Real full-duplex for simultaneous transmission and reception of radio signals

Almost all wireless system technologies use the scarce resource of the frequency spectrum to fulfill two typically bidirectional tasks. In cooperative communications the tasks are sending and receiving transmissions, for instance, while in non-cooperative communications the tasks may be reconnaissance and jamming. Just as it is difficult for a person to listen to the soft whisper of a distant speaker while shouting something to him at the same time, developers of wireless system technologies also face major challenges in performing both of these tasks truly simultaneously. For a long time, this was even considered technically impossible.

Classic approaches are therefore based on a strict separation of these two tasks at the antenna, even if the user perceives them as being performed simultaneously. The reality at the antenna, however, is that the two tasks are either being processed sequentially over time (time-division multiplex), or a separate frequency is used for each task (frequency-division multiplex).

Obviously, as a scarce resource, the frequency spectrum could be used much more efficiently (by a factor of 2, at least) if real simultaneity in the performance of both tasks were also possible at the antennas of wireless system technologies. Practical solutions to this challenge are becoming increasingly important, not least due to the advancement of digitalization in every aspect of our lives and the associated increasing need to exchange more and more data via wireless system technologies.

© Fraunhofer FKIE
Fig. 1: Disruptive self-interference components in a wireless communication system

Figure 1 illustrates the essential scientific challenge involved in real simultaneous execution of two bidirectional tasks at an antenna.

The figure illustrates two communication systems. The upper area shows signal processing in the transmit path and the lower area shows the corresponding processing in the receive path. Some signal processing usually takes place in the digital domain (outside of the dashed line) while some occurs in the analog domain (inside the dashed line). In both communication systems, the signal processing paths are combined by means of a so-called circulator and flow into a one and the same antenna.

The transmit signals are emitted by the antennas while the signal from the respective distant node is received at the same time.

Due to typical propagation attenuation, signals are received very weakly. At the same time, various disruptive self-interference components are superimposed onto the weak received signal in the receive path. This self-interference is usually not only significantly stronger in terms of power than the desired receive signal, but it also contains a great deal of linear and non-linear distortion. Due to these phenomena, researchers long considered it technically impossible to extract the weak signal of interest from the distorted composite signals so as to enable error-free receive-signal processing.

© Fraunhofer FKIE
Fig. 2: Analog (AC) and digital (DC) measures to compensate for disruptive self-interference components

The literature of recent years has presented some very promising approaches with the potential to yield a technical solution. These approaches are usually based on a two-stage process aimed at compensating for disruptive self-interference components.

The first stage has to take place in the analog domain due to the very different power levels of the disruptive self-interference components and the weakness of the desired receive signal. In the analog domain, one proposed method of compensation is to develop a circuit network in the carrier frequency consisting of a grid of delay lines, each with digitally controllable attenuators. Figure 2 shows the position of the analog compensation circuit (in green), including digital control of the attenuators.

Finally, suitable signal processing algorithms can be applied to further reduce the remaining disruptive self-interference components in the composite signal of the receive path following the analog compensation. The position of the digital compensation software is shown in light green in Figure 2.

These two compensation stages reduce the original self-interference signal sufficiently to enable the extraction of the weak, desired receive signal from the disrupted signal composite to enable error-free processing of the receive signal.

In the literature, wireless system technologies that apply measures aimed at utilizing the scarce resource of the frequency spectrum to perform two bidirectional tasks are referred to as »full duplex« (FD) and/or »same frequency simultaneous transmit and receive« (SF-STAR) technologies.

© Fraunhofer FKIE
Fig. 3: Circuit for analog compensation of self-interference in the 2.4 GHz frequency band
© Fraunhofer FKIE
Fig. 4: Experimental system at the FKIE Technology Forum 2018

In recent years, a technology demonstrator was developed at Fraunhofer FKIE enabling a proof of concept and live demonstration of the new possibilities promised in the literature. For this purpose, a complete wireless communication system based on software defined radio (SDR) technology was set up. While the SDRs and other hardware components, such as the circulators and antennas, are commercial off-the-shelf (COTS) components, all of the software and the analog compensation circuit are in-house developments.

Figure 3 shows the resulting analog compensation circuit specifically developed for the 2.4 GHz frequency band and approximately 100 mW transmission power, as well as two auxiliary circuit boards for measuring purposes.

The demonstrator was presented to the general public for the first time at the 2018 FKIE Technology Forum. The focus of the demonstration was on real, simultaneous bidirectional communication between two participants on the same frequency. The setup on display at the FKIE Technology Forum 2018 is shown in Figure 4.

Since these initial successes, all further research activities in this field have dealt with scientific questions that serve to increase the relevance of the technology for practical use, especially in the fields of defense and security. Pursuing practical applications is aimed at producing technology that is more mature in terms of aspects such as mobility (rapid adaptation when moving), frequency agility (low VUHF ranges below 500 MHz), transmission power (several watts), multi-node networks (relaying), etc.