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.

f10a.jpg

f10b.jpg

Figure 10 Extension of FOV using dual six port design: θ1 and θ2 (a), θ2-θ1 (b).37

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 efforts41, 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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