Performance Limits of Fluid Antenna Systems

Fluid antenna represents a concept where a mechanically flexible antenna can switch its location freely within a given space. Recently, it has been reported that even with a tiny space, a single-antenna fluid antenna system (FAS) can outperform an L-antenna maximum ratio combining (MRC) system in terms of outage probability if the number of locations (or ports) the fluid antenna can be switched to, is large enough. This letter aims to study if extraordinary capacity can also be achieved by FAS with a small space. We do this by deriving the ergodic capacity, and a capacity lower bound. This letter also derives the level crossing rate (LCR) and average fade duration (AFD) for the FAS.Comment: 4 pages, 5 figure

Fluid Antenna Systems

Over the past decades, multiple antenna technologies have appeared in many different forms, most notably as multiple-input multiple-output (MIMO), to transform wireless communications for extraordinary diversity and multiplexing gains. The variety of technologies has been based on placing a number of antennas at fixed locations which dictates the fundamental limit on the achievable performance. By contrast, this paper envisages the scenario where the physical position of an antenna can be switched freely to one of the N positions over a fixed-length line space to pick up the strongest signal in the manner of traditional selection combining. We refer to this system as a fluid antenna system (FAS) for tremendous flexibility in its possible shape and position. The aim of this paper is to study the achievable performance of a single-antenna FAS system with a fixed length and N in arbitrarily correlated Rayleigh fading channels. Our contributions include exact and approximate closed-form expressions for the outage probability of FAS. We also derive an upper bound for the outage probability, from which it is shown that a single-antenna FAS given any arbitrarily small space can outperform an L-antenna maximum ratio combining (MRC) system if N is large enough. Our analysis also reveals the minimum required size of the FAS, and how large N is considered enough for the FAS to surpass MRC.Comment: 26 pages, 5 figure

Performance of Joint Channel and Physical Network Coding Based on Alamouti STBC

This work considers the protograph-coded physical network coding (PNC) based on Alamouti space-time block coding (STBC) over Nakagami-fading two-way relay channels, in which both the two sources and relay possess two antennas. We first propose a novel precoding scheme at the two sources so as to implement the iterative decoder efficiently at the relay. We further address a simplified updating rule of the log-likelihood-ratio (LLR) in such a decoder. Based on the simplified LLR-updating rule and Gaussian approximation, we analyze the theoretical bit-error-rate (BER) of the system, which is shown to be consistent with the decoding thresholds and simulated results. Moreover, the theoretical analysis has lower computational complexity than the protograph extrinsic information transfer (PEXIT) algorithm. Consequently, the analysis not only provides a simple way to evaluate the error performance but also facilitates the design of the joint channel-and-PNC (JCNC) in wireless communication scenarios.Comment: 6 pages, 4 figures, accpete

Surface Wave Technique for RIS Applications: Invited Paper

Increasing demands on high-performance platforms in the sixth generation (6G) of mobile communications raise research focus on new transducers that can address both the coverage and power issues. One of the key technologies recently adopted to design and implement such promising features is metasurface (MTS). MTSs are two-dimensional low profile artificial materials realized by electrically small meta cells. Providing particular impedance boundary conditions (BC), they have a variety of applications that extensively increased their usage during the last decade. The need for reconfigurability and having a dynamic electromagnetic response resulted in introducing the reconfigurable intelligent surfaces (RISs) as one of the new popular MTSs in recent years [1]

Fluid Antenna System—Part II: Research Opportunities

The promising performance of fluid antenna systems (FAS) relies on activating the optimal port to access the spatial opportunity for favourable channel conditions for wireless communications. This nevertheless can imply enormous overheads in channel estimation and signal reception as the resolution of fluid antenna could be arbitrarily high. There is also the challenge of optimizing jointly the selected ports and beamforming when FAS combines with multiple-input multiple-output (MIMO) systems. This letter discusses some of these obstacles in FAS. Moreover, we present several research opportunities that, if addressed properly, FAS could synergize with other enabling mobile technologies for meeting the requirements of the sixth generation (6G)

Compact Ultra Massive Antenna Array: A Simple Open-Loop Massive Connectivity Scheme

This paper aims to present a simple multiple access scheme for massive connectivity that enables a large number of mobile user equipments (UEs) to occupy the same time-frequency channel without the need of precoding and power control at the base station (BS) and interference cancellation at each UE. The proposed approach does not even require the UEs to know their signal-to-interference ratios (SIRs) and each UE also needs only two radio-frequency (RF) chains to operate. The proposed scheme is inspired by the emerging concept of fluid antenna system (FAS) which enables high-resolution position-switchable antenna to be deployed at each UE. Instead of activating only one port of FAS for reception, each UE activates an ultra massive number of ports to receive the signal. The activated ports are chosen to ensure that the in-phase and quadrature components of the desired signal at the ports are added constructively while the interference signals superimpose randomly. This approach is referred to as compact ultra massive antenna array (CUMA) which can also be realized by deploying a dense, fixed massive antenna array at each UE. We derive the exact probability density function (pdf) of the SIR of a CUMA UE which leads to the data rate analysis. Simulation results demonstrate that even with mutual coupling and under finite scattering, more than 10 UEs can be supported by having a 25×13-port FAS of size 15 cm×8 cm at each UE. Considering quadrature phase shift keying (QPSK), CUMA delivers a network data rate of 10.7 bps per channel use serving 10 UEs at 26 GHz, and the rate is risen to 15.1 bps per channel use if 20 UEs are accommodated at 40 GHz with a 40×21-port FAS at every UE. In the case without mutual coupling and under rich scattering, CUMA can even support hundreds of UEs per channel use

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

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

Fast Fluid Antenna Multiple Access Enabling Massive Connectivity

Massive connectivity over wireless channels relies on aggressive spectrum sharing techniques. Conventionally, this may be achieved by sophisticated signal processing and optimization of applying multiple antennas and/or complex multiuser decoding at each user terminal (UT). Different from previous methods, this letter proposes a radical approach for massive connectivity, which employs fluid antenna at each UT to exploit the interference null, created naturally by multipath propagation and the randomness of UT’s data, on a symbol-by-symbol basis for multiple access. The proposed fast fluid antenna multiple access (f-FAMA) system adopts a large, distributed antenna array at the base station (BS) to transmit each UT’s signal from each of the BS antennas and lets each UT overcome the interference on its own using its fluid antenna. Our main contribution is a technique that estimates the best port of fluid antenna for reception at every symbol instance. The proposed approach needs only cross-correlation calculations and single-user decoding at each UT and requires no precoding at the BS. Simulation results demonstrate that for a BS with 16 antennas supporting 16 co-channel users, a multiplexing gain of 14.87 is achieved even when the channel has a strong line-of-sight (LoS) and multipath is few. The multiplexing gain can also rise to 24.36 if a 30-antenna BS is serving 30 co-channel users