| At present, with the demands of mobile phone users and network operators for audio and other multimedia data increasing, the data transmission rate of the mobile network also needs to be further improved, the technology of MIMO multi antenna is born at the right moment. MIMO is a multiple input multiple output technology, integrating multi antennas in a circuit board to transmit and receive mobile data. The capacity of wireless communication systems can be greatly improved when MIMO technology used in mobile phones and other small mobile terminals. However, how to design the MIMO multi antenna system which meets the requirement of the isolation level of the mobile phone is the hotspot and difficulty in the current design of the mobile phone antenna.This paper introduces the basic principle and the analysis method of the coupling between the MIMO antenna unit in mobile phone, and describes the influence of the correlation of the MIMO antenna unit on the system performance. According to the antenna radiation pattern and the incident field model, a method to calculate the envelope correlation coefficient of the antenna system is presented. This method considers more factors than the S parameter formula, which can not only be used in the engineering design of the mobile phone antenna, but can be used to calculate the envelope correlation coefficient of the simulation model.Based on the advantages of PIFA antennas of small size, light weight, small impact on the surrounding environment and small radiation damage to the human body, two 4*4MIMO mobile phone antenna systems are designed to work in a single frequency and multi frequency band respectively. The mobile phone antenna system working in single frequency,uses the rectangular radiation patch, the defective ground(DGS) structure and vertical polarization, making the antenna unit able to operate in three frequency bands. The results of simulation and envelope correlation calculation show that the S12 is less than-15 d B at the resonant frequency, and the correlation coefficient is less than 0.1. The mobile phone antenna system wo rking in multi bands is loaded with "U" type groove and a "L" type groove in the rectangular patch, with the "T" type slot on the floor. The results of simulation and envelope correlation calculation show that- 6d B bandwidth of the antenna system is covered by GSM, DCS and 4G-LTE high frequency band and envelope correlation coefficient between adjacent antenna elements in the frequency range is less than 0.2, so the antenna isolation level meets the requirements of the mobile phone antenna system well.Based on the characteristics of small size and wide band of monopole antennas, a new kind of 4*4MIMO mobile phone antenna system with large bandwidth is proposed. The antenna system adopts multi branch and defect structure, realizing the multi resonant mode of antenna elements. The results of simulation and calculation show that the antenna system works in the 1710-2690 MHZ band, and the return loss and isolation of the antenna can meet the performance requirements of the mobile phone. |
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