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

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 2.4GHz 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 3dB/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 105dB peak interport isolation. Moreover, the recorded interport isolation is more than 90dB and 95dB within 60MHz and 40MHz 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.


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][2][3]. is coupling from transmit (T x ) to receive (R x ) chain is termed as self-interference (SI) and overpowers the intended R x signals. e achieved self-interference cancellation (SIC) levels are termed as the figure of merit for IBFD transceiver design [2,3]. ese SIC levels are directly related to the transmitted power and T x bandwidth [4]. ese 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][2][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 R x chain [1][2][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 R x chain [1][2][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. e bistatic antenna topology (separate elements for T x and R x modes) can be used for IBFD transceiver where the spatial domain isolation through interantenna spacing provides reduced coupling between T x and R x ports. However, the T x and R x elements should be tightly packed for compact antenna structure which limits the amount of spatial isolation. Moreover, the T x and R x 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][6][7]. In that case, analog or RF domain active SIC topologies can be used with dual polarized antennas to achieve additional interport isolation [8][9][10][11].
e 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]. e 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. e differential excitation can be used either at one port (T x /R x ) or at both T x and R x 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][13][14][15][16][17][18][19]. In addition, the combination of differential excitation and balanced feeding networks is utilized to achieve improved isolation and reduced crosspolarization levels for dual polarized antennas. However, such antenna designs are mostly based on multilayered PCB structures and complex balanced feed networks [16][17][18][19]. e differential excitation based near-field SIC techniques utilize the two coupled signals from T x port to perform the difference operation at R x port. e achievable isolation levels through differential feeding are highly dependent upon the symmetry of T x and R x 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]. e 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]. e 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 T x or in R x path. Consequently, the insertion loss resulting from such series capacitors will be avoided. e 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 T x and R x 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. e 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 of λ o /4), dual polarized patches are demonstrated with improved T x -R x isolation through differential R x mode operation. e 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. e achieved isolation levels are provided through the combination of spatial isolation (path loss-based isolation), polarization diversity isolation, and isolation achieved through differential R x mode operation for the presented dual polarized bistatic antenna system.

Differentially Driven 2.4 GHz Dual Polarized
Bistatic Antenna e proposed dual polarized 2.4 GHz bistatic antenna system is shown in Figure 2. It comprised two patches where one patch with a single port is intended for T x mode and the second patch with two ports will be used for differentially driven R x mode. Each port is matched with square radiating element through quarter wave (λ g /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 T x and R x modes. Both elements are closely spaced and interelement spacing is only λ o /4, where λ o (125 mm) is the free space wavelength at 2.4 GHz frequency. A 1.6 mm thick single-layered FR-4 substrate (ε r � 4.4, tan δ � 0.02) has been opted for the design of presented antenna system. Due to the symmetric placement of R x patch with respect to T x patch, the same amount of T x power (self-interference) is coupled to each R x port of the second radiating element. Furthermore, both port 2 and port 3 (R x ports) are cross polarized with respect to port 1 (T x port). e polarization diversity and spatial separation provide around 47 dB isolation between each T x -R x pair of ports as endorsed through simulation results for the proposed antenna structure. e 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). e proposed differentially driven operation through both R x ports of the second patch can offer an effective suppression of SI (T x leakage) at R x port to obtain the additional isolation superimposed on inherent polarization, diversity isolation, and spatial isolation between T x and R x patches. is SIC mechanism can be illustrated through following signal flow analysis for the proposed, dual polarized IBFD bistatic antenna system.
As indicated in Figure 2, assume that S 21 and S 31 represent the magnitudes of interport coupling or T x leakage to each R x port, respectively. In that case, the currents flowing out of R x ports (I Rx1 and I Rx2 ) are related to T x port current (I Tx ) through the following equations: If an ideal differential circuit (lossless power combiner with perfect amplitude balance and 180°phase balance characteristics) is connected at both R x ports, the total current I Rx (total leakage signal) at the output of differential circuit is given as follows: (2) From (2), the ratio of the T x and R x currents can be defined as 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 T x and both R x ports (S 21 and S 31 ). 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 T x leakage should be generated at both R x ports which will result in perfect SIC operation if an ideal DFN is employed to provide infinite T x -R x isolation. However, the coupled signals from T x to R x 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, S 21 ≈ S 31 to offer very high SIC levels if a DFN with superior amplitude and out-ofphase balance characteristics is used for differential R x mode operation. Moreover, tunable attenuator and phase shifter can be placed at the inputs of DFN to adjust the magnitudes and phases of S 21 and S 31 to satisfy the SIC conditions stated in (3). is mechanism can be used to obtain the optimized T x -R x isolation through automatic tuning for practical scenarios. e 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 Figure 3. Figure 3 plots the difference (in dB) between two signals for different values of amplitude and out-of-phase errors. e 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 outof-phase response is essential to achieve improved SIC levels.
e simulated return losses and interport coupling results for the proposed bistatic antenna are presented in Figure 4. As can be seen from Figure 4, the ideal DFN with perfect amplitude and an out-of-phase characteristic can offer T x -R x isolation (negative of coupling) on excess of 100 dB within 75 MHz bandwidth (10 dB return-loss bandwidth of T x /R x 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. e coupler works as a differential power combiner to perform the intended (S 31 -S 21 ) process at its difference port (Δ port) when it is excited through a pair of R x 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. e 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. e simulated T x and differentially excited R x mode radiation characteristics of the proposed dual polarized bistatic antenna system are presented in Figure 5. e simulated results in Figure 5 demonstrate the vertical polarization for T x port excitation and horizontal polarization for differentially excited R x mode. As evident from Figure 5, the radiation performance of the antenna is not affected by differential excitation, as the E-field (E x ) components generated by differential feeding are additive as detailed in [15] and given by (4) for horizontal polarization. e differential feeding also suppresses the higher order modes to offer improved polarization purity or reduced cross-polarization levels for patch antenna [25].
where X is unit vector along the x-axis and E r represents the amplitude of electric field.

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 (ε r � 4.4, tan δ � 0.02). e DFN (ring hybrid) was connected at R x 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 T x and R x (∆ port of coupler) ports. e sum (Σ) port of the differential circuit (ring hybrid coupler) is terminated in a 50 Ω SMD resistor in order to avoid reflections. e overall size of the implemented prototype is 115 mm × 68 mm × 3.2 mm as shown in Magnitude (dB) Figure 4: e simulated S-parameters for proposed bistatic antenna system having single port T x patch and dual port R x patch. Figure 6. It is important to mention here that the DFN and radiating elements can be etched on the same side of a singlelayered 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. e implemented prototype can be interfaced with respective T x and R x chains of the full duplex transceiver. Moreover, the T x and R x ports of the presented antenna can be interchanged without affecting the interport isolation characteristics. However, the T x and R x 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]. e implemented bistatic antenna was characterized through return loss and interport isolation measurements in the antenna chamber. e simulated and measured S 11 (T x port), S 22 (R x port), and S 21 (T x to R x port coupling or negative isolation) results for physical model are presented in Figure 7. As clear from measurement results, the input (T x port) and output (R x 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. e 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. e 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. e radiation performance of the implemented prototype was endorsed through the gain measurements for each polarization or port excitation. e simulated and measured two-dimensional (2-D) copolarized and cross-polarized E-plane gain patterns for validation model are presented in Figure 8. ese radiation patterns have been recorded at 2.405 GHz frequency for excitation of the respective T x (port 1) and R x (differential port, i.e., port 2) ports. For the gain measurements of T x mode, the R x port of antenna was terminated in a 50 Ω load and T x port was excited through a signal generator. Similarly, for gain measurements of R x mode, the ∆ port of coupler was excited and T x port was connected with a 50 Ω termination. e 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 T x and R x ports. As expressed through experimental results, the low insertion loss of DFN employed for differentially driven mode does not degrade the radiation performance of R x . 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 T x and R x 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. ese results reflect the improved gain levels for intended polarization along with excellent polarization purity for each of T x and R x modes. e 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][27][28][29][30] as detailed in Table 1.
It is obvious from this comparison table, that the presented dual polarized IBFD antenna system offers better T x -R x interport isolation versus SIC bandwidth performance as   compared to the previously reported antenna designs in addition to reduced dimensions (0.9λ o × 0.6λ o )/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.
e 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. e 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.

Conclusion
A compact (0.9λ o × 0.6λ o ), 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. e 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 R x mode to achieve high port-to-port isolation through an effective SIC operation. e 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. e 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. e presented antenna system offers better isolation characteristics compared to the previously reported full duplex antennas. e 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 T x and R x ports). e DC interport isolation is required for various active antenna applications. us, the presented antenna can be employed for such active antenna applications without using series capacitors in T x path. Consequently, the RF power loss resulting from such series capacitors will be removed.

Data Availability
e data used to support the design and validation of the presented antenna are available from the corresponding author on request.

Conflicts of Interest
e authors declare that they have no conflicts of interest.