Читать книгу Advanced Antenna Array Engineering for 6G and Beyond Wireless Communications - Richard W. Ziolkowski - Страница 28

As frequency resources become more and more scarce, the issue of spectral efficiency has become a top priority for future generations of wireless communication systems. Consequently, in‐band full‐duplex (IBFD) radios are widely regarded as a key technology for the evolution of 5G and 6G systems. IBFD radios allow signal transmission and reception in the same frequency band and at the same time [32]. IBFD radios can double the data rate without using more frequency bands or more time, thus resulting in unprecedented spectrum efficiency enhancement. However, one major issue existing in full‐duplex radios is the suppression of in‐band self‐interference between the transmitters and receivers caused by mismatching at their ports, the mutual coupling between their antenna elements, and the scattering from objects in the environment in which they actually must work.

To realize a practical IBFD radio, the self‐interference from the colocated transmitter must be canceled first as it is typically much stronger than the intended received signal. For IBFD communication systems to operate, it usually requires more than 110–130 dB isolation between the transmitter and the receiver [33]. However, to cancel the self‐interference in IBFD systems satisfactorily, one needs a three‐stage solution in the antenna domain (or propagation domain), the analog domain, and the digital domain.

Clearly, no digital circuits can operate without adequate isolation and appropriate cancelations in the antenna and analog domains to bring the signal‐to‐noise‐and‐interference ratio down to an acceptable level. To this end, major efforts have been made in analog cancelation methods using adaptive circuits and antennas [34–37]. Reported antenna solutions aim to increase the isolation between the transmitter and receiver ports by virtue of spatial and polarization separation, use of metamaterials, and beam squinting. In principle, an ideal solution would be a combination of antenna‐decoupling techniques to be discussed in Chapter 3 and self‐interference cancelation circuits. Major challenges facing antenna researchers and engineers are wide bandwidth, limited antenna space, and low‐loss circuit designs.