MIMO Systems: Principles, Iterative Techniques, and Advanced Polarization - Adaptive Signal Processing in Wireless Communications - page 121

20

10

15

10 8

5

6

0

4

-5

2

-10

0

-15

-2

-20

50 100 150

AS(°)

FIgure 4.17 Difference between the dual-polarized and single-polarized channel capacities of
4 × 4 MIMO systems in the functions of XPD and AS.

diversity techniques are employed, the special correlation effect can be reduced or elimi-
nated when there is no pattern interference. Nevertheless, the cross-polarization dis-
crimination (XPD) becomes the most important parameter because XPD represents the
ratio of the co-polarized average received power to the cross-polarized average received
power. Then, for a high XPD value, less energy is coupled between the cross-polarized
wireless channels. At lower XPD and higher K -factor values [83], multipolarized antenna
arrays can give high capacity. However, at higher XPD and lower spatial correlation, a
single-polarized antenna array can provide even better results.
Figure 4.17 explains the difference between the dual-polarized and single-polarized
channel capacity (Δ C = C dual-polar - C single-polar ) of 4 × 4 MIMO systems versus XPD and
AS. Same angle spreads at Tx and Rx are considered in the simulation. For high XPD
and sufficiently large angle spread, we can note that the MIMO channel capacity of the
single-polarized antenna is superior to that of the dual-polarized antenna because of the
subchannel power loss. The Frobenius norm of the MIMO channel is used to investigate
the total channel power. It confirms that with a high XPD and low spatial correlation the
average transmission power of single-polarized isotropic antenna arrays is  H  F ≈ MN ,
but that of dual-polarized isotropic antenna arrays is always  H  F ≈ M θ N θ + M ϕ N ϕ for
high XPDs and  H  F ≈ M θ N ϕ + M ϕ N θ for low XPDs.
Figure 4.18 shows the XPD impact on 4 × 4 MIMO systems with single- and dual-
polarization configurations. We employ four azimuth isotropic antennas for the single-
polarization configuration and two elevation and two azimuth isotropic antennas for the
dual-polarization configurations, which are applied to a uniform linear antenna array with
a λ/2 antenna separation. It shows that the channel power of single-polarization configura-
tion augments significantly with respect to XPD until  H  F = 4 × 4 = 16, while that of dual-
polarization configuration keeps around  H  F = 2 × 2 + 2 × 2 = 8.