WO2001056197A2

WO2001056197A2 - Digital downstream communication system

Info

Publication number: WO2001056197A2
Application number: PCT/US2001/002503
Authority: WO
Inventors: Forrest M. Farhan
Original assignee: Scientific-Atlanta, Inc.
Priority date: 2000-01-28
Filing date: 2001-01-25
Publication date: 2001-08-02
Also published as: WO2001056197A3

Abstract

A broadband communication system includes an optical transmitter (410) for receiving an analog electrical signal and transmitting a digital optical signal. The transmitter (410) includes an analog filter (415) having a bandwidth for filtering the analog electrical signal to generate a bandwidth-limited analog signal having a lower frequency and an upper frequency. The bandwidth-limited analog signal is sampled by an analog-to-digital converter (415) at a sampling frequency that equals or exceeds the bandwidth of the analog filter (408), thereby generating a digital electrical signal that is shifted in frequency from the analog electrical signal. The lower frequency of the bandwidth-limited analog signal is greater than the sampling frequency used by the converter (415). A digital filter (422) then processes the output of the analog-to-digital converter (415) to pass the digital electrical signal that is frequency-shifted, and a laser transmitter (435) that transmits the digital electrical signal.

Description

DIGITAL FORWARD COMMUNICATION SYSTEM

Field of the Invention

This invention relates generally to broadband communications systems, and more specifically to systems for the distribution of video, digital, and other information signals from a transmitting station to number of receiving stations via optical fibers.

Background of the Invention

Cable television (CATV) systems typically include a headend section for receiving high frequency signals and demodulating them to baseband. In modern systems, signal sources may include satellites for digital and analog television programming, public telephony networks for voice telephony, and digital networks, such as the Internet, for the transfer of computer generated data. Regardless of origin, the headend transforms these signals to a composite broadband frequency division multiplexed analog signal that is transmitted via fiber optic cable to nodal stations, i.e., nodes, in the cable distribution plant. At the nodes, the optical signal is received and converted to a radio frequency (RF) electrical signal that is carried to individual subscribers by a "tree" network of coaxial conductors and amplifiers.

In the fiber optic portion of a typical prior art cable television transmission system, information is transported by amplitude modulation of a lightwave carrier in an analog fashion. Various factors influence the ability to accurately transmit and receive these signals. For example, optical nonlinearities degrade the signal-to-noise ratio of the signal as it travels through a fiber optic cable. Furthermore, analog modulation schemes are highly sensitive to variations in the input signal level and the electro-optical properties of the transmitting laser. As a result, nonlinearities in the optical signal may be caused or worsened by temperature fluctuations and/or changes of the electrical characteristics of the laser diode junction with environment and time. In the reverse path of a cable television system, these problems may be addressed by using an on/off keyed, digital reverse system, such as that described in U.S. patent application serial no. 09/102,344 to Farhan et al., entitled "Digital Optical Transmitter" ("Farhan") the teachings of which are hereby incorporated by reference. However, difference in bandwidth requirements (i.e., 750 MHz forward vs. 50 MHz reverse) can render the direct use of the Farhan invention impractical for transmission in the forward direction. Thus, what is needed is an improved technique capable of providing accurate and reliable transmission of optical signals within a cable television system.

Brief Description of the Drawings

FIG. 1 is a block diagram of a bidirectional hybrid fiber/coax broadband distribution system in accordance with the present invention.

FIG. 2 is a block diagram showing the input and output ports of an analog-to-digital converter in accordance with the present invention.

FIG. 3 is a graph showing the input and output spectra of an undersampled analog to digital conversion process in accordance with the present invention.

FIG. 4 is a block diagram of an optical transmission system in accordance with the present invention.

FIG. 5 is a block diagram of an alternative optical transmitter employed in a broadband communication system in accordance with the present invention.

Detailed Description of a Preferred Embodiment

Optical communications systems with their inherently high information carrying capacity are commonly utilized for the transport of video and other information signals from a transmitting station to one or more receivers. Two-way communication may also be accomplished by choosing separate transmission wavelengths for the oppositely directed signals. FIG. 1 shows the optical portion of a hybrid fiber/coax (HFC) broadband distribution system, such as a cable television system, that is bidirectional and in which the majority of the available bandwidth is devoted to the distribution of analog and television signals from a headend station 1 10 to individual subscribers 155. Forward, or downstream, distribution occurs in the frequency range of about 50 MHz to 750 MHz or higher. A smaller band of frequencies, typically stretching from about 5 to 50 MHz, is reserved for the communication in the reverse, or upstream, direction.

In FIG. 1, satellite transmissions of analog and digital television signals are received by the headend station 110 and converted to an intermediate radio frequency by one or more receivers 112. Optionally, a digital transceiver 1 14 and/or telephony transceiver 1 16 may provide bidirectional links between the headend station 110 and external systems that are usually land-based. The digital transceiver 1 14 can be connected to the Internet, and the telephony unit 1 16 communicates with a public telephone network. In each case, these units 1 14, 116 transform electronic signals at an intermediate frequency, or other acceptable format, to modulator/demodulator 120 via RF cables 118. The modulator/demodulator 120 processes and combines the forward- travelling RF signals in a manner suitable for transmission by the laser transmitter included within the transceiver 126.

Optical information signals are carried from the headend 1 10 by optical fibers 130 to one or more fiber optic nodes 142 that convert the optical signals to RF electronic signals. These RF signals are distributed to subscriber equipment 155 by RF branches that may include one or more coaxial transmission lines 144, distribution amplifiers 146, line extenders 148, taps 150, and coaxial service lines 152 that connect the individual subscribers 155 to the system.

In the reverse direction, subscriber generated information that may include special programming requests, Internet communications, and voice telephony are transmitted in the reverse or upstream direction through the coaxial portion of the HFC plant to the fiber optic node 142 via a separate frequency than that of the forward band. At the node 142, this information is converted to a form suitable for optical transmission and sent, via laser transmitter, to the optical transceiver 126 at the headend station 1 10. The optical receiver 126 within the headend 110 converts the received upstream optical signal to an electronic form that can be processed by the modulator/demodulator 120. This unit 120 identifies, splits, and reformats appropriate portions of the return signal in a manner that is compatible with cable program selectors, the Internet, and the public telephone network, as necessary.

Conventionally, optical transceivers that are located throughout the optical portion of an HFC plant transmit information using an analog modulation format, such as intensity or amplitude modulation. While capable of transmitting high-bandwidth information signals, this modulation technique has a number of drawbacks. Specifically, nonlinearities in the fiber optic cable produce a degradation in the signal quality with distance. Furthermore, analog modulation schemes are highly sensitive to variations in the input signal level and the electro-optical properties of the transmitting laser. For example, nonlinearities in the optical signal may be caused or worsened by temperature fluctuations and/or changes of the electrical characteristics of the laser diode junction with environment and time.

Accordingly, the reverse path of the optical plant of the present invention transmits signals in an on/off keyed digital format to avoid problems presented by return path analog transmissions. More specifically, the reverse-directed RF signals received by the nodes 142 are digitized using conventional analog-to-digital converters and transmitted as a serial bit stream to the headend station 1 10. Because the optical information signal consists of a series of l's and 0's, reductions in the signal-to-noise ratio due to nonlinearities in the fiber and laser transmitter are minimized.

The comparatively narrow bandwidth and low frequency of the return band (5 MHz - 40 MHz according to most standards) permit on/off keying to be realized in a straightforward and economical fashion. Key to this ease of deployment is the availability of component-level analog-to-digital converters with sampling rates exceeding 80 MHz. Operated at their maximum clock speed, these components can digitize any return signal without violating the Nyquist criterion. This well-known principle states that significant distortion will be present in the digitized signal in cases where the highest frequency in the input spectrum exceeds the sampling rate of the analog-to-digital converter (ADC). According to currently-accepted standards, the forward-directed information is generally transmitted at RF frequencies between 50 MHZ and 750 MHz. Since ADCs operate at clock speeds of about 100 MHz, digitization of the forward-directed RF signal violates the Nyquist criterion. Therefore, direct application of the Farhan reverse digital invention to the forward direction would be likely to lead to an decrease in the signal-to- noise ratio far in excess of any improvement afforded by on/off keyed transmission, at least in cases in which conventional ADCs are used.

Selected bands, however, of the forward RF spectrum may be digitized using conventional, component-level ADCs and subsequently transmitted by on/off keying without experiencing this problem. To better understand the system proposed in accordance with the present invention, consider conversion of an analog RF signal to a series of digital output words by an ADC, as illustrated in FIG. 2.

The ADC 200 includes an RF signal input 210 and a clock input 213. In operation, the RF input signal is sampled at a rate determined by the clock. For example, an ADC 200 with a sampling rate of 100 MHz would determine the amplitude of the RF input signal at 10 nsec intervals. These amplitudes are expressed as binary numbers that are output on the parallel bus 218 at the sampling rate.

FIG. 3 shows graphs of the spectra of an idealized input signal 310 and the ADC output 315 under conditions where the sampling frequency 320 exceeds the bandwidth 325 of the input signal but is significantly smaller than the lowest input frequency 330. In the figure, f represents the value of the minimum input frequency 330 and f_h represents the value of the maximum input frequency 335. Thus, the bandwidth 325 of the analog input signal is Δ=(f_h-f|). If this signal is digitized at a sampling rate, f_s, satisfying the conditions that 2Δ < f_s < f_\, the spectrum of the output signal will be distorted as shown in the bottom graph that corresponds to the ADC output 315.

More specifically, several lower frequency images 340 of the input signal 310 are generated. These images have the same spectral content as the input signal 310, but are displaced from it by integral multiples of the sampling rate. When f = (N * f_s), the lowest frequency image 345 is a baseband representation of the input signal 310 starting at 0 MHz. According to the system of the present invention, the properties of this distortion may be advantageously used to establish one or more forward-directed, on/off keyed channels in the optical portion of the broadband distribution system of FIG. 1. If a frequency band in the input spectrum with a width Δ that is less than half the ADC sampling frequency is selected using a conventional bandpass filter, the distorted output of the ADC will contain a baseband image of the input signal. This image may be isolated using a digital low pass filter and subsequently transmitted using known technology. Bandpass filters having different center frequencies may be used to establish additional channels up to the limiting case in which the entire desired input spectrum is covered.

FIG. 4 is a block diagram of an optical communications system 400 that utilizes the method described with reference to FIG. 3 to digitize RF analog signals for subsequent transmission in the frequency band between about 700 MHz and 750 MHz. According to a preferred embodiment of the present invention, an analog input signal with a frequency spectrum covering the band from 50 MHz to 750 MHz is applied to the input 405 of the optical transmitter 410, which can, for example, be located in a headend section of a broadband communication system. An analog bandpass filter 408 selects the desired frequency band to be transmitted via on/off keying. In this example, the desired band is 50 MHz wide, with a minimum frequency, f_\, equal to 700 MHz and a maximum frequency, f_h, of 750 MHz. An RF transmission line 412 carries signals lying within the selected frequency band to the input of the ADC 415.

The sampling frequency of the ADC 415 is fixed by the clock input 418 at 100 MHz, a value equal to or greater than twice the bandwidth of the analog input signal. The ADC 415 undersamples the input, thereby generating a distorted series of digital output values on the parallel output bus 420. As shown in FIG. 3, the frequency spectrum of the ADC output contains images of the input spectrum that are shifted to lower values by integral multiples of the sampling frequency. In the example system of FIG. 4, the fourteenth image has a minimum frequency of zero and and maximum frequency of 50 MHz. This can be verified by subtracting fourteen times the sampling frequency (50 MHz) from the minimum frequency of the analog input (700 MHz). In the case where f| - Nf_s > 0, say when fi-Nf_s = d, where f_s/2 > d > 0, a digital spectral shifting may be required in order to shift d closer to zero. This process may the channel bandwidth transport capacity. Unwanted frequencies are removed from the ADC output by the digital low-pass filter 422, and the resulting 0-50 MHz image of the original signal is output on the parallel bus 425. Data on this bus is serialized and framed in a conventional manner by the serializer 428 and, optionally, combined with other digital information that may be used to generate a clock signal at the receiver. The serial output 430 of the serializer 428 is connected to the input of the laser transmitter 435, which generates an on/off keyed optical signal for downstream distribution within the broadband communication system over an optical communication channel 440.

At the receiver 450, which may, for instance, be a node or hub, a photodiode detector 455 converts the optical signal to a serial electronic bitstream 458. A deserializer 460 converts the photodiode output to a parallel format that is sent to a digital-to-analog converter (DAC) 465 on the parallel bus 462. The deserializer 460 extracts transmitted clock frequency information to generate a local clock signal 464 for the digital to analog conversion process. The analog signal generated by the DAC 465 is filtered by a low pass filter 467 to remove components lying outside the 0 - 50 MHz band.

The local clock signal 464 is also used to synchronize a phase-locked loop 469 that generates a frequency equal to the shift between the transmitter input spectrum and that spectrum actually transmitted. In this example, the frequency shift is equal to 700 MHz. The output 470 of the phase-locked loop 469 is mixed with the filtered DAC output 472 using a conventional RF mixer 475. The output of the mixer 475 includes an image of the baseband analog signal that is shifted by 700 MHz. Frequency components lying outside the 700 MHz - 750 MHz band are removed from the mixer output by a conventional analog bandpass filter 478. The receiver output 480 will be an image of bandpass-filter-selected transmitter input signal.

While a 100 MHz clock rate has been used for this example, it should be noted that the teachings herein can be applied to any ADC process in which the input data is undersampled and the bandwidth of the input signal is less than the sampling rate. Thus, the development of ADC components with faster sampling rates may, at some point, make it desirable to use the teachings herein to transmit the entire 700 MHz-wide forward broadband signal, or larger sub-bands thereof, using on/off keyed digital modulation. Assuming, for example, that ADCs were available with sampling rates of greater than 200 MHz, such as 300 MHz, the forward broadband signal spectrum could be divided into five segments for transmission along a single fiber using wavelength or time division multiplexing.

Until ADCs having much greater sampling rates are available, the entire spectrum from 50 MHz to 750 MHz can be transmitted as shown in FIG. 5. As shown in FIG. 5, parallel processing branches within an optical transmitter 500 can be employed to divide the incoming analog signal spectrum into equal 50 MHz sub-bands. This can be done, for example, by providing the analog input 505 to multiple bandpass filters 510, each of which pass a separate 50 MHz segment of the signal. When bandpass filters 510 are used to pass 50 MHz segments and the incoming signal has a frequency range of 50 MHz to 750 MHz, fourteen bandpass filters would receive the analog signal. The first would pass signals at 50-100 MHz, the second would pass 100-150 MHz, the third would pass 150- 200 MHz, and so on. It will be appreciated that this parallel processing scheme can be used to pass an analog signal having a frequency spectrum that is different from, greater, or smaller than that set forth in this example.

The outputs of each bandpass filter 510 are then provided to an ADC 515, which digitizes the signal as explained with reference to FIGs. 3 and 4. The resulting digital signals are filtered by digital lowpass filters 520 and passed to an interleaver 525, which uses time division multiplexing to generate interleaved outputs. Next, the interleaved outputs are serialized and framed by device 530, subsequent to which the resulting serial bit stream is transmitted as a digital optical signal by the laser transmitter 535. In this manner, the entire forward band, or larger parts thereof, can be digitally transmitted throughout the broadband communication system.

In summary, the digital transmission system described above provides for the optical transmission of one or more bands of a broadband analog signal via optical fiber while preventing many of the problems inherent in prior art analog systems. As a result, information can be sent from the headend station to the subscriber in a more reliable and less expensive manner.

What is claimed is:

Claims

1. An optical transmitter for receiving an analog electrical signal and transmitting a digital optical signal, the optical transmitter comprising: an analog filter having a bandwidth for filtering the analog electrical signal to generate a bandwidth-limited analog signal having a lower frequency and an upper frequency; an analog-to-digital converter (ADC) for sampling the bandwidth-limited analog signal at a sampling frequency that equals or exceeds the bandwidth of the analog filter, thereby generating a digital electrical signal that is shifted in frequency from the analog electrical signal, wherein the lower frequency is greater than the sampling frequency; a digital filter for processing the output of the ADC to pass the digital electrical signal that is frequency-shifted; and a laser transmitter for transmitting the digital electrical signal.

2. The optical transmitter of claim 1, wherein the optical transmitter is included in a headend station of a broadband communication system.

3. The optical transmitter of claim 1, further comprising: a serializer coupled between the digital filter and the laser transmitter for serializing and framing the digital electrical signal for transmission.

4. The optical transmitter of claim 1, wherein the analog filter is a bandpass filter.

5. The optical transmitter of claim 1, wherein the digital filter is a lowpass filter.

6. A broadband communication system, comprising: an optical receiver for receiving a digital optical signal transmitted within the broadband communication system; and an optical transmitter for receiving an analog electrical signal and transmitting the digital optical signal for reception by the optical receiver, the optical transmitter comprising: an analog filter having a bandwidth for filtering the analog electrical signal to generate a bandwidth-limited analog signal having a lower frequency and an upper frequency; an analog-to-digital converter (ADC) for sampling the bandwidth-limited analog signal at a sampling frequency that equals or exceeds the bandwidth of the analog filter, thereby generating a digital electrical signal that is shifted in frequency from the analog electrical signal, wherein the lower frequency is greater than the sampling frequency; a digital filter for processing the output of the ADC to pass the digital electrical signal that is frequency-shifted; and a laser transmitter for transmitting the digital electrical signal as the digital optical signal.

7. The broadband communication system of claim 6, further comprising: an optical communication channel coupling the optical transmitter to the optical receiver.

8. The broadband communication system of claim 6, wherein the optical transmitter is included in a headend station.

9. The broadband communication system of claim 6, wherein the optical receiver is included in one of a node and a hub.

10. The broadband communication system of claim 6, wherein the optical transmitter further comprises: a serializer coupled between the digital filter and the laser transmitter for serializing and framing the digital electrical signal for transmission.

1 1. The broadband communication system of claim 6, wherein the analog filter included in the optical transceiver comprises a bandpass filter.

12. The broadband communication system of claim 6, wherein the digital filter comprises a lowpass filter.

13. The broadband communication system of claim 6, wherein the optical receiver comprises: a detector for receiving the digital optical signal and converting it to a digital electrical signal; a digital-to-analog converter (DAC) for converting the digital electrical signal to an analog electrical signal; an analog filter for filtering the analog electrical signal to generate a filtered signal; a mixer for mixing the filtered signal to shift it in frequency by an amount equal to the lower frequency of the bandwidth-limited analog signal processed by the optical transmitter; and a bandpass filter for filtering the output of the mixer to generate a signal representative of the analog electrical signal provided to the optical transmitter.