Pattern Reconfigurable MIMO Antennas for Multiband LTE Operation

Abstract

Nowadays, multiple antennas are becoming widely used in small mobile terminals as they can significantly improve wireless communication performance in terms of link reliability and spectral efficiency. Also, pattern reconfiguration is another new trend for antenna design as it enables the antennas to adapt to different propagation and user scenarios. However, due to the limited space in the terminals, it is difficult to implement both techniques and obtain good antenna performance, especially for frequency bands below 1 GHz. This is because the mobile chassis is often shared by different antennas as the main radiator, regardless of the antenna structure, which leads to high mutual coupling and correlation.

In this thesis, a dual-band (824-894 MHz and 1850-1990 MHz) MIMO antenna system with pattern reconfiguration at the low band was designed based on the theory of characteristic modes (TCM). The work began with the design of a single reconfigurable antenna that can be switched between an inverted-F antenna (IFA) mode and a bezel mode, with low envelope correlation of around 0.2 between the states. This was followed by the implementation of a second antenna (i.e., a single-side T-strip) for MIMO operation. The inclusion of the second antenna required the antenna system to be re-optimized, and several performance trade-offs were observed and investigated, including the tradeoff in the mutual coupling between the MIMO antennas in different states as well as the tradeoff between the mutual coupling and the inter-state correlation. The final design of the pattern reconfigurable MIMO antennas yields an inter-state correlation of around 0.3, and an intra-state correlation of below 0.1 and 0.2, respectively. All the studies were carried out in CST Microwave Studio and Matlab.

Popular Abstract

In modern days, mobile phones play quite an important role in people’s life. Besides sending messages and making phone calls, applications such as surfing web, listening to music, watching videos etc. becomes more and more popular. In order to meet the incredibly fast development of mobile market, faster and more stable wireless communication system are required. The theoretical download speed of LTE (4G standard) could reach 100 Mbps, which is 10000 times faster than that (9.6Kbps) for GSM (2G standard).

Antenna is a key element in wireless communication, which has great influence on the data rate. In Fig. 1, a Multiple Input Multiple Output (MIMO) system is presented with “x” representing the transmit side, “y” representing the receive side and “H” representing the channel. MIMO system is employed in modern LTE standard, which uses multiple antennas at both the transmitting and receiving sides, so that the data rate can be linearly increased without additional frequency spectrum and transmit power. Another technique regarding antenna is reconfiguration, especially pattern reconfiguration for reliable links. By directing the main beam to the signal and the null to the interference, the reliability of the communication link can be greatly increased. pand one of these techniques. In this way, transmitting speed, reliability and error rates could be apparently improved in MIMO system. Moreover, is another technique comes out in recent years which could modify automatically antenna itself into different states according to different needs (e.g. frequency band, radiation pattern) in different scenarios. As a result, better antenna performance is achieved. When it comes to real antenna design, size limitation can lead to high correlation between multiple antennas and also low reconfiguration effect. Thus, designing an efficient reconfigurable multiple antenna system is quite an important topic.

In this thesis, in order to design as efficient mobile antenna system, Theory of Characteristic Modes (TCM) is studied. TCM basically explains that any antenna structure inherently holds a set of orthogonal modes, and any current on the antenna can be expressed as the superposition of the weighted modes. By exciting different orthogonal modes, the TCM analysis can help design uncorrelated MIMO antenna and reconfigurable antennas with distinct patterns.



In this thesis, a dual-band (824-894 MHz and 1850-1990 MHz) MIMO antenna system with pattern reconfiguration at the low band was designed based on TCM. One antenna is a T-strip antenna along the length of the chassis. The second antenna can be reconfigured between bezel state and IFA state. These three antennas are situated on the edge over a 30 mm × 65 mm × 7 mm chassis, saving more space for other electrical devices in the handset. The simulated final structure is presented in Fig. 2. Simulations are carried out in commercial software Computer Simulation Technology (CST) studio, and the final simulated results are compared with the theoretical results from TCM to confirm whether the right modes are excited.

• author: Ma, Rui and Chountalas, Ioannis
• supervisor: Hui Li
• organization: Department of Electrical and Information Technology
• course: EITM02 20141
• year: 2015
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering
• report number: LU/LTH-EIT 2015-470
• language: English
• id: 8147169
• date added to LUP: 2015-11-24 14:12:43
• date last changed: 2015-11-24 14:12:43