Establishing Multi-User MIMO Communications Automatically Using Retrodirective Arrays

Communications in the mmWave and THz bands will be a key technological pillar for next-generation wireless networks. However, the increase in frequency results in an increase in path loss, which must be compensated for by using large antenna arrays. This introduces challenging issues due to power consumption, signalling overhead for channel estimation, hardware complexity, and slow beamforming and beam alignment schemes, which are in contrast with the requirements of next-generation wireless networks. In this paper, we propose the adoption of a retro-directive antenna array (RAA) at the user equipment (UE) side, where the signal sent by the base station (BS) is reflected towards the source after being conjugated and phase-modulated according to the UE data. By making use of modified Power Methods for the computation of the eigenvectors of the resulting round-trip channel, it is shown that, in single and multi-user multiple-input multiple-output (MIMO) scenarios, ultra-low complexity UEs can establish parallel communication links automatically with the BS in a very short time. This is done in a blind way, also by tracking fast channel variations while communicating, without the need for ADC chains at the UE as well as without explicit channel estimation and time-consuming beamforming and beam alignment schemes

Some new research trends in wirelessly powered communications

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Retrodirective Antenna Array Approach to Achieve Maximum Theoretical Beam Efficiency in Microwave Wireless Power Transfer

Efficient long range wireless power transfer (WPT) is realized if the distance between the source and receiver is less than the Fraunhoffer distance. This distance increases proportionally to the square root of the antenna size so to achieve efficient long range WPT, larger antennas are mandatory, but that comes with difficulty in implementing both the feeding network and beamforming control of the antenna. Several proposed implementations require power-hungry processors rendering implementation impractical. An alternative to reduce usage of digital processing is in the form of retrodirective antenna arrays. Its core operation is to track an incoming signal's direction of arrival and resend it to the same direction. This can be implemented by analog circuits. Retrodirective capability on both the generator and rectenna arrays creates a feedback loop that produces a high efficiency WPT channel. In this paper, we characterize the dynamics of this phenomenon using a discrete-time state-space model based on S-parameters and show that the system can naturally achieve maximum theoretical WPT efficiency. We further confirmed the theoretical analysis through a hardware experiment using a 12-port circuit board with measurable S-parameters mimicking a deterministic wireless channel. The results of the hardware experiment show agreement with the proposed theoretical framework.Comment: This work has been submitted to the IEEE for possible publication. Copyright may be transferred without notice, after which this version may no longer be accessible. 9 pages, 11 figures, 1 table, submitted to the IEEE Transactions on Theory and Techniques on September 22, 202

Cutting the last wires for mobile communications by microwave power transfer

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