Mimo-ofdm System, Adaptive Modulation Techniques

With the increasing requirements for high-data-rate multimedia services, multiple-input multiple-output (MIMO) and orthogonal frequency division multiplexing (OFDM) techniques have been given more and more interest. MIMO systems can increase the channel capacity and enhance the transmission reliability by employing multiple antennas at the transmitter and receiver. OFDM has the ability of transforming the frequency-selective channel into a collection of parallel flat-fading subchannels and therefore can be adopted in MIMO systems to combat multi-path effects. The MIMO-OFDM system is regarded as a potential candidate for 4G mobile systems.If the channel state information (CSI) is known at the transmitter, adaptive modulation techniques, specifically, adaptive bit and power allocation techniques can be applied to optimize the performance of MIMO-OFDM systems, such as transmit power, bit-error-ratio (BER) and so forth.In this paper, the adaptive modulation techniques for MIMO-OFDM systems are first studied. The MIMO-OFDM channel can be decomposed into a group of parallel subchannels by singular value decomposition (SVD). A low-complexity bit and power allocation algorithm is proposed based on this structure. To minimize the overall transmit power, we carry out a fixed bit and power allocation over these subchannels by exploring the characteristics of the ordered subchannel gains. The fixed bit and power assignment can be determined by greedy algorithm at the initial stage and utilized without change throughout the data transmission process. Simulation results show that the proposed algorithm has less than 1dB performance loss relative to the optimal algorithm when the number of OFDM subcarriers is large, but drastically reduces the complexity.Orthogonal frequency division multiple access (OFDMA) is an extension of OFDM, in which each user is assigned a subset of all subcarriers and each subcarrier is allocated exclusively to one user. A low-complexity adaptive subcarrier, bit and power allocation algorithm is proposed for OFDMA systems. The objective of the proposed algorithm is to minimize the overall transmit power with constraints on the data-rate and BER requirements of each user. In the proposed algorithm, a subcarrier allocation is first carried out, which includes two stages: an initial subcarrier allocation to guarantee the data-rate requirement and a residual subcarrier allocation...