Secured and Adaptive Space-Time Two-Path OFDM Systems

In this paper, a secured 2x2 space-time (ST) orthogonal frequency division multiplexing (OFDM) system is proposed with parallel cancellation (PC) two-path communication scheme to form a secured 2x2 STPC system in communication networks. These 2x2 STPC-OFDM systems not only secured but also mitigate the inter-carrier interference (ICI) in the mobile channels with a better bit error rate (BER) than that of a conventional 2x2 ST-OFDM system. The design is intelligently adaptive between two secured operation modes: either high data rate with ST-OFDM or high reliability with STPC in mobile fading channels having lower bit error rate (BER). The security is obtained via precoders of the transmitter on each path since the receiver (or users) must know a prior and perform the reversed operation. The intelligent decision of switching is obtained via channel estimation. It offers higher diversity order and better BER performance. Two orthogonal transforms, such as Zadoff-Chu transform (ZCT) and Walsh-Hadamard transform (WHT), are used as the precoder for security and simultaneously enhance the BER further. Although the proposed STPC-OFDM systems require two-path communication with the cost of reducing bandwidth efficiency, simulations demonstrate that the proposed 2x2 secured STPC-OFDM systems are robust to various channel environments. Moreover, by employing a larger signal alphabet size, it regains the bandwidth efficiency and has a better BER performance for fair data rate comparison with 2x2 ST-OFDM.