The concept of fluid antennas (FAs) has emerged as a promising solution to enhance the spectral efficiency of wireless networks, achieved by introducing additional degrees of freedom, including reconfigurability and flexibility. In this paper, we investigate the use of index-modulated (IM) transmissions within the framework of FA systems, where an FA position is activated during each transmission interval. This approach is motivated by the common characteristics exhibited by FAs and IM transmissions, which entails the use of a single radio-frequency chain. From this perspective, we derive a closed-form expression for the bit error rate of IM-FAs considering spatial correlation, demonstrating superior performance compared to conventional IM systems. To enhance the performance of IM-FAs in correlated conditions, channel coding techniques are applied. We first analyze a set partition coding (SPC) scheme for IM-FAs to spatially separate the FA ports, and provide a tight performance bound over correlated channels. Furthermore, the spatial SPC scheme is extended to turbo-coded modulation where the performance is analyzed for low and high signal-to-noise ratios. Our results reveal that through the implementation of channel coding techniques designed for FAs and IM transmission, the performance of coded IM-FAs exhibits notable enhancements, particularly in high correlation scenarios.

Reconfigurability is a desired characteristic of future communication networks. From a transceiver’s standpoint, this can be materialized through the implementation of fluid antennas (FAs). An FA consists of a dielectric holder, in which a radiating liquid moves between pre-defined locations (called ports) that serve as the transceiver’s antennas. Due to the nature of liquids, FAs can practically take any size and shape, making them both flexible and reconfigurable. In this paper, we deal with the outage probability of FAs under general fading channels, where a port is scheduled based on selection combining. An analytical framework is provided for the performance with and without errors due to post-scheduling delays. We show that although FAs achieve maximum diversity, this cannot be realized in the presence of delays. Hence, a linear prediction scheme is proposed that overcomes delays and restores the lost diversity by predicting the next scheduled port. Moreover, we design space-time coded modulations that exploit the FA’s sequential operation with space-time rotations and code diversity. The derived expressions for the pairwise error probability and average word error rate give an accurate estimate of the performance. We illustrate that the proposed design attains maximum diversity, while keeping a low-complexity receiver, thereby confirming the feasibility of FAs.

In this paper, we investigate the use of index-modulated (IM) transmissions within the framework of fluid antenna (FA) systems, where an FA port is activated during each transmission interval. The adoption of this approach is motivated by the common physical characteristic exhibited by both FAs and IM transmissions, which entails the use of a single radio-frequency (RF) chain. From this perspective, we derive a closed-form expression for the bit error rate (BER) of IM-FA systems in the presence of spatial correlation. Our results demonstrate that IM-FAs outperform conventional IM systems. Since the FA ports are relatively close to each other, we focus on correlation mitigation techniques to improve performance. Specifically, we first investigate two port selection strategies, namely the maximum norm-based and the Euclidean distance-based selection schemes, assuming full channel knowledge. Then, we introduce the concept of spatial set partition coding for IM-FAs to spatially separate the FA ports. Numerical results demonstrate that the performance of IM-FAs is further improved in the case of high correlation whenever we apply the proposed correlation mitigation strategies.

A desired characteristic of future communication networks is the notion of reconfigurability. For a wireless device, this can be realized through the employment of the so-called fluid antennas (FAs). An FA consists of a dielectric holder, in which a radiating liquid moves between pre-defined locations (called ports) that serve as the device's antennas. Therefore, due to the nature of liquids, an FA can essentially take any size and shape, making them both flexible and reconfigurable. In this paper, we study the outage probability of FAs where the scheduled port, based on selection combining, is subject to scheduling delays. An analytical framework is provided for the performance with and without estimation errors, as a result of post-scheduling delays. We show that even though FAs achieve maximum channel (spatial) diversity, this cannot be attained in the presence of delays.

In this paper, we investigate the performance of multiple-input multiple-output (MIMO) fading channels assisted by a reconfigurable intelligent surface (RIS), through the employment of partition-based RIS schemes. The proposed schemes are implemented without requiring any channel state information knowledge at the transmitter side; this characteristic makes them attractive for practical applications. In particular, the RIS elements are partitioned into sub-surfaces, which are periodically modified in an efficient way to assist the communication. Under this framework, we propose two low-complexity partition-based schemes, where each sub-surface is adjusted by following an amplitude-based or a phase-based approach. Specifically, the activate-reflect (AR) scheme activates each sub-surface consecutively, by changing the reflection amplitude of the corresponding elements. On the other hand, the flip-reflect (FR) scheme adjusts periodically the phase shift of the elements at each sub-surface. Through the sequential reconfiguration of each sub-surface, an equivalent parallel channel in the time domain is produced. We analyze the performance of each scheme in terms of outage probability and provide expressions for the achieved diversity-multiplexing tradeoff. Our results show that the asymptotic performance of the considered network under the partition-based schemes can be significantly enhanced in terms of diversity gain compared to the conventional case, where a single partition is considered. Moreover, the FR scheme always achieves the maximum multiplexing gain, while for the AR scheme this maximum gain can be achieved only under certain conditions with respect to the number of elements in each sub-surface.

In this paper, we present a non-coherent spatial modulation (SM) detection scheme appropriate for low-cost low-powered devices, where channel knowledge is restricted to the magnitude of the fading gains. We first derive a low-complexity energy detection metric for the multiple receive-antenna case based on the maximum-likelihood criterion. Next, we investigate a biased (non-negative) pulse amplitude modulation design and develop an accurate analytical framework for the symbol error rate at high signal-to-noise ratio. We compare the performance of the proposed scheme with that of the optimal/coherent maximum-likelihood receiver design. Numerical results show that the non-coherent SM outperforms the coherent SM technique for scenarios where non-negative one-dimensional constellations are employed.

In this paper, a partition-based scheme is investigated for multiple-input multiple-output (MIMO) fading channels assisted by an intelligent reflecting surface (IRS). In this scheme, the IRS elements are partitioned into sub-surfaces, which are periodically selected to assist the communication. Through the sequential activation of each sub-surface, an equivalent parallel channel in the time domain is produced. The proposed scheme has low implementation complexity and does not require knowledge of the channel state information. We analyze the performance of the proposed scheme in terms of outage probability and provide an expression for the diversity-multiplexing tradeoff. We show, through theoretical and numerical results, that the asymptotic performance of the considered network under the partition-based scheme can be significantly enhanced in terms of diversity gain compared to the conventional case, where a single partition is considered. Moreover, both maximum diversity and multiplexing gain can be achieved under certain conditions with respect to the number of elements in each sub-surface.

In this paper, we study the fundamental limits of a simultaneous wireless information and power transfer communication system, which is facilitated by an intelligent reflecting surface (IRS). Specifically, we propose a low-complexity optimization scheme that optimizes iteratively the input probability distribution at the transmitter side and the phase shift coefficient at the IRS to enhance the information-energy capacity region. By using alternating optimization and Lagrangian decomposition, analytical expressions for the optimal input distribution and the phase shifts are derived which decrease the computational complexity of the proposed scheme. A novel joint optimization scheme is also proposed inspired by the one in [1] for SWIPT systems. Numerical results show that the iterative optimization scheme almost achieves the same performance in comparison with the proposed joint optimization scheme while ensuring a reduced computation for high energy-harvesting thresholds.

In this paper, coding techniques for the multiple-input multiple-output multi-hop amplify-and-forward (MIMO-MH-AF) quasi-static fading channel with perfect feedback are proposed. First, the diversity orders that can be achieved over this channel are investigated. The study is based on the partitioning of the channel onto several paths whose individual diversity order add to attain the maximum diversity carried by the channel. Next, we study the performance of space-time bit-interleaved coded modulation (ST-BICM), consisting of a coded modulation concatenated to a space-time rotation, over the MIMO-MH-AF quasi-static fading channel with perfect feedback. We propose upper bounds on the diversity orders that a ST-BICM can attain if a limited feedback from the destination to all the nodes is available. These bounds provide a tradeoff between attainable diversity, detector complexity, and coding rates. Finally, word error rate performances of several schemes with up to three bits of feedback are compared to outage probabilities, which represents the fundamental limit over quasi-static fading channels.

We study the performance of space-time bit-interleaved coded modulation (ST-BICM) over quasi-static multiple-antenna fading channels with linear receivers. We show that, under iterative linear detection and decoding, an ST-BICM can achieve full diversity with a special class of space-time precoders. We then study the outage probability at the output of the linear detector that determines the theoretical performance of coded modulations with such receivers. Finally, symbol and word error rate performances under Monte Carlo simulations are shown.

In this paper, we propose a cooperative multiuser system for multiple-antenna terminals under quasi-static fading. The system employs a bit-interleaved code modulation scheme that combines space-time coding and cooperation based on the decode-and-forward protocol at the transmitter and iterative multiuser detection based on parallel interference cancellation and minimum mean square error filtering at the receiver. We investigate an approach that allows to provide diversity from different channels to each user up to full diversity. Frame error rate performance under Monte Carlo simulations is shown to confirm the effectiveness of the proposed schemes.

We study irregular binary turbo codes over nonergodic block-fading channels. We first propose an extension of channel multiplexers initially designed for regular turbo codes. We then show that, using these multiplexers, irregular turbo codes that exhibit a small decoding threshold over the ergodic Gaussian-noise channel perform very close to the outage probability on block-fading channels, from both density evolution and finite-length perspectives.

This paper considers the use of a sensor network to estimate the position of a transmitting radio based on the received signal strength at the sensors. A generic path loss model which includes the effects of spatially correlated shadowing is assumed. A weighted likelihood (WL) estimator is proposed, which can be seen as a simplified minimum mean square error (MMSE) estimator. This estimator can be used for localizing a source in a static scenario or it can provide the initial position estimate of a tracking algorithm. The performance of the WL estimator is simulated, and robustness to erroneous assumptions about path loss exponent, shadowing variance and correlation distance is demonstrated.

In this paper, we propose a class of irregular turbo codes that approach the capacity of the binary erasure channel. First, an analytic expression of the erasure probability of punctured recursive systematic convolutional codes is derived. This expression will then be used to study the density evolution of turbo codes over the binary erasure channel, that will allow for the design of capacity-approaching infinite-length irregular turbo codes. Next, a graph-optimal interleaver for finite-length irregular turbo codes is proposed. Finally, simulation results for different coding rates are shown.

In this paper, we consider the problem of coding for the half-duplex nonorthogonal amplify-and-forward (NAF) cooperative channel where the transmitter to relay and the interrelay links are highly reliable. We derive bounds on the diversity order of the NAF protocol that are achieved by a distributed space-time bit-interleaved coded modulation (D-ST-BICM) scheme under iterative APP detection and decoding. These bounds lead to the design of space-time precoders that ensure maximum diversity order and high coding gains. The word error rate performance of D-ST-BICM are also compared to outage probability limits.

We investigate a space-time coding scheme with four or more transmit antennas based on a matrix-Alamouti scheme in a quasi-static fading environment. The receiver consists of an Alamouti combiner followed by an iterative joint detection and decoding process. A priori probabilities fed back from the decoder are used to both get more reliable estimates of the transmitted symbols and efficiently remove inter-block interference. Interblock interference is removed using techniques known in multiuser communication. The theoretical bounds of this scheme are established through the study of its outage probability.