Digital Communications Corporation

Communications Satellite Corporation

Ova Gene Gabbard

Pradman P Kaul

John M Husted

Anders A Eklof

Pradeep Kaul

Andrew M Walker

William G Schmidt

Deepak Muzumdar

Joaquin Hangen

A TDMA multiplexer-demultiplexer includes a plurality of input/output ports and common control equipment. Each input port is capable of accepting information asynchronous to the TDMA clock and at typical terrestrial clock rates. The input/output ports perform all the necessary signal processing for a TDMA terminal including pulse stuffing, changing continuous asynchronous low rate information to high bit rate burst form as well as forward acting error correcting encoding, if desirable. By incorporating all the signal processing in the multiplexer-demultiplexer, much equipment duplication is eliminated along with eliminating a requirement for multiple high frequency control and timing signal line drivers and buffers. More particularly, the transmit side of each port includes a separate elastic buffer to raise the input rate to a synchronous common higher rate. In addition, on the receive side, clocking on the low speed side of the expansion buffer is derived from the station's transmit clock.

TDMA Multiplexer-demultiplexer with multiple ports

A communication terminal interface accepts, on different channels, information in a plurality of formats for communication purposes over a communication link which may be a TDMA satellite link. Each channel is accepted by a different interface unit which serves to format the information for communication over the system. This unit includes a compression-expansion buffer memory for allowing information received at a receive rate R.sub.s to be multiplexed and transmitted at a transmission rate T.sub.s which may be many times higher than the receive rate. In addition some units may include forward acting error correcting coding selected to provide the necessary corrected bit error rate for the type of information accepted by the unit and the capacity of the communication link over which the information is to be transmitted. The transmitter may employ differential encoding. To avoid increasing the error rate, which would normally result from this type of encoding, the encoded data is interleaved at the transmit side such that adjacent errors, after de-interleaving, on the receive side, are sufficiently displaced such that they may be corrected by the error decoding.

Error coding communication terminal interface

A phase locked loop with pre-set and squelch capability particularly adapted for burst type signals. In addition to the conventional phase detector, loop filter (LF) and voltage controlled oscillator (VCO) the loop includes a squelch switch connected between the phase detector and LF. A track and hold circuit is connected in series with another switch between output and input of the LF. Both switches and the track and hold circuit itself are controlled by the output of a level detector which indicates presence of an intermittent (i.e., bursty) carrier. The squelch switch is closed, connecting the phase detector to the LF, when the output of the level detector indicates the presence of a carrier burst. At the same time, the second switch is opened so that the LF is not affected by the output of the track and hold circuit. In addition, the track and hold circuit is enabled to track the error voltage output of the LF. At the conclusion of a burst the squelch switch is opened so that the noisy output of the phase detector will not affect the LF. In addition, the second switch is closed to allow the output of the track and hold circuit to reach the LF and finally, the track and hold circuit holds the previous error voltage output of the LF. Thus the error voltage, at the conclusion of the carrier burst, is provided by the track and hold circuit to the LF to maintain the VCO at the last known carrier frequency.

Phase locked loop with pre-set and squelch

A TDMA system in which each station times its respective data burst from a reference sync burst transmitted by a designated primary reference station over the satellite link. Another station transmits a secondary sync burst timed from the primary stations sync burst. To maintain synchronism at the satellite each station has a burst synchronizer operated by receipt of the primary reference sync burst. The secondary sync burst is also received by each of the stations and is capable of ensuring synchronization in case of failure of the primary stations's sync burst.

Double sync burst TDMA system

A stuff decision code word of multiple bit length is transmitted over a number of frames equal to the number of bits per stuff decision code word. Advantage is made of a unique word used for burst synchronization wherein a common code word is used for both stuff decision and burst synchronization. A hierarchy of bursts is established to identify the particular channel of information to which the stuff decision is associated.

Distributed bit stuff decision transmission

A clock for locally distributing a synchronized signal derived from a remotely transmitted signal. The clock normally operates in the slave mode, phase locked to the remotely transmitted signal, when that signal is deemed available. If it is not available the clock switches to a master mode in which it independently generates the necessary signal.

Growing community of inventors

Germantown, MD, United States of America

Ova Gene Gabbard