A proposed design for a 12-port 5G massive MIMO mobile terminal was presented for operating in sub-6 GHz range, specifically LTE band 42 (3400-3600 MHz), band 43 (3600-3800 MHz), and band 46 (5150-5925 MHz) . This device was shown to have good performance for 8x8 MIMO operation in bands 42/43 and 6x6 MIMO operation in band 46 by using several different antenna designs spaced around the edges of a mobile handset PCB. Here the performance of the antenna is simulated in XFdtd first with the handset alone and then in various configurations of either one or two hand grasping. The performance criteria evaluated are the return loss, isolation between antennas, the efficiency, gain, and the envelope correlation coefficient (ECC) which is an indicator of the diversity performance of antenna pairs.
The device is shown in Figure 1 where the 12 antenna elements are visible and labeled around the edges of the PCB. At the top and bottom left are dual band inverted PI-shaped antennas which operate in all bands under consideration. These antennas are sometimes labeled as IA in the figures that follow and are numbered as Antennas 1 and 2. Along both sides of the antenna are six longer inverted L-shaped open slot antennas (LA) for the lower (LTE 42/43) bands and four shorter inverted L-shaped open slot antennas (SA) for LTE band 46. The antennas alternate between LA and SA down the sides with three LA and two SA antennas per side. The numbering of antennas is 3 through 7 on the right (numbers 3, 5, and 7 are LA) and 8 through 12 on the left (with 8, 10, and 12 being LA). The dual band antennas are designed with a longer and shorter arm for the lower and higher frequency bands. In Figure 2, Antenna 1 is shown with conduction currents flowing at 3.6 GHz onto the longer arm and at 5.5 GHz on the shorter arm. Two elements of the LA and SA antennas are shown in Figure 3 for comparison purposes.
Simulation in Free Space
As one measure of performance, the S-parameters of various antennas will be considered to show the return loss and isolation of the antennas. In Figure 4, the return loss for antennas 1 and 2 (the top and bottom IA antennas) is shown along with the S21 between them. As can be seen, the antennas have good return loss values in the lower LTE 42/43 bands and the higher LTE 46 band. S21 remains low over both bands, which is expected since the antennas are on extreme ends of the PCB. The return loss of all the LA antennas in LTE bands 42/43 is shown in Figure 5. Figure 6 shows the return loss of the SA antennas in band 46 along with the isolation between adjacent SA antennas and between adjacent SA and IA antennas. The isolation between adjacent LA antennas (and between adjacent LA and IA antennas) for the lower frequency bands is shown in Figure 7. In all cases the results are within the design tolerances of return loss under -6 dB (3:1 VSWR) and isolation -11 dB or better.
The antenna performance is determined by viewing the efficiency and gain of the various elements over their bands of operation. In Figure 8 the efficiency of the LTE Band 42/43 antennas is shown and is reasonable for the IA antennas (1 and 2) and moderate for the LA antennas (3, 5, 7, 8, 10, and 12). In the LTE 46 band, the antenna performance is better, with efficiencies above 50% as shown in Figure 9. Gain is plotted in the XY plane (the plane of the device screen) for various combinations of antennas. In each case, the antenna gain patterns are designed to radiate away from the center of the device. In Figure 10, the gain for the IA antennas at LTE bands 42/43 are shown and can be seen to have a maximum gain at a 45 degree angle from the antenna which is toward the corner of the device screen. In Figure 11, the gain for the LA antennas in the LTE 42/43 band on the right side of the phone is shown and, in each case, the maximum gain is along the X axis, away from the screen center. In Figure 12, the gain for the SA antennas on the right side of the phone in LTE band 46 is shown with similar characteristics.
To evaluate the diversity performance of antenna pairs, an important measure beyond the isolation is the envelope correction coefficient (ECC). This measure is useful for determining how well a device will work for diversity and multiplexing applications where it is important that individual antennas perform independently from others. The basic criterion for ECC is a value less than 0.5. The ECC for various antenna pairs in the LTE 42/43 band and LTE 46 band are compared in Figures 13 and 14, respectively. All plots are well below the 0.5 limit with the maximum correlation being between Antennas 8 and 10 in the LTE 42/43 band at about 0.15.
Simulation with Hands
To further test the phone operation in more realistic conditions, hand models are introduced in both single hand and two hand configurations, as shown in Figure 15. The positions are for left and right hand holds, two hands at the side mode, and two hands with thumbs typing configuration. Depending on the hand position, certain antennas will be covered and the performance will be affected. However, since there are multiple antennas on both sides of the device, in most situations there are still several antennas available with good performance.
The S-parameter performance remains good for all hand positions, although in some cases the return loss is significantly affected. For example, in Figure 16 the return loss for the LTE band 42/43 antennas is shown for the left and right hold positions and there is are noticeable difference versus the phone alone return loss shown in Figure 5. Similarly, the SA antennas operating in the LTE band 46 are also changed by the left and right hand hold positions as shown in Figure 17. For the two hand hold positions, there is less disruption to the S-parameters for the same antennas since the hands are not directly covering the antennas, as shown in Figures 18 and 19.
As can be expected, the efficiency is greatly affected by the presence of the hands. The patterns are disrupted and power is lost in the hand tissue resulting in worse performance. An extreme example is shown in Figure 20 for the case of the two hands at the sides in LTE bands 42/43 where the peak efficiency is only around 35%. Other cases studied had better performance.
The gain patterns are understandably impacted by the hand positions on the phone. Figure 21 shows the three-dimensional patterns of the six LA antennas at LTE bands 42/43 in the case of the phone alone. When the left hand is holding the phone, as shown in Figure 22, some of the patterns, particularly antenna 10 by the thumb, are greatly impacted and show much less gain. For the higher frequency LTE band 46 SA antennas on the sides, there are fairly uniform patterns when the device is alone, such as in Figure 23. With the hands at the sides and the finger next to the two bottom antennas, as in Figure 24, the patterns on the bottom antennas (4 and 6) are reduced compared to the top antennas (9 and 11). For the two hand typing configuration and the same antennas, the top antennas show an increase in gain due to the presence of the thumbs in the middle of the screen as shown in Figure 25. In summary, the peak gains for all configurations are detailed in Tables 1 and 2.
As can be expected, the ECC results are also affected by the presence of the hands. In all cases though, the ECC remains well below the 0.5 threshold as desired. The worst-case ECC results are for the left and right hand hold positions in the LTE band 42/43 antennas where the ECC can reach as high as 0.4 as shown in Figures 26 and 27. All cases in LTE band 46 remain with an ECC below 0.2.
The 12-port antenna can be seen to deliver very good performance with a variety of use configurations. While some hand placements degrade the performance of antennas that are covered, there are other antennas available to maintain operation.
 Yixin Li, Chow-Yen-Desmond Sim, Yong Luo, and Guangli Yang, “12-Port 5G Massive MIMO Antenna Array in Sub-6GHz Mobile Handset for LTE Bands 42/43/46 Applications,” IEEE Access vol. 6, pp. 344-354, 2018.