Antenna development for MIMO and UHF RFID applications

Abstract

Multiple-input multiple-output (MIMO) wireless communication systems are capable of achieving significantly higher channel capacities than single-antenna systems. Furthermore, there has been a trend toward installing a larger number of cellular base stations and wireless access points (APs) over the same physical coverage area in order to meet increasing capacity demands. An in-room MIMO system—in which an AP and a user with a portable device (PD) occupy the same room—is considered. The associated channel is dominated by line-of-sight (LOS) components, which tend to increase the received signal-to-noise ratio (SNR) but can degrade MIMO performance. Therefore, the application of elevation-directional (θ-directional) AP antennas which emphasize wall-reflected non-line-of-sight (NLOS) components is investigated to improve in-room MIMO capacity. Simulation results based on the experimental set-up suggest an improvement in mean MIMO capacity on the order of 13 % coupled with a 4 % increase in mean relative MIMO gain if the AP antenna radiation patterns strike a balance between improving MIMO channel conditions and maintaining an appropriate SNR. The associated measurements revealed only a 5 % improvement in mean MIMO capacity and a small reduction in mean relative MIMO gain due to the increased multipath richness associated with the real environment. However, the measured results validated the predicted relationship between the main lobe directions of the AP antenna radiation patterns and the resulting in-room MIMO performance, suggesting that more significant improvements could be achieved for in-room MIMO systems experiencing less multipath richness. In the design of ultra-high frequency (UHF) radio-frequency identification (RFID) tags, the cost of the tag antenna is significant relative to that of the entire tag. As a result, cost-effective alternatives to copper as a UHF RFID tag antenna conductor have been explored in recent years. The feasibility of constructing such antennas using electrically conductive paper is investigated through simulations and experimental measurements. Results suggest that the conductivity of the paper is prohibitively low; however, reasonable performance would be achieved if the paper conductivity were increased by an order of magnitude.