Active quasi circulator: Comprehensive review and performance comparison

Designing circulator as an antenna interface device becomes a daunting task, particularly active‐quasi circulator. This article focuses on demonstrating the basic operation principle, design methods, technical parameters, and performance metrics of active quasi‐circulator. In addition, the study provides an analogy of the circuits and structures proposed by the researchers to enhance certain performance metrics. Active signal cancellation and passive signal cancellation are identified as the major design approaches. Tunable, wideband, and wideband‐tunable are the major types of circulators found in existing literature. Moreover, this article provides a performance comparison of the active‐quasi circulators available in existing literature. Several active quasi‐circulators were able to operate in high frequencies such as 60 GHz with acceptable isolation levels. On the other hand, several designs have over 30 dB isolation, which is a highly desired parameter. At last, the future design challenges associated with active‐quasi circulators have been discussed to provide insight into future research.

passive circulators, 5 with the Faraday rotation circulator being one prominent example. 6A device using the principle of Faraday rotator operates by using the Faraday effect, which is a magneto-optic phenomenon that occurs when microwaves are transferred through a substance in the presence of a static magnetic field that is oriented along its length.This incident causes a rotation in the polarization of the microwave signal.However, due to the drawbacks of ferrite being expensive, bulky, and difficult to integrate, active circulators have been developed to replace traditional bulky ferrite circulators, thus reducing costs and the system's size. 7n modern-day, semiconductor-based circuit components can be custom-designed to meet particular system requisites as a substitute for the conventional ferrite material-based circulator.In 1965, the concept of using semiconductor devices to create active circulators was introduced, which marked a significant departure from traditional circulators that relied on bulk physical properties. 8][11][12] According to a study conducted in Reference 13, active circulators using Monolithic Microwave Integrated Circuits (MMIC) offer better solutions for integrated system-on-a-chip (SOC) applications.They offer the benefit of increased bandwidth and high-frequency operation at a similar level of insertion loss compared to conventional systems.In contrast, ferrite circulators typically have limited bandwidth with comparatively higher insertion loss, especially at high frequencies.For instance, a conventional ferrite material-based circulator functioning at W-band typically possesses 1 W diminishing power handling, ∼2.1-dB insertion loss and ∼2% bandwidth (BW). 13Thus, to ensure high-frequency operation, MMIC active circulators capable of SoC implementation exhibit significant promise to assure reliable performance over a wide application range.
Nowadays, the design of quasi-circulator for full-duplex operations has gained significant interest among researchers.Operation principle of a traditional circulator and an active-quasi circulator is quite similar except that two out of the three ports (transmitter, antenna, and receiver) in a quasi-circulator are isolated in clockwise and counter-clockwise directions. 14This implies that power can either transmit from port 1 (transmitter) to port 2 (antenna) or from port 2 (antenna) to port 3 (receiver).However, power transmission is not allowed from port 3 (receiver) to port 1 (transmitter). 15ctive quasi-circulators have several advantages over traditional circulators, including their lower cost, lighter weight, smaller size, and compatibility with modern IC fabrication technologies.7][18] Therefore, active-quasi circulators offer an attractive option for a wide range of communication applications.
In existing literature, a detailed review of Active-Quasi circulator design and its performance metrics is missing.Numerous designs of active-quasi circulators were implemented over the course of time.Therefore, to address this research gap, it becomes essential to have a comprehensive overview and comparison of the circuits, design techniques, and performance metrics.This article presents an overall broad-gauge review of active-quasi circulator.As active quasi circulators have gained popularity in recent years, this review article can help to highlight the progress made in this field and identify areas for future research.Additionally, this article aims to serve as a valuable resource for researchers, engineers, and students who are interested in active quasi circulators and want to gain a deeper understanding of their principles, design, and applications.

THEORY AND PRINCIPLE OF OPERATION
In recent times, there has been a significant focus on circulators because of their significance in the advancement of full-duplex systems. 19Such sort of systems need to enable simultaneous signal transmission and reception at the same frequency using the same antenna. 20The illustration depicted in Figure 1 displays a quasi-circulator that is employed in a setup comprising a solitary antenna.The transmission signal generated by the transmitter is directed towards the antenna, and the response signal transmitted by the transponder is conveyed from the antenna to the receiver via the circulator.Additionally, the circulator serves to isolate the transmitter and receiver from each other.Existing circulators can be broadly classified into four different types: Ferrite, Tim-Varying, Non-Linear, and Active.A generic comparison of among the circulator types is presented in Table 1.
2][23] It can have multiple ports, but the one commonly used in multiplexing usually has three ports. 24Active circulators can be categorized into two major types: quasi-circulator and conventional three-way circulator. 25By analyzing the existing literature in References 11,26-68, theory and operation principle of active-quasi circulator is presented in the following sub-sections.

Theory
In Figure 2, difference between active 3-way and active-quasi circulator is shown. 13From the illustration, it can be observed that an active-quasi circulator has similar operation principle compared to the conventional three-way circulator.However, quasi-circulator does not allow signal transmission from port 3 (receiver) to port 1 (transmitter), due to which the S parameter S 13 = 0.These active circulators can be fabricated on a single chip and have been extensively researched, taking over traditional ferrite circulators.The distinction between quasi-circulators and symmetric circulators lies in their isolation properties.In another way, it can be said that quasi-circulators have bidirectional isolation for the transmitter and receiver, while the traditional symmetric circulators possess unidirectional isolation property.Although quasi-circulators have imperfect symmetry, they are suitable for full-duplex systems because connecting Transmitter (TX), Antenna (ANT), and Receiver (RX) to ports 1, 2, and 3 satisfies all relevant requirements.In full-duplex systems, RX-to-TX transmission is not significant because signals received at RX tend to be quite weak compared to the transmitted ones.To provide wide BW and for on-chip implementation, active nonreciprocal components have advantage of high BW and are suitable for monolithic integration.However, drawbacks such as high Noise Figure (NF) and low power handling capability are the major issues.Despite their limitations, modern versions provide adequate performance to satisfy a range of applications.

Operation principle
Two major operation principles of active quasi-circulator have been identified: Active Signal Cancellation and Passive Signal Cancellation.F I G U R E 3 Quasi-circulator using active mode cancellation technique: (A) out-of-phase divider; (B) in-phase-divider.

Active signal cancellation
The method of designing quasi-circulators using active in-phase or out-of-phase power dividers and combiners is referred to as active cancellation.Figure 3A demonstrates the example of an in-phase combiner and out-of-phase divider-based quasi-circulator, that utilizes active cancellation method.The main purpose of the out-of-phase divider is to separate the incoming signal into two distinct signals with a phase-shift of 180 • .On the other hand, the signals having opposite phases or 180 • phase-shift are rejected by the in-phase combiner since it only transmits signals having the same phases or 0 • phase-shift.Here, the ANT is attached to one of the transmission lines between the combiner and the divider, while a load having identical impedance of the ANT is connected to the other to ensure symmetry.RX is linked with the output of the combiner, and TX is connected to the input of the divider.By canceling the out-of-phase signal by the combiner, signal isolation between TX and RX is obtained.Figure 3B displays a quasi-circulator that is comparable to the one in Figure 3A.However, it uses a unilateral out-of-phase combiner and a unilateral in-phase divider. 69he TX is connected to the divider input, and the RX is connected to the combiner output, while ANT is connected at one of the lines between the divider and the combiner.To maintain symmetry, a load with the same impedance as ANT is connected at the second line between the divider and the combiner.Isolation between TX and RX is achieved by rejecting the out-of-phase signal at the output of the divider by the combiner.The S-parameter matrix for this quasi-circulator is identical to the one depicted in Figure 2A.
Over the course of time, different designs for the combiner/divider have been suggested, including basic circuits using single transistors and distributed amplifiers.However, circuits using passive cancellation have generally outperformed all such designs.

Passive signal cancellation
The method in which quasi-circulators utilize a passive power coupler or divider along with two amplifiers is referred to as passive cancellation.This technique is illustrated in Figure 4. 70 In multiband systems, there has been significant research conducted on the use of passive components like duplexers, power dividers, and directional couplers.Quasi-circulator function can be achieved by using a combination of active components with passive components such as coupler, 27 Wilkinson power divider, [70][71][72] and so forth.Amplifiers (Power Amplifier and Low-Noise Amplifier) are used in quasi-circulator employing passive signal cancellation method to block signal transmission from RX to ANT and ANT to TX.In addition, the passive elements establish an impediment between TX and RX ports.Here, the signal intended to get transferred through ANT is amplified by means of Power Amplifier (PA), while strong isolation for Low Noise Amplifier (LNA) is ensured using the passive elements.This implies that the signal bulk is transferred towards ANT.Wilkinson power divider divides the incoming signal from ANT.Then the signal reaches PA, which isolates TX route.Finally, RX signal is amplified by LNA.The unique unilateral characteristic of active components allows signal flow from input side of one element towards the output, not the other way around.Quasi-circulators that utilize passive components like the Wilkinson power divider exhibit better power-to-noise performance compared to those that use conventional designs, as discussed in Reference 12.This approach offers better NF and superior power handling in spite of incurring a 3 dB loss due to the passive component.

ACTIVE-QUASI CIRCULATOR TYPES
In upcoming wireless communication systems, it is essential for the quasi-circulator to support multiple frequency bands.
In existing literature, 11, three distinct types of circulators are observed: tunable, wideband, and dual. Each f the active-quasi circulator types have several benefits and drawbacks.Typical tunable active-quasi circulators can achieve excellent isolation level between the TX and RX ports (usually >30 dB), however at a narrow BW.In contrast, wideband circulator has the ability to function across large BW, however, at a lower isolation level (∼20 dB).

Tunable
Discrete components that can be adjusted have shown impressive narrow-band performance across a broad range of frequencies.This type of characteristic is typically necessary for quasi-circulators that need to be applied in wireless communication systems operating in multiple frequency bands.The Wilkinson power divider demonstrated in Figure 5A has been suggested to be used in these systems, but it is not appropriate for integrated circuits due to the size limitations imposed by the transmission lines used in the design. 71This issue has been addressed by applying two separate electronically tunable impedance transformers in lieu of the transmission lines, as shown in Figure 5B. 71Replacing the quarter-wave transmission lines helped reduce the circuit area.In this case, varactor diodes (Infineon BB857 series surface mount diodes) are used as the tuning components. 71Operation of this circuit can be explained using Figure 4. Here, PA is linked with port 1 and LNA is attached to port 3.This implies that port 1 is TX and port 3 is RX.As per odd-mode and even-mode analysis, input impedance is around 100 Ω.For a specific frequency and Z in , Capacitor and inductor values (L and C) can be determined for a phase-shift ΔΦ = ∕2.Moreover, to obtain optimal isolation, varactor capacitance is required to be tuned.Port 1-3 isolation (S 31 ), known as the main isolation of an active-quasi circulator is >50 dB across 53% of BW.Furthermore, all three ports have return losses >10 dB, while having an insertion loss of 3 ± 1 dB.In order to adjust the phase of a signal path and maintain a phase difference near 180 • between the out-of-phase and in-phase pathways at various frequency levels, a tunable capacitor has been inserted to a circulator's signal path, as illustrated in Figure 6. 51By tuning the capacitance value at different frequencies, high isolation (S 31 ) can be ensured.The tunable active-quasi circulator as shown in Figure 6 used Distributed Amplifiers (DA) comprised of transistors n 1 , n 2 , and n 3 .This DA is used to obtain the TX gain, which is denoted by S 21 .The gain stages produce output currents that are combined with the ANT port without any intervening steps.Meanwhile, any waves traveling in the opposite direction from the second stage and waves traveling in the same direction as the first stage are eliminated at the RX port.To maintain a high level of separation over a broad range of frequencies, a shunt capacitor bank is adjusted between the two amplifier stages to ensure that any out-of-phase signals are eliminated.Moreover, the gains can be adjusted separately to counterbalance the line losses that vary with frequency.Based on the measured outcomes, the tunable circulator exhibits isolation nulls between 5.3 and 7.3 GHz.The impact on reverse isolation (S 12 ), return losses (S 11 , S 22 , S 33 ), RX insertion loss (S 32 ), and TX gain (S 21 ) due to shunt capacitance is insignificant.
Tunable active quasi-circulators are effective in providing high isolation within a certain frequency range.However, tunable capacitor-based phase-shifters (T-networks) have a limited bandwidth and are suitable only for narrowband applications.The limited bandwidth of these phase shifters is inadequate for fulfilling the demands of 5G and future 6G wireless communications, which require high data rates and large bandwidths.

Wideband
The most commonly used method for designing wideband quasi-circulators is the DA architecture. 68In this method, distributed microwave/RF elements, having broad frequency range of operation are being utilized.Examples of such microwave/RF elements are duplexers and mixers.Figure 7 depicts a GaAs transistor based distributed circulator, 29

F I G U R E 7
Wideband active-quasi circulator in Reference 29.
the transmitter and receiver ports.The design of this distributed circulator involves combining a single-stage distributed balun and a single-stage distributed combiner, which amplifies the signals with an out-of-phase difference and a unity gain and then combines them in phase with equal gain to achieve the main isolation (S 31 ) between ports 1 and 3.The measured S-parameters show that this design achieves a return loss of >10 dB for all three ports, with insertion losses of around 0-1.5 dB for S 21 and S 32 .By employing a self-equalization method, the distributed circulator obtains ∼20 dB main isolation (S 31 ) ranging from 0.8 to 2.2 GHz.The other isolations S 12 , S 23 , and S 13 are greater than 20 dB.The use of feedback method is a common circuit design technique to improve circuit stability and increase bandwidth.The circulator in Figure 8 used a feedback pathway between the signal cancellation circuit and port 3. 55 This helped improve the TX-RX isolation.The feedback mechanism helped achieve an isolation of 27 dB at 0.8-6.8GHz frequency.The measured insertion loss S 21 lies in the range of −8 to −10 dB.If TX-RX isolations of the original design in Reference 41 and the feedback mechanism-based design in Figure 8 are denoted as S 31C and S 31F , respectively, then the relationship between S 31C and S 31F can be expressed as: where, 2 > 1, and R s is considered to be the characteristic impedance (50 Ω for the circulator in Reference 55).Here, the denominator portion of S 31F includes the squared term of S as mentioned in Reference 58.As the frequency increases, the denominator increases rapidly, resulting in a faster decrease in the value of S 31F compared to S 31C , particularly in the high-frequency range. 55Regardless of the frequency changes, S 31F | is α times smaller than S 31C .This implies that the circulator with feedback can attain greater isolation and a broader range of frequencies than the one without feedback.

Wideband tunable (WT) circulator
Although the traditional wideband quasi-circulators as described in Section 3.2 can operate at a large BW, they suffer from low levels of isolation between RX and TX.The typical RX-TX isolation is around 20 dB, which is quite often barely adequate.Applying this inadequate isolation in applications such as multiplexing or phase-shifting can result in high level of intermodulation distortion, 73,74 which contributes to performance degradation of the circulator.Due to this reason, this type of circulator has limited applications.The wideband quasi-circulators are appropriate for a multi-band system in which the bands are widely separated from one another.In contrast to the wideband circulators, tunable designs comparatively provide better isolation, however for a narrow BW.These above stated limitations imply that tunable and wideband circulators have their contradictory advantages and disadvantages in modern communication systems requiring multi-band operation.A combination of the two distinct approaches of active-quasi-circulator designs described in Sections 3.1 and 3.2 can provide wide BW and high isolation simultaneously within the same circuit.A circulator that integrates and exploits both tunable and wideband methods in a single circuit is referred as Wideband Tunable (WT) Circulator.Schematic of a WT circulator is shown in Figure 9, 42 where three distributed amplifiers (DA) have been used.Here, DA1 is employed to obtain wideband isolation by canceling the signal between the RX and TX terminals (port 3 and port 1).DA2 and DA3 ensure wideband transmission.DA2 passes wideband signal in port 1-2 pathway, whereas DA3 transfers the signal in port 2-3 route.The main objective is to attain the highest RX-TX isolation at midpoint or center frequency expressed as 0.5 c .Here,  c denotes the cutoff frequency.Within the BW, phase response and power have nonuniform characteristics for single-stage DAs.To compensate for this issue, device transconductance (g m ), gate-to-source capacitance (C gs ), and drain-to-source capacitance (C ds ) are optimized to enhance RX-TX isolation.The bias currents of the individual DAs are tuned to determine the C gs , C ds , and g m .The gate-to-source capacitance (C gs ) and effective device transconductance g m can be altered by changing the transistor gate voltage, which affects the power response of a single-stage DA.Moreover, in the T-network, change in phase across the gate and drain terminals is dependent on C ds and C gs .Thus, both phase and gain can be controlled by calibrating the gate voltage of the DAs, which helps provide greater level of isolation over the specific tuning range.
Figure 10 depicts another WT circulator. 58This structure is developed by employing the quasi-circulator in Reference 41.However, WT circulator in Reference 58 contains an extra identical circuit (denoted by red dotted lines in Figure 10) as shown in Reference 41.This additional circuit provides dual-interference cancellation to the quasi-circulator structure.The original structure in Reference 41 utilizes transistors n 1 and n 2 as the principal cancellation elements.However, the WT circulator in Reference 58 has the additional transistors n 4 and n 5 in the dual circuit path.The input transmits through both the principal and dual circuit pathways and the corresponding signals received at the end of these pathways are the same.Both of these signals from the principal and dual pathways are received at port 3 by the transistors n 3 and n 6 , respectively.Here, n 4 is a source-follower configured transistor which is responsible for providing the in-phase signal.On the other hand, n 5 is another source-follower configured transistor which provides the out-of-phase signal.Both of the signals combine at port 3, cancel each other out, and help provide better isolation.The measured principal isolation S 31 for this WT design is >36 dB with a BW of 6 GHz.In addition, within the BW range 1-7 GHz, the insertion losses for S 32 and S 21 are measured to be <9 and <10 dB, respectively.The primary aspect that determines the effectiveness of active quasi-circulators is the separation between the transmitter and receiver, which is denoted as S 31 .The values of S 31d for the dual active QC and S 31CC for the conventional active QC are derived and can be formulated as follows: where, R s = 50Ω and Therefore, S 31d is approximately enhanced by 2AB(E − F).

DESIGN CHALLENGES
Design challenges associated with active-quasi circulators are expressed in following sub-sections.

Isolation improvement
When selecting a circulator, it is crucial to ensure that it provides sufficient isolation for the intended use. 75Isolation is measured in decibels (dB) and indicates the degree of separation between the signal levels at different ports of the device.The higher the isolation value, the less the signal interference from one port to another.In the case of a quasi-circulator, the isolations among the ports are S 31 , S 12 , S 23 and S 13 .It is important for the circulator to provide a strong level of separation between transmitting and receiving signals, as this can significantly decrease any interference that might occur.By doing so, the receiver will not need to meet such stringent linearity requirements.

Bandwidth expansion
As 5G communication is being developed and the future of 6G communication is anticipated, there will be a significant rise in the need for active quasi-circulators that support broadband. 25The majority of active quasi-circulators rely on phase cancellation, which can result in a wide bandwidth if the in-phase and anti-phase paths possess identical frequency responses.

Insertion loss reduction
The term "insertion loss" refers to the amount of signal loss that occurs when it passes through the transmission line connecting the ports, and is typically measured in decibels.In general, the insertion loss tends to increase as the frequency of the signal increases.In the case of quasi-circulators, the insertion losses are denoted by the S 21 and S 32 parameters, as shown in Figure 2. 13 Lowering the insertion loss is beneficial for the active quasi-circulator, as it enhances the transmitter's transmission efficiency, decreases power usage, and reduces receiver noise.Reduction in insertion loss can be achieved by incorporating PA and buffers in the output signal pathway.In addition, structural changes in the circuit can help minimize signal transmission path loss.

Noise figure reduction
Circulator as a Microwave/RF system component serves as the receiving link's first component, and its noise level significantly affects the entire link's noise figure. 12As a result, NF reduction is a daunting task in present-day quasi-circulator design.Employing LNA design method can help reduce NF, in addition to transistor count reduction and gain enhancement on the TX signal path.

Chip area
The biggest drawback of a conventional ferrite circulator is its size due to which it cannot be integrated in the MMIC circuit. 13The future designers of active circulators have to keep in mind that the less area they take to incorporate the circulator; it will be much more helpful to align with the ongoing trend of designing faster communication systems.
Although due to Moore's Law it is evident that as we move forward, the upcoming technologies are going to offer smaller feature size.

Linearity improvement
The primary limitation of passive circulators that active quasi-circulators are designed to overcome is their lack of linearity.Achieving an input reference P 1dB of at least 30 dBm is a significant challenge for active circulators. 15To enhance linearity, high power-supply transistors or GaN devices can be utilized.Alternatively, new structures such as linear periodically-time-varying (LPTV) can be employed to enhance linearity.

Multiband operation
In future wireless communication systems, devices, circuits and systems need to have the ability to work in multiple BWs.Software-defined radio (SDR) is an example of a system that require multiband operation, where circulators can be applied to conduct the operation of switches and duplexers.A circulator, which is a critical component of the front-end circuit, facilitates the multiplexing of Receiver and Transmitter paths into a single-shared antenna.Here, the circulator is responsible for replacing the functions of switches and duplexers, which helps reduce circuit complexity.In addition, utilization of active-quasi circulators instead of passive ones reduces system area and power consumption.However, it must have the ability to ensure high isolation at multiple BWs.

Power handling
The ability to handle power is a crucial aspect of the quasi-circulator, which is typically assessed by determining its 1 dB compression point.This point represents the output power level at which the gain drops by 1 dB from its consistent value.

F I G U R E 11
Active-quasi circulator to improve power handling. 49 the power handling capability is weak, the circulator may be at risk of damage due to the high output power generated by the transmitter.
In quasi-circulator designs, the linear periodically-time-varying (LPTV) architecture is commonly used to enhance power handling, noise figure, and linearity.This approach has been studied extensively in literature sources. 49,52,56on-reciprocal linear time-varying (LTV) systems are also possible.Researchers have explored the use of N-path filters that have a phase difference between input and output clocks to introduce phase shift to signals passing through the filter.This behavior, known as phase non-reciprocity, can be explained by considering the two sets of switches as reciprocal quadrature mixers and regarding the baseband capacitors along with the source resistance at the two ports as a low-pass filter.
In an LTV active-quasi circulator, a 270 • phase shift is occurred by a 3/4 transmission line.This transmission line replaces one of the signal paths of the circulator.On the other hand, a 90 • phase-shift is generated using reciprocal quadrature mixers.The combined resulting signal has 180 • phase-shift, which is used to cancel the interference signal.Figure 11 shows the circuit diagram of an LTV based active-quasi circulator. 49lthough the LPTV-based active quasi-circulator exhibits desirable characteristics such as high-power processing capability, good linearity, and low noise figure, its narrowband nature makes it unsuitable for broadband applications.Therefore, it may not be well-suited for future high-speed and broadband wireless communication.

COMPARISON AND FUTURE RESEARCH TRENDS
Due to availability of a wide range of active quasi-circulator designs, it becomes necessary to have a summary of the designs.In addition, research trend based on the summary data needs to be identified as well.The following sub-sections elaborates on these topics.

Comparison
A comparative performance analysis of active-quasi circulators available in existing literature is shown in Table 2.In provides the technology information and performance data (return loss, insertion loss, isolation, power handling, power consumption, area coverage, and noise figure).

Discussion
As per extensive study conducted in this article, it is observed that Phase Cancellation is the widely used technique to realize Active-Quasi Circulators.In this type of circulator, as the signal proceeds through divider and combiner paths, amplitude and phase difference alter with frequency.As a result, moderate level of isolation, however, at a limited BW can be realized by this technique.In fact, it can be seen from Table 2 that most of the active-quasi circulators have low-moderate level of isolation with limited BW, compared to the conventional ferrite circulators.In addition, most high-frequency circulators are implemented with SiGe BiCMOS and 22 nm FDSOI processes.
From the performance comparison presented in Table 2, wideband active-quasi circulators with >100% BW have lower performance in terms of isolation (>15 dB).However, they have comparatively higher Insertion Loss.On the other hand, circulators with narrow BW can achieve high level of Isolation while maintaining a lower Insertion Loss.Thus, it becomes evident that performance tradeoffs occur among Isolation, BW, and Insertion Loss.

Future trends
Due to the trend towards smaller, more portable wireless communication systems, circulators need to be integrated and miniaturized.Due to the massive growth and acquisition of wireless communication systems operating in different frequency bands, quasi-circulators should be capable of multi-band operation.To ensure the desired isolation level, majority of the present-day quasi-circulators depend on phase-cancellation.But, while transmitting through anti-phase and in-phase routes, the amplitude and phase-difference in a signal may vary with frequency.Thus, circulator structures that rely on phase cancellation technique can achieve high isolation for a very limited BW.Hence, the majority of the designs presented in existing literature have the limitations of low isolation and narrow-BW operation.Tunable structures as discussed in Section 3.1 can provide better isolation at different frequency bands.However, due to the narrow-BW operation range, its applications are limited in systems that have large BW and high data transmission rate.Researchers attempted to overcome this issue by introducing feedback mechanism and WT method as described in Section 3.Although these techniques solved a part of the problem associated with active-quasi circulators, NF and linearity still remain a significant concern.LPTV structures came up with a solution to improve NF and linearity issues, however, the problems associated with BW, isolation and insertion loss remained almost the same.Therefore, developing active quasi-circulators with high isolation, broadband, and high linearity simultaneously is a significant challenge for future wireless communication systems.This challenge must be addressed to fully replace passive circulators with active circulators, leading to significant advances in highly integrated wireless communication systems.Five feasible directions to improve isolation, bandwidth, power processing, linearity, and noise figure are suggested based on an analysis of existing state-of-the-art technologies (Figures 9 and 10), including a discussion of the compromises among various performances.

APPLICATIONS
Quasi circulators have a wide range of applications across various fields, which are covered in the following sub-sections.

Full-duplex communication
In recent times, nonreciprocal components have garnered considerable attention due to their relevance in various microwave applications.One notable application is in full-duplex communication systems [75][76][77] (shown in Figure 12), where data packets can be transmitted and received simultaneously using a single carrier frequency.Full-duplex systems offer the advantage of doubling spectral efficiency compared to half-duplex systems using frequency-division duplexing (FDD) or time-division duplexing (TDD).TDD systems transmit and receive on different carriers.On the other hand, FDD systems transmit and receive on the same carrier, however, at separate time slots.The concurrent operation of transmission and reception in full-duplex wireless communication systems brings about a significant challenge in the form of self-interference.This means that RX of a node experiences interference from TX of the same node.
To address this problem, circulators offer a solution by providing isolation between the front-ends of TX and RX while sharing a common ANT.This arrangement, as shown in Figure 12 facilitates the establishment of low-loss unidirectional transmission channels between TX-ANT and ANT-RX.Additionally, to further minimize circulator's residual leakage, additional layers of self-interference cancellation can be implemented.In these layers, the process involves subtracting the receiver signal from the transmitter signal, with the subtraction being performed using a weighted version of the transmitter signal.This subtraction can take place either in analog or digital domain.

Software defined radios
The rapid expansion of multiband wireless communication has spurred the advancement of compact and cost-effective unique devices capable of handling several bands in the frequency range, in contrast to utilizing a number of typical devices, individually applied for a specific frequency band. 78One benefit of using several devices, with each of them assigned to a particular band, is the ability to optimize each device for its respective band (Figure 13A).Figure 13B presents a diagram of a typical SDR, which includes a broadband front-end and software-reconfigurable baseband. 79In SDRs, an individual device has the ability to function across a wide range of frequency bands.RX path in a typical SDR includes an analog-to-digital converter (ADC), a local oscillator, a down-conversion mixer, a low-noise amplifier (LNA), and a tunable bandpass filter (BPF). 80On the other hand, TX path comprises a power amplifier (PA), a tunable BPF, an up-conversion mixer, and a digital-to-analog converter (DAC).RX and TX pathways are combined together and connected to a shared antenna through a circulator, a critical component in the front-end circuitry.The functionality of switches and duplexers, as shown in Figure 13A are replaced by the circulator.By covering a wide BW, its primary function is to ensure desired isolation levels between the RX, TX, and ANT.

Quantum computing
2][83] Figure 14 illustrates a typical setup for controlling and measuring a superconducting qubit.In this setup, the state of the qubit is determined by probing it with an RF signal and measuring its reflected phase shift.The original signal and its reflection must be extremely weak to prevent any disturbance to the delicate state of the  qubit.As a result, the readout part contains a series of high-gain amplifiers to amplify the weak signal.Typically, the first stage employs a cryogenic Josephson junction parametric amplifier known for its low noise, approaching the quantum limit.The second stage often utilizes HEMT transistor-based amplifier operating at a temperature of 3-4 K.

Other applications
Both active QC and active circulator are specifically engineered to be compatible with contemporary RFIC and MMIC technology, making them well-suited for use in microwave transceiver front-ends.These components have the capability to act as isolators, offering valuable protection to sensitive devices like oscillators, by terminating one port by a properly matched load.
From Figure 15, it can be observed that circulator possesses a range of versatile applications.First, it is capable of functioning as an isolator, providing isolation between different ports.Second, it can serve as a duplexer, enabling simultaneous transmission and reception on a shared channel.Third it has the ability to act as a phase shifter, manipulating the phase of signals passing through it.Additionally, in the context of reflection amplifiers, it can be utilized as an amplification circuit. 25By terminating one of its ports with a matched load, the circulator can effectively function as an isolator, safeguarding sensitive components (Figure 15A).
The circulator's capacity to circulate signals renders it well-suited for deployment as a duplexer, which is a device employed to segregate TX and RX signals, enabling it to utilize a common antenna for both sending and receiving operations.Figure 15B illustrates an instance where a circulator is utilized as a duplexer.
By connecting multiple duplexers in a series arrangement, it is possible to construct a multiplexer that merges several narrowband channels into a single wide channel.Conversely, the reverse operation, known as demultiplexing, involves splitting a wideband channel into multiple narrowband channels.
The circulator possesses a unique characteristic that sets it apart from switch-based solutions.Unlike switches, the circulator allows for simultaneous transmission and reception of signals.Moreover, the circulator can be utilized to construct a narrow-band phase shifter, as demonstrated in Figure 15C.This type of phase shifter modifies the phase of a reflected wave by mechanically or electrically adjusting the position of a short circuit.Phased array systems frequently employ numerous phase shifters to shape specific beams based on predefined algorithms.Figure 15D shows the application of an active-quasi circulator as an amplifier circuit.

CONCLUSION
This study provides an assessment of various active-quasi circulator structures available in the existing literature by pointing out several technical methods to enhance performance parameters such as BW, isolation, linearity, NF, power handling, and so forth.Moreover, the article outlined several design methods and structures that can be suitable for modern multi-standard and multiband systems.The wideband circulator suffers from low isolation while tunable structures have narrow-BW issues.The advantages of tunable and wideband structures are integrated into WT circulators to mitigate isolation and BW issues.However, the designer needs to make the correct tradeoffs among linearity, broadband capacity, and isolation.Two circuit examples are provided for each of the circulator design method types for better understanding.
In addition, a comprehensive performance comparison table has been provided, which summarizes the performance metrics of active quasi-circulator designs available in the existing literature.Finally, this review article provides some insight on improving the characteristics and features of active-quasi circulators to enable readers and researchers to focus on the shortcomings of the present-day designs and architectures.

F I G U R E 2 S
-parameter.(A) Typical circulator.(B) Active-quasi circulator.

F I G U R E 4
Quasi-circulator using passive signal cancellation technique.

F I G U R E 5
Wilkinson power divider: (A) conventional circuit; (B) tunable equivalent circuit used in Reference 71.
which employs active signal cancellation method.Single-stage distributed Balun and combiners are employed to amplify the signals having a phase difference of 180 • .Later, the signals are combined together to provide the main isolation betweenF I G U R E 6 Tunable active-quasi circulator in Reference 51.

F I G U R E 8
Wideband active-quasi circulator in Reference 29.

F I G U R E 10
WT active-quasi circulator in Reference 58.

TA B L E 2
Performance comparison of active-quasi circulators.Full-duplex communication system.

F I G U R E 13
Radio architecture diagrams.(A) conventional multi-radio.(B) Software Defined Radio architecture that utilizes direct conversion (the SDR front-end is highlighted using dashed lines).F I G U R E 14 Superconducting Qubit control and measurement with circulator.

F I G U R E 15
Applications of circulator (A) as an isolator, (B) as a duplexer, (C) as a phase-shifter, (D) in reflection amplifier as an amplifier circuit.
Generic comparison among several approaches to implement circulator.