Digital modulation method and digital communication system

摘要

A digital communication method and system wherein in response to input 3-bit data, a phase and amplitude of a carrier signal are controlled so that a symbol at a given time is located at one of allowed signal points depending on the content of the symbol. The allowed signal points include six signal points where a phase shift equal to .+-.&pgr;/4, .+-.2&pgr;/4, or .+-.3&pgr;/4 occurs relative to the signal point at which the immediately previous symbol was located and where the amplitude is equal to a first level, and also include two signal points where a phase shift equal to .+-.2&pgr;/4 occurs relative to the signal point at which the immediately previous symbol was located and where the amplitude is equal to a second level lower than the first level. The digital communication method and system allows for a transmitter which does require a high- performance power amplifier, and a simplified receiver circuit. In addition, high-speed transmission can be achieved without expanding the bandwidth and high-reliability transmission is possible even in an environment having bad radio wave propagation as is the case in mobile communication.