Abstract: 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.

Abstract: 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).

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.

Abstract: 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.