHz) to attain a suitable RF frequency for transmission over the airwaves. The higher frequency signal can be provided by an oscillator block 240. Additional circuitry may be added to provide for greater frequency and phase accuracy. A combination of filters 221-222, amplifiers 223-224, local oscillator 225, and divider 226 are used to provide the appropriate signal processing, which is well known in the art.

The output of the applique 201 is a coax cable conveying the RF signal suitable for connection to an antenna 208. The transmitted power and transmitting antenna gain are chosen to provide a signal sufficient to reach one or more receiving antennas having an electrical path to the cable plant head end. It is desirable that the upstream radio transmitter is turned on if and only if there is information to be transmitted upstream. Hence, a switch 209 is used to enable transmission only if there is data to be transmitted. The cable modem also produces a control signal that is used to open up a blocking filter at the junction of the drop cable 204 to the subscriber and the cable tap 202 at the feeder 203. This control signal can be used to control switch 209 for turning on and off the RF transmitter in the applique 201. However, the use of this control signal is not mandatory. In an alternative embodiment, the head end unit adaptively controls the power level of the transmitted radio signal by sending a power level command over the downstream path to the cable modem 205. In turn, cable modem 205 transmits a corresponding power level control signal to a digital-to-analog converter (DAC) 220, which converts the digital signal into an analog signal for controlling the output power of amplifier 221.

The upstream RF signal is propagated over the airwaves and eventually picked up by a receiving antenna 106, as shown in

Figure 3. A receiver applique 107 associated with a head end unit 101 accepts the RF signal and down converts the signal back to the original 5-42 MHz frequency range. The down converted signal is then sent to a receiver in the head end unit 101 for further processing.

Figure 4 shows a more detailed circuit diagram of a receiver applique 107. RF signals originating from one or more transmitter appliques, propagating over the airwaves, are picked up by a receiving antenna 106. Antenna 106 can be one of any omni-directional antenna configurations, tuned to the frequency of interest. The received RF signals are first amplified by pre-amp 401 and then filtered by SAW filter 402. Additional gain is provided by amplifier 403. The RF signal is then mixed with a lower frequency reference signal by mixer 404. The reference signal is generated by a combination of voltage controlled oscillator 410, divider 411, mixer 412, and low pass filter 413 which is well known in the art. Alternatively, a simple oscillator block 420 may be used instead. The down converted 5-42 MHz signal is then filtered by SAW filter 306.

Figure 5 shows a block diagram of a standard one-way cable TV network that has been upgraded according to the present invention with a radio return link for enabling two-way communications. It can be seen that part of the CATV network can be comprised of standard one-way set-top boxes with no upstream capability; part can be upgraded with two-way cable modems utilizing an all-coax upstream path; and part can be upgraded with two-way cable modems utilizing applique/antennas for an over-the-air upstream path. Furthermore, the delineation between the various stages of a traditional headend all coax internal distribution, one-way coaxial cable distribution network, existing or new all coax distribution, and subscriber homes/offices is shown in this figure. A traditional head end cable TV network is shown as 101.

It consists of a headend digital communications controller 502 for controlling all digital data traffic to the downstream coaxial cable distribution network 506 via coaxial link 503. A standard analog TV programming source 504 also places an NTSC video signal onto the coaxial cable distribution network 506 via coax 505. A number of distribution or main amplifiers 523 are used to provide the requisite gain. Thereupon, the downstream signals are fed into one or more one-way line extender/bridging amplifiers or junctions 508-509, which routes the signals to the appropriate destinations. For example, a standard one-way set- top box/TV arrangement is shown as 513. Downstream are routed through interconnecting coupler 508, coax 510, coupler 509, coax 511, and splitter 512. A two-way cable modem is shown as 513. Its upstream path is through bi-directional coupler/amplifiers 509 and 508 onto a coaxial return cable 540. It should be noted that a hybrid fiber coax system can be used by adding a fiber terminal and a corresponding fiber node.

The cable modem 514 is also two-ways enabled. For its upstream side, a directional coupler 516 routes upstream signals from a two-way cable modem 514 to the transmitter applique 104. The transmitter applique 104 can be placed adjacent to the cable modem 514. Alternatively, transmitter applique 104 may be situated in a remote location a distance away from the cable modem 514. Furthermore, there may be instances whereby multiple two-way cable modems share one or more transmitter appliques. The upstream signal is up converted by transmitter applique 104 and transmitted by the antenna 105 as a radio signal propagated over the airwaves to a receiving antenna 106 connected to a receiver applique 107. Transmitting antenna 105 can be one of any directional antenna configurations used for transmitting RF signals. Note that directional antennas, particularly those having a high front to back ratio are especially useful for practice in the present invention. Transmitter antenna 105 may be mounted on a subscriber's window or remotely situated, such as on a telephone pole, depending on convenience and line-of- sight considerations.

Furthermore, more than one receiving antennas associated with the head end unit can be used to provide superior coverage. The receiver applique 107 down converts the radio signal back down to the 5-42 MHz frequency range and drops it onto cable 519. Cable 519 is connected into coupler 520 which routes the signal to the head end controller 502 on line 521. It should be noted that the cable modem 514 adjusts its upstream power level to insure that the signal arrives at an optimum level at the head end modem. A wide variety of protocols may be used in conjunction with the present invention. (See, for example, "The UPSTREAMS Protocol for HFC Networks" by Mark Laubach, submitted to the P801.14 Cable TV MAC and PHY Working Group, October 23, 1995) . In the currently preferred embodiment, the same protocol and data format is used with the radio return as is used by the upstream coaxial cable system. By using the same acquisition, ranging, QPSK modulation, and TDMA slotted approach for both radio and coax cable upstream paths, the cable network can be selectively upgraded.

A further embodiment of this invention places the upstream transmitter applique within or adjacent to a line extender, feeder, or trunk amplifier, to allow spanning over the presently one way parts of a mixed one way and two way cable system. This allows several subscribers may share a single upstream applique unit. Today, cable systems are set up for one way operation, the amplifier cases generally contain room for diplexing filters and an optional upstream amplifier module. Figure 6A shows a one-way line extender amplifier in a shielded case in a cable system. In most cases, plug-in diplexing filter modules can be later added for two-way operation, as shown in Figure 6B. And if upstream gain is needed, an upstream amplifier module can be further added, as shown in Figure 6C. The arrangement shown in Figure 7 shows that an upstream radio link can be added in lieu of the upstream amplifier module. Furthermore, cable TV amplifiers are generally mounted on a support unit and are in locations that can serve as antenna sites. Hence, a directive antenna can be pointed to a receiving antenna site offering significantly longer ranges than in-house antennas. The receiving antenna would generally be mounted at a receiver connected to a point along a two-way activated portion of the cable system placing both the transmitting and receiving antennas mounted in favorable sites for longer distance transmission. The connection of the receiver would terminate at an activated two-way location, allowing transmission to the cable head end in the 5-42 MHz range.

Figure 8 shows an exemplary transmitting antenna 801 as may be connected to a transmitter applique. Transmitting antenna 801 can be one of any directional antenna configuration used for transmitting RF signals. In this particular embodiment, the coax cable 802 extending from a transmitter applique is terminated to a reflector 803. Reflector 803 gives an electrical null. It can be fashioned as part of a rigid support rod which is pivotable 360 degrees about a mount 804. Thereby, the transmitting antenna can be situated so as to point in the direction of a receiving antenna. For more favorable transmitting environment, the mount can be fixed by means of vacuum cups 805 onto a non-conducting environment (e.g., a window) . A balun 806 is used as an interface between the reflector 803 and corporate feed 807. Corporate feed 807 is eventually split twice and terminated into four dipole 300 ohm elements 808. The doubling of power from each of dipoles 808 and elements 809, produces 6 dB of gain. Director elements 810, that are shorter than the dipoles 808, are used to direct the electrical characteristics of the antenna. In this particular application, director elements 810 are designed to have a center frequency of 915 MHz. The foregoing descriptions of specific embodiments of the present invention have been presented for purposes of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in light of the above teaching. The embodiments were chosen and described in order to best explain the principles of the invention and its practical application, to thereby enable others skilled in the art to best utilize the invention and various embodiments with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents.

Claims

CLAIMS We claim:

1. A two way cable TV network, comprising: a head end unit generating a downstream signal; a two-way cable modem coupled to receive said downstream signal transmitted through at least in part a length of coaxial cable to the two-way cable modem; a first frequency converter coupled to the two-way cable modem for heterodyning an upstream signal having a first frequency that is output from the two-way cable modem to a second frequency which is suitable for over the air transmission; a transmitting antenna coupled to the first frequency converter for transmitting the upstream signal at the second frequency over the air; a receiving antenna for receiving the over the air upstream signal; a second frequency converter coupled to the receiving antenna for heterodyning the received upstream signal from the second frequency back to the first frequency, wherein the upstream signal having the first frequency is sent from the second frequency converter to the head end unit for processing.

2. The cable TV network of Claim 1, further comprising a switch coupled to the two-way cable modem for disabling transmission of signals from the transmitting antenna unless the two-way cable modem outputs the upstream signal.

3. The cable TV network of Claim 1 or 2, further comprising an amplifier coupled to the first frequency converter for amplifying the upstream signal with a gain set according to a control signal generated from the head end unit.

4. The cable TV network of any one of the preceding claims, wherein the second frequency falls within an FCC license-free frequency range and/or wherein the network further comprises a one-way set-top box coupled to the head end unit by coaxial cable.

5. The cable TV network of any one of the preceding claims, further comprising a second two-way cable modem coupled to the head end unit, wherein both downstream and upstream signals to/from the second two-way cable modem are transmitted through a common coaxial cable and/or, wherein upstream signals being transmitted over the air by the first antenna and upstream signals output from the second two-way modem have the same protocol.

6. The cable TV network of any one of the preceding claims, wherein the first frequency converter is located adjacent to or a distance away from the two-way cable modem.

7. An apparatus for providing an upstream path of communications between a head end unit of a CATV network and a remote subscriber unit, comprising: a first heterodyne circuit coupled to the subscriber unit for up converting a signal generated by the subscriber unit from a first frequency to a second frequency suitable for RF transmission over the airwaves; a transmitter coupled to the first superheterodyne circuit for providing enough power to transmit the signal over the airwaves; a directional transmitting antenna coupled to the transmitter for propagating the RF transmission over the airwaves; a receiving antenna for receiving the RF transmission over the airwaves; a second heterodyne circuit coupled to the receiving antenna for down converting a signal received by the receiving antenna from the second frequency to the first frequency, wherein the down converted signal is sent by the second superheterodyne circuit to the head end unit for processing.

8. The apparatus of Claim 7, further comprising a switch coupled to the transmitter for enabling transmission of signals from the transmitting antenna only when the subscriber unit sends an upstream message.

9. The apparatus of Claim 7 or 8 , further comprising a power adjustment circuit coupled to the transmitter for controlling a power level of the RF transmission according to a command sent by the head end unit, wherein the head end unit can adaptively adjust the power level of RF transmissions sent by the transmitting antenna.

10. A method of converting a one-way CATV network having a downstream path for transmission of signals from a head end unit to a subscriber unit via coaxial cable into a two-way cable TV network having both the downstream path via coaxial cable and an over-the-air upstream radio return path, comprising the steps of: up converting an upstream signal having a first frequency generated by a two-way subscriber unit coupled to the coaxial cable into a second frequency, wherein the second frequency is suitable for over-the-air transmission; transmitting the signal over the air through a transmitting antenna ; receiving the transmitted signal by a receiving antenna; down converting a received signal from the receiving antenna from the second frequency to the first frequency; sending the received signal at the first frequency to the head end unit.

11. The method of Claim 10, further comprising the step of switching the transmitter on when the subscriber unit sends an upstream message and switching the transmitter off when there are no upstream messages being sent and/or, further comprising the step of adaptively setting a power level of the transmitted signal according to a command sent by the head end unit.

12. A two-way cable TV network comprising: a head end unit; a two-way cable modem associated with said head end unit; a frequency converter coupled to the two-way cable; and an antenna coupled to the frequency converter.