Richard A Stuart

US Patent:

41237102, Oct 31, 1978

Filed:

Nov 30, 1977

Appl. No.:

5/856072

Inventors:

Richard L. Stuart - Simpsonville MD
Arvind M. Bhopale - Placentia CA

Assignee:

Rixon, Inc. - Silver Spring MD

International Classification:

H04L 300

US Classification:

325 38A

Abstract:

A partial response QAM Modem is provided wherein a transmit signal is generated by modulating band limited, multilevel, partial response precoded data with inphase and quadrature carriers in inphase and quadrature phase channels, respectively, and summing the modulated outputs. The received signal is processed by a passband automatic complex equalizer. The equalized signal is delayed and then demodulated with recovered inphase and quadrature carriers to recover the inphase and quadrature phase baseband signals. The recovered baseband signals are sampled at appropriate instants to produce estimates of the transmitted symbols. These symbols are partial response decoded and processed by error correctors to improve the overall system performance. The error corrector outputs are decoded to convert the multilevel estimated data to binary data and the binary data of the two channels is summed to obtain estimates of the binary data transmitted.

US Patent:

46463054, Feb 24, 1987

Filed:

Sep 26, 1983

Appl. No.:

6/535636

Inventors:

Steven A. Tretter - Silver Springs MD
Robert P. Redick - Chevy Chase MD
Richard L. Stuart - Columbia MD
Paul E. Treynor - New Carrollton MD
David Gordon-Smith - Columbia MD

Assignee:

Case Communications, Inc. - Columbia MD

International Classification:

H04L 512
G06F 1110

US Classification:

371 43

Abstract:

A 14. 4 kilobit/second modem uses an encoding scheme in which groups of five bits are encoded as one of thirty-two (2. sup. 5) possible code groups. This is done by using quadrature amplitude modulation and a 6 by 6 space-state constellation which allows a maximum of thirty-six different points to be encoded. Since only thirty-two points are needed the four outer corner points of the constellation are not used. In order to achieve the desired 14. 4 KBPS data rate the baud clock must run at 2880 Hz. However, this bandwidth is very close to the maximum bandwidth available on voice-grade telephone lines. Accordingly, data encoding and data recovery techniques must be used which maximize the probabilities of correctly receiving the encoded data signals. These techniques include (1) data scrambling/descrambling; (2) assigning groups of five bits to constellation points, including performing rotational and Gray encoding; (3) a baud clock recovery scheme at the receiver which is performed prior to partial response encoding; and (4) a start-up technique using a three-level partial response ideal reference sequence during initial training at the receiver. In addition, the invention uses passband equalization, class I partial response encoding accomplished by the passband equalizer, and independent inphase and quadrature Viterbi decoders at the receiver.

US Patent:

43553974, Oct 19, 1982

Filed:

Oct 15, 1980

Appl. No.:

6/197038

Inventors:

Richard L. Stuart - Columbia MD

Assignee:

Rixon, Inc. - Silver Spring MD

International Classification:

H04L 300

US Classification:

375 17

Abstract:

A pair of full duplex modems (12, 14), each at one end of a two-wire voice grade telephone channel (16), simultaneously transmit synchronous data at high speeds in both directions. In each modem, the simultaneous transmission and reception are accomplished by frequency division. The high speed operation is obtained by odd/even bit stream division and full response binary to ternary level coding coupled with nine level quadrature amplitude modulation and correction framing techniques.

US Patent:

39834859, Sep 28, 1976

Filed:

Feb 28, 1975

Appl. No.:

5/554112

Inventors:

Richard L. Stuart - Simpsonville MD

Assignee:

Rixon Inc. - Silver Spring MD

International Classification:

H04L 2720

US Classification:

325 30

Abstract:

A multi-phase, multi-amplitude level modulation method, and modulator, is provided which produces a three-level, ten-phase modulated carrier. The carrier is modulated in three amplitude levels, viz. , off, intermediate and high and in ten phase positions, to produce sixteen signal states, one state corresponding to the carrier being off, five intermediate amplitude level states having five equally spaced phase positions, and ten high amplitude level states having ten equally spaced phase positions. A number of different four digit binary coding schemes for the sixteen signal states are disclosed together with other different coding and modulation techniques.

US Patent:

39462145, Mar 23, 1976

Filed:

Sep 17, 1974

Appl. No.:

5/506833

Inventors:

Richard L. Stuart - Simpsonville MD
Arvind M. Bhopale - Beltsville MD

Assignee:

Rixon, Incorporated - Silver Spring MD

International Classification:

G06F 1534

US Classification:

235152

Abstract:

A multi-level digital filter system applicable to modems is provided. The incoming data is read serially into M shift registers and read out in parallel by M further shift registers under the control of an M-bit clock, where M=log. sub. 2 N and N is the number of levels. After appropriate conditioning by a logic control circuit, the parallel outputs are filtered separately by M digital shaping filters. The shaping filters each comprise a chain of shift registers the outputs of which are weighted by a resistor network and summed to produce a desired time response, which in an exemplary embodiment is the inverse Fourier transform of an ideal low-pass filter. Summing of the filter outputs produces an N-level signal. Specific examples of three-, four-, five-, six- and eight-level systems are disclosed.

US Patent:

43800629, Apr 12, 1983

Filed:

Apr 22, 1981

Appl. No.:

6/256422

Inventors:

Richard L. Stuart - Columbia MD
Fred C. Killmeyer - Palm Bay FL

Assignee:

Rixon, Inc. - Silver Spring MD

International Classification:

H04B 156

US Classification:

370 20

Abstract:

A two-wire, carrier-type communication system is provided which enables simultaneous two-way transmission over two-wire circuits in the same spectrum. Carrier signals in quadrature are used to allow separation of signals within the same frequency spectrum. The carrier recovered at each subscriber station is used to demodulate the received signal and, after being phase shifted 90. degree. , is applied as carrier to the transmitter modulator. The transmitters at the subscriber stations are thus the same as that at the central office but are locked to the receiver and 90. degree. out of phase. A bidirectional control phase adjustment network at the central station adjusts the phase of the signal received thereby until the received carrier is 90. degree. out of phase with the transmit carrier.

US Patent:

44894163, Dec 18, 1984

Filed:

Mar 18, 1983

Appl. No.:

6/476682

Inventors:

Richard L. Stuart - Columbia MD

Assignee:

Rixon, Inc. - Silver Spring MD

International Classification:

H04B 304

US Classification:

375 13

Abstract:

In a system in which a central modem individually polls a number of remotely located modems for data, an adaptive equalizer in the central modem is initially trained to minimize communication link interference from a remote modem and develops a set of equalizer coefficients based on the initial training which is transmitted to the remote modem and stored there for future use while the central modem goes on to do the same with other remote modems. When a remote modem is polled, the stored equalizer coefficients are used to set the adaptive equalizer of the remote modem so the adaptive equalizer of the central modem does not have to be retrained for each individual remote modem.