METHOD FOR SPREAD SPECTRUM COMMUNICATIONS

Abstract: A method for secure radio pulse transmission through varying atmospheric condition which chooses different frequencies for transmitting a series of pulses of energy dependent upon the digital data content to be transmitted, provides for determining the frequency choices for a subsequent pulse transmission in response to digital data content of at least one previously transmitted pulse. Application includes h.f. band, data and fax, wireless communications.

Description: METHODFORSPREADSPECTRUMCOMMUNICATIONS Background of the Invention Field of the Invention 5 The present invention generally relates to spread spectrum, digital communications, and particularly to a method for determining frequencies for frequency hopping transmissions. Statement of the Prior Art Various algorithms exist for controlling digital pulse, spread spectrum 0 communications. The principal goal is that of encoding frequency selection for providing communications which are effective in a fading multipath environment and which are secure from both jamming and eavesdropping. Spread spectrum transmissions typically take narrow bandwidth symbols and spread them out over a much wider bandwidth than is necessary for the rate of data transmission. 5 Unfortunately, atmospheric multipath effects are frequency dependent and thus are accentuated by any spread spectrum approach. In frequency hopping, spread spectrum systems, the carrier frequency is changed in accordance with a predetermined sequence controlled by one of a variety of codes. The transmitted information is thereby contained in either the o transmitted frequency or in the frequency difference between the transmitted pulses and a reference frequency. The spread spectrum signal may be received, decoded and demodulated by a receiver constructed to employ the same frequency hopping code, once the same predetermined code is synchronized at the receiver with the transmission code. In one prior art method 5 described in US Patent No. 4,612,652, a matched pair of pseudo random number generators are used to determine the mark and space frequencies of both a transmitter and a receiver. The direct generator output and a delayed version thereof are used to provide separate random frequency sequences for each of the mark and space data signals, respectively. The separate generators o are synchronized between transmitter and receiver, resulting in synchronism of both the mark and space frequencies. Unfortunately, the required synchronism adds complexity to the transmitter and receiver and their interaction and can be the cause of transmission failures. Summary of the Invention Accordingly, it is an object of the present invention to provide a method of frequency hopping, spread spectrum communications. It is another object of the present invention to provide such a method 5 which is capable of compensating for multipath and fading transmission environments. It is a further object of the present invention to provide such a method which enables error correction and recovery of lost data. It is a still further object of the present invention to provide such a method 0 which does not require synchronism or synchronization between the transmitter and receiver. In one form, the present invention provides a method for frequency shift keying transmissions of the type including choosing different frequencies for transmitting a series of pulses of energy dependent upon the digital data content 5 to be transmitted, wherein the improvement comprises the step of determining the frequency choices for a subsequent pulse transmission dependent upon digital data content of at least one previously transmitted pulse. Brief Description of the Drawings The present invention is illustratively described in reference to the o appended drawings in which: Fig. 1 is a diagram of an encoding method configured in accordance with one embodiment of the present invention; and Fig. 2 is a diagram of a received transmission decoded in accordance with another embodiment of the present invention. 5 Detailed Description of the Drawings In one embodiment of the present invention, a wide frequency bandwidth is divided into a predetermined number of frequency channels or bins which are used to distribute sequentially transmitted pulses of a signal. The frequency selection or hopping pattern is partially predetermined and includes alternate o subsequent frequency paths depending upon the data content of each transmitted pulse. Normally the transmitted frequency represents a logical data value such as "1" or "0". In the subsequent frequency hop there are two pairs of frequencies with each pair corresponding to one of the possible data values of its immediate predecessor hop. Which pair of frequencies are used to transmit the subsequent data bit depends upon the data content of the previous hop. A "1 " sent on the previous hop will dictate use of one of the pairs and a "0" will dictate use of the other of the pairs. The individual channels in each frequency pair of the subsequent hop represent the same logical "1 " and "0" and the data content of the subsequent hop determines which channel of the determined pair is used. The present method is applicable to the transmission of more than one data bit at a time. Transmitting two data bits at once requires four possible pulse frequencies to represent the four possible conditions of two data bits. The subsequent pulse transmission uses four sets of four frequencies, or sixteen in all, and determining which set of four is used in response to the four possible data conditions of the previous bit. Fig 1 is a diagram showing a simplified encoding method in accordance with one embodiment of the present invention. Each row of the diagram represents a sequential pulse transmission interval. The diagram represents a single bit transmission rate meaning that only one logic bit is transmitted in each interval. The diagram is repetitive so that the four transmission intervals are repeated in sequential passes. Each logic value shown represents a separate frequency bin within a full transmission bandwidth. Logical pairs of bins are shown in parentheses, ( ). The example shows a total of thirty different frequency bins. Within each of the transmission intervals, rows, only one of the corresponding frequency bins is used to transmit a pulse. Which frequency bin is used depends upon the data content of the current bit and the data content of the bits transmitted in the previous intervals. The specific pair of frequency bins which is available for the subsequent transmission interval from each frequency bin is connected thereto by dotted lines. From the fourth transmission interval, each pair of frequency bins is treated as if it corresponds to the first transmission interval and the sequence repeats from the second transmission interval. Thus in the first interval, a pulse is transmitted in either of the bins shown representing the respective logic value. In the second transmission interval shown, which pair of bins is available depends upon the data content of the first interval. Which bin of the available pair is used depends upon the logic value to be transmitted in that second interval. This scheme continues to the transmission of a pulse in the fourth interval. The respective logic value of the pulse actually transmitted in the fourth interval is treated as the logical equivalent of the first interval and the pairs of frequency bins used in the actual fifth transmission interval are selected from the second transmission interval shown. By the method described, an actual transmission signal is identified by 5 determining the length of the sequence of transmission intervals in which pulses are detected in the proper frequency bins. Sequences of pulses which are not sufficiently sustained are discarded as non-signals. Further, an actual pulse transmitted in the fourth interval may be used to reconstruct pulses missing from the second and third intervals by simply following the dotted lines back through 0 the intervals. The actual frequencies which represent the frequency bins shown in Fig. 1 may be periodically reassigned to further prevent decoding of an intercepted message. Fig. 2 is a diagram representing an example of potential signal pulses 5 received over a portion of a message transmitted in accordance with one embodiment of the present invention. The horizontal axis represents a portion of the total bandwidth of the transmission with different horizontal positions representing different frequency bins or channels within that total bandwidth. The vertical axis of the diagram is labeled with successive pulse transmission intervals o to represent the successive frequency hops of the transmission. Each circle shown on the chart represents energy detected in the respective frequency bin and transmission interval, by a wideband receiver monitoring at least the portion of the bandwidth shown. The transmission shown in Fig. 2 is a two bit transmission rate for each frequency hop or transmission interval. That is, two 5 binary bits of data, representing a total of four possible logical states are transmitted in each interval. The first transmission interval shows energy detected in bins 10 and 12. The second transmission interval shows energy detected in the nine frequency bins for which there are circles shown. Those bins which are logical successors 0 to the bins 10 and 12 are respectfully connected thereto by dotted lines and are labeled in quotes (") with their logical value, i.e. "01 ". Because energy, represented by the circles, is shown to be present in all of the logical successors in the second interval as well as bins 10 and 12 of the first interval, subsequent transmission intervals are necessary to determine the correct data signal for the first and second intervals. In the third interval, energy is detected in the five frequency bins shown. Because the detected energy in the third interval can only be a logical successor to the energy in bin 14 of the second interval, it is known that bin 14 is the correct data signal for the second transmission interval. All of the remaining energy detected in the second interval can be discounted because none of the detected frequency bins have a logical successor in the third interval. Likewise the proper signal pulses may be traced back to bin 10 of the first interval. Also, the correct signal path continues to develop through subsequent transmission intervals in frequency bins 16-22. Simultaneously, another data path is shown to be received in energy detection circles 23-26. Because energy is continually detected in the proper frequency bins of subsequent intervals, it is determined to be a valid signal. In this manner simultaneous signals may be transmitted in a multi-user network. This second signal also demonstrates that the encoding is not time dependent and, therefore, does not require synchronization. The logical sequence of frequencies to be used is determined in response to the transmission bit rate and the available spectrum to be used, in a manner with makes full use of the available spectrum prior to allowing any frequency repetition. Such repetition limits the number of frequency hops which can be reconstructed if lost. The number of sequential pulses which can be transmitted prior to any frequency repetition is the number of hops that can be reconstructed from the method of the present invention. The use of frequency bins in this manner allows discontinuity of the full transmission bandwidth. Thus continuous emitters, located in the middle of the full transmission spectrum, only block their limited bandwidth portion of the spectrum. As mentioned, spread spectrum techniques in the H.F. band (3-30 MHz.) are readily susceptible to corruption from frequency dependent ionospheric conditions. Different frequencies penetrate the ionosphere to different altitudes prior to reflection back to earth and the same frequency may also be reflected from different altitudes. The first condition upsets the sequence of pulses transmitted at different frequencies and the second condition introduces additional, or multipath, signals which are likewise differently timed and out of sequence. The practice of the present invention therefore includes the periodic sounding of the ionosphere with a predetermined sequence of transmission pulses in as many as all of the separate frequency bins used in the full transmission bandwidth. Re-aligning each of the received pulses with the predetermined sequence provides delay and multipath corrections for the transmission environment. In one embodiment, this sounding may be executed every second to thereby allow compensation for the vast majority of varying ionospheric conditions. After the sounding, the actual data signal is transmitted and the received pulses are realigned in time in response to the sounding results and prior to decoding in accordance with the present invention. The encoding method of the present invention is also compatible with bit dispersion, which is an orthogonal or independent method of encoding. By example, the data bits of a signal to be transmitted are first aligned, at least theoretically, as sequential rows of a matrix. The data is then read from the matrix, for encoding in accordance with the present invention, as sequential columns of bits. Thus an entire missing column of data may be lost in transmission and it will show up in the received and decoded signal as single bits lost in sequential data strings. Such independent or orthogonal decoding techniques may be taken advantage of to iteratively decode, correct, recode and re-decode the signal and thereby reduce gaps in lost data. Conclusion The present invention provides a method for encoding spread spectrum pulse transmissions which are compensated for varying atmospheric conditions. The present invention further provides for recovery of lost signals which operates both independently and in combination with other known techniques to provide synergistic results when compared to the recovery ability of individual techniques. The present invention does not require any frequency or time sequence alignment between transmitter and receiver prior to transmission thus reducing complexity and transmission delay and failure. This transmitter/receiver independence further enables the reception of simultaneous signals by the same receiver thereby supporting radio network applications. The embodiments described above are intended to be taken in an illustrative and not a limiting sense. Various modifications and changes may be made to the above embodiments by persons skilled in the art without departing from the scope of the present invention as defined in the appended claims.

Claims: WHAT IS CLAIMED IS: 1. In a method for frequency shift keying transmissions of the type including choosing different frequencies for transmitting a series of pulses of energy dependent upon the digital data content to be transmitted, wherein the improvement comprises the step of determining the frequency choices for a subsequent pulse transmission dependent upon digital data content of at least one previously transmitted pulse. 2. The method of claim 1 , wherein the step of detecting the frequency choices for a subsequent pulse transmission are dependent upon a plurality of previously transmitted pulses. 3. The method of claim 2, wherein the different frequencies used for transmitting pulses are spread out over a predetermined spectrum, and further comprising the step of periodically determining characteristic transmission conditions of a transmission medium over the predetermined spectrum. 4. The method of claim 3, wherein the step of periodically determining transmission conditions includes transmitting, over the transmission medium, a predetermined sequence of pulses at predetermined frequencies spread over the predetermined spectrum, receiving the transmitted predetermined sequence of pulses and comparing the received pulses to the predetermined sequence to determine characteristic transmission conditions of the transmission medium.

Inventors: HERRICK DAVID L, HERRICK, DAVID, L.
Applicants: LOCKHEED CORP, LOCKHEED MARTIN CORPORATION
Application year: 1996
Main classification: H04K1
IPC codes: H04B1, H04L27