Performance enhancement schemes in wireless OFDM systems

Abstract

The demand for high capacity wireless mobile communication is rapidly growing. Orthogonal frequency division multiplexing (OFDM) technology promises to be a key technique for achieving the high data capacity and spectral efficiency requirements for wireless communication systems of the next generation. However, it is suffering from inter-carrier interference (ICI) caused by carrier frequency offset (CFO) and Doppler spread (DS). So, this thesis is mainly divided into two investigations: Capacity improvement and ICI cancellation. In the first investigation, we propose subcarrier multilevel transmit power allocation scheme and switching-level control scheme to increase bandwidth efficiency and to guarantee a required bit error rate. The system considered is OFDM-code division multiplexing under slowlyvarying frequency-selective fading channels. In the proposed schemes, modulation level and transmit power for each subcarrier are simultaneously controlled corresponding to channel condition to maximize transmitted bits per symbol while keeping both total transmit power and bit error rate constant. Simulation results and complexity analyses will verify that the proposed schemes are effective in terms of throughput and complexity. In the second investigation, we numerically analyze the negative effects of ICIs caused by CFO and DS on system performance in frequency domain, followed by the modelling of ICI components by only DS in time domain. This allows us to design a relatively simple receiver scheme that iteratively cancels the ICI. On the condition that DS exists alone, we propose new time domain equalizers to cancel the ICI in OFDM systems. One equalizer is more suitable for unignorably wide DS, and the other is more robust to highly wider DS. These equalizers consist of the following two-stages similar to each other. The first stage applies a suboptimal linear minimum mean squared error (MMSE) recursively employing partial rank of time impulse response mat...