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Non-Coherent Detection for Diffusive Molecular Communications

Vahid Jamali, Nariman Farsad, Robert Schober, and Andrea Goldsmith
Papers under Review IEEE Transactions on Communications, 2017

Abstract

We study non-coherent detection schemes for molecular communication (MC) systems that do not require knowledge of the channel state information (CSI). In particular, we first derive the optimal maximum likelihood (ML) multiple-symbol (MS) detector for MC systems. As a special case of the optimal MS detector, we show that the optimal ML symbol-by-symbol (SS) detector can be equivalently written in the form of a threshold-based detector, where the optimal decision threshold is constant and depends only on the statistics of the MC channel. The main challenge of the MS detector is the complexity associated with the calculation of the optimal detection metric. To overcome this issue, we propose an approximate MS detection metric which can be expressed in closed form. To reduce complexity even further, we develop a non-coherent decision-feedback (DF) detector and a suboptimal blind detector. Finally, we derive analytical expressions for the bit error rate (BER) of the optimal SS detector, as well as upper and lower bounds for the BER of the optimal MS detector. Simulation results confirm the analysis and reveal the effectiveness of the proposed optimal and suboptimal detection schemes compared to a benchmark scheme that assumes perfect CSI knowledge, particularly when the number of observations used for detection is sufficiently large.

Symbol Synchronization for Diffusion-Based Molecular Communications

Vahid Jamali, Arman Ahmadzadeh, and Robert Schober
Papers under Review IEEE Transactions on Nanobioscience, 2017

Abstract

Symbol synchronization refers to the estimation of the start of a symbol interval and is needed for reliable detection. In this paper, we develop several symbol synchronization schemes for molecular communication (MC) systems where we consider some practical challenges which have not been addressed in the literature yet. In particular, we take into account that in MC systems, the transmitter may not be equipped with an internal clock and may not be able to emit molecules with a fixed release frequency. Such restrictions hold for practical nanotransmitters, e.g. modified cells, where the lengths of the symbol intervals may vary due to the inherent randomness in the availability of food and energy for molecule generation, the process for molecule production, and the release process. To address this issue, we develop two synchronization-detection frameworks which both employ two types of molecule. In the first framework, one type of molecule is used for symbol synchronization and the other one is used for data detection, whereas in the second framework, both types of molecule are used for joint symbol synchronization and data detection. For both frameworks, we first derive the optimal maximum likelihood (ML) symbol synchronization schemes as performance upper bounds. Since ML synchronization entails high complexity, for each framework, we also propose three low-complexity suboptimal schemes, namely a linear filter-based scheme, a peak observation-based scheme, and a threshold-trigger scheme which are suitable for MC systems with limited computational capabilities. Furthermore, we study the relative complexity and the constraints associated with the proposed schemes and the impact of the insertion and deletion errors that arise due to imperfect synchronization.

SCW Codes for Maximum Likelihood Detection in Diffusive Molecular Communications without Channel State Information

Vahid Jamali, Arman Ahmadzadeh, Nariman Farsad, and Robert Schober
Papers under Review IEEE Transactions on Communications, 2017

Abstract

Instantaneous or statistical channel state information (CSI) is needed for most detection schemes developed for molecular communication (MC) systems. Since the MC channel changes over time, e.g., due to variations in the velocity of flow, the temperature, or the distance between transmitter and receiver, CSI acquisition has to be conducted repeatedly to keep track of CSI variations. Frequent CSI acquisition may entail a large overhead whereas infrequent CSI acquisition may result in a low CSI estimation accuracy. To overcome these challenges, we design codes which enable maximum likelihood sequence detection at the receiver without instantaneous or statistical CSI. In particular, assuming concentration shift keying modulation, we show that a class of codes, referred to as strongly constant-weight (SCW) codes, enables optimal CSI-free sequence detection at the expense of a decrease in data rate. For the proposed SCW codes, we analyze the code rate, the error rate, and the average number of released molecules. In addition, we study the properties of binary SCW codes and balanced SCW codes in further detail. Simulation results verify our analytical derivations and reveal that SCW codes with CSI-free detection outperform uncoded transmission with optimal coherent and non-coherent detection.

Cooperative Abnormality Detection via Diffusive Molecular Communications

Reza Mosayebi, Vahid Jamali, Nafiseh Ghoroghchian, Masoumeh Nasiri-Kenari, and Robert Schober
Papers under Review IEEE Transactions on Nanobioscience, 2017

Abstract

In this paper, we consider abnormality detection via diffusive molecular communications (MCs) for a network consisting of several sensors and a fusion center (FC). If a sensor detects an abnormality, it injects into the medium a number of molecules which is proportional to the sensed value. Two transmission schemes for releasing molecules into the medium are considered. In the first scheme, referred to as DTM, each sensor releases a different type of molecule, whereas in the second scheme, referred to as STM, all sensors release the same type of molecule. The molecules released by the sensors propagate through the MC channel and some may reach the FC where the final decision regarding whether or not an abnormality has occurred is made. We derive the optimal decision rules for both DTM and STM. However, the optimal detectors entail high computational complexity as log-likelihood ratios (LLRs) have to be computed. To overcome this issue, we show that the optimal decision rule for STM can be transformed into an equivalent low-complexity decision rule. Since a similar transformation is not possible for DTM, we propose simple low-complexity sub-optimal detectors based on different approximations of the LLR. The proposed low-complexity detectors are more suitable for practical MC systems than the original complex optimal decision rule, particularly when the FC is a nano-machine with limited computational capabilities. Furthermore, we analyze the performance of the proposed detectors in terms of their false alarm and missed detection probabilities. Simulation results verify our analytical derivations and reveal interesting insights regarding the trade-off between complexity and performance of the proposed detectors and the considered DTM and STM schemes.

Molecular Communication using Magnetic Nanoparticles

Wayan Wicke, Arman Ahmadzadeh, Vahid Jamali, Harald Unterweger, Christoph Alexiou, and Robert Schober
Papers under Review IEEE Wireless Communications and Networking Conference, 2018

Abstract

In this paper, we propose to use magnetic nanoparticles as information carriers for molecular communication. This enables the use of an external magnetic field to guide information-carrying particles towards the receiver. We show that the particle movement can be mathematically modeled as diffusion with drift. Thereby, we reveal that the key parameters determining the magnetic force are particle size and magnetic field gradient. As an example, we consider magnetic nanoparticle based communication in a blood vessel. For this bounded environment, we derive an analytical expression for the channel impulse response subject to fluid flow and magnetic drift. Numerical results, obtained by particle-based simulation, validate the accuracy of the derived analytical expressions. Furthermore, adopting the symbol error rate as performance metric, we show that using magnetic nanoparticles facilitates reliable communication, even in the presence of fluid flow.

On the Design of Matched Filters for Molecule Counting Receivers

Vahid Jamali, Arman Ahmadzadeh, and Robert Schober
Journal Papers IEEE Communications Letter, 2017

Abstract

In this paper, we design matched filters for diffusive molecular communication systems taking into account the following impairments: signal-dependent diffusion noise, inter-symbol interference (ISI), and external interfering molecules. The receiver counts the number of observed molecules several times within one symbol interval and employs linear filtering to detect the transmitted data. We derive the optimal matched filter by maximizing the expected signal-to-interference-plus-noise ratio of the decision variable. Moreover, we show that for the special case of an ISI-free channel, the matched filter reduces to a simple sum detector and a correlator for the channel impulse response for the diffusion noise-limited and (external) interference-limited regimes, respectively. Our simulation results reveal that the proposed matched filter considerably outperforms the benchmark schemes available in literature, especially when ISI is severe.

Optimal Relay Selection for the Parallel Hybrid RF/FSO Relay Channel: Non-Buffer-Aided and Buffer-Aided Designs

Marzieh Najafi, Vahid Jamali, and Robert Schober
Journal Papers IEEE Transactions on Communications, 2017

Abstract

Hybrid radio frequency (RF)/free space optical (FSO) systems are among the candidate enabling technologies for the next generation of wireless networks since they benefit from both the high data rates of the FSO subsystem and the high reliability of the RF subsystem. In this paper, we focus on the problem of throughput maximization in the parallel hybrid RF/FSO relay channel. In the parallel hybrid RF/FSO relay channel, a source node sends its data to a destination node with the help of multiple relay nodes. Thereby, for a given relay, the source-relay and the relay-destination FSO links are orthogonal with respect to each other due to the narrow beam employed for FSO transmission, whereas, due to the broadcast nature of the RF channel, half-duplex operation is required for the RF links if self-interference is to be avoided. Moreover, we consider the two cases where the relays are and are not equipped with buffers. For both cases, we derive the optimal relay selection policies for the RF and FSO links and the optimal time allocation policy for transmission and reception for the RF links. The proposed optimal protocols provide important insights for optimal system design. Since the optimal buffer-aided (BA) policy introduces an unbounded end-to-end delay, we also propose a suboptimal BA policy which ensures certain target average delays. Moreover, we present distributed implementations for both proposed optimal protocols. Simulation results demonstrate that a considerable gain can be achieved by the proposed adaptive protocols in comparison with benchmark schemes from the literature.

Diffusive Mobile Molecular Communications Over Time-Variant Channels

Arman Ahmadzadeh, Vahid Jamali, Adam Noel, and Robert Schober
Journal Papers IEEE Communications Letter, 2017

Abstract

This letter introduces a formalism for modeling time-variant channels for diffusive molecular communication systems. In particular, we consider a fluid environment where one transmitter nano-machine and one receiver nano-machine are subjected to Brownian motion in addition to the diffusive motion of the information molecules used for communication. Due to the stochastic movements of the transmitter and receiver nanomachines, the statistics of the channel impulse response change over time. We show that the time-variant behaviour of the channel can be accurately captured by appropriately modifying the diffusion coefficient of the information molecules. Furthermore, we derive an analytical expression for evaluation of the expected error probability of a simple detector for the considered system. The accuracy of the proposed analytical expression is verified via particle-based simulation of the Brownian motion.

Capacity of the Gaussian Two-Hop Full-Duplex Relay Channel with Self-Interference

Nikola Zlatanov, Erik Sippel, Vahid Jamali, and Robert Schober
Journal Papers IEEE Transactions on Communications, 2016

Abstract

In this paper, we investigate the capacity of the Gaussian two-hop full-duplex (FD) relay channel with self-interference. This channel is comprised of a source, an FD relay, and a destination, where a direct source-destination link does not exist and the FD relay is impaired by self-interference. We model the self-interference as an additive Gaussian random variable whose variance is proportional to the amplitude of the transmit symbol at the relay. For this channel, we derive the capacity and propose an explicit capacity-achieving coding scheme. Thereby, we show that the optimal input distribution at the source is Gaussian and its variance depends on the amplitude of the transmit symbol at the relay. On the other hand, the optimal input distribution at the relay is discrete or Gaussian, where the latter case occurs only when the relay-destination link is the bottleneck link. The derived capacity converges to the capacity of the two-hop ideal FD relay channel without self-interference and to the capacity of the two-hop half-duplex (HD) relay channel in the limiting cases when the self-interference is zero and infinite, respectively. Our numerical results show that significant performance gains are achieved using the proposed capacity-achieving coding scheme compared to the achievable rates of conventional FD relaying and HD relaying.

Channel Estimation for Diffusive Molecular Communications

Vahid Jamali, Arman Ahmadzadeh, Christophe Jardin, Heinrich Sticht, and Robert Schober
Journal Papers IEEE Transactions on Communications, 2016

Abstract

In molecular communication (MC) systems, the expected number of molecules observed at the receiver over time after the instantaneous release of molecules by the transmitter is referred to as the channel impulse response (CIR). Knowledge of the CIR is needed for the design of detection and equalization schemes. In this paper, we present a training-based CIR estimation framework for MC systems which aims at estimating the CIR based on the observed number of molecules at the receiver due to emission of a sequence of known numbers of molecules by the transmitter. Thereby, we distinguish two scenarios depending on whether or not statistical channel knowledge is available. In particular, we derive maximum likelihood (ML) and least sum of square errors (LSSE) estimators which do not require any knowledge of the channel statistics. For the case, when statistical channel knowledge is available, the corresponding maximum a posteriori (MAP) and linear minimum mean square error (LMMSE) estimators are provided. As performance bound, we derive the classical Cramer Rao (CR) lower bound, valid for any unbiased estimator, which does not exploit statistical channel knowledge, and the Bayesian CR lower bound, valid for any unbiased estimator, which exploits statistical channel knowledge. Finally, we propose optimal and suboptimal training sequence designs for the considered MC system. Simulation results confirm the analysis and compare the performance of the proposed estimation techniques with the respective CR lower bounds.

Optimal Buffer-Aided Relaying with Imperfect CSI

Vahid Jamali, Noha Waly, Nikola Zlatanov, and Robert Schober
Journal Papers IEEE Communications Letters, 2016

Abstract

In this paper, we consider a two-hop relay channel that consists of a source, a half-duplex decode-and-forward relay, and a destination. We derive the optimal buffer-aided relaying policy based on imperfect channel state information (CSI) and under longterm per-node power constraints for the source and the relay. In particular, the proposed protocol determines the optimal state of the relay node, namely the transmit, receive, or silent state, based on the imperfect CSI of the involved links, the quality of the CSI, and the required acquisition overhead. Our simulation results verify the analysis and reveal that there is an optimal trade-off between the required acquisition overhead and the quality of the CSI that maximizes the throughput.

Link Allocation for Multiuser Systems With Hybrid RF/FSO Backhaul: Delay-Limited and Delay-Tolerant Designs

Vahid Jamali, Diomidis S. Michalopoulos, Murat Uysal, and Robert Schober
Journal Papers IEEE Transactions on Wireless Communications, vol. 15, no. 5, pp. 3281-3295, May 2016.

Abstract

In this paper, we consider a cascaded radio frequency (RF) and hybrid RF/free space optical (FSO) system where several mobile users transmit their data over an RF link to a decode-and-forward relay node (e.g., a small cell base station) and the relay forwards the information to a destination (e.g., a macro-cell base station) over a hybrid RF/FSO backhaul link. The relay and the destination employ multiple antennas for transmission and reception over the RF links while each mobile user has a single antenna. The RF links are orthogonal to the FSO link but half-duplex with respect to each other, i.e., either the user-relay RF link or the relay-destination RF link is active. For this communication setup, we derive the optimal fixed and adaptive link allocation policies for sharing the transmission time between the RF links based on the statistical and instantaneous channel state information (CSI) of the RF and FSO links, respectively. Thereby, we consider the following two scenarios depending on the delay requirements: 1) delay-limited transmission where the relay has to immediately forward the packets received from the users to the destination, and 2) delay-tolerant transmission where the relay is allowed to store the packets received from the users in its buffer and forward them to the destination when the quality of the relay-destination RF link is favorable. Our numerical results illustrate the effectiveness of the proposed communication architecture and link allocation policies, and their superiority compared to existing schemes, which employ only one type of backhaul link.

Buffer-Aided Relaying with Discrete Transmission Rates for the Two-Hop Half-Duplex Relay Network

Wayan Wicke, Nikola Zlatanov, Vahid Jamali, and Robert Schober
Journal Papers IEEE Transactions on Wireless Communications, 2016

Abstract

We consider the two-hop half-duplex (HD) relay network, where the source-to-relay and relay-to-destination links are impaired by block fading. The relay is equipped with a buffer, which enables the relay to receive or transmit in each time slot independent of previous time slots. As a practical constraint, source and relay can transmit only at rates taken from predefined and finite sets. Thereby, it is assumed that for each time slot the instantaneous qualities of the two links are available. For this network, we derive the optimal scheduling of reception and transmission at the relay and the optimal rate selection at source and relay such that the throughput is maximized. Since the optimal protocol introduces unbounded delay, we also propose a buffer-aided protocol which limits the delay. For this delay-limited protocol, we study the achieved delay and throughput by modeling the queue at the buffer as a Markov chain. Our numerical results show that the throughputs achieved with the proposed buffer-aided protocols for discrete transmission rates are significantly larger than the throughputs achieved with conventional relaying protocols where the HD relay switches between reception and transmission in a strictly alternating manner.

Buffer-Aided Diamond Relay Network with Block Fading and Inter-Relay Interference

Renato Simoni, Vahid Jamali, Nikola Zlatanov, Robert Schober, Laura Pierucci, and Romano Fantacci
Journal Papers IEEE Transactions on Wireless Communications, 2016

Abstract

A simple diamond half-duplex relay network composed of a source, two decode-and-forward half-duplex relays, and a destination is considered, where a direct link between the source and the destination does not exist. For this network, we study the case of buffer-aided relays, where the relays are equipped with buffers. Each relay can receive data from the source, store it in the buffer, and forward it to the destination when the channel conditions are advantageous. Thereby, buffering enables adaptive scheduling of the transmissions and receptions over time, which allows the network to exploit the diversity offered by the fading channels. For the considered half-duplex network, four transmission modes are defined based on whether the relay nodes receive or transmit. In this paper, we derive the locally optimal scheduling of the transmission modes over time and investigate the achievable average rate when the relays are affected by inter-relay interference. Since the proposed buffer-aided transmission policies introduce unbounded delay, we provide a sub-optimal buffer-aided transmission policy with limited delay. Moreover, for inter-relay interference cancellation, we consider two coding schemes with different complexities. In the first scheme, we employ dirty paper coding, which entails a high complexity whereas in the second scheme, we adopt a low-complexity technique based on successive interference cancellation at the receiving relay nodes and optimal power allocation at the transmitting nodes. Our numerical results show that the proposed protocols, with and without delay constraints, outperform existing protocols for the considered network from the literature.

Achievable Rates for the Fading Half-Duplex Single Relay Selection Network Using Buffer-Aided Relaying

Nikola Zlatanov, Vahid Jamali, and Robert Schober
Journal Papers IEEE Transactions on Wireless Communications, vol. 14, no. 8, pp. 4494-4507, Aug. 2015.

Abstract

In the half-duplex single relay selection network, comprised of a source, M half-duplex relays, and a destination, only one relay is active at any given time, i.e., only one relay receives or transmits, and the other relays are inactive, i.e., they do not receive nor transmit. The capacity of this network, when all links are affected by independent slow time-continuous fading and additive white Gaussian noise (AWGN) , is still unknown, and only achievable average rates have been reported in the literature so far. In this paper, we present new achievable average rates for this network, which are larger than the best known average rates. These new average rates are achieved with a buffer-aided relaying protocol. Since the developed buffer-aided protocol introduces unbounded delay, we also devise a buffer-aided protocol which limits the delay at the expense of a decrease in rate. Moreover, we discuss the practical implementation of the proposed buffer-aided relaying protocols and show that they do not require more resources for channel state information acquisition than the existing relay selection protocols.

Bidirectional Buffer-Aided Relay Networks With Fixed Rate Transmission—Part I: Delay-Unconstrained Case

Vahid Jamali, Nikola Zlatanov, and Robert Schober
Journal Papers IEEE Transactions on Wireless Communications, vol. 14, no. 3, pp. 1323-1338, March 2015.

Abstract

In this paper, we consider bidirectional relay networks in which two users exchange information only via a relay node, i.e., a direct link between both users is not present. We assume that channel state information at the transmitter is not available and/or only one coding and modulation scheme is used due to complexity constraints. Thus, the nodes transmit with a fixed predefined rate regardless of the channel state. In general, the nodes in the network can assume one of three possible states in each time slot, namely, the transmit, the receive, and the silent state. Most of the existing bidirectional relaying protocols assume a prefixed schedule for the sequence in which the states of the nodes are used. In this paper, we abandon the restriction of having a fixed and predefined schedule and consider the selection of the states of the nodes as a degree of freedom that can be exploited for performance optimization. To this end, the relay has to be equipped with two buffers for storage of the information received from the two users. In Part I of this paper, we propose a delay-unconstrained protocol that, based on the qualities of the involved links, selects the optimal states of the nodes in each time slot such that the sum throughput is maximized. In Part II, several delay-constrained protocols are proposed and analyzed. Numerical results show that the proposed protocols significantly outperform the existing bidirectional relaying protocols in the literature.

Bidirectional Buffer-Aided Relay Networks With Fixed Rate Transmission—Part II: Delay-Constrained Case

Vahid Jamali, Nikola Zlatanov, and Robert Schober
Journal Papers IEEE Transactions on Wireless Communications, vol. 14, no. 3, pp. 1339-1355, March 2015.

Abstract

This is the second part of a two-part paper considering bidirectional relay networks with half-duplex nodes and block fading where the nodes transmit with a fixed transmission rate. In Part I, it was shown that a considerable gain in terms of sum throughput can be obtained by optimally selecting the transmission modes or, equivalently, the states of the nodes, i.e., the transmit, the receive, and the silent states, based on the qualities of the involved links. To enable adaptive transmission mode selection, the relay has to be equipped with two buffers for storage of the data received from the two users. The protocol proposed in Part I was delay unconstrained and provides an upper bound for the performance of practical delay-constrained protocols. In this paper, we propose two heuristic but efficient delay-constrained protocols, which can approach the performance upper bound reported in Part I, even in cases where only a small delay is permitted. The proposed protocols not only consider the instantaneous qualities of the involved links for adaptive mode selection but also take the states of the queues at the buffers into account, i.e., the number of packets in the queues. The average throughput and the average delay of the proposed delay-constrained protocols are evaluated by analyzing the Markov chain of the states of the queues. Numerical results show that the proposed protocols outperform existing bidirectional relaying protocols for delay-constrained transmission.

On the Design of Fast Convergent LDPC Codes for the BEC: An Optimization Approach

Vahid Jamali, Yasser Karimian, Johannes Huber and Mahmoud Ahmadian Attari
Journal Papers IEEE Transactions on Communications, vol. 63, no. 2, pp. 351-363, Feb. 2015.

Abstract

The complexity-performance trade-off is a fundamental aspect of the design of low-density parity-check (LDPC) codes. In this paper, we consider LDPC codes for the binary erasure channel (BEC), use code rate for performance metric, and number of decoding iterations to achieve a certain residual erasure probability for complexity metric. We first propose a quite accurate approximation of the number of iterations for the BEC. Moreover, a simple but efficient utility function corresponding to the number of iterations is developed. Using the aforementioned approximation and the utility function, two optimization problems w.r.t. complexity are formulated to find the code degree distributions. We show that both optimization problems are convex. In particular, the problem with the proposed approximation belongs to the class of semi-infinite problems which are computationally challenging to be solved. However, the problem with the proposed utility function falls into the class of semi-definite programming (SDP) and thus, the global solution can be found efficiently using available SDP solvers. Numerical results reveal the superiority of the proposed code design compared to existing code designs from literature.

Adaptive Link Selection for Cognitive Buffer-Aided Relay Networks

Mostafa Darabi, Vahid Jamali, Behrouz Maham, and Robert Schober
Journal Papers IEEE Communications Letters, vol. 19, no. 4, pp. 693-696, April 2015.

Abstract

In this paper, we consider a block fading underlay cognitive radio network where the primary network (PN) consists of a source and a destination, and the secondary network (SN) has three nodes, namely a source, a half-duplex decode-and-forward relay, and a destination. We propose a novel link selection protocol for the SN such that the throughput of the SN is maximized while the average or instantaneous interference to the primary destination is kept below a certain threshold. In particular, in the proposed protocol, the secondary relay (SR) decides optimally when to transmit data, receive data, and be silent. To this end, the SR is equipped with a buffer for the storage of information. In the proposed policy, the interference from the PN to the SN is also taken into account and canceled by an opportunistic interference cancelation scheme. Our simulation results show that for a given interference threshold, the proposed link selection protocol outperforms the existing relaying policies reported in the literature in terms of thesecondary throughput.

Achievable Rate of the Half-Duplex Multi-Hop Buffer-Aided Relay Channel With Block Fading

Vahid Jamali, Nikola Zlatanov, Heba Shoukry, and Robert Schober
Journal Papers IEEE Transactions on Wireless Communications, vol. 14, no. 11, pp. 6240-6256, Nov. 2015.

Abstract

The half-duplex (HD) multi-hop relay channel consists of a source, multiple HD relays connected in series, and a destination where links are present only between adjacent nodes. In this paper, we focus on decode-and-forward relays and assume that the links are impaired by block fading and additive white Gaussian noise. We design a new protocol which, unlike the conventional protocols for the multi-hop relay channel, does not adhere to a fixed and predefined pattern of using the transmit, receive, and silent states of the nodes. In particular, the proposed protocol selects the optimal states of the nodes and the corresponding optimal transmission rates based on the instantaneous channel state information (CSI) of the involved links in each fading block such that the achievable average rate from source to destination is maximized. To enable adaptive scheduling of the states of the nodes, the relay nodes have to be equipped with buffers for temporary storage of the information received from the preceding node. Additionally, we discuss and address two practical challenges arising in the implementation of the optimal protocol, namely the unconstrained end-to-end delay due to data buffering at the relays and the required CSI overhead. Numerical results confirm the superiority of the proposed buffer-aided protocols compared to existing multi-hop relaying protocols.

Achievable Rate Region of the Bidirectional Buffer-Aided Relay Channel With Block Fading

Vahid Jamali, Nikola Zlatanov, Aissa Ikhlef, and Robert Schober
Journal Papers IEEE Transactions on Information Theory, vol. 60, no. 11, pp. 7090-7111, Nov. 2014.

Abstract

The bidirectional relay channel, in which two users communicate with each other through a relay node, is a simple but fundamental and practical network architecture. In this paper, we consider the block fading bidirectional relay channel with a decode-and-forward relay and propose efficient transmission strategies that exploit the block fading property of the channel. We assume that a direct link between the two users is not present and consider two transmission modes: 1) the multiple-access mode (both users transmit to the relay) and 2) the broadcast mode (the relay transmits to both users). Most existing relaying protocols assume a fixed schedule for using these transmission modes. In contrast, we abandon the restriction of having a fixed and predefined schedule and propose to optimize the selection of the transmission modes and the associated transmission rates based on the instantaneous channel state information (CSI) of the involved links. Thereby, we consider two different types of transmit power constraints: 1) a fixed transmit power for each node and 2) a per-node long-term power constraint. To enable the use of a nonpredefined schedule for transmission mode selection, the relay has to be equipped with two buffers for storage of the information received from both users. We develop new relaying protocols based on adaptive mode selection and provide the corresponding achievable long-term rate regions. In particular, based on the CSI of the involved links, the optimal transmission mode as well as the optimal transmission rates and/or the transmit powers of the nodes are chosen in each time slot to maximize the weighted sum rate of both users. By varying the weights assigned to the users, the boundary surface of the achievable long-term rate region of the proposed protocol can be obtained. In addition, we discuss and address two practical challenges for the implementation of the proposed protocols, namely, the availability of the knowledge of the channel statis- ics required for the implementation of the optimal protocols, and the increase of the end-to-end delay due to the data buffering. Numerical results confirm the superiority of the proposed buffer-aided protocols compared with existing bidirectional relaying protocols.

Optimization of Linear Cooperation in Spectrum Sensing Over Correlated Log-normal Shadow Fading Channels

Vahid Jamali, Nima Reisi, Mahmoud Ahmadian Attari, Soheil Salari
Journal Papers Wireless Personal Communications, Springer, vol. 72, pp. 1691-1706, Apr. 2013.

Abstract

The objective of cooperative spectrum sensing is to collaboratively detect the presence of the primary user by the aid of multiple secondary users. It is known that the performance of such a framework substantially depends on the fading assumption. In this paper, we propose an advanced framework for linear cooperative spectrum sensing in cognitive radio networks over correlated log-normal shadow fading channels. Considering the realistic sensing and reporting channels which are not addressed in similar works, motivates us to propose a novel approximation for correlated log-normal sum based on moment generating function calculation and moment matching method. Furthermore, the linear cooperative spectrum sensing coefficients are computed based on the optimization of the deflection criterion. This results in a framework with reasonable complexity which is suitable for practical applications. Simulation results show the excellent agreement between the exact and approximated statistics and the superior performance compared with conventional equally gain combiner.

Cluster-Based Cooperative Spectrum Sensing over Correlated Log-Normal Channels and Noise Uncertainty in Cognitive Radio Networks

Nima Reisi, Mahmoud Ahmadian Attari, Vahid Jamali, and Soheil Salari
Journal Papers IET Communications, vol. 6, no. 16, pp. 2725-2733, Nov. 6 2012.

Abstract

In this study, the authors consider the problem of cooperative spectrum sensing (CSS) based on linear combination of observations over correlated log-normal shadow-fading channels. To reduce the effects of imperfect reporting channels, a cluster-based CSS framework and a new cluster head selection algorithm are proposed. Using the received energies (as local observations) from different clusters, the fusion centre can make the final decision by linearly combining the noisy cluster observations. To calculate the combination weights, the authors come across the problem of joint distribution approximation of sum of the correlated log-normal random variables corresponding to different clusters. A joint moment generating function (MGF) matching algorithm is proposed in this study to estimate the summations by a single log-normal vector. Monte Carlo simulations confirm the accuracy of the proposed MGF-based approach in estimating the desired statistics and efficiency of the cluster-based spectrum-sensing algorithm in terms of primary signal detection.

Bayesian-based Cooperative Framework for Spectrum Sensing in Cognitive Radio Networks

Vahid Jamali, Nima Reisi, Mahmoud Ahmadian Attari, and Soheil Salari
Journal Papers International Journal of Information and Communication Technology Research, vol. 3, no. 3, Jun. 2011.

Abstract

Energy detection has been adopted as an alternative spectrum sensing method for cognitive radios due to its low computational complexity and not requiring a priori information about the signal to be detected. However, noise uncertainty and hidden terminal problem make energy detector practically challenging specially in low signal-to-noise ratio (SNR) regime. Collaboration among multiple cognitive radios has been recognized as a practical strategy to improve the reliability of spectrum sensing. In this paper, a cooperative spectrum sensing framework is proposed to blindly determine the occupancy of a wideband spectrum. Specifically, contrary to conventional energy detector, the proposed method does not require any knowledge of noise variance to detect the presence of primary signals. Moreover, diversity achieved by cooperation enables the framework to maintain a reasonable performance even in low SNR values. Simulation results confirm the effectiveness of our proposed method in improving both the probabilities of detection and false alarm.

Statistical Analysis of Time-Variant Channels in Diffusive Mobile Molecular Communications

Arman Ahmadzadeh, Vahid Jamali, and Robert Schober
Conference Papers IEEE Global Communications, 2017

Abstract

In this paper, we consider a diffusive mobile molecular communication (MC) system consisting of a pair of mobile transmitter and receiver nano-machines suspended in a fluid medium, where we model the mobility of the nano-machines by Brownian motion. The transmitter and receiver nano-machines exchange information via diffusive signaling molecules. Due to the random movements of the transmitter and receiver nano-machines, the statistics of the channel impulse response (CIR) change over time. We introduce a statistical framework for characterization of the impulse response of time-variant MC channels. In particular, we derive closed-form analytical expressions for the mean and the autocorrelation function of the impulse response of the channel. Given the autocorrelation function, we define the coherence time of the time-variant MC channel as a metric that characterizes the variations of the impulse response. Furthermore, we derive an analytical expression for evaluation of the expected error probability of a simple detector for the considered system. In order to investigate the impact of CIR decorrelation over time, we compare the performances of a detector with perfect channel state information (CSI) knowledge and a detector with outdated CSI knowledge. The accuracy of the proposed analytical expression is verified via particle-based simulation of the Brownian motion.

C-RAN with Hybrid RF/FSO Fronthaul Links: Joint Optimization of RF Time Allocation and Fronthaul Compression

Marzieh Najafi, Vahid Jamali, Derrick Wing Kwan Ng, and Robert Schober
Conference Papers IEEE Global Communications, 2017

Abstract

This paper considers the uplink of a cloud radio access network (C-RAN) comprised of several multi-antenna remote radio units (RUs) which send the data that they received from multiple mobile users (MUs) to a central unit (CU) via a wireless fronthaul link. One of the fundamental challenges in implementing C-RAN is the huge data rate required for fronthauling. To address this issue, we employ hybrid radio frequency (RF)/free space optical (FSO) systems for the fronthaul links as they benefit from both the large data rates of FSO links and the reliability of RF links. To efficiently exploit the fronthaul capacity, the RUs employ vector quantization to jointly compress the signals received at their antennas. Moreover, due to the limited available RF spectrum, we assume that the RF multiple-access and fronthaul links employ the same RF resources. Thereby, we propose an adaptive protocol which allocates transmission time to the RF multiple-access and fronthaul links in a time division duplex (TDD) manner and optimizes the quantization noise covariance matrix at each RU such that the sum rate is maximized. Our simulation results reveal that a considerable gain in terms of sum rate can be achieved by the proposed protocol in comparison with benchmark schemes from the literature, especially when the FSO links experience unfavorable atmospheric conditions.

SCW Codes for Optimal CSI-Free Detection in Diffusive Molecular Communications

Vahid Jamali, Arman Ahmadzadeh, Nariman Farsad, and Robert Schober
Conference Papers IEEE International Symposium on Information Theory (ISIT), 2017

Abstract

Instantaneous or statistical channel state information (CSI) is needed for most detection schemes developed in the molecular communication (MC) literature. Since the MC channel changes, e.g., due to variations in the velocity of flow, the temperature, or the distance between transmitter and receiver, CSI acquisition has to be conducted repeatedly to keep track of CSI variations. Frequent CSI acquisition may entail a large overhead whereas infrequent CSI acquisition may result in a low CSI estimation quality. To cope with these issues, we design codes which facilitate maximum likelihood sequence detection at the receiver without instantaneous or statistical CSI. In particular, assuming concentration shift keying modulation, we show that a class of codes, referred to as strongly constant-weight (SCW) codes, enables optimal CSI-free sequence detection at the cost of decreasing the data rate. For the proposed SCW codes, we analyze the code rate and the error rate. Simulation results verify our analytical derivations and reveal that the proposed CSI-free detector for SCW codes outperforms the baseline coherent and non-coherent detectors for uncoded transmission.

Symbol Synchronization for Diffusive Molecular Communication Systems

Vahid Jamali, Arman Ahmadzadeh, and Robert Schober
Conference Papers IEEE International Conference Communications (ICC), 2017

Abstract

Symbol synchronization refers to the estimation of the start of a symbol interval and is needed for reliable detection. In this paper, we develop a symbol synchronization framework for molecular communication (MC) systems where we consider some practical challenges which have not been addressed in the literature yet. In particular, we take into account that in MC systems, the transmitter may not be equipped with an internal clock and may not be able to emit molecules with a fixed release frequency. Such restrictions hold for practical nanotransmitters, e.g. modified cells, where the lengths of the symbol intervals may vary due to the inherent randomness in the availability of food and energy for molecule generation, the process for molecule production, and the release process. To address this issue, we propose to employ two types of molecules, one for synchronization and one for data transmission. We derive the optimal maximum likelihood (ML) symbol synchronization scheme as a performance upper bound. Since ML synchronization entails high complexity, we also propose two low-complexity synchronization schemes, namely a peak observation-based scheme and a threshold-trigger scheme, which are suitable for MC systems with limited computational capabilities. Our simulation results reveal the effectiveness of the proposed synchronization schemes and suggest that the end-to-end performance of MC systems significantly depends on the accuracy of symbol synchronization.

Cooperative Wireless Backhauling

Vahid Jamali, Nikola Zlatanov, and Robert Schober
Conference Papers International ITG Conference System, Communication, and Coding 2017.

Abstract

We consider wireless backhauling for a scenario where two small-cell base stations (SC-BSs) employ the same time and frequency resources for offloading their data to a common macrocell base station (MC-BS). The two SC-BSs allocate a part of the shared resource to exchange data in order to be able to cooperate to boost the backhaul capacity. For this scenario, we develop the optimal transmission strategy which, based on the channel state information, determines whether the SC-BSs should exchange data and cooperate or transmit their data independently to the MC-BS. Our numerical results demonstrate the superiority of the proposed cooperative wireless backhauling protocol compared to existing protocols in the literature.

Non-Coherent Multiple-Symbol Detection for Diffusive Molecular Communications

Vahid Jamali, Nariman Farsad, Robert Schober, and Andrea Goldsmith
Conference Papers ACM International Conference on Nanoscale Computing and Communication (NanoCom), 2016.

Abstract

Most of the available works on molecular communication (MC) assume that the channel state information (CSI) is perfectly known at the receiver for data detection. In contrast, in this paper, we study non-coherent multiple-symbol detection schemes which do not require knowledge of the CSI. In particular, we derive the optimal maximum likelihood (ML) multiple-symbol (MLMS) detector. Moreover, we propose an approximated detection metric and a sub-optimal detector to cope with the high complexity of the optimal MLMS detector. Numerical results reveal the effectiveness of the proposed optimal and suboptimal detection schemes with respect to a baseline scheme which assumes perfect CSI knowledge, particularly when the number of observations used for detection is sufficiently large.

Capacity of the Gaussian Two-Hop Full-Duplex Relay Channel with Self-Interference

Nikola Zlatanov, Erik Sippel, Vahid Jamali, and Robert Schober
Conference Papers IEEE Global Communications, 2016

Abstract

In this paper, we investigate the capacity of the Gaussian two-hop full-duplex (FD) relay channel with self-interference. This channel is comprised of a source, an FD relay, and a destination, where a direct source-destination link does not exist and the FD relay is impaired by self-interference. We model the self-interference as an additive Gaussian random variable whose variance is proportional to the amplitude of the transmit symbol at the relay. For this channel, we derive the capacity and propose an explicit capacity-achieving coding scheme. Thereby, we show that the optimal input distribution at the source is Gaussian and its variance depends on the amplitude of the transmit symbol at the relay. On the other hand, the optimal input distribution at the relay is discrete or Gaussian, where the latter case occurs only when the relay-destination link is the bottleneck link. The derived capacity converges to the capacity of the two-hop ideal FD relay channel without self-interference and to the capacity of the two-hop half-duplex (HD) relay channel in the limiting cases when the self-interference is zero and infinite, respectively. Our numerical results show that significant performance gains are achieved using the proposed capacity-achieving coding scheme compared to the achievable rates of conventional FD relaying and HD relaying.

Ion Pump Based Bio-Synthetic Modulator Model for Diffusive Molecular Communications

Hamidreza Arjmandi, Vahid Jamali Arman Ahmadzadeh, Robert Schober, Masoumeh Nasiri Kenari
Conference Papers International Workshop on Signal Processing Advances in Wireless Communications (SPAWC), 2016.

Abstract

In diffusive molecular communication (DMC), the transmitter has to be able to control the release of signaling molecules for modulation of the information bits. In natural cells, pumping ions is an important control mechanism for releasing molecules which is carried out by ion pumps embedded in the membrane. The activity of the ion pumps is controlled by a driving parameter. In particular, light driven pumps are controlled by light intensity and enable a high degree of spatial and temporal control for modulation functionality. In this paper, a modulator based on ion pumps is proposed for DMC which controls the release rate of the molecules from the transmitter by modulating a light intensity signal. The pumping process of the ion pump is modeled by a Markov model based on which the stochastic nature of the modulated signal, i.e., the release rate of the ions from the transmitter is analyzed. A simple on-off keying modulation scheme is realized based on the proposed modulator. Our numerical results show that a realistic transmitter can not release ions instantaneously nor deterministically.

Channel Estimation Techniques for Diffusion-Based Molecular Communications

Vahid Jamali, Arman Ahmadzadeh, Christophe Jardin, Heinrich Sticht, and Robert Schober
Conference Papers IEEE International Conference on Communications (ICC), 2016.

Abstract

In molecular communication (MC) systems, the expected number of molecules observed at the receiver over time after the instantaneous release of molecules by the transmitter is referred to as the channel impulse response (CIR). Knowledge of the CIR is needed for the design of detection and equalization schemes. In this paper, we present a training-based CIR estimation framework for MC systems which aims at estimating the CIR based on the observed number of molecules at the receiver due to emission of a sequence of known numbers of molecules by the transmitter. In particular, we derive maximum likelihood (ML) and least sum of square errors (LSSE) estimators. We also study the Cramer Rao (CR) lower bound and training sequence design for the considered system. Simulation results confirm the analysis and compare the performance of the proposed estimation techniques with the CR lower bound.

Adaptive Relay Selection Protocol for the Parallel Hybrid RF/FSO Relay Channel

Marzieh Najafi, Vahid Jamali, and Robert Schober
Conference Papers IEEE International Conference on Communications (ICC), 2016.

Abstract

Hybrid radio frequency (RF)/free space optical (FSO) systems are among the candidate enabling technologies for the next generation of wireless networks since they benefit from the advantages of both the FSO subsystem, e.g. high data rates, and the RF subsystem, e.g. high reliability in terms of link connectivity. In this paper, we focus on the problem of throughput maximization in the parallel hybrid RF/FSO relay channel. In the parallel hybrid RF/FSO relay channel, a source node sends its data to a destination node with the help of multiple relay nodes. Thereby, the source-relay and the relay-destination FSO links are orthogonal with respect to each other due to the narrow beam employed for FSO transmission whereas the RF links are half duplex with respect to each other due to the broadcast nature of RF signals. We derive the optimal relay selection policies for transmission and reception for the RF and FSO links and the optimal time allocation policy to the RF relay reception and transmission links. Simulation results demonstrate that a considerable gain can be achieved by the proposed adaptive protocol in comparison with benchmark schemes from the literature.

Novel Protocol with Improved Outage Probability Performance for the Fading Two-Hop Half-Duplex Relay Channel

Nikola Zlatanov, Vahid Jamali, Kwan Ng., and Robert Schober
Conference Papers IEEE International Conference on Communications (ICC), 2016.

Abstract

Based on the coding scheme recently introduced in [1], we propose new communication protocols with improved outage probability performance for the fading two-hop half-duplex (HD) relay channel. This channel is comprised of a source, a HD relay, and a destination, where a direct source-destination link does not exist. For this channel, we assume that the transmitting nodes do not have transmitter-side channel state information and therefore have to transmit with fixed rate. As a result, outages may occur. We propose protocols for the cases when feedback from receiving to transmitting nodes is not possible and restricted to one bit of feedback information per time slot, respectively. In the proposed protocols, the relay's silent symbol intervals, when the relay receives, carry information which improves the reliability of both the source-relay and relay-destination links. In contrast, in existing protocols, the relay's silent symbol intervals, when the relay receives, are a priori known to the destination and thereby cannot carry information. Our numerical results show that the proposed protocols achieve significant performance gains in terms of outage probability compared to existing protocols in the literature. These gains are only the result of using a different coding scheme than existing protocols.

Mixed RF and Hybrid RF/FSO Relaying

Vahid Jamali, Diomidis S. Michalopoulos, Murat Uysal, and Robert Schober
Conference Papers IEEE Global Communications Workshops (Globecom), 2015.

Abstract

In this paper, we consider a mixed RF and hybrid RF/FSO system where several mobile users transmit their data over an RF link to a relay node (e.g. a small cell base station) and the relay forwards the information to a destination (e.g. a macro cell base station) over a hybrid RF/FSO backhaul link. The relay and the destination employ multiple antennas for transmission and reception over the RF links while each mobile user has a single antenna. The RF links are orthogonal with respect to the FSO link and half-duplex with respect to each other, i.e., either the user-relay RF link or the relay-destination RF link is active. For this communication setup, we derive the optimal policy for sharing the RF transmission time between the RF links. Our numerical results show the effectiveness of the proposed communication architecture and link allocation policy, and their superiority compared to existing schemes which employ only one type of backhaul link.

Outage Analysis of q-Duplex RF/FSO Relaying

Vahid Jamali, Diomidis S. Michalopoulos, Murat Uysal, and Robert Schober
Conference Papers International Symposium on Wireless Communication Systems (ISWCS), 2015.

Abstract

In this paper, we propose and analyze a novel q-duplex radio frequency/free space optical (RF/FSO) relaying protocol for a mixed RF and hybrid RF/FSO communication system. In our scheme, several mobile users transmit their data over an RF link to a relay node (e.g. a small cell base station) and the relay forwards the information to a destination (e.g. a macro cell base station) over a hybrid RF/FSO backhaul link. The RF links are full-duplex with respect to the FSO link and half-duplex with respect to each other, i.e., either the user-relay RF link or the relay-destination RF link is active. Depending on the channel statistics, the q-duplex relaying protocol may reduce to full-duplex relaying, when the quality of the FSO link is sufficiently high, or to half-duplex relaying, when the FSO link becomes unavailable due to severe atmospheric conditions. We derive an analytical expression for the end-to-end outage probability of the proposed protocol when the fading for the user-relay RF link, the relay-destination RF link, and the relay-destination FSO link are modelled as Rayleigh, Ricean, and Gamma-Gamma distributed, respectively. Our simulation results confirm the analytical derivations and reveal the effectiveness of the proposed q-duplex protocol and its superiority compared to existing schemes.

On the Capacity of the Two-Hop Half-Duplex Relay Channel

Nikola Zlatanov, Vahid Jamali, and Robert Schober
Conference Papers IEEE Global Communications (Globecom), 2015.

Abstract

Although extensively investigated, the capacity of the two-hop half-duplex (HD) relay channel is not fully understood. In particular, a capacity expression which can be evaluated straightforwardly is not available and an explicit coding scheme which achieves the capacity is not known either. In this paper, we derive a new expression for the capacity of the two-hop HD relay channel based on a simplified converse. Compared to previous results, this capacity expression can be easily evaluated. Moreover, we propose an explicit coding scheme which achieves the capacity. To achieve the capacity, the relay does not only send information to the destination by transmitting information-carrying symbols but also with the zero symbols resulting from the relay's silence during reception. As examples, we compute the capacities of the two-hop HD relay channel for the cases when the source-relay and relay-destination links are both binary-symmetric channels (BSCs) and additive white Gaussian noise (AWGN) channels, respectively, and numerically compare the capacities with the rates achieved by conventional relaying where the relay receives and transmits in a codeword-by-codeword fashion and switches between reception and transmission in a strictly alternating manner. Our numerical results show that the capacities of the two-hop HD relay channel for BSC and AWGN links are significantly larger than the rates achieved with conventional relaying.

Buffer-Aided Diamond Relay Network with Block Fading

Renato Simoni, Vahid Jamali, Nikola Zlatanov, Robert Schober, Laura Pierucci, and Romano Fantacci
Conference Papers IEEE International Conference on Communications (ICC), pp. 1982-1987, 2015.

Abstract

A simple diamond half-duplex relay network composed of a source, two half-duplex relays, and a destination is considered, where no direct link between the source and the destination is exists. For this network, we investigate the achievable rate when the relays are equipped with buffers. Buffer-aided relays can receive data from the source, store it in their buffers, and forward it to the destination when the channel conditions are more advantageous. Thereby, buffering enables adaptive scheduling of the transmissions and receptions over time, which allows the network to better exploit the diversity offered by the fading channel. For the considered network, because of the half-duplex relays, four transmission modes are employed based on whether the relay nodes receive or transmit. Considering these four transmission modes, in this paper, we derive the optimal transmission mode selection policy such that the received data rate at the destination is maximized. Furthermore, based on numerical examples, we show that the proposed protocol outperforms the existing protocols for the considered network in the literature.

Adaptive Resource Allocation for the Fading Interference Channel with Source Cooperation

Marzieh Najafi, Vahid Jamali, and Robert Schober
Conference Papers IEEE Iranian Workshop on Communication and Information Theory (IWCIT), pp. 1-6, 2015.

Abstract

In this paper, we consider the Gaussian interference channel consisting of two source-destination pairs where the sources cooperate to transmit their messages to their respective destination. We assume block fading and that the nodes are constrained to half-duplex operation. For this channel, assuming availability of channel state information (CSI), we determine the optimal adaptive scheduling of reception and transmission at the source nodes. Thereby, the proposed protocol chooses one of the information flows, decides whether to transmit directly or cooperatively to the corresponding destination, and specifies the required transmission strategy such that the achievable ergodic rate region is maximized. Numerical results reveal that the proposed protocol can effectively realize the diversity offered by the fading processes of the different links in the interference channel.

Buffer-Aided Relaying with Discrete Transmission Rates

Wayan Wicke, Nikola Zlatanov, Vahid Jamali, and Robert Schober
Conference Papers IEEE Canadian Workshop on Information Theory (CWIT), pp. 186-189, 2015.

Abstract

We consider a slow fading three-node network consisting of a source, a half-duplex decode-and-forward relay, and a destination, where a direct link between the source and the destination does not exist. We assume that the half-duplex relay is equipped with a buffer and employs in each time slot adaptive scheduling of reception and transmission based on the instantaneous qualities of the source-to-relay and relay-to-destination links. Furthermore, as a practical constraint, we assume that in each time slot, the source and the relay can select transmission rates only from a discrete and finite set of available transmission rates. For this network, we design the optimal scheduling of reception and transmission at the relay and the optimal selection of the transmission rates at the source and the relay such that the throughput is maximized. Our numerical results show that the maximum throughput achieved with the proposed buffer-aided protocol for discrete transmission rates is significantly larger than the throughput achieved with conventional relaying protocols with continuous transmission rates.

A Delay-Constrained Protocol with Adaptive Mode Selection for Bidirectional Relay Networks

Vahid Jamali, Nikola Zlatanov, and Robert Schober
Conference Papers IEEE Global Communications (Globecom), pp. 4162-4167, 2014.

Abstract

In this paper, we consider a bidirectional relay network with half-duplex nodes and block fading where the nodes transmit with a fixed transmission rate. Thereby, user 1 and user 2 exchange information only via a relay node, i.e., a direct link between both users is not present. Recently in [1], it was shown that a considerable gain in terms of sum throughput can be obtained in bidirectional relaying by optimally selecting the transmission modes or, equivalently, the states of the nodes, i.e., the transmit, the receive, and the silent states, in each time slot based on the qualities of the involved links. To enable adaptive transmission mode selection, the relay has to be equipped with two buffers for storage of the data received from the two users. However, the protocol proposed in [1] was delay-unconstrained and provides an upper bound for the performance of practical delay-constrained protocols. In this paper, we propose a heuristic but efficient delay-constrained protocol which can approach the performance upper bound reported in [1]. Moreover, the average throughput and delay of the protocol are evaluated by analyzing the Markov chain of the states of the queues.

An Efficient Complexity-Optimizing LDPC Code Design for the Binary Erasure Channel

Vahid Jamali, Yasser Karimian, Johannes Huber and Mahmoud Ahmadian Attari
Conference Papers International Symposium on Turbo Codes and Iterative Information Processing (ISTC), pp. 238-242, 2014.

Abstract

The complexity-performance trade-off is a fundamental aspect of the design of low-density parity-check (LDPC) codes. In this paper, we consider LDPC codes for the binary erasure channel (BEC), use code rate for performance metric, and number of decoding iterations to achieve a certain residual erasure probability for complexity metric. The available complexity-optimizing problems in the literature for the BEC are either non-convex or belong to the class of semi-infinite problems which are computationally challenging to be solved. Hence, in this paper, we first propose a lower bound on the number of iterations for the BEC. Moreover, a simple but efficient utility function corresponding to the number of iterations is developed. Using this utility function, an optimization problem w.r.t. complexity is formulated to find complexity-optimized code degree distributions. We prove that the considered problem with the proposed utility function falls into the class of semi-definite programming (SDP) and thus, the global solution can be found efficiently using available SDP solvers. Numerical results reveal the superiority of the proposed code design compared to existing code designs from literature.

Adaptive Mode Selection for Bidirectional Buffer-Aided Relay Networks with Block Fading – Fixed Rate Transmission

Vahid Jamali, Nikola Zlatanov, and Robert Schober
Conference Papers IEEE International Conference on Communications (ICC), pp. 5831-5837, 2014.

Abstract

In this paper, we consider the problem of sum throughput maximization for bidirectional relay networks with block fading. Thereby, user 1 and user 2 exchange information only via a relay node, i.e., a direct link between both users is not present. We assume that channel state information at the transmitter (CSIT) is not available and/or only one coding and modulation scheme is used at the transmitters due to complexity constraints. Thus, the nodes transmit with a fixed predefined rate regardless of the channel state information (CSI). In general, the nodes in the network can assume one of three possible states in each time slot, namely the transmit, receive, and silent state. Most of the existing protocols assume a fixed schedule for the sequence of the states of the nodes. In this paper, we abandon the restriction of having a fixed and predefined schedule and propose a new protocol which, based on the CSI at the receiver (CSIR), selects the optimal states of the nodes in each time slot such that the sum throughput is maximized. To this end, the relay has to be equipped with two buffers for storage of the information received from the two users. Numerical results show that the proposed protocol significantly outperforms the existing protocols.

Adaptive Mode Selection and Power Allocation in Bidirectional Buffer-aided Relay Networks

Vahid Jamali, Nikola Zlatanov, Aissa Ikhlef and Robert Schober
Conference Papers IEEE Global Communications (Globecom), pp. 1933-1938, Dec. 2013.

Abstract

In this paper, we consider the problem of sum rate maximization in a bidirectional relay network with fading. Hereby, user 1 and user 2 communicate with each other only through a relay, i.e, a direct link between user 1 and user 2 is not present. In this network, there exist six possible transmission modes: four point-to-point modes (user 1-to-relay, user 2-to-relay, relay-to-user 1, relay-to-user 2), a multiple access mode (both users to the relay), and a broadcast mode (the relay to both users). Most existing protocols assume a fixed schedule of using a subset of the aforementioned transmission modes, as a result, the sum rate is limited by the capacity of the weakest link associated with the relay in each time slot. Motivated by this limitation, we develop a protocol which is not restricted to adhere to a predefined schedule for using the transmission modes. Therefore, all transmission modes of the bidirectional relay network can be used adaptively based on the instantaneous channel state information (CSI) of the involved links. To this end, the relay has to be equipped with two buffers for the storage of the information received from users 1 and 2, respectively. For the considered network, given a total average power budget for all nodes, we jointly optimize the transmission mode selection and power allocation based on the instantaneous CSI in each time slot for sum rate maximization. Simulation results show that the proposed protocol outperforms existing protocols for all signal-to-noise ratios (SNRs). Specifically, we obtain a considerable gain at low SNRs due to the adaptive power allocation and at high SNRs due to the adaptive mode selection.

Cooperative Spectrum Sensing with Per-User Power Constraints

Vahid Jamali, Bizan Golkar, Soheil Salari, Mahmoud Ahmadian, and Elvono S. Sousa,
Conference Papers IEEE 23rd International Symposium on Personal, Indoor and Mobile Radio Communications(PIMRC), pp. 1559-1564, Sep. 2012.

Abstract

In collaborative spectrum sensing, the presence of the primary user is detected at a central entity, known as the fusion center. This center collects the information from the secondary users and decides on the occupancy of the desired frequency band. In the conventional strategy, the secondary users transmit their initial observations toward the fusion center with their maximum transmit powers. In this paper, however, we consider the problem of beamforming among the secondary users with individual power constraints. Correlated shadow fading has been considered in the channel gains between the primary transmitter and the secondary users as well as the channel gains between the secondary users and the fusion center. We consider the problem of maximizing the probability of detection for a required probability of false alarm. Most previous works have considered the total power constraint while in practical scenarios, each secondary user has a limited battery lifetime. An algorithm is developed which efficiently solves the problem via second order cone programming (SOCP) in an iterative manner. An approximation of the original problem is studied which reduces the computational complexity of the iterative procedure. The Monte Carlo simulations confirm the effectiveness of the spectrum sensing framework compared to the conventional strategy.

Linear Cooperation for Spectrum Sensing over Correlated Log–Normal Shadow Fading Channels

Vahid Jamali, Soheil Salari, N. Reisi, and Jean-Pierre Cances,
Conference Papers International Symposium on Wireless Personal Multimedia Communications (WPMC), pp. 1-5, Oct. 2011.

Abstract

In this paper, we propose an advanced framework for linear cooperative spectrum sensing in cognitive radio networks over correlated log-normal shadow fading channels. Considering the realistic sensing and reporting channels which are not addressed in similar works, motivates us to propose a novel approximation for correlated log-normal sum. Furthermore, the linear cooperative spectrum sensing coefficients are computed based on the deflection criteria. This results in a framework with reasonable complexity which is suitable for practical applications. Simulation results show the excellent agreement between the exact and approximated statistics.

Interference Minimization Approach for Distributed Beamforming in Cognitive Two-way Relay Networks

Seyed Hamid Safavi, Ramezan A. Sadeghzadeh, Vahid Jamali, and Soheil Salari,
Conference Papers IEEE Pacific Rim Conference on Communications, Computers and Signal Processing (PacRim), pp. 532-536, Aug. 2011.

Abstract

This paper investigates the problem of interference minimization which restricts the secondary users (SUs) quality of service (QoS) while coexisting the primary users (PUs), using distributed beamforming for a bidirectional cognitive relay network. We consider a network which consists of two secondary transceivers and K cognitive relay nodes and a primary network with a transmitter and receiver, all equipped with single-antenna. For effective use of spectrum we propose a two-step two-way relaying for cognitive relay networks. Our aim is to design the beamforming coefficients for a bidirectional cognitive relay network through interference minimization approach subject to two constraints on each transceiver QoS which is solved using convex optimization. Our simulation results demonstrate that our distributed relay beamforming scheme improve network performance significantly so that the interference power is decreased by increasing the number of relay nodes while QoS of the secondary network is guaranteed.