Joint Time-Frequency Diversity in the Context of Spread-Spectrum Systems

We discuss a new method for realizing the diversity in spread-spectrum communications over fast-fading multipath channels. Maximum Ratio Combining (MRC) can add the synchronized tributary signals in the weighted approach to obtain the maximum diversity gain. The signal distortion caused by the Doppler shift broadened spectrum cannot be eliminated. The diversity receiver used in existing systems suffers from significant performance degradation due to the rapid channel variations encountered under fast fading. We show that the Doppler spread caused by temporal channel variations actually provides other versatile means that can be further utilized to resist fading. This paper proposes a receiving method based on time-frequency cooperative processing. Joint time-frequency representation is a powerful tool. With precise synchronization and channel estimation, even the relatively small Doppler spread encountered in practice can be used for significant diversity gains by our approach. The framework is suitable for multiple mobile wireless multiple access systems and can provide significant performance improvements over existing systems.