Study On Bit Loading For OFDM Systems

OFDM(Orthogonal Frequency Division Multiplexing) has outstanding advantages because that it could utilize frequency resource sufficiently and robust against channel delay spread.It's suitable for high speed and broadband wireless communication. In broadband OFDM system over mobile fading channels, each individual subchannel has its own attenuation. So each subchannel has different channel capacity. A more efficient radio resource allocation and management scheme should be proved to transmit with higher spectrum and power efficiency and higher system performance. Aadaptive allocation of resource gains more and more attention and becomes one of the focuses in OFDM research . The thesis researches on adaptive resource allocation in OFDM systems and proposes an improved adaptive algorithm of allocation bits and energy.And the applications of the algorithm in MIMO-OFDM and OFDMA systems are studied.The main research work includes: 1.Analyzing the advantages and disadvantages of OFDM and key techniques firstly,then contrast several famous allocation algorithm for different optimizing object.To reduce transmit power and improve the performance of system, an adaptive bit and power allocation algorithm for a given transmission speed is proposed.This algorithm loads bits on subchannels according to the subchannels signal to noise ratios initially,then adjusts total number of bits one by one to maintain stable transmission speed.Researches show that this algorithm has faster converging speed than traditional algorithms that load bits one by one,and could save power over non-adaptive allocation and enhance performance of the systems. 2.Employing the above adaptive loading algorithm on Multiple-Input Multiple-Output (MIMO)systems.Applies adaptive modulation on separate parallel subchannels which get by making use of the SVD on MIMO channel.This could enhance both the performance and transmission rate geminated with increasing the number of antennas.Compared with applications in SISO systems, using the algorithm in MIMO systems could improve the BER performance more obviously and has higher frequency efficiency. 3.Applying the above algorithm in OFDMA systems.The same sunchannel has different attenuation for different users,so subchannels should be allocated to multiusers firstly.The allocation could use channel frequency resource efficiently.Then employ the above algorithm to load bits and energy on the subchannels allocated to each user. Analysis shows that adaptive allocation in OFDMA could save transmit power than equal allocation in traditional FDMA systems,and the performance is improved greatly. The studies of the thesis indicate that application of adaptive technique could enhance the performance of OFDM systems. The adaptive algorithm can be used over multipath frequency selective fading channels,and could meet the requests of improving frequency efficiency and system performance in broadband wireless systems.