Study On Novel Transmitter Diversity Schemes For MIMO-OFDM Systems

MIMO (Multiple-Input Multiple-Output) technique utilizing multiple antennas to realize multiple transmission and multiple receiving, can exploit space resource adequately and can improve channel capacity without any loss in bandwidth and transmitting power. So it can meet the need of high data transmission in future wireless communications. OFDM (Orthogonal Frequency Division Multiplexing) as a kind of multi-carrier transmission can utilize spectrum resource efficiently. Furthermore, the whole bandwidth is divided into many narrow subcarriers, which can combat frequency selective fading. So the combination of MIMO and OFDM could be core solution for 4th Generation Mobile Communication. Aiming at MIMO-OFDM system, four novel transmitter diversity schemes are designed and studied, and two robust detection algorithms are proposed for V-BLAST (Vertical Bell Labs Layered Space-Time) OFDM systems based on array signal processing theory in this dissertation. Then these methods are verified by theoretical analysis and computer simulations. The primary contributions included in this dissertation can be summarized as follow:1. A novel QO-GLSFTBC (Quasi-Orthogonal Group Layered Space-Frequency-Time Block Coding)-OFDM scheme with full-rate and full-diversity is presented for improving the performance of quasi-orthogonal codes. The four transmit antennas are divided into two groups (2 of each group), after SFBC (Space-Frequency Block Coding), the input signals are divided into two groups. After STBC (Space-Time Block Coding) and OFDM modulation, the two groups are transmitted by corresponding antennas. At the receiver, using mutual orthogonality between subcarriers to suppress group interference, then the two groups are decoded, respectively. Because each group is an Alamouti coding, it can achieve full rate and full diversity. Combining the two groups can still achieve full rate and full diversity. Compared to the constellation-rotated quasi-orthogonal codes, the newly proposed scheme has the advantage of not expanding the signal constellation at each transmit antenna. Furthermore, the encoding and decoding process of the proposed method is made of linear processing and only requires simple operation. Both theoretical analysis and simulation results show the validity of the proposed method.2. A novel GLSFTBC-OFDM transmitter diversity scheme based on LCP (Linear Constellation Precoding) is proposed for wireless communications over frequency -selective fast fading channels. That is LCP-GLSFTBC-OFDM. The proposed approach is based on a four-level design of user codes: the first level which is based on OFDMA (Orthogonal Frequency-Division Multiple Access) deals with group interference and intersymbol interference (ISI), the second level which is based on STBC (Space-Time Block Coding) results in space diversity, the third level which is based on SFBC (Space-Frequency Block Coding) obtains time diversity, and the fourth level which is based on LCP results in frequency diversity. The proposed method overcomes the shortcomings of the complexity of STBC matrix when the number of transmit antennas is more than 2 and the weakness of conventional GLSTBC-OFDM, which requires quasi-static fading, furthermore, both encoding and decoding process of the proposed scheme are based on linear processing and require simple operation. Theoretical analysis and simulation results show the validity of the proposed scheme.3. Two multiuser transmitter schemes are proposed for MIMO-OFDM systems. The first one is CDMA multiuser transmitter scheme, which is based on subcarrier grouping to suppress group interference. That is GLSFBC (Group Layered Space-Frequency Block Coding)-OFDM-CDMA (Code-division multiple-access). It is based on a three-level design of user codes: the top level (based on OFDMA) deals with group interference and intersymbol interference (ISI), the middle level (based on SFBC) results in space-frequency diversity, and the lower level (based on CDMA) handles multiuser interference. The second one is OFDMA multiuser transmitter scheme, which is based on CDMA to suppress group interference. That is GLSFBC-CDMA-OFDMA. It is also based on a three-level design of user codes: the top level (based on OFDMA) deals with multiuser interference and intersymbol interference (ISI), the middle level (based on CDMA) handles group interference, and the lower level (based on SFBC) results in space-frequency diversity. Both of them only need one receive antenna to distinguish multiple users and suppress group interference simultaneously, so the complexity of the receiver decreases remarkably (the conventional group interference cancellation methods need to operate on the equivalent channel response matrix corresponding to each subcarrier, and need more than one receive antenna). Theoretical analysis and simulation results confirm the validity of the two multiuser transmitter schemes.4. Two robust detection algorithms based on array signal processing theory are proposed for V-BLAST OFDM systems with channel estimation error. The first one is projecting onto the signal subspace. The covariance matrix of receive data is eigendecomposed, and the signal subspace is obtained. Then the channel vector (with error) is projected onto the subspace, more accurate channel vector is obtained. According to Capon beamforming algorithm, the filter coefficients are calculated. The second one is according to the mutual orthogonality between the signal subspace and the noise subspace. The covariance matrix of receive data is eigendecomposed, and the noise subspace is obtained. According to the mutual orthogonality between the signal subspace and the noise subspace, a cost function is optimized, then the optimal estimation of the actual channel response is obtained. According to Capon beamforming algorithm, the filter coefficients are calculated. Simulation results show that the performance of the proposed algorithms are better than ZF (Zero-Forcing) and MMSE (Minimum Mean Square Error) algorithms under channel estimation error.