The present invention relates generally to methods for coherently demodulating information symbols from communication signal bursts subject to fading communications channel distortion. More specifically, but without limitation thereto, the present invention relates to a method for estimating a fading communications channel.
Reasons for experiencing fading in mobile communications include multiple scattering of the radio waves by structures in the vicinity of a mobile unit. Each scattered component plane wave has an associated Doppler shift that depends on the mobile unit speed, the carrier frequency, etc. Estimating and compensating for fading communications channel gain is desirable so that a communications system may coherently demodulate a burst of transmitted information symbols. A problem with current block phase estimation (BPE) techniques is the introduction of non-linearities in removing the modulation that may result in substantial degradation of performance in the presence of strong fading channels.
Another approach uses single reference symbols distributed uniformly within a data burst for a communications channel with Rayleigh fading. In Rayleigh fading, all the signal energy is distributed in scattered paths. Because satellite communications are characterized by a direct, line-of-sight signal path that is not scattered, it would be preferable to use a fading channel model that accommodates combinations of scattered and unscattered or dominant path signals.
The present invention advantageously addresses the needs above as well as other needs by providing a method and apparatus for estimating and compensating for a fading communications channel.
In one embodiment, the invention may be characterized as a method for estimating and compensating for a fading communications channel that includes the steps of generating matched filter data from a received data burst, interpolating between reference symbols to generate a fading channel estimate from the matched filter data, applying a phase correction to the matched filter data from the fading channel estimate to recover free data from the matched filter data, and performing decision feedback on the free data to improve the fading channel estimate.
In another embodiment, the invention may be characterized as an apparatus for estimating and compensating for a fading communications channel that includes a matched filter for generating matched filter data from a received data burst, a reference symbol interpolator coupled to the matched filter for interpolating between reference symbols to generate a fading channel estimate from the matched filter data, a first fading channel compensator coupled to the matched filter for applying a phase correction to the matched filter data from the fading channel estimate to generate phase corrected data, and a first demodulator coupled to the fading channel compensator for demodulating free data from the phase corrected data.