Pulse or digital communications – Transceivers – Modems
Patent
1994-02-17
1996-07-02
Chin, Stephen
Pulse or digital communications
Transceivers
Modems
375316, 375340, H04B 138, H04L 2706
Patent
active
055330487
ABSTRACT:
A system and method for reducing amplitude distortion in data signals transmitted via cellular analog communication systems are embodied in several forms. In one embodiment, pre-emphasis and de-emphasis functions of the cellular system are offset by complementary de-emphasis and post-emphasis circuits to preclude clipping of high frequency components of the data signal by the limiter. Alternatively, the limiter circuit is selectively bypassed when a data signal is transmitted. Alternatively, a pilot tone is combined with the incoming data signal. The amplitude of the pilot tone is constant and always sets the gain of the limiter, thereby keeping the gain of the limiter circuit constant. Alternatively, a pair of signals at different frequencies are transmitted on a telephone line, and the relative amplitudes of the return echoes are measured to determine the frequency roll-off of the line and adjust the spectral shaping accordingly. Alternatively, a pair of modems vary their respective spectral characteristics until the eye quality values of the two modems are satisfactory. Alternatively, a receiving modem performs an auto-equalization routine which varies to ignore outer points on a signal constellation that are likely to have a reduced amplitude because of limiting. Alternatively, the threshold amplitudes of an auto-equalizer are varied in accordance with phase changes that correspond to new data points by varying the threshold or by varying the amplitude of the data compared to the threshold.
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Bocure Tesfaldet
Celeritas Technologies, Ltd.
Chin Stephen
LandOfFree
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