Abstract: A data transmitting QAM modem with partitions incoming data into unequal frames. Some of the bits in each frame are encoded using fractional rate encoding and then recombined the rest of the bits to form frames of equal bits. Trellis encoding is then performed on the frames of equal bits. Advantageously the number of points in the QAM constellation is also increased to an integer divisible by a power of 2 to reduce the complexity of fractional rate encoding.

Abstract: A method and system for a synchronized pseudorandom privacy modem makes use of a pseudorandom signal generator to index into a table of N signal spaces thereby altering gain and phase modifiers so or to modify both the gain and phase of portions of the data signal being transmitted from a QAM modem and thus transmit an encrypted signal pattern to the other end of the communications channel. The modem transmitter circuit includes a data scrambler, and QAM signal point mapper, connected in series, as well as a pair of mixers having inputs from the pseudorandom signal generator and functioning to modify the gain and phase of portions of the QAM signal point mapper output signal before they are pulse amplitude modulated, filtered, converted to analog form, and transmitted over the communications channel. Complementary circuit elements in the receiver, which have been synchronized with those of the transmitter by baud rate counters at both locations, demodulate and decode the received encrypted signal.