With development of communications technologies, a transmission rate required by a communications system increases exponentially. Because communication spectrum resources are increasingly scarce, spectrum utilization of a system needs to be fully improved. Manners are studied to improve spectrum efficiency of various communications systems, for example, a microwave backhaul system, a copper wire system, a satellite communication system, a wireless communications system, and a fiber-optic communications system.
Higher order modulation is the first approach used to develop high spectrum efficiency in a communications system. Although the system spectrum efficiency can be greatly improved by using the higher order modulation, the system is required to provide a larger signal-to-noise ratio (SNR). In other words, with a same SNR, a higher modulation order indicates a higher system bit error rate.