Harris Corporation

Daniel D McRae

Frank A Perkins

Earl Ford Smith

William Nelson Furman

Joseph Bibb Cain

Robert C Davis

Edward B Glover

George C Clark

Nicholas C Szuchy

A frequency management scheme for a multistation H.F. communication network establishes communication channels among network stations over respective ones of K frequencies, so that network stations may communicate with one another. Any station desiring to communicate with another station, repeatedly transmits, on each of plural ones of the K communication frequencies in sequence, a probe message comprised of three successive symbols, each of which is defined in accordance with a prescribed pseudo random M-ary code, in the form of multi-chip sequence, that represents the address of the station to whom the communication is directed. The first symbol is effectively unmodulated to permit monitoring equipment of a receiving station of the network to settle out. The second symbol represents the address of the transmitting station. The third symbol identifies the number of times that the frequency which carries the probe message has been transmitted. Each idle station monitors the level of activity of each of the K communication frequencies by successively dwelling on each frequency for the length of time required for any message-transmitting station to step through all K frequencies of the network. During each dwell time, the idle station looks for its address. If a probe message is directed to that monitoring station, the addressed station transmits back to the probing station a prescribed three symbol response message.

Frequency management system for use in multistation H.F. communication

An 'avalanche' relay communication network is configured of a plurality of transceiver stations spread out over a geographic area of interest to establish multipath communication diversity among the stations. The transceiver equipment at each station has the capability of simultaneous transmission over the same frequency through a 'common knowledge' network timing scheme such as TDMA and has the capability of taking advantage of received multipath signals. Communications between an originating station and an intended recipient station are achieved by the originating station modulating onto an HF carrier a digital packet formatted to contain the number of times the message is to be repeated and a means of establishing the quality of the received message. All stations which have correctly received the packet repeat that same message on the same carrier frequency at the same preestablished future absolute time (or times, based on the number of repeats).

HF avalanche relay communication technique

An auxiliary phase tracking subsystem for compensating for fast phase changes in an adaptive transversal filter obtains a measure, from the received signal, of channel phase variations and corrects the phase of each received sample in accordance with this phase measure. The channel tracking weights are used in conjunction with the known transmitted symbols and the data decisions to 'predict' the n received samples, PR.sub.i, for each frame. The average difference in phase, DP, between the predicted samples, PR.sub.i, and the actual received samples, R.sub.i, is then used to determine the phase correction for each received sample prior to using it for making estimates. The received samples, R.sub.0, R.sub.1, ------, R.sub.n-1, are then corrected to obtain the corrected samples, CR.sub.0, CR.sub.1, ------, CR.sub.n-1, as follows: ##EQU1## and K.sub.1 and K.sub.2 are constants. The result of this is to phase-correct each received sample by some constant phase, K.sub.1 DP, plus an additional correction which increases linearly with time during the frame. This latter correction is, in effect, a 'frequency offset' correction and when accumulated from frame-to-frame serves the function of doppler tracking as well as phase tracking.

Phase tracking correction scheme for high frequency modem

Source intelligence within a predetermined bandwidth is normally provided to a transmission channel having a bandwidth constraint of the predetermined bandwidth. For privacy transmission, a first pulse train indicative of the source intelligence is derived and is enciphered by addition of pseudo-random values to the pulses, either directly or modulo some specified constant. The enciphered pulse train is converted to first and second symbol pulse trains at subharmonic frequencies of the first pulse train. Energy within a selected spectrum is derived from each symbol pulse train, such as by filtering, and used to modulate first and second channels of a quadrature modulator having a carrier frequency centered within the predetermined bandwidth. A suppressed carrier quadrature modulated output is transmitted along with a carrier frequency component. Receiving means include a quadrature demodulator and means for reconstructing symbol pulse trains and means for reconstructing an additively enciphered pulse train from symbol pulse trains. The enciphered pulse train is provided to an adaptive filter and deciphering means for provision of an output to an intelligence utilization means. A training mode of operation is also provided in which calibrating information is transmitted from enciphering means to the deciphering means. Weights of the adaptive filter may be adjusted during the training mode, and the adaptive filter is synchronized to sample each transmitted symbol several times.

Apparatus for privacy transmission in system having bandwidth constraint

In a communication system for transmitting information over a dispersive link, a transmitted message is assembled to contain alternately arranged known and unknown pluralities of data symbols. At the transmitter, the known data symbols may be produced by a maximal length PN sequence generator, with an identical generator being provided at the receiver for reproducing the known symbol sequences to enable data recovery to be achieved. In the received signal processing equipment, the received message is subjected to a prescribed data recovery algorithm containing a transversal filter function and a data estimate refinement scheme which takes advantage of the a priori knowledge of data symbols of those portions of the transmitted message between which unknown data symbols are located. A transmitted message contains a plurality of successive frame, each frame containing N known data symbols followed by M unknown data symbols. Thus preceding and succeeding each set of M unknown data symbols there are respective sets of N known data symbols that enable the characteristics of the transversal filter function, and thereby the data estimates of the unknown symbols, to be updated and adaptively refined. Synchronization of the local PN sequence generator at the receiver with the PN sequence of which the successive sets of N known data symbols are composed in the transmitted message is achieved through a prescribed tone sequence/initialization scheme, subsequent to which unknown data is transmitted and processed at the receiver in accordance with the data recovery algorithm.

Technique for high rate digital transmission over a dynamic dispersive

Equipment-generated and dynamic crosstalk are effectively compensated by a signal equalization technique which involves the cascading of equalizers of respectively different lengths and respectively different loop gains. The first equalizer to which a received data sequence is applied is a long equalizer having a large number of stages, the term long being relative to the time dispersion of the high frequency channel. The gain of this long equalizer is adjusted to a low value that is consistent with the basic acquisition requirements of the transmission system, but is such that it averages through rapid variations of the high frequency channel. The second equalizer is short relative to the first but is sufficiently long to cover the span of the time dispersion of the high frequency channel. The first equalizer includes a data storage device which receives and stores successively received data samples. Each data sample is multiplied by a controllable weighting factor and the weighted samples are summed to provide an estimate of a received data signal. This estimate is then supplied to the second equalizer which may either be of the form of a transversal equalizer, like the first, or a decision-configured equalizer. At the output of the second equalizer there is obtained a second estimate which is compared with a reference value to form an error signal. The error signal is correlated with the data stored in the first and second equalizers in order to update the weighting factors of the respective equalizer. The reference value can be a preset value or one determined from the data. The gain factor for correlating the error signal with the data stored in the second equalizer is considerably greater (at least an order of magnitude and preferably two or more orders of magnitude) than that for the first equalizer.

Cascaded equalizer technique

The influence of memoryful non-linearities on a serial data sequence is combatted by initially analyzing a transmitted test or training data sequence, the contents of which are known and, from this analysis, a decoding scheme, referred to as a look-up table, is prepared. The look-up table contains a set of averaged threshold parameters with which subsequently received data signals, the values of which are unknown, are to be compared. From these comparisons, decisions are made as to what data signals were actually transmitted. Further, the decisions may be refined by carrying out a second iteration of comparisons on the previous choice for the sequence of transmitted data signals, thereby enhancing the reduction in error rate. Implementation of the above technique may incorporate a storage device such as a conventional random access memory into which threshold values for a sequence of training data signals are written during a training transmission and, for each identification of the same data symbol, its threshold value in memory is updated by averaging the new value with each of the received preceding values, so that, at the end of the training sequence, the memory contains threshold comparison values representative of the average value of each data symbol transmitted.

Technique for controlling memoryful non-linearities

Amplitude variations in a multi-amplitude transmission system are tracked by monitoring amplitude variations over a substantial data time span through the use of a technique in which the percentage of time that a maximum-valued reference amplitude is exceeded is made equal to the percentage of time that a minimum-valued amplitude is not obtained. For this purpose, received data samples are multiplied by an adjustable gain constant to obtain an amplitude product. The amplitude product is then compared with reference values corresponding to the maximum and minimum amplitudes. If the amplitude product exceeds the maximum amplitude reference, the gain constant is decreased, and if the amplitude product is smaller than the minimum amplitude reference, the gain constant is increased. To implement this technique on a percentage basis, a storage device, such as an up/down counter, is loaded with a prescribed number corresponding to the percentage range over which the gain adjustment is measured and the contents of the counter are controllably incremented or decremented depending upon received gain products. Modification of the gain constant can be in accordance with a fixed value or one which is a function of the difference between the amplitude product and the maximum or minimum reference values.

Technique for tracking amplitude fades for multi-amplitude signalling

The adverse effects of node slips on the operation of an adaptive equalizer are corrected by determining phase and amplitude errors in the received vector signal relative to phase and amplitude estimates, based on node decisions. The phase and amplitude error signals are aligned in a prescribed fashion with the tip of the received signal vector and, from this alignment, respective X and Y components of the error signals are obtained and fed back to the equalizer as projected X and projected Y errors for updating the equalizer weights.

Technique for supplying reference for an adaptive equalizer when

To eliminate the effects of phase jitter, successively measured phase values received in an N-phase transmission system are stored and compared with a phase node reference plot to derive values representative of the respective phase nodes to which the measured phase values are closest and the degree of deviation from these 'closest' phase node values. The degree of deviation for a prescribed number of successive phase measurements is averaged over this number to obtain a mean phase error. The successive phase deviations are then adjusted by this mean deviation and, based upon the adjusted deviation values, the originally chosen node values may be changed to one of their immediately adjacent nodes. This process is carried out for each phase measurement in the sequence, so as to obtain a phase adjustment factor, by which each of the respective nodes relative to what the measured phase values were originally measured is shifted. This shifting effectively rotates the phase node reference plot by an amount which minimizes the mean square error of the phase node measurements for the prescribed number of phase measurements. The difference between the adjusted node values of the two adjacent phases located midway in the sequence is then taken to determine the transmitted symbol for that point in the transmission. The above process is then continuously repeated for successive sequences of prescribed numbers of phase measurements by advancing through the successive measurements one at a time and making whatever adjustments are necessary on the phase node reference plot after each sequence analysis.

Technique for combatting jitter in multiple phase transmission system

An equalizer-dependent timing recovery system monitors the equalizer weighting coefficient pattern over the entire time span of the equalizer, and differential changes at opposite ends of the pattern are used as a basis for adjusting the receiver symbol timing clock. The magnitudes of a plurality of equalizer weighting coefficients at the beginning of the equalizer are summed and the total is compared with the summed magnitudes of a plurality of equalizer weighting coefficients at the end of the equalizer. Depending upon the sign of the difference between the two totals, the phase (or frequency) of the receiver symbol clock will be adjusted so as to shift the equalizer weighting coefficient pattern in a direction such that the magnitude of the weights at both ends of the equalizers are approximately the same. Circuitry for implementing the above scheme includes a pair of adders coupled to sets or plural weighting coefficient stages for opposite ends of the equalizer. The outputs of these adders are applied to a subtractor or difference circuit and the sign of the result is controllably gated to an add/delete pulse circuit, which controllably increases or decreases the frequency of symbol timing recovery circuitry by injecting or removing pulses from a stable clock stream prior to frequency division, thereby controllably advancing or retarding the receiver symbol timing clock.

Technique for obtaining symbol timing for equalizer weights

Demodulation of M-ary continuous-phase FSK signals is effected in accordance with phase measurements taken at symbol transition times. The phase error caused by intersymbol interference is estimated by knowledge of the type and degree of distortion which is principally dependent upon the frequency difference between successive data symbols and phase distortion characteristics of the channel filter. Pairs of phase nodes are selected at each symbol transition time and, from each pair, a tentative choice is made by using the expected phase distortion based on adjacent phase node pairs. Refined choices are then made using the estimated phase distortion based on adjacent tentative choices. Differences between successive tentative choices of phase node selections yield a sequence of frequencies making up the FSK signal. Circuitry implementation embraces the use of phase-locked loop detectors and digital decision logic including programmable read-only memories.

Crosstalk corrector and decision device for FSK

A coherent phase detector using a frequency discriminator for reducing a frequency encoded signal to baseband, thereby avoiding the necessity of generating a coherent RF phase reference signal. A baseband processor is provided to compensate for error components arising in the frequency discriminator output. The baseband processor includes feedback loops for DC level correction and automatic gain correction. A novel scheme is also described for reducing the influence of interbit crosstalk on the operation of the baseband processor.

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West Melbourne, FL, United States of America

Daniel D McRae