Low-Proﬁle and Closely Spaced Four-Element MIMO Antenna for Wireless Body Area Networks

: A compact four-element multiple-input multiple output (MIMO) antenna is proposed for medical applications operating at a 2.4 GHz ISM band. The proposed MIMO design occupies an overall volume of 26 mm × 26 mm × 0.8 mm. This antenna exhibits a good impedance matching at the operating frequency of the ISM band, whose performance attributes include: isolation around 25 dB, envelope correlation coefﬁcient (ECC) less than 0.02, average channel capacity loss (CCL) less than 0.3 bits/s/Hz and diversity gain (DG) of around 10 dB. The average peak realized gain of the four-element MIMO antenna is 2.4 dBi with more than 77 % radiation efﬁciency at the frequency of interest (ISM 2.4 GHz). The compact volume and adequate bandwidth, as well as the good achieved gain, make this antenna a strong candidate for bio-medical wearable applications.


Introduction
Due to the tremendous demand for high data rate and large channel capacity, developing multiple-input multiple-output (MIMO) antenna systems is given more attention by industry and academia. The key goals of the MIMO system is to accomplish multiband operation, enhanced bandwidth, improved gain/efficiency, good channel capacity and diversity performance [1].
To achieve a high quality data transmission, the multi-input multi-output (MIMO) antenna technology has been promoted and has become an eye-catching choice for antenna designers. However, a good MIMO antenna should meet several important factors, such as ensuring high isolation between elements, while obtaining a good impedance characteristics and highly efficient individual antennas in [32,33,37], improved ECC with comparison to refs [30,35], and better power gains over the designs in [33,36,38]. Moreover, the proposed antenna has been embedded on the body and its performance compared with free space antennas.

Antenna Design And Concept
The proposed design consists of four p-shaped monopole radiators as shown in Figure 1. The proposed ISM 2.4 GHz MIMO antenna is printed on FR-4 dielectric substrate (permittivity of 4.3, loss tangent of 0.025, and thickness of 0.8 mm). The proposed geometry of the four-element MIMO monopole antenna is considered as one of the smallest designs in wearable antennas with a total size of 26 × 26 × 0.8 mm 3 . As can be seen in Figure 1, the proposed antenna is fed by a single 50 Ω microstrip line designed and printed on FR-4 substrate. Initially, a single-element antennas was investigated. The dimensions of the antenna were optimized at the desired frequency (2.4 GHz). The fabricated prototype of the single-element antenna is portrayed in Figure 2a. The simulated and measured reflection coefficients of the antenna are shown in Figure 2b. We see that the simulated and measured results have the same resonant frequency. The bandwidth of the fabricated prototype is smaller than the simulated one but still wider than the targeted ISM band.
The four p-shaped elements are symmetrically placed with a separation distance of 4.5 mm (0.06λ). The very small ground plane is printed on the lower side of the substrate, where this appellation is used in [39][40][41]. The ground plane was defected into four rectangle parts printed underneath each radiating element. The schematic views of the considered radiating element (front side) and partial ground (back side) are shown in Figure 1, along with the optimized dimensions. The present design is simulated with the aid of CST [42]. The detailed geometrical parameters and their sizes are listed in Table 2.

Equivalent Circuit Model
In order to evaluate the true behavior of the proposed four elements MIMO antenna, an equivalent lumped circuit model is generated using advance design system (ADS). The equivalent circuit model is illustrated in Figure 3. Each monopole radiator of the MIMO antenna system is represented by the parallel RLC resonant circuit [43,44]. Each monopole is excited by a 50 Ω terminal. The associated coupling between the MIMO elements is represented by an LC circuit [45,46]. The equivalent circuit is fine tuned to extract the optimized circuit values. The optimized circuit parameters are listed in Figure 3. The s-parameters of the circuit and EM model are compared in Figure 4. We can see that the plotted curves present satisfactory agreement.

Simulation and Measurement Results
The computed and measured s-parameters of the antenna are demonstrated in Figure 5. It is observed that the proposed MIMO antenna has simulated −10 dB impedance bandwidth at the 2.4 GHz ISM band, where it shows isolations between the adjacent and diagonal ports of around −25 dB. Reflection Coefficient (dB)

Frequency (GHz)
Sim: S11/S22/S33/S44 Meas: S11/S22/S33/S44 For proof-of-concept, the four-element MIMO antenna model is fabricated as depicted in Figure 6. The MIMO antenna was printed over FR-4 substrate with thickness of 0.8 mm. Four rectangle copper parts were printed underneath each radiating element to mimic the partial ground plane of the antenna. Four SMA connectors were exploited to feed the four-element radiators as illustrated in Figure 6. The measured s-parameters of the proposed MIMO antenna in Figure 5 show a good agreement with the simulated outcomes, with an insignificant move in the resonant frequency, which can be attributed to several factors such as the fabrication process and error and port/cable coupling losses.

Current Distribution
To validate the outcome of the mutual coupling between the antenna elements of the proposed antenna obtained in Figures 4, 5 and 7, we can refer to the antenna surface current as indicated in Figure 8. Generally, to understand the influence of the coupling between the antenna elements using this approach, one of the four ports should be excited, whereas the other three ports remain terminated with 50 Ω load as shown in Figure 8. The space between the ports is considered as an important factor in order to figure out how much the antennas are being isolated. Generally speaking, when the antenna elements are largely separated, this will lead to a high isolation.
In the case of Figure 8a, only port 1 is excited. As observable from Figure 8a, the higher value of current concentrates on the feed arm, as there is no current linked to the adjacent ports. Therefore, its effect can be seen in terms of the S12 (or S34) isolation parameter. When port 2 is excited and the other three ports are terminated with 50 Ω load, the current only exist on this port while being negligible on the remaining three. The same case is illustrated when port 3 and port 4 are only excited as indicated in Figure 8c,d. From Figure 8, it can be observed that the surface current is mainly focused over the excited port. Therefore, the measured and simulated isolations for the proposed MIMO antenna that were shown in Figures 5 and 7 are more than 10 dB.
Two planes, i.e., E-plane and H-plane at 2.4 GHz are taken in order to examine the radiation patterns of the proposed antenna as illustrated in Figure 9, when one port is excited and the others are matched to a 50 Ω load. As it is noted, the E and H plane simulated field patterns of the present MIMO antenna are shown at the ISM 2.4 GHz resonant frequencies, which are generated by the CST software package. The measured radiation is taken and carried out inside an anechoic chamber in the presence of a standard horn antenna (as transmitter) and the power meter (to record the received field). In the measurement process, any one port is chosen to work as a receiver, while the other ports are loaded with 50 Ω load, for the reason of avoiding any signal pick-up. Such a procedure is repeated with every port of the proposed MIMO design. The measured and CST simulated radiation patterns are compared in Figure 9 in the case of the two planes. Thanks to the symmetric geometry of the proposed antenna that helps in accomplishing a stable radiation pattern, we consider an appropriate design for use in communication systems. However,certain irregularities between the simulated and measured patterns are observed and this may be attributed to the fabrication process errors, and the losses of the ports/cables.

Antenna ON-Body Investigation
The human body can absorb electromagnetic (EM) waves (termed as specific absorption rate, SAR), generated by antenna operated in its vicinity. If this absorption of EM waves exceeds some standard limits, the temperature of the tissues will rise [47]. These limits are standardized by the electromagnetic field (EMF) regulation authorities (ICNIRP, IEEE, and FCC). Considering public safety concerns, we have calculated the specific absorption rate (SAR) according to IEEE C95.1-2005 as shown in Figure 10. The antenna was placed on the chest, arm, and leg of a real realistic Ella model while maintaining some gap to avoid direct contact with tissue as shown in Figure 10. Figure 10a-c show the SAR distributions on the chest, arm, and leg of the Ella model. We selected small areas for SAR calculation instead of full body to reduce simulation time. The peak 10-g SAR values 20 W/kg, 8.36 W/kg, and 7.17 W/kg at 1 W input power to each array element on the chest, arm, and leg, respectively. The peak SAR values exceed the standard limits; however, the devices operating near the body use little power. The standard maximum allowable power (EIR) for such devices is set in the mW range [48][49][50]. To restrict the SAR value under the limit for our proposed antenna, the maximum input powers to the antenna are 100 mW, 239.2 mW, and 278.9 mW for the chest, arm, and leg, respectively, which are much greater than the standard radiated limit (in mW range). Based on the above SAR calculated value, our antenna is safe if the input power is less than than 100 mW, 239.2 mW, and 278.9 mW for the chest, arm, and leg cases, respectively. Additionally, the antenna's performance in terms of reflection coefficient, gain, and radiation pattern is evaluated in the vicinity of the human body. The reflection coefficient of the antenna in free space and in on-body worn scenarios is illustrated in Figure 11a. We can see that the resonant frequency shifts a bit towards the lower frequency side due to body loading effects [51]; however, the antenna still retains a good impedance bandwidth and covers the targeted ISM band (2.4 GHz). Figure 11b,c illustrate the radiation pattern of the antenna in free space and in the vicinity of the human body. We can see that the front-to-back ratio of the antenna in Phi = 0 0 is enhanced due to the fact that skin behaves as an extension of the ground plane [52]. The antenna gain is also one of the diversity parameters. The MIMO antenna gain is a far-field parameter and is measured in an Anechoic chamber using the two standard horn antennas and proposed structure. The simulated and measured gains on the free space and on-body paradigms were carried out as depicted in Figure 12. In the case of our design, the minimum value of the computed and measured peak gain is varied between 2.20 dBi and 2.5 dBi. The simulated antenna gain for the free space and on-body analysis at 2.4 GHz resonant frequency is around 2.49 dBi, while the measured gain in the case of free space and on-body worn at the same band is about 2.20 dBi.   Figure 13a illustrates the outcomes of the computed envelope correlation coefficient (ECC) between the ports of the MIMO antenna system in both scenarios, namely, the free space and on-body worn. ECC of the MIMO system portrays how independent the individual elements of the MIMO system are in terms of radiation pattern. It is always desirable in a MIMO system that each element is independent of the other; however, zero value of ECC is difficult in practical applications. In this case, the three ECC values may be accomplished by substituting the appropriate terms (S 11 = S 22 = S 33 = S 44 , and S 12 = S 21 = S 34 = S 43 , and S 13 = S 31 = S 24 = S 24 , and S 14 = S 41 = S 23 = S 32 ) into the following equations, and can be written as shown in Equations (1)- (3): From Figure 13a, the ECC between the ports is lower than 0.03 for the band of interest. Thus, this value of ECC is reasonable and comparable to the ones achieved in [53], which show a good indication for promising diversity performance. The diversity gain also is deemed as a paramount diversity parameter [54,55], which may be accomplished in terms of maximum theoretical diversity gain (10 dB) and correlation coefficient exploiting Equation (4)- (6). Diversity gain refers to improvements in signal-to-interference ratio by applying any diversity scheme. As the antenna shows diversity gain with higher values, this means a better isolation is achieved and vice versa. The plots of the diversity gain of the present MIMO antenna in the case of free space and on-body investigations are shown in Figure 13b. From Figure 13b, it is noticed that the values of DG are about 10 dB at the operating band for both paradigms.
The capacity capacity loss (CCL) provides information regarding the maximum limit of the message rate up to which the message can be continuously transmitted without any loss over a communication channel. In theory, the channel capacity can be enhanced by increasing the number of antennas of the MIMO system. Nevertheless, the presence of Rayleigh-fading MIMO channels will induce loss of channel capacity. This loss can be calculated from the correlation matrices given in [55,56]. In the case of a 2 × 2 MIMO system, assuming that only the receiving antenna patterns are correlated and assuming the worst scenario where high signal-to-noise ratio is occurring, the channel capacity loss (CCL) can be evaluated by using the following equation [54][55][56]: where Ψ R is the receiving antenna correlation matrix that is given by: The CCLs of the proposed radiator for both scenarios are shown in Figure 14a. Figure 14a reveals that the proposed MIMO antenna achieves acceptable CCL values, which vary from 0.2 to 0.3 around the targeted band of ISM 2.4 GHz (ideally, CCL should be less than 0.4 bps/Hz within the entire operating band). Thus, our investigation proves that good impedance matching and isolation between the two antenna elements lead to low capacity loss in the case of free space as well as on-body worn devices.
The efficiency also describes the diversity behaviour of the proposed MIMO antenna. The values of the radiation efficiencies for the 2.4 GHz ISM band in both cases are around 77%, as shown in Figure 14b.

Conclusions
A compact four-element MIMO antenna is designed and presented for the ISM band. The P-shaped radiating elements are realized on a low-cost FR-4 substrate. The proposed antenna occupies a compact envelope dimension of 0.20λ • × 0.20λ • × 0.006λ • , where λ • is the wavelength of the operating frequency. The MIMO antenna design has been simulated and also fabricated. Both the simulations and measurements showed high isolation of around −25 dB at the targeted band of 2.4 GHz . In addition, the envelope correlation coefficient, antenna gain, efficiency and channel capacity loss and other performances are also provided. The proposed antenna has also been placed on a human body, where its performance was investigated and showed acceptable results compared to the free space antenna design. Therefore, the proposed MIMO antenna has been shown to be a strong candidate for use in medical applications, as well as for mobile and satellite communications.