Mimo-ofdm System Under The Beam-forming Technology

MIMO is one of the promising techniques in B3G systems. MIMO systems enable to grow linearly in transmission rate with the minimum of the number of antennas at either end. And OFDM is widely studied because of its high spectrum efficiency and capability to combat multipath fading. Combining the MIMO and OFDM together, as MIMO-OFDM system, is the promising strategy in the next generation communication system. In the close-loop system, because the transmitter can knows the channel state information, so, it can adopts precoding algorithms at the transmitter to greatly increase the channel capacity and transmission performance. So, the precoding for MIMO and OFDM systems are studied in this thesis.The traditional beamforming in MIMO-OFDM systems uses the subcarriers characterized by the largest eigenvalues to transmit the OFDM block symbols. In contrast to the previous one, the optimal beamforming in MIMO-OFDM systems use the best subchannels in all optional orthogonal subchannels. But when feedback from receiver delays, the optimal beamforming in MIMO-OFDM will appear self-interference in the subcarriers that have more than one subchannels. The self-interference will impact the performance severely especially when the normalized Doppler frequency is large. The following works is introduced in this paper:An improved detection algorithms is introduced to reduce the self-interference. By equaling the model of the subcarriers with has 2 subchannels to a V-BLAST model, then, it's easy to eliminate the self-interference.Moreover, we adopt channel coding to the system and output soft information. But in the V-BLAST model, the noise is not white noise, so, we use a method to whiten the color noise. Then, we can use traditional soft information algorithms in V-BLAST model.For the beamforming based on codebook, we can also adopt dynamic subcarriers allocation. As the same, it can detect by equalling the subcarriers that have several codes to MIMO-OFDM V-BLAST model.This dynamic subcarriers allocation algorithm of codebook beamforming is still robust when consider the efforts of feedback delay.We introduced a adaptive codebook allocation algorithm based on subcarriers at last, and simulate this method's performance.