Spectral efficient transmit diversity techniques without cyclic prefix for fading relay channels

This paper proposes spectral efficient relay-assisted transmit diversity techniques, i.e., space-time and space-frequency block codes (STBC/SFBC), for single carrier frequency-domain equalization (SC-FDE). The proposed systems achieve spatial diversity over frequency selective channels in a distributed fashion without cyclic prefix (CP), which increases spectral efficiency of conventional relay-assisted systems. In practical mobile applications, the CP-less distributed STBC (D-STBC) SC-FDE suffers from the time selectivity of wireless fading channels, resulting in a deviation from the basic assumption of Alamouti codeword. In order to obtain a reliable performance over doubly selective channels, the CP-less D-SFBC SC-FDE and its efficient implementation are presented. Further, the performance of these two distributed SC transmissions is analytically compared over fast fading (i.e., time selective) environments. For evaluation of mean square error over fast fading channels, we present a channel model that captures the time-varying nature of wireless channels. It is shown through analysis that the CP-less DSFBC SC-FDE outperforms the CP-less D-STBC SC-FDE when there exists a severe Doppler spread. Timing synchronization and highly accurate channel estimation of the proposed systems are also addressed for practical implementations. Simulation results show that the proposed systems approach the lower bound of full-CP systems, thus increasing the spectral efficiency.