| In Band Full Duplex(IBFD) transmission, as one of the core technologies in the Generation Fifth(5G) mobile communication system, differs from the conventional transmission modes such as Frequency Division Duplex(FDD) and Time Division Duplex(TDD). It realizes the synchronous receiving and transmitting signal of the communication node with the same frequency resource in the same time slice. So the IBFD transmission mode can theoretically provide two times the bandwidth utilization than the existing traditional duplex modes for the wireless communicat ion system, which will have a very wide range of application prospects for its high speed transmission of information today.However, when the communication node works in the IBFD mode, transmitter and receiver transmit data in the same frequency band simultaneously, leading to Self-Interference(SI) at receivers. Since the distance between receiving antenna and transmitting antenna of one single user is relatively close, SI has less experience of free space path loss. In a result, the interference intensit y at the receiving antenna is larger, which seriously affects the performance of the receiver sampling decision. Whether truly effective inhibition of SI will be the most critical factor in the future development and practical application of IBFD transmiss ion technology.Around this issue, this paper proposes an interference suppression scheme of spread spectrum transmission based on Complementary Codes(CC) for effectively eliminating the interference in the nodes. Different from the one dimensional spread spectrum sequences, CC is one kind of two-dimensional(2D) sequence with ideal correlation properties. While its ideal autocorrelation characteristic will fully suppress Multipath Interference(MPI) in fading channels, its ideal cross correlation can eliminate Multiple Access Interference(MAI) and SI.In this paper, a general mathematical model of the full duplex multi user communication system is constructed based on the above 2D complementary codes. The general process of spreading and despreading of CC s is depicted. The general expression of the transmit signal is given in the single carrier serial time division transmission mode. Then, the general expression of the received signal is derived combining with frequency selective slow fading Rayleigh chann el. On the basis of CC correlation properties and the received signal expression, interference suppression for SI, MAI and MPI in the system is analysed. The results show that the multi-user communication system based on the 2D complementary code can completely eliminate all kinds of interference. Moveover, Bit Error Rate(BER) and Outage Probability(OP) closed expression are given for system transmission reliability analysis. At last the simulation results show that within the fixed node SI intensity, full duplex multiuser communication system using 2D complementary code has lower BER and OP than the traditional one-dimensional(1D) spreading sequences like Gold sequences, Walsh sequences and Kasami sequences. Under the condition of the fixed transmitted signal to noise ratio, the error and outage performance can be stable for CCs within the fixed node SI intensity increasing. In contrast, error and outage performance of the traditional 1D spread spectrum sequences will be worse.Finally, in this paper, 2D complementary code will be extended to three-dimensional(3D) complementary code, combined with Multiple Input Multiple Output(MIMO) transmission technology. Then the general mathematical model of the full duplex multi user communication system based on t he extended 3D complementary code is constructed, and the general forms of the transmitted signal, the fading channel and the received signal are derived. Observing the receiving signal expression, it is easy to find that MIMO multi-antenna structure makes MAI, MP, SI stacked in the form of multiple antennas. However, in the region of zero correlation, the effective suppression of interferences can be achieved by the ideal correlation properties of the 3D complementary codes. Then, the analysis BER and OP i s similar to 2D complementary codes. Finally, the simulation results show that the BER and OP of the 3D complementary code are better than those of the traditional 1D spread spectrum sequences. Compared with the single antenna structure of 2D complementary code, the spatial diversity or multiplexing gain provided by the multi antenna structure of the three-dimensional complementary code will effectively improve the transmission performance of the system. |
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