In this paper, the performance of generalized channel inversion (GCI) technique for a multiuser MIMO system operating under antenna mutual coupling conditions is investigated. The investigation is performed via theoretical derivations and computer simulations. The theoretical derivations show that the presence of mutual coupling may result in an increased performance in terms of sum rate capacity. The simulation experiments confirm the theoretical findings in examples of three groups of numerical results. These results show that the existence of mutual coupling in an array antenna at base station (BS) results in an increased sum rate capacity when antenna interelement spacing at BS is smaller than 0.55

Multiple-input multiple-output (MIMO) technology has received enormous interest due to its potential in providing high and robust data transmission for wireless communications [

Research interests in multiuser MIMO systems have mainly focused on uplink multiple-access channel (MAC) and downlink broadcasting channel (BC). In this respect, the optimal encoding and decoding strategies for a MIMO MAC have been broadly studied. The information theory hints that the broadcasting case is by far the most challenging. In this case, an interuser interference occurs due to the spatially multiplexed transmitted signals at BS. It has been shown via theoretical derivations that the dirty paper coding (DPC) [

To overcome the hurdle associated with the computing intensive nonlinear precoding techniques, linear precoding techniques have been investigated for use in multiuser MIMO systems. Block diagonalization (BD) [

The investigations concerning GCI have been presented for the case of simplified operation of array antennas at BS and MSs. In this case, the antenna elements are assumed to operate in an ideal manner with mutual coupling neglected. In practice, physical limitations in mobile terminals as well as in BS force antenna elements in an array to be placed with a small interelement spacing. This requirement is governed by compact MIMO transceivers especially on the user side. The resulting electromagnetic interactions between the antenna elements affect the performance of MIMO links. The effect of mutual coupling on array signal processing has been investigated in many works, such as [

In this paper, the impact of antenna mutual coupling is investigated when a multiuser MIMO system employs a generalized channel inversion approach for its operation. The investigations include a theoretical analysis and computer simulations.

The paper is structured as follows. Section

A narrowband downlink multiuser MIMO system with the BS serving

In (

In a rich scattering environment, the correlation for any pair of dipole elements with spacing

The interactions between the entire set of antennas within an array and scattering objects can be described by the impedance matrix

Once the mutual coupling matrix for the transmit array at BS and receive arrays at MSs are obtained, we can have mutual coupling taken into account by building effective MIMO channels. As a result, the effective MIMO channel between the BS and the

With spatial correlation and mutual coupling taken into account, the received signal at

Inter-user interference cancellation relies on precoding techniques. To completely eliminate the inter-user interference, linear precoding techniques require that

With spatial correlations and mutual coupling taken into account, the effective wireless links between the BS and the

The generalized inversed MIMO channel presented by (

As a result of (

Equation (

It is worth noting that the inter-user elimination given by (

To investigate the impact of antenna mutual coupling on the performance of a multiuser MIMO system employing generalized channel inversion, the Monto-Carlo simulations are performed. In the assumed system, the BS is equipped with 6 transmit dipole antennas, and there are 2 MSs each having 3 receive dipole antennas. Transmit power is equally allocated to the 2 MSs. Both for BS and MSs, the dipole antennas are assumed to be 0.5

The numerical results are divided into three groups, in which only transmit correlation and mutual coupling are considered, only receive correlation and mutual coupling are considered, and where both transmit and receive correlation and mutual coupling are considered. In the simulations, the unit for interelement spacing is one wavelength and denoted by

In the first set of simulations, spatial correlation and mutual coupling at BS are considered. We assume that there are no spatial correlations or mutual coupling effects at the MSs. The simulations are designed to show the impact of mutual coupling at the BS side on the sum rate capacity of a multiuser MIMO system. Figure

Sum rate capacity with regards to interelement spacing at BS for a 2 × (3 × 6) multiuser MIMO system with and without mutual coupling taken into account.

It can be seen from results presented in Figure

Figures

Surface of sum rate capacity with regard to interelement spacing at BS and SNR for a 2 × (3 × 6) multiuser MIMO system without mutual coupling taken into account.

Surface of sum rate capacity with regard to interelement spacing at BS and SNR for a 2 × (3 × 6) multiuser MIMO system with mutual coupling taken into account.

Cases of with and without mutual coupling taken into account are considered. In both cases, the sum rate capacity is shown with respect to SNR and interelement spacing at the BS. The peaks between 0.2–0.4

In the second set of simulations, spatial correlation and mutual coupling at MSs are considered, by varying the interelement spacing at the MSs. It is postulated that the receive array at each of the MSs has the same interelement spacing and there are no spatial correlation and mutual coupling effects at the BS. This group of simulations is designed to investigate the impact of mutual coupling at MSs’ side on the sum rate capacity of a multiuser MIMO system.

Figure

Sum rate capacity with regard to interelement spacing at MSs for a 2 × (3 × 6) multiuser MIMO system with and without mutual coupling taken into account.

The presented results show a similar trend to the results given in Figure

Figures

Surface of sum rate capacity with regard to interelement spacing at MSs and SNR for a 2 × (3 × 6) multiuser MIMO system without mutual coupling taken into account

Surface of sum rate capacity with regard to interelement spacing at MSs and SNR for a 2 × (3 × 6) multiuser MIMO system with mutual coupling taken into account.

In Figure

In the third set of simulations, spatial correlations and mutual coupling at both BS and MSs are considered, by varying the interelement spacing at the BS and MSs. We assume that the receive array at each of the MSs has the same interelement spacing. The SNRs for both cases are fixed to 20 dB. This group of simulations are designed to reveal the joint impact of mutual coupling at both the BS and MSs on the sum rate capacity of a multiuser MIMO system.

Figure

Surface of sum rate capacity with transmit and receive spatial correlation only for a 2 × (3 × 6) multiuser MIMO system at SNR = 20 dB.

As expected from the previous simulations, the results in Figure

Figure

Surface of sum rate capacity with transmit and receive spatial correlation and mutual coupling for a 2 × (3 × 6) multiuser MIMO system at SNR = 20 dB.

As expected from the previous simulations, the results presented in Figure

The paper has reported investigations into the performance of a multiuser MIMO system employing generalized channel inversion scheme. The investigations have taken into account spatial correlation and antenna mutual coupling effects which are present in compact array antennas. The undertaken theoretical analysis has shown that the presence of antenna mutual coupling may have positive impact on the performance of the multiuser MIMO system in terms of sum rate capacity. The simulation results have confirmed these theoretical findings by showing that when the antenna interelement spacing at BS is smaller than 0.55