27,593 research outputs found

    Fluid Antenna System—Part III: A New Paradigm of Distributed Artificial Scattering Surfaces for Massive Connectivity

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    Reconfigurable intelligent surface (RIS) has recently emerged as a promising technology to extend the coverage of a base station (BS) in wireless communication networks. However, the adoption of RIS comes with the challenges of highly complex joint optimization of the multiple-input multiple-output (MIMO) precoding matrix at the BS and the phase shifters of the RIS as well as estimation of the cascaded channels. To circumvent this, this letter presents a new paradigm that uses RISs as distributed artificial scattering surfaces (DASSs) to produce a rich scattering environment that enables fluid antenna system (FAS) to prevent multiuser interference at each user equipment (UE). The use of fluid antenna multiple access (FAMA) liberates MIMO and RIS and greatly simplifies their optimization. Our simulation results show that with DASS, slow FAMA can obtain a high multiplexing gain without precoding and phase shifter design when the direct link does not exist. In the presence of the direct link, nonetheless, BS precoding becomes essential. Our results further reveal that fast FAMA with 20 DASSs can accommodate 64 co-channel UEs to achieve a multiplexing gain of 59.3 without precoding at the BS nor RIS phase shifter optimization and the direct link

    Dispensing with channel estimation: differentially modulated cooperative wireless communications

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    As a benefit of bypassing the potentially excessive complexity and yet inaccurate channel estimation, differentially encoded modulation in conjunction with low-complexity noncoherent detection constitutes a viable candidate for user-cooperative systems, where estimating all the links by the relays is unrealistic. In order to stimulate further research on differentially modulated cooperative systems, a number of fundamental challenges encountered in their practical implementations are addressed, including the time-variant-channel-induced performance erosion, flexible cooperative protocol designs, resource allocation as well as its high-spectral-efficiency transceiver design. Our investigations demonstrate the quantitative benefits of cooperative wireless networks both from a pure capacity perspective as well as from a practical system design perspective

    Fronthaul-Constrained Cloud Radio Access Networks: Insights and Challenges

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    As a promising paradigm for fifth generation (5G) wireless communication systems, cloud radio access networks (C-RANs) have been shown to reduce both capital and operating expenditures, as well as to provide high spectral efficiency (SE) and energy efficiency (EE). The fronthaul in such networks, defined as the transmission link between a baseband unit (BBU) and a remote radio head (RRH), requires high capacity, but is often constrained. This article comprehensively surveys recent advances in fronthaul-constrained C-RANs, including system architectures and key techniques. In particular, key techniques for alleviating the impact of constrained fronthaul on SE/EE and quality of service for users, including compression and quantization, large-scale coordinated processing and clustering, and resource allocation optimization, are discussed. Open issues in terms of software-defined networking, network function virtualization, and partial centralization are also identified.Comment: 5 Figures, accepted by IEEE Wireless Communications. arXiv admin note: text overlap with arXiv:1407.3855 by other author
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