Gte Automatic Electric Laboratories Incorporated

Lockheed Martin Corporation

Gte Automatic Electric Incorporated

Gte Network Systems Corporation

Adam Lender

Henry H Olszanski

A system and method of modulation of digital data to create a low probability of intercept communication system. The method comprises the steps of receiving digital data (or analog data and converting it into digital data), modulating the digital data signal, scrambling the digital data signal using here an eight position shift register with appropriately positioned feedback elements, converting the modulated-scrambled signal into a direct-sequence pseudo noise spread spectrum signal, converting the spread spectrum signal into a gaussian type signal, resulting in a white and gaussian noise-like characteristics. The system and method employ proved and innovative correlative techniques in converting data into a noise-like waveform. This insures a higher degree of immunity to interception, and provides an unmatched level of privacy and security over all types of communication media.

System and method for converting data into a noise-like waveform

An incoming bit stream is digitally processed to obtain both in-phase and quadrature-phase 7-level correlative signals. The two 7-level correlative signals are separately band limited to a passband having a bandwidth that is about 1/8th of the bit rate of the original bit stream, and these band limited signals are then separately analog modulated, respectively, using in-phase and quadrature-phase components derived from a line carrier oscillator. The modulation products are then combined in a summing circuit to derive a signal-sideband 7-level correlative signal.

Method and apparatus for converting binary information into a high

A transmitter converts an incoming bit stream into a single-sideband signal for transmission over a band-limited channel such as a digital radio. The incoming bit stream is digitally processed to obtain both in-phase and quadrature-phase 3-level correlative signals. The two correlative signals are separately band limited to a passband having a bandwidth that is about one-fourth the bit rate of the original bit stream, and these band-limited signals are then separately analog modulated, respectively, using in-phase and quadrature-phase carrier frequencies which are derived from a carrier oscillator. The products from the analog modulation process are then combined in a summing circuit to derive the single-sideband signal. At the receiver, the single-sideband signal is analog demodulated using a carrier frequency that is at either end of the single-sideband frequency spectrum, i.e., C.+-.1/4T Hz where C is the carrier, frequency in Hz and 1/T is the cut-off frequency in Hz and is numerically equal to the bit rate in bits per second (BPS). The lower sideband is selected by a low-pass filter and the signal thus obtained is modified duobinary. By rectifying and slicing the modified duobinary signal the original data is recovered.

Method and apparatus for converting binary information into a

Single errors can be detected and corrected in a signal employing a 3-level modified duobinary code. Error correction is predicated on the concept of maximum likelihood of error for the bit having the maximum departure from normal amplitude (error differential). A converter accepts the modified duobinary signal, decodes the signal to obtain a binary output signal and determines the error differential for each bit. Bits of the binary output signals are temporarily stored in a sequential storage device. The error differential is applied to an input of an error analyzer. An error detector determines if an error has occurred during an error correction interval which interval is established by successive extreme levels of the modified duobinary signal. The error analyzer tracks the location of the bit having the largest error differential during each error correction interval. A plurality of bit correctors, one for each time slot of the sequential storage device, operate in combination with the error detector and the error analyzer to alter the bit having the highest error differential following detection of an error.

Error correction for signals employing the modified duobinary code

Modified duobinary regenerative repeaters are tested in situ by means of a pulse pattern generator connected to the repeatered span line, a plurality of audio filters, one at each repeater site, a return transmission path and a measuring device for determining the character of the signal output from a selected repeater.

Modified duobinary regenerative repeater testing arrangement

The correlation properties of a 7-level correlative waveform permit detection of errors due to irregularities and impairments in the transmission facility, where these errors violate the correlation patterns of the waveform. The received 7-level correlative waveform is first decoded to obtain a pair of binary outputs which are representative of the original binary signal at the transmitter, except for errors that might have occurred during the transmission period. The pair of binary signals are operated on to obtain coded binary signals and their delayed counterparts. These coded binary signals are used to establish the effect of presence or absence of a top or bottom level that would result from the formation of a nonbinary correlative signal. If an error has occurred during transmission, the extreme levels of the locally generated signal will not normally coincide with those of the received signal. To determine if an error has occurred it is necessary to compare the extreme levels of the incoming waveform with those locally generated. Thus, the decoder provides the bottom and top level outputs which are compared with the extreme levels of the equivalent signal from the binary coded signals, and when occurrences of extreme levels of the two signals differ, it is an indication that an error has occurred. The error indication results in an output signal from the error detector circuitry.

Apparatus and method for detecting errors in a 7-level correlative signal

Apparatus and method for generating a multilevel correlative signal wherein the multiple levels are digitally generated. A serial binary bit stream is first converted into two parallel binary bit streams each at half the bit rate of the original serial binary bit stream. The parallel bit streams are encoded in accordance with the relation B = C - .DELTA..sup.2 C MOD 4; where B is the original waveform, C is the resultant waveform, and .DELTA..sup.2 indicates two units delay or 2T seconds where 1/T is the parallel speed in digits per second. Next the binary encoded bit streams are digitally converted to a 7-level waveform; and, finally the 7-level waveform is transformed into an analog signal which retains the 7-levels and provides a band limited 7-level signal related on a one-to-one basis to the binary input signal.

Apparatus and method for generating a high-accuracy 7-level correlative

A modified duobinary signal is separated into two (odd and even) pulse trains, each of which is representative of a bipolar pulse train. Each representative bipolar pulse train has a bit rate one-half that of the modified duobinary signal. Violations of the modified duobinary coding rules appear as violation of the bipolar coding rules. The bipolar violations which occur are detected separately for the odd and even pulse trains. The detected errors are read into an error combiner and are read out serially at the original modified duobinary bit rate.

Error detector for modified duobinary signals

In PCM transmission, it is well known to transmit 24 voice-grade channels at a bit rate of 1.544 Megabits per second (Mb/s). The number of channels which may be transmitted in about the same bandwidth is doubled by the use of correlative level coding, such as the modified duobinary technique. Because of the degradation experienced by the modified duobinary signal in passing over cable pairs, periodic regeneration of the signals is required for effective transmission. The present invention is directed to a regenerative repeater which equalizes, amplifies, reshapes, and retimes the received modified duobinary signal so as to provide a regenerated signal which is nearly identical with that originally transmitted.

Method and apparatus for regenerating a modified duobinary signal

An end-office repeater accepts a high-speed bipolar signal, converts the bipolar signal into a unipolar signal, scrambles the unipolar signal, and encodes the scrambled unipolar signal into a modified duobinary signal for transmission over a cable pair. Before cable loss would severely degrade the duobinary signal, line repeaters and/or inter-office repeaters are installed at predetermined intervals so as to equalize and regenerate the modified duobinary signal. By so doing, the signal is recovered with acceptable phase jitter, which permits recovery of the original high-speed bipolar signal at the distant end of a repeatered span line. The receiving-end office repeater equalizes and regenerates the modified duobinary signal, decodes the signal to obtain a unipolar signal, descrambles the unipolar signal, and converts the unipolar signal into a bipolar signal. Detection of errors in the modified duobinary signal may be obtained without adding redundant digits. However, errors which may occur during transmission may be lost by the decoding to a unipolar signal. Thus, errors detected from violations of the coding rule of the modified duobinary signal are inserted, by means of an error transfer network, into the bipolar signal and appear as violations of the bipolar code.

Modified duobinary repeatered span line

A binary signal is first digitally transformed to obtain both an altered binary signal and its complement. The altered and altered complementary signals are next each delayed by two bit intervals of the original waveform to obtain both delayed altered and delayed altered complementary signals in binary form. Then, the altered and delayed altered complementary signals are gated to obtain the first gated binary output signal wherein a second level output signal is obtained whenever the two input signals are both first level signals and the first gated signal is a first level signal for all other input combinations. The altered complementary and delayed altered signals are similarly gated to obtain a second gated output signal in binary form. Finally, the first and second gated output signals are compared to obtain a three-level digital modified duobinary signal.

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Adam Lender