Abstract:
A method and apparatus for simultaneously communicating a first data stream along with a second data stream. A first carrier is modulated with a first data stream and a feature of the modulated first carrier is then imposed under the control of a second independent data stream. The modulated first carrier with the imposed feature is then demodulated to provide the first data stream. The distinguishable feature of the modulated first carrier is then detected to provide the second data stream. The first and the second data streams are clocked at rates which are integer multiples of each other. The distinguishable feature of the modulated first carrier includes the amplitude of the modulated first carrier or the frequency of the modulated first carrier. M-ary information is transmitted by transmitting groups of 3 or more bits simultaneously by using distinguishable amplitude or frequency values of a feature imposed on a first carrier.

Description:
TECHNICAL FIELD 
     The present invention generally relates to the transmission of binary digits (bits), and, more particularly, to the use of the presence, absence or values of features of the means of transmission to represent single bits or groups of bits. 
     BACKGROUND OF THE INVENTION 
     Bit streams are used to represent the outputs of microphones, cameras, and a variety of other transducers. Bit streams are the input and output forms of computers. When the origin of the message of interest is distant from the user, then a means of transporting the bit streams is required. Commonly, this means is optical or electromagnetic. The means can be guided (fibers, cables, pairs of wire), or it can be unguided (radiation). Radiation makes use of carriers consisting of sinusoidal waves. Modulation of the sinusoidal carrier enables the transmission of the bit stream(s). 
     A simple form of a feature of a modulated carrier is its presence. That is, in a simple case, the presence of a sinusoid or the presence of optical energy (light) represents a binary 1, for example. The absence of these represents a binary 0. This is a standard means of transmitting bit streams over glass fibers and of recovering bit streams from compact disk recordings, for example. In radio communications, this simple form is called on-off amplitude modulation or on-off keying (OOK). In this case, only one bit stream is sent per carrier. 
     The amplitude, frequency or phase and combinations thereof of a sinusoidal carrier can be modulated to distinguish one bit from the other or a group of bits from another group. A variety of well known modulation schemes are treated in the literature. Types of modulation are characterized by bandwidth efficiency and power efficiency. Bandwidth efficiency relates to the band of frequency spectrum occupied by the modulated carrier for a given rate of bit transmission (bit rate in bits per second). Power efficiency relates to the probability of any bit received in error, or bit error ratio (BER), as a function of receiver input signal to noise ratio (SNR). 
     Conventional means of bit stream transmission do not consider use of multiple co-channel carriers to transmit one or more bit streams simultaneously. In this manner, spectrum can be reused thereby improving bandwidth efficiency. Modification of a bit by the inclusion of a recognizable feature is not treated as a means of increasing data throughput within a prescribed band of frequency. Theoretical comparisons of the types of carrier modulation rarely consider the complexity of the transmitter circuitry needed to generate and radiate the modulated carrier and the receiver circuitry required to recover the bit stream from the carrier. Therefore, because of the cost and availability of spectrum, and because of the cost associated with complexity with a small number of units, there is a need to define and develop data transmission systems which are simple, robust and bandwidth efficient. 
     U.S. Pat. Nos. 4,859,958 and 4,992,747, invented by Glen A. Myers, the inventor of the present invention, are each incorporated by reference in the present application as though fully set forth herein. In these patents, a means for demodulating all of several co-channel FM carriers is described. 
     U.S. Pat. No. 5,038,115, co-invented by the inventor of the present invention, is also incorporated by reference in the present application as though fully set forth herein. In this patent, phase tracking of input terminal signals is described. In one embodiment of the phase tracking circuit of U.S. Pat. No. 5,038,115, a phase tracking circuit makes use of two phase-locked loops electrically connected in a feed forward manner. 
     U.S. Pat. No. 5,329,242 invented by the inventor of the present invention, is also incorporated by reference as though fully set forth herein. In this patent, demodulating a frequency modulated signal using the time intervals between zero crossings of a received carrier signal is described. Averaging and mapping techniques are used to improve estimates of the message signal. 
     U.S. Pat. Nos. 5,541,959 and 5,570,395, invented by the inventor of the present invention, are also incorporated by reference as though fully set forth herein. These patents describe, analytically and geometrically, the effect of adding two sinusoids of different frequency. 
     U.S. Pat. No. 5,606,581, invented by the inventor of the present invention is also incorporated by reference as though fully set forth herein. This application described a method and apparatus for creating a replica of a dominant carrier. 
     U.S. Pat. No. 5,554,955, invented by the inventor of the present invention, is also incorporated by reference as though fully set forth herein. This patent describes method and apparatus for removing the effects of co-channel interference from the message on a dominant frequency modulated carrier and for recovering the message from each of two co-channel carriers. 
     U.S. patent application Ser. No. 08/705,721 by the inventor of the present invention is also incorporated by reference as though fully set forth herein. This application describes method and apparatus for recovering the independent bit streams from each of two co-channel frequency modulated carriers. 
     SUMMARY OF THE INVENTION 
     The present invention relates to method and apparatus for creation and use of a feature of a modulated carrier to represent a bit or a group of bits. The presence of the feature represents one of the two possible binary digits and the absence of the feature represents the other binary digit. 
     This invention considers the addition of a feature to an existing bit stream representation to permit the transmission of other bit streams whose binary digits are recognized by distinguishable features imposed on the first bit stream. The presence or absence of a feature imposed on a bit stream allows the simultaneous transmission of a second independent bit stream. 
     For example, consider an optical transmission means whereby a carrier of one color (blue) represents a binary one and that of another color (green) a binary zero. Transmit a second independent bit stream by shifting these colors slightly when the bit of the second stream is one digit and no shift for the other digit. When detected in the receiver, the original and slightly shifted colors are distinguished to recover the first bit stream. The shift or absence of the shift in colors when detected provides the second bit stream. Simplest operation in this manner occurs when the two bit streams are clocked at the same rate or at rates which are integer multiples of each other. 
     Accommodating two independent bit streams in this manner is equivalent to sending two bits simultaneously as a group from one bit stream. This reduces the switching rate of the carrier which translates to a reduction in required bandwidth or occupied spectrum. 
     Using additional available parameters of the carrier (intensity in the optics example) along with a range of possible values of each parameter permits the accommodation of several independent bit streams sent simultaneously on a single carrier. Alternatively, the grouping of several bits of one or more streams can be sent at once using a single carrier. 
     In accordance with a preferred embodiment of the present invention, use is made of the appropriate addition of a second co-channel carrier to generate the feature of interest. This is particularly attractive in radio communications where bandwidth efficient transmission means are important. By judicious choice of the amplitude separation and frequency separation of the two carriers, many differing and distinguishable feature voltages can be generated by design. This method of constructing features results in simple transmitter circuitry and simple receiver circuitry as well as simple and flexible communications as compared with other systems. 
     The present invention provides a method of simultaneously communicating a first data stream along with a second data stream. The invention includes the steps of: modulating a first carrier with a first data stream to provide a modulated first carrier; imposing a feature on the modulated first carrier to provide a distinguishable feature of the modulated first carrier which represents a second data stream; transmitting the modulated first carrier with the distinguishable feature through a transmission medium. The method further includes the steps of: receiving the transmitted modulated first carrier with the distinguishable feature to provide a received modulated first carrier with the feature; demodulating the received modulated first carrier with the distinguishable feature to provide the first data stream; and detecting the distinguishable feature of the received modulated first carrier to provide the second data stream. 
     The first and the second data streams are clocked at rates which are integer multiples of each other. 
     The invention further includes the step of simultaneously transmitting M-ary information by transmitting groups of 2 or more bits simultaneously wherein each group of 2 or more bits defines a symbol and wherein the number of distinct symbols equals 2 n  where n is the number of bits in a group. 
     A system is provided according to the invention for transmitting and receiving independent bit streams from each of two co-channel frequency-modulated carriers. The system includes a transmitter having: a first system input terminal for receiving a first input bit stream; a first frequency modulator having an input terminal coupled to the first system input terminal for receiving the first input bit stream and having an output terminal at which is provided a first sinusoidal signal which is frequency modulated by the first input bit stream; a summer having a first and a second input terminal and an output terminal, wherein the output terminal of the first frequency modulator is coupled to the first input terminal of the summer; an inverter having an output terminal and an input terminal coupled to the first system input terminal for receiving the first input bit stream; a second frequency modulator having an input terminal coupled to the output terminal of the inverter and having an output terminal at which is provided a second sinusoidal signal which is frequency modulated by the inverted first input bit stream; an electronic switch having a signal input terminal which is coupled to the output terminal of the second frequency modulator and having an output terminal which is coupled to the second input terminal of the summer, wherein the electronic switch includes a control terminal for receiving a control signal which opens and closes the electronic switch; a second system input terminal coupled to the control terminal of the electronic switch for receiving a second input bit stream which controls the electronic switch. The power of the first sinusoidal signal is greater than the power of the second sinusoidal signal. A receiver includes a frequency demodulator having an input terminal for receiving a replica of the output signal of the summer, the frequency demodulator having an output terminal at which is provided a frequency-demodulated signal which is comprised of the bit stream on the stronger carrier on which are superimposed voltage spikes wherein said voltage spikes have characteristics which include amplitude characteristics and rate characteristics. A limiter has an input terminal coupled to the output terminal of the frequency demodulator, said limiter having an output terminal at which is provided an output signal corresponding to the first input bit stream. A spike characteristic detector has an input terminal coupled to the output terminal of the frequency demodulator for receiving the frequency-demodulated signal, said spike characteristic detector having an output terminal at which is provided output signal levels corresponding to the characteristics of said voltage spikes. A comparator has a first input terminal with a reference voltage coupled thereto, said comparator having a second input terminal coupled to the output terminal of the spike characteristic detector, said comparator having an output terminal at which is provided an output signal corresponding to the second input bit stream. independent bit streams are recovered from a single composite power multiplexed sinusoidal carrier. 
     Another embodiment of a receiver uses an envelope detector having an input terminal for receiving a replica of the output signal of the summer; the envelope detector having an output terminal at which is provided an amplitude-demodulated signal which indicates the state of the second input bit stream. A comparator having a second input terminal coupled to the output terminal of the envelope detector, said comparator having an output terminal at which is provided an output signal corresponding to the second input bit stream. In this second manner, the two independent bit streams are recovered from a single composite power multiplexed sinusoidal carrier. 
     The transmitter may include an envelope conditioner. The spike characteristic detector is a peak voltage detector for the voltage spikes of the frequency-demodulated signal. The peak voltage detector includes a full-wave rectifier having an analog voltage multiplier having one input terminal coupled to the output terminal of the frequency demodulator and having another input terminal coupled to the output terminal of the limiter, said analog voltage multiplier having an output terminal. The spike characteristic detector can also be a rate detector for the voltage spikes of the frequency-demodulated signal. The input bit streams are clocked from a common timing reference signal in the transmitter. 
     The invention provides a method for transmitting and receiving independent bit streams from each of two co-channel frequency-modulated carriers. A first embodiment of the method includes the steps of: frequency-modulating a first sinusoidal signal with a first input bit stream to provide a first frequency-modulated carrier signal; inverting the first input bit stream; frequency-modulating the inverted first input bit stream to provide a second frequency-modulated carrier signal; electronically switching on and off the second frequency-modulated carrier signal with a second input bit stream, wherein the power of the first frequency-modulated signal is greater than the power of the second frequency-modulated signal; summing the first and the second frequency-modulated carrier signals to provide a combined transmitter signal; frequency demodulating a replica of the combined transmitter signal to provide a frequency-demodulated signal, wherein the frequency-demodulated signal is comprised of the first input bit stream; limiting the frequency-demodulated signal to provide an output signal corresponding to the first input bit stream; detecting the spike characteristics of the frequency-demodulated signal to provide output signal levels corresponding to the characteristics of said voltage spikes; comparing the output signal levels corresponding to the characteristics of said voltage spikes to a reference signal in a comparator to provide an output signal corresponding to the second input bit stream; wherein the two independent bit streams are recovered from the single composite sinusoidal carrier. 
     A second embodiment of the method includes amplitude demodulating a replica of the combined transmitter signal to provide an amplitude-demodulated signal, wherein the amplitude-demodulated signal indicates the state of the second input bit stream; comparing the amplitude-demodulated signal levels to a reference signal in a comparator to provide an output signal corresponding to the second input bit stream; the first input bit stream is recovered in the manner of the first embodiment of the method; wherein the two independent bit streams are recovered from a single composite sinusoidal carrier. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The accompanying drawings which are incorporated in and form a part of this specification, illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention. 
     FIG. 1 is a block diagram of a transmitting system for the case of two bit streams. 
     FIG. 2 is a block diagram of a transmitting system for the case of n bit streams. 
     FIG. 3 is a block diagram of a particular transmitting system that uses an auxiliary sinusoid to create a feature for the case of two bit streams. 
     FIG. 4 is a block diagram of a particular transmitting system that uses an auxiliary sinusoid to create a feature for the case of n bit streams. 
     FIG. 5 is an example of a 4-ary signal constellation. 
     FIG. 6 is an example of an 8-ary signal constellation. 
     FIG. 7 is an illustrative example of a feature generating function. 
     FIG. 8 is an example of an illustrative input bit stream d d (t) to the transmitter. 
     FIG. 9 is an example of another illustrative input stream d s (t) to the transmitter. 
     FIG. 10 is an example of the values of the peak frequency deviation of the dominant carrier and the subdominant carrier. 
     FIG. 11 is an example of the function F(t) for the bit stream of FIGS. 4 &amp; 5. 
     FIG. 12 is a block diagram of a transmitter according to the invention. 
     FIG. 13 is an example of the values of the envelope E(t). 
     FIG. 14 is a block diagram of a receiver having a spike peak-voltage feature detector according to the invention. 
     FIG. 15 is an illustrative example of the signal output of a full-wave rectifier having as input the output of the frequency demodulator of FIG.  14 . 
     FIG. 16 is an illustrative example of a 4-ary voltage constellation. 
     FIG. 17 is an illustrative example of an 8-ary voltage constellation showing transitions when one bit stream is clocked at twice the rate of the other. 
     FIG. 18 is an illustrative example of an 8-ary voltage constellation identifying the symbols representing a group of three bits of a single bit stream or one bit each of three bit streams. 
     FIG. 19 is a graph indicating possible values of frequency of the dominant carrier and the three subdominant carriers. 
     FIG. 20 is an example of a constellation of an 8-ary system for the combination of a dominant carrier with another one of three subdominant carriers of differing frequency. 
     FIG. 21 is a graph of F(t) for the combination of a dominant carrier with another one of three possible subdominant carriers of differing frequency. 
     FIG. 22A is a block diagram of a transmitter for a system which uses an envelope as a feature. 
     FIG. 22B is a block diagram of a receiver having an envelope feature detector. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     This invention considers use of features of a single, composite modulated carrier to transmit two bit streams simultaneously, or, in general, to transmit groups of 2, 3 or more bits simultaneously. This grouping of bits defines symbols. A group of two bits results in four possible combinations of bits. These are 00, 01, 10 and 11. A group of three bits results in 8 possible combinations or symbols, etc. When the bits are grouped to define symbols prior to transmission, then the process in called M-ary communications where M is the number of possible symbols to be sent. Therefore, there are binary, 4-ary, 8-ary, 16-ary, etc. communication systems. 
     FIG. 1 is a block diagram of a transmitting system whereby a first bit stream frequency modulates a first carrier. A second bit stream effects the combining of the first modulated carrier with a suitable voltage to create a composite carrier which carries the first bit stream and possesses a feature whose presence or absence represents the second bit stream. 
     As shown in FIG. 2, in the case of n bit streams, the first bit stream frequency modulates a first carrier . The remaining bit streams together effect the combining of the first modulated carrier with one of several suitable voltages to create a composite carrier which carries the first bit stream and possesses one of several possible features whose distinction indicates the state of the remaining bit streams. 
     FIG. 3 is a block diagram of a particular realization of the system of FIG. 1 whereby an auxiliary sinusoid is used as a suitable voltage to create a composite carrier. 
     FIG. 4 is a block diagram of a particular realization of FIG. 2 whereby an auxiliary sinusoid having a range of possible values of amplitude and frequency is used as a suitable voltage added to the first modulated carrier to create a composite carrier. 
     Conventional M-ary systems include M-ary PSK (phase shift keying) which are special cases of quadrature amplitude modulation (QAM). QAM uses two carriers of the same frequency and separated 90 degrees in phase (sine and cosine carriers). Each carrier is then modulated in amplitude. The two carriers are added to generate a composite carrier whose phase and amplitude combinations number m=the number of symbols which must be distinguished in the receiver. 
     FIG. 5 is a planar representation of 4-ary PSK. Each of the two carriers has one value of amplitude and two values of phase (0 degrees and 180 degrees). FIG. 6 is such a representation of 8-ary PSK. Note that for the case of 8-ary PSK, each carrier (sine and cosine) has two values of phase (0 and 180 degrees) and two values of amplitude. Because all 8 of the points that represent the composite carrier, which is the sum of the two quadrature carriers, lie on a circle, then the peak amplitude of this composite carrier is constant. Only phase modulation of the transmitted carrier exists in this case. This is but one form of QAM. There are many others. In a more general form of QAM, both the phase and amplitude of the composite carrier varies. 
     Plots like FIGS. 5 and 6 are called signal constellations. The design of such constellations is part of the creation of M-ary signaling systems. 
     In U.S. Pat. No. 5,554,955 by the inventor of the present invention, which is incorporated by reference as though fully set forth herein, the inventor of the present invention describes analytically the effect of adding two sinusoids of different frequency. The additive combination is another sinusoid having amplitude and frequency which change with time. The exact relationships for the envelope or peak amplitude variations E(t) and the instantaneous frequency deviation F(t) from the value of carrier frequency are shown to be 
     
       
         E(t)=A d ·[1+r 2 +2r cos φ(t)] ½   (1) 
       
     
     
       
         F(t)=k d ·d d (t)+[k d ·d d (t)−k s ·d s (t)][g(r,φ)]  (2) 
       
     
     where 
     r=A s /A d    
     A s =peak amplitude of the subdominant (weaker) carrier 
     A d =peak amplitude of the dominant (stronger) carrier 
     φ(t)=φ d (t)−φ s (t) 
     θ d (t)=phase variation of the dominant carrier due to its message 
     θ s (t)=phase variation of the subdominant carrier due to its message 
     k d =a property of a dominant carrier frequency modulator which converts volts to Hz 
     k s =a property of a subdominant carrier frequency modulator which converts volts to Hz 
     d d (t)=(½π)[dφ d (t)/dt]=bit stream message carried by the dominant sinusoid 
     d s (t)=(½π)[dφ s (t)/dt]=bit stream message carried by the subdominant sinusoid 
     
       
         g(r,φ)=−{r·[r+cos φ(t)]/[1+r 2 +2r cos φ(t)]}.  (3) 
       
     
     The function g(r,φ) is plotted as FIG. 7 when r=0.707 (r=−3 dB) and for φ in the range (0, 4π). This function consists of spikes with cusps between the spikes. 
     The form of g(r,φ) can be used as a feature to represent 2, 3 or more bits simultaneously. 
     As a first example, consider two bit streams represented as two voltage levels. Assume the two streams have the same bit rate and are clocked from a common clock so that the voltage transition times are the same. Of course, there may or may not be a voltage transition at a transition time. 
     FIG. 8 illustrates a two-level bit stream d d (t) which is applied by FM to a dominant carrier. FIG. 9 illustrates a second independent two-level bit stream d s (t) which is carried by the presence of a feature voltage on the bit stream of FIG.  8 . 
     An example of the creation of a feature voltage results by applying the following rule. When d s (t) represents a binary 1, then add a second (subdominant) carrier to the dominant carrier. This generates a variation of F(t) described by the term involving g(r,φ) in Equation (2). The carrier frequencies of the two sinusoids are approximately equal at f c  Hz. When the subdominant carrier is added, it has a frequency deviation from the carrier frequency opposite that of the dominant carrier. If the frequency deviation of the dominant carrier is ±Δf d , then the ±Δf d , then the difference in frequency of the two carriers has magnitude equal 2Δf d  when they coexist. When d s (t)=binary 0, transmit only the FM dominant carrier. 
     FIG. 10 is an example of the possible frequencies of the two carriers for the case of a binary one of each bit stream where it is assumed a 1 is represented by a positive value of frequency deviation from the carrier frequency. The long arrow indicates the dominant carrier and the short arrow the subdominant carrier. When d s (t) is a binary zero, then the short arrow is absent in FIG. 10 according to the rule. 
     Said rule results in a form of F(t) shown in FIG. 11 for the bit streams of FIGS. 8 and 9. The graph of F(t) consists of the bit stream on the dominant carrier plus spikes as suggested by Equations (2) and (3). The function g(r,φ) is the source of the spikes. The peak voltage value of said spikes is related to r and to [k d ·d d (t)−k s ·d s (t)] as given by Equation (2) where k d ·d d (t) is just Δf d . The deviation of the subdominant carrier is k s ·d s (t) which is −Δf d  according to the rule. Therefore, Equation (2) assumes the simpler form 
     
       
         F(t)=k d ·d d (t)+(±2Δf d )[g(r,φ)] when r≠0.  (4) 
       
     
     The rule used in this example can be summarized analytically as follows. When d s (t)=binary 0, then A s =0 volts which implies r=0 and then F(t)=k d ·d d (t). When d s (t) =binary 1, then F(t) is given by Equation (4). 
     In U.S. Pat. Nos. 5,541,959 and 5,570,395, the inventor of the present invention shows by equations and by use of a pinwheel diagram that peak frequency deviations placed like those of FIG. 10 are desirable because the spikes occurring at the output of any frequency demodulator have a polarity like that of the bit stream on the dominant carrier. 
     It is not necessary that the bit stream on the subdominant carrier have a rate greater than that on the dominant carrier. The rate of the bit stream on the dominant carrier can be greater than that on the subdominant carrier without affecting system operation. In this invention, it is only necessary that the bit stream rate on one carrier be an integer multiple of the bit stream rate on the other. The integer can be 1, 2, 3, etc. 
     Furthermore, it is not necessary that the bit streams experience abrupt transitions. The bit streams or the composite carrier can experience filtering (shaping) with the filter (shaper) output then applied to the frequency modulator, or the modulated carrier can experience bandpass filtering. The results are the same. Said results are that the bits at the output of any frequency demodulator in the receiver experience gradual transitions. 
     FIG. 12 shows a block diagram of a transmitter  100 . The input bit stream d d (t) at a rate R d  bps to be placed on the dominant carrier is applied to a frequency modulator  102 , such as the Valpey Fisher VF940 series, to generate a dominant frequency modulated sinusoid. The input bit stream is also inverted in an inverter  104  and applied to the input terminal of another frequency modulator  106 . A second independent bit stream d s (t) at the same rate, in this example, controls an electronic switch  108 , such as a CD4066. When the voltage d s (t) represents a binary 1 (in this example), the switch  108  is closed allowing a subdominant sinusoid frequency modulated by −d d (t) to be added to the dominant FM sinusoid in a summer  110 . This generates a single composite carrier. This composite carrier is then converted in frequency and amplified in a usual manner suitable for the application. An antenna completes the transmitter system. 
     An optional envelope conditioner  112  in FIG. 12 reduces the peak-to-peak value of any amplitude variation of the composite carrier prior to radiation. This improves power efficiency of the transmitter and noise performance of the radio communication system. Equation 1 describes the envelope variation of the sum of two sinusoids. 
     FIG. 13 shows the form of the envelope E(t) when r=−3 dB and for φ in the range (0, 4π). The envelope can be conditioned in a variety of ways. A class C amplifier removes much of the envelope variations by virtue of its operation. A class D amplifier consisting of a hard limiter followed by a bandpass filter will provide a constant amplitude carrier at the output of the filter. A fast automatic gain control (AGC) circuit can also act as an envelope conditioner. 
     The bandwidth of the signal after the envelope conditioner is somewhat greater than that of its input signal. This increased bandwidth results in considerable improvement in noise performance of a practical system. 
     FIG. 14 shows a block diagram of a receiver  120  which is the companion to the transmitter shown in FIG.  12 . The received signal is converted in frequency and amplified in an IF amplifier  122  and then limited in a hard limiter  124 , as in any ordinary FM receiver, to prepare the received signal for demodulation in a frequency demodulator  126 , such as Signetics NE604 integrated circuit. Since F(t) is the instantaneous frequency deviation of the composite carrier which is radiated and since the output of any frequency demodulator is a voltage linearly related to the instantaneous frequency of the sinusoidal input, then F(t) represents the voltage appearing at the output of any frequency demodulator in a receiver. 
     The output signal of the frequency demodulator  126  for an example in this invention is F(t) which is shown as FIG.  11 . The bit on the dominant carrier can be recovered by passing the output of the frequency demodulator through a baseband hard limiter  128  which functions as an analog-to-digital converter (ADC) since the limiter output is ‘high’ when its input voltage is &gt;0 (or some other predetermined value) and ‘low’ when its input voltage is &lt;0. Inspection of FIG.  11  and comparison with FIG. 8 establishes that the output of the hard limiter is the bit stream d d (t) carried by the dominant sinusoid. 
     The series connection of any frequency demodulator and a hard limiter is commonly used to recover bit streams from single frequency modulated carriers. Consequently, a quality of this invention is operation as an ordinary single carrier per channel system or as a two carrier per channel system with no change in receiver design. A switch in the transmitter selects the mode of operation. 
     The spikes superimposed on d d (t) are the feature representing the bits of d s (t). The presence of a feature represents a 1 of d s (t) and its absence a 0. The existence of the peak voltage value of the spikes superimposed on the message of the dominant carrier can be used to indicate the presence of the feature voltage. The presence of these peak values can be determined in a variety of ways. In one embodiment, the output of any frequency demodulator is passed into a full-wave rectifier  130  to generate a unipolar voltage. The rectified version of FIG. 11 is shown as FIG.  15 . An embodiment of a full-wave rectifier is an analog voltage multiplier, such as the Analog Devices AD734 integrated circuit, having the limiter output as one input and the output of any frequency demodulator as the other input. 
     A common peak voltage measuring circuit used in AM radios is an envelope detector  132 . The voltage of FIG. 14 when applied to an envelope detector generates two distinct voltage levels which can be discerned and converted to digital form by a comparator  134 . An Elantec 2625 operational amplifier is configured as a comparator. The other input to the comparator is a reference voltage V T  having value approximately equal the mean of the two different peak values of the voltage of FIG.  15 . The output of a comparator is the other independent bit stream d s (t) which can be verified by comparing the peak values of the voltage of FIG. 15 with the bit stream in FIG.  9 . Other embodiments of peak voltage measuring circuits and comparators can be used in this application to recover bit stream d s (t). 
     Another characteristic of this invention is the absence of any adjustments in receiver operation. The reference or threshold voltage used to distinguish peak values of spike voltages is determined from knowledge of r and Δf d  and said reference voltage is then set prior to receiver operation. 
     Any edge jitter occurring in the recovered voltages representing the independent bit streams d d (t) and d s (t) can be removed by recovering a clock from the output of the hard limiter. Said clock can be used with usual digital circuitry to generate clocked wave forms d d (t) and d s (t) like those present in the transmitter. 
     The previous example which uses a subdominant carrier added to a FM dominant carrier to generate a feature voltage can be extended to M-ary signaling. The same two dimensional constellation concept shown in FIGS. 5 and 6 applies. In the case of QAM, in-phase and quadrature carriers are used to provide two dimensions. In this invention, two system variables are also used to define the points in the constellation. 
     With two dimensional constellations, it is necessary to have baseband voltages appearing at two nodes in the receiver. With QAM, separate coherent demodulation of the in-phase and quadrature carriers provides the two needed voltages. In one embodiment of this invention, the output of the frequency demodulator is one voltage and the output of the envelope detector in FIG. 14 is the other voltage. 
     The two variables used to define the constellation in a preferred embodiment of this invention are amplitude ratio r of the two carriers and peak frequency deviation Δf d  of the dominant carrier. The variable Δf d  determines the ‘pedestal’ (constant) values of the output F(t) of the frequency demodulator. See FIG.  11 . This pedestal value is k d d d (t) in Equation 4. The feature voltage g(r,φ) in Equation 4 has peak amplitude determined by r. The feature voltage when superimposed on the pedestal value is the output F(t) of the frequency demodulator. The output of the envelope detector is the peak value of F(t). 
     FIG. 16 shows the constellation for a 4-ary embodiment of this invention with bit assignments. The abscissa is a measure of the pedestal portion of the output of the frequency demodulator. Mark this as F P . The ordinate is a measure of the output of the envelope detector. Mark this as E F , the envelope of F(t). A previous example treated the simultaneous transmission of two bit streams. That example is a 4-ary version of a preferred embodiment of this invention. Only one value of r and two values of carrier frequency are needed to realize the four points in the constellation as indicated in the previous example. 
     The symbols in FIG. 16 are defined by the following rule. The first bit of the group defines the carrier frequency deviations as + or −. The second bit determines whether the second carrier is added. A binary one means the subdominant carrier has been added which gives rise to an interference voltage having spikes whose peak value is detected. Therefore, in FIG. 16, the symbol  11  has coordinates (Δf d , E F 1). Similarly, the symbol 00 has coordinates (−Δf d , −E F 2). This rule is that of the previous example. 
     In the receiver of FIG. 14, the output of the envelope detector is shown as always a positive voltage. This is consistent with the use of full-wave rectification and a single envelope detector as shown in FIG.  14 . Two envelope detectors (positive envelope and negative envelope) can be used to generate a link between circuitry and the negative values of the ordinate shown in FIG.  16 . 
     An 8-ary embodiment of the present invention can assume various forms. In one case, to send a group of 3 bits when there are two independent bit streams, it is convenient to clock one bit stream at twice the rate of the other. That is, while one bit of a stream is sent, two bits of the other are sent. The group of 3 is then defined as the one bit plus the two bits. 
     The transmitter and receiver hardware of one 8-ary embodiment of the present invention is essentially the same as that when the bit streams are clocked at the same rate. The only change is the addition of a clock of twice frequency in the transmitter and the creation of two clocks in the receiver. These tasks are accomplished easily with readily available hardware. Either the bit stream carried by the dominant sinusoid or the bit stream carried by the feature voltage can be clocked at twice the rate of the other. The choice depends primarily on the use to be made of the two bit streams. 
     Clocking one stream at twice the rate of the other increases the switching rate of the state of the composite carrier which is radiated. Therefore, there is an increase in bandwidth required to obtain the higher data throughput rate. 
     To depict this 8-ary embodiment of the present invention with the usual planar constellation, it is necessary to indicate transitions from one point in the constellation to the other. FIG. 17 is shown as an example. In the case shown, the left-most bit of the group of 3 is represented by the carrier deviation. The middle bit indicates the initial state of the two bit group and the right-most bit indicates the final state of the two bit group. Consider the group  101 . Let +Δf d  represent a binary 1. Let the presence of a feature voltage (superimposed spikes) represent a binary 1. So,  101  is positioned at F=+Δf d  (left bit) with a small initial value of envelope E F  (middle bit) which then transitions to a large value of envelope E F  (right bit) as shown in FIG.  17 . If the middle and right bits are the same, then a point in the constellation transitions to itself (no change). 
     This example considers the subdominant carrier as being switched at twice the rate. In this case, the transitions in the constellation are vertical. When the dominant carrier is switched at twice the rate, then the abscissa becomes one value of deviation or the other and the transitions become horizontal in the constellation plane. 
     An 8-ary embodiment of the present invention can be used to send three bits at a time when there is but one bit stream involved. This requires 8 points of the constellation formed in the F P , E F  plane. A possible use of these two variables is shown as FIG.  18 . Four values of frequency and two values of r are used. A binary one for the left-most bit of the group implies positive values of F P . A large excursion of F p  corresponds to a binary one for the middle bit. A right-most bit of one implies r≠0 (beat spikes are present). 
     This same 8-ary embodiment of the present invention having FIG. 18 as an example can also be used to send three independent bit streams simultaneously where each bit stream is clocked at the same rate. The left-most bit of a symbol in FIG. 18 can be that of the first stream. The middle bit, that of the second stream. The right-most bit, that of the third stream. 
     An alternative embodiment of an 8-ary system according to this invention can be generated from inspection of Equation (2). There, [k d ·d d (t)−k s ·d s (t)] is the instantaneous difference in frequency of the dominant carrier and the subdominant carrier. In previous inventions by the inventor of this invention, this is called a beat frequency f B  Hz. Therefore, Equation (2) becomes 
     
       
         F(t)=k d ·d d (t)+f B ·g(r,φ).  (5) 
       
     
     The peak value of the spikes due to g(r,φ) are determined by f B  and r. Differing values of f B  result in differing peak values of the voltage spikes for a fixed value of r. A means of obtaining different values of f B  is having available subdominant sinusoids of differing values of frequency. 
     As an example, consider the selection of available frequencies shown in FIG.  19 . The long arrow represents the dominant carrier. The short arrows indicate available subdominant carriers. The constellation of FIG. 20 for this example is formed by setting E F =1 volts when F P =k d ·d d =Δf d . A single value of r=0.6 (−4.4 dB) for all three possible choices of frequency of the subdominant sinusoid gives values of E F =2, 3 and 4 volts. FIG. 21 shows the output F(t) of any frequency demodulator for this example. Twelve bits are included in FIG.  21 . The value of 1 volt represents the group  100 . The spikes having peak value of 2 volts are formed from the sum of sinusoids 1 and 2 in FIG. 19, and 2 volts represents the group  101 . Those of 3 volts, from the sum of sinusoids 1 and 3 representing  111  and those of 4 volts, from the sum of sinusoids 1 and 4 representing  110 . 
     In the transmitter, the left most bit of the group determines whether the dominant carrier is deviated +Δf d  or −Δf d  Hz from the carrier frequency. If the remaining two bits are 00, no subdominant sinusoid is added. Combinations 01, 10 and 11 determine the frequency of the subdominant sinusoid which is added to the dominant sinusoid. In the receiver, the output of the envelope detector consists of the four voltage levels 1, 2, 3 and 4 volts. These must be distinguished to recover the 4 possible symbols. Similarly for the negative envelope detector. 
     The 8-ary embodiments represented in FIGS. 18 and 20 can be extended to 16-ary, 32-ary, etc. representations of radio communication systems using feature voltages generated by the addition of two sinusoids by selecting various sets of values of r, dominant carrier frequency deviation Δf d  Hz and/or beat frequency f B  Hz. 
     A 16-ary embodiment is possible using just the four frequencies of FIG. 19 when the amplitudes of the subdominant carriers are taken so that the peak value of the sinusoid labeled  4  is less than that of the sinusoid labeled  3  which is less than that of the sinusoid labeled  2 . The following eight combinations of sinusoids will yield eight different positive values of E F : (1 only), (2 only), (1 and 2), (1 and 3), (1 and 4), (2 and 3), (2 and 4), (3 and 4). Reversing the order of the sinusoids of FIG. 19 will yield a mirror set of eight different negative values of E F . The result is  16  distinguishable values of E F . 
     The envelope variation of FIG. 13, which is created by the addition of a subdominant carrier and a dominant carrier, can be used as a feature rather than the frequency variation represented by FIG. 7. A possible rule is as follows. When d s (t) is a binary 1, add a subdominant carrier to the dominant carrier to create an envelope variation. In the receiver, distinguish the envelope variation from the constant value of the dominant carrier only. Either the maximum or the minimum of the envelope variation can be used to recognize the feature. Envelope detection of the output of the IF amplifier in FIG. 14 recovers the constant value of the dominant carrier or the envelope variation of the sum of the two sinusoids. A comparator circuit provides a digital indication of the presence or absence of the envelope feature. The presence of the envelope feature translates to a decision of a binary 1 for d s (t). A suitable threshold voltage is applied to the comparator. 
     FIG. 22A shows a transmitter for a system which uses an envelope as a feature. The transmitter is similar to that of FIG. 12 except that when the envelope is used as a feature, then the envelope conditioner circuit of FIG. 12 is not used. 
     FIG. 22B shows a receiver having an envelope feature detector. The receiver circuit of FIG. 14 is modified. The receiver is similar to that of FIG. 14 except that when the envelope is used as a feature, the full-wave rectifier is not used. An envelope detector operates in parallel with the frequency demodulator (rather than in series when using the frequency variation as a feature). The envelope detector input is the output of the IF amplifier. The envelope detector output is applied to a comparator which has as its input a binary indication of the bit stream d s (t). FIG. 22B is a block diagram of a receiver having envelope feature detection. The receiver of FIG. 22B is similar to that of FIG. 14 with the exception that the input signal to the envelope detector  132  is obtained directly from the IF amplifier  122 . 
     In an embodiment of the present invention, the feature is generated by adding a subdominant carrier to the dominant carrier. A feature can also be generated by modifying a bit stream (changing the two-level voltage representation) prior to carrier modulation, for example. A feature may also be generated by suitable additional modulation of a carrier for one of the binary digits or symbols. 
     In an embodiment of the present invention, the feature detector is an envelope detector. Various other feature detectors can be used depending on the nature of the feature voltage. Examples include matched filters, pattern recognizers and neural networks. coding methods can be applied to the constellations generated using feature voltages. 
     In various alternative embodiments of the invention, all or parts of the transmitter functions as well as all or parts of the receiver functions for recovering the individual bits of the various independent bit streams are obtained using computational routines provided by software in a programmed computational device. These routines emulate the receiver and transmitter functions. The organization and structure of such routines use conventional block diagrams of transmitters and receivers as flow charts for signal processing. Each of the functions of frequency demodulation, limiting, feature detection and voltage comparison is achieved using a variety of algorithms for each of such functions. 
     If necessary, an analog-to-digital converter (ADC) can be used to interface a continuous input voltage signal to computer code words processed by such routines. Similarly, a digital-to-analog converter (DAC) can be used to interface computer code words to a continuous output voltage. 
     Alternatively, a computer implemented routine can operate on the output F(t) of a frequency demodulator and the outputs of other demodulators to recover the bit streams directly by suitable sampling of F(t). Processing with appropriate algorithms these samples to distinguish the various feature voltages and/or determine their presence or absence will identify the bit or group of bits transmitted. 
     While the present invention has been particularly shown and described with respect to a certain preferred embodiment thereof, it should be readily apparent to those of ordinary skill in the art that various changes and modifications in form and detail may be made without departing from the spirit and scope of the invention as set forth in the appended claims. In particular, for example, other circuit configurations can be used to extract from the output of any frequency demodulator voltages necessary to generate the transmitted bit streams. Sampling and digital signal processing circuits can be used to accomplish the required result as described in the invention. Also, circuitry may be simplified when other prior information about the bit streams and any possible relations thereof are known. The principles of the invention apply to bit streams having unrelated clock rates. The invention can be used when the bit streams are filtered or shaped prior to carrier modulation. The invention illustratively disclosed herein may be practiced in the absence of any element which is not specifically disclosed herein. The invention is not limited to the embodiments disclosed herein and may be practiced using other techniques such as, for example, DSP or software implementations of circuit or system functions.