This work investigates multi-hop serial relaying half-duplex (HD) networks comprising one source, one destination and$K$decode-and-forward (DF) relays. Buffer-aided (BA) relays are considered where finite size buffers are added to relays. For this setup, we propose a novel BA relaying strategy that selects the node that must transmit over one epoch in a block fading environment. Selecting a node depends on the buffer state information of all relays and on the availability of the hops. Moreover, the proposed relaying scheme is controlled by an adjustable parameter that allows the system to achieve multiple levels of tradeoff between the average packet delay (APD) and the outage probability (OP). A Markov chain model is adopted to evaluate the system’s performance and simple closed-form expressions were derived for the APD and OP based on an asymptotic analysis. The performance analysis proves the ability of the suggested relaying scheme to achieve significant OP and APD gains with small buffer sizes. Two variants of the proposed relaying scheme are particularly appealing. The APD-prioritizing variant achieves the smallest APD of$2K$without improving the diversity order. The second OP-prioritizing variant reaches the full diversity order of$K+1$while increasing the asymptotic APD to$K(K+3)$.

Integrating device-to-device (D2D) cooperation with mobile edge computing (MEC) for computation offloading has proven to be an effective method for extending the system capabilities of low-end devices to run complex applications. This can be realized through efficient computing data offloading and yet enhanced while simultaneously using multiple wireless interfaces for D2D, MEC and cloud offloading. In this work, we propose user-centric real-time computation task offloading and resource allocation strategies aiming at minimizing energy consumption and monetary cost while maximizing the number of completed tasks. We develop dynamic partial offloading solutions using the Lyapunov drift-plus-penalty optimization approach. Moreover, we propose a task admission solution based on support vector machines (SVM) to assess the potential of a task to be completed within its deadline, and accordingly, decide whether to drop from or add it to the user’s queue for processing. Results demonstrate high performance gains of the proposed solution that employs SVM-based task admission and Lyapunov-based computation offloading strategies. Significant increase in number of completed tasks, energy savings, and cost reductions are resulted as compared to alternative baseline approaches.

This paper tackles the performance analysis and relaying strategies for two-relays buffer-aided (BA) parallel relaying cooperative free space optical (FSO) networks. We first provide an asymptotic analysis of the existing selective relaying scheme that simultaneously activates the strongest source-relay (S-R) and relay-destination (R-D) optical links. We then propose an improvement of this scheme and highlight on its advantages by deriving closed-form accurate expressions for the asymptotic values of the outage probability (OP) and average packet delay (APD). The theoretical evaluation and numerical analysis demonstrate the capability of the improved scheme in enhancing the diversity order with finite buffer sizes unlike the selective relaying scheme that achieves diversity gains only with infinite-size buffers entailing infinite delays. The OP performance is significantly enhanced for all network topologies whether the relays are closer to S, closer to D or placed at the same distance from S and D. On the other hand, the reaped APD gains depend on the positions of the relays.

In this paper, we consider two-way relaying where two users exchange information through a decode-and-forward (DF) buffer-aided (BA) relay. We formulate a generic BA relaying protocol that is based on both the channel and buffer states and that can be parameterized by two parameters. Through a Markov chain analysis, we show how these parameters can be selected for the sake of minimizing the outage probability (OP) and average packet delay (APD) for asymptotic values of the signal-to-noise ratio. The performed optimization sheds more light on the impact of the buffer sizes on the triad of diversity order, coding gain and APD that can be contemplated. In particular, we prove that equipping the relay with two buffers of size three each is sufficient for extracting the full capabilities of two-way BA relaying. Depending on the network setup, one or both buffer sizes can be further reduced to two at the expense of a reduced coding gain without affecting the diversity order and asymptotic APD. Simulations under the generalized κ-μ fading demonstrate the appropriateness of the performed optimization of the relaying parameters and buffer sizes.

In this paper, the problem of Quality-of-Service (QoS) differentiation is studied in the context of Buffer-Aided (BA) cooperative Free Space Optical (FSO) communication systems. This is particularly true since the existing relevant literature overlooked the possibility of preferential treatment of data packets based on their delay requirements. Inspired by this observation, this paper proposes to classify the data packets emanating from the source node into either Delay Tolerant (DT) packets or Non Delay Tolerant (NDT) ones and to service these packets according to this classification at the relay node. Priority queueing is first introduced in the relay's buffer with a view to improving the delay experienced by NDT packets. Class of service mutation is then proposed as a starvation mitigation strategy to better manage the interesting dynamics resulting from the co-existence of packets having different QoS requirements in the same buffer. The various performance measures of interest for the explored QoS-aware communication system are both evaluated mathematically based on a Markov chain analysis and validated through extensive simulations. An asymptotic analysis is also carried out highlighting the dependence of the performance on the system parameters in an intuitive manner.

In this paper, we propose a novel relay selection strategy for half-duplex decode-and-forward cooperative networks with an arbitrary number of buffer-aided (BA) relays. Unlike most of the existing predetermined relaying protocols, the proposed strategy is adjustable in the sense that it is controlled by K threshold levels. A Markov chain (MC) analysis is adopted for evaluating the outage probability (OP) and average packet delay (APD) of the proposed scheme. Through an asymptotic analysis, we highlight on the impact of the K controlling parameters on the triad of OP, APD and diversity order that can be contemplated. While most of the existing schemes are designed to achieve fixed APD and diversity order values, the proposed scheme can achieve all diversity orders ranging from K to 2K while compromising the asymptotic APD that will range from 2 to 2K+2. We also target the optimization of the buffer size and we prove that a buffer size of three is sufficient for extracting the full capabilities of the BA network. Simulations over Rayleigh fading channels demonstrate the performance gains and the OP-APD tradeoffs that can be attained.

This work targets the performance analysis of optical spatial modulation (OSM) for free-space optical (FSO) communication systems under the exact Poisson photon-counting detection model. We derive the error probability in the case of receivers equipped with multiple apertures where we analyze and compare different combining schemes. This paper also focuses on the energy harvesting (EH) in OSM FSO systems. In particular, we evaluate the error performance and EH levels with the conventional pulse position modulation (PPM) and with the more recent EH-efficient pulse gap modulation (PGM). We also advise novel averaging-based and switching-based EH architectures for multi-aperture receivers and we compare the EH capabilities of these schemes.

The massive increase of IoT connected devices is imposing various challenges at different dimensions due to the constrained capabilities of devices and huge requirements of applications. Attributed to the benefits it offers in terms of time and cost, pushing security solutions to the network edge has attracted tremendous interest. Most of the hashing algorithms are centralized focusing on optimizing hashing techniques while considering enough processing and power capabilities. However, hashing algorithms in the context of IoT world suffer from the limitations of devices in terms of battery power and computing capabilities, thus distributed hashing becomes a necessity to enable it within IoT environment. In this paper, we introduce a trust-aware based model that provides efficient and trusted distribution of hashing computation among mobile edge resources. We formulate the distribution model as an integer linear programming problem while taking into consideration various parameters and constraints. Experimental results explore the efficiency of our proposed approach with respect to the literature.

Ever since the idea of buffers was incorporated in wireless communications, buffer‐aided relaying has become an emerging breakthrough in the world of transmitting and receiving signals. Equipping the relays with buffers adds a new degree of freedom capable of enhancing numerous quality‐of‐service (QoS) metrics including throughput, outage probability, power efficiency and physical‐layer security. The QoS enhancement is achieved by compromising the end‐to‐end delay that is inflicted by storing the packets in the relays' buffers until a suitable source–relay or relay–destination link is selected. In this context, the selection of a relay for transmission/reception is important since it governs the QoS‐delay trade‐off that can be contemplated. In this survey, the authors review and analyse various link selection protocols in buffer‐aided relaying systems. These relaying strategies are categorised and contrasted according to their performance levels, limitations, applications, system model assumptions and complexity. Finally, they discuss current challenges, and highlight future research directions.

In this paper, we consider a three-node free space optical (FSO) cooperative network with an energy harvesting (EH) decode-and-forward (DF) relay. The relay is equipped with an energy buffer and implements the harvest-store-use (HSU) architecture where the harvested energy is accumulated and used for forwarding the information from the source node to the destination node. We propose a novel HSU-based relaying strategy based on the simultaneous lightwave information and power transfer (SLIPT) concept where the relay harvests energy from the optical signal transmitted by the source. Firstly, we analyze the performance of the considered system by modeling the energy buffer as a continuous-space Markov chain (MC) for the sake of deriving the limiting distribution of the stored energy. Secondly, we discretize the state space and use the resulting discrete-space MC to derive the steady-state distribution of the buffer content. Thirdly, we propose an approximate discrete-space MC for capturing the energy buffer dynamics in a simple manner. The third approach is useful for relating the outage probability to the channel coefficients and average amounts of harvested energy in a simple closed-form manner. Simulation results validate the presented analytical evaluation and demonstrate the performance improvement that is achieved by the proposed scheme.

This work targets the performance analysis of buffer-aided (BA) Free-Space Optical (FSO) relaying with a single decode-and-forward (DF) relay. In particular, we study the impact of activating the direct source-destination link on the system performance. Based on a Markov chain analysis, we derive closed-form expressions for the system outage probability (OP) and average packet delay (APD). Moreover, we carry out an asymptotic analysis and evaluate the achievable diversity order. Results show that while activating the direct link reduces the APD in all scenarios, this activation might either increase or decrease the diversity order depending on the relay position.

This paper investigates two pulse-based Optical Spatial Modulation (OSM) schemes as cost-efficient solutions for multi-aperture Free-Space Optical (FSO) communications with Intensity-Modulation and Direct-Detection (IM/DD). Namely, we consider Optical Space Shift Keying (OSSK) where information is encoded in the index of the pulsed optical source and Spatial Pulse Position Modulation (SPPM) where additional bits determine the position of the transmitted optical pulse resulting in higher transmission rates. A performance analysis is carried out over gamma-gamma channels with the exact Poisson photon-counting detection model. Exact Symbol Error Probability (SEP) expressions, simple upper bounds and the achievable transmit diversity orders are derived for both the open-loop and closed-loop scenarios. Based on the presented performance analysis, a transmit aperture selection scheme capable of maximizing the transmit diversity order is proposed for OSSK and SPPM in the closed-loop case. Results show that for open-loop OSSK, open-loop SPPM and closed-loop OSSK, the transmit diversity order does not depend on the severity of scintillation unlike the closed-loop SPPM case.

This paper investigates a hybrid radio frequency (RF)/visible light communication (VLC) ultra-small cell network consisting of multiple optical angle-diversity transmitters, one multi-antenna RF access point (AP) and multiple terminal devices. In the network, the optical transmitters play the primary role and are responsible for delivering information and power over the visible light while the RF AP acts as a complementary power transfer system. Thus, we propose a novel collaborative RF and lightwave resource allocation scheme for hybrid RF/VLC ultra-small cell networks. The proposed scheme aims to maximize the communication quality-of-service provided by the VLC under a constraint of total RF and light energy harvesting performance while keeping illumination constant and ensuring health safety. This scheme leads to the formulation of two optimization problems that correspond to the resource allocation at the optical transmitters and the RF AP. Both problems are optimally solved by appropriate algorithms. Moreover, we propose a closed-form suboptimal solution with high accuracy to tackle the optical transmitters' resource allocation problem, as well as an efficient semi-decentralized method. Finally, simulation results illustrate the achievable performance of the investigated system and the effectiveness of the proposed solutions.

In this paper, we consider the problem of Multiple-Input-Single-Output (MISO) Free-Space Optical (FSO) communications with intensity-modulation and direct-detection (IM/DD). Unlike the commonly adopted Gaussian-noise approximation, we adopt the exact Poisson statistics for the number of photons falling on the receiver. In particular, we compare three popular MISO schemes; namely, repetition coding (RC), optical spatial modulation (OSM) and spatial multiplexing (SMux). For each one of these schemes, we derive the optimal maximum-Iikelihood (ML) decoder and we contrast the schemes in terms of rate, performance, complexity, channel state information requirements and tolerance to channel estimation errors.

This work targets a comprehensive understanding of the impact of the serial‐relaying and multiple‐input‐multiple‐output (MIMO) fading mitigation techniques on the performance of indoor power line communication (PLC) systems. While these techniques were studied extensively under additive Gaussian noise, we aim at highlighting the implications of Nakagami‐like noise on the achievable performance levels. The author derive approximate closed‐form expressions for the bit error rate (BER) under Rayleigh fading and Nakagami‐ m background noise where the obtained approximations are very close to the exact BERs. The derived expressions are useful for evaluating the achievable diversity orders and for relating the BER to the main parameters of the PLC system in a direct and intuitive way. In this context, we analytically prove that MIMO doubles the diversity order with different combining schemes while the diversity order of a system under Nakagami‐like noise is only half the diversity order that would have been obtained under Gaussian noise. The author also shows that multi‐hop relaying does not enhance the diversity order.

In this paper, we consider the problem of multiple-input multiple-output (MIMO) free-space optical communications under the Poisson photon-counting detection model. Aiming for high bit rate objectives, we consider the spatial-multiplexing (SMux) solution with M-ary pulse-position modulation, where we propose appropriate optimal and suboptimal decoders and evaluate their complexities. Such novel decoder designs are needed, since the widely spread Gaussian noise-based MIMO decoders are not suitable for the Poisson model. We also carry out an asymptotic performance analysis. This analysis guides a candidate constellation confinement where transmissions are limited to some selected information vectors of the multidimensional SMux constellation. This confinement meets the objective of compromising the multiplexing gains for the sake of improved error-rates. The analyzed SMux solutions with both the unconfined and confined constellations transmit at higher data rates compared with the existing single-aperture systems, MIMO systems with repetition coding, and MIMO systems based on spatial modulation.

In this paper, we analyze multi-hop free-space optical (FSO) communications in the context of decode-and-forward serial relaying, where the relays are equipped with finite-size buffers. Based on a Markov chain analysis, we derive closed-form asymptotic expressions for the system outage probability (OP) and average packet delay (APD) for an arbitrary number of relays N, and an arbitrary buffer size L. The closed-form evaluation links the system performance to the various network parameters in a simple and intuitive manner, and it is useful for offering clear insight on the impact of the relay placement and the selection of the buffer size for practical FSO systems. We prove that buffer-aided multi-hop systems can reap a diversity gain that ranges from [(Nτ/2)] + 1 to Nτ+ 1 compared with multi-hop buffer-free systems while the asymptotic APD values can range from Nτto (L - 1)Nτfor L ≥ 2. Our analysis also highlights the optimal solutions capable of concurrently minimizing the OP and APD.

This paper tackles the performance analysis of multiuser diversity (MD) free space optical (FSO) communication systems. In particular, we evaluate the performance of the n-th best user point-to-multipoint selection scheme and the performance of the multipoint-to-multipoint user grouping scheme. We derive approximate bit error rate (BER) expressions under lognormal and gamma-gamma scintillation. The approximation technique is based on the recurrence relations between the probability density functions (pdf's) of order statistics and on moment matching methods for deriving the parameters of the approximating pdf. Results highlight the accuracy of the derived expressions in predicting the BER performance.

In this study, the authors evaluate the bit error rate (BER) performance of multichannel power line communication (PLC) systems under lognormal fading and Nakagami‐ m background noise. The information signal propagates over two parallel, but correlated channels while maximum ratio combining and equal gain combining are implemented at the receiver side. By appropriately approximating the tail of the probability density function of a single Nakagami‐ m noise term and of the weighted sum of two Nakagami‐ m noise terms, they derive accurate expressions of the BER. They prove that single‐channel and multichannel PLC systems both benefit from an infinite diversity order and that this quantity increases logarithmically with the signal‐to‐noise ratio. In this case, the advantage of the diversity combining techniques resides in their impact on the average energy of the fading gain; a quantity that they derive analytically by applying the lognormal‐sum approximation.

In this paper, we propose a novel multipoint-to-multipoint cooperative diversity scheme for multiuser free-space optical (FSO) communications. This scheme is motivated by the inherit immobility and directivity of FSO communications where several transceivers mounted on a given building can be easily interconnected for achieving enhanced diversity levels through joint encoding/decoding. In this case, the N users are clustered in a number of groups depending on the state of the FSO network. Users of a given group transmit the same encoded symbol comprising the information of all these users while maximum-ratio combining is implemented at the receive apertures. We present an asymptotic analysis of the bit error rate and derive the achievable diversity orders over the gamma-gamma turbulence-induced fading channels affected by pointing errors. Results show that while grouping all users in one group achieves the highest diversity order, this partitioning is suboptimal for practical ranges of the signal-to-noise ratio. In this context, the partitioning of the N users turns out to be of crucial importance where the optimal grouping outperforms the all-user grouping by several orders of magnitude.

In this paper, we propose novel 2×2 and 3×3 noncoherent space-time (ST) codes for impulse-radio ultra-wideband communications with pulse position modulation (PPM). The code design is based on existing coherent ST codes, where the associated constellation is confined in order to achieve full diversity with analog energy detectors. A comprehensive performance analysis guides this confinement and highlights the maximum rates that can be achieved by this specific layered code word structure with two and three transmit antennas. The proposed codes attain these maximum rates without introducing any expansion to the unipolar PPM signal set.

In this paper, we consider the problem of relay-assisted free space optical (FSO) communications in the case where the relays are equipped with buffers of finite size. The high directivity of the FSO links clearly distinguishes cooperative FSO networks from their radio frequency (RF) counterparts thus motivating the design of FSO-specific buffer-aided (BA) cooperative protocols. We propose three novel decode-and-forward relaying protocols that are adapted to the nature of FSO transmissions and are capable of achieving different levels of tradeoff between outage probability, average packet delay and system complexity: 1) the BA selective relaying protocol that can be implemented in the presence of channel state information (CSI) and that outperforms the RF max-link protocol with a reduced delay; 2) the BA all-active relaying protocol that can be implemented in the absence of CSI and constitutes the simplest protocol with the best delay performance at the expense of a degraded outage performance; and 3) the BA load-balanced selective protocol where supplementary FSO communications are triggered along the inter-relay links for a more balanced distribution of the packets among the buffers. While the last protocol incurs the highest signaling complexity, it results in significant performance gains with a delay that is comparable to that of the BA selective protocol. A Markov chain analysis is adopted for evaluating the system outage probability and the average packet delay where the corresponding state transition matrices are derived in the cases of both symmetrical and asymmetrical networks.

In this paper, we consider the problem of space-time (ST) coding for the noncoherent ultra-wideband (UWB) systems with pulse position modulation (PPM). In particular, we propose the first known family of ST codes that lends itself to optimal decoding with the energy detectors. Energy detection avoids the sampling of the received signal where the sampling rates can be prohibitively high and escapes from the challenging task of estimating the dense multi-path UWB channel. In the absence of design criteria that are adapted to the problem under consideration, we derive two novel design criteria and propose the novel construction accordingly. The proposed code is based solely on permutations, and hence it does not introduce any constellation expansion on the totally-real and unipolar PPM signal set. In other words, the components of the constructed codewords are limited to either zero or one indicating the absence or presence of an UWB pulse in the corresponding PPM slot, respectively. The above characteristics result in a remarked simplicity of the ST encoder/decoder making it an appealing solution to low-cost and low-power UWB systems. For a system equipped with n transmit antennas, we prove that the proposed code is capable of achieving a full transmit diversity order with M-PPM constellations for M>n+1 while transmitting at the rate of (1/n)log2(nM-1) bits per channel use.

In this paper, we study the impact of inter-relay co-operation on the performance of decode-and-forward (DF) cooperative free space optical (FSO) communication systems with any number of relays. The idea of inter-relay co-operation (IRC) was introduced very recently where the relay-relay links are activated for further boosting the system performance. We evaluate the outage probability under forward and forward-backward IRC that constitute the two variants of this transmission strategy. We also derive the diversity orders that can be achieved over a composite channel model that takes both turbulence-induced fading and misalignment-induced fading into consideration. We present a comprehensive asymptotic analysis that is effective for tackling the usefulness of IRC with an arbitrary number of relays and for deriving the network conditions under which implementing IRC in any of its variants can be beneficial for enhancing the diversity order of the FSO system. The introduced framework answers the question on what is the optimal solution for a particular FSO network (among the parallel-relaying solution with no IRC, forward IRC or forward-backward IRC).

In this paper, we consider the problem of differential space-time (ST) block coding in the context of impulse-radio ultra-wideband (IR-UWB) communications with two transmit antennas and Pulse Position Modulation (PPM). We propose a novel family of codes where the information is encoded differentially through the number of transmitted pulses, the positions of these pulses and the locations of the non-idle symbol durations within each ST block. In order to maintain a low complexity of the UWB transmitter, unipolar transmissions are ensured by avoiding all forms of amplitude-domain encoding. The proposed scheme provides a full scalability capability where depending on the value taken by the scale parameter Θ, different levels of compromise between data rates and error rates (along with the decoding complexity) can be achieved. Finally, the proposed scheme lends itself to optimal detection with cross-correlation receivers that can be implemented in the analog domain in the absence of all forms of channel state information at the transmitter and receiver sides.

This paper targets the performance analysis of multiple-input-multiple-output (MIMO) point-to-point single-aperture relay-assisted and MIMO relay-assisted free-space optical (FSO) communications. The objective of the paper is to compare the different FSO diversity methods via an outage probability analysis over gamma-gamma turbulence channels in the case of independent fading among the different apertures of the communicating nodes. Through a well-tailored calculation method, we approximate the gamma-gamma distribution and the gamma-gamma sum-distribution by appropriate gamma distributions. This method resulted in simple closed-form outage probability expressions that are accurate for average-to-large values of the signal-to-noise ratio (SNR) and in asymptotic expressions that are useful for evaluating the diversity gains and coding gains. In particular, we derived the diversity gains of MIMO repetition coding (RC), MIMO transmit laser selection (TLS), MIMO all-active relaying (AR) and MIMO selective-relaying (SR). We prove that MIMO-RC and MIMO-TLS, on one hand, and MIMO-AR and MIMO-SR, on the second hand, achieve the same diversity gain and we evaluate the coding gain of TLS with respect to RC and of SR with respect to AR. By comparing the MIMO and cooperative methods with point receivers, we reach the central result that it is always better, from a diversity gain point of view, to add more apertures to the source and destination rather than adding more relays in their vicinity despite the fact that the fading variance along FSO links decreases with the distance. For receivers that implement aperture averaging, we derive conditions under which MIMO techniques are unconditionally better than cooperative techniques with any network topology.

In this paper, we consider the problem of full-duplex (FD) relaying in the context of impulse-radio ultra-wideband (IR-UWB) communications. In particular, we propose two novel distributed space-time block codes (STBCs) suitable for the amplify-and-forward (AF) cooperation protocol with one and two relays. Despite the fact that FD relaying results in significant levels of interference between the transmit and receive antennas of each relay, it introduces new concepts to the problem of distributed STBC design. Compared to half-duplex (HD) STBC, FD-STBC is subject to an additional constraint that imposes the structure of the codewords while it offers the predominant advantage that resides in the possibility of including a smaller number of information symbols per codeword for achieving a full rate. We take advantage of this potential for constructing fully-diverse, full-rate and totally-real IR-UWB FD-STBCs that outperform the existing HD-STBCs for all practical levels of the residual self loop interference. In fact, for the sake of transmitting at full rate, the best known distributed HD-STBCs for the non-orthogonal AF protocol require the joint encoding/decoding of $4N_{r}$ symbols where $N_{r} $ is the number of relays. On the other hand, the proposed FD-STBCs require embedding only $N_{r} +2$ symbols per codeword for transmitting at full rate. Following from this fact, not only the decoding complexity is reduced, but also the coding gains are improved resulting in enhanced performance levels.

In this paper, we present a general technique for constructing minimal-delay unitary differential Space-Time (ST) block codes for Pulse Position Modulation (PPM) with an arbitrary number of transmit antennas and signal set cardinality. A typical application corresponds to Multiple Input Multiple Output (MIMO) Impulse-Radio Ultra-Wideband (IR-UWB) systems where neither the transmitter nor the receiver knows the channel. The proposed scheme is a pulse-based solution where the information is encoded differentially through the relative shifts of the pulses in one block with respect to the pulse positions in the previous block where each block extends overPsymbol durations withPstanding for the number of transmit antennas. This technique of time-domain encoding avoids all types of amplitude constellation expansions and achieves a full transmit diversity order while maintaining a single unipolar pulse transmission per symbol in a way that is completely equivalent to single-antenna PPM communications. We also propose a simplified decoder that can be associated with the proposed ST code and we perform a detailed complexity analysis that allows to quantify the reduction in the number of operations offered by this simplified decoding strategy. Finally, the results are validated numerically and through a semi-analytical evaluation of the conditional symbol error rate.

In this paper, we study the effect of the presence of inter-relay connections on the performance of cooperative free-space optical (FSO) networks. Unlike the existing literature where conventional cooperation in FSO systems is realized in two steps through source-relay (S-R) followed by relay-destination (R-D) communications, we propose and analyze a novel three-stage cooperation methodology. The proposed scheme is based on exploiting the potential presence of FSO links between neighboring relays for the sake of complementing the S-R and R-D steps with an intermediate inter-relay communication step that serves in enhancing the fidelity of signal reconstruction at the relays before retransmitting to the destination. For the proposed scheme, we derive closed-form expressions for the conditional error probability with any number of relays. We also propose a closed-form power allocation strategy (PAS) in the case where the background noise is negligible. This PAS is based on minimizing an asymptotic upper-bound on the conditional error probability by applying the method of Lagrange multipliers with the solution satisfying the Karush-Kuhn-Tucker (KKT) conditions. Results show that the presence of inter-relay connections, associated with an appropriate cooperation scheme and PAS, can significantly boost the performance of relay-assisted FSO systems.

In this paper, we consider the problem of relay-assisted free-space optical (FSO) transmissions. We evaluate the diversity orders that can be achieved by simple-DF and selective-DF relaying protocols over quantum-limited FSO systems that are subject to Rayleigh fading. We prove that for a Nr-relay system, selective-DF captures the full spatial diversity order of Nr+1 while simple-DF achieves a reduced order of ⌈Nr/2⌉+1 making this scheme highly suboptimal for FSO communications.

In this paper, we propose a novel cooperation strategy for coherent Impulse-Radio Ultra-Wideband (IR-UWB) communication systems with one relay. While non-orthogonal cooperation in narrow-band wireless networks often requires deploying distributed space-time codes with joint encoding of several symbols at the source and relays and joint decoding of these symbols at the destination, the proposed non-orthogonal cooperation scheme constitutes the first known symbol-by-symbol-based scheme where cooperation is entirely realized within one symbol duration. This follows from the fact that the proposed strategy is adapted to the structure of the Pulse Position Modulation (PPM) that constitutes the most popular modulation scheme associated with UWB transmissions. We also propose a simple and efficient power allocation strategy that further boosts the performance of the proposed cooperation strategy. The error performance of the proposed scheme is evaluated analytically while simulations performed over the IEEE 802.15.3a UWB channel model are provided to support the theoretical results.

In this paper, we consider the problem of relay-assisted free-space optical (FSO) transmissions with any number of relays. At a first time, we consider the quantum limited scenario and analyze the simple Decode-and-Forward (SDF) protocol where all symbols received at a certain relay are retransmitted to the destination. We also propose a novel Selective-DF (SelDF) protocol that can be implemented without requiring any kind of channel state information (CSI) neither at the transmitter nor at the receiver sides. We derive the diversity orders that can be achieved by SDF and SelDF over gamma-gamma fading channels and highlight the superiority of SelDF. We also prove that for some network configurations, SDF might achieve the same diversity order as SelDF. On the other hand, for most of the network configurations, the SDF protocol, that was studied extensively in the context of N_r-relays radio-frequency (RF) systems and one-relay FSO systems, is not suitable for FSO systems with more than one relay since it results in reduced diversity orders. At a second time, the above protocols are extended to FSO systems that are corrupted by background radiation. For SelDF, we propose a novel metric that captures the fidelity with which a certain message is detected at the relay thus determining whether this relay will participate in the cooperation effort or not.

In this paper, we propose a novel cooperation strategy for Impulse-Radio Ultra-Wideband (IR-UWB) communication systems with binary Pulse Position Modulation (PPM). Unlike Decode-and-Forward (DF) strategies where a group of PPM symbols, or at least one PPM symbol, is decoded and forwarded to the receiver, the proposed scheme inspects the PPM slots in a sequential manner. In particular, only the UWB pulses that are received at the relay with a sufficient level of fidelity are retransmitted in subsequent PPM slots. This technique limits cooperation to only one symbol duration while avoiding any form of joint decoding at the relay and destination nodes. The proposed scheme corresponds to a selective-relaying protocol where the relay imposes a certain level of selectivity on the symbols to be retransmitted. The error performance of the proposed scheme is evaluated theoretically. Based on this performance analysis, we derived an optimal pulse selection strategy at the relay.

In this paper, we propose an accurate method for evaluating the performance of Ultra-Wideband systems over the IEEE 802.15.3a channel model. Depending on the severeness of fading, the method consists of approximating the distribution of the captured channel energy by either a Coxian, gamma-mixture or lognormal distribution based on a least square fitting criterion. The proposed approximations lend themselves to simple mathematical analysis and turn out to be useful in evaluating the performance, in terms of average bit-error-rate (BER) and outage probability, over the IEEE 802.15.3a channels.

In this paper, we investigate the cooperative diversity technique as a candidate solution for combating turbulence-induced fading over Free-Space Optical (FSO) links. In particular, a one-relay cooperative diversity scheme is proposed and analyzed for non-coherent FSO communications with intensity modulation and direct detection (IM/DD). The error performance is derived in semi-analytical and closed-form expressions in the presence and absence of background radiation, respectively. Results show the enhanced diversity orders that can be achieved over both Rayleigh and lognormal fading models.

In this paper, we consider the application of Multipulse Pulse Position Modulation (MPPM) as a power-efficient modulation scheme for Impulse-Radio Ultra-Wideband (IR-UWB) communications and we propose the first known MPPM-specific 2×2 space-time code that is convenient for this modulation. The proposed rate-1 code achieves a full transmit diversity order with all MPPM constellations without introducing any constellation expansion. We also show that the proposed code profits from a low decoding complexity in the absence of interference between the different modulated UWB pulses.

In this paper, we propose an accurate method for evaluating the performance of Ultra-Wideband (UWB) coherent receivers over the IEEE 802.15.3a channel model. Depending on the severeness of fading, the method consists of approximating the distribution of the captured channel energy by either a Coxian, gamma-mixture or lognormal distribution based on a least square fitting criterion. The proposed approximations lend themselves to simple mathematical analysis and turn out to be useful in evaluating the performance, in terms of average bit-error-rate (BER) and outage probability, over the IEEE 802.15.3a channels.

In this paper, we consider the problem of applying the Multiple-Input-Multiple-Output (MIMO) techniques on Impulse-Radio Time-Hopping Ultra-Wideband (IR-TH-UWB) communications. In particular, we propose two novel Space-Time (ST) block codes that are suitable for UWB. The proposed encoded MIMO-UWB schemes present the main advantage of conveying the information only through the positions of the very short unipolar UWB pulses. The constraint of unipolar transmissions keeps the transceiver structures very simple since it imposes no additional constraints on the RF circuitry to control the amplitudes or the phases of the sub-nanosecond UWB pulses. Consider the case where the transmitter is equipped with P antennas and where M PPM modulation positions are available. The first proposed scheme achieves a full transmit diversity order for M ges P while transmitting at the rate of log2(M) bits Per Channel Use (PCU). The second scheme is fully diverse with any number of antennas and transmits at a rate of M log2(P)/P bits PCU. The proposed codes permit to achieve different levels of compromise between complexity and performance since scheme 1 necessitates M-dimensional Maximum-Likelihood (ML) decoding while scheme 2 necessitates MP-dimensional decoding. We also present a comprehensive analysis on the enhancement in terms of the data rate achieved at a certain communication distance based on realistic indoor channel models and on an exact system model that takes inter-pulse-interference and intersymbol- interference into consideration.

In this paper, we propose orthogonal Space-Time (ST) codes for binary Pulse Position Modulations (PPM). Unlike the well known orthogonal ST codes, the proposed schemes verify the additional constraint of achieving a full transmit diversity order without introducing any phase rotations. This renders the proposed codes suitable for Free-Space Optical (FSO) communications with direct detection and for Ultra-WideBand (UWB) communications. At the receiver side, optimal detection can be achieved with linear operations and the proposed codes can be also applied with On-Off Keying (OOK).

We present a general technique for constructing minimal-delay rate-1 Space-Time (ST) block codes for Pulse Position Modulations (PPM) with an arbitrary number of transmit antennas. We show that the novel idea of time-domain constellation extension as well as introducing joint position and symbol permutations achieves a full transmit diversity order while maintaining unipolar transmissions. This renders the proposed scheme suitable for low-cost Time-Hopping Ultra- Wideband (TH-UWB) systems as well as Free-Space Optical (FSO) communications with direct detection.

In this paper, we consider the problem of space-time (ST) coding with unipolar pulse position modulations (PPM) and propose a novel ST code that satisfies a large number of construction constraints rendering it superior to the existing PPM encoding schemes. In particular, the proposed 2 times 2 code achieves a full transmit diversity order while transmitting at a rate of 1 PPM-symbol per channel use. The proposed scheme can be associated with M-ary PPM constellations for all even values of M without introducing any constellation expansion. This renders the proposed scheme suitable for low cost carrier-less ultra-wideband (UWB) systems where information must be conveyed only by the time delays of the modulated sub-nanosecond pulses without introducing any amplitude amplifications or phase rotations. Finally, the proposed scheme can be associated with a reduced complexity optimal maximum-likelihood (ML) decoder that takes the structure of the proposed code into consideration in order to simplify the decoding procedure. We also propose a simple diversity-preserving suboptimal decoder that requires approximately half the number of multiplications compared to the ML decoder. Possible extensions to transmitters equipped with three antennas are also discussed in situations where a certain number of feedback bits is available.

One of the major challenges for optical network operators is related to realizing profitable networks that preserve the overall satisfaction of their end customers. This necessitates ensuring a controlled network resource usage while decreasing connection blocking probability. However, optical networks are witnessing the introduction of an increasing number of new services, each having different requirements. As such, the evolution towards resource efficient connection setup management strategies that provide differentiated call-blocking probability becomes inevitable. Building on the previous observation, we propose in this paper a novel connection setup management strategy. This latter aims to differentiate the blocking probability among different classes of lightpath requests, while avoiding the starvation of the lower class lightpaths. The main idea behind our proposal is to privilige higher class lightpaths via preemption and to protect the lower class lightpaths through rerouting. In this manner, the proposed strategy serves the double objective of minimizing the overall connection blocking probability and of realizing quality of service (QoS) differentiation. The previous claim is asserted through our simulation results that gauge the main benefits behind our proposed connection setup strategy. The simulation results drawn from the National Science Foundation Network (NSFNET) and the European Optical Network (EON) show that the proposed strategy achieves both differentiated and low blocking probabilities.

In this paper, we consider the problem of space-time (ST) coding with unipolar pulse position modulations (PPM). The proposed code satisfies a large number of construction constraints that render it superior to the existing PPM encoding schemes. In particular, the proposed 2 times 2 code achieves a full transmit diversity order while transmitting at a rate of 1 PPM-symbol per channel use. The proposed scheme can be associated with M-ary PPM constellations for all even values of M without introducing any constellation extension. This renders the proposed scheme suitable for low cost carrier-less ultra-wideband (UWB) systems where information must be conveyed only by the time delays of the modulated sub-nanosecond pulses without introducing any amplitude amplification or phase rotation. Finally, the proposed scheme is symbol-by-symbol decodable where the information can be reconstituted by performing simple linear operations at the receiver side. A possible extension to transmitters equipped with three antennas is also discussed in situations where a certain number of feedback bits is available.

In this paper, we propose a new Multiple-Input-Multiple-Output (MIMO) transceiver for Transmitted-Reference Impulse-Radio Ultra-Wideband (TR-IR-UWB) communications. The proposed scheme is capable of achieving full spatial and multi-path diversity orders while assuring a simple symbol-by-symbol decodability with analog autocorrelation receivers that do not require any kind of channel estimation. This approach results in high performance levels while keeping the natural advantages of simple and low-cost TR-UWB receivers that do not require local oscillators or analog-to-digital converters.

In this paper, we consider the problem of space-time (ST) coding with pulse position modulation (PPM). While all the existing ST block codes necessitate rotating the phase or amplifying the amplitude of the transmitted symbols, the proposed scheme can be associated with unipolar PPM constellations without introducing any additional constellation extension. In other words, full transmit diversity can be achieved while conveying the information only through the time delays of the modulated signals transmitted from the different antennas. The absence of phase rotations renders the proposed scheme convenient for low- cost carrier-less multiple-input-multiple-output (MIMO) time- hopping ultra-wideband (TH-UWB) systems and for MIMO free-space optical (FSO) communications with direct detection. In particular, we propose two families of minimal-delay ST block codes that achieve a full transmit diversity order with PPM. Designate by n the number of transmit antennas and by M the number of modulation positions. For a given set of values of (n, m), the first family of codes achieves a rate of 1 symbol per channel use (PCU) which is the highest possible achievable rate when no constellation extensions are introduced. The second family of codes can be applied with a wider range of (n, m) at the expense of a reduced rate given by: R=1/n+n-1/n log2(M-1)/n log2(M).

In this paper, we extend the Amplify-and-Forward cooperative diversity scheme to the context of impulse radio ultra wideband (IR-UWB). In particular, we present the construction of three families of minimal-delay and totally-real distributed algebraic space-time (ST) codes suitable for IR-UWB. The first family encodes adjacent symbols and is based on totally-real cyclic division algebras. The second family encodes the pulses used to transmit one information symbol and permits to achieve high performance levels with lower complexity. Both families of codes achieve full rate, full diversity with non-vanishing determinants for various number of relays. These schemes can be associated with pulse position modulation (PPM), pulse amplitude modulation (PAM) and hybrid pulse position and amplitude modulation (PPM-PAM). The third family of codes is information-lossless and does not require any pulse repetitions. It is specific to M-PPM-M'-PAM with M ges 3 and for all values of M'. Simulations performed over realistic indoor UWB channels are provided to verify the theoretical results.

In this paper, we consider the problem of maximum-likelihood (ML) detection with multi-antenna impulse radio ultra-wideband (IR-UWB) communications in the cases where the information is modulated onto either the positions or the positions and amplitudes of the transmitted UWB pulses. The proposed solution assures good convergence times since the structure of these sparse and multi-dimensional constellations is taken into consideration. While all the known ML decoders can not be applied with these constellations that do not have a lattice structure, the proposed solution assures an optimal detection.

In this paper, we propose for the first time a 2 × 2 space-time (ST) block code that can be applied with pulse position modulation (PPM) without introducing any constellation extension. This encoding scheme is adapted to low cost carrierless time-hopping ultra-wideband (TH-UWB) systems where information must be conveyed only by the time delay of the modulated signal that has a very low duty cycle. The proposed scheme permits to achieve full transmit diversity with M-ary PPM constellations for all values of M. Moreover, it permits to overcome the additional phase rotations or amplitude amplifications that are introduced by all the known ST codes such as the Alamouti code (S.M. Alamouti, 1998), the rate-1 codes proposed in B.A. Sethuraman et al. (2003), (A. J. Hammons and H. El Gamal, 2000) for PSK and the diversity schemes proposed in L. Yang and G.B. Giannakis (2004) for TH-UWB systems with PAM or PPM.

In this work, we propose novel families of space-time (ST) block codes that can be associated with impulse radio ultra-wideband (IR-UWB) communication systems. The carrier-less nature of this nonconventional totally real transmission technique necessitates the construction of new suitable coding schemes. In fact, the last generation of complex-valued ST codes (namely, the perfect codes) cannot be associated with IR-UWB systems where the phase reconstitution at the receiver side is practically infeasible. On the other hand, while the perfect codes were considered mainly with quadrature amplitude modulation (QAM) and hexagonal (HEX) constellations, IR-UWB systems are often associated with pulse-position modulation (PPM) and hybrid PPM-PAM (pulse-amplitude modulation) constellations. In this paper, instead of adopting the classical approach of constructing ST codes over infinite fields or for the perfect codes), we study the possibility of constructing modulation-specific codes that are exclusive to PPM and PPM-PAM. The proposed full-rate codes are totally real, information lossless, and have a uniform average energy per transmit antenna. They permit to achieve a full diversity order with any number of transmit antennas. In some situations, the proposed schemes have an optimal nonvanishing coding gain and satisfy all the construction constraints of the perfect codes in addition to the constraint of being totally real. Simulations performed over realistic indoor UWB channels showed that the proposed schemes outperform the best known codes constructed from cyclic division algebras.

This paper presents the construction of new totally-real space-time coding schemes suited for carrier-less ultra-wideband transmissions. These schemes are associated with pulse position modulation and with hybrid pulse amplitude and position modulation where the input data is modulated onto both the pulse amplitudes and positions. The new schemes have a uniform average transmitted energy per antenna and achieve full rate and full diversity with hybrid M-PPM-M'-PAM for all values of M' and for M ges 3, M ges 5 and {M = 5, M ges 7} with n = 2, 3 and 4 transmit antennas respectively

In this paper, we present a new full rate scheme for exploiting diversity in multi-antenna ultra-wideband (UWB) systems. The independent data streams that are transmitted simultaneously from the different antennas are rendered orthogonal by the use of modified Hermite pulses. At a second time, appropriate real rotations enhance the transmit diversity level. We also consider, for the first time, the problem of maximum likelihood (ML) detection with multi-dimensional PPM and hybrid PPM-PAM constellations that can be used in conjunction with the proposed scheme

In this paper, we extend the amplify-and-forward (AF) cooperative diversity scheme to the context of impulse radio ultra wideband (IR-UWB). In particular, we apply a space-time (ST) coding scheme based on totally-real cyclic division algebras in order to achieve full diversity with no data rate losses. At a second time, the pulse repetitions are exploited in order to enhance the performance of the first scheme and to propose a new scheme that achieves comparable performance with lower complexity. These schemes are associated with pulse position modulation (PPM), pulse amplitude modulation (PAM) and hybrid pulse position and amplitude modulation (PPM-PAM)

In this paper, we present the construction of full-rate, fully diverse, and totally real space–time (ST) codes for ultra-wideband (UWB) transmissions. In particular, we construct two families of codes adapted to real carrierless UWB communications that employ pulse position modulation, pulse amplitude modulation, or a combination of the two. The first family encodes adjacent symbols and is constructed from totally real cyclic division algebras. The second family encodes the pulses used to convey one information symbol, and permits achieving high performance levels with reduced complexity. The first family of codes achieves only a fraction of the coding gain of the second one. Moreover, these coding gains are independent from the size of the transmitted constellation. For time-hopping multiple-access channels, the amplitude spreading code associated with the second family of codes is taken to be user-specific. In this case, a simple design criterion is proposed, and spreading matrices constructed according to this criterion permit reducing the level of multiple-access interference (MAI). Performance over realistic indoor UWB channels verify the theoretical claims, and show high performance levels and better immunity against MAI.

In this paper, we present the construction of full rate, fully diverse, and totally real space-time (ST) codes for ultra-wideband (UWB) transmissions. In particular, we construct two families of codes adapted to real carrierless UWB communications that employ pulse position modulation, pulse amplitude modulation, or a combination of the two. The first family encodes adjacent symbols and is constructed from totally real cyclic division algebras. The second family encodes the pulses used to convey one information symbol, and permits achieving high performance levels with reduced complexity. The first family of codes achieves only a fraction of the coding gain of the second one. Moreover, these coding gains are independent from the size of the transmitted constellation. For time-hopping multiple-access channels, the amplitude spreading code associated with the second family of codes is taken to be user-specific. In this case, a simple design criterion is proposed, and spreading matrices constructed according to this criterion permit reducing the level of multiple-access interference (MAI). Simulations performed over realistic indoor UWB channels verify the theoretical claims and show high performance levels and better immunity against MAI