WO2005057816A1

WO2005057816A1 - Modulator and demodulator for passive optical network

Info

Publication number: WO2005057816A1
Application number: PCT/EP2004/053234
Authority: WO
Inventors: Matthew Brocco; Suhas Gandhi; Charles Kristofek
Original assignee: Siemens Aktiengesellschaft
Priority date: 2003-12-08
Filing date: 2004-12-02
Publication date: 2005-06-23

Abstract

A system and method for carrying a return path signal from a cable system set top box (64) to a cable television head end via a passive optical network (12). The return path signal is received by a demodulator (110) for demodulating and decoding the signal. A digit representation of the return path signal is carried over the passive optical network (12) to an optical terminal unit (42) operative in conjunction with a modulator (116) for producing a radio frequency signal in response thereto for transmittal to the cable television head end.

Description

MODULATOR AND DEMODULATOR FOR PASSIVE OPTICAL NETWORK

CROSS-REFERENCE TO RELATED APPLICATIONS The present application claims the benefit of the provisional patent application filed on December 8, 2003, and assigned application number 60/527,889.

FIELD OF THE INVENTION The present invention relates generally to passive optical networks, and more particularly to a modulator and demodulator for use on a passive optical network carrying cable access television signals.

BACKGROUND OF THE INVENTION The use of fiber optic-based networks to carry information signals continues to grow in popularity worldwide. Such networks employ optical data transmitters at a source node for converting electrical signals to optical signals for transmission over the network. At a destination node the optical signals are converted back to electrical form for processing and extracting the information signal. Optical networks are classified as active or passive. An active optical network comprises intermediate nodes (disposed between the source and destination nodes) for regenerating the optical signal using a receiver/transmitter (i.e., transceiver) that provides optical-to-electrical-to-optical conversion and amplification of the signal in electrical form. A passive optical network (PON) architecture does not employ intermediate nodes, but instead comprises beam splitters and filters to direct the optical signal to its intended destination node. The length of the PON is typically limited to about 20 km. As is well known, a fiber optic cable has a much higher information carrying capacity than copper wire, including the ubiquitous unshielded twisted copper pair commonly used for providing dial-up telephone service, due to a higher bandwidth capacity and lower signal attenuation when compared with copper conductors. Fiber is also more reliable and has a longer useful life than copper conductors. Since fiber does not emit any electromagnetic radiation, it is a more secure transmission medium than copper. The passive optical network (PON), including for example a B-PON (broadband passive optical network) or a G-PON (gigabit passive optical network), provides multiple data transmission paths, each capable of delivering high-bandwidth data services to multiple users. An exemplary B-PON comprises 32 (extensible to 64) such data paths, each data path comprising one fiber optic cable. A G-PON comprises, for example, 64 or 128 data paths. A standardized PON network management protocol controls and manages transmission and receiving of signals across the passive optical network. In addition to the fiber optic cable, the PON further comprises optical splitters and combiners for directing information signals propagating between an optical line terminal (OLT) at a network head end and a plurality of optical network units (ONU's) located at or proximate a subscriber's site. Each ONU connects to several subscribers, permitting each subscriber to access the passive optical network and connect to other networks through the PON. According to current network standards, the fiber optic, path on a PON network operates at data rates of 155 Mbps, 622 Mbps, 1 .25 Gbps, and 2.5 Gbps. Bandwidth allocated to each customer from this aggregate bandwidth can be statically or dynamically assigned to support voice (both telephone service and voice-over-internet protocol service), data, video, television and multimedia applications. The conventional PON topology comprises a shared upstream signal path and a broadcast downstream path. Downstream data includes an address header and is broadcast from the OLT to all ONU's. All ONU's receive the broadcasted data and each employs an address matching process to identify data intended for it. Use of a shared PON data path for upstream traffic requires apportioning the available bandwidth among all users and requires a scheme to avoid data collisions. One technique for managing the upstream traffic employs a TDMA (time division, multiple access) protocol in which dedicated transmission time slots are granted to each ONU. All ONU's are time synchronized and each transmits data only during its assigned time slot. Upstream data received by the OLT from an ONU is processed and forwarded to its intended destination, including other networks outside the PON. The PON is terminated at or near subscriber sites, and the optical signal transformed to electrical form for processing by subscriber premises equipment. Various

PON termination configurations are known, including FTTH (fiber to the home),

FTTBusiness (fiber to the business), FTTB/C (fiber to the building/curb) and FTTCab (fiber to the cabinet). Figure 1 illustrates a fiber to the premises (FTTP) network 10 to which the teachings of the present invention can be applied. As can be appreciated by those skilled in the art, the teachings can be applied to other PON configurations. The network 10 comprises a passive optical network (PON) 12 providing an optical communications link between a central office 14 of a telecommunications provider and users or subscribers. In another embodiment, a broadband passive optical network (B-PON) or a gigabit passive optical network (G-PON) is substituted for the PON 12. Although not apparent from Figure 1 , the

PON 12 provides various classes of service to subscribers, wherein each class identifies a data traffic flow priority across the PON 12 and within the optical network units. According to one embodiment of the FTTP network 10, information signals are carried over the network at wavelengths of 1310 nm, 1490 nm and 1550 nm. Typically, the 1310 nm signal carries upstream traffic, including return path signals as discussed below, and the 1490 and 1550 nm signals carry downstream traffic. Multimedia and video signals are carried at 1550 nm while data and telephone service is provided on 1490 nm. Signals at these wavelengths experience relatively low attenuation as they propagate through the fiber optic cable. Digital data on the PON is encoded as a digital bit stream and the video signals comprise an analog modulated signal on their respective optical wavelengths on the PON. Voice, data, video and multimedia broadband services are supplied from the Internet and switched networks 20 connected to the central office 14 for distribution to users through the PON 12. The Internet and switched networks 20 are representative of various networks capable of sending information to and receiving information from the users. Such networks are known to those skilled in the art and include, but are not limited to: video and multimedia networks, TDM/PTN networks, ATM networks, IP networks, telephone networks and cable television (CATV) networks. Thus at the user's site, the FTTP network 10 provides multiple information services including: video and multimedia signal delivery, telephone service and Internet access. A single-family optical network unit 21 (also referred to as a single family unit, SFU) terminates the PON 12 and converts the optical signals carried on the PON 12 to electrical signals. Typically, the ONU 21 is mounted on or proximate the premises to provide a single residence or business user with access to the PON 12. Since the PON (i.e., the fiber optic cable) extends to the single user premises, the network is referred to as a fiber to the home (FTTH) or a fiber to the building/curb (FTTB/C) network. At each user's site, customer premises terminal equipment (CPTE) 24 connects to an appropriate output port of the ONU 21 for receiving the electrical signals therefrom.

According to one embodiment, the ONU 21 comprises separate output ports for telephone service (also referred to as telephone voice), video and data (e.g., Internet access) for each subscriber. Alternatively, any of the these information services can be provided in digital form over the data (i.e., digital) port, in which case this single port supplies multiple service channels (e.g., four channels in one embodiment), including a service channel for video, telephone and data services. In one embodiment, an optical network unit (ONU) 22 (also referred to as a multiple distribution unit, MDU) provides xDSL service to a plurality of subscribers or users via an ADSL or a VDSL terminal (also referred to as a port) on the ONU 22 that connects to a corresponding VDSL modem 26 or an ADSL modem 28 at the subscriber's site. The ONU 22 also supports other DSL-type connections, generically referred to as xDSL connections.

Thus the ONU 22 functions as an access multiplexer, providing multiple xDSL subscribers with access to the PON 12 via the xDSL connection between the ONU 22 and the VDSLADSL modem 26/28. The CPTE 24 connected to the VDSL/ADSL modem 26/28 represents equipment that responds to the digital bit stream signals carried over the PON 12, e.g., a television, telephone and/or a computer and operates according to the internet protocol (IP). The optical network unit 22 is located proximate the users or subscribers, e.g., in a telephone closet of a multi-dwelling or multi-office complex, and is connected to the VDSL/ADSL modems 26/28 via an appropriate conductor suitable for carrying xDSL signals, such as a twisted pair of copper conductors. Although it is theoretically possible to extend the fiber optic cable to each subscriber in a multi-subscriber premises, and thus utilize a single-family optical network unit 21 as described above to terminate the fiber, it is commercially more pragmatic to extend the fiber to the premises, terminate the fiber with an ONU 22, and use existing copper conductors within the premises to provide each subscriber with an xDSL connection between the ONU 22 and the subscribers space within the premises. Installing fiber optic cable to each subscriber in the premises would be an expensive and labor-intensive undertaking. Although only a single ONU 22 is shown in Figure 1 , those skilled in the art recognize that multiple ONU's 22 are connected to the PON 12 to provide telephone, multimedia and data services to the users via an xDSL connection to an xDSL modem, e.g., the VDSL/ADSL modems 26/28. The central office 14 comprises an optical line terminal (OLT) 42 operative as an optical transceiver for broadcasting data, multimedia, video and voice signals to the ONU's 22 and for receiving data, multimedia and voice signals from the CPTES 24 via the ONU's 22. The OLT 42 also provides a network management function for managing the ONU's 22, executing its network management functions in accordance with an ONT management and control interface (OMCI) standard. As is known, cable television systems (also referred to as cable access television, CATV) distribute television program signals via coaxial cable arranged in downstream tree and branch networks. To ensure adequate signal quality at the receiving end, coaxial cable distribution systems require a large number of high bandwidth signal amplifiers. For example, as many as 40 amplifiers may be required between the cable system head end and an individual subscriber's home. Certain cable television systems also provide enhanced data services comprising an upstream data channel that allows the subscriber to send data to the head end over the coaxial cable distribution system. For example, an upstream data path is provided for enhanced pay-per view, video-on-demand, interactive television, interactive games, image networking, video conversing and video telephony. Newer cable systems comprise a hybrid fiber-coax network, wherein an optical fiber extends from the head end and terminates in a node connected to 500-2000 individual subscribers via a coaxial cable network. Figure 2 illustrates a hybrid fiber-coax network 60 comprising transceivers 62 disposed at network nodes. Radio frequency signals supplied from a head end of the CATV system are broadcast to a plurality of subscriber set top boxes 64 via an optical fiber 66 coupled to a coaxial cable 68 via the transceivers 62. The transceivers 62 convert optical signals traveling downstream on the optical fiber 66 to electrical signals for processing by the set top boxes 64. Such signals typically comprise television program video signals. Each transceiver 62 supports up to about 2000 subscribers, depending on the bandwidth allocated to each subscriber. Subscribers can communicate in the upstream direction with the cable system head end, for example to implement enhanced data services, by entering commands to the set top box 64. The return channel commands are received by the appropriate transceiver 62 for conversion to optical signals for transmission over the optical fiber 66. After traversing the hybrid-fiber network, the return channel signals are converted to electrical signals by the appropriate transceiver 62A, demodulated by a demodulator 70 and supplied to the

CATV head end as a base band signal. By convention and/or industry standard, a spectrum of 5-42 MHz is set aside for the return path video signal in the conventional hybrid fiber-coax network 60. Thus in Figure 2, each transceiver must be capable of transmitting a signal within the 5-42 MHz spectrum in the upstream direction over the optical fiber 66. Typically, such transceivers 62 employ dense wavelength division multiplexing techniques. Although the hybrid fiber-coax network 60 may reduce the number of required network amplifiers when compared to the coaxial cable distribution system, the extensive coaxial cable portion of the network is not without problems. Since the coax network is essentially a shared bus, any noise ingress or nonlinearity can detrimentally affect many subscribers. Also, a significant number of amplifiers may be required in the coaxial network to supply a signal of adequate signal strength to the most distant subscriber. It is also known to supply CATV signals over a PON, such as the PON 12 of Figure

1. In such an application, the Internet and switched networks 20 further comprise a CATV head-end. Thus the PON 12 provides CATV signals in addition to high-speed Internet access and telephone service, all in digital form according to the Internet protocol standard. Figures 3 and 4 illustrate exemplary customer premises terminal equipment 24. In Figure 3, the ONU 21 receives and converts the optical signals to electrical signals for input to a telephone 90, a computer 92 and the set top box 64 that controls the reception of television program signals by a television or display 96. According to one embodiment, the ONU 21 comprises separate output ports for telephone service, video and data, with each output port supplying the signal over a conventional conductor based on the service type, e.g., a twisted conductor pair for telephone service, a coaxial cable for the television signal and an Ethernet-based conductor for the data. Alternatively, any of the these information services can be provided in digital form, according to the Internet protocol standard, from the data (i.e., digital) output port, in which case this single data port supplies multiple service channels (e.g., four channels in one embodiment), including a service channel for video, telephone and data services. The ONU 21 also converts the electrical signals from the telephone 60, the computer 92 and the set top box 64 to optical signals for transmission over the PON 12 in the upstream direction.

In the Figure 4 embodiment, the xDSL modem 26/28 is connected to a network 71 , for example an Ethernet network, for connection to the computer 92. The telephone 90 and the set top box 64 are connected directly to the ONU 22 for receiving telephone and video signals transmitted over the PON 12. Also, the return path signal from the set top box 64 to the cable television head end, which is generated in the set top box 64 in response to user-entered commands, is input to the ONU 22 for transmission over the PON 12.

SUMMARY OF THE INVENTION In one embodiment, the present invention comprises a method for sending a return path radio frequency signal from a set top box to a cable television head end over an optical network. The method comprises producing a base band digital signal in response to the return path signal, converting the base band signal to an optical signal, transmitting the optical signal over the optical network, producing a modulated radio frequency signal in response to the optical signal and transmitting the modulated radio frequency signal to the cable television head end. According to another embodiment, the present invention comprises a system for transmitting a return path signal from a set top box to a cable television head end. The system comprises a demodulator for producing an optical signal in response to the return path signal, a passive optical network for carrying the optical signal and a modulator for producing a radio frequency signal in response to the optical signal, wherein the radio frequency signal is transmitted to the cable television head end.

BRIEF DESCRIPTION OF THE DRAWINGS The present invention can be more easily understood and the advantages and uses thereof more readily apparent, when the following detailed description of the present invention is read in conjunction with the figures wherein:

Figure 1 illustrates a prior art passive optical network and associated components to which the teachings of the present invention can be applied. Figure 2 illustrates a prior art hybrid fiber-coaxial cable system. Figures 3 and 4 illustrate customer terminal premises equipment for use with the passive optical network of Figure 1. Figure 5 illustrates a passive optical network including a modulator and a demodulator according to the teachings of the present invention. Figure 6 illustrates the optical network unit and the optical terminal unit of Figure 5 in additional detail. Figures 7 - 11 illustrate certain components of the demodulator of Figures 5 and 6 in additional detail. Figures 12 — 15 illustrate certain components of the modulator of Figures 5 and 6 in additional detail. In accordance with common practice, the various described device features are not drawn to scale, but are drawn to emphasize specific features relevant to the invention.

Reference characters denote like elements throughout the figures and text.

DETAILED DESCRIPTION OF THE INVENTION Before describing in detail the particular method and apparatus related to a video return path (i.e., upstream path) for a passive optical network supplying downstream CATV signals to subscribers according to the present invention, it should be observed that the present invention resides primarily in a novel and non-obvious combination of elements and process steps. So as not to obscure the disclosure with details that will be readily apparent to those skilled in the art, certain conventional elements and steps have been presented with lesser detail, while the drawings and the specification describe in greater detail other elements and steps pertinent to understanding the invention. According to the teachings of the present invention, in addition to carrying the downstream video program signals to the subscriber's set top box 64, the upstream path from the subscriber's set top box 64 to the CATV head end is also provided by the PON 12. The signal carried on the upstream path, referred to as a video return channel or return channel, is referred to as a video return signal or a return path signal. In response to user- entered command, the set top box generates a digital signal that modulates a radio frequency carrier having a frequency of between about 5 — 42 MHz. According to the present invention, the analog modulated signal is converted to digital form, demodulated, decoded to extract the data packets carrying the information and converted to an optical signal for transmission over the PON 12. At the receiving end, the video return signal is modulated back to an analog radio frequency signal, representative of the radio frequency modulated signal produced by the set top box, and forwarded to the CATV head end for processing. Extending the capability of the PON 12 to carry return path signals, as taught by the present invention, permits utilization of the PON 12 in lieu of the hybrid fiber-coax system 60 to provide enhanced CATV services to subscribers. As illustrated in Figure 5, the set top box 64 is connected to the optical network unit

21/22 via a splitter/combiner 98. Video program signals carried downstream over the PON 12 are received by the set top box 64 and supplied to the television 96 in response to user commands entered to the set top box 64, e.g., television channel selection. User-entered commands intended for the CATV head end, e.g., for controlling video programs supplied to the subscriber, are carried upstream over the PON 12 as an optical signal on the 1310 nm upstream channel as described further below. Conventional set top boxes 64 generate the return path signal in the form of a radio frequency signal modulated by an information signal (typically a digital information signal) representing the user's command. When used with the coaxial cable CATV system, the radio frequency signal is carried over the coaxial cable back to the head end for effecting a response to the user's command, such as supplying the user with a selected pay-per-view movie for displaying on the television 96. When used with the hybrid coax-fiber CATV system, the radio frequency set top box signal is carried over the coaxial cable to a node joining the fiber optic cable to the coaxial cable. Here the radio frequency signal is converted to an analog optical signal for transmission over the optical fiber to the CATV head end. Returning to Figure 5, according to the teachings of the present invention, the ONU 21/22 converts the radio-frequency modulated signal produced by the set top box 64 to digital form, demodulates (within a demodulate 1 10) and processes the signal and supplies a base band digital signal to the ONU 21/22. Once demodulated, the return path signal is presented to the ONU 21/22 in the same format as the other digital signals supplied thereto by the subscriber, i.e., digital signals from other components of the customer premises terminal equipment 24. According to one embodiment, the return path signal is carried over the PON 12 via the 1310 nm wavelength (i.e., digital modulation of the 1310 nm wavelength signal by the base band digital signal) for receiving at the CATV head end as further described below. To transparently support existing set top boxes 64 and CATV head end equipment, in one embodiment the invention further comprises a modulator 116 (e.g., disposed within the OLT 42) for receiving the return path signal in the form of an optical signal, transforming the signal to electrical form and modulating a radio frequency carrier signal in response thereto. The modulated radio frequency signal supplied to the CATV head end, as indicated in Figure 5, thus is in the same format as the return path video signal received at the head end over a prior art coaxial or hybrid coax-fiber CATV system. At the CATV head end the return path video signal is demodulated and processed in accordance with the information contained therein. The PON 12 provides timing synchronization between the modulator 1 16 and the demodulators 1 10 to permit the modulator 1 16 to transparently reproduce the set top box signal at the OLT-side of the PON 12. Also, a power of the modulated radio frequency signal (also referred to as the radio frequency head end signal) bears a direct relationship to a power of the set top box signal. By providing representing the set top box signal power in the modulated signal, ranging tests can be performed at the CATV head based on that power level. Such ranging tests are known in the art as conducted on prior art CATV systems. As a result of the ranging tests, the CATV head end can issue a command to the set top box to increase or decrease the power level of the set top box signal. According to one embodiment of the invention, the modulator 116 is co-located with the OLT 42 on one of the OLT'S PON interfaces, thereby providing a data path between the modulator 116 and each ONU 21/22 that is connected to the OLT 42 via the PON 12. The OLT 42 provides network management functions for the modulator 116, as it provides network management functions for the ONU'S 21/22, using the existing PON OMCI management protocol and thereby avoiding the need for a separate network control scheme for modulator 116. Treating the modulator 116 as an ONU 21/22 simplifies network management, as the modulator 116 can be managed by the OLT 42 as it manages the ONUS 21/22 according to the OMCI protocol. It may be necessary to augment the network management protocol with additional managed entities related to configuration and control of the modulator 116, such that optional/configurable elements of the modulator 116 can be controlled by the OLT 42. Although the demodulator 1 10 is illustrated as an element of the ONU 21/22 and the modulator 1 16 is illustrated as an element of the OLT 42, these are not necessarily required configurations as in another embodiment the demodulator 110 and the modulator 116 comprises separate elements interfacing with the ONU 21/22 and the OLT 42, respectively. A more detailed illustration of the interface between the set top box 64 and the ONU 21/22 is depicted in Figure 6. The signal from the set top box 64 (only one shown in Figure 6) is converted from analog- modulated (QPSK or QAM, for example) to digital format in an A/D converter 140 and supplied to the demodulator 110. A host processor 142 controls the demodulator 110 by issuing commands over a conductor 144 and receiving data from the demodulator over a conductor 146. The host processor 142 transmits the optical signal over the PON 12 in the form of one or more ATM (asynchronous transfer mode) cells.

The return path signal is carried over the PON 12 to a host processor 148 in the OLT 42. Depending on the features of the set top box 64, the origin of he return path signal, i.e., the identity of the set top box 64 that sent the signal, is determined by the timing information provided in the set top box signal or by virtual connection information in the ATM data cell that encapsulates the set top box signal. The host processor 148 supplies the received data to the modulator 1 16 over a conductor 150 and controls the modulator 116 by issuing commands thereto over a conductor 152. Analog modulated signals produced by the modulator 116 in response to the return path signals are input to a D/A converter 156 for producing a radio frequency output signal that is supplied to the CATV head end. Thus a command issued by the subscriber to the set top box 64 has been carried over the PON 12 to the CATV head end. Various symbol rates are supported by the modulator 110 and the demodulator 116 as required to interface with set top boxes 64 supplied by different vendors. These symbol rates include, but are not limited to (in kilosymbols per second): 128, 160, 320, 640, 772, 1280, 1544 and 2560. According to different embodiments of the present invention, the upstream bandwidth, channel bandwidth and channel spacing are, respectively: 8-26.5 MHz, 781.25 MHz and 192 MHz, or 8-42 MHz, 129.54 MHz and 192 MHz. To transparently integrate the demodulator 110 and the modulator 116 with the PON 12, the signal received by the CATV head end must maintain accurate timing and power levels relative to the return path signal produced by the set top box 64. According to certain CATV standards, each set top box 64 is assigned a time slot during which it can transmit a signal to the CATV head end. Thus, in a typical CATV system both power and timing are calibrated and negotiated between the head end and each set top box 64. When the set top box signal is carried over the PON 12, as in the present invention, the power level and timing information originating with the set top box signal must be maintained or supplied by the signal generated in the modulator 116. According to a preferred embodiment, timing synchronization is performed by the host processors 142 and 148. As is known in the art, in certain CATV systems, each set top box 64 in the CATV system is assigned a time slot during which it transmits signals to the CATV head end. To account for the delay between sending the signal from the set top box 64 and its receipt at the head end, a time offset is assigned to each set top box to ensure that the signal is received at the head end during the correct time interval. The offset for each set top box is determined during ranging tests conducted by the head end equipment. To maintain the proper timing relationships for set top box signals carried over the PON 12 according to the present invention, the demodulator 1 10 comprises a 32-bit timing counter (in one embodiment) that is controlled according to a system clock of the PON 12. According to one embodiment, the demodulator counter is incremented with a programmable phase step on each system clock tick by the host processor 142. Each packet of the set top box signal is time-stamped by the timing counter. This timing information is carried over the PON 12 as a component of the return path signal. Upon receipt at the modulator 116, a delay interval is added to the return path signal such that all set top box signals from the plurality of set top boxes 64 in the CATV system maintain their proper timing relationship relative to all other set top box signals and their constituent packets. Thus this technique assures a deterministic data transmission time between the PON 12 and the CATV head end. In one implementation of this timing scheme, the modulator 116 comprises a similar 32-bit timing counter to determine the time of receipt of a received packet. The modulator timing counter is incremented with a programmable phase step on each system clock. When the modulator timing counter value matches the demodulator timing counter value, the packet is transmitted from the modulator 116 to the CATV head end. The host processor 148 supplies the timing information, as received from the PON 12, to the modulator 116. To ensure accurate power negotiation between the head-end and the set top box 64, a power level of the set top box signal as received by the demodulator 110 is replicated at the modulator 116. According to one embodiment, a power level value or estimate is determined by the demodulator 110 and bits representative of the power level are included within the signal transmitted to the modulator 116 over the PON 12. The power level of the modulated signal is determined in response to the set top box signal power. An exemplary block diagram of the demodulator 110 is illustrated in Figure 7. The A/D converter 140 (see Figure 6) digitizes the set top box signal and supplies a digital data stream to the demodulator 110 for generating a digital base band signal therefrom and performing certain digital signal processing functions thereon to retrieve the information signal. The information signal is supplied to the host processor 142 for modulating light at a wavelength of 1310 nm for transmission over the PON 12. The demodulator 110 comprises a digital down converter 190, comprising a mixer

192 for down converting the digital signal to base band and a decimator 194 for decimating the base band signal, in one embodiment, to a 2x over-sampled base band signal. The base band signal is fed to a base band processor 196 comprising a symbol tracking and recovery element 198 for detecting a unique word that precedes each data packet and determines the initial phase angle to optimize symbol recovery. The sampled data is input to a bit slicer 200 for de-mapping the data symbols into bits. From the bit siicer 200, the data is input to a forward error correction block 202 where Reed-Solomon error correction and de-randomizer functions are performed to extract the data from the bit stream. As is known by those skilled in the art, other error detection and correction techniques can implemented by the forward error correction block 202. The data is then passed to a host interface 204 for interfacing with the host processor 142 and controlling the various components of the demodulator 110 by issuing commands over a conductor 206. An AGC controller 207 responsive to a signal level of the digital signal from the A/D converter 140 provides an AGC control signal for the A/D converter 140 and the various elements of the demodulator 1 10 as is known in the art. According to one embodiment, the demodulator operates according to a first-in, first-out process relative to the set top box signals received from the plurality of set top boxes 64 as illustrated in Figure 4.

A block diagram of the digital down converter 190 as shown in Figure 8, comprises a numerically controlled oscillator (NCO) 220 and two multipliers 222 and 224 (that comprise the mixer 192 of Figure 7) for decomposing input data into I and Q base band components. Decimation is provided by a filter chain, including one or more cascaded integrated comb (CIC) filters 226 followed serially by a compensation finite impulse response (CFIR) filter

228 and a root raised cosine (RRC) filter 230. Those skilled in the art recognize that other techniques and filter types can be employed to provide the decimation function. The numerically controlled oscillator 220 generates a quadrature local oscillator signal for down converting the digitized set top box signal. According to one embodiment the NCO 220 supports two tuning inputs for the phase step. One input provides an initial frequency and the second provides a phase increment as may be required to avoid the accumulation of phase error depending on the type of modulation employed by the set top box 64. For DQPSK modulation it is not necessary to track the carrier as the symbols are differentially encoded and no phase error is accumulated. In an embodiment employing QAM, for example, it may be necessary to implement carrier tracking using a Costas loop or a similar approach. In certain embodiments it is desired to minimize the amount of data to be processed and to filter out adjacent channels. Thus the filters 226 decimate and filter the data. To support a large dynamic range of symbol rates, in one embodiment the filters 226 comprises two cascaded integrated comb filters. The decimation rate of the filters 226 can be made programmable, in response to a command from the host interface 204 on the conductor 206, based on the symbol rate of the set top box signal. The compensation FIR filter 228 reduces the data rate by an additional factor of two and provides better rejection than the CIC filters 226. Since the pass band of the CIC filters 228 is not perfectly flat, the compensation FIR filter 228 compensates the CIC filter roll off.

The RRC filter 230 (in one embodiment having programmable coefficients loaded by the host interface 204 over the conductor 206). The appropriate coefficients depend on the decimated symbol rate as well as the roll-off factor for a particular transmission. The ideal total decimation factor represents the divisor required to develop exactly 2x over-sampled data at the output of the RRC filter 230 using a 100 MHz A/D sample clock. Since the ideal decimation factors are fractional, the decimation rates for the CIC filters 226 and the CFIR filter 228 are calculated to yield a down-converted over-sampled rate as close to the exact integer of 2x as possible. The difference between the ideal 2x over-sampled data and the actual decimation rates represent a need for a fractional interpolator that is implemented in the base band processor 196. A block diagram of the base band processor 196 is shown in Figure 9. The base band processor 196 receives the 2x over-sampled base band data for producing the raw binary data to be processed by the forward error correction block 202. The base band processor 196 provides multi-phase interpolation for fraction symbol resolution, initial timing estimation, symbol timing track and bit slicing. Multiphase interpolators 250 process the incoming I and Q base-band data, which is approximately 2x the symbol rate and interpolate the data to a higher rate, such as 8x or

16x of the symbol rate. The interpolated data is used to determine which phase of the symbol time yields the best bit error rate. Several techniques can be used, one embodiment utilizes a polyphase FIR structure. Two correlation banks 252, each comprising a plurality of correlators (where the specific number of correlators depends on the multiphase interpolation rate) are each responsive to one of the I and Q channels to detect the unique word that indicates the initial phase angle. According to one embodiment, each of the I and Q phases is correlated to determine the phase with the highest correlation, from which the initial symbol time is determined. To avoid carrier recovery, the signal is differentiated prior to correlation so that the correlation is independent of carrier phase rotation.

To maintain a system with a single system clock, the actual symbol rate likely will not be one of the exact phases out of the interpolator. As a result, the input data to the correlator will take "phase steps" to maintain an average timing of one sample per symbol at the input of each correlator bank 252. During each symbol time the maximum output from each correlator in each of the correlation banks 252 is compared in a maximum correlation detector 256. The highest correlation value and its phase is provided at the output of the correlation detector 256. The correlation value is checked against a threshold to determine if the unique word has been received and to signal the beginning of a burst. The phase serves as the initial estimate for symbol timing recovery. If a higher precision is necessary, two adjacent correlator values to the maximum correlator value can be used and fit to a parabola, with the apex indicating the best symbol sampling time. The I and Q channel values are supplied to a timing error detector 257 for providing an input to a symbol tracker 258 as shown. The reception of the unique word is time-stamped based on the receiver global timing counter for use later by the modulator 116 to ensure proper timing utilization of the TDMA system. The sampler block consists of a symbol NCO that runs at the symbol rate. Once a burst is detected in the correlator block, the initial phase of the symbol time is forced into the phase accumulator of the symbol NCO. At each rollover of the NCO a new symbol is sampled and fed to the bit-slicer to perform the constellation de-mapping. The sampler can be implemented using a linear interpolator to further interpolate between the 16 phases of the symbol time that is fed in at the input. The bit slicer 200 performs the symbol-to-bit-mapping and determines the received power. According to one embodiment, the symbols are de-mapped using a CORDIC rectangular to polar converter 260, that allows both an angle and a magnitude calculation, and a differential demapper 261. The angle value is used to determine the output bit stream using preset values of the differential carrier phase. The magnitude is integrated over the burst time and a value representative thereof is appended to the end of the packet that is transmitted over the PON 12. The modulator 116 uses this value to reconstruct the signal with substantially the same power level as was received at the demodulator 1 10. Figure 10 illustrates a block diagram of the forward error correction (FEC) block 202, comprising a programmable de-randomizer 280 followed by a variable Reed-Solomon decoder 282. According to one embodiment, the Reed-Solomon decoder 282 is programmable to support various error correction modes. In addition, if an error is detected or if the decoding process fails, one or more flags are appended to the end of the packet for analysis by the host processor 148 at the receiving end. The output of the Reed Solomon decoder 282 is the decoded data that is buffered in a memory buffer 286 before transfer to the host interface 204 for transmission over the PON 12. A block diagram of the host interface 204 of the demodulator 1 10 is shown in Figure 1 1. The host interface 204 communicates with the host processor 142 and provides control, monitoring and data transfer for the demodulator 110. The host interface 204 comprises a receiver global timing counter 290 and a command and control block 294 for providing commands to the demodulator elements as described above in response to various status signals received from other demodulator components. Packet transmission control is achieved using a packet control signal (issued by the command and control block 294) to gate the data signal from the forward error correction block 202 through an AND gate 296. The receiver global timing counter 290 determines the time at which the set top box signal was received and provides this information to the command and control block 294 for time stamping the demodulator signal. Additional information provided by the demodulator 1 10 is appended to the upstream decoded data to form the PON data packet, comprising a preamble and appended data words. Note that the set top box signal also comprises a data packet (referred to as the set top box data packet) including a preamble, that may include an identification of the set top box. The PON data packet comprises the set top box data packet repackaged into an ATM cell for transmission over the PON 12. The appended data words within the PON packet include additional information that includes but is not limited to the time stamp as determined by the receiver global timing counter 290, the power level of the set top box signal, forward error correction detected errors and forward error correction failures. The PON data packet is a fixed length for all combinations of data rates and FEC encoding, as determined by the ATM protocol. A block diagram of the modulator 116 including its external interfaces, a host interface 310, a forward error correction block 312 and a symbol processor 314 is shown in Figure 12. The optical signal received via the passive optical network 12 is transformed from an optical to an electrical signal prior to processing by the modulator 116. In one embodiment, the modulator is absent such that the PON data packet received at the OLT is sent to the CATV head end in digital base band form, that is, the signal is not modulated to a radio frequency format. To avoid fractional rate conversion within the modulator 116, it may be necessary to fix all interpolation factors at an integer multiple of the input clock. As a result, a plurality of different reference clock frequencies are required to support the various rates. A programmable clock synthesizer 318 generates the plurality of reference clocks input to the modulator 116 and the host processor 148. In another embodiment, a plurality of crystals 320 are used in lieu of the clock synthesizer 318. The host interface 310 comprises a transmit global timing counter 322 that determines the current time (based on a PON clock) and provides same to the host processor 148. The global timing counter 322 also determines the time at which the set top box signal was sent based on the time stamp in the received PON packet, which includes the time stamp. According to the teachings of the present invention, a difference between the time stamp of the set top box signal and the radio frequency signal produced by the modulator equals a predetermined fixed interval. Thus the host processor 148 delays sending the modulator signal until the fixed delay interval has elapsed. Since the communications link between the CATV head end and the set top box can be a time slotted link, preserving the time at which the set top box signal was sent, by inserting the time stamp in the demodulator signal, allows the CATV head end to maintain that time slotted relationship, after the known delay period is accounted for. The host interface 310 sends received data packets to the forward error correction block 314 for error correction and randomization of the data. The output signal from the forward error correction block 314 is supplied to the symbol processor 314 that maps data bits into symbols and interpolates the data either by 2x or 8x as determined by the interpolation rate employed in the demodulator 110. An output signal from the symbol processor 326 feeds a quadrature up-converter 340 that in one embodiment includes additional interpolation, quadrature modulation, filtering and D/A conversion. A block diagram of the forward error correction block 312 is illustrated in Figure 13. Data from the host interface 310 is buffered in a memory buffer 350 until a full packet has been received. The full packet is passed to a variable rate Reed-Solomon encoder 352. In one embodiment, both the code size and the number of check words are programmable and are set by the contents of control registers within the modulator 116. Also, the Reed-

Solomon encoder 352 is variable with respect to packet size. Data from the Reed-Solomon encoder 352 is sent to a randomizer 354 to implement a scrambling polynomial (several are known in the art) that equalizes the number of one bits and zero bits. The unique word is not scrambled and is inserted as a packet header via a packet control signal from the host interface 310 to an AND gate 356 that gates the data signal from the randomizer 354.

Data from the symbol processor 314 is supplied to the quadrature up-converter 340. The symbol processor 314 comprises a symbol mapper 370 (see Figure 14) that maps the bits received from the forward error correction block 312 into symbols, and interpolating filters 372 that interpolate the data either by 2x or 8x, as determined by the input symbol rate. A functional block diagram of one embodiment of the quadrature up converter 340, shown in Figure 15, accepts I and Q data time-division-multiplexed onto the same bus and de-multiplexes the data in a demultiplexer 380 into two separate paths for processing. Each path comprises a CFIR filter 382 that compensates for the roll off incurred by a subsequent CIC filter 384. Interpolators 388 are disposed between the CFIR filter 382 and the CIC filter 384 in each data path. Output signals from the CIC filters 384 are modulated in a quadrature modulator 390 that receives sine and cosine waveforms from an integrated direct digital synthesizer or

NCO 392. According to one embodiment, the NCO 392 is tunable to provide the appropriate carrier waveform frequency via a control register within the NCO 392. A sinx/x compensation filter 396 precedes a digital-to-analog converter 156. An output scalar 399 and multiplier 400, included in one embodiment as required, cooperate to scale the power level to the power level of the set top box signal as received at the demodulator 110. The scaled signal is supplied to the digital-to-analog converter 156 for producing the radio frequency signal sent to the CATV head end. While the present invention has been described with reference to preferred embodiments, it will be understood by those skilled in the art that various changes may be made and equivalent elements may be substituted for the elements thereof without departing from the scope of the present invention. The scope of the present invention further includes any combination of elements from the various embodiments set forth herein. In addition, modifications may be made to adapt a particular situation to the teachings of the present invention without departing from its essential scope. Therefore, it is intended that the invention not be limited to the particular embodiments disclosed, but that the invention includes all embodiments falling within the scope of the appended claims.

Claims

WHAT IS CLAIMED IS: 1 . A method for sending a return path radio frequency signal from a set top box to a cable television head end over an optical network, comprising: producing a base band digital signal in response to the return path signal; converting the base band signal to an optical signal; transmitting the optical signal over the optical network; producing a modulated radio frequency signal in response to the optical signal; and transmitting the modulated radio frequency signal to the cable television head end.

2. The method of claim 1 wherein the return path signal comprises an analog signal, and wherein the step of producing the base band signal further comprises converting the return path signal to digital form and decoding the return path signal to produce the base band signal.

3. The method of claim 1 wherein the step of producing the modulated radio frequency signal further comprises producing a digital electrical signal in response to the optical signal, modulating the digital electrical signal to a radio frequency and converting the modulated electrical signal to analog form.

4. The method of claim 3 wherein a power-related parameter of the modulated electrical signal is responsive to a power-related parameter of the return path signal.

5. The method of claim 1 wherein the optical signal comprises a component representative of a power-related parameter of the return path signal.

6. The method of claim 5 wherein a power-related parameter of the modulated radio frequency signal is responsive to the component.

7. The method of claim 6 wherein the power-related parameter comprises a power level of the modulated radio frequency signal.

8. The method of claim 1 wherein the optical signal comprises a component representative of a timing parameter of the return path signal.

9. The method of claim 9 wherein the optical network provides a timing signal for determining the timing parameter.

10. The method of claim 1 wherein the modulated radio frequency signal bears a predetermined timing relation to the return path signal.

1 1. The method of claim 10 wherein the modulated radio frequency signal is transmitted to the cable television head end after a predetermined fixed delay from generation of the return path radio frequency signal.

12. The method of claim 1 wherein video program signals are supplied to the set top box from the cable television head end over the optical network.

13. The method of claim 1 wherein the optical network comprises a passive optical network.

14. A method for sending a return path radio frequency signal from a set top box to a cable television head end over an optical network, comprising: converting the return path signal to a digital format signal, wherein the digital format signal comprises one or more of a power-related parameter and a timing-related parameter of the return path signal; converting the digital format signal to a base band signal; recovering symbols from the base band signal; determining data bits from the recovered symbols; appending error correction bits to the data bits; forming an ATM cell from the data bits and the error correction bits; modulating a light source in response to the ATM cell to form an optical signal; transmitting the optical signal over an optical network; and transforming a received optical signal to an electrical signal in the form of base band data bits for forwarding to the cable television head end..

15. A system for transmitting a return path signal from a set top box to a cable television head end, comprising: a demodulator for producing an optical signal in response to the return path signal; a passive optical network for carrying the optical signal; and a modulator for producing a radio frequency signal in response to the optical signal, wherein the radio frequency signal is transmitted to the cable television head end.

16. The system of claim 15 wherein the passive optical network comprises a fiber optic cable, a optical line terminal and a plurality of optical network units, and wherein the demodulator is disposed in one of the plurality of optical network units.

17. The system of claim 16 wherein the modulator is operative in conjunction with the optical line terminal.

18. The system of claim 15 wherein the demodulator determines a power parameter of the return path signal, and wherein a parameter of the radio frequency signal is responsive to the power parameter.

19. The system of claim 18 wherein the power parameter comprises a power level of the return path signal.

20. The system of claim 15 wherein the optical signal comprises a component representative of at least one of a timing parameter of the return path signal and a power parameter of the return path signal.

21. The system of claim 20 wherein the passive optical network provides a timing signal for determining the timing parameter.

22. The system of claim 15 wherein the radio frequency signal bears a predetermined timing relation to the return path signal.

23. A system for transmitting a return path signal from a set top box to a cable television head end, comprising: a demodulator for producing a base band optical signal in response to the return path signal; a passive optical network for carrying the optical signal; a transceiver for transforming the optical signal to a digital base band signal, wherein the digital base band signal is provided to the cable television head end; and wherein at least one of a power parameter and a timing parameter of the return path signal is provided to the cable television head end.

24. The system of claim 23 wherein the at least one of the power parameter and the timing parameter is provided to the cable television head end as a component of the digital base band signal.