The multiport circuit theory was initially developed in the 1970s by scientists for accurate automatized measurements of the complex reflection coefficients, in microwave network analysis [
Since 1994, the multiport techniques were further developed for microwave and millimeter-wave radios [
Basically, the multiport is a passive circuit composed of several couplers interconnected by transmission lines and phase shifters. Its specific architecture and design are strongly related to the target application and the operating frequency. The multiport acts as an interferometer; its output signals are linear combinations of phase shifted input signals. By using the appropriate circuit design and appropriate devices connected to the output ports, this circuit can provide specific parameters, such as reflection coefficient, distance or modal measurements, phase and frequency analysis, quadrature down-conversion, or direct modulation of microwave/millimeter-wave frequencies.
As originally designed for automated measurements of the complex reflection coefficient, the multiport has a local oscillator input, a measurement port, and four outputs [
The new application fields require a different architecture of the circuit and specific modules to be connected at its ports. The S-parameter matrix of the multiport circuit reveals that there are two clusters of ports, 1 to 4 and 5 and 6. Inside each cluster, all the ports are perfectly matched and isolated, one versus the others [
The use of multiport technology in RF design is a good choice, especially if the operating frequency is in the high microwave or millimeter-wave range. The dimensions of the multiport circuit fabricated in miniature hybrid microwave integrated circuit (MHMIC) technology, usually around
This special issue highlights, through several examples and multiple references, some of the modern applications of the multiport technology and significant advances in fabrication procedures, in the recent years.
This special issue dedicated to multiport technology hosts several papers covering last advances in six-port receivers, demodulators, radar sensing, and ultrawide band (UWB) phase noise measurements.
An interesting question in multiport technology is how many ports should be used to fulfill a given system specification. In their paper entitled “
Finally, the implementation of all architectures is demonstrated; furthermore, detailed measurement results are presented. These results indicate that TPD outperforms FPD in terms of residual DC offset, IMD2, noise figure, and sensitivity as well as error vector magnitude. Moreover, TPD has advantages over quadrature demodulators in the case of RF carrier aggregation. Thus, the authors conclude that the presented TPD architecture is a potential candidate for future long term evolution (LTE-A) standard.
The use of the 60-GHz band has attracted a great deal of interest over the last few decade, especially for its use in future compact transceivers dedicated to high-speed wireless applications in indoor environments (57–64 GHz).
In this context, C. Hannachi et al., the authors of the paper “
The work is focused on the design and characterization of a novel rounded Wilkinson power divider, a 90° hybrid coupler, a rat-race coupler, and a novel six-port circuit. The authors describe in detail the equipment, the calibration technique, and the multiple challenges in millimeter-wave measurements. A series of microphotographs show a typical fabricated ceramic die and details of all circuits, as prepared for probe station testing.
Measurement results prove that the proposed circuits are UWB components. The supplementary insertion losses, amplitude, and phase unbalancements are considered more than acceptable to build modulators/demodulators for modulation schemes having up to 16 symbols (BPSK to 16 QAM, PSK, or dual star). Keeping in account the 7 GHz bandwidth allowed for V-band communication systems, the data rates can reach quasioptical values.
Apart from its ease of use, simplicity of implementation, and low power requirements, which are due to the passive circuit implementation nature of six-port based receiver, other benefits drove interests in using six-port technology in receivers’ design for applications using lower frequencies than millimetre wave. Indeed, the wide frequency bandwidth coverage, configurability, and low cost are some of the additional features that make this architecture very attractive for the implementation of the software defined radio (SDR) concept in the wireless communication frequency bands. The utilization of six-port concept as a RF front end in communication receivers was investigated by A. Hasan and M. Helaoui; in their paper “
Besides the broad range of applications already introduced in this paper, another very promising topic is ranging and direction finding. In particular for industry grade distance measurements, the six-port operated as a microwave interferometer shows extraordinary performance, compared to common radar architectures.
In the paper entitled “
The authors present a compact 24 GHz six-port radar sensor with reliable high-integration of all subcomponents in one single module. This sensor has been developed in cooperation with industry. After a general system overview and the theoretical description of the implemented six-port network, a short introduction to the digital signal processing strategy is given. Simulation results of the subcomponents are furthermore validated by measurements. The demonstrated results of the system’s performance show a maximum relative error of
J. M. Ávila-Ruiz et al. propose in their paper new six-port for low-power oscillator phase noise measurements. This paper entitled “
The presented phase noise measurement system combines two phase noise measurement approaches, that is, the delay line discriminator method and the phase noise measurement method based on the quadrature
After briefly introducing the state-of-the-art phase noise measurements, the authors provide a detailed theoretical analysis that demonstrates the complex processing of signals performed by the multiport system. It is shown that the required phase noise information is embedded in the quadrature
The results presented by J. M. Ávila-Ruiz et al. reconfirm the multiport technology in its original application proposed in the 1970s as an accurate, flexible measurement set-up based on interferometric signal processing.
In the last 40 years, since the multiport (six-port) circuit is used, dozens of architectures have been proposed. The operating frequencies, fabrication technologies, and applications do not cease to evolve. Through several well-chosen examples, the special issue highlights some of the multiport technology modern applications.
These papers and related references demonstrate that multiport technology is a good competitor of the conventional approaches, especially in the microwave and millimeter-wave frequencies, where circuit dimensions, closely related to the guided wavelength, are in the cm range or smaller. Broadband circuits can be designed to cover a wide frequency spectrum and to allow quasioptical data rates for wireless communication. Impressive resolution and high accuracy can be obtained in range or angle-of-arrival measurements. Calibration and impairment mitigation techniques can also be used to improve demodulation results, especially in low microwave spectrum, where analog to digital convertors (ADC) and digital signal processing (DSP) techniques are available today.
Future work targets, among others, improvements in multiport circuit design for each specific application, the increasing of the operating frequencies, and, why not, the use of multiport systems in THz frequency range.