Research On The Adaptive CO-STBC/SFBCS In MIMO-OFDM Systems

The multi-input-multi-output(MIMO) technology utilizes multiple transmit and receive antennas to fully exploit the spatial resources,which can exponentially improve the channel capacity but do not need extra frequency resources or power of transmit antennas.The orthogonal frequency-division multiplexing(OFDM) is one of the multi-carrier narrow-band transmission technologies which features its orthogonal subcarriers.It can utilize the frequency resources effectively.Therefore,the MIMO-OFDM system integrating the advantages of the two technologies above can significantly increase the tolerance of wireless system toward the noise,disturbance and multipath by effectively using time diversity,frequency diversity and spatial diversity.The MIMO-OFDM technology has great potentials in improving the transmit speed and reliability of wireless link,thus has become a hot technology in the 4^th Generation communication.The space-time block coding(STBC) is a practical implementation of the MIMO system,and it has attracted much attention in recent years.The quasi-orthogonalized STBC(QO-STBC) is developed based on the orthogonalized STBC(OSTBC) which can acquire full speed at the cost of incomplete orthogonality.However,the non-orthogonal coding matrix of QO-STBC can be retrieved to orthogonal by applying channel-orthogonalized STBC(CO-STBC) or channel-orthogonalized space-frequency block coding(CO-SFBC).In this paper,the adaptive CO-STBC/SFBCs in the MIMO-OFDM system are the main topics to be discussed.The general organization of this paper is as below.First,the CO-STBC/SFBC technologies is introduced,mainly focusing on the orthogonalization of channels in QO-STBC/SFBC.Such orthogonalization can be achieved by rotating some elements in the non-orthogonal coding matrix of QO-STBC/SFBC by utilizing the channel information fed back from the receive antennas.Some principles for designing QO-STBC/SFBC schemes are also provided. Beside of this,two novel forms of coding matrix for CO-STBC/SFBC are proposed based on the existing CO-STBC/SFBC coding matrices.Simulation results demonstrate that the CO-STBC/SFBC schemes can significantly improve the performance of QO-STBC/SFBC at full coding speed.Second,the quantization schemes of different CO-STBC/SFBC schemes are investigated.In the actual communication systems,the quantization for the feedback phase angles in the CO-STBC/SFBC schemes is always followed by the quantization errors,and the values of such errors differ from different CO-STBC/SFBC schemes applied.In this paper,the scopes of quantization errors in CO-STBC/SFBC are discussed in details.The theoretical analysis and the related simulations all demonstrate that the performances of the QO-STBC/SFBC systems with over 3 feedback bits are very close to that in the unquantized system.In order to reach the same performance,the number of feedback bits required in the proposed CO-STBC/SFBC Scheme B is always one more than that in the proposed CO-STBC/SFBC Scheme A.Next,the SFBC-OFDM system based on the antenna selection technology is investigated,and some performance indices including average signal-to-noise ratio (SNR),outage probability and bit error rate(BER) are analyzed within different antenna selection schemes such as transmit antenna selection(TAS),receive antenna selection(RAS) and transmit and receive antenna selection(TRAS).This paper also makes further research on the implementation of TAS in the CO-SFBC-OFDM systems in order to seek better performance in the whole architecture.Simulation results demonstrate that the overall system performance can be further improved by utilizing TAS in the CO-SFBC-based systems.At the end of this paper,a comparison towards the performances between the SFBC-OFDM systems utilizing adaptive modulation and antenna selection is conducted.The adaptive modulation technology is then applied to the CO-SFBC systems,and the CO-SFBC-OFDM system based on the adaptive coding and modulation technology is simulated and analyzed.Simulation results demonstrate that the performances of both the SFBC-OFDM system and the CO-SFBC-OFDM system can be improved by utilizing adaptive modulation technologies,and such adaptive modulation technology can bring more gain improvement than the antenna selection technology.