Itt Manufacturing Enterprises, Inc.

International Telephone and Telegraph Corporation

Itt Corporation

Marvin A Epstein

Joseph M Fine

Gary V Blois

Peter Cummiskey

Paul A Gilmour

Richard Kim

Bryan S Majkrzak

John Bertrand

Lawrence J Doyle

C Chiu Chan

There is disclosed a bit sync search and frame sync search system operative with a digital data signal as transmitted by a digital radio transmitter. The bit search is implemented by detecting a predetermined phasing signal which is incorporated in the digital signal and which has a repetitive bit pattern of ones and zeroes. The phasing signal is first detected by providing an in-phase and quadrature component signal and correlating those signals to provide an output signal indicative of the bit pattern in the phasing signal. After the phasing signal has been provided and an oscillator associated with a receiving apparatus is compensated according to the detected phasing signal, a tracking mode is entered, whereby a frame signal is captured and the system generates histograms of data bit transitions for producing an error signal indicative of the difference of the transmitted clock rate and the sampling portion of a received bit. In this manner, by adjusting the clock according to the error signal produced by the histogram process one can be assured that the sampling rate at the receiver will occur relatively at the center of each bit to therefore provide reliable decoding or detection of the received digital data signal in the presence of the noise.

Synchronization and tracking in a digital communication system

There is disclosed in a digital communications system for communicating between two radios by transceiving a signal comprising a first type control signal, a second type data traffic signal and a third type voice signal, a remote communication interface for providing transmission therebetween comprising a duplexer/bridge network for transceiving the signal, means for determining the signal type to be transmitted, low pass filters and analog to digital and digital to analog converters for processing received and transmitted signals, and a digital signal processor for modulating and demodulating the signal for transmission/reception according to the signal type. There is also disclosed means for adjusting the clock sampling frequency during the demodulation process to obtain bit sync, bit tracking and frequency tracking. The communication interface is operable to transceive either the modulated traffic, control, or voice signal, or combined modulated traffic and control signal, or combined modulated voice and control signal.

Radio remote interface for modulating/demodulating data in a digital

There is disclosed an improved frequency hopping synchronization and tracking system for bit syncing and frame syncing a digital data signal as transmitted by a radio transmitter. A detection means responsive to incoming data samples representative of the phasing signal portion of the digital data signal partitions the data samples into alternating first and second sample sequences and correlates the samples against a frequency hop modulation code reference sequence to provide an initial data signal detection. A post-detection means responsive to the initial detection and to the frequency hop modulation code reference sequence correlates the incoming data samples and compares the spectral power of the correlated data samples to a threshold value to provide bit sync of the detected data signal. Tracking means responsive to the bit sync and to the input samples captures a frame signal and generates histograms of data bit transitions for producing an error signal indicative of the difference of the transmitted clock rate and the sampling position of the received bit. A clock means responsive to the error signal produced by the histogram process can be adjusted according to the error signal to enable sampling of each received data bit to occur relatively at the center of each bit to ensure reliable decoding or detection of a digital data signal in a noisy channel at the receiver.

Frequency hopping synchronization and tracking in a digital

There is disclosed in a frequency hopping digital communication network having a multimode radio transmitter for transmitting a data signal and a multimode radio receiver tuned for receiving the same, the multimode transmitter and receiver operable in a first standard mode and a second enhanced mode, each mode having a late-net acquisition library period and an in-net acquisition library period, a method for obtaining a non-noisy frequency channel over which to receive the data signal transmitted across the radio network. The method comprises the steps of successively averaging signal samples received over a particular frequency channel and time interval corresponding to the receiver mode and library period to determine the noise level of the particular reception frequency channel. The averaged signal samples are then compared to a predetermined noise threshold to determine whether the frequency channel is noisy. If the frequency channel is determined to be noisy, the receiver is tuned to a new reception frequency channel, the new frequency channel being selectable from a group of permissible frequency channels and responsive to the receiver mode and library period and in a predetermined sequence. The steps are repeated until the noise threshold is no longer exceeded.

Noisy channel avoidance method in a digital communication system

There is disclosed a bit sync search and frame sync search system operative with a digital data signal as transmitted by a digital radio transmitter. The bit search is implemented by detecting a predetermined phasing signal which is incorporated in the digital signal and which has a repetitive bit pattern of ones and zeroes. The phasing signal is first detected by providing 94 in-phase and quadrature component signals and correlating those signals to provide an output signal indicative of the bit pattern in the phasing signal. After the phasing signal has been provided and an oscillator associated with a receiving apparatus is compensated according to the detected phasing signal, a tracking mode is entered, whereby a frame signal is captured and the system generates histograms of data bit transitions for producing an error signal indicative of the difference of the transmitted clock rate and the sampling portion of a received bit. In this manner, by adjusting the clock according to the error signal produced by the histogram process one can be assured that the sampling rate at the receiver will occur relatively at the center of each bit to therefore provide reliable decoding or detection of the received digital data signal in the presence of the noise.

In a digital communications system for communicating between two radios by transceiving a signal comprising a first type control signal, a second type data traffic signal, and a third type voice signal, a remote communication interface for providing transmission therebetweeen comprising a duplexer bridge network for transceiving the signal, low pass filters, and analog to digital and digital to analog converters for processing received and transmitted signals respectively, and a digital signal processor for modulating and demodulating the signal for transmission or reception according to the signal type. A control signal output from the digital signal processor enables adjustment of the clock sampling frequency during the reception and demodulation process to obtain bit sync, bit tracking and frequency tracking. The communication interface is operable to transceive either the modulated traffic, control, or voice signal, or combined modulated traffic and control signal, or combined modulated voice and control signal.

There is disclosed in a frequency hopping digital communications system having a transmitter for transmitting a data signal via a noisy transmission path, and a receiver responsive to the transmitted signal for receiving the same, a method for removing bias in the data signal at the receiver comprising the steps of: forming a sample amplitude histogram from the received data signal samples for a given hop frequency; correlating the amplitude histogram against a stored reference amplitude histogram to obtain a correlated output signal having a peak correlation position; determining the position difference between the peak correlation position and the correlation center position to provide an offset signal corresponding to the difference and indicative of an estimated bias offset value; and applying the estimated bias offset value to the received signal samples in response to the offset signal to obtain an output signal indicative of an unbiased received data signal.

Method for removing bias in a frequency hopping digital communication

A system and method for maintaining a precise time standard among a system of orbiting satellites is disclosed. In an illustrative embodiment, atomic clock data is circulated among the satellites via RF crosslinks. Each satellite uses the received data as input to a Kalman process which acts to minimize the mean squared error among the satellite clocks to form a set of 'ensemble clocks'. The resulting ensemble clock values are then transmitted to an earth station where an offset between the ensemble clocks and Universal Time is computed. The offset is transmitted from the earth station to the satellites where it is used by the satellites to lock their on-board clocks to Universal Time, thereby creating a corrected system time. The corrected system time is transmitted, via RF crosslinks, to satellites not having operational on-board clocks. The satellites without atomic clocks employ phase locked loops to anchor their clocks to the corrected system time as it is received over the crosslinks.

RF link control of satellite clocks

An echo cancellation apparatus is included in a variable rate modem. The echo cancellation apparatus includes a local echo canceler, a remote echo canceler and a quadrature echo canceler unit. The local and remote echo cancelers both operate to cancel echoes in a similar manner employing topped delay lines or transversal filters wherein the taps of each delay line are adjusted to accommodate both local and remote echo cancellation. The local echo canceler and the remote echo canceler share many programs in common. However, the remote echo canceler operates with a phase lock loop which essentially derives a signal obtained from the quadrature and local echo cancelers which signal is further correlated with the output residuals from the local echo canceler to develop samples applied to the remote echo canceler and to derive a frequency which is indicative of a offset carrier frequency to enable the remote echo canceler to cancel remote echoes. Remote echoes are due to frequency shifts occurring remote from the modem, such as those caused by changes in satellite positions or changes in frequency sources. The remote echo canceler operates to cancel echoes which are greater than 15 milliseconds in delay, as compared to local echoes, which are canceled by the local echo canceler responding to echoes which are delayed 15 milliseconds or less.

Local and remote echo canceling apparatus particularly adapted for use

Multirate wire line modem apparatus operable at either of two rates in either transmission or reception modes is provided according to the teachings of the instant invention. Full duplex operation and echo cancellation are utilized for both voice and data. Structurally an IOP processor acts as a system controller in controlling transmission and reception digital signal processors which provide the independent transmission and reception functions of the resulting multirate modem apparatus.

Multirate wire line modem apparatus

An improved modem system for HF data transmission over a varying communication channel transmits data in packets formatted into successive frames, each having two identical training sequences following a sequence of data. Each training sequence has a time length longer than the expected time length of the multipath dispersion in the communication channel, in order to isolate one training sequence in each frame from multipath interference from the preceding data sequence. The 'clean' training sequence allows the receiving modem system to derive very accurate channel estimates for each frame by autocorrelation to the known symbol sequence stored at the receiver. By interpolating across the channel estimates for the current and previous frames, the modem system can track the communication channel by interpolated estimates for any part or any point in time of the data sequence. An improved data recovery algorithm divides the received data sequence in each frame into a series of data blocks, each of which is processed through a respective decision loop using the interpolated channel estimates corresponding to the time position of the data block in the data sequence. The interpolated channel estimate for each data block is used to set the tap values for the filter functions of the corresponding decision loop.

Adaptive modem for varying communication channel

A communication system is protected against jamming by modifying a conventional spread spectrum communication system by inserting errors, in the form of digital noise, into the pseudo-noise digital code used to generate the spread spectrum carrier. A bidirectional system is provided so that when jamming occurs, as a consequence of breaking the corrupted spread spectrum code, the pseudo-noise digital code and/or the percent of errors is changed in a prearranged manner to compensate for the jamming and/or reduce the likelihood of breaking the corrupted spread spectrum code. A conventional correlation receiver is employed to recover the intelligence.

Secure spread spectrum communication system

Baudot code characters each having start and stop bits are transmitted in the form of a MSK digital data signal having a stop-start triangle corresponding to adjacent stop and start bits of two adjacent Baudot characters. The MSK signal is correlated with MSK correlation waveforms to produce first and second correlation signals. Each of the first and second correlation signals are coupled to a different one of two delay lines each having three output taps separated by one bit time. The output signals from the output taps at a given time are appropriately summed to provide inphase and quadrature triangle signals. These triangle signals are operated on by an arithmetic unit to calculate many time and phase conditions of the MSK signal at one time and to produce third, fourth and fifth correlation signals as a function of assumed bit times. These third, fourth and fifth correlation signals are operated on by a ratio calculator to produce a ratio signal proportional to the ratio of a minimum magnitude squared to a maximum magnitude squared for all carrier phase angles for each of the assumed bit times. A synchronization detected signal is produced when the ratio signal value is less than a given amplitude threshold.

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Monsey, NY, United States of America

Marvin A Epstein