This invention is applicable to communications systems generally, and is especially applicable to, and is described below in the context of, TDMA (time division multiple access) cellular communications systems compatible with EIA/TIA document IS-54-B: Cellular System Dual-Mode Mobile Station--Base Station Compatibility Standard (Rev. B). For convenience and brevity, such a system is referred to below simply as an IS-54 system. In such a system, data is communicated in time slots each comprising a sync (synchronization) word of 14 symbols followed by an information sequence. The sync word is used among other things to facilitate carrier recovery. The manner in which carrier recovery is implemented has a direct impact on the performance of the system.
In cellular communications systems, carrier recovery is made difficult by fading and interference or noise. Although conventional PLL (phase locked loop) carrier recovery systems are well understood and widely used in wireless communications systems, they do not perform satisfactorily in noisy (or low SNR (signal-to-noise ratio)) and fading channel environments, such as can occur in cellular communications systems.
An object of this invention is to provide an improved method of and apparatus for use in carrier recovery using a known sync word in a received communications signal.