Variable Sub-Carrier OFDM Using OOK-7PSK Modulation in AWGN Channel

Orthogonal Frequency Division Multiplexing (OFDM) is a technique that provides high data rate communications, but it is sensitive to the carrier frequency offset (CFO) caused by the misalignment in carrier frequencies between transmitter and receiver, but also sensitive to the receiver In-phase and Quadrature (IQ) imbalance due to manufacturing inaccuracies. Both CFO and IQ imbalance will cause Inter-Carrier Interference (ICI) among the sub-carriers and degrade the Carrier-to-Interference Ratio (CIR). In this paper, we propose an OFDM system using On-Off-Keying 7-Phase-Shift-Keying (OOK-7PSK) modulation, which consists of a 7-PSK constellation plus a point at the origin, to reduce the ICI. Its performance is evaluated by both theoretic analysis and simulations under both CFO and IQ imbalance impairments. The numerical and simulation results show that the OFDM system using OOK-7PSK is more robust against both CFO and IQ imbalance compared to the OFDM system using conventional 8-PSK. This proposed OOK-7PSK modulation can not only reduce Inter-Carrier Interference, but also improve the power efficiency.