Communications Satellite Corporation

Comsat Corporation

Ajit Shenoy

Lin-Nan Lee

Harvey Chalmers

Neal D Becker

Farhad B Verahrami

Michael K Eng

Peter N Johnson, Jr

A satellite communication system including a portable satellite terminal is provided which utilizes C/K.sub.u -band and spread spectrum technology to drastically reduce the antenna and terminal sizes. Both CDMA and time division multiplexing (TDM) are used to permit efficient all-digital voice, facsimile and data operations in either full mesh or star modes. A single Network Controller can handle both Star and Mesh mode traffic on the same or several transponders. Narrowband CDMA/frequency division multiple access is used for inbound transmission from the terminal while outbound transmissions from the hub utilize TDM/FDMA. Furthermore, the system is scalable in fixed bandwidth increments to support network size evolution. All signalling for call setup and takedown is done in-band on the same carrier that is used for the traffic, thereby eliminating the need for a separate signalling channel and frequency retuning. Only a small set of distinct PN code sequences, independent of the actual number of user terminals in the network, are required for the entire network.

CDMA system architecture for satcom terminals

The correct sampling timing phase is obtained by a digital data receiver by averaging samples of the preamble of a received data burst and using the computed averages as addresses to a memory which stores correction values for the timing phase. The output of the memory is sent to a phase correction circuit for correcting the phase of the presently received data burst.

Method and apparatus for clock recovery for digitally implemented modem

A low-bit-rate, low-cost, all-digital preambleless demodulator for maritime and mobile data communications operates under severe high noise conditions, fast Doppler frequency shifts, large frequency offsets, and multipath fading. Sophisticated algorithms, including an FFT-based burst acquisition system, a cycle-slip resistant carrier phase tracker, an innovative Doppler tracker, and a fast acquisition symbol synchronizer, provide reliable burst reception. The compact DSP-based demodulator includes an input buffer receiving a complex sampled baseband input signal and providing a baseband output to a coarse frequency estimator fast Fourier transform (FFT) or discrete Fourier transform (DFT) module which produces a first estimation of the carrier frequency. A fine frequency estimator FFT or DFT module receives the first estimation and provides a second estimation of the carrier frequency. An extra coarse frequency estimator FFT or DFT module may be provided between the buffer and the coarse frequency estimator.

Digitally implemented fast frequency estimator/demodulator for low bit

An analog TV system includes a transmitter which produces a horizontal unique pulse sequence during each horizontal blanking interval. The pulse sequence is amplitude modulated between two levels to define a horizontal unique word for horizontal synchronization. The frequency of the pulse sequence is selected for use by the receiver section as the color subcarrier phase reference. The average level of the pulse sequence is set at the transmitter and used at the receiver for DC restoration in a clamping circuit. The peak amplitude of the pulses of the pulse sequence define a reference level against which a receiver produced local reference is compared for AGC. As four separate functions are accomplished by a single unique pulse sequence transmitted during the horizontal blanking interval, which four functions are typically accomplished using four separate signals transmitted sequentially during the horizontal blanking interval, a substantial portion of the horizontal blanking interval remains available for other data.

Horizontal synchronization, clock synchronization, D.C. restoration and

An analog TV system includes a transmitter which produces a horizontal unique pulse sequence during each horizontal blanking interval. The pulse sequence is amplitude modulated between two levels to define a horizontal unique word for horizontal synchronization. The frequency of the pulse sequence is selected for use by the receiver section as the color subcarrier phase reference. The average level of the pulse sequence is set at the transmitter and used at the receiver for DC restoration in a clamping circuit. The peak amplitude of the pulses of the pulse sequence define a reference level against which a receiver producing local reference is compared for AGC. As four separate functions are accomplished by a single unique pulse sequence transmitted during the horizontal blanking interval, which four functions are typically accomplished using four separate signals transmitted sequentially during the horizontal blanking interval, a substantial portion of the horizontal blanking interval remains available for other data.

Horizontal synchronization, clock synchronization, D. C. restoration and

A serial Viterbi decoder in which a single set of circuits is sequentially used for all the encoder states to calculate the maximum probability path, as measured against received signals, of unencoded signal sequences to reach each encoder state. The path to any encoder state having the maximum probability is traced back to output the oldest encoded signals.

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Germantown, MD, United States of America

Ajit Shenoy