Abstract:
In a spread spectrum communication system, sequence signals are developed in both a transmitter and receiver, in each case being derived from a clock signal through digital count-down and logic circuitry including a programmable counter which receives control number defining groups of signals from a memory under control of an indexing counter. A sequence signal in the transmitter is applied through a filter to produce a filtered signal which is propagated as a sequence of variations of alternating polarity with predetermined varying durations occurring in a predetermined distinctive order. A received signal and sequence signal of the same form as in the transmitter are applied to a product detector the output of which is applied to an integrator. Loop circuitry is provided for synchronizing the generation of the sequence signal in the receiver with that in the transmitter. A plurality of channels may be provided, using common synchronizing and other circuitry.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to a communication system and more particularly to a communication system which uses a distribution of propagated energy over a wide frequency spectrum in a manner such as to obtain reliable operation while minimizing interference with propagation of other signals. The system is readily implemented through the use of digital circuitry and is otherwise readily and economically manufacturable. 
     2. Background of the Prior Art 
     The prior art includes disclosures of various type of spread spectrum communication systems which, so far as is known, appear to be quite complicated and expensive and of questionable reliability. 
     SUMMARY OF THE INVENTION 
     This invention was evolved with the general object of providing an improved communication system in which propagated energy is distributed over a wide range of frequencies in a manner such as to obtain reliable communication while minimizing interference with other communications in a medium. 
     The system is usable for propagation of radio waves through the atmosphere but may be used for propagation .[.or.]. .Iadd.of .Iaddend.other types of waves through the same or another medium. 
     In accordance with the invention, wavetrains are propagated from a transmitting point to a receiving point, each wavetrain comprising a sequence of alternations having predetermined varying durations and propagated in a predetermined distinctive order. Energy received at a receiving point is sampled at sampling times having durations and an order corresponding to the predetermined varying durations of the transmitted wavetrains. 
     Important features of the invention relate to the use of digital circuitry for establishing the durations and sequence of the propagated wavetrains and also of the sampling operation, to synchronization of sampling at the receiving point with the generation of the propagated wavetrains and to provisions for simultaneous transmission on a plurality of channels. 
     This invention contemplates other objects, features and advantages which will become more fully apparent from the following detailed description taken in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic block diagram of a communication system constructed in accordance with the invention; 
     FIG. 2 is a schematic block diagram of a sequence generator usable in both transmitting and receiving portions of the system of FIG. 1; 
     FIG. 3 illustrates waveforms produced in operation of the system, for explanation thereof; and 
     FIG. 4 is a block diagram of an alternative synchronizing system, also showing how a single receiver might be used to provide a plurality of communication channels. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     Reference numeral 10 generally designates a communication system which is constructed in accordance with the principles of this invention and which includes a transmitter 11 and a receiver 12 which may be connected to antennas 13 and 14 as shown for transmission of signals therebetween. 
     The transmitter 11 comprises a sequence generator 15 supplied with signals from a clock 16 and operative to generate sequences of signals having fundamental frequency components distributed across a certain frequency range, e.g. from 600 to 1200 .[.Hz.]. .Iadd.KHz.Iaddend.. The signal sequences are applied through a band-pass filter 17, which passes on the fundamental components, and through an amplitude modulator 18 to the antenna 13. The amplitude modulator 18 may be responsive to a relatively low frequency analog or digital signal such as an audio signal or a digital signal having a bit rate of 2400 bits per second or less. 
     The receiver 12 comprises a sequence generator 20 which may be substantially identical to the sequence generator 15. It is supplied with a clock signal from a voltage controlled oscillator 21 which is controlled by a voltage from a potentiometer 22. A product detector 23 has one input connected through an input circuit 24 to antenna 14, a second input connected to the output of the sequence generator 20 and an output connected through an integrator circuit 25 to an output terminal 26. When the potentiometer 22 is properly adjusted, an output signal is developed at the terminal 26 which corresponds to the digital or analog signal applied to the modulator 18 of the transmitter 11. 
     FIG. 2 shows a circuit usable for the sequence generator 15 of transmitter 11 and for the sequence generator 20 of the receiver 12. A clock signal, from clock 16 or the voltage controlled oscillator 21, is applied through a gate 27 to a four stage counter 28 and through gate 29 to a four stage programmable counter 30. The gate 27 is enable in a reset condition of a control flip-flop to apply clock pulses to the counter 28 and after 16 pulses, the counter 28 sets the flip-flop 32 to disable gate 27 and enable gate 29 and to then apply clock pulses to the programmable counter 30. After a count of from 1 to 16 pulses, the counter 30 applies a reset signal to the flip-flop 32 as well as to the counters 28 and 30. 
     The total number of pulses required to develop an output from counter 30 is from 17 to 32, depending upon the number loaded therein. This number is controlled from member 34 which may be a 128×4 memory, for example, connected to a seven stage address counter 35. The address counter 35 is indexed after every other output of counter 30 by a signal from a flip-flop 36 which operates as a divide-by-two stage and which also generates output signals. After indexing of the address counter 35, a read signal is applied through a delay circuit 37 to the memory 34. 
     With this arrangement, the flip-flop 36 develops an output signal which is of one value, e.g. high, for from 17 to 32 clock pulses depending upon the number at one address location in the memory 34, and which is of a second value, e.g. low, for the same number of clock pulses. Then the address counter 34 is indexed to load another number in the programmable counter 30. Thus from 34 to 64 clock pulses are required for each cycle of operation of the flip-flop 36. 
     If all address locations of memory 34 contained the same number, any number from 0 to 15 (decimal), and if the clock signal frequency is 38.4 MHz, the flip-flop 36 would be operative to generate a square wave at a certain fixed frequency of from 1.1294 to 0.6 MHz. However, in the sequence generators 15 and 20 of the illustrated embodiment, the locations in memory 36 contain various values such as to operate to generate signals which have frequency components distributed over the 0.6 to 1.2 MHz range. Preferably, each group of sixteen consecutive memory locations contains values from 0 to 15 (decimal), with no two locations in the same group containing the same value. As an example, the first group contains value as indicated in the following Table I which also shows the cumulative total of clock pulse at each transition time. 
     
                       TABLE I______________________________________Number in                  CumulativeMemory (decimal)          Number plus 16                      Total______________________________________8                  24           24                           4812                 28           76                          10411                 27          131                          15813                 29          187                          2166                  22          238                          26010                 26          286                          3124                  20          332                          3522                  18          370                          3885                  21          409                          4301                  17          447                          4640       (full count)              32          496                          5287                  23          551                          57415                 31          605                          63614                 30          666                          6963                  19          715                          7349                  25          759                          784______________________________________ 
    
     The other seven groups of sixteen four-bit memory locations may preferably also contain values of from 0 to 15, but in orders which are different from the order of the first group and from one another. Each group requires 784 clock pulses regardless of the order of the values. 
     FIG. 3 is a timing diagram in which waveform 40 is that of the signal generated by the sequence generator 15 of the transmitter 11 in response to reading of the first group of sixteen memory locations as listed in Table I. 
     Waveform 41 is that at the output of the band-pass filter 17, after removal of DC components and components having a frequency above 1.2 MHz. The transmitted signal has the same waveform, but may be changed in amplitude in accordance with an analog or digital signal applied to the modulator 18 
     The received waveform may be identical to waveform 41, under ideal conditions, and is applied through the input circuit 24 to one input of the product detector 23. Input circuit 24 may simply be a direct coupling but preferably includes a band-pass filter having a pass band from 0.6 to 1.2 MHz, an amplifier and suitable automatic gain control circuitry to limit the amplitude of the signal applied to the one input of the product detector 23. 
     The voltage controlled oscillator 21 is controlled by adjustment of the potentiometer 22 to cause the sequence generator 20 of the receiver to apply a signal to the second input of the product detector 23 which has a waveform identical to the waveform 41, identical values being stored in the memories of the sequence generators 15 and 20. As a result, the product detector 23 produces a signal having a waveform 42 as shown in FIG. 3, which is the product of waveforms 40 and 41. 
     For comparison purposes, a waveform 43 is shown which is that of a signal having a fixed frequency of 0.864 MHz and a waveform 44 is shown which is the product of waveforms 40 and 43. Waveform 44 fluctuates between positive and negative values and the sum thereof over any substantial interval of time is zero or very nearly so. Waveform 42, on the other hand, is always positive and the sum thereof over any interval of time is a positive value proportional to the duration of the time interval. The integrator 25 operates to sum or integrate the output of the product detector over a time interval which is as long as possible while permitting variations in response to amplitude modulations to be detected. It may comprise a simple series resistor and parallel capacitor combination with a time constant of on the order of 100 microseconds, for example. 
     Proper operation of the system shown in FIGS. 1-3 requires that the transmitter clock 16 and receiver oscillator 21 be very stable and also requires very careful adjustment of the potentiometer to obtain exact synchronization. To avoid the need for stable circuits and adjustment, synchronizing signals might be sent separately. For example, a 34.6 MHz carrier modulated by sync pulses at a 6272 rate might be transmitted to be detected at the receiver, with the received 34.6 MHz carrier being used to develop the clock signal and with detected sync pulses being used to rest the sequence generator of the receiver. 
     An alternative synchronizing system is shown in FIG. 4 which also shows how a single receiver might be used to provide a plurality of communication channels. As shown, an additional sequence generator 50 is provided together with an associated product detector 51 and integrator 52. A reset signal is developed upon reset of the address counter of the sequence generator 20 and is applied through a reset line 54 to the sequence generator 50 to reset its address and other counters. A signal from the input circuit is applied through a line 55 to the product detector 51. A clock signal from oscillator 21 is applied through a line 56 and a pulse-swallower 57 to the sequence generator 50, the pulse-swallower 57 being operated from a divide-by-64 counter 58 connected to the output of the sequence generator 50. 
     Sequence generator 50 is thus operated at a clocking frequency which is slightly slower than that of the sequence generator, while being reset at the same time, and the values stored in its memory are identical to those in the memory of the sequence generator 20. Assuming that the reset of the sequence generators 20 and 50 is synchronized with that of the sequence generator 15 of the transmitter, the output of the integrator 52 will be equal to that of the integrator 25 when the transmitting clock frequency is midway between the frequency of the VCO applied to the generator 20 and the slightly lower frequency applied to the sequence generator 50 from the pulse-swallower 57. If under such conditions, the VCO frequency increases, the output of the integrator 52 will increase and the output of the integrator 25 will decrease. In the system as shown, such outputs are applied to a comparator 60 which applies an output signal through a resistor 61 to a circuit point 62 which is connected to the VCO 21 and also through a resistor 63 to ground. The polarity of the signal applied is such as to adjust the VCO frequency toward a frequency which produces the balanced condition. 
     This balancing operation requires that the VCO frequency be relatively close to the optimum frequency. To insure that this condition will be reached, a searching operation is performed through a D/A converter 65 which is connected through a resistor 66 to the circuit point 62 and which is connected to a counter 67 driven through a gate 68 from the output of the sequence generator 50. The result is the application of a triangular stepped voltage through resistor 66 to the circuit point 62 to cause the VCO frequency to slowly sweep in small steps through a certain range. When the frequency is close to the optimum value, to produce relatively high outputs from the integrators 25 and 52, a threshold circuit 70 applies a disabling signal to the gate 68, after which the VCO is controlled from the output voltage of the comparator 60 superimposed upon the output from the D/A converter 65. 
     As indicated in FIG. 4, the reset, input and clock signals on lines 54, 55 and 56 may be applied to additional integrators to produce additional output signals. The memories of such additional generators may contain values different from the values in generators 20 and 50, and from each other, to thus provide such additional independent channels as may be desired. For example, 8 additional channels may be provided for &#34;parallel&#34; transmission of digital data between the data bus of an 8 bit processor and a printer, modem or other peripheral. 
     It will be understood that modifications and variations may be effected without departing from the spirit and scope of the novel concepts of this invention