A pilot assisted channel estimation OFDM system has a transmitter that sends predetermined pilot symbols in a set of dedicated time and frequency pilot points for receipt by a receiver. The receiver uses the pilot symbols to help decode the signal. The pilot assisted channel estimation OFDM is sensitive to time and frequency synchronization errors that cause baseband phase rotations in the channel. The phase rotations reduce channel time and frequency coherence, thereby increasing the interpolation error of pilot assisted channel estimations. The interpolation error can dramatically degrade the channel estimation of a single path channel system and is even more pronounced in multiple path channel systems.
Various methods are presently used to synchronize and reduce the interpolation error in OFDM systems including blind algorithms and wideband synchronization pilot signals transmitted intermittently in short time periods. The blind algorithms use a cyclical prefix instead of the pilot symbols to synchronize the OFDM system. The wideband synchronization pilot signals use an additional intermittent pilot signal to synchronize the OFDM system. The additional intermittent pilot symbol increases the pilot symbol overhead and is incapable of tracking any fast time variations in the channel. These methods when applied to multipath systems simply assume there is a single time and frequency offset to be estimated and do not directly consider multipath effects. These systems cannot synchronize to all of the channels simultaneously since each channel can have a different arrival time and frequency offset. Furthermore, these cannot adequately process signals having a delay or a Doppler spread.
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