# A Compact, Bistatic Antenna System with Very High Interport Isolation for 2.4 GHz In-Band Full Duplex Applications

Authors: Haq Nawaz; Nosherwan Shoaib; Ahmad Umar Niazi; Shabbir Majeed Chaudhry
Journal: International Journal of Antennas and Propagation
 (2021)

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## Abstract

This paper presents a compact, dual polarized bistatic (two closely spaced transmit and receive radiators) patch antenna with excellent interport isolation performance. The presented antenna system employs differential receive mode operation for the cancellation of self-interference (SI) to achieve very high interport isolation for 2.4 GHz in-band full duplex (IBFD) applications. The presented antenna is based on two closely spaced radiators and a simple 3 dB/180° coupler for differentially excited receive mode operation. The 3 dB/180° coupler performs as a passive self-interference cancellation (SIC) circuit for the presented antenna. The small form-factor structure is realized through via interconnections between the receiving patch and SIC circuit. The prototype of the presented antenna characterizes better than 105 dB peak interport isolation. Moreover, the recorded interport isolation is more than 90 dB and 95 dB within 60 MHz and 40 MHz bandwidths, respectively. The measured gain and cross-polarization levels reflect superior radiation performance for the validation model of the proposed antenna. The presented antenna offers DC interport isolation too, which is required for active antenna applications. The novelty of this work is a compact (small form-factor) antenna structure with very high peak interport isolation along with wider SIC bandwidth as compared to previously reported antennas for full duplex applications.


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## Body

## 1. Introduction

For the realization of in-band full duplex (IBFD) wireless communications with its full potentials, the RF coupling at receive chain resulting from its own transmitter must be fully suppressed [1–3]. This coupling from transmit (Tx) to receive (Rx) chain is termed as self-interference (SI) and overpowers the intended Rx signals. The achieved self-interference cancellation (SIC) levels are termed as the figure of merit for IBFD transceiver design [2, 3]. These SIC levels are directly related to the transmitted power and Tx bandwidth [4].These high SIC levels for intended IBFD operation can be achieved through multiple SIC techniques at various stages across the transceiver including SIC at receiver’s front end, analog/RF domain SIC, and the digital baseband SIC topologies [1–3]. Moreover, comparatively large amount of SIC levels should be achieved at receiver’s front end (antenna stage) in order to preserve the dynamic range of analog to digital converter in Rx chain [1–3, 5]. In addition, achieving high SIC levels at receiver’s front end will reduce the complexity of SIC topologies employed on subsequent stages across the Rx chain [1–3]. For example, as illustrated in Figure 1, for the case of transceiver with required isolation levels of 110 dB, if the antenna stage SIC is around 80 dB, only 30 dB SIC is required from SIC topologies employed at digital baseband stage to achieve the required aggregate SIC levels of 110 dB for the realization of IBFD communication without using complex analog domain SIC techniques.Figure 1
Achieving around 110 dB self-interference cancellation (SIC) for an IBFD transceiver with 80 dB and 30 dB SIC at antenna and digital baseband stages, respectively.The bistatic antenna topology (separate elements for Tx and Rx modes) can be used for IBFD transceiver where the spatial domain isolation through interantenna spacing provides reduced coupling between Tx and Rx ports. However, the Tx and Rx elements should be tightly packed for compact antenna structure which limits the amount of spatial isolation. Moreover, the Tx and Rx operation can be realized through orthogonal polarization to exploit the intrinsic isolation of polarization diversity. However, the polarization isolation alone is not enough at antenna stage to meet the required aggregate SIC levels for IBFD operation [5–7]. In that case, analog or RF domain active SIC topologies can be used with dual polarized antennas to achieve additional interport isolation [8–11]. The performance of such SIC techniques is highly dependent on the characteristics of active SIC circuitry. Moreover, these SIC techniques are normally narrow band to provide SIC for few MHz bandwidths [8, 9].The differential feeding or excitation is a very effective SIC mechanism (passive SIC) to obtain high port to port isolation without degradation in radiation characteristics of antennas. The differential excitation can be used either at one port (Tx/Rx) or at both Tx and Rx ports. Moreover, the differential excitation based near-field SIC techniques can achieve improved levels of interport isolation for IBFD monostatic (shared antenna) or bistatic (separate antenna) antenna systems [12–19]. In addition, the combination of differential excitation and balanced feeding networks is utilized to achieve improved isolation and reduced cross-polarization levels for dual polarized antennas. However, such antenna designs are mostly based on multilayered PCB structures and complex balanced feed networks [16–19]. The differential excitation based near-field SIC techniques utilize the two coupled signals from Tx port to perform the difference operation at Rx port. The achievable isolation levels through differential feeding are highly dependent upon the symmetry of Tx and Rx ports of IBFD antennas and the performance of employed differential feeding network (DFN). In addition, the propagation domain coupling between the radiating element(s) and DFN limits the achievable SIC levels too [19]. The 3 dB/180° ring hybrid coupler (rat-race coupler) is considered a good choice as DFN due to its superior amplitude and out-of-phase balance response to achieve very high SIC levels [12, 13, 19].The dual polarized antennas with high interport RF isolation along with DC isolation are required for active integrated antenna applications [20, 21]. Such antennas are also used for realization of retrodirective antenna arrays [22] or amplifying-reflect type of arrays [23]. Such antennas with DC interport isolation will avoid the DC blocking series capacitors required either in Tx or in Rx path. Consequently, the insertion loss resulting from such series capacitors will be avoided. The cross-polarized antennas with high interport decoupling can also mitigate the fading effects [24].However, most of the previously reported bistatic IBFD antenna configurations have large dimensions or degraded levels of interport isolation levels when the spacing between the Tx and Rx elements is reduced [14]. Achieving the high isolation levels through the compact antenna structures without compromising its radiation performance and electrical dimensions is still a challenging task. The motivation of this work is to realize an IBFD antenna with high interport isolation through a small form-factor (reduced dimensions) antenna structure.In this work, a 2.4 GHz bistatic antenna system based on two closely spaced (interelement spacing ofλo/4), dual polarized patches are demonstrated with improved Tx-Rx isolation through differential Rx mode operation. The differentially driven SIC mechanism for presented antenna system is illustrated through design equations. A simple 3 dB/180° ring hybrid coupler with superior amplitude and out-of-phase balance characteristics has been used as DFN for effective SIC operation to achieve improved levels of interport isolation. The achieved isolation levels are provided through the combination of spatial isolation (path loss-based isolation), polarization diversity isolation, and isolation achieved through differential Rx mode operation for the presented dual polarized bistatic antenna system.


## 2. Differentially Driven 2.4 GHz Dual Polarized Bistatic Antenna

The proposed dual polarized 2.4 GHz bistatic antenna system is shown in Figure2. It comprised two patches where one patch with a single port is intended for Tx mode and the second patch with two ports will be used for differentially driven Rx mode. Each port is matched with square radiating element through quarter wave (λg/4) transmission lines as depicted in Figure 2. Due to the symmetric dimensions of both patches, they resonate at the same frequency of 2.4 GHz for Tx and Rx modes. Both elements are closely spaced and interelement spacing is only λo/4, where λo (125 mm) is the free space wavelength at 2.4 GHz frequency. A 1.6 mm thick single-layered FR-4 substrate (εr = 4.4, tanδ=0.02) has been opted for the design of presented antenna system. Due to the symmetric placement of Rx patch with respect to Tx patch, the same amount of Tx power (self-interference) is coupled to each Rx port of the second radiating element.Figure 2
(a) The topology of dual polarized bistatic patch antenna system based on a single port transmit patch and two ports receive patch for differential receive mode operation to suppress the self-interference. (b) 3 dB/180° ring hybrid coupler as a SIC circuit.Furthermore, both port 2 and port 3 (Rx ports) are cross polarized with respect to port 1 (Tx port). The polarization diversity and spatial separation provide around 47 dB isolation between each Tx-Rx pair of ports as endorsed through simulation results for the proposed antenna structure. The polarization isolation is around 37 dB at 2.4 GHz as reported in [5] and spatial isolation is 10 dB for physical separation (interelement distance) of 31.25 mm. Moreover, the interport isolation is better than 35 dB for 70 MHz bandwidth (10 dB return-loss bandwidth for each port). The proposed differentially driven operation through both Rx ports of the second patch can offer an effective suppression of SI (Tx leakage) at Rx port to obtain the additional isolation superimposed on inherent polarization, diversity isolation, and spatial isolation between Tx and Rx patches. This SIC mechanism can be illustrated through following signal flow analysis for the proposed, dual polarized IBFD bistatic antenna system.As indicated in Figure2, assume that S21 and S31 represent the magnitudes of interport coupling or Tx leakage to each Rx port, respectively. In that case, the currents flowing out of Rx ports (IRx1 and IRx2) are related to Tx port current (ITx) through the following equations:(1)IRx1=ITx∗S31,IRx2=ITx∗S21.If an ideal differential circuit (lossless power combiner with perfect amplitude balance and 180° phase balance characteristics) is connected at both Rx ports, the total current IRx (total leakage signal) at the output of differential circuit is given as follows:(2)IRx=12IRx1+ej180°∗IRx2=ITx2S31−S21.From (2), the ratio of the Tx and Rx currents can be defined as(3)ITxIRx=2S31−S21.As evident from (3), the achievable port to port isolation levels for the proposed bistatic antenna system depend on the amplitude and out-of-phase balance properties of the differential network in addition to the RF coupling between Tx and both Rx ports (S21 and S31). It is important to mention here that this coupling depends upon the polarization and free pace path loss between the two closely spaced patches.As stated earlier, due to the symmetry of antenna structure, the same amount of Tx leakage should be generated at both Rx ports which will result in perfect SIC operation if an ideal DFN is employed to provide infinite Tx-Rx isolation. However, the coupled signals from Tx to Rx patch are dependent upon the environmental factors and manufacturing accuracy for the implemented antennas. Moreover, the propagation domain coupling between radiating elements and DFN degrades the SIC level too. Consequently, in practical scenarios, S21 ≈ S31 to offer very high SIC levels if a DFN with superior amplitude and out-of-phase balance characteristics is used for differential Rx mode operation. Moreover, tunable attenuator and phase shifter can be placed at the inputs of DFN to adjust the magnitudes and phases of S21 and S31 to satisfy the SIC conditions stated in (3). This mechanism can be used to obtain the optimized Tx-Rx isolation through automatic tuning for practical scenarios.The effects of the amplitude and out-of-phase errors (imbalances) of DFN on SIC capabilities can be analyzed through the subtraction of two sinusoidal signals as shown in Figure3. Figure 3 plots the difference (in dB) between two signals for different values of amplitude and out-of-phase errors. The magnitude and phase errors correspond to the magnitude and phase response of employed DFN, while the resulting difference is termed as SIC levels. As obvious from Figure 3, the SIC potential of DFN relies totally on both magnitude and phase response of the DFN. In other words, the SIC levels are throttled to very low levels when the amplitude and an out-of-phase errors are increased. Consequently, a DFN with well-balanced amplitude and an out-of-phase response is essential to achieve improved SIC levels.Figure 3
The effects of the amplitude and out-of-phase errors or imbalances of DFN on the achievable SIC levels.The simulated return losses and interport coupling results for the proposed bistatic antenna are presented in Figure4. As can be seen from Figure 4, the ideal DFN with perfect amplitude and an out-of-phase characteristic can offer Tx-Rx isolation (negative of coupling) on excess of 100 dB within 75 MHz bandwidth (10 dB return-loss bandwidth of Tx/Rx patches). However, the isolation levels of better than 90 dB for 75 MHz bandwidth can be obtained when the proposed 3 dB/180° ring hybrid coupler is used as DFN. The coupler works as a differential power combiner to perform the intended (S31–S21) process at its difference port (Δ port) when it is excited through a pair of Rx ports of the antenna. As endorsed through simulation results, the proposed 3 dB/180° ring hybrid coupler has a well-balanced amplitude and out-of-phase balance response for intended bandwidth to offer better than 50 dB SIC levels. The proposed 3 dB/180° ring hybrid coupler and its dimensions are shown in Figure 2, but its simulation results are not presented here for brevity.Figure 4
The simulatedS-parameters for proposed bistatic antenna system having single port Tx patch and dual port Rx patch.The simulated Tx and differentially excited Rx mode radiation characteristics of the proposed dual polarized bistatic antenna system are presented in Figure 5. The simulated results in Figure 5 demonstrate the vertical polarization for Tx port excitation and horizontal polarization for differentially excited Rx mode. As evident from Figure 5, the radiation performance of the antenna is not affected by differential excitation, as the E-field (Ex) components generated by differential feeding are additive as detailed in [15] and given by (4) for horizontal polarization. The differential feeding also suppresses the higher order modes to offer improved polarization purity or reduced cross-polarization levels for patch antenna [25].(4)ERx=ERx1+ej180°ERx2=ErX^−Er−X^=2ErX^,where X^ is unit vector along the x-axis and Er represents the amplitude of electric field.Figure 5
The simulated surface currents intensity and 3D gain patterns atf = 2.4 GHz for Tx mode and differential driven Rx mode through respective port excitation.


## 3. Antenna Implementation for Experimental Demonstration

For experimental demonstration, the 2.4 GHz ring hybrid coupler and radiating structure (antenna elements) were etched on 1.6 mm thick FR-4 substrate (εr = 4.4, tanδ=0.02). The DFN (ring hybrid) was connected at Rx ports through holes with ground plane sandwiched between two substrate layers. Both structures are electromagnetically isolated due to interlayer ground plane. Two SMA connectors were soldered at the respective Tx and Rx (∆ port of coupler) ports. The sum (Σ) port of the differential circuit (ring hybrid coupler) is terminated in a 50 Ω SMD resistor in order to avoid reflections. The overall size of the implemented prototype is 115 mm × 68 mm × 3.2 mm as shown in Figure 6. It is important to mention here that the DFN and radiating elements can be etched on the same side of a single-layered printed circuit board (PCB). However, this will result in a bistatic antenna system with larger dimensions or sizes. Moreover, the direct electromagnetic coupling between the two structures will degrade the interport isolation performance of the IBFD antenna. The implemented prototype can be interfaced with respective Tx and Rx chains of the full duplex transceiver. Moreover, the Tx and Rx ports of the presented antenna can be interchanged without affecting the interport isolation characteristics. However, the Tx and Rx ports of antenna connected to remote radio (transceiver) should be interchanged too in order to match the polarization for the respective links at local and remote nodes or transceivers [15, 19].Figure 6
The validation model (prototype) of 2.4 GHz differential fed IBFD bistatic patch antenna implemented on double layered FR-4 substrate. (a) Top side. (b) Bottom side.The implemented bistatic antenna was characterized through return loss and interport isolation measurements in the antenna chamber. The simulated and measuredS11 (Tx port), S22 (Rx port), and S21(Tx to Rx port coupling or negative isolation) results for physical model are presented in Figure 7.Figure 7
The simulated and experimentally recordedS11, S22, and interport coupling results (S21) for implemented dual polarized bistatic antenna.As clear from measurement results, the input (Tx port) and output (Rx port) return losses for implemented antenna are better than 18 dB and 12 dB at the center resonating frequency of 2.42 GHz. In addition, the validation model or prototype achieves the overlapping 10 dB return-loss bandwidth of 70 MHz (2.365 GHz to 2.435 GHz) for both input and output ports. The measured isolation (negative of coupling) for implemented antenna was determined as better than 90 dB for 60 MHz bandwidth which spans over 2.365 GHz to 2.425 GHz. Furthermore, higher than 95 dB isolation has been recorded for 40 MHz bandwidth which ranges from 2.38 GHz to 2.42 GHz as clearly marked in Figure 7. The measured peak isolation is in excess of 105 dB at 2.405 GHz frequency as indicated in Figure 7. Consequently, for the 60 MHz bandwidth, 45–50 dB isolation is contributed by SIC circuit on the top of polarization diversity and spatial isolation.The radiation performance of the implemented prototype was endorsed through the gain measurements for each polarization or port excitation. The simulated and measured two-dimensional (2-D) copolarized and cross-polarized E-plane gain patterns for validation model are presented in Figure8. These radiation patterns have been recorded at 2.405 GHz frequency for excitation of the respective Tx (port 1) and Rx (differential port, i.e., port 2) ports. For the gain measurements of Tx mode, the Rx port of antenna was terminated in a 50 Ω load and Tx port was excited through a signal generator. Similarly, for gain measurements of Rx mode, the ∆ port of coupler was excited and Tx port was connected with a 50 Ω termination. The sum (Σ) of coupler is already terminated in 50 Ω SMD resistor. As clearly indicated and evident from Figure 8, the recorded gains for intended polarizations are better than 4.8 dBi for each of Tx and Rx ports. As expressed through experimental results, the low insertion loss of DFN employed for differentially driven mode does not degrade the radiation performance of Rx. Moreover, the gain improvements due to differential excitation compensate the resulting insertion loss of DFN. Consequently, almost similar measured gain levels have been observed for both Tx and Rx polarizations. Moreover, the recorded/measured cross-polarization levels for the implemented antenna are suppressed to better than −40 dB for half power beam width (HPBW) of 75 degrees. These results reflect the improved gain levels for intended polarization along with excellent polarization purity for each of Tx and Rx modes.Figure 8
The measured Tx and Rx modes copolarization and cross-polarization gain levels for the presented antenna at 2.405 GHz frequency.The interport isolation performance and dimensions of presented antenna are compared with some of the previously reported 2.4 GHz dual polarized IBFD antennas [14, 26–30] as detailed in Table 1.Table 1
The comparison of interport isolation and dimensions of presented antenna with some of the previously reported dual polarized, full duplex antennas [14, 26–30].
ReferenceCenter freq. (GHz)Peak isolation (dB)Isolation (dB)/B.W. (MHz)Antenna size (L∗W) ∗ λoSIC topology[14]a2.57064/110(1.1∗0.6)DFN[14]b2.57560/160(1.0∗0.6)DFN[26]2.5≥7540/220(1.1∗0.7)Polarization diversity[27]2.45≥5030/300(1.2∗0.8)Analog SIC[28]2.45≥9040/65Not applicableAnalog SIC[29]2.4∼5047/75(1.5 × 1.4)DFN[30]3.35≥5540/250Not givenUsing circulatorsThis work2.40≥10590/60 and 95/40(0.9∗0.6)Polarization diversity + DFNIt is obvious from this comparison table, that the presented dual polarized IBFD antenna system offers better Tx-Rx interport isolation versus SIC bandwidth performance as compared to the previously reported antenna designs in addition to reduced dimensions (0.9λo × 0.6λo)/small form factor of our antenna structure. Moreover, the presented antenna system provides very high peak isolation compared to the closely spaced antennas which were reported earlier. The 60 MHz SIC (isolation) bandwidth with more than 90 dB isolation makes this antenna a promising choice for deployment in 2.4 GHz ISM band applications having three full duplex bidirectional channels. The potential applications include 2.4 GHz wireless local area network (WLAN) with 20 MHz bandwidth for each of the three full duplex channels, where the propagation domain isolation between forward and reverse channel is achieved through polarization diversity.


## 4. Conclusion

A compact (0.9λo × 0.6λo), dual port, bistatic antenna system with high port to port isolation is demonstrated for 2.4 GHz same frequency full duplex or IBFD wireless applications. The presented antenna is comprised of two closely spaced dual polarized patches. It employs a nicely balanced 3 dB/180° ring hybrid coupler as a differential feeding network for Rx mode to achieve high port-to-port isolation through an effective SIC operation. The differential feeding based SIC operation is also described through mathematical equations in order to illustrate the effects of amplitude and phase imbalances of differential feeding network on the achievable SIC levels. The differential feeding network offers around 45–50 dB SIC on the top of 45–50 dB isolation provided by polarization diversity (dual polarization) and interelement spacing. The presented antenna system offers better isolation characteristics compared to the previously reported full duplex antennas. The implemented antenna offers compact size along with SIC capabilities better than 90 dB for 60 MHz bandwidth and can be readily integrated with 2.4 GHz radios for realization of low power full duplex wireless operation, without using additional complex SIC topologies. In addition to high interport RF isolation, the presented antenna also offers DC interport isolation (as no conduction path exists between Tx and Rx ports). The DC interport isolation is required for various active antenna applications. Thus, the presented antenna can be employed for such active antenna applications without using series capacitors in Tx path. Consequently, the RF power loss resulting from such series capacitors will be removed.



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