Online Spotlight: A Survey of Six Port Network Techniques for Direction Finding Applications

The six port network has emerged as a successful receiver architecture with application in metrology, communications, remote sensing, displacement/misalignmentdetection and direction finding due to its precise phase measurement resolution and accuracy. Over the years various design techniques have been used depending upon the application and desired performance specifications. In this article, a comprehensive taxonomy of six port architectures and design techniques is presented under various categories such as topology, number of layers and prominent design features. Furthermore, a comprehensive comparative analysis among different six port design techniques is carried out based on key performance characteristics including operating bandwidth, design complexity, insertion loss, return loss, transmission coefficients, phase performance and physical characteristics. Advantages gained by multilayer and multi-section approaches in terms of bandwidth and compactness are discussed. Basic operation is presented making use of simple analytical expressions suitable for a direction finding (DF) or angle of arrival (AOA) detection system. Various operational enhancements with respect to direction finding are also included, such as extended field of view (FOV) and dual angle detection (azimuth and elevation). Open problems and future research directions are suggested based on the survey.

A direction finding system measures the AOA of an intercepted electromagnetic (EM) signal with respect to the host platform or some reference axis. Locating a signal source is of critical importance both from a civilian and military perspective. There are numerous commercial DF applications such as search and rescue, navigation, automotive radars, misalignment detection, astronomy and telecommunications.

Military applications of direction finding or AOA estimation may be more obvious. In electronic warfare, a direction finding receiver is integrated with an electronic intelligence (ELINT) receiver providing AOA information which helps in initial sorting of an intercepted signal. Similarly, a moving intercept platform with a DF system can triangulate hostile radars based on several converging lines of bearing (LOBs), which results in the formulation of an electronic order of battle (EOB). Moreover, a DF system can provide early warning of an impending attack and help the host platform take appropriate evasive maneuvers. In a potent electronic attack system, AOA information of a hostile emitter can also be used to jam it effectively.

AOA estimation or direction finding has been accomplished using different architectures and techniques since the advent of radio waves. The basic techniques can be divided into Amplitude Comparison, Phase Comparison (Interferometry), Time Difference of Arrival and Frequency Difference of Arrival. Single or multiple channel DF receivers can be used depending upon the complexity of the system. It is well established that phase interferometry provides the best angular resolution and accuracy.^{1, 2}

Phase interferometry can be implemented in both the digital signal processing (DSP) and microwave domains as shown in Figure 1. DSP based solutions require large antenna arrays consisting of multiple antenna elements, coherent receivers to preserve phase and amplitude information, multiple analog-to-digital converters (ADCs) and DSPs adding to overall cost and complexity. Some AOA estimation algorithms used extensively are Multiple Signal Classification (MUSIC), Estimation of Signal Parameters using Rotational Invariance Technique (ESPRIT), Maximum Likelihood Estimation (MLE) and Correlative Phase Interferometry.^{1, 2}

Alternatively, most DF processing can also be carried out in the microwave domain. In multiple microwave-based DF solutions, a four port phase comparator is used to detect the phase difference between incident signals received at its two input ports from which AOA is determined. The phase difference is computed from the I/Q signals at its two output ports. This technique provides reasonable angular accuracy; however, the use of mixers in its architecture adds inaccuracy in phase detection.³ Moreover, at high frequencies the design of active mixers and direct conversion components becomes difficult. Phase noise and DC offset are also generated at a mixer output port due to local oscillator (LO) leakage and amplitude imbalance between RF and LO ports. Due to these inherent limitations, an alternate receiver architecture based on a six port phase comparator has been adopted in DF applications for the last decade. This phase detector design eliminates mixer inaccuracies by using only passive elements.

SIX PORT NETWORK BASICS

The advent of six port technology dates back to 1972 when Engen⁴ described its applications in power measuring devices; however, as planar microstrip design and fabrication technology was not mature enough at that time, it did not gain much popularity. It is only in the last decade that the engineering community has started to realize six port applications in numerous fields.

A six port network can also be called an RF interferometer.⁵ The overall architecture of a six port network, along with input/output signals at each component, is shown in Figure 2. It is an alternative homodyne receiver architecture that provides vector measurement of the two complex input signals by taking scalar measurements at its four output ports. It achieves this through the superposition of the two input signals, producing relative phase shifts of 0, 90, 180 and 270 degrees between the incident signals at its four output ports.^5-8 These superpositions and relative phase shifts are carried out using a network of 0/180 degree power dividers, quadrature hybrid couplers and 90 degree phase shifters. Depending upon the inherent phase shift between the incident signals, they interfere constructively or destructively at the four output ports. Diode power detectors are then used at the output ports to convert high frequency signals to baseband voltages. The diodes behave as ideal square law detectors, i.e. the voltage at the output of the detector is proportional to the square of the magnitude of the input waves.

Assume the incident complex signals at Ports 1 and 2 to be Aejφ1 and Bejφ2 respectively. A, B are the magnitudes and φ1, φ2 are the incident phases. The superimposed signals at the four output ports of the six port network are shown. The following equations show the baseband signals at the output of power detectors. These are achieved by taking the square of the magnitude of the complex output signals at the four output ports assuming ideal square law detectors.

The quadrature and in-phase pairs (Q+, Q- & I+, I-) are then fed to differential amplifiers to generate quadrature and in-phase signals respectively. Sum and difference trigonometric identities are applied to extract the quadrature sine and in-phase cosine components of the incident signal complex ratio.

The phase difference (Δφ = φ 2 - φ 1) between the two complex signals Ae^jφ1 and Be^jφ2 can be easily computed using the following expression:

The relationship between the phase difference and the AOA (θ) can be found as:

where L is the distance between two antenna elements and θ is the angle of incidence which can be found using Equations 7 and 8.

Hence, in a direction of arrival (DOA) detection system, the six port network acts like a high resolution phase comparator extracting the I/Q components from the two incident signals. These I/Q signals are then digitized and used in the DSP domain to calculate the angle of arrival. Determining the distance between antenna elements requires a tradeoff between unambiguous FOV and angular resolution/accuracy. It is chosen according to the specific application or in some cases dual baselines are used to achieve both.

APPLICATIONS

The six port network has been one of the biggest innovations in RF and microwave technology in the last decade with numerous applications (see Figure 3). Initially, it was used in portable vector network analyzers⁹ and reflectometers; however it has recently been used in industrial applications such as precise direction finding and ranging.⁷ In the field of communications, a six port homodyne receiver architecture can be utilized for direct up conversion/down conversion and modulation/demodulation. It can also be used in 5 G to support ultra high data rates.¹⁰ Six port networks have direct applications in optical coherent receivers at extremely high frequencies where the design of conventional architectures becomes very complex. In medical science, vibration and remote heartbeat/respiratory rate sensing have been achieved with the six port’s precise phase resolution.¹¹ Wide applicability is possible because of its numerous advantages, including:

• High phase resolution.
• Absence of phase noise as no LO is required.
• Only passive RF components are used.
• Low size, weight, power and cost (SWaP-C).
• Good performance at high frequencies (microwave and mmwave bands) providing high data rates suitable for ultra-wideband applications.
• 6 port RF front end can be calibrated.

However, there are design tradeoffs that must be considered:

• High insertion loss results in reduced overall dynamic range. It is dependent on having a high gain LNA stage.
• Amplitude and phase imbalances in the transmission coefficients of a six port network result in I/Q gain and phase errors, which must be compensated. Hence, I/Q gain and phase equalization is done by calibration at the DSP level.
• Diode power detectors are nonlinear devices and contribute additional errors; however these can also be compensated through calibration at the DSP level.

TOPOLOGIES AND DESIGN TECHNIQUES

A comprehensive survey (see Table 1) focuses on six port network design techniques. Over the years, researchers have used different subcomponents and design methodologies to achieve relative quadrature phase shifts at the four output ports, the signature of a six port network. This survey focuses particularly on broadband designs that can support multimode and multiband applications and analyzes them based on relevant parameters such as operating bandwidth, design complexity, layers, reflection and transmission coefficients (both simulated and measured), phase performance and dimensions. The techniques reviewed are categorized by the following design topologies:

• Three quadrature hybrid couplers (QHCs) & in- phase/out-of-phase power divider (PD).
• Four PDs & 90 degree phase shifters.
• Four QHCs & 90 degree phase shifter.

Figure 4 is a classification taxonomy that groups each technique by basic topology, single/double layer design and prominent design feature. A separate categorization for operational enhancements in six port DF designs is included and discussed later.

Topology 1 (Three QHCs and one PD)

Topology 1 consisting of three couplers and a divider is the one most widely used. This topology provides the minimum insertion loss (6 dB theoretical) as the input signal passes through only two components. The basic setup consists of two RF inputs feeding 2 LNAs (for extending dynamic range), then the six port network followed by four power detectors feeding into two differential amplifiers to generate the I/Q signals for phase detection. Using this architecture, six ports have been designed in single (see Figure 5) and multilayers (see Figure 6).

The Following are examples six port single layer substrate board designs:

Single section branch line coupler (BLC):

Ibrahim et al.12 assess a six port network from 5 to 8 GHz for DOA detection. It uses a single section BLC and Wilkinson PD. The bandwidth of the BLC is enhanced with λ/4 transformers at all ports. The measured S parameters of fabricated components is used to form a six port ADS simulation model; the six port circuit, as a whole, is not built. The results are tabulated in Table I. The phase imbalance is 90 +5 degrees. Amplitude imbalance among the transmission coefficients is 2 dB, which is slightly higher than desired.

Mohsin et al.¹³ report on a 6 to 9 GHz six port network using single section BLCs with inside folded stubs, increasing the bandwidth from 10 to 50 percent. A single stage Wilkinson PD is used.

Koelpin et al.⁶ describe an integrated six port receiver at 24 GHz (see Figure 5) with a six port circuit in the middle comprising single stage BLCs, bandpass filters, power detectors and baseband amplification circuitry.

Double section BLC design:

Askari and Kamarei¹⁰ use a double section coupler design in a six port circuit for 5 G applications. The use of a double section coupler provides a broad bandwidth between 21 and 30 GHz. In multi-section branch line couplers, however, impedances of the vertical branch lines increase considerably. This decreases the widths of corresponding microstrip lines to micrometers, making them difficult to fabricate.

Abdullah et al.’s¹⁴ design of a complex ratio measurement system uses a six port network between 2.5 and 5.2 GHz. Double section BLCs are used with slots in the ground plane to increase bandwidth. Additional stubs on all three ports of the Wilkinson PD also serve to enhance bandwidth.

Elliptical patch offset port coupler design:

A six port based phase correlator is used for DOA detection using elliptical patch offset port couplers.^{15, 16} The six port network is simulated and experimentally tested from 3 to 8 GHz. Wide bandwidth is achieved using an elliptical patch quadrature coupler design. Quadrature phase at the output port is achieved by varying the major axis and the angle of the output ports with respect to the major axis. The minor axis and step impedance width and length are optimized to achieve the best isolation and return loss. A double stage Wilkinson PD enhances the bandwidth. Although wide bandwidth is achieved, reported phase performance is 30 degrees, which is inadequate for precise phase detection applications.

In the following examples, multilayer design is used, where two or more substrate boards are joined to achieve compact size and wider bandwidth:

Elliptical microstrip slot directional coupler (MSDC):

Ultra-wideband (UWB) six port networks from 3.1 to 10.6 GHz demonstrated by Bialkowski et al.¹⁷ use multilayer microstrip slot transitions.^18-21 Two 0.5 mm substrates are joined such that ground is formed at the center with signal layers at top and bottom. The multilayer coupler has input and output ports on both sides of the combined substrates. The physical dimensions of the elliptical microstrip patches and ground slots depend upon the even and odd mode impedances of the coupled structure which in turn depend upon the desired coupling values. A similar slot transition coupler is designed by Abbosh and Bialkowski²² using multi-section elliptical microstrip patches.

Two different multilayer UWB 180 degree PDs are designed and fabricated. One uses a stripline input port and two microstrip output ports on different layers. The other²³ uses a microstrip slotline transition technique for the PD. The advantage of the second design is that its microstrip input port can easily be integrated with other microstrip components. A multioctave bandwidth and a compact size of 43x43 mm is achieved using the multilayer microstrip slot transition.

Wei et al.¹⁸ use a similar elliptical slot coupled coupler, however a multi-section Wilkinson replaces the multilayer PD. The two components are integrated with an impedance controlled via having a fractional bandwidth of 190 percent. The complete network operates from 0.5 to 4 GHz.

Stripline multi-section cascaded design:

Li et al.²⁵ demonstrate a wideband six port network operating from 2 to 8 GHz. The use of broadside coupled multi-section stripline couplers²⁶ yields a very wide bandwidth. A four section Wilkinson power divider achieves a wider bandwidth in suspended stripline while providing low dissipation, good temperature performance and batch consistency. The 3 dB quadrature couplers are realized using two 8.34 dB couplers in series using coupled stripline technology. A via hole is provided to connect one of the output ports of divider with a bottom broadside coupled stripline layer. The multi-section coupler design includes a 7-section quarter wave transformer, which also improves bandwidth. Individual components transmission and reflection coefficients are measured,²⁶ and measurements of the integrated six port network are in Table I. This design provides a multioctave bandwidth at the cost of larger size. The network is shown in the Figure 7.

Rectangular meandering:

Winter et al.²⁷ propose a multilayer six port design based upon a rectangular meandering (zig-zag pattern) where the size can be reduced by 79 percent, making MMIC implementation possible. Like Abbosh and Bialkowski,²² the signal layers are on the top and bottom with ground in the center. Twelve vias are incorporated at the input and output ports to connect the top and bottom signal layers. The strength of this design in comparison with others is its compactness with final dimensions of (0.025 x 0.19) λ at 2 GHz.

Topology 2 (power dividers and 90 degree phase shifters)

In this topology, the six port circuit is realized using power dividers and 90 degree phase shifters; however, the signal must travel through three components, which increases theoretical insertion loss to 9 dB.

The single layer substrate design in the following not only provides ease of fabrication but also eliminates the alignment related errors inherent in multilayer designs.
90 degree phase shift by vertical microstrip to co-planer waveguide (CPW) transition:

Ibrahim et al.²⁸ use only in-phase and quadrature phase Wilkinson power dividers versus the conventional topology. Wide bandwidth from 3.5 to 9 GHz is achieved using dual section vertical interconnect microstrip to coplanar waveguide (CPW) transitions as 90 degree phase shifters.²⁹ This single layer approach eliminates a matched termination, which is present in other designs. The top and bottom layers can be seen distinctly in Figure 8. The authors use six in-phase and four quadrature phase power dividers. Each signal passes through two dividers and one combiner.

A broad band 90 degree phase shift is achieved with a dual vertical interconnect microwave to CPW transition.²⁹ This uses two pairs of broadside coupled elliptical sections on the top and bottom layers. The bottom layer sections are connected by a small strip of microstrip line. The differential phase shift can be controlled by tuning the minor axis of the broadside coupled elliptical sections. As previously mentioned, the downside of this design is greater insertion loss which is reported to be 11 +2 dB.

Ground slotted technique:

Yusof et al.³⁰ use the ground slotted technique in a single board to achieve ultra-wide bandwidth from 3.1 to 10.6 GHz. The six port network is designed using four in-phase two section power dividers and two 90 degree power dividers. A 90 degree phase shift is achieved using two parallel coupled microstrip lines on the top layer and a corresponding zig-zag pattern slot in the ground layer. Double section in-phase power dividers with rectangular slots in the ground plane further enhance the bandwidth (see Figure 9). This achieves the same ultra-wide bandwidth as the slot coupled approach, however it has a greater insertion loss of 10 +2 dB, limiting its use in high sensitivity applications.

Topology 3 (Four QHCs and 90° phase shifter)

In this topology, the six port circuit is realized using four QHCs and a 90 degree phase shifter.

Single layer design
Ring hybrid coupler design:

A new beam direction finding circuit based on six port technology employs four ring QHCs along with an integrated delay line quadrature phase shifter to realize a six port network fabricated using miniaturized hybrid microwave integrated circuit (MHMIC) technology.^31-33 The complete setup provides multiband operation at 2.45, 5.8 and 9.4 GHz. The network is simulated in ADS and fabricated. The results include I,Q signal graphs versus phase difference between two incident RF signals and the angle of arrival. The authors describe how a quadrature signal increases from zero to maximum with the increase in angle of incidence. Hence, an alternate trivial method of finding DOA has been suggested by just monitoring the quadrature signal.

Multilayer design
Microstrip slot coupled phase shifter:

Moscoso et al.³⁴ use four multilayer MSDCs along with a slot coupled double section quadrature phase shifter, which is the prominent feature of this design. The authors claim that the use of four identical MSDCs help to cancel most of the structure’s intrinsic phase errors. This claim is supported by a measured phase imbalance of just 2.5 degrees, which is due solely to the quadrature phase shifter.

Operational enhancements in six port design

This article focuses primarily on direction finding, so the operational enhancements discussed below identify improvements in six port network design for DF systems.

Dual angle detection:

Hussain and Sharawi³⁵ describe a dual channel six port network for angle of arrival detection in both azimuth and elevation employing a planar antenna array with four patch antennas in a square configuration. It has a multilayer PCB design in which the top and bottom layers are used for channels 1 and 2 of a six port network. It is designed to operate from 1.68 to 2.25 gHz with a bandwidth of 570 MHz. The authors selected a Topology 3 six port design using hybrid ring quadrature couplers and a 90 degree delay line phase shifter. A compact size of (0.43 x 0.43 x 0.01)λ at 2 GHz achieves a dual angle capability utilizing a multilayer architecture.

As disclosed in a US patent, Sharawi et al.³⁶ use a similar architecture for a direction finding system using a multilayer PCB with a four board vertical stack. Two six port networks are printed on the inner two and bottom layer for azimuth and elevation angle detection respectively, while four planar patch antennas are on the top layer. Horizontal pairs are used for azimuth angle detection and vertical pairs for elevation angle detection. The system is designed for multiband operation. It gives satisfactory performance at 1.8, 2.1 and 5.8 GHz over a bandwidth of 60 MHz at each spot frequency.

Wide FOV:

Vinci et al.³⁷ use the concept of a dual six port network to counter the angle ambiguity problem. It consists of two identical six port receivers connected with two different pairs of antenna elements with unequal distances between them. The shorter distance (less than λ/2) is ideal for unambiguous angle measurement. The unambiguous FOV is greater in this case; however, angular resolution and accuracy are compromised. Greater distance between the antenna elements reduces the unambiguous FOV, but it increases angular resolution and accuracy.

Vinci et al. (see Figure 10) explain the advantage of the dual six port network with dual baselines. Individually θ₁ (for antenna distance d₁) and θ₂ (for antenna distance d₂) have limited periodicities or FOV. However, their difference function θ₂-θ₁ has a longer period (-80° to +80°), extending the unambiguous FOV.

Mmwave band designs:

Koelpin et al.⁵ describe a six port receiver design in the Industrial, Scientific and Medical (ISM) bands at 24 and 61 GHz. A dual six port at 24 GHz simultaneously achieves a wide FOV and good angular resolution.

Vinci et al.³⁸ describe single/dual six port based DOA detection systems at 24 and 77 GHz. The 77 GHz system is designed specifically for misalignment angle detection in automotive radars. The system is designed with an integrated silicon germanium (SiGe) six port receiver and a passive antenna network. Its FOV is limited to 10 degrees. The maximum error in DOA detection is less than 0.03 degrees.

Askari and M. Kamarei¹⁰ describe the possible application of six port receivers in a 5 G envi- ronment. Their six port receiver is assessed between 21 and 30 GHz at data rates up to 6.7 Gbps. Testing with UWB orthogonal frequency division multiplexing (OFDM) and quadrature amplitude multiplexing (QAM) input signals is claimed to provide satisfactory phase constellation diagrams over the wide bandwidth.

DSP Calibration:

The six port architecture cancels some of the noise, parasitics and DC offsets due to the differential amplifiers that provide I and Q signals; however, calibration of the whole network in the DSP domain is essential due to the amplitude and phase imbalances created inside the six port network. Different calibration techniques such as calibration of baseband voltages, calibration in the I/Q domain and detector linearization have been discussed.³⁹ Linz et al.⁴⁰ use curve fitting as a tool for I/Q gain compensation using simple analytical formulas. A few research efforts^{41, 42} define calibration parameters based on Fourier analysis in which a variable phase shifter is used and then Fourier series coefficients based on periodic variations of output power ratios are computed. Using these parameters and measured power ratios, the desired incident complex signal ratios are determined.

OPEN ISSUES AND FUTURE RESEARCH

Six port networks perform signal processing in the analog domain using passive microwave components with little dependency on the DSP. Hence a simple microcontroller (e.g. Arduino) based solution would suffice and reduce overall system cost and complexity.

This article classifies various broadband six port design techniques; nevertheless, a broadband power detector circuit is equally significant in ensuring the overall instantaneous bandwidth and dynamic range of the receiver chain. This can be realized with a reactive broadband matching network at the six port output. Integrated RF power detectors provide greater sensitivity, dynamic range and stability over temperature versus classical diode-based solutions. Similarly, frequency independent antennas such as planar sinuous or spiral can be integrated with the six port network for a multi-octave DF solution.

Angular accuracy/resolution is the most critical performance parameter in a DOA detection system, however, analysis on this criterion has remained a neglected area in most of the six port based DF papers. This can be done with a test setup containing rails for linear displacement of the transmitting antenna.

UWB has been achieved using multilayer and multi-section designs from 2 to 11 GHz. Ku band (12 to 18.5 GHz) may also be covered using a separate six port channel operating in the upper band. As six port receivers are portable and better meet SWaP constraints they find applications in airborne intercept platforms as well; however, the antenna assembly and integrated six port receiver should be located at the wingtips or at places where reflections and multipath is minimized.

Six port based DF systems should be tested in high density signal environments with multiple incident simultaneous signals and evaluated against different radar modulation schemes such as LFM and Barker coding. Existing six port DF systems are typically tested in ideal noise free environments at very short ranges. Long range and multioctave (2-18 GHz) performance will determine if they are suitable for military DF applications.

CONCLUSION

Six port design techniques are reviewed and classified with respect to design topology, single/multiple layers and prominent design features. Designs are evaluated in terms of bandwidth, complexity and compactness. The applicability of six port circuits to direction finding is presented both analytically and from a practical perspective. It is concluded that a microstrip slot transition approach provides a very wideband and compact solution; however, the multilayer architecture and transition designs add complexity.

Once calibrated for static offsets and I/Q errors, a six port network can provide the very high angular accuracy and fine resolution required of DOA detection systems. The six port phase comparator has a limited unambiguous FOV, however it can be extended using dual six port channels used with unequal baselines. Similarly, dual angle detection (azimuth & elevation) is also possible using planar antenna arrays integrated with dual six port circuits. Hence the six port network offers a low cost, compact, precise and broadband DF solution as compared to DSP based systems.

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