Research On MIMO-OFDM Key Technologies

Broadband wireless communication is the main direction of the development of modern communications, and the keep growth of the demand for high-speed service stimulates the emergence of new communication technologies continually. Most of these technologies come from innovative applications of existing technology or the combinations of variety of existing technologies. Orthogonal frequency division multiplexing (OFDM) technology transforms the high-speed data transmission to the low-speed data transmission by increasing the symbol period, which effectively transforms the complex frequency selective channels to the flat fading channel, thus improving the frequency domain channels. However, the performance of the OFDM systems deteriorates for the presence of multipath. Multiple-input multiple-output (MIMO) technology makes full use of the multipath signals in space to improve the communication system capacity or enhance the system robustness without the need to increase system bandwidth. The implementations of MIMO technology are based on flat fading channels, which limits the use of MIMO technology in broadband communications. The combination of MIMO and OFDM technology greatly overcomes the limitations while keeps their advantages. Therefore, MIMO-OFDM technology becomes the research hotspot in recent years, and was selected as the core technology in the next generation mobile communications and network access standards.In this paper, the key technologies for MIMO-OFDM system are studied, including the capacity of MIMO system, the channel estimation of MIMO-OFDM system and the implementations MIMO technology such as space-time coding. Simulations are provided to verify the correctness of the proposed methods, and conclusions are obtained, which are offered as references for the development of wireless communications. The following aspects are focused on.The research methods of MIMO channel capacity and related mathematical models are given, and MIMO system capacity with co-channel interference are discussed. This paper studies the factors affecting the capacity of MIMO systems. The system capacity increases with the increasing of the effective physical radio frequency links in space, and the number of the effective physical radio frequency links is equal to the matrix rank of actual physical channel in space. Based on former analysis, the MIMO system capacity is studied under multi-user environment. The impact of a few interferers each with high power and a large number of interferers each with low power on system capacity are analyzed, and the relationships between outage capacities and the number of interference users, transmit antennas of each interference user and interference to noise ratio (INR) are obtained.The channel estimation technology of MIMO-OFDM systems is studied and the corresponding channel estimator is designed. The maximum likelihood estimate (MLE) and minimum mean squared error (MMSE) linear estimation methods are discussed in this paper. The difference between MIMO-OFDM channel estimation and the previous single-input single-output (SISO)-OFDM channel estimation is analyzed, and the transformation between the two is gotten. After that, a MIMO-OFDM system channel estimator is designed. The performance of the designed channel estimator is simulated under the spatial channel model (SCM) proposed by the third generation partnership project (3GPP),which shows the advantages of the designed channel estimator.The research on space-time coding technology is gone deep into, and the interference cancellation algorithm for space-frequency block code (SFBC)-OFDM systems is proposed. Several space-time coding techniques are discussed in the paper. Analyzing their principles and characteristics, we can see that they are the best choice for different communication systems. For the characteristics of space-time coding system, the interference cancellation method for SFBC-OFDM systems is proposed in this paper. The simulation results show that the proposed detector effectively mitigates the effects of inter-symbol interference (ISI) and inter-carrier interference (ICI) under selective fading channel with different fading factors, which resulting in the improvement of system bit to error rate.