Research On Ostbc Techniques In Cognitive Radio System Based Mimo-Ofdm

Cognitive radio allows non-authorized users using the idle licensed bands, taking full advantage of those idle spectrums through time and space. CR is an effective solution to resolve the contradiction between the growing demands for wireless communication applications with the increasingly tense radio spectrum resources. As the cognitive radio technology develops, there are various deformations of cognitive radio which have better performance and can be applied to different environments. For example, the integration of cognitive and MIMO (Multiple-Input Multiple-output)、the integration of cognitive and OFDM (Orthogonal Frequency Division Multiplying、the integration of cognitive and STC (Space Time Coding) and etc. In short, the cognitive radio technology demonstrates broad application prospects and becomes a core technology of communication researchers’concern, its realization and perfection is just around the corner. This paper researches OSTBC techniques in cognitive radio system based MIMO-OFDM. The main work is as follows:First, the OSTBC scheme in cognitive radio based MIMO-OFDM is investigated. A structure of cognitive MIMO system based on the OSTBC is proposed. After mapping, the source signal was fed OSTBC encoding module, the encoded signal carried by the beam forming module, the main beam is formed in the matrix direction of a desired user, while in the other matrix directions forming nulls, which can simultaneously achieve spatial diversity and spatial multiplexing effect at the same time. Finally, the signal is divided into a number of data streams and loaded into the respective sub-carriers of OFDM. The signal is transmitted via radio channels. A reverse operation is run at the receiving end to recover the original signal. Throughout the process, cognitive subsystem requires collaboration, it must use the channel estimation information to complete spectrum sensing, channel allocation, transmit power selection and sub-carrier allocation work according to the specific needs of users. The simulation results show that applying OSTBC in cognitive system based MIMO-OFDM can significantly improve the signal-to-noise ratio when compared to non-coding system.Second, the pre-coding scheme based OSTBC in cognitive radio system is investigated. The performance of traditional MIMO system will drop significantly because of the correlation between channels, and therefore we need to combine pre-coding techniques with STC. On the basis of other scholars’study on the subject, this paper proposes an extension of the pre-coding scheme. The scheme allows both the primary user and cognitive user using multiple antennas. The correlation of antennas is taken into consideration at both ends. In order to obtain the optimal pre-coding matrix, we proposed an algorithm based on Lagrange double decomposition, i.e. under a set of restrictions of per sub-carrier transmit power at the CR transmitter, a set of interference power thresholds at primary user receivers and the interference power limit of primary users, taking advantage of ellipsoid method to find a set of optimal pre-coding matrix, so that we can get the minimum average pair-wise error probability. Then, we apply the proposed algorithm based the Lagrange double decomposition to design the optimal pre-coding matrix according to different correlation of the send and receive antennas at cognitive users side. The simulation results show that the performance of proposed pre-coding scheme is still better than the unified power distribution system even in the case of interference power limit.Finally, on the basis of the first two chapters, the last part of this paper uses MATLAB software build a pre-coding based OSTBC software simulation platform in cognitive MIMO-OFDM system, and eventually formed a GUI(graphical user interface). This platform can achieve the theoretical BER curves of pre-coding based OSTBC under different conditions such as the channel conditions(Rayleigh, Gaussian),different modulation schemes(BPSK,QPSK,8PAK,16QAM),and the number of transmit and receive antennas(1*2,1*4,2*1,2*2,3*1,3*4,4*1,4*4). Users can select a group of parameters, then seeing the advantages and disadvantages of pre-coding based OSTBC in different environment more intuitive. In the end, the article use specific examples to demonstrate the implementation process of the software platform and the simulation results. Using the simulation platform, we can realize human-machine interaction, and also provide a great convenience for the subsequent research.