Nonreciprocal structures introduce different responses under the change of the direction of the incident field. Such nonreciprocal platforms are one of the key elements of modern full-duplex wireless telecommunication systems. The advent of metasurfaces over the past decade has resulted in remarkable advances to ultra compact nonreciprocal apparatuses. Conventionally, ferrite-based magnetic materials have been used for nonreciprocity realization. Such magnetic materials, however, are cumbersome, costly, incompatible with printed circuit board technology, and unsuitable for high frequencies. In our recent work, we have proposed an architecture in which a metasurface is formed by a set of active and nonreciprocal chains. The proposed nonreciprocal reflective metasurface represents an original and efficient nonmagnetic apparatus for full-duplex nonreciprocal beamsteering and amplification (Figure 1a).

Our work has answered two key questions: (i) How to create a reflective beamsteering metasurface? (ii) How can nonmagnetic non-reciprocal signal amplification be achieved?

(i) How to create a reflective beamsteering metasurface?

We propose a reflective surface comprising chains of cascaded supercells. Each chain of this metasurface is composed of cascaded supercells. These supercells are formed by a set of gradient reciprocal phase shifters, unilateral transistors, and single-fed and double-fed microstrip radiating patch elements (Figure 1b). The patch antenna elements are double-fed microstrip patch antennas to allow the flow of the reflection of power in the desired direction inside the metasurface. However, the first and last patch antenna elements are single-fed patches. 

In the forward problem, the incoming wave from the right side impinges on the chains of the metasurface under an angle of incidence which is inside the reception beam of the metasurface. The reception beam of the metasurface is determined by the gradient phase shifters in each supercell. Hence, the wave is received by the chain, acquires a power gain and is reflected at the desired angle of reflection, instead of the specular reflection angle as in conventional reciprocal reflective surfaces. In contrast, in the backward problem, the incoming wave from the left side impinges on the metasurface under an angle of incidence which is outside the reception beam of the metasurface. Therefore, the wave is not received by the metasurface and is reflected without significant reflection gain or with loss. 
The metasurface integrates a dielectric layer sandwiched between two conductor layers. The bottom conductor layer acts as the ground plane of the patch antenna elements and also includes the direct current (DC) signal path of the unilateral circuits. The top conductor layer includes patch antenna elements, transistors, and phase shifters. The dielectric layer separates the two conductor layers from each other. The transistor radio frequency (RF) circuit includes two decoupling capacitors, and the DC biasing circuit of the transistor includes a choke inductor, two bypass capacitors and one biasing resistor. A DC signal biases the transistors to create a gradient nonreciprocal phase shift profile.

(ii) How can nonmagnetic non-reciprocal signal amplification be achieved?

Our study showed that the gradient phase shifters can be utilized in a reflective metasurface for efficient beamsteering. These gradient phase shifters specify the angles of incidence and reflection. Integration of such gradient phase shifters with unilateral transistors yields a controllable nonreciprocal phase shifter. As a result, this ultra-thin reflective metasurface can provide directive and diverse radiation beams, large wave amplification, steerable beams by simply changing the bias of the gradient active nonmagnetic nonreciprocal phase shifters, and is immune to undesired time harmonics. Having accomplished all these functionalities in the reflective state, the metasurface represents a conspicuous apparatus for efficient, and controllable wave processing. The frequency bandwidth of this non-reciprocal reflective surface can be further enhanced via engineering approaches for the bandwidth enhancement of patch radiators. Figures 2a to 2d plot the experimental results for full-duplex beam steering functionality of the metasurface for wave incidence from different angles of incidence.

The proposed metasurface can be mounted on a wall or on a smart device in a seamless way. Such surfaces are capable of massive MIMO beam-forming, as no excessive RF feed lines and matching circuits are required. Additionally, the metasurface functionality and operation can be fully controlled and programmed via biasing of unilateral devices and phase shifters, as well as tunable patch radiators. Highly directive and reflective full-duplex nonreciprocal-beam operation is a very promising feature of the proposed metasurface to be utilized for a low-cost high capability and programmable wireless beam-forming. The metasurfaces can become the core of an intelligent connectivity solution for signal enhancement in WiFi , cellular, satellite receivers and IoT sensors. It provides fast scanning between users while providing full-duplex multiple access and signal coding.