A Study Of Precoding In Ofdm Systems

Orthogonal frequency division multiplexing (OFDM) is becoming a hotspot of study and considered as the core technology of the 4th generation mobile communication systems for its robustness to frequency selective fading and high spectral efficiency. OFDM systems transfer a multi-path channel into a flat fading channel, so we merely need single-tap equalization in the receiving end to recover the transmitted data. Nevertheless, under severe conditions of propagation, some of the subcarriers could suffer high attenuation which causes a raise in the bit error rate, and the performance of OFDM systems is severely degraded for ICI (inter-carrier interference) in fast time-varying channel. The thesis mainly investigates how to decrease the bad effect of high attenuation and ICI caused by fast time-varying by precoding technology. The main work of the thesis is listed as follows:1. The thesis elaborates the method of modeling of wireless fading channel and the basic principle of OFDM systems and how the ICI comes into being. Then the thesis builds the precoded OFDM system models, which contain independent channel information precoded model and precoded model based on channel information. All the researches followed are based on the corresponding model.2. The independent channel information precoded OFDM systems based on unitary matrix are studied. The thesis analyses and compares the performance of the precoded OFDM system based on unitary matrix with the conventional OFDM system by utilizing receivers based on two criterion--ZF(Zero-Force) and MMSE (Minimum-Mean-Square-Error),respectively. Then a varying dimension Hadamard matrix precoding scheme based on MMSE receiver is proposed. The size of Hadamard matrix is not restricted by the number of subcarries, so the scheme improves the flexibility of precoding. And the performance of the scheme is improving with increasing of the dimension.3. Precoded OFDM systems based on the channel information are investigated in the thesis. The present pecoding algorithms based on channel information are applied in OFDM systems over slow and fast time-varying channels. The thesis compares the performance of them and proposes combined Hadamard matrix and TXMMSE precoding schemes which include invarying dimension Hadamard matrix precoding sheme and varying dimension Hadamard matrix precoding scheme. And the two schemes improve the performance of OFDM systems effectively by the combined precoding of Hadamard matrix and TXMMSE.