Reseach On Low-complexity Nonlinear Precoding Algorithm

In recent years, with the growing demand for communication, the channel capacity and transfer rate of mobile communication systems should achieve a higher lever. The technology of orthogonal frequency-division multiplexing(OFDM) and multiple-input multiple-output(MIMO) has become a research focus in the field of mobile communications, and is widely used in Long Term Evolution(LTE), as well as in the fourth generation(4G) mobile communication systems. Among them, the biggest advantage of MIMO technology is that it can exponentially enhance the channel capacity, without any increase of spectrum resources and antenna transmission power. So it becomes one of the core technologies of LTE and 4G systems. Precoding can be used to solve the problem of interference among the antennas, which is introduced by MIMO system.Linear and non-linear ways are two different methods to precode the transmitted data. The former applies fundamental linear signal detection technologies at receiver to transmitter directly, and pre-substracts interference among antennas. This scheme can be implemented easily and has low complexity, but the signal-to-noise ratio(SNR) will be reduced at receiver, which will cause the deterioration of system performance. The latter nonlinearizes signal before transmission, pre-substracts the interference and at the same time averts the decreasing of SNR at receiver. So such scheme can get a larger performance gain, but has higher complexity which concentrate on solving the perturbation vector. Therefore, how to reduce the complexity of nonlinear precoding is the main content of this paper.In the current traditional MIMO broadcast channel precoding schemes, zero-forcing precoding has the least complexity, but also has the worst performance. Sphere encoding has the most complexity, but it can achieve the optimal performance. To propose a precoding algorithm which has a low complexity and an excellent performance is extremely difficult. So the key to the research of low-complexity nonlinear precoding is how to get a tradeoff between the contradiction of performance and complexity.This paper introduces linear and nonlinear precoding and analyses the advantages and disadvantages of these two schemes in detail. Then futher studies the key problem of nonlinear precoding: solving perturbation vector. Through analysis and comparison of some existing solving algorithms, two schemes that can reduce the complexity effectively are proposed: precoding switching scheme based on the condition number of channel matrix and improved sphere encoding scheme. Through simulation and performance analysis of the complexity, the two proposed schemes are proved to be able to get a better tradeoff between performance and complexity.