Degradation of MIMO-capacity due to correlation and efficiency of practical antennas

TLDR: The purpose of the present paper is to compare the performance of three different practical linear MIMO arrays, each with 4 radiating elements, but having different element spacing.

Abstract: Handheld terminals such as mobile phones incorporate more and more multimedia functionality. This is a challenge for the terminal designers, since higher download speed is demanded. Therefore, a lot of research effort is currently spent on approaches to enhance download speed. One approach is to use multiple antennas on both the transmitting and receiving sides. If there are M antenna ports on the transmit side and N on the receive side, there will be MxN signal channels through the multipath environment, thereby allowing for higher data speed than if only one single channel is used. This is commonly referred as a MIMO-system. Multipath environments can be emulated in a reverberation chamber and hence the performance of a MIMO system can be measured there [1]. It is not obvious how to design good antennas for MIMO systems, since classical array theory can not be used. The maximum available capacity of the MIMO antenna system in an isotropic Rayleigh environment can in principle be calculated and used as a cost function in the optimization process of the antenna. However, most antenna engineers do not have any feeling for how to optimize the MIMO capacity and can not give a good starting point for the optimization process. Hence, it will be useful to find how classical antenna performance parameters like radiation efficiency and mutual coupling affect the MIMO-capacity, as shown initially in [2] by characterization both in reverberation chamber and by simulation. The correlation between the antenna ports in the multipath environment also affect the performance but was shown to be a minor effect compared to the radiation efficiency. The reverberation chamber has also been used to characterize the MIMO antennas in [3] and [4]. The purpose of the present paper is to compare the performance of three different practical linear MIMO arrays, each with 4 radiating elements, but having different element spacing. Previously, a circular array of monopoles was studied for different element spacing in [2].