A network interface is presented that receives packet data from a shared medium and accomplishes the signal processing required to convert the data packet to host computer formatted data separately from receiving the data packet. The network interface receives the data packet, converts the analog signal to a digitized signal, and stores the resulting sample packet in a storage queue. An off-line processor, which may be the host computer itself, performs the signal processing required to interpret the sample packet. In transmission, the off-line process converts host-formatted data to a digitized version of a transmission data packet and stores that in a transmission queue. A transmitter converts the transmission data packet format and transmits the data to the shared medium.

Off-line broadband network interface

A transmitter encodes a number 2N data bits by using N data bits to select one of 2N levels of a cosine wave and the other N data bits to select one of 2N levels of a sine wave. The modulation attains the cosine wave levels at instants offset by half an N-bit symbol interval with respect to the sine wave levels, and is called Offset QAM (OQAM). A received OQAM signal is amplified, filtered and digitized at a sampling rate of preferably only one sample per N-bit half-symbol interval. Successive N-bit half-symbols comprise information modulated alternately on a cosine and a sine carrier wave. The receiver may remove this successive rotation by applying successive derotations of successive digitized samples by like amounts. The derotated samples are then correlated with known sync half-symbols. The sync correlations determine a set of channel coefficients describing the dependence of each digitized sample on one or more unknown half-symbols. The computed channel estimates are then used to predict the expected received sample values for all possible sequences of successive unknown half-symbols that are to be decoded. Received samples are then compared with all possible expected values and a measure of disagreement or error metric computed. That sequence with the lowest cumulative error metric is used to determine the decoded symbol output. In other aspects of the invention, dual mode transmitters and receivers are disclosed that enable different types of modulation to be alternatively utilized in a same apparatus.

Transmitter/receiver for gmsk and offset-qam

In a digital radio communication system using at least two regular channel and two or more standby channels for transmitting a test signal therethrough in the standby condition thereby permitting the regular channel to be synchronously switched to one of the standby channels, the improvement wherein a channel switching system for monitoring the standby channels is provided at the receiving side of the digital communication system with: control signal generator means operating so that when two or more standby channels are in standby condition, it generates a control signal to designate them in turn, and when only one standby channel is in standby condition, it generates a control signal to designate the channel; synchronizing switching means for selectively outputting the test signal transmitted via each of the standby channels designated by the control signal; and standby channel monitoring means for detecting an output from the synchronizing switching means to monitor the communication quality of the standby channel placed in standby condition on the basis of the result detected.

Channel switching system

The system for recovering symbol timing offset and carrier frequency error from an orthogonal frequency division multiplexed (OFDM) signal includes a receiver circuit for receiving an OFDM modulated signal representing a series of OFDM symbols, and providing a received signal to an output thereof. A peak development circuit is included for developing a signal having a plurality of signal peaks representing symbol boundary positions for each received OFDM symbol, where each of the signal peaks is developed responsive to an amplitude and phase correspondence produced between the leading and trailing portions of each of the received OFDM symbols. The system includes a circuit for enhancing the signal peak detectability, which includes a circuit for additively superimposing and then filtering the signal peaks, to produce an enhanced signal peak having an improved signal-to-noise ratio. A circuit for establishing a temporal position indicative of the symbol boundary position from at least one of the enhanced signal peaks output from the signal enhancing module is also provided. Further, a circuit for recovering the received OFDM signal carrier frequency error corresponding to the temporal position is included.

System and method for recovering symbol timing offset and carrier frequency error in an OFDM digital audio broadcast system

A distributed artificial intelligence data acquisition and equipment control system forms a local area network (LAN) over the AC power lines (83) of a vessel for communicating shipboard engineering casualty and damage control information. Each node (10E, 12E, etc.) in the LAN includes a power line carrier current (PLCC) transceiver, a slave processing unit (22) for interfacing with local sensors or equipment, and a master processing unit (28) for controlling the formation, transmission and reception of OQPSK LAN messages. Nodes in the LAN are strategically positioned at various locations throughout the ship and are configured to function either as a Terminal Unit or as a Data and Control Unit. Terminal Units are equipped with a touch screen display (124) and an intelligent controller (128) that provides a user-friendly interactive operator interface through a series of display screen prompts and includes a data base of ship casualty and damage control information stored in a local non-volatile random access memory.

Distributed intelligence engineering casualty and damage control management system using an ac power line carrier-current lan

Bit timing references are derived at a receiver for a carrier modulated by OQPSK. A carrier is transmitted with a constant reference phase modulation during a first interval. During a subsequent, second interval the carrier is transmitted with rotational phase modulation representing binary bit values so that orthogonal components phase modulate the carrier with a predetermined bit sequence. A receiver responds to the constant phase modulated carrier to lock the frequency and phase of a phase lock loop oscillator of a carrier recovery loop. Synchronization at the receiver is achieved in response to a reception of the rotational phase modulation, while maintaining the frequency and phase lock. A carrier recovery loop of the receiver is activated to a decision directed operation after bit synchronization has been achieved. Decision directed operation is achieved by sampling and comparing components derived in orthogonal channels of the carrier recovery loop while the receiver is responsive to the rotational phase modulation. The bandwidth of a filter used in achieving bit synchronization is reduced after there has been a decay of transients in the carrier tracking loop resulting from a transition between locking of the loop and initiation of the decision directed operation. Then the bandwidth of a filter in the carrier tracking loop is adjusted as a function of the quality of the received rotational modulated suppressed carrier.

Mark J. Dapper

Papers

Method of and apparatus for transmitting and recovering offset QPSK modulated data