Holographic beamforming to moving users: distributed antenna systems versus reflective intelligent surfaces

Instead of conventional beamforming, modern multi-antenna systems focus the energy in three dimensions on the desired users while creating zeros at locations where the transmitted signal is undesired or where interferers are located. On the other hand, massive MIMO systems achieve such focusing by deploying a massive number of antenna elements at the base station and by relying on multipath propagation, making such setups attractive in outdoor communication scenarios. On the other hand, distributed antenna systems enable focusing in indoor environments while also countering shadowing and blocking by surrounding users by a large number of coherently fed antennas interconnected by an optical backbone integrated in the infrastructure. Moreover, both massive MIMO and distributed antenna systems may be supplemented with intelligent reflective surfaces to improve coverage in difficult to reach areas.

Yet, correct focusing the desired signals while reducing interference remains challenging for moving users. Given that the coherence time of the wireless channel is inversely proportional to the operating frequency, the challenge of real-time array processing and signal generation becomes even more daunting when moving towards the millimeter wave and terahertz frequency bands, as envisaged in beyond-5G wireless communication systems.

This PhD project aims to develop and experimentally validate novel real-time array processing and signal generation for focusing signals on desired users while creating zeros to unintended or interfering users. Therefore, first reliable channel estimation is required between the different users and all the antenna elements forming the distributed antenna system. The developed algorithms should be implemented such that they converge in a time much smaller than the channel coherence time. Next, based on the estimated channel, the antenna weights should be chosen such that each transmitted data signal is focused onto the intended user while being suppressed at all other users. After thoroughly testing the real-time algorithms for channel detection and array processing, they should be implemented on the distributed antenna system testbed available within imec. This will enable testing the new formalism in different realistic scenarios involving mobile users.

Once the focusing technique for mobile users has been successfully demonstrated, we will investigate how to further improve the coverage of the system by adding intelligent reflective surfaces. Based on the interest of the candidate, these can be designed and developed in-house or one can rely on commercially available prototypes. Research will focus on the interaction between the distributed antenna system and the intelligent reflective surface. This will require extending the channel estimation algorithm and adapting the array processing algorithm to optimally cooperate with the training algorithm for the reflective surface.