U. S. Patent Apr. 14,1987 Sheet 5 of 6
DISGUISED TRANSMISSION SYSTEM AND
METHOD
TECHNICAL FIELD
The present invention relates generally to a disguised transmission system and method and more particularly to a disguised transmission system and method wherein the transmitted power and data information rate increase and decrease together to simulate fading.
BACKGROUND ART
In certain situations, it is desirable to enable data to be transmitted from a particular location in such a manner as to be disguised from receivers in the vicinity. In 15 particular, personnel in hostile territory frequently desire to transmit a data signal to a remote, friendly receiver without detection by enemy receivers in the vicinity of the transmission site. Such a transmission scheme is desirably employed by military personnel, in 20 behind the front activities, from portable transmitters, such as "man packs", or by clandestine "civilian" personnel from fixed transmission sites. It is also desirable for a transmitter for such purposes to be relatively jam proof and for the data signal to be coded, to decrease 25 the possibility of deciphering a message, in the event that the disguised signal is detected.
DISCLOSURE OF THE INVENTION
In accordance with the present invention, a disguised 30 signal transmission system and method involves transmitting a data signal at a variable information rate with variable power so the information rate and transmitted power increase and decrease together and the transmitted data signal simulates fading. Fading is particularly 35 simulated in high frequency (i.e., 3-30 MHz) transmission links because of the ionospheric high frequency fading effect. A hostile surveillance receiver in the vicinity of the transmitter that picks up the transmitted, disguised signal detects the signal as if it were transmit- 40 ted from a great distance. Hence, the surveillance receiver does not attempt to jam the transmitted signal because the transmitted signal appears to be at a sufficiently great distance that it can not be jammed. Thereby, the transmitter and the personnel associated 45 therewith are not detected by personnel associated with the hostile surveillance receiver.
In one embodiment, the transmitted data signal includes a series of data bits, each having the same energy by virtue of the product of the power and duration of 50 every bit being constant. A transmitter in accordance with this embodiment includes a generator for deriving a pseudo-random sequence having a predetermined chip rate. In response to the sequence derived from the generator, a signal indicative of the number, K, of chips 55 of the sequence for each data bit is derived. The value of K is an integral submultiple of the number of chips derived from the sequence in a predetermined interval. In response to the sequence and the signal indicative of K, a data bit from a source of information data bits is 60 clocked each time the number of bits in the sequence equals K. Transmitter means, preferably in the high frequency band, derives an angle modulated wave, i.e., frequency modulated or phase modulated, having a modulation extent controlled by the clock bits and hav- 65 ing a variable power level proportional to K. The transmitter means is responsive to a signal derived from an EXCLUSIVE OR gate means responsive to the
clocked data bits and the sequence. Because the value of K is an integral submultiple of the number of chips derived from the sequence, transitions from the EXCLUSIVE OR gate means are synchronized with transitions from the sequence.
A receiver for the disguised wave demodulates the wave to derive a base band replica of the binary data bits supplied to the receiver with an occurrence rate determined by the sequence. A generator of the pseudorandom sequence is synchronized to derive a synchronized sequence. EXCLUSIVE OR gate means responsive to the synchronized sequence and the base band replica derives a first signal, which in a perfect noiseless link has a value and duration equal to that of every data bit derived from the EXCLUSIVE OR gate means at the transmitter. In response to the base band replica, a second signal having a value indicative of K is derived. An accumulator adds the binary values of the output of the gate means at the receiver over an interval of K, under the control of the second signal. Because the link is not noiseless the output of the receiver gate means may not correspond accurately with the input of transmitter gate means. To compensate for this possible inaccuracy, the receiver includes a decision means to derive a replica of the binary data bits supplied by the data source to the transmitter.
In the previously discussed embodiment K chips, each having the same power, are transmitted during each data bit. After one data bit has been transmitted, the transmitter power under most circumstances suddenly changes to a new power level for the next transmitted data bit, having a duration of K' chips. Such sudden changes in the power level can likely be used by intelligent surveillance receivers to identify the transmitter, unless a very large number of discrete power and data bit duration levels are employed, in turn implying a very large number of bits in the transmitter digital hardware. A second disadvantage of maintaining the energy per transmitted bit absolutely constant is that there is a very unnatural power versus time relationship. High power portions of the transmission are skewed toward being shorter than the lower power, i.e., faded, portions. Thus, the high power portions result in a burst like transmission.
To obviate these disadvantages of the first named embodiment, a second embodiment of the invention provides for minor energy variations during each data bit, enabling the transmitted signal power to match Rayleigh or other fading statistics exactly without quantizing the transmitted power levels. To provide the Rayleigh or other fading statistics without quantizing the power level, a digital noise generator derives a sequence which serves as a fading waveform source. The particular statistics and spectral characteristics of the digital noise source are not important as long as they simulate Rayleigh or other fading characteristics, but the noise source characteristics must be predetermined to enable a receiver to detect the transmitted variable duration and amplitude data bits; both the amplitude and duration of the transmitted data bits are controlled by the fading waveform source.
A dynamic digital low pass filter responds to the digital noise generator to transform the noise generator sequence into a smooth function of time. The dynamic digital low pass filter provides a first-order approximation of the fading characteristics of a high frequency channel. For typical high frequency channels, the filter
has a cut-off frequency in the range of 0.5 to 2.0 Hertz. To provide the most realistic simulation of high frequency fading, the bandwidth of the low pass filter is changed during operation by changing the filter coefficients.
A receiver for the transmitted signal includes digital noise sequence generator circuitry and dynamic digital low pass filter circuitry having identical characteristics with those of the transmitter. The characteristics of the receiver noise source and low pass filter are synchronized with those of the transmitter.
The output of the dynamic digital low pass filter typically has a nearly Gaussian probability. The nearly Gaussian probability output of the digital low pass filter is mapped to a density function which represents high frequency fading. If the transmitted power is directly controlled, a chi-square controller, having two degrees of freedom, determines the amplitude of the transmitted power. However, if the transmitted power is varied directly by controlling the voltage of the transmitter, a Rayleigh distribution controls the amplitude of the transmitter power. To convert a chi-square distribution into a Rayleigh distribution, it is merely necessary to determine the square root of the chi-square distribution. Because of the relatively low frequency nature of the Rayleigh and chi-square distributions relative to the duration of a transmitted data bit, only minor power variations can occur over the length of a transmitted data bit.
To enable the duration of the transmitted data bits to be approximately inversely related to the amplitude of the power in the data bits, the density function representing fading controls the duration of each bit. To this end, the density function representing fading is applied as an address input to a read only memory which maps values of the density function representing fading into the number of pseudo noise chips included in a single transmitted data bit. The read only memory responds to the density function representing fading to produce an output signal representing the number of chips in a data bit, i.e., the read only memory derives a signal indicative of K. As in the first embodiment, a data bit from a source of data bits is clocked each time the number of bits in the sequence equals K. An angle modulated wave 45 having a modulation extent controlled by the clock bits and a variable power level determined by the density function representing fading is transmitted. A receiver responsive to the transmitter functions similarly to that described with regard to the first embodiment, but includes suitable circuitry for determining the duration of each data bit in response to a density function representative of fading. Thereby, the beginning and end of each data bit at the receiver can be determined, for detection purposes.
It is, accordingly, an object of the present invention to provide a new and improved system for and method of enabling data to be transmitted from hostile territory without arousing suspicion of a surveillance receiver in the vicinity of the transmitter.
Another object of the present invention is to provide a transmitter system and method wherein a hostile surveillance receiver responsive to the transmitter detects the transmitter signal in such a manner as to believe that the transmitter is at a sufficiently great distance that the signal can not be jammed.
A further object of the invention is to provide a data communication system and method wherein a wave is
"modulated with data in such a manner that the wave is disguised with characteristics simulating fading.
Still a futher object of the invention is to provide a transmission system and method wherein a data signal is transmitted at a variable information rate with variable power to simulate fading, particularly ionospheric high frequency fading.
The above and still further objects, features and advantages of the present invention will become apparent upon consideration of the following detailed description of several specific embodiments thereof, especially when taken in conjunction with the accompanying drawing.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a block diagram of one embodiment of a transmitter in accordance with the invention;
FIG. 2 is a series of waveforms helpful in describing the operation of FIG. 1;
FIG. 3 is a block diagram of a receiver responsive to a signal transmitted from the transmitter of FIG. 1;
FIG. 4 is a block diagram of a second embodiment of a transmitter in accordance with the invention;
FIG. 5 is a waveform of a typical output of the transmitter of FIG. 4; and
FIG. 6 is a block diagram of a receiver responsive to the transmitter of FIG. 4.
BEST MODE FOR CARRYING OUT THE
INVENTION
Reference is now made to FIG. 1 of the drawing wherein there is illustrated a serial pseudo-random number (PN) generator 11, driven by square wave clock source 12, having a frequency fc. In a preferred embodiment, fc=l-10 kHz, whereby pseudo-random noise generator 11 derives a predictable random sequence having a chip rate of 10 kHz, and the pseudo-random noise generator derives a bit sequence in the range of between 215 —1 and 230— 1, depending upon the application. As is well known to those skilled in the art, one chip is derived from PN generator 11 in response to each cycle of source 12. If the bit sequence is in the range of 215—1 bits in length, the apparatus of the present invention can be employed for mobile, man pack type devices because the bit sequence is on the order of 3 seconds for a 10 kHz chip rate. For a bit sequence of 230— 1, the sequence requires approximately 100,000 seconds to complete, for a chip rate, of 10 kHz; such a generator is suitable for non-real time communication links wherein a receiver can store a coded binary data sequence resulting from the output of generator 11.
The pseudo-random number sequence derived by generator 11 is applied to digital low pass filter 13, having a clock input responsive to the 10 kHz output of clock source 12. Filter 13 typically has an effective cut-off frequency of 10 Hertz, to provide a receiver correlation time of approximately 100,000 milliseconds. Filter 13 has a relatively low, 10 Hertz cut-off frequency to enable a transmitted wave derived from the transmitter of FIG. 1 to have only relatively small changes in power, to assist in simulating a fading high frequency signal transmitting characteristic. Filter 13 responds to the serial output signal of pseudo-random number generator 11 and clock source 12 to derive a multi-bit, parallel output word once for each cycle of clock 12; typically, each output word of filter 13 includes eight bits, to represent the accumulated value of the output of generator over the previous 0.1 second,
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