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Diffusion-based molecular communication (DBMC) systems allow messenger molecules to reach the intended receiver using laws of diffusion. The Brownian motion of messenger molecules introduces delay in the system causing the problem of inter-symbol interference (ISI). Along with ISI, the problem of inter-link interference (ILI) comes into picture in the diffusion-based molecular multiple-input multiple-output (DB-MoMIMO) systems. To combat against ISI and ILI issues, index modulation (IM) approaches are recently proposed in the literature for improving the performance of DB-MoMIMO systems. Despite the existence of IM approaches, problems of ISI and ILI still persist due to the slow random motion of molecules in DB-MoMIMO channels. Motivated by this, a novel coded modulation approach called molecular spatio-temporal coded modulation (MSTCM) is proposed in this paper which shows promising improvement in the performance of DB-MoMIMO systems by encoding the information both in space and time axes. Further, if system has access to two types of molecules, the dual-type modulation schemes are discussed for the same. Moreover, the encoder and decoder designs for the proposed mapping strategy are provided. In addition, a theoretical bit error rate expression for the MSTCM scheme is derived and simulation results are shown to corroborate the theoretical ones.

Relay-assisted free space optical (FSO) systems offer several advantages such as cooperative diversity, mitigating turbulence-induced fading, and broad coverage area. Despite these manifold advantages, they can still be affected by unfavorable activities of jammer. Therefore, it is of high importance and need to investigate random jamming specially for security-constrained cooperative FSO communication protocol. This article develops a new system model of relay-assisted FSO system and studies a novel mathematical framework to analyze the performance of decode-and-forward protocol-based cooperative FSO system in presence of random relay jamming effects. In presence of jammer, the probability density functions (pdfs) of different additive noises, i.e., a mixture of negative exponential and Gaussian random variables (RVs) and mixture of negative exponential RVs are derived. A closed-form expression of the average bit error rate (ABER) is analytically evaluated by using the derived pdfs in the presence of random relay jamming effects. In presence of jammer, an approximate and simple expression of a threshold-based detector is obtained. By studying the ABER performance comparison, it is shown that the proposed approximate detector works satisfactorily compared to the maximum-likelihood detector in presence of random relay jammers. In addition to that, ABER performance of a general relay FSO system with an arbitrary number of relays is also obtained.

Free space optical (FSO) channel provides a significantly large transmission capacity. However, in the presence of random jamming attacks, the efficacy of FSO communication channel can be affected critically. In this paper, the discrete-input continuous-output memoryless channel (DCMC) capacity of a single-input single-output FSO communication system is investigated under the random jamming effects. The ergodic channel capacity analysis is performed over additive non-Gaussian noise channel (jamming noise channel). The derived closed-form expression of DCMC capacity for the considered FSO system and also, the effects of partial-band jamming and broadband jamming over the FSO channel capacity are verified through simulation results. It is observed that the FSO channel capacity is very sensitive to the degree of jamming and the ergodic channel capacity alters crucially even for small change in jamming probability. The derived results are utilized to study the considered network in the presence of a rational jammer whose ability to jam is limited by its battery capacity. The presence of a rational jammer is addressed by modelling the interaction between the FSO transmitter and the jammer as a non-cooperative game. The closed-form expressions of the equilibrium of the proposed game are derived and verified through simulation. The game-theoretic solution provides a performance improvement of 74% in comparison to uniform power allocation.

In a free-space optical (FSO) communication system, a conventional transmit laser selection (CTLS) scheme selects the best transmit aperture to send the information bits and offers the best possible error performance. However, its performance is highly dependent on the feedback link, and even a slight error in the feedback information can cause a significant loss to the diversity order and bit error rate (BER). To mitigate this problem, in this research, a novel clustering-based TLS scheme, named two TLS (TTLS), is proposed and analyzed. Note that the word two in TTLS corresponds to the transmission corresponding to the two transmit apertures. TTLS scheme divides the arbitrary number of transmit apertures/lasers into two clusters and then selects the best transmit aperture from each cluster. It is observed by analysis that the TTLS scheme offers a lesser loss in the diversity order as compared to CTLS under imperfect feedback link. Therefore, a significant performance gain can be achieved by TTLS as compared to CTLS under a practical (imperfect/erroneous feedback link) scenario. Further, to retain the full diversity order under the imperfect feedback link, a modified error-tolerant weighting scheme (METWS) is also proposed. It is noticed from the asymptotic analysis that METWS offers full diversity order and outperforms CTLS and repetition coding schemes under both perfect and imperfect feedback links.

In this paper, we investigate the physical layer security based on space shift keying (SSK) in which bit-to-spatial mapping of antenna indices dynamically adapts according to the partial channel state information (CSI) of the legitimate channel at the transmitter (Tx). Further, the Tx also adapts transmit power allocation to each transmit antenna according to partial CSI. Since an eavesdropper (Eve) is not (usually) aware of the legitimate CSI, it cannot successfully decode the antenna index; thus, transmission over the legitimate channel is secured from the wiretapping of Eve. An important virtue of the proposed scheme is that the Tx does not require full CSI of the legitimate and eavesdropper channels to secure the confidential information. The proposed work focuses on the secrecy rate (SR) and bit error rate (BER) of the considered multiple-input single-output system over Rayleigh fading channels. Approximate expressions of Bob’s data rate for 2-ary and 4-ary SSK modulations are derived in closed-form. Furthermore, an upper-bound of Bob’s data rate for $N_{a}$ -ary SSK modulation is also derived. In order to obtain the secrecy rate for the proposed scheme, we also evaluate Eve’s data rate. Analytical results of SR and BER demonstrate the robustness of the proposed scheme. All numerical results are thoroughly verified by using extensive Monte Carlo simulations.

Reconfigurable intelligent surface (RIS) is an emerging technology that can achieve reconfigurable radio propagation environments for beyond 5G/6G wireless systems by smartly tuning the signal reflection via a large number of low-cost passive reflecting elements. In this letter, we investigate the effect of co-channel interference (CCI) on RIS-based dual-hop mixed free-space optical (FSO)-radio frequency (RF) communication systems. The source node, equipped with multiple FSO apertures, transmits subcarrier intensity modulated symbols on the FSO link subjected to Gamma-Gamma (GG) distributed atmospheric turbulence with pointing errors (PEs). It is assumed that the relay-RIS and RIS-destination links follow Rayleigh distribution. The destination node is corrupted by multiple CCI (each following Rayleigh distribution). The performance of the considered system is evaluated by deriving novel closed-form expressions for the outage probability (OP) and bit error rate (BER). It is shown through numerical results that RIS-based dual-hop mixed FSO-RF system provide a significant performance enhancement in comparison to a traditional dual-hop mixed FSO-RF system even in presence of CCI.

In this paper, an artificial noise (AN) injection technique is incorporated in a free-space optical (FSO) communication system with the aim of enhancing the secrecy performance of the system. An intensity modulated direct detection (IM/DD) FSO link which is subjected to Malaga ( $\mathcal {M}$ ) distributed turbulence with pointing errors is considered in this paper. The performance of FSO systems is evaluated by deriving novel closed-form expressions for the secrecy outage probability (SOP), strictly positive secrecy capacity (SPSC), and throughput of the system. By formulating a constrained optimization problem, we discuss an optimal power allocation strategy for throughput maximization in the considered system. It is shown through the results that the proposed technique is very effective in improving the secrecy performance of FSO systems.

International audience; Emerging applications such as connected farms, wildness monitoring, smart cities, and Factory of the Future leverage emerging Low Power Wide Area Networks (LPWAN), allowing a good trade-off between radio range, data rate, and energy consumption. However, only few theoretical studies of these recent technologies are available to help network designers to optimize real field deployments or even achieve real-time adaptation of transmission parameters. A new approach based on Marcum function is proposed in this paper to estimate-in a fast and accurate manner-the performance in terms of Bit Error Rate of LoRa, one of the most used LPWAN technologies. The method is proposed for Gaussian channels, over challenging propagation environments, i.e., Ricean and Nakagami fadings. Simulation results show that our proposed approximation reduces the approximation error about one order of magnitude compared to existing ones and can be computed by classical software. INDEX TERMS Accurate approximation, fading channels, Internet of Things, LoRa performance, low-power wide area networks, Marcum function.

In this paper, a cyber-physical system (CPS) is considered, whose state estimation is done by a central controller (CC) using the measurements received from a wireless powered sensor network (WPSN) over fading channels. An adversary injects false data in this system by compromising some of the idle sensor nodes (SNs) of the WPSN. Using the WPSN for transmitting supervision and control data, in the aforementioned setting, makes the CPS vulnerable to both error and false data injection (FDI). The existing techniques of launching stealthy FDI attack are not applicable to the aforementioned network due to the random nature of wireless channels, which is used for both transmitting control and false data. The objectives of the adversary and the CC to launch stealthy FDI attack and to detect the same, respectively, are found to be depending on the powers they use for transmitting data over wireless channels. The transmit powers of the CC, and the adversary that fulfill their respective objectives are derived by modeling their interaction as a Bayesian Stackelberg game. Based on their objectives, novel utility functions are defined for the CC and the adversary. Subsequently, the equilibrium of the proposed game is obtained by solving a non-convex bi-level quadratic-quadratic program. Finally, the analytical results are verified and compared with other state-of-art techniques by applying them in a realistic smart grid simulations.

In this article, a two-dimensional channel characterization of a molecular motor signal in a diffusive fluid environment is provided by deriving the first hitting time probability density function of the motor signal to an absorbing lattice point. A temporal modulation scheme to establish communication between two nodes using the motor signal is also proposed. Detection of the motor signal is achieved by using the maximum likelihood detector with erasures for a negligible or no-interference scenario. On the other hand, for motor signal reception with inter-symbol interference, the reception process is modeled as a hidden Markov model, and a Viterbi sequence detection algorithm is devised for decoding the sequence of input Markov states. Efficient criteria to determine the memory length of a diffusive molecular communication (MC) system are defined, which address the problem of choosing a detector for a given MC system. It is demonstrated using numerical results that the hitting probability and, consequently, the error performance of the motor signal worsens with an increase in the unbound probability ( $\epsilon /2$ ). Furthermore, it is also quantitatively established, as to, why in nature directed signals are preferred for communication over purely random diffusive signals at larger distances.

In this article, we propose a limited feedback-based space-shift keying (SSK) modulation scheme where transmitter (Tx) obtains partial channel state information (CSI) via a feedback link from receiver (Rx). More specifically, the Tx does not receive perfect information of channel gain, but is aware only of channel gain ordering. Based on the received partial CSI, the Tx adapts the spatial constellation and allocates different transmit weights to each transmit antenna representing spatial constellation points. Furthermore, the power adapted spatial constellation points are encoded by using gray coding scheme, which results in unit hamming distance between adjacent spatial constellation points. Next, a multiple-input single-output (MISO) system with maximum–likelihood detection is investigated. Accurate expressions for the average bit error rate (ABER) for ${2}\times {1}$ and ${4}\times {1}$ MISO systems over Rayleigh fading channel are derived. Further, an approximate expression for the ABER of a generalized MISO system is also obtained. It is shown analytically that the ABER performance of the proposed SSK scheme can be improved by applying a non-uniform power allocation to make transmit links more distinguishable at the Rx. Moreover, we demonstrate analytically that the proposed scheme attains a diversity order which is twice of the conventional SSK scheme. Numerical results, corroborated by Monte Carlo simulations, verify the proposed analysis and clearly highlight that, the proposed scheme significantly outperforms existing closed-loop spatial constellation-based schemes relying on full CSI in Tx and conventional open-loop SSK scheme.

Evolution and accessibility of smart-phones have led to a huge demand in network bandwidth. The ubiquitous use of smart-phones in high-speed trains poses a unique challenge in delivering high-speed internet service on board. This challenge can be overcome by employing free-space optics, an alternative to radio-frequency technology. Previous coverage models for ground-train communications employed single laser systems with a larger divergence angle to cover a larger distance. Larger divergence angles lead to larger geometric losses, which may result in a non-reliable communication link. This study proposes a sectorised multi-beam coverage model with a smaller divergence angle to reduce the impact of geometric losses in the system. This study also proposes two receiver (Rx) architectures for Rxs deployed on the train. Along with geometric losses, the atmospheric attenuation is taken into consideration for the FSO link. The performance of the ground-train communications system in terms of bit-error-rate is evaluated under weak turbulence conditions via numerical simulation.

In this article, we propose a double differential (DD) modulation technique for low earth orbit (LEO)-based land mobile satellite (LMS) communication to overcome the carrier frequency offset, which is an inevitable effect with the LEO satellites. The symbol error rate (SER) expressions for the considered scheme over Shadowed–Rician fading channels with phase-shift keying (PSK) constellations are presented. Since a closed-form expression cannot be derived for quadrature PSK and higher order PSK constellations, an upper bound and a lower bound of SER are derived. The error performance of the DD modulated LMS system is compared to that of coherent modulation-based and single differential modulation-based LMS systems, and a significant performance gain of the proposed DD modulation-based system is observed. Furthermore, we have also studied the effects of elevation angle and LEO trajectory on the error performance of the considered system.

In this article, we summarize the research conducted for YFRFs of Visvesvaraya PhD Programme. The research work is mainly focused on the design and analysis of wireless communication networks. We also explore the spectral and energy efficiency aspects of future generation networks like smart grid communications. This research is useful for improving the performance of current and future generation of communication systems.

In this paper, we study the impact of correlation on the bit error rate (BER) and the channel capacity of a free-space optical (FSO) multiple-input-multiple-output (MIMO) system employing optical space shift keying (OSSK) over a fading channel. In order to study a practical correlated channel, we consider the effect of channel correlation due to both small-and large-scale eddies and show that the use of OSSK over correlated FSO channel can lead to an improved system performance with increasing correlation level of upto 0.9. In this work, we first develop an analytical framework for different performance metrics of the OSSK multiple-input single-output system with correlation and then extend our investigation by proposing an asymptotically accurate mathematical framework for MIMO. We also validate all the analytical results using MATLAB simulations. Finally, we develop an experimental setup of FSO with two correlated links to study the throughput and latency of the links at different turbulence levels.

In this paper, the effect of jamming in free space optical (FSO) link, is evaluated by deriving closed-form expressions of the bit error rate (BER) and outage probability (OP) for single-input single-output (SISO) and multiple-input single-output (MISO) FSO systems. The effects of partial-band jamming and broadband jamming over the error performance of the considered FSO systems are also analyzed. The jammer behaves as a random noise source, following the negative exponential distribution–due to the atmospheric turbulence. Therefore, in the presence of jammer, the error performance of FSO systems is governed by the additive negative exponential noise. It is shown by a rigorous analysis that a MISO FSO system can significantly mitigate the effect of jamming. Specifically, we consider a $2\times 1$ FSO system for analysis, and demonstrate the improvement in BER and OP performance an FSO system can gain by using an additional spatial dimension, in the presence of a random jammer. The mitigation of jamming, due to implementation of arbitrary transmit apertures, is also verified through simulation results; which lies in complete agreement with the analytical results. Also, the error performance under the jamming effect is studied numerically over the Gamma-Gamma fading channel incorporated with pointing error effect.

The motion of molecules in a constrained fluid environment is defined by specified boundary conditions. Deriving analytical channel characterization of a bounded fluid environment is challenging as the solution must satisfy the given boundaries. In this letter, we derive the probability distribution function (PDF) of molecular motion inside a rectangular container with reflective and absorbing boundaries. Using the molecular motion’s PDF, we deduce an analytical expression for the PDF of the first arrival time (FAT) of the molecules to an absorbing wall. Furthermore, we also evaluate the capacity for a temporally modulated molecular communication (MC) system in the bounded environment.

The diversity-multiplexing tradeoff (DMT) in a multiple-input multiple-output (MIMO) free-space optical (FSO) communication with limited channel state information at the transmitter (CSIT) is investigated. Using the limited CSIT based power and rate control strategy, we optimally allocate the power among the good and bad channels in such a fashion that the DMT performance of the system enhances significantly unlike the no-CSIT based MIMO-FSO DMT. In this way, a new limited CSIT based technique/model with optimal power and rate control strategy is proposed to enhance the DMT performance. The optimal DMT is studied for two different transmission scenarios: single-rate and adaptive-rate transmission. It is shown that how the optimal DMT is influenced when the concept of minimum guaranteed multiplexing gain in the forward link is taken into account. It is illustrated that power control based on the feedback plays a vital role in attaining the optimal DMT, and rate adaptation is significant in obtaining a high diversity gain, especially at high rates. Moreover, the analysis of upper and lower bounds on the optimal DMT is done by giving useful insights. Furthermore, a novel study based on the optimal tradeoff between the degrees of freedom and the number of transmit apertures in a coherent MIMO-FSO channel is also done. To validate the results of the proposed model, we compare the derived results with the no-CSIT based MIMO-FSO DMT. It is observed that the proposed technique/model outperforms the no-CSIT based MIMO-FSO DMT.

Introduction: In this paper, we consider a fundamental problem of testing faced by the health staff on testing COVID 19 patients. In general, the number of testing kits or testing capabilities is limited, and the number of COVID 19 patients is much larger than that. This paper tries to answer this fundamental question that how to perform COVID 19 testing to reduce the death rate. Materials and Methods: We consider that the survival rate of patients is dependent over two parameters: their age and severity of symptoms at a time. Based on these observations, we propose a survival rate function. Using the survival rates, the death rate for an individual is obtained. MATLAB was used for plotting the graphs. Results: It is shown by numerical results that by using the proposed mechanism, the average death rate can be significantly reduced. Conclusion: It is proposed to conduct the testing for those patients who lie in the high death rate regimen.

We consider a cognitive radio (CR) network with simultaneous wireless information and power transfer (SWIPT) system having one base station (BS) with multiple antennas that acts as a secondary transmitter, one desired information receiver (IR) acting as secondary receiver, multiple primary users (PUs) having licenses band of spectrum, and multiple energy harvesting receivers (EHRs). The EHRs harvests energy from the BS and eavesdrop the information from signal. In order to extract the information and then harvest energy from that signal, power splitting is considered in each EHR. On the basis of an imperfect channel state information (CSI) in IR, selection of the best transmit antenna at the BS is made. The effect of path loss on the system is also considered. The derivation of analytical expression in closed-form is provided for the exact secrecy outage probability (SOP) of the network. The derived expression highlights the variation in the secrecy outage of the considered CR SWIPT system relying on imperfect CSI with respect to the system parameters, which is depicted through numerical analysis. Furthermore, we derive the asymptotic SOP and study the effects of different system parameters on the secrecy diversity order and secrecy array gain.

In this paper, a novel signal-space diversity (SSD) technique based multipoint-to-multipoint cooperative scheme for a multiuser free-space optical communication system is proposed. In SSD technique, constellation points of the transmitted signal are rotated by a certain angle and in-phase and quadrature components of two different symbols are interleaved to obtain diversity from the signal space. In the proposed scheme, the diversity of each cooperative user gets improved through joint encoding/decoding of the rotated symbols using subcarrier intensity modulation (SIM). The joint encoding/decoding can be done by connecting all the users' transceivers by wires that ensure the availability of the information of a user to all other users. The average symbol error rate for the proposed scheme under log-normal and gamma-gamma channel models is evaluated analytically and corroborated through Monte Carlo simulations. It is reported through intensive analyses and comparisons that the proposed cooperative scheme not only achieves full diversity order but also outperforms non-cooperative and repetition coding schemes.

Optical spatial modulation (OSM) is a multiple-input–multiple-output technique wherein the overall information is transferred through spatial and signal domains simultaneously. A problem in OSM is the necessity for channel state information at receiver (CSIR) that is obtained by complex channel estimation procedure. Apart from this, a small error in CSIR can cause a significant loss in the error performance of the OSM scheme. In this paper, in order to avoid the requirement of CSIR, a new scheme named as differential OSM (DOSM) is proposed; the average symbol error rate for the DOSM scheme with pulse-amplitude modulation is assessed under negative exponential and gamma–gamma channel models. A special and simplified version of DOSM, called as differential optical space shift keying is also discussed. It is noticed from the analysis that bypassing the requirement of CSIR leads to a small degradation in the error performance of DOSM scheme as compared to its traditional counterpart scheme with perfect CSIR. On the other hand, the proposed scheme outperforms OSM scheme with imperfect CSIR. Further investigations thoroughly examine the interplay between the error performance and spectral efficiency of the proposed scheme.

In this paper, the groundwork on jamming effect and its alleviation mechanisms in free space optical (FSO) communication are studied over Gamma-Gamma (GG) fading channels along with the pointing error (PE) effects. A closed-form expression of the bit error rate (BER) is evaluated analytically for a single-input single-output (SISO) FSO system in the presence of jammer. The worst case jamming condition for maximization of error is calculated numerically. The jammer channel is considered to be GG distributed. Being the most dominating noise, the jamming signal is acted as the only noise source in the considered FSO set-up. Therefore, this study is performed over additive GG noise channel. The error performances are investigated for different atmospheric turbulence (AT) regimes (weak to strong) and for different PE parameters of jamming noise. Moreover, to combat the jamming effect, a multiple-input single-output (MISO) FSO system is considered. A closed-form expression of the BER of the multiple transmit channels over the GG noise is analytically calculated. The analytical results for both SISO and MISO FSO systems are verified with the help of the simulation results obtained by MATLAB software. It is established that MISO FSO performs better than SISO FSO system in terms of BER performance under the influence of jamming noise. Furthermore, many important observations are made upon the BER performances for different AT and PE parameters for SISO and MISO FSO systems.

In this work, the authors consider the communication network of a power substation, where multiple intelligent electronic devices (IEDs) transmit their delay sensitive control and monitoring messages to a common receiver over a wireless network; in the presence of a broadband jammer, which is capable of jamming multiple channels simultaneously. The objective of the IEDs is to successfully transmit their messages within a specified time, whereas the jammer wants to obstruct IED's transmission. A novel utility function is designed for the players, that addresses the time-critical nature of communication. Due to the conflicting interest of the IEDs and the jammer, they model the interaction between them as a repeated Bayesian zero-sum game, which also addresses the repeated interaction among the IEDs and the jammer, and the unavailability of exact information about the jammer. The equilibrium strategies for both the scenarios of perfect and imperfect monitoring are derived and verified through simulation results. Further, the performance of the proposed game model in various scenarios is thoroughly compared in the result section. Finally, the efficacy of the proposed defence strategies is tested in a practical communication network of a power substation under jamming, which is simulated in Optimised Network Engineering Tool (OPNET).

In this paper, a partially informed transmitter-based optical space shift keying (PIT-OSSK) scheme is proposed, wherein the transmitter only possesses the knowledge of channel gain ordering. This partial information is used to adapt the OSSK constellation and power allocation (PA). The adaptation of OSSK constellation reduces the number of bits in error between two consecutive constellation points, which in turn lowers the overall bit error rate (BER) of the proposed scheme. On the other hand, it is demonstrated by a mathematical analysis that the PA achieves improved diversity order as compared to the conventional OSSK scheme. These modifications make the PIT-OSSK scheme significantly better than the conventional OSSK scheme in terms of the BER under both indoor and outdoor conditions. For a single receiver aperture, accurate closed-form expressions of average BER (ABER) for two and four transmitter aperture-based PIT-OSSK schemes are derived under negative-exponential channel model. Moreover, a very tight approximate closed-form expression of the ABER for an arbitrary number of transmitter apertures is also developed. All the derived expressions are verified via Monte-Carlo simulations. Furthermore, the proposed strategy is extended and examined for an arbitrary number of receiver apertures by the simulation-based results under different channel models.

In this letter, we derive the first hitting time probability density function (PDF) of bacteria in a one-dimensional fluid environment. The derived PDF is then employed to obtain the PDF of the random absorbing ordinate on an infinite absorbing wall in the two-dimensional fluid geometry. A novel method of modeling the interference ridden fluid channel as a finite-state Markov channel (FSMC) is proposed. Information rates and the capacity for the bacterial communication system are derived using the FSMC modeling of the fluid channel.

In this study, the authors propose a novel virtual full-duplex relaying scheme, which is referred to as centre-partition max-link relaying by using buffer-aided half-duplex relays for multi-hop decode-and-forward cooperative networks. In the proposed scheme, they aim to select two independent available links for data packet transmission in the same time slot, which, in turn, improves the overall performance of the system. By modelling the evolution of buffer state transition as a Markov chain, the performance of the system is analysed in terms of outage probability and end-to-end average packet delay. The state transition matrix of the Markov chain is constructed and its steady-state distribution is obtained to derive the outage probability. Numerical results demonstrate that the proposed scheme has a potential to provide significant outage performance with reduced average packet delay as compared to that of buffer-aided multi-hop decode-and-forward relaying networks using the conventional max-link scheme.

Due to the rapid growth of free-space optical (FSO) communication over the last few years, there is an exigency to develop diversity-multiplexing tradeoff (DMT) metric in order to compare and design new multiple-input multiple-output (MIMO) FSO schemes. This paper introduces the DMT for MIMO-FSO systems that utilize the intensity modulation and direct detection technique for which the channel gains are real and positive, contrary to MIMO radio-frequency (MIMO-RF) systems where the channel gains are complex. Log-normal, gamma–gamma, and negative exponential channel models are used in this paper to cover weak to saturated turbulence environments. It is shown that at zero multiplexing gain ( $r$ ), the log-normal channel offers maximum but finite diversity order that depends on the block length within which the channel matrix is constant. On the other hand, negative exponential channel offers minimum diversity order. However, for $r \geq 1$ , DMT curves under all channel models are of the same nature; and for a given $r$ , the diversity is half as that observed in the MIMO-RF channels. To gain additional insights into the DMT of an MIMO-FSO system, we also develop upper and lower bound expressions of DMT and determine the minimum block length at which these bounds are identical.

We propose to use 1-bit feedforward information at the free-space optical (FSO) receiver to improve its error performance. We concentrate upon M -ary pulse amplitude modulation (PAM) signaling and design an error-optimal PAM constellation for providing efficient 1-bit feedforward information to the receiver. A maximum likelihood detector is derived, and the symbol error rate of the considered FSO system is analytically obtained in closed form by using a union-bound approach. It is shown by using the analysis and simulation that 1-bit feedforward can significantly improve the performance of PAM in FSO systems.

In this paper, the effect of jamming in a free-space optical (FSO) communication system that employs optical space shift keying (OSSK) is studied. The optical link suffers from saturated atmospheric turbulence (AT). Hence, the channel fading coefficients follow negative exponential probability density function (PDF). Along with additive Gaussian noise, the jammer, under consideration, is an additional source of noise, and jamming signals also suffer from saturated AT. Thus, the jamming channel also follows the negative exponential PDF. A multiple-input single-output (MISO) FSO system is used to mitigate the effects of jammer and channel fading. Explicitly, we employ a 2 × 1 MISO-OSSK system for analysis and derive a closed-form expression of bit error rate (BER) of the considered system. Moreover, the jammer’s effect is also studied numerically under the gamma-gamma (GG) fading channel.

Multiple-input multiple-output (MIMO) systems have recently been envisioned in the molecular communication (MC) realm, due to their desirable properties of improving the system performance. Despite the existence of robust index modulation (IM) schemes, intersymbol interference (ISI) and interlink interference (ILI) problems still impact the system performance due to the inherent nature of diffusion-based molecular multipleinput multiple-output (Mo-MIMO) systems. Inspired by this, a novel coded modulation scheme named spatio-temporal molecular coded modulation (STMCM) is proposed in this paper. The proposed STMCM scheme can combat ISI and ILI-caused errors effectively, improving the overall error performance of Mo-MIMO systems.

In this paper, we study the effect of a beamforming scheme for a multiple-input-single-output (MISO) uplink free-space optical (FSO) communication system with nonorthogonal multiple access (NOMA) over negative exponential fading channels. 1-bit feedback about the channel state information is required by the proposed beamforming scheme. The feedback is considered to be error-free in this work; however, an error due to successive interference cancellation (SIC) is taken into consideration. It is inferred by bit error rate (BER) analysis that a higher weight given to the channel with higher channel gain and a slightly lower weight given to the channel with lower channel gain gives the best performance as compared to all arbitrary schemes in a 2 × 1 FSO-NOMA system. Further, it is also analytically shown that using a 1-bit feedback-based beamforming scheme in an FSO-NOMA system suppresses the effect of SIC error; hence the BER performance of both the transmitters (TXs) is the same, which is not the case for a conventional NOMA scheme. A simplified asymptotic upper bound of BER of the proposed scheme is obtained by using the order statistics, and an optimized beamforming vector is found by minimizing this upper bound. It is then established analytically as well as through simulations that the beamforming vector is independent of average signal-to-noise ratio as long as the two channels remain independent and identically distributed. We further compare the proposed scheme with an FSO-NOMA system without feedback and beamforming and with single-input single-output FSO system using 4-ary pulse amplitude modulated signaling. It is shown by simulations that the proposed scheme outperforms both of them and has a huge coding gain advantage. A numerical analysis of this scheme is also provided for gamma-gamma (GG) and log-normal (LN) turbulence regime. The proposed scheme is extended to 3 × 1 and 4 × 1 MISO systems, and it is revealed that the performance of the system degrades as the number of TX increases.

In order to send the information bits, a conventional transmit laser selection (CTLS) scheme selects the best transmit aperture. Due to the optimal transmit aperture selection, CTLS offers the best possible error performance in a free-space optical (FSO) communication system. Moreover, this optimal error performance is observed under the perfect feedback link scenario. However, a slight error in the feedback information can cause a significant loss in the diversity order. This deteriorates the error performance. To overcome this problem, we propose and analyze a novel grouping-based TLS scheme, called two TLS (TTLS) scheme. In the proposed TTLS scheme, the transmit apertures are divided into two groups and the best aperture in each group is used for transmission of the data. Furthermore, it is observed by the intensive analysis that the proposed TTLS scheme offers a lesser loss in the diversity order as compared to CTLS under the erroneous/imperfect feedback link. Therefore, a significant amount of performance gain can be achieved by the proposed TTLS scheme as compared to the CTLS scheme under a practical (erroneous feedback) scenario.