Pulse or digital communications – Systems using alternating or pulsating current – Angle modulation
Reexamination Certificate
1998-11-03
2001-10-16
Pham, Chi (Department: 2631)
Pulse or digital communications
Systems using alternating or pulsating current
Angle modulation
C375S265000
Reexamination Certificate
active
06304614
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates to the transmission of digitally formatted data via a communication system employing a carrier modulated with phase shift keying (PSK) and, more particularly, to removal of phase ambiguities introduced in the analog signal transmission of the communication system for the case of modulation of the digitally formatted data by trellis coded modulation with Viterbi decoders.
A typical communication system includes a transmitter having an oscillator for generation of a carrier, and a receiver having an oscillator employed in reception of the carrier. A modulator within the transmitter is operative to modulate the carrier with a modulation such as phase shift keying, and a demodulator in the receiver is operative to demodulate the modulated carrier to extract data transmitted via the communication system. Such a communication system may be subject to phase ambiguities with consequent errors in the signal transmission, due to the presence of noise at the transmission end and/or the reception end of the communication system. Resolution of phase ambiguity in trellis coded modulated data is disclosed in U.S. Pat. Nos. 5,428,631 of Zehavi and 5,233,630 of Wolf which employ various types of encoding and decoding to resolve the phase ambiguity. Each of these patents also disclose components of a communication system with coding and decoding of Mary PSK modulated signals.
With the wide use of such communication systems in radio-telephony and in other areas which can benefit from a minimization of size and complexity of the electronic equipment, it is desirable to reduce the complexity of the circuitry. A problem arises in that the coding and decoding circuitry of the foregoing communication system is greater than that which is desired.
SUMMARY OF THE INVENTION
This invention provides a system including coding and decoding circuits for resolution of the foregoing phase ambiguities by a methodology which attains a simplification of circuitry from that employed in existing systems. In a typical M-ary PSK modulation communication system, an oscillator providing a carrier and a modulator of phase of the carrier are located at the transmission end of the communication system, and further oscillator or carrier recovery circuitry in conjunction with phase demodulation circuitry are located at the reception end of the communication system. An error correcting coder, such as a convolutional encoder, precedes the modulator for reducing the effect of noise in inducing phase errors. A corresponding decoder appears at the reception end of the communication system. The invention employs, as an ambiguity removal circuit, a differential encoder and decoder which automatically remove the possible phase ambiguities, and operate in conjunction with the error correcting encoder and decoder. Each of the ambiguity-removal differential encoder and the decoder act as an operator upon its input signal.
In the practice of the invention, it is recognized that both the error correcting encoder and the ambiguity encoder should immediately precede the modulator. The invention meets this challenge by placing the ambiguity removal circuitry between the error correcting encoder and modulator, and by constructing the ambiguity removal circuitry as a combination of differential encoder and inverse differential encoder. The use for ambiguity removal of both the differential encoder and the inverse differential encoder operates to remove the phase ambiguity while making the ambiguity operation transparent to the output of the error correction encoder. This retains the benefit of placing the error correcting encoder immediately before the modulator.
As a further feature of the invention, the ambiguity encoder with its inverse ambiguity encoder are located at the transmitting end of the communication system, and a corresponding decoder and its inverse decoder are located at the reception end of the communication system. In the ambiguity removal circuit, the encoder function may be accomplished by a mapper (such as a read-only memory) in combination with feedback signal delays, and the decoder comprises a mapper in combination with feed-forward signal delays.
In accordance with another feature of the invention, it has been observed that, in a PSK communication system such as 4-PSK or 8-PSK, the inverse encoder function need be placed only on one of the lower bits, and is not necessary for the higher bits. A simplification of the inverse function can then be obtained by means of simply a one-dimensional differential encoder following the error correcting encoder. Further simplification of the circuitry at both the transmission end and the reception end by replacing the mapper in each of the differential encoder and decoder with circuitry having logic gates.
REFERENCES:
patent: 5757856 (1998-05-01), Fang
patent: 5878085 (1999-03-01), McCallister et al.
patent: 5995551 (1999-11-01), McCallister et al.
patent: 6125136 (2000-09-01), Jones et al.
Abbaszadeh Ayyoob D.
Fonnesbeck Dale D.
L-3 Communications Corp.
Nguyen Dung X.
Perman & Green LLP
Pham Chi
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