Asynchronous Launch Mimo-ofdm Signal Design And Detection

With the development of increasing mobile phone user and wireless multimedia service, how to meet the demand of increasing wireless multimedia service with the spectrum limit, is the core problem of IMT-Advanced, the next generation mobile communications system.Multiple-Input Multiple-Output (MIMO) can promote remarkably the transmission speed, and Orthogonal Frequency Division Multiplexing (OFDM) can withstand frequency-selective fading, reduce the complexity of channel equalizer. The combination of these two technologies, named MIMO-OFDM, utilizing sufficiently space, time, frequency resource, will satisfy the demand of IMT-Advanced.The classic BLAST (Bell Laboratories Layered Space-Time) based MIMO signal is transmitted and received synchronously at each antenna. This synchronous transmission MIMO scheme constrain by the detection arithmetic: Maximum Likelihood (ML) detection can achieve full receive diversity, but the complexity is high in the extreme, which is difficult to realize in engineering; Zero-forcing detector can lower the complexity at the express of performance, and even need to gurantee that the number of receive antennas should not be smaller than the number of transmit antennas; other ameliorated detectors, taking Minimum Mean Square Error for example, are the tradeoff schemes between spacial diversity and complexity, which neither solve the problem of full receive diversity with low complexity detector, nor the limitation of the number of transmission and receive antennas.Aiming at the deficiency of synchronous transmission MIMO, this thesis discusses the linear detection arithmetic of BLAST-based asynchronous transmission MIMO, proposes the optimal linear detector, and analyzes its diversity order. The analysis and simulation result shows that the optimal linear detection arithmetic is equivalent to MRC-MMSE (maximum ratio combination-minimum mean square error) detection, which can achieve the maximum receive diversity, and break through the constraint that the number of receive antennas must not be smaller than that of transmit antennas.To solve the problem of asynchronous transmission of BLAST-based MIMO-OFDM, this thesis extends the asynchronous transmission scheme in point-to-point channel to frequency-selective fading channel. Utilizing OFDM scheme, this thesis proposes an asynchronous transmission MIMO-OFDM scheme with frequency-delay, and then discusses its linear detection arithmetic and performance. Analysis and simulation result shows that the performance of asynchronous transmission MIMO-OFDM scheme with frequency-delay outperforms that of synchronous transmission scheme. Asynchronous transmission scheme can achieve full receive diversity using linear detector, and break through the limit of the number of antennas at the same time.The scheme discussed above can provide excellent performance; however, its complexity is extremely high. The next part ameliorates the asynchronous transmission scheme, proposes a novel asynchronous transmission MIMO-OFDM scheme with Cyclic Prefix/Postfix, and discusses the linear detection arithmetic and corresponding performance. Analysis and simulation result shows that the performance of asynchronous transmission MIMO-OFDM scheme with Cyclic Prefix/Postfix is parallel to the scheme with frequency delay, but the former can provide far lower complexity, which is close to the synchronous transmission scheme.The research results in this thesis provide a space-time coding strategy to meet the physical layer technology needs in the next generation mobile communications system. By employing the asynchronous transmission strategy, the receiver can use the low complexity detector to realize the full spatial multiplexing and receive diversity over the frequency-selective fading MIMO channels. Hence, this asynchronous transmission architecture can be applied in the LTE (Long Term Evolution), IMT-Advanced systems.