Search

Optical wireless communication (OWC) over atmospheric turbulence and pointing errors is a well-studied topic. Still, there is limited research on signal fading due to random fog in an outdoor environment for terrestrial wireless communications. In this paper, we analyze the performance of a decode-and-forward (DF) relaying under the combined effect of random fog, pointing errors, and atmospheric turbulence with a negligible line-of-sight (LOS) direct link. We consider a generalized model for the end-to-end channel with independent and not identically distributed (i.ni.d.) pointing errors, random fog with Gamma distributed attenuation coefficient, double generalized gamma (DGG) atmospheric turbulence, and asymmetrical distance between the source and destination. We develop density and distribution functions of signal-to-noise ratio (SNR) under the combined effect of random fog, pointing errors, and atmospheric turbulence (FPT) channel and distribution function for the combined channel with random fog and pointing errors (FP). Using the derived statistical results, we present analytical expressions of the outage probability, average SNR, ergodic rate, and average bit error rate (BER) for both FP and FPT channels in terms of OWC system parameters. We also develop simplified and asymptotic performance analysis to provide insight on the system behavior analytically under various practically relevant scenarios. We demonstrate the mutual effects of channel impairments and pointing errors on the OWC performance, and show that the relaying system provides significant performance improvement compared with the direct transmissions, especially when pointing errors and fog becomes more pronounced. [ABSTRACT FROM AUTHOR]

A hybrid satellite-terrestrial relay network (HSTRN) has been envisioned as a promising communication architecture for future wireless communications, which can prominently reduce the impact of shadow fading and expand service coverage. In this paper, we conduct theoretical performance analysis for a dual-hop communication system, consisting of multiple relays and multiple users in the HSTRN with the amplify-and-forward protocol. For the shadowing effect and the multipath effect, we introduce the shadowed-Rician distribution and Nakagami- $m$ fading to characterize the channel models for the satellite-relay and the relay-user links, respectively. Additionally, the opportunistic scheduling scheme is employed, where the optimal relay and the optimal user are selected from the alternative nodes with the maximum instantaneous signal-to-noise ratio. Thereafter, the analytical expressions including ergodic capacity and average symbol error rate (SER) are derived. More specifically, we also present the asymptotic behavior of the average SER at the high signal-to-noise ratio (SRN) regime. The theoretical analysis is validated by the numerical results. Moreover, the results also reveal that the performance of the system can be improved by increasing the number of relays and users under different channel parameters and various modulation schemes. [ABSTRACT FROM AUTHOR]

We consider two users A and B wanting to exchange their messages MA and MB via a relay in a wireless communication network. To this aim, we have two methods: direct forward (DF) and computation and forward (CAF). In the first method, after an orthogonal multiple access (OMA) or non-orthogonal multiple access (NOMA) phase, the receiver decodes the two messages. In the second method, after a NOMA phase, the receiver decodes only the (modulo) sum. In the latter case the capacity region (computation region) is not known. This paper compares the two methods. To such aim, we propose a unified coding framework and identify a lower bound of achievable computation rates. We consider three constellations for the symmetric two-user Gaussian channel with Eisenstein constellation showing highest performance. We also show that a minimum signal-to-noise ratio is required for our CAF method to outperform DF, and we obtain the numerical values of such minimum ratios for the three constellations. Finally, we show achievability of our bounds, which is proved to require affine codes for computation, which has not been proved before. Overall, our results are useful for communication system designers towards practical implementation of multiple access methods in integrated wireless systems. [ABSTRACT FROM AUTHOR]

Cooperative beamforming is a potentially useful technique for enhancing link reliability in modern wireless relay communications. To aid the beamforming design, the signal-to-noise ratio (SNR) of the source-to-relay (S-R) channel links must be known at the destination node. Abdallah and Papadopoulos in IEEE Trans. Signal Processing, vol. 56, no. 10, 2008, proposed a beamforming system with relay-assisted SNR acquisition; that is, the S-R link SNR is first quantized at each relay and is then forwarded to the destination. An optimal quantizer design problem was proposed in that paper, aiming at reducing the system bit error rate; however, only the binary quantization case was considered therein, with the quantization threshold computed by a suggested rule of thumb. In this paper, we study the aforementioned quantizer design problem in the general multiple-bit setting. We show that the solutions to the first-order necessary condition for optimality can be obtained as a fixed point of a certain nonlinear map over the feasible threshold set. The existence and uniqueness of the fixed point are established by using, respectively, the Brouwer’s and Schauder’s fixed point theorems. Finally, we show that the fixed point thus obtained yields the globally optimal set of quantization thresholds. To the best of our knowledge, this paper is the first which leverages fixed point theories to rigorously solve SNR quantization problems in the context of wireless relay communication. [ABSTRACT FROM AUTHOR]

Non-orthogonal multiple access (NOMA) and spectrum sharing (SS) are two emerging multiple access technologies for efficient spectrum utilization in future wireless communications standards. In this paper, we present the performance analysis of a NOMA-based cooperative relaying system (CRS) in an underlay spectrum sharing scenario, considering a peak interference constraint (PIC), where the peak interference inflicted by the secondary (unlicensed) network on the primary-user (licensed) receiver (PU-Rx) should be less than a predetermined threshold. In the proposed system the relay and the secondary-user receiver (SU-Rx) are equipped with multiple receive antennas and apply selection combining (SC), where the antenna with highest instantaneous signal-to-noise ratio (SNR) is selected, and maximal-ratio combining (MRC), for signal reception. Closed-form expressions are derived for the average achievable rate and outage probabilities for SS-based CRS-NOMA. These results show that for large values of peak interference power, the SS-based CRS-NOMA outperforms the CRS with conventional orthogonal multiple access (OMA) in terms of spectral efficiency. The effect of the interference channel on the system performance is also discussed, and in particular, it is shown that the interference channel between the secondary-user transmitter (SU-Tx) and the PU-Rx has a more severe effect on the average achievable rate as compared to that between the relay and the PU-Rx. A close agreement between the analytical and numerical results confirm the correctness of our rate and outage analysis. [ABSTRACT FROM AUTHOR]

To improve the secrecy performance of the untrusted relay system with a multiple-antenna destination, a novel full-duplex destination jamming scheme with optimal antenna selection (FDJ-OAS) is proposed in this paper. The approximate closed-form expressions of the average achievable secrecy rate are derived for the proposed scheme. To attain more insightful results, the approximated closed-form expressions for the average achievable secrecy rate and the optimal power allocation factor for FDJ-OAS are derived in the case with a large-scale antenna array, and simple power allocation solutions are also provided in the high and low SNR regimes, respectively. Furthermore, we derive the asymptotic expressions for the secrecy outage probability under different asymptotic conditions for the proposed scheme, and the analysis results indicate that the secrecy diversity order of FDJ-OAS is significantly superior to that of the comparable schemes. Simulation results show that the analytical curves match well with the simulation results, and the advantage of FDJ-OAS is also clearly demonstrated through performance comparison. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the secrecy performance of a dual-hop cooperative relaying network, where a source node communicates with a destination node via an amplify-and-forward relay node and via direct link in the presence of a passive eavesdropper. In this network, the eavesdropper exploits the advantages of both direct and relay links. We assume that the source, relay, and eavesdropper nodes are mobile, and the destination node is stationary. Moreover, we consider that the source-relay, relay-destination, and source-destination links experience Rayleigh fading, whereas source-eavesdropper and relay-eavesdropper links undergo double-Rayleigh fading. Examples of such scenario are encountered in vehicle-to-vehicle communication and/or vehicle-to-infrastructure communication under cooperative vehicular networks. In particular, we derive new tight closed-form expressions for the secrecy outage probability and ergodic secrecy capacity of the considered system under mixed fading environments. In addition, we present an effective secrecy diversity order within the main-to-eavesdropper ratio ranges of practical interest, and demonstrate that the secrecy diversity order of 2 can be achieved. Finally, the numerical and simulation results corroborate our analytical findings, and show the occurrence conditions for secrecy outage probability and ergodic secrecy capacity floors, and impacts of channel conditions and relay and eavesdropper positions on the system secrecy performance. [ABSTRACT FROM AUTHOR]

In this paper, we study the connectivity between two fixed terminal nodes within a geometrically bounded network of relay nodes and fixed infrastructure nodes. We assume that a communication path, between the terminal nodes, can be established by direct connection (1-hop), via a relay node (2-hop) or via the infrastructure network; the choice is based on link viability in a Rayleigh fading environment. We adopt a probabilistic approach to our analysis considering a homogeneous spatial distribution of relay nodes and an inhomogeneous spatial distribution of infrastructure nodes. Our analysis of the relationship between relay and infrastructure densities for a prescribed outage probability shows that reliance on infrastructure connectivity can be appreciably reduced by employing direct and 2-hop connectivity. We further show that, as the relay density increases, optimum connectivity is achieved by employing a non uniform spatial thinning of the infrastructure, which is dependent upon the geometric extent of the network. Our analysis provides an insight into multi-mode connectivity in bounded network domains and has application in many fields including cellular and vehicle-to-infrastructure (V2X), which are often modelled as having finite extent and where reducing the reliance on fixed infrastructure can effectively reduce operating expenditure costs. [ABSTRACT FROM AUTHOR]

Incremental relaying is a spectral-efficient relaying approach which introduces itself as one of the possible solutions to the spectrum scarcity problem for future generations of cellular communications systems. In this paper, we study threshold amplify-and-forward incremental relaying for correlated fading channels, i.e., fading channels for which the channel gain amplitude varies during the time interval of a block transmission; however, the variation occurs in a correlating fashion. From this aspect, these channels are different from both block-fading and fast-fading channels. Due to the correlating nature of the channels, it is possible to compress channel state information (CSI) sequences and employ the compressed CSI sequences as side information to carefully select the bits transmitted by the relay. This idea is already employed in previous works to increase the spectral efficiency; however, those works consider block-fading channels and also assume maximal ratio combining (MRC) at receiver. In case of MRC, explicit expressions for the end-to-end signal-to-noise ratio (SNR) exist, which enable packet error rate analysis by aid of Moment Generating Functions of the SNR. However, to apply MRC, the receiver requires all channel gains (including source-relay channel gain). In case of correlated channels, communicating all real-valued channel gains, requires a large overhead. This fact renders applicability of MRC for correlated channels. We apply the more practical Equal-Gain Combining method. We derive series approximations for symbol error rate, given M-QAM and MPSK modulations, and assuming Nakagami- $m$ fading channels. We also analyze the spectral efficiency by assuming Markov models for the channels. We compare the spectral efficiency in case of correlated and fast-fading channels and show that, unlike fast-fading channels, correlated fading channels allow for a spectrally efficient scheme. [ABSTRACT FROM PUBLISHER]

In this paper, a novel cooperative non-orthogonal multiple access (NOMA) system is proposed, where one near user is employed as decode-and-forward relaying switching between full-duplex (FD) and half-duplex (HD) mode to help a far user. Two representative cooperative relaying scenarios are investigated insightfully. The first scenario is that no direct link exists between the base station (BS) and far user. The second scenario is that the direct link exists between the BS and far user. To characterize the performance of potential gains brought by the FD NOMA in two considered scenarios, three performance metrics outage probability, ergodic rate, and energy efficiency are discussed. More particularly, we derive new closed-form expressions for both exact and asymptotic outage probabilities as well as delay-limited throughput for two NOMA users. Based on the derived results, the diversity orders achieved by users are obtained. We confirm that the use of direct link overcomes zero diversity order of far NOMA user inherent to FD relaying. In addition, we derive new closed-form expressions for asymptotic ergodic rates. Based on these, the high signal-to-noise ratio (SNR) slopes of two users for FD NOMA are obtained. Simulation results demonstrate that: 1) the FD NOMA is superior to the HD NOMA in terms of outage probability and ergodic sum rate in the low SNR region; and 2) in delay-limited transmission mode, the FD NOMA has higher energy efficiency than the HD NOMA in the low SNR region; However, in delay-tolerant transmission mode, the system energy efficiency of the HD NOMA exceeds the FD NOMA in the high SNR region. [ABSTRACT FROM PUBLISHER]

Physical transceivers have hardware impairments that create distortions which degrade the performance of communication systems. The vast majority of technical contributions in the area of relaying neglect hardware impairments and, thus, assume ideal hardware. Such approximations make sense in low-rate systems, but can lead to very misleading results when analyzing future high-rate systems. This paper quantifies the impact of hardware impairments on dual-hop relaying, for both amplify-and-forward and decode-and-forward protocols. The outage probability (OP) in these practical scenarios is a function of the effective end-to-end signal-to-noise-and-distortion ratio (SNDR). This paper derives new closed-form expressions for the exact and asymptotic OPs, accounting for hardware impairments at the source, relay, and destination. A similar analysis for the ergodic capacity is also pursued, resulting in new upper bounds. We assume that both hops are subject to independent but non-identically distributed Nakagami-m fading. This paper validates that the performance loss is small at low rates, but otherwise can be very substantial. In particular, it is proved that for high signal-to-noise ratio (SNR), the end-to-end SNDR converges to a deterministic constant, coined the SNDR ceiling, which is inversely proportional to the level of impairments. This stands in contrast to the ideal hardware case in which the end-to-end SNDR grows without bound in the high-SNR regime. Finally, we provide fundamental design guidelines for selecting hardware that satisfies the requirements of a practical relaying system. [ABSTRACT FROM AUTHOR]

Considering a dual-hop energy-harvesting (EH) relaying system, this paper advocates novel relaying protocols based on adaptive time-switching (TS) for amplify-and-forward and decode-and-forward modes, respectively. The optimal TS factor is first studied, which is adaptively adjusted based on the dual-hop channel state information (CSI), accumulated energy, and threshold signal-to-noise ratio (SNR), to achieve the maximum throughput efficiency per block. To reduce the CSI overhead at the EH relay, a low-complexity TS factor design is presented, which only needs single-hop CSI to determine the TS factor. Theoretical results show that, in comparison with the conventional solutions, the proposed optimal/low-complexity TS factor can achieve higher limiting throughput efficiency for sufficiently small threshold SNR. As the threshold SNR approaches infinity, the throughput efficiency of the proposed optimal/low-complexity TS factor tends to zero in a much slower pace than that of the conventional solutions. Simulation results are presented to corroborate the proposed methodology. [ABSTRACT FROM PUBLISHER]

This paper analyzes the impact of statistical delay constraints on the achievable rate of a two-hop wireless communication link in which the communication between a source and a destination is accomplished via a buffer-aided relay node. It is assumed that there is no direct link between the source and the destination, and the buffer-aided relay forwards the information to the destination by employing the decode-and-forward scheme. Given statistical delay constraints specified via maximum delay and delay violation probability, the tradeoff between the statistical delay constraints imposed on any two concatenated queues is identified. With this characterization, the maximum constant arrival rates that can be supported by this two-hop link are obtained by determining the effective capacity of such links as a function of the statistical delay constraints, signal-to-noise ratios at the source and relay, and the fading distributions of the links. It is shown that asymmetric statistical delay constraints at the buffers of the source and relay node can improve the achievable rate. Overall, the impact of the statistical delay tradeoff on the achievable throughput is provided. [ABSTRACT FROM PUBLISHER]

This paper analyzes the performance of energy-constrained dual-hop amplify-and-forward relaying systems with multi-antenna nodes in the presence of multiple co-channel interferers at the destination. To maximize the overall signal-to-interference-plus-noise ratio, as well as the harvested energy so as to mitigate the severe effects of fading and enable long-distance wireless power transfer, hop-by-hop information and energy beamforming is proposed where the transmitted signal is steered along the strongest eigenmode of each hop. The wirelessly powered relay scavenges energy from the source information radio frequency signal through energy beamforming, where both the time-switching receiver and power-splitting receiver are considered, and then uses the harvested energy to forward the source message to the destination. To this end, tight lower and upper bound expressions for the outage probability and ergodic capacity are presented in closed form. These are employed to investigate the throughput of the delay-constrained and delay-tolerant transmission modes. In addition, the asymptotic high signal-to-noise ratio outage probability and ergodic capacity approximations are derived, where the achievable diversity order is also presented. Numerical results sustained by Monte Carlo simulations show the tightness of the proposed analytical expressions. The impact of various parameters, such as energy harvesting time, power-splitting ratio, source transmit power, and the number of antennas on the system throughput, is also considered. [ABSTRACT FROM PUBLISHER]

This paper investigates the impact of practical operation conditions such as channel information imperfection (CII), independent nonidentical (i.n.i) fading distributions, strict power constraints (i.e., peak transmit power constraint and primary outage constraint), and primary interference on outage performance of opportunistic relay selection (ORS) in underlay cooperative cognitive networks (UCCNs). Toward this end, the power of secondary transmitters is first established to meet strict power constraints and account for primary interference and CII. Then, exact closed-form outage probability expressions for the secondary destination employing the maximum ratio combining (MRC) and the selection combining (SC) are proposed to promptly evaluate the effect of these conditions and provide useful insights into performance limits. Numerous results illustrate significant system performance deterioration due to primary interference and CII, performance saturation phenomenon in the secondary network, performance compromise between the secondary network and the primary network, significant performance improvement with respect to the increase in the number of involved relays, a large gap between the lower outage bound (MRC's outage performance) and the upper outage bound (SC's outage performance), and the advantage of utilizing direct channel between the source and the destination. [ABSTRACT FROM PUBLISHER]

In this paper, we consider a dual-hop amplify-and-forward (AF) multiple-input–multiple-output (MIMO) relay network, where the source, relay, and destination are each equipped with multiple antennas. The relay is untrusted if it is willing to forward the signal to the destination and, at the same time, acts as a potential eavesdropper to interpret the message from the source. Since there exists no direct link between the source and the destination, a positive secrecy rate cannot be obtained. Addressing this issue, we propose a joint destination-aided cooperative jamming and precoding at both the source and the relay scheme: joint source, relay, and destination precoding (JP). We target at maximizing the secrecy rate by jointly designing the source, relay, and destination precoding matrices and propose an alternating iterative optimization algorithm to tackle the nonconvex problem. Then, a comprehensive study on the asymptotic performance is conducted in a high signal-to-noise ratio (SNR) regime. In particular, we present simple closed-form expressions for the two key performance parameters in the asymptotic secrecy rate, i.e., the high-SNR slope and the high-SNR power offset. Finally, numerical results are conducted to demonstrate the validity of the proposed secure scheme and its performance analysis. [ABSTRACT FROM PUBLISHER]

A novel transmission scheme for cooperative relay networks is presented in this paper. The proposed scheme is based on the non-orthogonal selection decode-and-forward protocol with an additional assumption of having a low rate feedback channel from the destination to relays. Benefited from the feedback information, an optimal distributed code that has an extremely short delay equal to four is constructed, and the same code is applicable to networks with the arbitrary number of relays to yield optimal cooperative diversity. The proposed code is sphere decodable with a decoding complexity again independent of the number of relays in high SNR regime. In particular, when operating at multiplexing gain ≥(1/2), the lattice decoder at the destination has a zero complexity exponent, meaning a constant decoding complexity and independent of transmission rate. Analyses for the decoding complexity of other existing diversity-optimal distributed codes are also provided. It is shown that these codes have a linear growth in delay and an exponential growth in decoding complexity as the number of relays increases. [ABSTRACT FROM PUBLISHER]

Though a cooperative broadcast scheme has been proposed for fading environments, it has two defects: First, it only handles a packet flow from a single source node in the network, but does not consider the scenario of multiple packet flows simultaneously broadcasted from different source nodes. Second, it only allows a single relay node to forward a packet in each time slot, though multiple relay nodes forwarding in a time slot can significantly reduce broadcast latency. In this paper, we aim achieve low-latency multi-flow broadcast in wireless multi-hop networks with fading channels. To describe the interference among the transmission in different flows, we incorporate the Rayleigh fading model to the signal to noise ratio (SNR) model. Then, we introduce a cooperative diversity scheme which allows multiple relays forwarding in a time slot to reduce broadcast latency. We then formulate an interesting problem: In a fading environment, what is the optimal relay allocation schedule to minimize the broadcast latency? We propose a warm up heuristic algorithm for single-flow cooperative broadcast, based on which, we further propose a heuristic algorithm for multi-flow cooperative broadcast. Simulation results demonstrate that the two algorithms achieve lower broadcast latency than a previous method. [ABSTRACT FROM PUBLISHER]

This paper proposes a new technique for soft information relaying, which is based on a soft decode–compress–forward relay protocol. The proposed system provides a means of using distributed low-density parity-check (LDPC) coding in conjunction with higher order modulation, such as pulse amplitude modulation (PAM) and quadrature amplitude modulation (QAM), which is effective even under poor source–relay link conditions. Ordinarily, such schemes suffer from error propagation to the destination caused by incorrect decoding at the relay when the signal-to-noise ratio (SNR) on the source–relay link is low; however, our system avoids this problem by generating soft versions of the additional (parity-bearing) PAM symbols for transmission from the relay. The proposed technique of soft compression does not suffer from parity log-likelihood ratios (LLRs) converging to zero, as do many soft re-encoding techniques for turbo and LDPC codes. In the case of Gray-coded PAM/QAM signaling, we also propose a method of performing exact expectation-based soft modulation with low computational complexity. Furthermore, we propose a new model, which we refer to as the soft scalar model, for the overall source-to-destination channel encountered by the constellation symbols, and this model is used at the destination to compute LLRs for joint decoding of the distributed LDPC code. Simulation results demonstrate that the proposed scheme can provide good coding gain, diversity gain, and spectral efficiency under poor source–relay SNR conditions. [ABSTRACT FROM AUTHOR]

Compute-and-forward (C&F) is a promising new approach to interference management, enjoying several advantages over other information-theoretic schemes. C&F usually requires channel state information (CSI) at the receivers so that an “optimal” scaling factor can be computed for the purposes of decoding. In this paper, a blind C&F scheme—i.e., one not requiring CSI—is developed. Rather than attempting to compute the optimal scaling factor, this new scheme seeks one or more “good” scalars, i.e., scalars that allow correct decoding despite possibly being suboptimal. The region of all such good scalars is characterized. To find a good scalar, a computationally efficient scheme is proposed which involves error-detection, a hierarchically organized list, as well as a use of the smoothing lemma from lattice theory. Simulation results show that our blind C&F scheme achieves—for a class of nested lattice codes—the same throughput as its CSI-enabled counterpart at the expense of, approximately, a two-fold increase in computational complexity in the high-throughput region. Moreover, our blind C&F scheme can be applied to multisource multirelay networks with a good performance/complexity tradeoff. [ABSTRACT FROM AUTHOR]

In this paper, we consider the adaptive-gain $M$-relay amplify-and-forward (AF) cooperative system with both conventional relaying $(cr),$ as well as a best-relay-selection $(br)$ scheme, and investigate its performance over time-selective fading, due to nodes' mobility, in the presence of channel estimation errors. We first derive exact closed-form expressions for the destination's direct path and overall end-to-end instantaneous signal-to-noise ratios (SNRs). We also derive closed-form tight approximate expressions for the system average bit error rate, outage probability, and Shannon capacity, which are general functions of the cooperating nodes' relative speeds, in terms of the links' correlation parameters, the estimation errors' variances, and the receivers' tracking loops' channel estimation rates, in terms of the transmitted block length. Our analysis reveals that due to the cooperating nodes' mobility, the system performance metrics are severely degraded and, in some particular network circumstances, experience asymptotic limits. In addition, when both of the source and the destination are static, the system performance does not experience asymptotic limits although the relays are in motion. On the other hand, despite that the relays are static, the mobility of either the source or the destination severely impacts the system performance by asymptotic limits, where the difference between their impacts depends on the symmetrical conditions of the network two-hop fading gain power. In addition, we found that the $br$ provides higher asymptotic error limits than that of the $cr$ scheme. We also assumed the special case when the network receivers are equipped with fast estimation loops and showed their effect of reducing the degradation impacts of the nodes' mobility. Simulation and numerical results are provided to verify the accuracy of the derived analytical expressions. [ABSTRACT FROM PUBLISHER]

The end-to-end performance of cooperative beamforming in dual-hop relaying systems operating over $\eta-\mu$ fading channels is investigated. In the analysis, the source–destination node pair is equipped with multiple antennas, whereas the fixed-gain relay has a single antenna. The $\eta-\mu$ fading channel has Nakagami- $m$, Rayleigh, one-sided Gaussian, and Nakagami- $q$ (Hoyt) channels as special cases. In this paper, novel statistical expressions for the generalized moments of the end-to-end signal-to-noise ratio (SNR), cumulative density function (cdf), probability density function (pdf), and moment-generating function (mgf) are derived and evaluated. Furthermore, the obtained results are used to derive new and simple expressions for the pdf, the outage probability, and the mgf of the end-to-end SNR over Nakagami- $m$ fading channels. In addition, we show that the derived expression for parameter $C$ over the Nakagami- $m$ fading channel is incorrect, and hence, it leads to inaccurate results. Also, we study the impact of the fading parameters $\eta$ and $\mu$ on the end-to-end system performance. Simulation results show that, in contrast to the fading parameter $\eta$, $\mu$ has a significant impact on the system performance. [ABSTRACT FROM PUBLISHER]

This paper tackles the problem of minimum-energy cooperative transmission in wireless networks under the assumption of limited channel state information at the transmitters. It is assumed that only the average statistics of the fading channels are known to the transmitters. The objective here is to jointly optimize the set of relays and the transmission powers for both the broadcasting and cooperative transmission phases while satisfying probabilistic signal-to-noise ratio (SNR) constraints at the relays and at the destination node. Increasing the broadcasting power expands the set of potential relays and decreases the required power for cooperative transmission. Hence, there is a compromise in the selection of the power values, which is addressed in this work using a chance-constrained optimization framework. A closed-form approximate solution is also presented, which provides a low-complexity transmission scheme for energy-harvesting wireless networks. Simulations are presented to demonstrate the efficacy of the proposed approach and the approximate solution. [ABSTRACT FROM AUTHOR]

This paper investigates the diversity order of beamforming schemes in frequency-selective multiple-input multiple-output (MIMO) amplify-and-forward (AF) relaying channels with a nonnegligible direct link between the source and the destination. In this system, orthogonal frequency-division multiplexing and bit-interleaved coded modulation schemes are adopted to exploit multipath diversity. Due to a nonconvex property of the problem, the optimal performance of flat-fading MIMO relaying channels, which is imperative to derive the diversity order, is still unknown. To this end, we first identify upper and lower bounds of the signal-to-noise ratio (SNR) of the flat-fading channels, which lead to upper and lower bounds of diversity order of coded beamforming in frequency-selective channels. Then, we establish the diversity order as a closed form by showing that these bounds are tight. In addition, our analysis provides an insightful guideline for code constructions to achieve a proper diversity gain of the system. The accuracy of our analysis is validated through simulation results. [ABSTRACT FROM PUBLISHER]

For clustered wireless networks, the conventional dual-hop cooperation technique is inefficient since relays with imbalanced channels to the source and the destination may become the bottleneck of the overall transmission. In this paper, a novel cooperative framework called the three-stage relaying (TSR) scheme is proposed to address this problem. In TSR, relays are divided into two clusters/groups, and a virtual multiple-input–multiple-output (MIMO) antenna array is formed by introducing the transmission between the two relay groups. This “relay-to-relay” communication enables transmissions over shorter distances, which naturally breaks the bottleneck emerging in the dual-hop cooperation scheme. An optimization problem is formulated to deal with the relay selection for these two groups, aiming at maximizing the received signal-to-noise ratio (SNR) at the destination, subject to the number-of-relay constraint. This problem turns out to be nontrivial due to the coupling of the relays between two groups. We tackle this challenging problem by decomposing it into two subproblems: One deals with the selection of transmitting relays, and the other deals with the selection of receiving relays. We then propose two heuristic algorithms that achieve the tradeoff between the optimality of the solution and the computational complexity. Through extensive numerical simulations, we show the superiority of TSR over the dual-hop cooperation schemes in both the clustered and centered wireless networks in terms of symbol error rate and throughput. [ABSTRACT FROM PUBLISHER]

In this paper, an energy harvesting dual-hop relaying system without/with the presence of co-channel interference (CCI) is investigated. Specifically, the energy constrained multi-antenna relay node is powered by either the information signal of the source or via the signal receiving from both the source and interferer. In particular, we first study the outage probability and ergodic capacity of an interference free system, and then extend the analysis to an interfering environment. To exploit the benefit of multiple antennas, three different linear processing schemes are investigated, namely, 1) Maximum ratio combining/maximum ratio transmission (MRC/MRT), 2) Zero-forcing/MRT (ZF/MRT) and 3) Minimum mean-square error/MRT (MMSE/MRT). For all schemes, both the systems outage probability and ergodic capacity are studied, and the achievable diversity order is also presented. In addition, the optimal power splitting ratio minimizing the outage probability is characterized. Our results show that the implementation of multiple antennas increases the energy harvesting capability, hence, significantly improves the systems performance. Moreover, it is demonstrated that the CCI could be potentially exploited to substantially boost the performance, while the choice of a linear processing scheme plays a critical role in determining how much gain could be extracted from the CCI. [ABSTRACT FROM PUBLISHER]

In this paper, we consider multiuser multihop relay communication systems, where the users, relays, and the destination node may have multiple antennas. We address the issue of source and relay precoding matrices design to maximize the system mutual information (MI). By exploiting the link between the maximal MI and the weighted minimal mean-squared error (WMMSE) objective functions, we show that the intractable maximal MI-based source and relay optimization problem can be solved via the WMMSE-based source and relay design through an iterative approach which is guaranteed to converge to at least a stationary point. For the WMMSE problem, we derive the optimal structure of the relay precoding matrices and show that the WMMSE matrix at the destination node can be decomposed into the sum of WMMSE matrices at all hops. Under a (moderately) high signal-to-noise ratio (SNR) condition, this WMMSE matrix decomposition significantly simplifies the solution to the WMMSE problem. Numerical simulations are performed to demonstrate the effectiveness of the proposed algorithm. [ABSTRACT FROM PUBLISHER]

In this paper, we consider the transceiver and relay design for the multiple-input multiple-output (MIMO) cellular two-way relay channel (cTWRC), where a multi-antenna base station (BS) exchanges information with multiple multiantenna mobile stations via a multi-antenna relay station (RS). We propose a novel two-way relaying scheme to approach the sum capacity of the MIMO cTWRC. A key contribution of this work is a new nonlinear lattice-based precoding technique to precompensate the interstream interference, so as to achieve efficient interference-free lattice decoding at the relay. We derive sufficient conditions for the proposed scheme to asymptotically achieve the sum capacity of the MIMO cTWRC in the high signal-to-noise ratio (SNR) regime. To fully exploit the potential of the proposed scheme, we also investigate the optimal power allocation at the BS and the RS to maximize the weighted sum-rate of the MIMO cTWRC in the general SNR regime. It is shown that the problem can be formulated as a monotonic program, and a polyblock outer approximation algorithm is developed to find the globally optimal solution with guaranteed convergence. We demonstrate by numerical results that the proposed scheme significantly outperforms the existing schemes and closely approaches the sum capacity of the MIMO cTWRC in the high SNR regime. [ABSTRACT FROM PUBLISHER]

In this paper, the end-to-end performance of a wireless relay transmission system that employs amplify-and-forward (AF) relays and operates in an interference-limited Nakagami- $m$ fading environment is studied. The wireless links from one relay node to another experience Nakagami- $m$ fading, and the number of interferers per hop is Poisson distributed. The aggregate interference at each relay node is modeled as a shot-noise process whose distribution follows an $\alpha$-stable process. For the considered system, analytical expressions for the moments of the end-to-end signal-to-interference ratio (SIR), the end-to-end outage probability (OP), the average bit-error probability (ABEP), and the average channel capacity are obtained. General asymptotic expressions for the end-to-end ABEP are also derived. The results provide useful insights regarding the factors affecting the performance of the considered system. Monte Carlo simulation results are further provided to demonstrate the validity of the proposed mathematical analysis. [ABSTRACT FROM PUBLISHER]

This paper investigates the achievable rate and ergodic capacity of a non-orthogonal amplify-and-forward (NAF) half-duplex multi-relay network where multiple relays exploit channel state information (CSI) to cooperate with a pair of source and destination. In the first step, for a given input covariance matrix at the source, we derive an optimal power allocation scheme among the relays via optimal instantaneous power amplification coefficients to maximize the achievable rate. Given the nature of broadcasting and receiving collisions in NAF, the considered problem in this step is non-convex. To overcome this drawback, we propose a novel method by evaluating the achievable rate in different sub-domains of the vector channels. It is then demonstrated that the globally optimal solution can be derived in closed-form. In the next step, we establish the ergodic channel capacity by jointly optimizing the input covariance matrix at the source and the power allocation among the relays. We show that this is a bi-level non-convex problem and solve it using Tammer decomposition method. This approach allows us to transform the original optimization problem into an equivalent master problem and a set of sub-problems having closed-form solutions derived in the first step. The channel capacity is then obtained using an iterative water-filling-based algorithm. Finally, we analyze the capacity-achieving input covariance matrix at the source in high and low signal-to-noise ratio (SNR) regimes. At sufficiently high SNRs, it is shown that the transmit power at the source should be equally distributed in all broadcasting and cooperative phases. On the other hand, in low SNR regions, the source should spend all its power in the broadcasting phase associated with a relay having the strongest cascaded source-relay and relay-destination channels. [ABSTRACT FROM AUTHOR]

We study the outage performance of orthogonal frequency-division multiplexing dual-hop opportunistic amplify-and-forward relaying in the presence of I/Q imbalance (IQI) in all nodes. We derive a closed-form expression for the end-to-end outage probability for the general case where each node suffers from a different IQI level and assuming that the distance between source and relay is not the same for all relays. Furthermore, we consider the more general case where there is a direct link between the source and the destination and we derive a closed-form expression for the end-to-end outage probability for both maximum-ratio and selection combining at the destination. To gain more insights, we analyze a special case where all relays lie on the perpendicular line midway between source and destination and all nodes experience the same IQI level. Our simulations show excellent match to the analytical results and both demonstrate that uncompensated IQI can be detrimental but it can also be effectively mitigated using a few opportunistic relays. In summary, the main contributions of this paper are analyzing the effect of IQI on the outage probability of an opportunistic relaying system and determining the number of relays needed to effectively mitigate uncompensated IQI. [ABSTRACT FROM AUTHOR]

The performance analysis of a distributed access wireless network, in which nodes are randomly distributed [namely, a random wireless network (RWN)] over a large area, is challenging. An effective mathematical modeling of these networks can help network designers to predict whether a given set of quality-of-service (QoS) requirements can be satisfied prior to the real network deployment. Previous models for the performance analysis of multihop RWNs fail to give accurate results due to the assumption of a single-hop, limited, and a priori known number of stations; negligence of the hidden terminals; and the use of a Boolean logic collision model. In this paper, we develop an alternative probabilistic modeling approach to analyze the performance of these networks in medium-access-control (MAC) and network layers. Using the proposed modeling approach and obtained metrics, we can identify a set of optimal parameters to maximize the performance of the networks. [ABSTRACT FROM PUBLISHER]

Cooperative communications has received tremendous interests in wireless systems. The communication performance of a single source–destination pair can be enhanced via the help of relays in traditional cooperative communications. Recent advances in interference cancelation (IC) can further enhance the performance of parallel cooperative transmissions between multiple source–destination pairs. Although some works have been done on cooperative communications, most existing works are focused on link-level physical-layer issues. Consequently, the potential benefits of cooperative communications on network-level upper layer issues, such as topology control, are largely ignored in existing works. In this paper, we study the impacts of cooperative communications, particularly IC-based cooperative communications, on topology control in wireless ad hoc networks (WANETs). We propose a distributed cooperative topology control scheme with opportunistic IC (COIC) to improve network capacity in WANETs by jointly considering both upper layer network capacity and physical-layer cooperative communications with IC. We show that the benefits brought by cooperative communications are opportunistic, which rely on network structures and channel conditions. These opportunistic advantages have significant impacts on network capacity, and our proposed COIC can effectively capture these opportunities to substantially improve network capacity. [ABSTRACT FROM PUBLISHER]

The performance of relay deployments depends significantly on the capacity of the wireless relay link between a relay node (RN) and its serving base station (BS). Exploiting the deployment flexibility of RNs, relay site planning (RSP) can be utilized to overcome the limitations of the relay link. In particular, RSP is carried out by selecting RN deployment locations from a discrete set of alternatives considering the signal-to-interference-plus-noise ratio (SINR) on the relay link as the selection criterion. In this paper, we present an analytical framework for RSP that can be used for planning and dimensioning of two-hop cellular relay networks operating over composite fading/shadowing channels in the presence of cochannel interference. Nakagami–lognormal distribution is used to model the relay link, whereas the access link between a mobile terminal (MT) and its serving RN is modeled by Rician–lognormal distribution. As these composite models do not have closed-form distribution functions, we utilize mixture gamma (MG) distribution to accurately approximate them. Further, the total cochannel interference in the considered multicellular system is approximated using the moment-generating function (MGF) matching method. Accordingly, we present closed-form expressions for the distributions of relay-link SINR, link rates, and end-to-end rate. In addition, RSP is shown to effectively decrease the amount of fading (AoF) and, thus, mitigate the detrimental effects of composite fading/shadowing. Thorough results reveal significant performance improvements, which justify the use of RSP in cellular relay networks. [ABSTRACT FROM PUBLISHER]

We consider the Gaussian “diamond” or parallel relay network, in which a source node transmits a message to a destination node with the help of N relays. Even for the symmetric setting, in which the channel gains to the relays are identical and the channel gains from the relays are identical, the capacity of this channel is unknown in general. The best known capacity approximation is up to an additive gap of order N bits and up to a multiplicative gap of order N^2, with both gaps independent of the channel gains. In this paper, we approximate the capacity of the symmetric Gaussian N-relay diamond network up to an additive gap of 1.8 bits and up to a multiplicative gap of a factor 14. Both gaps are independent of the channel gains and, unlike the best previously known result, are also independent of the number of relays N in the network. Achievability is based on bursty amplify-and-forward, showing that this simple scheme is uniformly approximately optimal, both in the low-rate as well as in the high-rate regimes. The upper bound on capacity is based on a careful evaluation of the cut-set bound. We also present approximation results for the asymmetric Gaussian N-relay diamond network. In particular, we show that bursty amplify-and-forward combined with optimal relay selection achieves a rate within a factor O(\log^4(N)) of capacity with preconstant in the order notation independent of the channel gains. [ABSTRACT FROM AUTHOR]

CoopMAC has been recently proposed as an efficient medium-access-control (MAC)-layer protocol for cooperative wireless networks. This protocol allows high-data-rate terminals to assist low-data-rate terminals by relaying their data. The backward compatibility of this protocol with well-known distributed local area networks (LANs), its adaptable functionality, and its capability to flatten the received signal-to-noise ratio (SNR) level over time make CoopMAC a desirable choice for cooperative communications. In spite of the prevalence and importance of this protocol, not enough attention has been paid to this protocol so far. Therefore, in this paper, we develop an analytical model to evaluate the performance of this protocol. Our proposed model is centered around a 2-D Markov chain, similar to that introduced by Bianchi for IEEE 802.11. This model empowers us to derive the probability density function (pdf) of medium-access delay in CoopMAC from which any higher statistical moments can be obtained. Using this obtained pdf, both local and global network performance metrics can be improved. [ABSTRACT FROM PUBLISHER]

In this paper, we consider an interference multiple-input multiple-output (MIMO) relay system where multiple source nodes communicate with their desired destination nodes concurrently with the aid of distributed relay nodes all equipped with multiple antennas. We aim at minimizing the total source and relay transmit power such that a minimum signal-to-interference-plus-noise ratio (SINR) threshold is maintained at each receiver. An iterative joint power control and beamforming algorithm is developed to achieve this goal. The proposed algorithm exploits transmit-relay-receive beamforming technique to mitigate the interferences from the unintended sources in conjunction with transmit power control. In particular, we apply the semidefinite relaxation technique to transform the relay transmission power minimization problem into a semidefinite programming (SDP) problem which can be efficiently solved by interior point-based methods. Numerical simulations are performed to demonstrate the effectiveness of the proposed iterative algorithm. [ABSTRACT FROM PUBLISHER]

In this paper, we consider a secrecy relaying communication scenario where all nodes are equipped with multiple antennas. An eavesdropper has the access to the global channel state information (CSI), and all the other nodes only know the CSI not associated with the eavesdropper. A new secrecy transmission protocol is proposed, where the concept of interference alignment is combined with cooperative jamming to ensure that artificial noise from transmitters can be aligned at the destination, but not at the eavesdropper due to the randomness of wireless channels. Analytical results, such as ergodic secrecy rate and outage probability, are developed, from which more insightful understanding of the proposed protocol, such as multiplexing and diversity gains, can be obtained. A few special cases, where outage probability cannot be decreased to zero regardless of SNR, are also discussed. Simulation results are provided to demonstrate the performance of the proposed secrecy transmission protocol. [ABSTRACT FROM AUTHOR]

Cooperative techniques have been shown to significantly improve the performance of wireless networks. Despite being a mature technology in single communication link scenarios, their implementation in wider, and practical, networks poses several challenges which have not been fully identified and understood so far. In this paper, the implementation of cooperative communications in non-centralized ad hoc networks with sensing-based channel access is extensively discussed. Both analysis and simulation are employed to provide a clear understanding of the mutual influence between the link layer contention mechanism and relaying protocols. Results show that sensing-based channel access significantly hampers the effectiveness of cooperation by biasing the spatial distribution of available relays, and by inducing a level of spatial and temporal correlation of interference that diminishes the diversity improvement on which cooperative gains are founded. Moreover, the efficiency reduction entailed by several practical protocol issues related to carrier sense multiple access which are typically neglected in the literature is thoroughly investigated. [ABSTRACT FROM PUBLISHER]

In this paper, we investigate data dissemination in delay tolerant networks (DTNs) via roadside wireless local area networks (RS-WLANs). The data dissemination service is destined to a group of nomadic nodes roaming in a large network region with a low node density. The local nodes within the coverage area of an RS-WLAN can provide packet caching and relaying capabilities. We consider a cooperative data dissemination approach where information packets are first pre-downloaded to the local nodes within the RS-WLAN before the visit of a pedestrian nomadic node, and then opportunistically scheduled to transmit to the nomadic node upon its arrival. In order to resolve the channel contention among multiple direct/relay links and exploit the predictable traffic characteristics as a result of packet pre-downloading, a double-loop receiver-initiated medium access control (DRMAC) scheme is proposed. The MAC scheme can achieve spatial and temporal diversity via the outer-loop and inner-loop MAC, respectively. A receiver initiated mechanism is used to reduce the signalling overhead, where the ACK message is used as an invitation of channel contention. An analytical model is established to evaluate the performance of the proposed MAC scheme. Numerical results demonstrate that the proposed MAC scheme can significantly improve the number of delivered packets from an RS-WLAN to a nomadic node as compared with the existing MAC schemes. [ABSTRACT FROM PUBLISHER]

In this paper, we propose a set of joint transmit diversity selection (TDS) and relay selection (RS) algorithms based on discrete iterative stochastic optimization for the uplink of cooperative multiple-input–multiple-output (MIMO) systems. Decode-and-forward (DF) and amplify-and-forward (AF) multirelay systems with linear minimum mean square error (MSE), successive interference cancelation, and adaptive reception are considered. The problems of TDS and RS are expressed as MSE and mutual information (MI) joint discrete optimization problems and solved using iterative discrete stochastic algorithms. Such an approach circumvents the need for exhaustive searching and results in a range of procedures with low complexity and increased speed of convergence that can track the optimal selection over an estimated channel. The proposed schemes are analyzed in terms of their complexity, convergence, and diversity benefits and are shown to be both stable and computationally efficient. Their performance is then evaluated via MSE, MI, and bit error rate comparisons and shown to outperform conventional cooperative transmission and, in the majority of scenarios, match that of the optimal exhaustive solution. [ABSTRACT FROM PUBLISHER]

In this paper, we propose and analyze a multihop transmission scheme based on detect-and-forward (DF) relay protocol and network coding (NC) for two-way relay channels. In this scheme, the odd relays perform hard detection and then forward the detected signals to the next hop, whereas the even relays perform NC on the detected signals from the two adjacent nodes and broadcast them to the next hop. By separating the network into multiple two-hop subsystems, we develop closed-form expressions for bit error rate (BER) in flat Rayleigh fading channels. Our results are given as both lower bound and asymptotic expression based on an accurate upper bound of the end-to-end signal-to-noise ratio (SNR). It is shown that the proposed scheme has the same asymptotic BER performance and a much higher throughput compared with the conventional bidirectional relay scheme based on four transmission phases. Therefore, the proposed scheme is efficient for practical wireless applications. Simulation results are provided to validate the analysis. [ABSTRACT FROM PUBLISHER]

In this paper, we consider a general cooperative wireless sensor network (WSN) with multiple hops and the problem of channel estimation. Two matrix-based set-membership (SM) algorithms are developed for the estimation of complex matrix channel parameters. The main goal is to significantly reduce the computational complexity, compared with existing channel estimators, and extend the lifetime of the WSN by reducing its power consumption. The first proposed algorithm is the SM normalized least mean squares (SM-NLMS) algorithm. The second is the SM recursive least squares (RLS) algorithm called BEACON. Then, we present and incorporate an error bound function into the two channel estimation methods, which can automatically adjust the error bound with the update of the channel estimates. Steady-state analysis in the output mean-square error (MSE) is presented, and closed-form formulas for the excess MSE and the probability of update in each recursion are provided. Computer simulations show good performance of our proposed algorithms in terms of convergence speed, steady-state mean square error, and bit error rate (BER) and demonstrate reduced complexity and robustness against time-varying environments and different signal-to-noise ratio (SNR) values. [ABSTRACT FROM PUBLISHER]

This paper considers transmission schemes in multiaccess relay networks (MARNs) where J single-antenna sources send independent information to one N-antenna destination through one M-antenna relay. For complexity considerations, we propose a linear framework, where the relay linearly transforms its received signals to generate the forwarded signals without decoding and the destination uses its multiantennas to fully decouple signals from different sources before decoding, by which the decoding complexity is linear in the number of sources. To achieve a high symbol rate, we first propose a scheme called ConcurrentS\rightarrowR\rightarrowD-IC_D in which all sources' information streams are concurrently transmitted in both the source-relay link and the relay-destination link. In this scheme, distributed space-time coding (DSTC) is applied at the relay, which satisfies the linear constraint. DSTC also allows the destination to conduct the zero-forcing interference cancellation (IC) scheme originally proposed for multiantenna systems to fully decouple signals from different sources. Our analysis shows that the symbol rate of ConcurrentS\rightarrowR\rightarrowD-IC_D is 1/2 symbols/source/channel use and the diversity gain of the scheme is upperbounded by M-J+1. To achieve a higher diversity gain, we propose another scheme called ConcurrentR\rightarrowD-IC_D in which the sources time-share the source-relay link. The relay coherently combines the signals on its antennas to maximize the signal-to-noise ratio (SNR) of each source, then concurrently forwards all sources' information. The destination performs zero-forcing IC. It is shown through both analysis and simulation that when N\geq 2J-1, ConcurrentR\rightarrowD-IC_D achieves the same maximum diversity gain as the full TDMA scheme in which information streams from each source are assigned to orthogonal channels in both links, but with a higher symbol rate. [ABSTRACT FROM AUTHOR]

Due to the half-duplex transmission nature of cooperative protocols, almost all cooperative diversity systems suffer from low spectral efficiency. In general, adaptive modulation can offer a good tradeoff between spectral efficiency and bit-error-probability (BEP) performance of wireless communication systems. In this paper, we propose two adaptive modulation schemes for the cooperative system with detect-and-forward relaying that simultaneously exploit the cooperative diversity gain and improve the spectral efficiency of the system. In the first scheme, which is similar to conventional selection relaying, the relay decides independently whether to forward the source messages to the destination or not by comparing the instantaneous received signal-to-noise ratio (SNR) with a predetermined fixed threshold, which is independent of the instantaneous modulation scheme. In contrast, in the second scheme, the relay and the destination jointly decide whether the relay will participate in the second phase of the protocol or remain silent. This second scheme can be viewed as a combination of selection and incremental relaying and achieves higher spectral efficiency in comparison with its nonincremental counterparts. We evaluate the performance of the protocols in terms of the end-to-end BEP, the average spectral efficiency, and the outage probability for both identically and nonidentically distributed Rayleigh fading environments. In our analysis, we consider M-ary phase-shift keying ( M-PSK) modulation, M-ary pulse-amplitude modulation (M-PAM), and M-ary quadrature-amplitude modulation ( M-QAM) schemes and examine the impact of error propagation from the relay to the destination on the end-to-end performance of the system in detail. Moreover, we derive a closed-form expression for the optimal relaying threshold that effectively mitigates error propagation and minimizes the end-to-end BEP of the system. The high-SNR analysis demonstrates that both proposed schemes provide full diversity. [ABSTRACT FROM PUBLISHER]

In this paper, we propose algorithms to jointly optimize the multiple multiantenna relays which assist multipoint-to-multipoint communications in wireless networks. Assuming that the knowledge of the second order statistics of the channels such as covariance matrices are available, the multiple-input-multiple-output (MIMO) relays are designed using two different methods: 1) minimize the sum of the powers of the relays while fulfilling the signal-to-interference-plus-noise ratio (SINR) requirements for all destinations and 2) maximize the minimum of the SINRs of all destinations satisfying the transmit power constraint of each MIMO relay. Furthermore, considering the fact that the covariance matrices of the channels between the MIMO relays and destinations are subject to uncertainty due to feedback and quantization errors, the robust versions of the aforementioned methods based on worst-case concept are proposed. It is shown that the proposed nonrobust as well as robust designs are nonconvex optimization problems but they can be solved accurately and efficiently using the standard semidefinite relaxation and randomization techniques. Computer simulations verify the improved performance of the robust designs over nonrobust methods. [ABSTRACT FROM AUTHOR]

Since the electromagnetic spectrum resource is becoming more and more scarce, improving spectral efficiency is becoming extremely important for the sustainable development of wireless communication systems and services. Integrating cooperative relaying techniques into spectrum-sharing cognitive radio systems sheds new light on higher spectral efficiency. In this paper, we analyze the end-to-end performance of cooperative amplify-and-forward (AF) relaying in spectrum-sharing systems. In order to achieve the optimal end-to-end performance, the transmit powers of the secondary source and the relays are optimized with respect to average interference power constraints at primary users and Nakagami-m fading parameters of interference channels (for mathematical tractability, the desired channels from secondary source to relay and from relay to secondary destination are assumed to be subject to Rayleigh fading). Also, both partial and opportunistic relay-selection strategies are exploited to further enhance system performance. Based on the exact distribution functions of the end-to-end signal-to-noise ratio (SNR) obtained herein, the outage probability, average symbol error probability, diversity order, and ergodic capacity of the system under study are analytically investigated. Our results show that system performance is dominated by the resource constraints and it improves slowly with increasing average SNR. Furthermore, larger Nakagami-m fading parameter on interference channels deteriorates system performance slightly. On the other hand, when interference power constraints are stringent, opportunistic relay selection can be exploited to improve system performance significantly. All analytical results are corroborated by simulation results and they are shown to be efficient tools for exact evaluation of system performance [ABSTRACT FROM PUBLISHER]

The hybrid relay selection (HRS) scheme, which adaptively chooses amplify-and-forward (AF) and decode-and-forward (DF) protocols based on the decoding results at the relay, is very effective to achieve robust performance in wireless relay networks. This paper analyzes the frame error rate (FER) of the HRS scheme in general wireless relay networks without and with utilizing error control coding at the source node. We first develop an improved signal-to-noise ratio (SNR) threshold-based FER approximation model. Then, we derive an analytical average FER expression as well as a high SNR asymptotic expression for the HRS scheme and generalize to other relaying schemes. Simulation results exhibit an excellent agreement with the theoretical analysis, which validates the derived FER expressions. [ABSTRACT FROM AUTHOR]

This paper analyzes the performance of cooperative relay networks using adaptive decode-and-forward (DF) relaying in two-wave with diffuse power (TWDP) fading channels. Among the multiple parallel relays that participate in transmission, only the relays that successfully decode the source message retransmit the signals to the destination. At the destination, the direct signals from the source and the indirect signals via the relays are combined using maximal-ratio combining (MRC). We first derive a new closed-form expression for the outage probability, which encompasses the Rayleigh fading and Rician fading as special cases. We then derive a tight lower bound on the outage probability to reduce computational complexity. The correctness of our derivation is validated through Monte Carlo simulations. It is shown that increasing the number of relays decreases the outage probability at high signal-to-noise ratio (SNR), but increases the outage probability at low to medium SNR. [ABSTRACT FROM AUTHOR]

In this paper, we investigate a communication system in which unmanned aerial vehicles (UAVs) are used as relays between ground-based terminals and a network base station. We develop an algorithm for optimizing the performance of the ground-to-relay links through control of the UAV heading angle. To quantify link performance, we define the ergodic normalized transmission rate (ENTR) for the links between the ground nodes and the relay, and derive a closed-form expression for it in terms of the eigenvalues of the channel correlation matrix. We show that the ENTR can be approximated as a sinusoid with an offset that depends on the heading of the UAV. Using this observation, we develop a closed-form expression for the UAV heading that maximizes the uplink network data rate while keeping the rate of each individual link above a certain threshold. When the current UAV relay assignments cannot meet the minimum link requirements, we investigate the deployment and heading control problem for new UAV relays as they are added to the network, and propose a smart handoff algorithm that updates node and relay assignments as the topology of the network evolves. [ABSTRACT FROM PUBLISHER]