Abstract:
This invention is a new communication system in which multichannel broadcast digital services are distributed to each user with the broadcast services signal riding in the passband above a digital baseband signal. The system can deliver more than 1 Gbps additional bandwidth to each subscriber. The passband bandwidth will accommodate growth in downstream services including video on demand, higher speed web downloads including improved streaming audio and video, HDTV, interactive video, and personalized video. The invention requires only a single fiber path and a single optical receiver for each user or group of users. A single fiber, single optical receiver system is much less expensive than two systems, one transmitting baseband and the other passband. A single receiver is greatly cost beneficial to achieving economical fiber to the home.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application claims the benefit of U.S. Provisional application Serial No. 60/112,480 filed by the present applicants on Dec. 16, 1998 and U.S. Provisional Application Serial No. 60/147,666 filed by the present applicants on Aug. 6, 1999. 
    
    
     FIELD OF THE INVENTION 
     This invention relates to optical communication systems and, in particular, to an optical communication system which optically combines baseband signals and passband signals and transmits the combined signals over a common optical fiber. 
     BACKGROUND OF THE INVENTION 
     An ever increasing communication need of today is to deliver multimedia services such as voice, data, high speed internet access, video conferencing, video on demand, and broadcast television video to small businesses and residences. Cost is the prominent issue for the deployment of such networks. Among various technologies that are currently available and being deployed, optical fiber extending to users—Fiber to the Home (FTTH)—is the preferred technology to meet present and future needs. Service providers are taking fiber as deep into their networks as their costs allow. 
     Two different optical fiber communication systems have evolved for carrying information in digital formats to homes and businesses. One system delivers information by a digitally modulated series of light pulses. These are referred to as baseband signals. A second system uses a plurality of frequency separated carriers. Each carrier is modulated to transmit a digital signal. These are passband signals. Each system has its own specialized equipment, its own physical plant and its own standards. 
     FIG. 1A schematically illustrates a baseband system  10  comprising a central office  11  providing optical fiber connections to a plurality of homes  12  and businesses  13 . High power optical signals at single or multiple wavelengths are transmitted over a plurality of access fibers  15 A,  15 B,  15 C to respective optical power splitters and/or wavelength demultiplexers  16 A,  16 B,  16 C, and at each power splitter or demultiplexer, e.g.,  16 B, the high power signal is divided into a plurality of lower power or separate wavelength signals and transmitted over a respective plurality of end user fibers  17 A and  17 B. These signals are called downstream signals. The downstream signals are typically a digitally modulated baseband series of light pulses centered in the 1.3-1.6 μm wavelength band. Signals from the end users to the central office, called upstream signals, are typically digitally modulated baseband pulses in the same 1.3-1.6 wavelength band but at different wavelength from the downstream wavelength. They are transmitted in the reverse direction over the same fibers. The upstream signals can be buffered and time division multiplexed for burst transmission at the power splitters, e.g.,  16 B. Since this system does not employ any active electronic or photonic component between the central office and the users, it is called a Passive Optical Network (PON). 
     FIG. 1B illustrates a simplified baseband modulation scheme. Typically, a digital  1  is represented by a light pulse in the series. A digital  0 , by the absence of a pulse in a pulse position. Alternatively, the signal can be inverted with a pulse representing digital  0  and its absence representing 1. 
     FIG. 2A schematically illustrates a passband system  20  comprising a hub  21 , and a plurality of fibers  22 A,  22 B,  22 C connecting the hub to a respective plurality of fiber nodes  23 A,  23 B and  23 C. Each node is connected, as by a plurality of fibers or coaxial cables  24 A and  24 B to a plurality of homes  12  and businesses  13 . 
     FIG. 2B illustrates the radio frequency spectrum of a typical digitally modulated passband signal. The signal comprises a plurality of different radio frequency (RF) carriers spaced apart in frequency (e.g. 6 MHz spacing in the NTSC system). Each of the carriers is modulated among a plurality of states to carry a higher order digital signal to encode plural bits for each modulation state. The modulation can be amplitude modulation, frequency modulation, phase modulation or a combination of them. 
     Digital passband signals are conventionally transmitted using two RF carriers that are frequency locked but 90 degrees out of phase. The two carriers are said to be in quadrature. The two carriers are separately amplitude modulated (AM), and the modulated carriers are combined to form a single RF output having both amplitude information corresponding to their vector sum and phase information corresponding to their vector angle. The technique is known as quadrature amplitude modulation or QAM. 
     FIG. 2C illustrates the simplest case of QAM which occurs when each of the carriers has only two states (e.g. +V and −V). One carrier, is considered the reference carrier and is called the in-phase channel. Its amplitude is represented along the horizontal axis of FIG.  2 (C). The other carrier, 90° out of phase, is called the quadrature channel. Its amplitude is represented along the vertical axis. As can be seen from the diagram, if each carrier has two states (+V, −V), then there are four possible combined outputs, each of which can represent two bits of information: (0,0), (0,1), (1,0), (1,1). This simple modulation scheme is known as quadrature phase shift keying (QPSK). 
     Similar modulation schemes can be based on amplitude modulation of the carriers among a larger number of states. For example if both carriers can be modulated among four amplitudes, the combined output can represent 4×4=16 states, and the modulation is called 16 QAM modulation. Modulation using 8×8=64 states is 64 QAM. With an increasing number of modulation states, the required signal-to-noise ratio also increases. 
     In the past few years there has been an international effort from service providers and system manufacturers to define common specifications aimed at the extension of fiber all the way to homes and businesses to deliver existing and future services. These specifications are now part of International Telecommunication Union (ITU) standard G.983.1 
     According to G.983.1, all services are transported in baseband format in both the upstream and downstream directions on a power splitter-based system. In one variant of the network, a shared 155-Mbps baseband signal is transported downstream in the 1.5-μm band and the same bit rate is sent upstream in the 1.3-μm band on a single fiber. For low cost, a single transmitter in the central office and a single fiber can serve up to 32 users if the fiber is all the way to the user&#39;s premises. The number of users can even be greater if the receiver is at the curb and electrical signals are distributed to multiple dwellings. The G.983.1 specification calls for a minimum logical reach of at least 20 km and an optical power budget consistent with that reach. The specified downstream receiver sensitivity at a bit error ratio of &lt;10 10  is −30 dBm for Class B operation and −33 dBm for Class C. 
     A downstream capacity of 155 Mbps shared among 32 end users is more than adequate for interactive services such as voice, data, or interactive video, but can be quickly exhausted by multichannel broadcast video, especially if high definition TV (HDTV) is to be delivered. One approach to dealing with broadcast video delivery in G983.1 is to increase the downstream bandwidth from 155 to 622 Mbps. This approach is very expensive and complicates video channel switching. Alternatively, video signals can be delivered on a separate fiber using a separate transmitter and a separate receiver. This approach is even more expensive. Accordingly, there is a need for a new approach which improves the performance and lowers the cost. 
     An optical communication system for gracefully combining both baseband and passband signals on a common fiber is described in applicant&#39;s U.S. patent application Ser. No. 09/432,936 filed Nov. 3, 1999 and entitled “Optical Communication System Combining Both Baseband and Passband Signals”, which is incorporated herein by reference. In this system, the baseband and passband signals are electrically combined, and the combined signal modulates an optical output signal at the Central office. The optical signal can be sent over an optical fiber to a remote power splitter where it is passively power split among a plurality of fibers to respective end users. Within the power budgets of ITU-T G.983.1, this architecture can support the QPSK modulation format that satellite TV uses for class B operation with a PIN diode receiver or class C operation with an APD receiver. For terrestrial transmission of broadcast digital services, most service provider&#39;s, such as providers of cable TV, or wireless cable TV (MMDS services), use 64 QAM or higher order modulation. Compared to QPSK, delivery of 64 QAM modulated signal requires about 13 dB more signal to noise ratio in the electrical domain which means 6.5 dBm more optical power at the receiver. Alternatively, the receiver should be at least 6.5 dB more sensitive. Experimental data show that to deliver a 64 QAM modulated passband signal on top of a 155 Mbps baseband signal in class B of G.983.1, an APD based receiver is required. An APD is much more expensive than a PIN diode. The APD operation requires a supply of typically more than 50V with much more complex voltage and temperature stabilization circuitry as compared to a PIN diode that requires less than 5V and much simpler circuitry. 
     Telecommunication and Cable TV service providers would prefer to use 64 QAM or higher order modulation for broadcast digital services to make use of their existing video infrastructure and to use the bandwidth efficiency of 64 QAM that can deliver up to 6 bits per Hertz as opposed to a maximum of 2 bits per Hertz by QPSK. To deliver 6.5 dBm more optical power for 64 QAM and to remain within the power budgets of ITU-T G983.1 for the baseband signal, there is a need for a different architecture (system) that can combine baseband and passband signals with an even higher level of performance. 
     SUMMARY OF THE INVENTION 
     This invention is a new communication system in which multichannel broadcast digital services are distributed to each user with the broadcast services signal riding in the passband above a digital baseband signal. The system can deliver more than 1 Gbps additional bandwidth to each subscriber. The passband bandwidth will accommodate growth in downstream services including video on demand, higher speed web downloads including improved streaming audio and video, HDTV, interactive video, and personalized video. The invention requires only a single fiber path and a single optical receiver for each user or group of users. A single fiber, single optical receiver system is much less expensive than two systems, one transmitting baseband and the other passband. A single receiver is greatly cost beneficial to achieving economical fiber to the home. 
     Although the result of the invention is to add digital video and other bandwidth demanding services on the system described in G983.1, it can also be used in other architectures where specifications or requirements differ from G983.1. For example, upstream or downstream data rate and optical wavelengths and the required receiver sensitivity and bit error rates may vary according to the specific application. The invention can also be used in point to point transmission of baseband and passband signals on a single fiber and receiver. Furthermore, the optical receiver or ONT does not have to be at the customer premises. It can be outside on the curb. From the curb, the baseband and passband services can be delivered to subscribers sharing that ONT on twisted copper wires or coaxial cables, in an architecture popularly known as Fiber to the Curb (FTTC). For twisted copper wires, the services can be delivered using any of the conventional digital subscriber line techniques. For coaxial lines, hybrid fiber-coaxial (HFC) technology is used. 
     In an exemplary embodiment, an optical communication system comprises a first optical transmitter for generating an optical baseband signal, a second optical transmitter for generating an optical passband signal, an optical power coupler for combining the signals, a length of optical transmission fiber for transmitting the combined optical signal, and one or more receivers optically coupled to the fiber. In an advantageous system, an optical power splitter is optically coupled to the transmission fiber for power splitting the transmitted signal among a plurality of end-user fibers, and, for each user or group of users, an optical receiver is coupled to the user fiber. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The advantages, nature and various additional features of the invention will appear more fully upon consideration of the illustrative embodiments described in connection with the accompanying drawings. In the drawings: 
     FIGS. 1A and 1B illustrate features of a typical baseband communication system; 
     FIGS. 2A,  2 B and  2 C illustrate features of a typical passband communication system; 
     FIG. 3 is a schematic block diagram of an exemplary optical communication system combining both baseband and passband signals; 
     FIG. 4 is a schematic diagram of an optical transceiver for the system of FIG. 3; 
     FIG. 5 is a diagram of a test system combining both baseband and passband signals; and 
     FIGS. 6-9 are graphical illustrations showing the results of tests using the system of FIG.  5 . 
     It is to be understood that these drawings are for purposes of illustrating the concepts of the invention and, except for the graphs, are not to scale. 
    
    
     DETAILED DESCRIPTION 
     FIGS. 1 and 2, which relate to conventional baseband and conventional passband systems were described in the Background of the Invention. 
     FIG. 3 is a schematic block diagram of an optical communication system  30  for combining both baseband signals and passband signals on one or more common optical transmission fibers  32 . In essence, the system  30  comprises a first optical transmitter  40  responsive to an input electrical baseband signal from a baseband source  28  and a second optical transmitter  41  responsive to an input electrical passband signal from a passband source  29 . Transmitter  40  can be a digital transmitter (e.g. directly modulated digital laser) digital laser, and transmitter  41  can be an analog transmitter (e.g. an analog laser or an externally modulated digital laser). The transmitters  40 ,  41  separately generate optical signals containing the information of their respective input signals. The optical signals are then optically combined in an optical combiner such as an optical power coupler  42 , and the combined optical signal is transmitted to one or more optical receivers  35 . At least one optical receiver  35  can detect and demodulate both the baseband signal and the passband signal. It is contemplated that the transmitters  40 , 41  will be located at a central office. It is also possible that the electrical to optical conversion of the passband signal may occur deeper in the network, presenting an optical passband signal at the central office. 
     In the preferred arrangement shown in FIG. 3 the passband signal comprises broadcast digital signals that will be virtually the same for all users whereas the baseband signal is dedicated to a particular group of users. Accordingly, it may be advantageous to amplify the optical passband signal in an erbium doped fiber amplifier (EDFA)  43  and apply the amplified output to an optical power splitter  44 . This provides plural optical passband signals that can be combined with other dedicated baseband signals for other groups of users (not shown). 
     In one embodiment, the baseband signal is a digitally modulated time division multiplexed (TDM) 155 Mbps baseband signal conforming to the power budgets and other specifications of the ITU industry standard G983.1. The passband signal is a quadrature phase-shift keying frequency division multiplexed (FDM) signal carrying greater than 1 Gbps of information. The baseband and passband signals are converted by transmitter lasers  40  and  41  into two different wavelengths in the 1.5 μm band such that the optical interaction between the two wavelengths does not cause any distortion of the baseband and passband signals which are combined in the optical domain. Although the preferred embodiment of this invention uses a single optical receiver for both baseband and passband signals for low cost, a variant of this invention may use an integrated optical module at the receiver to separate the passband signal for detection by a separate O/E receiver or for delivery to another location with a separate fiber. 
     An important advantage of this system is that the optical transmitters  40  and  41  can be separately optimized for their respective signals. Such optimization eliminates a 1 to 2 dB transmitter-related optical power penalty that would be incurred if a single transmitter were used, and it improves system performance. For example, the optical path for the passband signal can include an additional erbium doped fiber amplifier  44  to optimize passband transmission. 
     FIG. 4 schematically illustrates an advantageous optical transceiving arrangement  35  for end-users. The arrangement receives the transmitted combined optical signal from a fiber  34  through a MUX  50 . The combined signal is applied to a receiving element  51 , such as a PIN diode, and the resulting combined electrical signal is fed into a preamplifier  52 . The preamplified combined electrical signal is then split by diplexer  53  into its baseband portion and its passband portion. 
     To permit two-way communication with the end user, the transceiver  35  may advantageously include a transmitter laser  54  for receiving electrical signals from the users and generating an optical return signal at wavelength different from the optical input signal. This return signal is presented to the multiplexer  50  and sent back toward the central office by fiber  34 . The input downstream signals are preferably at two spaced apart wavelengths in the 1.48-1.60 μm range and the output upstream signal is preferably at 1.3 μm. 
     The invention may now be better understood by consideration of the following specific examples and tests. In the examples and tests, which are intended for those skilled in the art, the acronyms used are expanded when they are first used and in the definitions section at the end of this specification. 
     The following examples are provided for the purpose of illustration only. The examples should not be construed as limiting the invention. 
     EXAMPLE 1 
     This example is to demonstrate this invention&#39;s feasibility. The experimental detail combined with the description above shows advantages of various possible components and methods. 
     FIG. 5 schematically illustrates the test apparatus wherein an electrical baseband modulation signal applied to a DFB laser transmitter  40  generates an optical baseband signal. An electrical 64 QAM passband signal derived from a MMDS signal is converted to an optical passband signal in a second DFB laser  41 , and the baseband and passband optical signals are combined in a 2×2 optical power combiner  42 , transmitted over a common fiber  32  to a power splitter  33 . An arm  34  of the splitter transmits a portion of the received optical signal to an end-user receiver  35  including a diplexer  53 . The receiver  34  was a Lucent Technologies 1319 O/E receiver. The receiver comprises a PIN detector and a transimpedance amplifier (TIA). A diplexer  53  is used to separate the baseband and passband signals at the output of the receiver. Electrical circuitry pertinent to the particular passband signal used and to the tests conducted (but not to the actual systems using the invention) will be described in connection with these tests. 
     We have used a 64-QAM MMDS video signal for the passband signal. The signal comprises 31 carriers with 4 empty slots in 222-408 MHz. Each carrier is 6 MHz wide and carries 5.063 M symbols/s. At one of the empty slots at 279 MHz, we inserted a 6 MHz wide 64-QAM test carrier for bit error rate and constellation studies from a Broadcom modulator. The power level of the test carrier was adjusted to the level of the MMDS carriers. The DAVIC scheme was used for forward error correction (FEC) as offered by the Broadcom modulator  60 . For the passband signal, we used both a directly modulated DFB digital laser transmitter and a commercially available transmitter from Harmonics Lightwave that is optimized for analog signals. 
     The outputs of the receiver  35  are shown in FIGS.  6 ( a ),  6 ( b ) and  6 ( c ). FIG.  6 ( a ) shows the composite signal at the output of the receiver. This composite includes both the baseband and the passband signals with a notable separation between them. 
     FIG.  6 ( b ) shows the high frequency output of the diplexer comprising the MMDS digital video carriers in 222-408 MHz. This constitutes the passband signal. 
     FIG.  6 ( c ) shows the low frequency output of the diplexer constituting the 155 Mbps baseband signal. The diplexer separated the baseband and passband signals with negligible insertion loss for either signal. 
     FIG. 7 compares the system performance of an analog transmitter optimized for analog signals with a digital transmitter for the passband signal. The analog transmitter was obtained as a commercial product from Harmonic Lightwave. The circuit of this transmitter had been designed for optimum analog video modulation. The digital transmitter has a DFB laser on a laser test fixture. FIG. 7 plots the bit error rate of the received signal versus the received optical power. The measurements were taken on the 279 MHz test carrier with all MMDS carriers present but without the baseband signal present. Data are shown for both digital and analog transmitters and before and after forward error correction. The forward error correction in the DAVIC scheme provides about 4 dB optical coding gain for a bit error rate of 10 −10 . The coding gain decreases with increasing bit error rate. The data shows that the analog transmitter gives a 2-dB optical power improvement in system performance as compared to the digital transmitter. 
     Qualitatively, no impairment was observed in the picture quality or sound quality of the 285 MHz channels on TV for a test channel bit error rate of less than 5×10 −3  before forward error correction. This corresponds to −26.5 dBm passband optical power at the receiver. After FEC, this corresponds to about 10 −10  BER. It is believed that a system built for a 10 −6  BER before FEC will result in an essentially error free signal after FEC. This requires about −24.5 dBm optical signal at the receiver in the absence of a baseband signal. 
     FIGS.  8 ( a )- 8 ( c ) plot the logarithmic error probability of the passband in the presence of respectively different baseband optical power levels. FIG.  8 ( a ) is measured with −28 dBm of baseband optical power, FIG.  8 ( b ) with −25 dBm and FIG.  8 ( c ) with −22 dBm optical powers. The optical power penalty on passband due to the presence of baseband varies with the baseband power. The optical power penalty is 0.2 dB, 0.6 dB and 0.75 dBm for baseband powers of −28 dBm, −25 dBm and −22 dBm, respectively. 
     FIG. 9 shows the logarithmic error probability of the baseband with and without the presence of a passband signal at −24 dBm and −25 dBm optical power. Based on these measurements and other measurements at high passband optical power, it is believed that the presence of a passband signal at less than −11 dBm optical power adds negligible noise in the baseband region. The baseband power penalty is less than 0.5 dB. 
     As the baseband optical power increases beyond −22 dBm, the noise due to baseband harmonics increases in the RF range of the passband. This increases the power penalty on passband as shown in FIGS.  8 ( a )- 8 ( c ). Thus baseband harmonics should be minimized by filtering the baseband signal and biasing the baseband laser in a linear region. This may reduce the extinction ratio in the baseband below the FSAN required 10 dB. Nonetheless, the system can operate at less than −22 dBm for either type of signal. 
     These tests show that a digitally modulated passband signal can be delivered over the same optical fiber along with a baseband signal. This system can use most existing transmission equipment and baseband transmission power budgets (e.g. ITU-T G983.1). Approximately 1-5 Gbps transmission rates can be achieved compatible with existing components. This additional bandwidth can be used for broadcast and switched video and for other bandwidth demanding services. It can also be used to provide a dedicated channel of desired bandwidth to any subscriber. The proposed technique also provides a graceful upgrade of currently installed passive optical networks without incurring significant additional costs or power penalty. If some subscribers want to receive baseband data only and no video or any other passband services, the system is compatible with baseband only ONU with negligible power penalty. 
     It will be clear to those skilled in the art that many modifications and variations may be made to the embodiments as described without substantially departing from the spirit and scope of the present invention. It is intended that all such modifications and variations be included herein within the scope of the present invention, as is set forth in the appended claims. 
     
       
         
               
             
               
               
               
             
           
               
                   
               
               
                 DEFINITIONS 
               
             
          
           
               
                   
                 Acronym 
                 Meaning 
               
               
                   
                   
               
               
                   
                 APD 
                 avalanche photodetector 
               
               
                   
                 BER 
                 bit error rate 
               
               
                   
                 DAVIC 
                 digital audio visual council 
               
               
                   
                 DSS 
                 digital satellite service 
               
               
                   
                 EDFA 
                 erbium doped fiber amplifier 
               
               
                   
                 FDM 
                 frequency division multiplexing 
               
               
                   
                 FEC 
                 forward error correction 
               
               
                   
                 FTTC 
                 fiber to the curb 
               
               
                   
                 FTTH 
                 fiber to the home 
               
               
                   
                 FDM 
                 frequency division multiplexed 
               
               
                   
                 Gbps 
                 gigabits per second 
               
               
                   
                 HDTV 
                 high definition television 
               
               
                   
                 ITU 
                 International Telecommunications Union 
               
               
                   
                 LNB 
                 low noise block 
               
               
                   
                 Mbps 
                 megabits per second 
               
               
                   
                 MMDS 
                 multichannel multipoint distribution 
               
               
                   
                   
                 system 
               
               
                   
                 O/E 
                 optical-to-electrical 
               
               
                   
                 OMD 
                 optical modulation depth 
               
               
                   
                 OMI 
                 optical modulation index 
               
               
                   
                 ONT 
                 optical receiver 
               
               
                   
                 ONU 
                 optical network unit 
               
               
                   
                 PON 
                 passive optical network 
               
               
                   
                 QAM 
                 quadrature amplitude modulation 
               
               
                   
                 QPSK 
                 quadrature phase shift keying 
               
               
                   
                 RF 
                 radio frequency 
               
               
                   
                 R-S coding 
                 Reed-Solomon coding 
               
               
                   
                 SNR 
                 signal-to-noise ratio 
               
               
                   
                 STB 
                 set top boxes 
               
               
                   
                 TDM 
                 time division multiplexing