| Orthogonal Frequency Division Multiplexing (OFDM), a form of multi-carrier modulation technique, has become one of the most promising technologies for the next-generation communication systems to support high-speed high-quality digital transmission and reception due to its robustness to frequency selective fading, high spectral efficiency, and low computational complexity. However, conventional OFDM systems are sensitive to the carrier frequency offset (CFO) and in-phase and quadrature (IQ) imbalance, which cause the inter-carrier interference (ICI) among sub-carriers and greatly degrade the system performance. Traditionally, algorithms for the ICI reduction mainly base on channel estimation and compensation. In this research, we deal with the ICI reduction from the modulation formats. Three different types of modulation methods in OFDM systems are proposed to reduce the ICI due to both the CFO and IQ imbalance. As we know, in a conventional OFDM system, the modulation symbols on each of the sub-carriers are transmitted at the same time. Thus, the total number of sub-carriers and the separation between sub-carriers are fixed at all times over each OFDM symbol interval. When the CFO and IQ imbalance exist, all sub-carriers will contribute to the ICI. The basic idea of the proposed schemes is to reduce the total number of sub-carriers used for transmission in order to increase the average separation among sub-carriers. Namely, some sub-carriers can be either selected to transmit the signal, or not selected for use. Consequently, variable number of sub-carriers is used to transmit the signal in the proposed OFDM systems. When the CFO and IQ imbalance exist, not all sub-carriers will contribute to the ICI, so that the ICI can be consequently reduced. The analyses of symbol error rate (SER) are provided, respectively. Simulation results show that when the ICI resulting from both the CFO and IQ imbalance is significant, the OFDM systems using our proposed modulation methods outperform a conventional OFDM system in terms of the SER. |
