Cyclic delay diversity (CDD) is a simple and yet effective transmit diversity technique. Thanks to its strong power saving capability, CDD is very promising for green Internet of Things (IoT) networks. However, the reduced degrees of freedom in data transmission and the overhead of cyclic prefix lead to unsatisfactory spectral efficiency (SE), hindering the direct application of CDD to IoT networks where short-packet communication prevails. To tackle this problem, in this article, we propose to convey additional information via the varying cyclic delays for improving the SE of the CDD system based on the concept of index modulation (IM), which applies to both OFDM and cyclic-prefixed single-carrier (CPSC) signals. The methods to generate all possible cyclic delays and the optimal receivers are designed for both proposed systems. To aim at low computational complexity, we further propose a suboptimal receiver for the CDD-CPSC-IM system. Moreover, a closed-form upper bound on the bit error rate (BER) of the CDD-OFDM-IM system is derived, from which the coding gain is characterized. Simulation results in terms of BER corroborate the analysis and the superiority of the proposed systems over the pure CDD-OFDM and CDD-CPSC systems, uncovering the potential of CDD in the application to green IoT networks.

Delays; OFDM; Receivers; Modulation; Indexes; Transmitting antennas; Frequency diversity; Cyclic delay diversity (CDD); index modulation (IM); single-carrier; permutation

South China University of Technology