Modified conjugate beamforming for cell-free massive MIMO

We present a modification of conjugate beamforming for the forward link of cell-free massive MIMO networks. This modification eliminates the self-interference and yields a performance that, without forward pilots, closely approaches what would be achieved with such pilots in place. The simplicity of conjugate beamforming is preserved, with no need for matrix inversions, at the expense of fading-rate coordination among the access points.The work of A. Lozano was supported in part by MINECO/FEDER, UE under Project TEC2015-66228-P, and in part by the European Research Council through H2020 Framework Programme/ERC under Grant 694974

Random vs structured pilot assignment in cell-free massive MIMO wireless networks

This paper addresses the performance improvement that structured pilot assignment policies can bring about, relative to a random pilot assignment, in cell-free massive MIMO wireless networks. It is shown that structured policies can deliver a multiple-fold reduction in the pilot overhead required to keep pilot contamination at some acceptably low level. While the implementation of the structured policies considered in the paper might require some degree of centralized control, their performance also captures what distributed nonrandom pilot assignment schemes (e.g., greedy or collision-detecting algorithms) can hope to approach.The work of A. Lozano was supported by Project TEC2015-66228-P (MINECO/FEDER, UE) and by the European Research Council under the H2020 Framework Programme/ERC grant agreement 694974

Subset MMSE receivers for cell-free networks

This paper formulates linear MMSE receivers that are both network- and user-centric for the uplink of cell-free wireless networks with centralized processing. Precisely, every user’s reception involves a distinct subset of access points (APs) while every AP participates in the reception of a distinct subset of users, hence the moniker subset MMSE receivers . These subsets, defined on the basis of the large-scale channel gains between users and APs, capture the most relevant signal and interference contributions while disregarding those whose processing is cost-ineffective and whose associated channel estimations would incur unnecessary overheads. With that, subset reception approaches the performance of network-wide MMSE reception, offering a multiple-fold improvement over cellular and matched-filtering counterparts, while being scalable in terms of cost and channel estimation. Moreover, because the subsets overlap considerably, they can sometimes be advantageously combined and the computation of the corresponding receivers can share a hefty amount of processing.M. Attarifar and A. Abbasfar are with the University of Tehran, North Kargar St., Tehran, Iran (e-mail: [email protected]; [email protected]). Their work is supported by the Iranian National Science Foundation (INSF) Grant #98012057. A. Lozano is with Univ. Pompeu Fabra, 08018 Barcelona (e-mail: [email protected]). His work is supported by the European Research Council under the H2020 Framework Programme/ERC grant agreement 694974, and by MINECO’s Projects RTI2018-102112 and RTI2018-101040

Random vs structured pilot assignment in cell-free massive MIMO wireless networks

This paper addresses the performance improvement that structured pilot assignment policies can bring about, relative to a random pilot assignment, in cell-free massive MIMO wireless networks. It is shown that structured policies can deliver a multiple-fold reduction in the pilot overhead required to keep pilot contamination at some acceptably low level. While the implementation of the structured policies considered in the paper might require some degree of centralized control, their performance also captures what distributed nonrandom pilot assignment schemes (e.g., greedy or collision-detecting algorithms) can hope to approach.The work of A. Lozano was supported by Project TEC2015-66228-P (MINECO/FEDER, UE) and by the European Research Council under the H2020 Framework Programme/ERC grant agreement 694974

Modified conjugate beamforming for cell-free massive MIMO

We present a modification of conjugate beamforming for the forward link of cell-free massive MIMO networks. This modification eliminates the self-interference and yields a performance that, without forward pilots, closely approaches what would be achieved with such pilots in place. The simplicity of conjugate beamforming is preserved, with no need for matrix inversions, at the expense of fading-rate coordination among the access points.The work of A. Lozano was supported in part by MINECO/FEDER, UE under Project TEC2015-66228-P, and in part by the European Research Council through H2020 Framework Programme/ERC under Grant 694974

Spatial-wideband effect in line-of-sight MIMO communication

Comunicació presentada a IEEE International Conference on Communications (ICC 2022), celebrat del 16 al 20 de maig de 2022 a Seül, Corea del Sud.This paper addresses the spatial-wideband effect in line-of-sight multiple-input multiple-output channels. This effect arises once the bandwidth is large enough that the differences in propagation delays for distinct transmit-receive antenna pairs cease to be negligible relative to the symbol period; this, in turn, gives rise to intersymbol interference. The impact of this effect is quantified as a function of the relevant geometric parameters (range, array orientations, antenna spacings) and a family of scalable solutions is proposed to counter it. In particular, a solution based on per-antenna delay lines at transmitter and receiver is shown to be highly effective, and a criterion is derived to set those delay lines as a function of the channel.This work was supported by the European Research Council under the H2020 Framework Programme/ERC grant agreement 694974, by the ICREA Academia program, and by the UPF-Fractus Chair on Tech Transfer and 6G