Abstract:
A communication system and more particularly to a variable rate differential phase shift keying (DPSK) communication system with minimal hardware that does not have power or performance penalties associated with known DPSK modulation systems is disclosed. The DPSK modulation system in accordance with the present invention includes a transmitter, which includes a carrier signal source, a phase modulator and a DPSK encoder for modulating a carrier signal. The modulated carrier signals may be amplified, for example, in optical communication systems by a rare earth element doped fiber amplifier. The signals are continuously transmitted to a multi-rate receiver through a communication channel, for example, free space. The multi-rate receiver includes a single demodulator, for example, a single optical interferometer, used for multiple integer sub-harmonic data rates which demodulates the modulated signal. The demodulated signals are detected, for example, in optical communication systems by an arrangement of photodiodes, and the detected signals are applied to, for example, a clock and data recovery circuit that is tuned as a function of data rate, for example, by way of a switched filter circuit. The switched filter circuit may include a plurality of low-pass filters that are selected as a function of the data rate. Since the carrier signal is continuously transmitted, a phase reference is available to demodulate all received power and the peak transmitted power is approximately equal to the average transmitted power even at data rates corresponding to bit times that are large compared to the differential time delay of the demodulator.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is related to the following commonly-owned application: “A Multi-Rate Variable Duty Cycle Modem for Use in an Optical Communications System”, Ser. No. 09/522,802, filed on Mar. 10, 2000. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to a communication system and more particularly to a variable rate differential phase shift keying (DPSK) system which includes a continuous transmitter and a multi-rate receiver with a single demodulator. 
     2. Description of the Prior Art 
     Fixed rate differential phase shift keying (DPSK) digital communication systems are known to have performance comparable to coherent phase shift keyed systems but without the need for a coherent phase reference in the receiver. In such DPSK digital communication systems, the received signal waveform is demodulated, for example, by splitting the received signal in two parts, adding a time delay to one of the two parts and recombining the two signals. The delayed version of the received signal provides the necessary phase reference. The time delay is typically equal to the period of one data bit. One possible implementation of such a demodulator for optical communications systems employing DPSK signaling is an interferometer, such as a Mach-Zehnder interferometer, with unequal optical paths such that the difference in the optical path delay between the two legs of the interferometer is equal to the time of one bit. 
     Communication systems using DPSK signaling and capable of operating at multiple data rates are known but are hardware intensive and normally require a separate demodulator for each data rate. Each individual demodulator introduces a differential time delay corresponding to the desired bit time for that data rate. In order to solve this problem, commonly-owned U.S. patent application Ser. No. 09/522,802, filed on Mar. 10, 2000, discloses a variable duty cycle DPSK communication system which operates at multiple data rates. Although the variable duty cycle approach permits the use of a single demodulator, there are other problems with this approach. First, it suffers a power penalty because the received power sent during the first bit time for each block of data can not be demodulated because it lacks a phase reference, thus it is wasted. The wasted power approaches one half of the total transmitted power for data rates with low duty cycles. Also, the increase in the ratio of the peak power to the average transmitted power for such variable duty cycle waveforms increase the dynamic range requirements on many signal path components. Such components must tolerate proportionately higher peak power than would otherwise be required. Thus, there is a need for a DPSK communication system which can operate at multiple data rates which minimizes transmitted power requirements without corresponding performance loss and also minimizes hardware. 
     SUMMARY OF THE INVENTION 
     Briefly, the present invention relates to a communication system and more particularly to a variable rate differential phase shift keying (DPSK) communication system with minimal hardware that does not have power or performance penalties associated with known DPSK modulation systems. The DPSK modulation system in accordance with the present invention includes a transmitter which includes a carrier signal source, a phase modulator and a DPSK encoder for modulating a carrier signal. The modulated carrier signals may be amplified, for example, by a rare earth element doped fiber amplifier. The signals are transmitted to a multi-rate receiver through a communication channel, for example, free space. The multi-rate receiver includes a single demodulator, for example, a single optical interferometer, used for multiple integer sub-harmonic data rates, which demodulates the modulated signal. The demodulated signals are detected, for example, by an arrangement of photodiodes, and the detected signals are applied to, for example, a clock and data recovery circuit that is tuned as a function of the data rate, for example, by way of a switched filter circuit. The switched filter circuit includes a plurality of low-pass filters that are selected as a function of the data rate. Since the carrier signal is continuously transmitted, a phase reference is available to demodulate all received power and the peak transmitted power is approximately equal to the average transmitted power even at data rates corresponding to bit times that are large compared to the differential time delay of the demodulator. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     These and other advantages of the present invention will be readily understood with reference to the following specification and attached drawing wherein: 
     FIG. 1 is a block diagram of a typical satellite data communication system. 
     FIG. 2 is a block diagram of the optical communication system in accordance with the present invention. 
     FIGS. 3A-3I represent exemplary waveforms of the optical communication system illustrated in FIG.  2 . 
     FIG. 4 is a schematic diagram of an exemplary DPSK encoder which forms a part of the present invention. 
     FIG. 5 is an exemplary optical DPSK demodulator for use with the present invention. 
     FIG. 6 is an exemplary switched filter circuit for use with the present invention. 
     FIG. 7 is an exemplary clock and data recovery circuit for use with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The present invention relates to a communication system and in particular to a multi-rate optical DPSK communication system which may be used as part of a satellite communication system, for example, an intersatellite link, as illustrated in FIG.  1 . However, the principles of the present invention are applicable to any multi-rate communication system using DPSK signaling to exchange data between two locations via a transmission medium. Examples of such a transmission media include but are not limited to electrical cable, optical fiber or free space. 
     FIG. 1 represents an exemplary satellite communication system, generally identified with the reference numeral  20 . The satellite communication system  20  includes at least one satellite  22  which may be used to complete a virtual connection between ground stations, for example, the ground stations  24 ,  25  and  26 . In particular, the ground stations  24  and  25  are transmitting ground stations while the ground station  26  is a receiving ground station. In general, data is up-linked to the satellite  22  by way of the transmitting ground stations  24  and  25  and down-linked to the receiving ground station  26 . The virtual connection may be made between the ground stations  24  and  26  or between the ground stations  25  and  26 . In order to extend geographic coverage of the satellite communication system  20 , a second satellite  28  may be provided. The second satellite  28  is in communication with the first satellite  22  by way of an intersatellite communication link  30 . As shown, the satellite  28  is in direct communication with a transmitting ground station  32  and a receiving ground station  34 . By providing the intersatellite link  30 , the satellite  28  can communicate with the ground stations  24 ,  25  and  26  in addition to the ground stations  32  and  34 . Similarly, the satellite  22  is able to communicate with the ground stations  32  and  34  in addition to the ground stations  24 ,  25  and  26 . 
     FIG. 2 illustrates an optical communication system, generally identified with reference numeral  36 , in accordance with the present invention. The optical communication system  36  includes a transmitter, shown within the dashed box  38 , and a receiver, shown within the dashed box  40 . As shown, the transmitter  38  and receiver  40  include optical antennae used when the transmission medium between the transmitter  38  and receiver  40  is free space, for example, in an application of an intersatellite communication link as discussed above. In applications where the transmission medium between the transmitter  38  and receiver  40  is not free space, the transmitter  38  and receiver  40  may be connected together by way of an optical fiber (not shown) which forms the transmission medium. In applications, in which the carrier signal source is not an optical signal source, for example, radio frequency DPSK communications systems, the transmission medium may be free space or a suitable conductor for the radio frequency signals of interest, for example, coaxial cable. All of such embodiments are within the broad scope of the present invention. 
     Turning back to FIG. 2, the transmitter  38  includes a source of an optical carrier signal. The optical source  42  is a single frequency, narrow line width source, for example, a Northern Telecom, Ltd. Model LC155CD-20 distributed feedback laser. Other single frequency optical sources are also suitable for providing the optical carrier signal. 
     The optical carrier source  42  is optically coupled to a phase modulator  44  by way of an optical link  46 , for example, an optical fiber. The optical phase modulator  44 , for example, a Sumitomo Osaka Cement Co. Model T-PM1.5-20 or other such device capable of controlling the optical phase of the carrier signal source, is used to modulate the optical carrier signal from the laser source  42 . In particular, the phase modulator  44  modulates the phase of the optical carrier signal by either shifting the carrier phase 180 degrees or not at all depending on the output of differential phase shift key (DPSK) encoder  46 . More particularly, the external phase modulator  44  is under control of the DPSK encoder  46 . As shown, the DPSK encoder  46  receives the incoming data as indicated by the arrow  48 . The system  36  is a multi-rate system and thus is able to receive full rate data or sub-harmonic rate data. 
     An exemplary DPSK encoder  46  is illustrated in FIG.  4 . The DPSK encoder may include a D type flip-flop  49  and dual input exclusive-OR gate  50 . The output of the D flip-flop is applied to one input of the exclusive-OR gate. The data to be modulated is applied to the other input. The output of the exclusive-OR gate  50  is applied to the D input of the D flip-flop  49 . Essentially, the DPSK encoder  46  receives the incoming data and performs an exclusive-OR function on the previously encoded data before latching. 
     A clock source  50  is used to apply a full rate clock signal to the DPSK encoder  46 . The clock source  50  may have a waveform  51 , for example, as shown in FIG. 3A, which illustrates an exemplary full rate input clock. The clock source  50  is controlled by the clock reference as indicated by the arrow  52 . The clock reference  52  is typically provided by the equipment that produces the incoming data  48 . If such a clock reference is not available, it may be generated using the techniques employed by the receiver  40  to regenerate a clock signal from the incoming data  48 . In the case that the frequency of the clock reference  52  is not equal to the full rate, the clock source  50  may include conventional circuitry by which the clock reference frequency is multiplied up to the full rate frequency, for example, by phase-locked loop frequency synthesis or other such frequency multiplication techniques known to those having ordinary skill in the art, to produce a full rate input clock. 
     In some applications, the modulated carrier signals from the external phase modulator  44  may be amplified, for example, by a rare earth element doped fiber amplifier, for example, an IPG Laser GmbH Model EAD-1000. The optical phase modulator may be optically coupled to the optical amplifier  54  by way of an optical fiber  56 . 
     The optical amplifier  54  is coupled to an optical antenna  58  by way of an optical fiber  60 . The optical antenna  58  converts the phase modulated optical signal from the optical amplifier  54  to a form that may be applied to the transmission medium connecting the transmitter  38  and the receiver  40 . In applications where the transmission medium between the transmitter  38  and receiver  40  is free space, an exemplary optical antenna  58  converts the phase modulated optical signal from the optical amplifier  54  to a free space optical beam. In this case, an exemplary optical antenna is a Cassegrain telescope or other similar optical device known to those having ordinary skill in the art. If the transmission medium between the transmitter  38  and receiver  40  is an optical fiber, the optical antenna  58  is not required. In such applications, the optical fiber  60  may be connected directly to the receiver  40 . 
     The optical communication system  36  is adapted to operate at multiple data rates, for example, 1/n times the full data rate for integer values of n. An exemplary full rate binary data sequence  62  is illustrated in FIG.  3 B. The full rate data waveform  64  corresponding to the binary data sequence  62  is illustrated in FIG. 3C, where the two levels in the waveform represent either a logical “1” or a logical “0”. The encoded full rate waveform  66  illustrated in FIG. 3D represents the result of applying the binary data sequence  62  to the DPSK encoder  46  in FIG. 4 with the DPSK encoder  46  clocked by the full rate clock  51 . The encoded full rate waveform  66  also represents the relative phase of the optical carrier at the output of the optical phase modulator  44  in FIG. 4, with a logical “1” corresponding to a phase change of the optical carrier of 180 and a logical “0” corresponding to no phase change. 
     An exemplary binary data sequence  68  at ¼ of the full data rate is illustrated in FIG.  3 F. The ¼ rate data waveform  70  corresponding to the binary data sequence  64  is illustrated in FIG.  3 G. As discussed above, the two levels in the ¼ rate data waveform  70  represent either a logical “1” and logical “0”. The encoded ¼ rate waveform  72  illustrated in FIG. 3H represents the result of applying the binary data sequence  68  to the DPSK encoder  46  in FIG.  4 . As discussed above, the DPSK encoder  46  is clocked by the full rate clock  51  even though the data rate of the binary data sequence  64  is ¼ of the full data rate. The encoded ¼ rate waveform  72  also represents the relative phase of the optical carrier at the output of the optical phase modulator  44  in FIG. 4, with a logical “1” corresponding to a phase change of the optical carrier of 180 and a logical “0” corresponding to no phase change. 
     The transmitted optical signals from the transmitter  38  are received by the receiver  40 . In applications where the transmission medium is free space, the receiver may include an optical antenna  74  which collects the transmitted beam from the transmit optical antenna  58  and couples it into an optical fiber  76 . As discussed above, in applications where the optical communication system is used in optical fiber systems where the transmission medium is an optical fiber, the receive optical antenna  74  is eliminated and a single optical fiber may be coupled directly between the optical amplifier  54  and the receiver  40 . The waveforms  66  and  72  are illustrative of the full rate and quarter rate modulated signals received by the receive optical antenna  74 . 
     Since there are losses in the optical transmission, the optical fiber  76  may be coupled to a low noise optical amplifier  78 , which may also be rare earth element doped fiber amplifier, as discussed above. The output of the low-noise amplifier  78  may be coupled to an optical DPSK demodulator  80  by means of an optical fiber  82 . In some applications, as in the case that the losses of the transmission medium between the transmitter  38  and the receiver  40  of the optical communication system are sufficiently low, the optical fiber  56 , which connects the optical phase modulator  44  to the optical amplifier  54 , may be connected directly to the optical DPSK demodulator  80 . 
     An exemplary optical DPSK demodulator is an unbalanced Mach-Zehnder interferometer  84 , shown on FIG. 5, similar in function to a device supplied by Photon Integration Research, Inc., under Model FDM-3G-1.5-M. Incoming light from an optical fiber  86  is divided into two signals by an optical coupler  88  and recombined by another optical coupler  90  after the two signals have propagated down a pair of optical paths  92  and  94 . The optical paths  92  and  94  may be constructed such that the difference in the optical path delay between them is equal to the period of one bit. The unbalanced Mach-Zehnder interferometer  84  will thus have an output  96  that is proportional to the coherent sum of the optical signal at the input and a time-delayed version of the input signal. If the two versions of the input signal are in phase, they will constructively interfere and produce a high intensity output signal. If the two versions of the input signal are out of phase, they will destructively interfere and produce a low intensity output signal. The unbalanced Mach-Zehnder interferometer  84  therefore converts the differential phase of the input optical signal into an intensity, thereby performing the DPSK demodulation function for a data rate whose bit period corresponds to the difference in optical path delay between optical paths  92  and  94 . Commonly-owned U.S. patent application Ser. No. 09/236,981, “Apparatus and Method for Tuning an Optical Interferometer”, filed on Jan. 26, 1999 describes a method for precisely controlling the optical path difference of such an interferometer. Other interferometer implementations may also be used to demodulate optical DPSK signals, including other Mach-Zehnder interferometer configurations. 
     The optical output from the DPSK demodulator  80  may be coupled to a detector  98  by way of an optical fiber  100 . The detector  98  converts the demodulated optical DPSK signal into an electrical signal representative of the differential phase of the optical signal received by the receiver  40 . An exemplary detector is described in U.S. Pat. No. 6,064,507, “High Speed Differential Optoelectronic Receiver”. Other detector implementations may also be used. 
     FIG. 3E illustrates the demodulated and detected waveform  102  produced by the detector  98  when the exemplary full rate binary data sequence  62  is applied to the transmitter  38  and received by the receiver  40 , which includes an optical DPSK demodulator  80  configured to introduce a differential time delay corresponding to one bit period at the full data rate of the communication system  36 . The two levels in the demodulated and detected waveform  102  correspond to, for example, either constructive interference or destructive interference between the optical signals in the two optical paths  92  and  94  of the unbalanced Mach-Zehnder interferometer  84 , as described above. The associated “high” and “low” optical signal levels are converted by the detector  98  to equivalent electrical signals levels representing either a logical “1” or a logical “0”. 
     FIG. 31 illustrates the demodulated and detected waveform  104  produced by the detector  98  when the exemplary quarter rate binary data sequence  68  is applied to the transmitter  38  and received by the receiver  40 , which, as in the full data rate example above, includes an optical DPSK demodulator  80  configured to introduce a differential time delay corresponding to one bit period at the full data rate of the communication system  36 . The two levels of the demodulated and detected waveform  104  represent either a logical “1” or logical “0” as described above. 
     The ability of the optical communication system  36  to communicate using DPSK signaling at variable sub-harmonic rates using a single demodulator  80  is apparent upon noting that: 1) the exemplary full rate demodulated and detected waveform  102  produced internal to the receiver  40  is a replication of the associated full rate data waveform  62  input to the transmitter  38 ; 2) the exemplary quarter rate demodulated and detected waveform  104  produced internal to the receiver  40  is a replication of the associated full rate data waveform  68  input to the transmitter  38 ; and 3) both the exemplary detected and demodulated waveforms are produced by a single optical DPSK demodulator  80  configured to introduce a differential time delay corresponding to one bit period at the full data rate of the communication system  36 . 
     Since the components in the transmitter  38  and receiver  40  introduce noise and produce other imperfections, for example, due to band limiting of the electrical circuits, the performance of the communication system  36  may be enhanced by coupling the detector  98  to a switched filter circuit  106 , which includes a plurality of selectable filters, by way of an electrical conductor  108 . An exemplary switched filter circuit  106  is shown in FIG.  6  and includes an input switch  112 , an output switch  114  and a plurality of filters  116 . The plurality of filters  116  may consist of as many as one filter for each data rate. A signal applied to the switched filter circuit  106  by way of an electrical conductor  118  is directed by the input switch  112  to the output switch  114  through one filter out of the plurality of filters  116 . The output switch  114  directs the filtered signal to the output of the switched filter circuit  110  as indicated by the arrow  120 . Such switched filter circuits are known by those with ordinary skill in the art. The characteristics of any individual filter among the plurality of filters  116  may be designed to match or approximate the filter characteristics that optimize the performance of the receiver  40  at specific data rates. 
     The output of the switched filter circuit  106  is coupled to the clock and data recovery circuit  122 , by way of an electrical conductor  124 . The data recovery circuit  122  regenerates the clock, as indicated by arrow  126 , and the detected data, as indicated by arrow  128 . An exemplary clock and data recovery circuit  130  is illustrated in FIG.  7 . The input to the clock and data recovery circuit  130 , indicated by the arrow  132 , is applied to a squaring circuit  134 , for example, a step recovery diode, and to a D type flip-flop  136 . The output of the squaring circuit is coupled to switched filter circuit  138 . The switched filter circuit  138  may include a plurality of bandpass filters, with one bandpass filter, tuned to the clock frequency, for each desired data rate. The output of the switched filter circuit is the recovered clock signal, as indicated by the arrow  136 , which is applied to the D type flip-flop  136 . The output of the D type flip-flop  136  is the recovered data, as indicated by arrow  142 . Clock and recovery circuits are known to those with ordinary skill in the art. Other clock and data recovery circuits, including those that recover the clock directly from the output of the detector  98 , are possible. 
     Obviously, many modifications and variations of the present invention are possible in light of the above teachings. Thus, it is to be understood that, within the scope of the appended claims, the invention may be practiced otherwise than as specifically described above. 
     What is desired to be secured by a Letters Patent is as follows.