Secondary channel for digital modems using spread spectrum subliminal induced modulation
First Claim
1. A method employing digital modems for transmitting a secondary channel over a communications connection such as a telephone line comprising the steps of:
- generating a primary channel signal;
encoding said primary channel signal;
generating a secondary channel signal;
FSC encoding said secondary channel signal;
rotating the FSC encoded signal through phase shifts generated by a trellis encoder at the primary baud rate to spread said FSC encoded signal'"'"'s frequency spectrum;
combining said primary channel signal and said rotated FSC encoded signal; and
transmitting said combined signals over said communications connection.
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Accused Products
Abstract
A system and method are provided for use of a secondary channel in high speed (greater than 2400 bits per second) wire line modems to provide low speed (around 110 bits per second) non-interfering communications for control or monitoring functions. The secondary channel is used to monitor, configure and test the modems in the network using commands sent over the secondary channel. Secondary channels are also used in low duty cycle applications such as in alarms or teletype data transmission. The invention provides a spread spectrum secondary channel which occupies the same bandwidth as the primary data channel and is immune to jamming tones that would interfere with conventional frequency shift keying secondary channels. The spectrum spreading is achieved through use of the primary channel'"'"'s trellis encoder. Phase shifts of 0°, 90°, 180° or 270° are induced depending upon the trellis state. This rotation simplifies decoding which as a result can be done in the reference system of the decoded primary trellis signals instead of the baseband equalizer reference system. The primary trellis decoder automatically resolves the 90° references. There is no penalty in terms of additional circuitry or software because the transmitter spreading eliminates the need for additional rotation in the receiver which is required for non-spread signals.
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Citations
8 Claims
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1. A method employing digital modems for transmitting a secondary channel over a communications connection such as a telephone line comprising the steps of:
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generating a primary channel signal; encoding said primary channel signal; generating a secondary channel signal; FSC encoding said secondary channel signal; rotating the FSC encoded signal through phase shifts generated by a trellis encoder at the primary baud rate to spread said FSC encoded signal'"'"'s frequency spectrum; combining said primary channel signal and said rotated FSC encoded signal; and transmitting said combined signals over said communications connection. - View Dependent Claims (2, 3, 4)
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5. A system employing digital modems for transmitting a secondary channel over a communications connection such as a telephone line comprising:
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means for generating a primary channel signal; a trellis encoder connected to an output of said primary channel signal generating means; means for generating a secondary channel signal; an FSC encoder connected to an output of said secondary channel generating mean;
said trellis encoder having an output connected to an output of said FSC encoder for rotating the FSC output signal through the trellis encoder stated rotation at the primary baud rate to spread the frequency spectrum of said FSC output signal;means for combining said spread FSC output signal with another output from said trellis encoder, said another output constituting an encoded primary channel signal; and means for transmitting said combined signals over said communications connection. - View Dependent Claims (6, 7, 8)
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Specification