Composite Analysis

Antenna spatial correlation actually matters at the receiver ends for it is the receiving signals that propagate through the multiple scattering rich environments and create the signal diversity. On the other hand, radiation efficiency is actually more important when investigating multiple antenna systems at transmit mode.

We first pay attention to the spatial correlation calculated from three kinds of termination networks in Figure 4.6, 4.8 and 4.10 respectively. Because antenna spatial correlation is actually also dependent on the AoA scenarios as discussed in Chapter 3, we therefore adopt Equation (4.11) as the reference case for it assumes the AoA scenario is 3-D uniform distribution. According to the resulting performance, the input-impedance termination can actually decorrelate the signals between the dual

antenna elements. This agrees well with the result where we let ZL=Zin* in Equation (4.11). Since lower correlation between two antenna elements leads to higher signal diversity, an inference can be made that Zin* case offers the best diversity performance than the other two termination strategies. What’s more, the correlation value of 50-Ohm case and Z11* case will be high (0.45 for 50-Ohm case and 0.57 for Z11* case) only at element spacing < 0.2 λ. Fortunately, the value will be lower than 0.05 as long as element spacing is larger than 0.3 λ, which can be viewed as decorrelation as well for both cases. A conclusion can be drawn that antenna spatial correlation can be high only at very close antenna element spacing which is about less than 0.3 λ for 50-Ohm case and Z11* case; in order to decrease the antenna spatial correlation at very close antenna element spacing, Zin* case can be a good candidate to reach totally decorrelation and high diversity performance.

TARC-based radiation efficiency for three termination cases is shown in Figure 4.7, 4.9 and 4.11 respectively. We first define a parameter called radiation swing margin (RSM) which describes the radiation efficiency swing resulting from the variation of different port-excitation phases. The largest RSM happens when antenna element spacing is 0.1 λ for the first two cases: RSM of 50-Ohm case is about 4 dB which ranges from -1 to -5 dB, RSM of Z11* case is about 5.5 dB which ranges from -0.5 to -6 dB. Another interesting phenomenon is there are some crossing points occurring in the first two cases. The first crossing point of 50-Ohm and Z11* cases occur at antenna element spacing =0.4 λ. They can be interpreted that there exists element spacing which is immune from the variation of different excitation phases. A key to the design of MIMO antennas is therefore provided for we may find element spacing beneficial for radiation efficiency which will not be impacted by the random variation of signal excitation phases between ports. Moreover, we can observe RSM would be greatly reduced as element spacing becomes larger, and that can be

interpreted as when the mutual coupling effect is reduced, the radiation efficiency will become much less sensitive to the phase variation of input signal.

Figure 4.11 brings a perfectly good result for there exists no RSM if the termination of the antenna system is chosen to be input-impedance match termination.

That exactly represents this kind of termination is immune from the random variation of different port-excitation phases at any antenna element spacing. More explicitly, this is a consequence of the input impedance match termination network taking into account not only self impedance but also mutual coupling effect. This termination technique makes the new scattering matrix composed of very low return loss (S11) and moderate insertion loss (S12) which is dependent on the antenna element spacing, and finally leads to the radiation efficiency which is immune from phase variation.

Furthermore, the “o”-marked curve in Figure 4.11 actually represents the conventional radiation efficiency which shares exactly the same trend with the TARC-based radiation efficiency. This again proves the TARC-based radiation efficiency is general for radiation efficiency analysis, and will become the conventional type for the phase-invariant case.

A composite comparison table is shown as in TABLE 4.1. From the comparison, although the first crossing points of 50-Ohm and Z11* cases is at 0.4 λ, the corresponding radiation efficiency is about -0.6 and -0.35 dB, which means at least 8

% of the incident power either reflects back or is absorbed by the load of the adjacent antenna element. On the contrary, since Zin* case results in phase-invariant radiation efficiency, how much power will radiate is then the only concerned topic. However, even if Zin* case is phase-invariant, too close antenna element spacing still results in undesirable radiation performance. For example, when Zin* cases is at 0.1 λ, more than 46 % of input power either reflects back or is absorbed by the load of the adjacent antenna element and not a good solution in the desire of high radiation

efficiency. On the other hand, when Zin* cases is at 0.4 λ, the radiation efficiency has the similar performance with that of Z11* case.

TABLE 4.1 COMPOSITE ANALYSIS TABLE FOR THREE TERMINATION NETWORKS

Chapter 5

Concluding Remarks

In the thesis, we proposed two new electromagnetic analysis strategies to evaluate the performance of multiple antenna systems. The first is a new antenna spatial correlation formulation, and the other is a new analysis strategy of radiation efficiency combined with TARC. All the simulation results are provided using a dipole pair as the benchmark.

We first introduced the proposed 2-D approximate spatial correlation formulation of arbitrary AoA scenarios, and derived a 3-D spatial correlation incorporating antenna mutual coupling and AoA scenarios. The new 3-D antenna spatial correlation formulation not only effectively reduces computation complexity without sacrificing much accuracy but also offers a more detailed analysis presented in the parameterized manner, which can be combined with the proposed approximate spatial correlation formulation of arbitrary AoA scenarios.

Secondly, the new suggested analysis of radiation efficiency combined with TARC evaluates how the radiation efficiency may change when the antenna ports excite signals with different phases. Furthermore, the survey of how termination networks impact on radiation efficiency and spatial correlation is conducted based on a dual dipole system. A composite analysis strategy including antenna spatial correlation and the TARC-based radiation efficiency is offered as the gauge of if a

given multiple antenna system is well designed.

Two suggested strategies make a composite analysis between antenna spatial correlation and radiation efficiency since a multiple antenna system operates in both receiving and transmit modes under the same hardware setup of the multiple antenna system. With the proposed analysis strategies, the design of the multiple antenna system can be more efficient and persuasive before physical implementation.

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