49 research outputs found
Single-User Performance of Direct-Sequence Code-Division Multiple-Access Using Relay Diversity and Power Allocation
No full textIn this contribution, the authors investigate the single-user-bound performance of a direct-sequence code-division multiple-access (DS-CDMA) system, where one source mobile terminal (MT) communicates with its base-station (BS) with the assistance of multiple relays. The authors assume that the communications channels experience both propagation pathloss and fast fading, and that the channels from the source MT to the BS and relays as well as that from the relays to the BS may experience different fast fading modelled correspondingly by Nakagami-m distributions. In the study, they assume both the single-user combining using maximal ratio combining and the multiuser combining, which are derived based on the maximum signal-tointerference-plus-noise ratio criterion. The bit-error-rate (BER) performance of the DS-CDMA is investigated associated with considering the locations of the relays as well as the power-allocation among the source MT and relays. From the study and simulation results, it can be shown that the achievable BER performance of the DS-CDMA depends on the locations of the relays and also on the power-allocation among the source MT and relays. When the relays of a source MT are chosen from a different area, the power-allocation should also be adjusted correspondingly in order to achieve the minimum BER. Furthermore, when optimum power allocation is assumed, the BER performance of the DS-CDMA can be significantly improved, when increasing the number of relays assisting the source MT
Performance of Relay-Aided DS-CDMA Downlink Systems Communicating over Nakagami-m Fading Channels
No full textEfficiently Scheduling Parallel DAG Tasks on Identical Multiprocessors
No full textParallel real-time embedded applications can be modelled as directed acyclic graphs (DAGs) whose nodes model subtasks and whose edges model precedence constraints among subtasks. Efficiently scheduling such parallel tasks can be challenging in itself, particularly in hard real-time systems where it must be ensured offline that the deadlines of the parallel applications will be met at run time. In this paper, we tackle the problem of scheduling DAG tasks on identical multiprocessor systems efficiently, in terms of processor utilisation. We propose a new algorithm that attempts to use dedicated processor clusters for high-utilisation tasks, as in federated scheduling, but is also capable of reclaiming the processing capacity lost to fragmentation, by splitting the execution of parallel tasks over different existing clusters, in a manner inspired by semi-partitioned C=D scheduling (originally devised for non-parallel tasks). In the experiments with synthetic DAG task sets, our Segmented-Flattened-and-Split scheduling approach achieves a significantly higher scheduling success ratio than federated scheduling.Version submitted to RTNS 2024, on 16/08/2024 (with some typos fixed
Mixed Criticality Scheduling of Probabilistic Real-Time Systems
No full textIn this paper we approach the problem of Mixed Criticality (MC) for probabilistic real-time systems where tasks execution times are described with probabilistic distributions. In our analysis, the task enters high criticality mode if its response time exceeds a certain threshold, which is a slight deviation from a more classical approach in MC. We do this to obtain an application oriented MC system in which criticality mode changes depend on actual scheduled execution. This is in contrast to classical approaches which use task execution time to make criticality mode decisions, because execution time is not affected by scheduling while the response time is. We use a graph-based approach to seek for an optimal MC schedule by exploring every possible MC schedule the task set can have. The schedule we obtain minimizes the probability of the system entering high criticality mode. In turn, this aims at maximizing the resource efficiency by the means of scheduling without compromising the execution of the high criticality tasks and minimizing the loss of lower criticality functionality. The proposed approach is applied to test cases for validation purposes
Design and implementation of ambiently powered Internet of Things-That-Think with asynchronous inference
No full textThis work was supported by the Hellenic Foundation for Research and Innovation (H.F.R.I.) through the “First Call for H.F.R.I. Research Projects to support Faculty Members and Researchers and the Procurement of High-Cost Research Equipment” under Project 2846.
This article was presented in part at 5th IEEE International Workshop on Wireless Communications and Networking in Extreme Environments (WCNEE), International Conference on Distributed Computing in Sensor Systems (DCOSS), Pafos, Cyprus, July 2021, pp. 458–465 [DOI: 10.1109/DCOSS52077.2021.00077]. (Vasileios Papageorgiou, Athanasios Nichoritis, and Panagiotis Vasilakopoulos contributed equally to this work.) (Corresponding author: Aggelos Bletsas.)Summarization: This work offers design and implementation of in-network inference, using message passing among ambiently powered wireless sensor network (WSN) terminals. The stochastic nature of ambient energy harvesting dictates intermittent operation of each WSN terminal and as such, the message passing inference algorithms should be robust to asynchronous operation. It is shown, perhaps for the first time in the literature (to the best of our knowledge), a proof of concept, where a WSN harvests energy from the environment and processes itself the collected information in a distributed manner, by converting the (network) inference task to a probabilistic, in-network message passing problem, often at the expense of increased total delay. Examples from Gaussian belief propagation and average consensus (AC) are provided, along with the derivation of a statistical convergence metric for the latter case. A k-means method is offered that maps the elements of the calculated vector to the different WSN terminals and overall execution delay (in number of iterations) is quantified. Interestingly, it is shown that there are divergent instances of the in-network message passing algorithms that become convergent, under asynchronous operation. Ambient solar energy harvesting availability is also studied, controlling the probability of successful (or not) message passing. Hopefully, this work will spark further interest for asynchronous message passing algorithms and technologies that enable in-network inference, toward ambiently powered, batteryless Internet of Things-That-Think.Presented on: IEEE Internet of Things Journa
Energy-Efficient Channel-Dependent Cooperative Relaying for the Multiuser SC-FDMA Uplink
No full textFirst-hop-quality-aware dynamic resource allocation for amplify-and-forward opportunistic relaying assisted SC-FDMA
No full textIn this paper we exploit the benefits of the diversity gains arising from a cluster of opportunistic relays (OR) and from the independently fading subcarriers of multiple users. Our goal is to improve the energy-efficiency of the OR assisted single-carrier frequency-division multiple-access (SC-FDMA) uplink using amplify-and-forward (AF), where the direct transmission (DT) link is unavailable. By assuming that the pilot aided channel quality information (CQI) of all the users may be exchanged amongst the cooperating relays, we propose two joint dynamic resource allocation (DRA) schemes based on the so-called ’first-hop quality awareness’. Our results demonstrate that compared to the DT benchmark, the proposed joint DRA schemes are capable of achieving a power reduction of 10dB for a single-antenna base station (BS) receiver, albeit for a multi-antenna BS the power-reduction remains more modest
Distributed beamforming and power allocation for cooperative networks.
No full textCooperative diversity systems rely on using relay nodes to relay copies of transmitted information to the destination such that each copy experiences different channel fading, hence increasing the diversity of the system. However, without proper processing of the message at the relays, the performance of the cooperative system may not necessarily perform better than direct transmission systems. In this paper, we proposed a distributed beamforming and power allocation algorithm which substantially improves the diversity of the system with only very limited feedback from the destination node. We also derive outage probability as well as study the outage behavior of this scheme
Optimal Cooperative MAC Protocol with Efficient Selection of Relay Terminals
No full textA new cooperative protocol is proposed in the context of wireless mesh networks. The protocol implements ondemand
cooperation, i.e. cooperation between a source terminal
and a destination terminal is activated only when needed. In that case, only the best relay among a set of available terminals is re-transmitting the source message to the destination terminal. This typical approach is improved using three additional features. First, a splitting algorithm is implemented to select the best relay. This ensures a fast selection process. Moreover, the duration of the selection process is now completely characterized.
Second, only terminals that improve the outage probability of the direct link are allowed to participate to the relay selection. By this means, inefficient cooperation is now avoided. Finally, the destination terminal discards the source message when it fails to decode it. This saves processing time since the destination terminal does not need to combine the replicas of the source message: the one from the source terminal and the one from the best relay. We prove that the proposed protocol achieves an optimal performance in terms of Diversity-Multiplexing Tradeoff
(DMT)
On-Demand Decode and Forward Cooperative MAC for VoIP in Wireless Mesh Networks
No full textThe employment of wireless mesh networking in real- life scenarios has attracted substantial research interest in recent years and in this context VoIP has become a ubiquitous application. However, it has been demonstrated that VoIP transmissions over a multihop network may still remain inadequate in terms of their high packet-loss ratio and network-induced delays. To alleviate these limitations, we propose a novel distributed MAC-layer cooperation protocol, which is based on the decode-and-forward regime, whilst relying on the lowest possible control packet overhead. Furthermore, we employ several improvements across the protocol stack, including an improved PHY-layer, based on three-stage turbo-style differential detection, packet aggregation in the MAC-layer, as well as on adjusting the retransmission limit of each packet in order to reduce the delay imposed when employing cooperation. We characterize our improved system in a Wireless Mesh Network (WMN) scenario using the OMNeT++ network simulator and compare it to an 802.11g-based benchmarker. As a benefit of these techniques, we have observed up to 10-fold reduction in the energy consumption per bit, despite increasing the number of simultaneous calls supported by up to 9, when the number of hops between the sources and destination is 6
