Athanasios S. Lioumpas

Uplink scheduling for Machine-to-Machine communications in LTE-based cellular systems

Cellular systems are expected to play a fundamental role in the future Machine-to-Machine (M2M) networks, which could inherit crucial benefits from the former, such as ubiquitous coverage and global internetworking. However, unique features of M2M communications, such as the larger number of connected devices and the diversity of applications, require specific enhancements to these cellular systems. Because of the nature of most of M2M applications, a large number of uplink transmissions is expected, rendering uplink scheduling as an essential issue towards supporting M2M communications via cellular networks. We propose two scheduling schemes for the uplink of LTE-based cellular systems, which take into account both the channel conditions and the maximum allowed delay of each device requesting to be served. In contrast to other scheduling algorithms no classes of devices are formed, but the exact delay constraint of each device is considered, approaching the requirements of M2M communications in a more realistic way. This way, the number of effectively served requests increases, while it becomes possible to exploit the exact delay constraints in order to put the devices in sleep modes, until next transmissions. It is also shown that dividing the devices into a limited number of QoS classes deteriorates the systems performance.

Analytical modelling and performance evaluation of realistic time-controlled M2M scheduling over LTE cellular networks

Supporting emerging machine-to-machine (M2M) communications over Long-term Evolution (LTE)/LTE Advanced cellular networks in an efficient way will be beneficial for both telecommunication communities. The first step to migrate to an M2M-enabled cellular standard is to provide these new services through the existing architectures and protocols, while maintaining seamless backward compatibility. To this end, we thoroughly examined a key LTE Medium Access Control entity, which is the packet scheduler, and proposed solutions based on the time-controlled M2M feature, to deal with the diverse M2M traffic characteristics and quality-of-service requirements. Starting from the single M2M class case, we extended our study to more realistic scenarios, involving more M2M classes with diverse quality-of-service requirements. We defined analytical models for predicting the system performance on the basis of queueing theory concepts and considered the interaction between classes with different priorities. The proposed analytical models are validated through extensive system-level simulations. On the basis of the insight obtained from our analytical approach, we modified an existing scheduling algorithm to improve the performance of low-priority M2M device groups, and we demonstrated its superior performance both experimentally and analytically. Copyright

Evolution of packet scheduling for Machine-Type communications over LTE: Algorithmic design and performance analysis

Providing LTE connectivity to emerging Machine-to-Machine (M2M) applications imposes several challenges to the operation and optimization of current and future 3GPP mobile broadband standard releases. Scheduling in an efficient way M2M traffic over the existing LTE MAC infrastructure is decisive for the smooth evolution towards an M2M-enabled LTE system. The large number of connecting devices compared to classical LTE terminals, and their vastly diverse quality-of-service requirements, call for the design of new packet scheduling schemes tailored to the M2M paradigm. To this end, we propose low complexity and signaling scheduling policies which periodically grant access to the M2M devices. In particular, we first propose an analytical model for predicting the QoS performance of M2M services when the fixed periodic scheduling algorithm is employed. Next we propose a modification to this scheme, which exploit queueing-dynamics and finally we examine QoS-differentiation issues when devices are grouped into clusters. Interesting performance-complexity trade-offs are exposed. The results of our study may aid the system designer in tuning and optimizing M2M traffic scheduling.

Partitioning of Distributed MIMO Systems Based on Overhead Considerations

Distributed-Multiple Input Multiple Output (D-MIMO) networks is a promising enabler to address the challenges of high traffic demand in future wireless networks. A limiting factor that is directly related to the performance of these systems is the overhead signaling required for distributing data and control information among the network elements. In this paper, the concept of orthogonal partitioning is extended to D-MIMO networks employing joint multi-user beamforming, aiming to maximize the effective sum-rate, i.e., the actual transmitted information data. Furthermore, in order to comply with practical requirements, the overhead subframe size is considered to be constrained. In this context, a novel formulation of constrained orthogonal partitioning is introduced as an elegant Knapsack optimization problem, which allows the derivation of quick and accurate solutions. Several numerical results give insight into the capabilities of D-MIMO networks and the actual sum-rate scaling under overhead constraints.

Energy efficient AF relaying under error performance constraints with application to M2M networks

In this paper, we present a machine to machine (M2M) communication scheme, where the source communicates with the corresponding destination node with the help of one or more relay nodes. The objective is to minimize the total consumed power at the relays under specific performance constraints, while also considering the individual power limitations of each relay. In order to solve the original problem, we transform into an equivalent convex optimization problem. Simulations results are provided to validate the theoretical analysis and to illustrate the desired tradeoff between error performance and total energy consumption.

Energy efficient cooperative scheduling based on sleep-wake mechanisms

In this paper, we present a Machine to Machine (M2M) communication scheme, where the source communicates with the corresponding destination with the help of multiple relays. Relays that participate into the communication process are selected according to three alternative selection criteria, while the non selected relays sleep for periods of fixed length. Thus, the proposed method combines cooperative communication techniques and sleep wake mechanisms. In order to enhance further energy efficiency and prolong network lifetime, we seek to determine the optimal energy allocated at each active relay, according to the minimization of the total energy consumed at the relays, under specific error performance constraints. Simulation results are provided to illustrate the performance of the proposed method.

A hybrid contention/reservation medium access protocol for Wireless Sensor Networks

Most Wireless Sensor Networks (WSNs) applications require that a large number of low-complexity energy-constrained sensors forward the sensed information to a sink node, which gathers, processes and forwards the information to the end-user. Although, this approach is beneficial for the sensor nodes, since most of the complexity is delegated to the sink node, it is also the origin of severe bottleneck effects near to sink. Having a large number of sensors trying to forward their data to a single sink node leads to increased traffic intensity, congestion and increased packet loss probabilities. The key point in coping with this problem is the medium access control (MAC) protocols that handle the sensors traffic, with respect to specific performance optimization criteria. Following the general research trend, which focuses on hybrid contention/reservation MAC protocols, we present a novel hybrid protocol, which decides on the access mode to be used, based on the trade-off between the expected throughput and protocol complexity. The expected throughput can be predicted by exploiting an analytical framework grounded on the queueing theory, which evaluates the performance of both contention based and contention-free access schemes. Extensive system simulation results validate the theoretical derivations and the ability of the proposed hybrid MAC scheme to balance between performance and complexity.

Mitigating shadowing effects through cluster-head cooperation techniques

In many situations the performance of wireless communication systems decreases especially when they operate over multipath fading channels subject also to shadowing. In this sense, cluster-based networks have been introduced as an efficient solution, offering coverage extension and energy saving. In this study, the authors investigate new cluster-head (CH) selection algorithms, where the nodes can select different CHs, according to the corresponding signal strength. In addition, it is shown that if CHs are equipped with multiple antennas, the negative consequences of fading/shadowing can be further reduced. The performance of this scheme is theoretically investigated over correlated Nakagami-m fading channels, which are also subject to shadow fading, modelled by gamma distribution. The derived statistical metrics are used to obtain numerical evaluated results for the outage and the average bit error probabilities. These results are complemented by computer simulated ones, which validate the accuracy of the proposed analysis.

Enhancing the efficiency of cluster-based networks through MISO techniques

Towards the realization of the Internet of Things (IoT), cellular networks are expected to play a fundamental role, providing the ubiquitous coverage and global internetworking. However, due to the physical limitations, namely energy consumption or hardware complexity, of many of these objects, the direct communication with the cellular infrastructure is hindered. In this sense, cluster-based networks have been introduced as an efficient solution, offering coverage extension and energy savings. The energy efficiency and performance of these networks can be further enhanced if the devices can choose between two or more cluster-heads towards their connection to the infrastructure. In this paper, we propose a novel cluster-head (CH) selection algorithm, where the nodes can switch between different CHs, according to the corresponding signal strength, in order to maintain a predefined quality of service constraint. We show that the network reliability significantly increases, especially when considering mobile scenarios, where the connection to a CH may be not feasible, due to shadowing. In addition, the CHs are equipped with multiple antennas for enhanced performance. The performance of this scheme is theoretically investigated over correlated Nakagami-m multipath fading channels, subject also to shadowing. By considering Gamma distributed shadow effects, convenient expressions for important statistical metrics are obtained. The theoretical analysis is accompanied by representative performance evaluation results, complemented by equivalent computer simulated ones, which validate the accuracy of the proposed analysis.